U.S. patent application number 15/086612 was filed with the patent office on 2016-11-03 for bioerodible wraps and uses therefor.
The applicant listed for this patent is University of Pittsburgh - Of the Commonwealth System of Higher Education. Invention is credited to Mohammed S. El-Kurdi, Yi Hong, Lorenzo Soletti, John J. Stankus, David A. Vorp, William R. Wagner.
Application Number | 20160317280 15/086612 |
Document ID | / |
Family ID | 39674763 |
Filed Date | 2016-11-03 |
United States Patent
Application |
20160317280 |
Kind Code |
A1 |
El-Kurdi; Mohammed S. ; et
al. |
November 3, 2016 |
Bioerodible Wraps and Uses Therefor
Abstract
A tubular tissue graft device is provided comprising a tubular
tissue and a restrictive fiber matrix of a bioerodible polymer
about a circumference of the tubular tissue. The matrix may be
electrospun onto the tubular tissue. In one embodiment, the tubular
tissue is from a vein, such as a saphenous vein, useful as an
arterial graft, for example and without limitation, in a coronary
artery bypass procedure. Also provided is method of preparing a
tubular graft comprising depositing a fiber matrix of a bioerodible
polymer about a perimeter of a tubular tissue to produce a tubular
tissue graft device. A cardiac bypass method comprising bypassing a
coronary artery with a tubular tissue graft device comprising a
vein and a restrictive fiber matrix of a bioerodible polymer about
a circumference of the vein also is provided.
Inventors: |
El-Kurdi; Mohammed S.;
(Pittsburgh, PA) ; Hong; Yi; (Pittsburgh, PA)
; Soletti; Lorenzo; (Pittsburgh, PA) ; Stankus;
John J.; (Campbell, CA) ; Vorp; David A.;
(Pittsburgh, PA) ; Wagner; William R.; (Gibsonia,
PA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
University of Pittsburgh - Of the Commonwealth System of Higher
Education |
Pittsburgh |
PA |
US |
|
|
Family ID: |
39674763 |
Appl. No.: |
15/086612 |
Filed: |
March 31, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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14601523 |
Jan 21, 2015 |
9333068 |
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15086612 |
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12022430 |
Jan 30, 2008 |
9237945 |
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14601523 |
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60898356 |
Jan 30, 2007 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
D10B 2509/06 20130101;
A61L 2300/414 20130101; C12N 2533/40 20130101; D10B 2401/12
20130101; A61L 27/18 20130101; A61L 27/34 20130101; A61L 27/3691
20130101; A61L 27/3826 20130101; A61L 27/58 20130101; C12N 5/0068
20130101; D01D 5/0076 20130101; A61F 2210/0004 20130101; A61L
2430/22 20130101; A61F 2210/0076 20130101; A61L 27/3804 20130101;
A61F 2/07 20130101; A61L 27/3625 20130101; D01D 5/0007 20130101;
A61L 2300/604 20130101; A61F 2250/0067 20130101; A61F 2/06
20130101; A61L 27/507 20130101; A61L 27/54 20130101; D10B 2331/10
20130101; A61F 2240/001 20130101; A61L 27/3683 20130101; A61L 27/34
20130101; C08L 67/04 20130101 |
International
Class: |
A61F 2/06 20060101
A61F002/06; A61L 27/36 20060101 A61L027/36; D01D 5/00 20060101
D01D005/00; A61L 27/18 20060101 A61L027/18; A61L 27/38 20060101
A61L027/38; A61F 2/07 20060101 A61F002/07; A61L 27/50 20060101
A61L027/50; A61L 27/54 20060101 A61L027/54 |
Goverment Interests
STATEMENT REGARDING FEDERAL FUNDING
[0002] This invention was made with government support under Grant
Nos. HL65745 and HL069368, awarded by the National Institutes of
Health. The government has certain rights in the invention.
Claims
1-17. (canceled)
18. A graft device for a patient, comprising: a harvested vein
segment comprising an outer surface; a fiber matrix comprising one
or more electrospun synthetic fibers; wherein the one or more
electrospun synthetic fibers are deposited about the harvested vein
segment by positioning the fiber matrix in substantial contact with
the outer surface; and wherein the fiber matrix restricts expansion
of the resulting graft device when grafted.
19. The graft device according to claim 18, wherein the one or more
electrospun synthetic fibers are deposited about the outer surface
of the harvested vein segment while the harvested vein segment is
positioned about a spinning mandrel.
20. The graft device according to claim 18, wherein the harvested
vein segment comprises a saphenous vein.
21. The graft device according to claim 18, wherein the one or more
electrospun synthetic fibers comprise at least one polymer
comprising urethane linkages.
22. The graft device according to claim 18, wherein the one or more
electrospun synthetic fibers comprise a polymer derived from a
material selected from the group consisting of: an alpha-hydroxy
acid; a polylactide; a poly(lactide-co-glycolide); a
poly(L-lactide-co-caprolactone); a polyglycolic acid; a
poly(dl-lactide-co-glycolide); a poly(l-lactide-co-dl-lactide); a
polymer comprising a lactone monomer; a polycaprolactone; a polymer
comprising carbonate linkages; a polycarbonate; a polyglyconate; a
poly(glycolide-co-trimethylene carbonate); a
poly(glycolide-co-trimethylene carbonate-co-dioxanone); a polymer
comprising urethane linkages; a polyurethane; a poly(ester
urethane) urea; a poly(ester urethane) urea elastomer; a polymer
comprising ester linkages; a polyalkanoate; a polyhydroxybutyrate;
a polyhydroxyvalerate; a polydioxanone; a polygalactin; a natural
polymer; chitosan; collagen; elastin; alginate; cellulose;
hyaluronic acid; gelatin; and combinations thereof.
23. The graft device according to claim 18, wherein the one or more
electrospun synthetic fibers are electrospun from a fluid
comprising between 1% and 15% polymer by weight.
24. The graft device according to claim 18, wherein the fiber
matrix is formed over less than the entire outer surface of the
harvested vein segment.
25. The graft device according to claim 18, wherein the restriction
of expansion of the graft device provided by the restrictive fiber
matrix decreases over time.
26. The graft device according to claim 18, wherein the fiber
matrix comprises a porous mesh of the one or more electrospun
synthetic fibers.
27. The graft device according to claim 18, wherein the fiber
matrix comprises an anisotropic arrangement of the one or more
electrospun synthetic fibers.
28. The graft device according to claim 18, wherein the graft
device comprises an arterial bypass graft device.
29. The graft device according to claim 18, further comprising one
or more cells, active agents, or a combination thereof, associated
with the fiber matrix.
30. The graft device according to claim 18, further comprising the
one or more active agents that comprise a drug.
31. The graft device according to claim 30, wherein the drug
comprises a drug selected from the group consisting of: a
non-steroidal anti-inflammatory drug; an antibiotic; an
anticlotting factor; an immunosuppressant; a glucocorticoid; a drug
acting on an immunophilin; an interferon; a TNF binding protein; a
taxane; a statin; a nitric oxide donor; a nitric oxide precursor;
and any combination thereof.
32. The graft device according to claim 30, wherein the drug
comprises a drug selected from the group consisting of: an NSAID;
salicylic acid; indomethacin; sodium indomethacin trihydrate;
salicylamide; naproxen; colchicine; fenoprofen; sulindac;
diflunisal; diclofenac; indoprofen sodium salicylamide;
antiinflammatory cytokines; antiinflammatory proteins; steroidal
anti-inflammatory agents; heparin; Pebac; enoxaparin; aspirin;
hirudin; Plavix; bivalirudin; prasugrel; idraparinux; warfarin;
coumadin; clopidogrel; PPACK; GGACK; tissue plasminogen activator;
urokinase; streptokinase; a glucocorticoid; hydrocortisone;
betamethasone; dexamethasone; flumethasone; isoflupredone;
methylpred-nisolone; prednisone; prednisolone; triamcinolone
acetonide; an antiangiogenic; fluorouracil; paclitaxel;
doxorubicin; cisplatin; methotrexate; cyclophosphamide; etoposide;
pegaptanib; lucentis; tryptophanyl-tRNA synthetase; retaane; CA4P;
AdPEDF; VEGF-TRAP-EYE; AG-103958; Avastin; JSM6427; TG100801; ATG3;
OT-551; endostatin; thalidomide; becacizumab; neovastat; an
antiproliferative; sirolimus; perillyl alcohol; famesyl transferase
inhibitors; FPTIII; L744; antiproliferative factor; Van 10/4; 5-FU;
Daunomycin; Mitomycin; azathioprine; chlorambucil; mofetil;
vasoactive intestinal polypeptide; an antibody; a drug acting on
immunophilins; cyclosporine; zotarolimus; everolimus; tacrolimus;
an interferon; a TNF binding protein; a taxane; docetaxel; a
statin; atorvastatin; lovastatin; simvastatin; pravastatin;
fluvastatin; rosuvastatin; a nitric oxide donor; a nitric oxide
precursor; Angeli's Salt; L-Arginine; Free Base; Diethylamine
NONOate; Diethylamine NONOate/AM; Glyco-SNAP-1; Glyco-SNAP-2;
(O)--S-Nitroso-N-acetylpenicillamine; S-Nitrosoglutathione; NOC-5;
NOC-7; NOC-9; NOC-12; NOC-18; NOR-1; NOR-3; SIN-1; Hydrochloride;
Sodium Nitroprusside; Dihydrate; Spermine NONOate; Streptozotocin;
an antibiotic; acyclovir; afloxacin; amphotericin B; atovaquone;
azithromycin; ciprofloxacin; clarithromycin; clindamycin;
clofazimine; dapsone; diclazaril; doxycycline; erythromycin;
ethambutol; fluconazole; fluoroquinolones; foscarnet; ganciclovir;
gentamicin; iatroconazole; isoniazid; ketoconazole; levofloxacin;
lincomycin; miconazole; neomycin; norfloxacin; ofloxacin;
paromomycin; penicillin; pentamidine; polymixin B; pyrazinamide;
pyrimethamine; rifabutin; rifampin; sparfloxacin; streptomycin;
sulfadiazine; tetracycline; tobramycin; trifluorouridine;
trimethoprim sulphate; Zn-pyrithione; silver salts such as
chloride, bromide, iodide and periodate; and any combination
thereof.
33. The graft device according to claim 18, wherein a thickness of
the fiber matrix is from 150 to 200 micrometers (.mu.m).
34. The graft device according to claim 18, wherein the harvested
vein segment is a living harvested vein segment.
35. The graft device according to claim 34, wherein a tissue
viability of the living harvested vein segment is not reduced by
the one or more electrospun synthetic fibers deposited
thereabout.
36. The graft device according to claim 18, wherein a compliance of
the graft device is similar to a native carotid artery.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This is a Continuation of U.S. patent application Ser. No.
14/601,523, filed Jan. 21, 2015, which is a Continuation of U.S.
patent application Ser. No. 12/022,430, filed Jan. 30, 2008, now
U.S. Pat. No. 9,237,945, issued Jan. 19, 2016, which claims the
benefit of U.S. Provisional Patent Application No. 60/898,356,
filed Jan. 30, 2007, each of which is incorporated herein by
reference in its entirety.
[0003] Coronary artery disease, leading to myocardial infarction
and ischemia, is currently the number one cause of morbidity and
mortality worldwide. Current treatment alternatives consist of
percutaneous transluminal angioplasty, stenting, and coronary
artery bypass grafting (CABG). CABG can be carried out using either
arterial or venous conduits and is the most effective and most
widely used treatment to combat coronary arterial stenosis, with
nearly 500,000 procedures being performed annually. In addition
there are approximately 80,000 lower extremity bypass surgeries
performed annually. The venous conduit used for bypass procedures
is most frequently the autogenous saphenous vein and remains the
graft of choice for 95% of surgeons performing these bypass
procedures. According to the American Heart Association, in 2004
there were 427,000 bypass procedures performed in 249,000 patients.
The long term outcome of these procedures is limited due to
occlusion of the graft vessel or anastomotic site as a result of
intimal hyperplasia (IH), which can occur over a timeframe of
months to years.
[0004] Development of successful small diameter synthetic or tissue
engineered vascular grafts has yet to be accomplished and use of
arterial grafts (internal mammary, radial, or gastroepiploic
arteries, for example) is limited by the short size, small diameter
and availability of these vessels. Despite their wide use, failure
of arterial vein grafts (AVGs) remains a major problem: 12% to 27%
of AVGs become occluded in the first year with a subsequent annual
occlusive rate of 2% to 4%. Patients with failed AVGs will die or
require re-operation.
[0005] IH accounts for 20% to 40% of all AVG failures within the
first 5 years. Several studies have determined that IH develops, to
some extent, in all mature AVGs and this is regarded by many as an
unavoidable response of the vein to grafting. IH is characterized
by phenotypic modulation, followed by de-adhesion and migration of
medial and adventitial smooth muscle cells (SMCs) and
myofibroblasts into the intima where they proliferate. In many
cases, this response can lead to stenosis and diminished blood flow
through the graft. It is thought that IH may be initiated by the
abrupt exposure of the veins to the dynamic mechanical environment
of the arterial circulation.
[0006] Vein segments transposed to the arterial circulation for use
as bypass grafts are exposed to increased blood flow and
intraluminal pressure (Porter K E, Nydahl S, Dunlop P, Varty K,
Thrush A J, and London N J. The development of an in vitro flow
model of human saphenous vein graft intimal hyperplasia. Cardiovasc
Res. 1996; 31(4): 607-14), and cyclic wall motion (including
bending, twisting and stretching) due to their attachment to the
beating heart in the case of CABGs (Vorp D A, Severyn D A, Steed D
L, and Webster M W. A device for the application of cyclic twist
and extension on perfused vascular segments. Am J Physiol. 1996;
270(2 Pt 2): H787-95). Since veins are much thinner walled and more
fragile than arteries, they experience significantly greater
stresses in the arterial circuit than those to which they are
accustomed in the venous circuit. Indeed, Liu and Fung showed that
the average circumferential wall stress (CWS) in an AVG immediately
upon reestablishing arterial flow could be 140-fold that in a vein
under normal circumstances (Fuchs J C, Mitchener J S, and Hagen P
O. Postoperative changes in autologous vein grafts. Ann Surg. 1978;
188(1): 1-15). This dramatic increase in CWS is due to the AVG
being distended to its maximum diameter under arterial pressure.
The tissue responds to this perceived injury by thickening, which
is thought to be an attempt to return the stress to venous levels.
However, this response is uncontrolled and can over-compensate,
leading to stenosis instead of the desired thickening or
"arterialization" of the vein segment.
[0007] It has been suggested that the hyperplastic response by AVGs
is a direct result of a "cellular shock" that occurs as a result of
their abrupt exposure to the arterial biomechanical environment
(Angelini G D, et al. Distention promotes platelet and leukocyte
adhesion and reduces short-term patency in pig arteriovenous bypass
grafts. J Thorac Cardiovasc Surg. 1990; 99(3): 433-9; Campbell P A,
et al. Vein grafts for arterial repair: Their success and reasons
for failure. Ann R Coll Surg Engl. 1981; 63(4): 257-60; Campeau L L
J, et al. Natural history of saphenous vein aortocoronary bypass
grafts. Mod Concepts Cardiovasc Dis. 1984; 53: 59-63; Fuchs J C,
Mitchener J S, and Hagen P O. Postoperative changes in autologous
vein grafts. Ann Surg. 1978; 188(1): 1-15; Huynh T T, et al.
Alterations in wall tension and shear stress modulate tyrosine
kinase signaling and wall remodeling in experimental vein grafts. J
Vasc Surg. 1999; 29(2): 334-44; Liu S Q et al. Changes in the
organization of the smooth muscle cells in rat vein grafts. Ann
Biomed Eng. 1998; 26(1): 86-95; Ramos J R, et al. Histologic fate
and endothelial changes of distended and nondistended vein grafts.
Ann Surg. 1976; 183(3): 205-28; Resnick N and Gimbrone M A.
Hemodynamic forces are complex regulators of endothelial gene
expression. The Faseb J. 1995; 9(10): 874-82; Sumpio B. Hemodynamic
forces and vascular cell biology. Austin: R. G. Landes Company.
1993; Szilagyi D E, et al. Biologic fate of autogenous vein
implants as arterial substitutes: Clinical, angiographic and
histopathologic observations in femoro-popliteal operations for
atherosclerosis. Ann Surg. 1973; 178(3): 232-46; and Zwolak R M, et
al. Kinetics of vein graft hyperplasia: Association with tangential
stress. Journal of Vascular Surgery: Official Publication, the
Society For Vascular Surgery [and] International Society For
Cardiovascular Surgery, North American Chapter. 1987; 5(1):
126-36). Preventing acute distension of AVGs by adding an external
structural support (or sheath) has seemingly improved the patency
of vein grafts (Huynh T T, et al. J Vasc Surg. 1999; 29(2): 334-44;
Cabrera Fischer E I, et al. Reduced elastic mismatch achieved by
interposing vein cuff in expanded polytetrafluoroethylene femoral
bypass decreases intimal hyperplasia. Artif Organs. 2005; 29(2):
122-30; Ducasse E, et al. Interposition vein cuff and intimal
hyperplasia: An experimental study. Eur J Vasc Endovasc Surg. 2004;
27(6): 617-21; Huynh T T, et al. External support modulates g
protein expression and receptor coupling in experimental vein
grafts. Surgery. 1999; 126(2): 127-34; Jeremy J Y, et al. A
bioabsorbable (polyglactin), nonrestrictive, external sheath
inhibits porcine saphenous vein graft thickening. J Thorac
Cardiovasc Surg. 2004; 127(6): 1766-72; Karayannacos P E, et al.
Late failure in vein grafts: Mediating factors in subendothelial
fibromuscular hyperplasia. Ann Surg. 1978; 187(2): 183-8; Kohler T
R, et al. The effect of rigid external support on vein graft
adaptation to the arterial circulation. J Vasc Surg. 1989; 9(2):
277-85; Liu S Q, et al. Partial prevention of monocyte and
granulocyte activation in experimental vein grafts by using a
biomechanical engineering approach. J Biomech. 1999; 32(11):
1165-75; Liu S Q, et al. A possible role of initial cell death due
to mechanical stretch in the regulation of subsequent cell
proliferation in experimental vein grafts. Biomech Model
Mechanobiol. 2002; 1(1): 17-27; Mehta D, et al. External stenting
reduces long-term medial and neointimal thickening and platelet
derived growth factor expression in a pig model of arteriovenous
bypass grafting. Nat Med. 1998; 4(2): 235-9; Parsonnet V, et al.
New stent for support of veins in arterial grafts. Arch Surg. 1963;
87: 696-702; Vijayan V, et al. Long-term reduction of medial and
intimal thickening in porcine saphenous vein grafts with a
polyglactin biodegradable external sheath. J Vasc Surg. 2004;
40(5): 1011-9; and Vijayan V, et al. External supports and the
prevention of neointima formation in vein grafts. Eur J Vasc
Endovasc Surg. 2002; 24(1): 13-22). However, due to one or more
fundamental limitations, these previous approaches have not
resulted in a clinically viable means for improving AVG patency.
All of these previous approaches utilized adventitially placed
wraps/sheaths that were biodurable, and/or loose-fitting.
[0008] The Role of Biomechanics in the Development of Intimal
Hyperlasia
[0009] IH is defined by an increase in the thickness of the inner
layer of a blood vessel, typically as a result of an increased
number and/or size of cells in the intima, followed by deposition
of massive amounts of ECM by these cells. The cells contributing to
this response are predominantly SMCs of medial and adventitial
origin. IH occurs both physiologically during development as in the
closure of the ductus arteriosus, and pathologically as a result of
vascular injury. It is thought that AVG IH may be initiated by the
abrupt exposure of the veins to the dynamic mechanical environment
of the arterial circulation (Dobrin P B, Littooy F N, and Endean E
D. Mechanical factors predisposing to intimal hyperplasia and
medial thickening in autogenous vein grafts. Surgery. 1989; 105(3):
393-400). However, while increased levels of CWS has been shown to
promote IH formation (Huynh T T, Davies M G, Trovato M J, Svendsen
E, and Hagen P O. Alterations in wall tension and shear stress
modulate tyrosine kinase signaling and wall remodeling in
experimental vein grafts. J Vasc Surg. 1999; 29(2): 334-44 and
Gusic R J, Myung R, Petko M, Gaynor J W, and Gooch K J. Shear
stress and pressure modulate saphenous vein remodeling ex vivo. J
Biomech. 2005; 38(9): 1760-9), increased levels of shear stress
tend to modulate it (Huynh T T, Davies M G, Trovato M J, Svendsen
E, and Hagen P O. Alterations in wall tension and shear stress
modulate tyrosine kinase signaling and wall remodeling in
experimental vein grafts. J Vasc Surg. 1999; 29(2): 334-44; Gusic R
J, Myung R, Petko M, Gaynor J W, and Gooch K J. Shear stress and
pressure modulate saphenous vein remodeling ex vivo. J Biomech.
2005; 38(9): 1760-9; Goldman J, Zhong L, and Liu S Q. Negative
regulation of vascular smooth muscle cell migration by blood shear
stress. Am J Physiol Heart Circ Physiol. 2006; Jiang Z, Berceli S
A, Pfahnl C L, Wu L, Goldman D, Tao M, Kagayama M, Matsukawa A, and
Ozaki C K. Wall shear modulation of cytokines in early vein grafts.
J Vasc Surg. 2004; 40(2): 345-50; Jiang Z, Wu L, Miller B L,
Goldman D R, Fernandez C M, Abouhamze Z S, Ozaki C K, and Berceli S
A. A novel vein graft model: Adaptation to differential flow
environments. American Journal of Physiology. Heart and Circulatory
Physiology. 2004; 286(1): H240-5; and Morinaga K, Okadome K, Kuroki
M, Miyazaki T, Muto Y, and Inokuchi K. Effect of wall shear stress
on intimal thickening of arterially transplanted autogenous veins
in dogs. J Vasc Surg. 1985; 2(3): 430-3). These two biomechanical
factors, seemingly causing opposing hyperplastic responses by AVGs,
were carefully explored by Dobrin et al., who showed that the
increased circumferential stretch plays a more significant role in
promoting intimal thickening than the increased shear stress does
in preventing it (Dobrin P B, Littooy F N, and Endean E D.
Mechanical factors predisposing to intimal hyperplasia and medial
thickening in autogenous vein grafts. Surgery. 1989; 105(3):
393-400). In another study that motivates this work, Zwolak et al.
suggested a regulatory role for biomechanical wall stress in the
arterialization of AVGs (Zwolak R M, Adams M C, and Clowes A W.
Kinetics of vein graft hyperplasia: Association with tangential
stress. Journal of Vascular Surgery: Official Publication, the
Society For Vascular Surgery [and] International Society For
Cardiovascular Surgery, North American Chapter. 1987; 5(1):
126-36). Jiang et al. demonstrated that increased wall shear
stress, in the absence of an increase in wall tension, reduced the
hyperplastic response in AVGs (Jiang Z, Wu L, Miller B L, Goldman D
R, Fernandez C M, Abouhamze Z S, Ozaki C K, and Berceli S A. A
novel vein graft model: Adaptation to differential flow
environments. American Journal of Physiology. Heart and Circulatory
Physiology. 2004; 286(1): H240-5). The in vivo work by Liu et al.
has shown that by reducing the level of CWS in AVGs, via placement
of a permanent polytetrafluoroethylene sheath, the hyperplastic
response can be reduced (Cabrera Fischer E I, Bia Santana D,
Cassanello G L, Zocalo Y, Crawford E V, Casas R F, and Armentano R
L. Reduced elastic mismatch achieved by interposing vein cuff in
expanded polytetrafluoroethylene femoral bypass decreases intimal
hyperplasia. Artif Organs. 2005; 29(2): 122-30; Liu S Q, Moore M M,
Glucksberg M R, Mockros L F, Grotberg J B, and Mok A P. Partial
prevention of monocyte and granulocyte activation in experimental
vein grafts by using a biomechanical engineering approach. J
Biomech. 1999; 32(11): 1165-75; and Liu S Q, Ruan Y Y, Tang D, Li Y
C, Goldman J, and Zhong L. A possible role of initial cell death
due to mechanical stretch in the regulation of subsequent cell
proliferation in experimental vein grafts. Biomech Model
Mechanobiol. 2002; 1(1): 17-27). It is clear from these previous
studies that the biomechanical environment of an AVG plays a
significant role in the development of IH. In particular, the CWS
appears to regulate the formation of IH, and controlling this was
the focus of the approach described in this study.
Molecular and Cellular Processes Associated with Intimal
Hyperplasia
[0010] Once injury is perceived by a vein, the hyperplastic
response is set into motion and can be described by five distinct
but interrelated cell processes: 1) Phenotypic modulation of
adventitial and medial SMCs from a contractile and quiescent state
with low proliferative potential to a synthetic state with high
proliferative potential; 2) De-adhesion of SMCs or alteration of
focal adhesions with other cells and the ECM; 3) Migration of SMCs
from the outer layers through the basement membrane to the intima,
which requires selective reassembling of focal adhesions that allow
the cell to "walk" along the ECM; 4) Proliferation; and 5)
Remodeling of the tissue, reflecting the changes in ECM composition
caused by the synthetic SMCs secreting collagen, elastin,
fibronectin, etc., as well as matrix degrading enzymes such as the
various matrix metalloproteinases (MMPs). In order to inhibit the
initiating events of AVG IH, it is probable that one must take into
account each of these five processes. A schematic depicting the
chain of events associated with IH is shown in FIG. 1.
Phenotypic Modulation
[0011] Modulation of SMC phenotype is a prominent feature in the
pathogenesis of IH. Plaques abundant with modified SMCs have been
found in the intima as early as the second week after grafting.
Fully differentiated adult SMCs demonstrate low turnover as
demonstrated by low proliferation and apoptosis rates. However, 48
hours after arterial injury, 15-40% of SMCs are mitotic. This
abrupt shift in functionality is related to the fact that SMCs can
exist in a spectrum of phenotypes, spanning from fully synthetic to
fully contractile. Synthetic SMCs respond to regulatory signals and
cytokines, and are capable of ECM turnover as well as growth factor
production. On the other hand, contractile SMCs respond to
vasomotor signals and control vessel tone. AVGs exhibit neointimal
formation within the first two months by the migration and
proliferation of synthetic SMCs and by subsequent, sustained ECM
accumulation, including type I collagen production, in the
prolonged presence of the de-differentiated type SMCs.
[0012] The phenotypic state of SMCs is regulated at least in part
by mechanical forces, as demonstrated by the observation that
cyclic stretch induces a substrate-dependent modulation of
proliferation and h-caldesmon expression in vitro. In vivo studies
have also shown the importance of mechanical injury on the
phenotype of SMCs. Balloon inflation injury to the media was shown
to promote ECM synthesis by SMCs as well as to decrease alpha actin
content. Several reports have shown that neointimal SMCs of veins
transposed to the arterial circulation are phenotypically altered,
supporting the notion that the change from the venous to the
arterial environment triggers phenotypic alteration. Further
evidence comes from ex vivo organ culture studies where, for
example, cyclic stretch was found to be necessary to maintain the
contractile function of SMCs in cultured rat portal veins. Goldman
et al. exposed rat vena cava to arterial pressures (Goldman J,
Zhong L, and Liu S Q. Degradation of alpha-actin filaments in
venous smooth muscle cells in response to mechanical stretch.
American Journal of Physiology. Heart and Circulatory Physiology.
2003; 284(5): H1839-47), which led to a large increase in medial
circumferential strain and a concomitant reduction in the SMC
filamentous actin coverage. Clearly, the changes in the mechanical
environment related to vein grafting can lead to phenotypic
alterations of the mural SMCs, possibly contributing to the
development of IH.
[0013] Indicators of a synthetic phenotype include the presence of
increased quantities of Golgi complex and rough endoplasmic
reticulum, and decreased quantities of filamentous actin. A
contractile phenotype is demonstrated by the presence of an intact
contractile apparatus indicated by the expression of contractile
proteins such as smoothelin, h-caldesmon, smooth muscle myosin
heavy chain, and large quantities of filamentous actin.
De-Adhesion and Migration
[0014] Cellular de-adhesion is one of the earliest responses in the
IH cascade. This process refers to an alteration in a cell's
adhesion to the ECM from a state of strong adherence, with focal
adhesions and stress fibers, to a state of weaker adherence,
characterized by a restructuring of focal adhesions and stress
fibers while maintaining a spread cell shape. SMC de-adhesion will
of course allow SMC migration and proliferation which will
contribute to neointima formation.
[0015] While there are many important proteins involved in the
regulation of cellular adhesion, we focused our attention on
matricellular proteins, which function as adaptors and modulators
of cell matrix interactions (Bornstein P. Diversity of function is
inherent in matricellular proteins: An appraisal of thrombospondin
1. J Cell Biol. 1995; 130(3): 503-6 and Sage E H and Bornstein P.
Extracellular proteins that modulate cell-matrix interactions.
Sparc, tenascin, and thrombospondin. The Journal of Biological
Chemistry. 1991; 266(23): 14831-4), and intracellular adhesion
proteins, which have been shown to localize to cellular focal
adhesion sites (Nikolopoulos S N and Turner C E. Integrin-linked
kinase (ilk) binding to paxillin ldl motif regulates ilk
localization to focal adhesions. The Journal of Biological
Chemistry. 2001; 276(26): 23499-505 and Tu Y, Wu S, Shi X, Chen K,
and Wu C. Migfilin and mig-2 link focal adhesions to filamin and
the actin cytoskeleton and function in cell shape modulation. Cell.
2003; 113: 37-47). Tenascin C (TN-C), thrombospondin 1,2 (TSP), and
secreted protein acidic and rich in cysteine (SPARC) are
matricellular proteins that exhibit highly regulated expression
during development and cellular injury (Murphy-Ullrich J E. The
de-adhesive activity of matricellular proteins: Is intermediate
cell adhesion an adaptive state? J Clin Invest. 2001; 107(7):
785-90). Mitogen inducible gene 2 (Mig-2) and integrin linked
kinase (ILK) are intracellular proteins involved in cellular shape
modulation (Nikolopoulos S N and Turner C E. Integrin-linked kinase
(ILK) binding to paxillin ldl motif regulates ilk localization to
focal adhesions. The Journal of Biological Chemistry. 2001;
276(26): 23499-505 and Tu Y, Wu S, Shi X, Chen K, and Wu C.
Migfilin and Mig-2 link focal adhesions to filamin and the actin
cytoskeleton and function in cell shape modulation. Cell. 2003;
113: 37-47) and integrin mediated signal transduction (Wu C and
Dedhar S. Integrin-linked kinase (ILK) and its interactors: A new
paradigm for the coupling of extracellular matrix to actin
cytoskeleton and signaling complexes. J Cell Biol. 2001; 155(4):
505-10), respectively. The actions of TN-C, TSP, and SPARC on the
cytoskeleton and focal adhesions are basically indistinguishable
(Greenwood J A, Theibert A B, Prestwich G D, and Murphy_Ullrich J
E. Restructuring of focal adhesion plaques by pi 3-kinase.
Regulation by ptdins (3,4,5)-p(3) binding to alpha-actinin. J Cell
Biol. 2000; 150(3): 627-42 and Murphy-Ullrich J E, Lightner V A,
Aukhil I, Yan Y Z, Erickson H P, and Hook M. Focal adhesion
integrity is downregulated by the alternatively spliced domain of
human tenascin. J Cell Biol. 1991; 115(4): 1127-36). However, these
three proteins each have unique receptors and have similar but
separate signaling pathways to produce a state of intermediate
adhesion, which is a precursor to cell migration (Murphy-Ullrich J
E. The de-adhesive activity of matricellular proteins: Is
intermediate cell adhesion an adaptive state? J Clin Invest. 2001;
107(7): 785-90). Mig-2 and ILK have also been implicated in
cellular adhesion (Nikolopoulos S N and Turner C E. Integrin-linked
kinase (ILK) binding to paxillin ldl motif regulates ilk
localization to focal adhesions. The Journal of Biological
Chemistry. 2001; 276(26): 23499-505 and Tu Y, Wu S, Shi X, Chen K,
and Wu C. Migfilin and Mig-2 link focal adhesions to filamin and
the actin cytoskeleton and function in cell shape modulation. Cell.
2003; 113: 37-47). Specifically, Mig-2 has been shown to
participate in the connection between cell matrix adhesions and the
actin cytoskeleton as well as to modulate cell shape (Tu Y, Wu S,
Shi X, Chen K, and Wu C. Migfilin and mig-2 link focal adhesions to
filamin and the actin cytoskeleton and function in cell shape
modulation. Cell. 2003; 113: 37-47). Recent studies have indicated
that ILK serves as a mediator in integrin mediated signal
transduction (Wu C. Integrin-linked kinase and pinch: Partners in
regulation of cell-extracellular matrix interaction and signal
transduction. Journal of Cell Science. 1999; 112 (Pt 24): 4485-9).
Furthermore, both Mig-2 and ILK are required for maintaining focal
adhesions (Nikolopoulos S N and Turner C E. Integrin-linked kinase
(ilk) binding to paxillin ldl motif regulates ilk localization to
focal adhesions. The Journal of Biological Chemistry. 2001;
276(26): 23499-505 and Tu Y, Wu S, Shi X, Chen K, and Wu C.
Migfilin and mig-2 link focal adhesions to filamin and the actin
cytoskeleton and function in cell shape modulation. Cell. 2003;
113: 37-47). By examining the changes in the levels of TN-C, TSP,
SPARC, Mig-2, and ILK, we believe that we will be able to make
conclusions about the state of adhesion of SMCs within the vein
segments. A schematic showing the intracellular localization of
TN-C, TSP, SPARC, Mig-2 and ILK is shown in FIG. 2.
[0016] A prerequisite for SMC migration in vivo is degradation of
surrounding matrix proteins. Matrix metalloproteinases
(specifically, MMP-1, MMP-2, and MMP-9) can selectively degrade
various components of the vascular ECM (Galis Z S, Muszynski M,
Sukhova G K, Simon_Morrissey E, Unemori E N, Lark M W, Amento E,
and Libby P. Cytokine-stimulated human vascular smooth muscle cells
synthesize a complement of enzymes required for extracellular
matrix digestion. Circulation Research (Online) 1994; 75(1): 181-9;
Newby A C, Southgate K M, and Davies M G. Extracellular matrix
degrading metalloproteinases in the pathogensis of
arteriosclerosis. Basic Res Cardiol. 1994; 89(Suppl 1): 59-70;
Porter K E, Naik J, Turner N A, Dickison T, Thompson M M, and
London J M. Simvastatin inhibits human saphenous vein neointima
formation via inhibition of smooth muscle cell proliferation and
migration. J. Vasc. Surg. 2002; 36: 150-7; and Southgate K M,
Davies M, Booth R F, and Newby A C. Involvement of
extracellular-matrix-degrading metalloproteinases in rabbit aortic
smooth-muscle cell proliferation. Biochem J. 1992; 288 (Pt 1):
93-9). MMPs have been shown to be critical for the development of
arterial lesions by regulating SMC migration. The balance between
MMPs, their activator (MT-1 MMP) (Lafleur M A, Hollenberg M D,
Atkinson S J, Knauper V, Murphy G, and Edwards D R. Activation of
pro-(matrix metalloproteinase-2) (pro-mmp-2) by thrombin is
membrane-type-mmp-dependent in human umbilical vein endothelial
cells and generates a distinct 63 kda active species. Biochem J.
2001; 357(Pt 1): 107-15), and their inhibitors (specifically,
TIMP-1, TIMP-2, TIMP-3, and TIMP-4) determines the level of ECM
degradation (Meng X, Mavromatis K, and Galis Z S. Mechanical
stretching of human saphenous vein grafts induces expression and
activation of matrix-degrading enzymes associated with vascular
tissue injury and repair. Exp Mol Pathol. 1999; 66(3): 227-37).
Numerous studies have shown that MMPs and TIMPs play a significant
role in the early stages of IH in response to altered hemodynamics
and vascular injury (George S J, Baker A H, Angelini G D, and Newby
A C. Gene transfer of tissue inhibitor of metalloproteinase-2
inhibits metalloproteinase activity and neointima formation in
human saphenous veins. Gene Ther. 1998; 5(11): 1552-60; George S J,
Johnson J L, Angelini G D, Newby A C, and Baker A H.
Adenovirus-mediated gene transfer of the human TIMP-1 gene inhibits
smooth muscle cell migration and neointimal formation in human
saphenous vein. Hum Gene Ther. 1998; 9(6): 867-77; and Lijnen H R,
Soloway P, and Collen D. Tissue inhibitor of matrix
metalloproteinases-1 impairs arterial neointima formation after
vascular injury in mice. Circ Res. 1999; 85(12): 1186-91). For
example, after 6 hours of ex vivo perfusion with arterial
hemodynamics, expression of MMP-2 and MMP-9 was increased in human
saphenous veins (Mavromatis K, Fukai T, Tate M, Chesler N, Ku D N,
and Galis Z S. Early effects of arterial hemodynamic conditions on
human saphenous veins perfused ex vivo. Arterioscler Thromb Vasc
Biol. 2000; 20(8): 1889-95). Other organ culture studies of human
saphenous vein have shown increased production of MMP-9 and
increased activation of MMP-2 (Porter K E, Thompson M M, Loftus I
M, McDermott E, Jones L, Crowther M, Bell P R, and London N J.
Production and inhibition of the gelatinolytic matrix
metalloproteinases in a human model of vein graft stenosis. Eur J
Vasc Endovasc Surg. 1999; 17(5): 404-12; Porter K E, Naik J, Turner
N A, Dickison T, Thompson M M, and London J M. Simvastatin inhibits
human saphenous vein neointima formation via inhibition of smooth
muscle cell proliferation and migration. J. Vasc. Surg. 2002; 36:
150-7; and George S J, Zaltsman A B, and Newby A C. Surgical
preparative injury and neointima formation increase MMP-9
expression and MMP-2 activation in human saphenous vein. Cardiovasc
Res. 1997; 33(2): 447-59) under arterial conditions. Broad spectrum
MMP inhibitors such as simvastatin have been shown to inhibit
neointima formation in this model (Porter K E, Naik J, Turner N A,
Dickison T, Thompson M M, and London J M. Simvastatin inhibits
human saphenous vein neointima formation via inhibition of smooth
muscle cell proliferation and migration. J. Vasc. Surg. 2002; 36:
150-7 and Porter K E, Loftus I M, Peterson M, Bell P R, London N J,
and Thompson M M. Marimastat inhibits neointimal thickening in a
model of human vein graft stenosis. Br J Surg. 1998; 85(10):
1373-7).
[0017] Mechanical forces can influence SMC de-adhesion and
migration by directly regulating the above factors. For example,
MMP-1 expression is increased in venous SMCs exposed to pulse
pressure compared to static controls (Redmond E M, Cahill P A,
Hirsch M, Wang Y N, Sitzmann J V, and Okada S S. Effect of pulse
pressure on vascular smooth muscle cell migration: The role of
urokinase and matrix metalloproteinase. Thrombosis &
Haemostasis. 1999; 81(2): 293-300), while MMP-2 mRNA levels are
increased in mouse SMCs exposed to cyclic stretch (Grote K, Flach
I, Luchtefeld M, Akin E, Holland S M, Drexler H, and Schieffer B.
Mechanical stretch enhances mRNA expression and proenzyme release
of matrix metalloproteinase-2 (MMP-2) via nad(p)h oxidase-derived
reactive oxygen species. Circulation Research. 2003; 92(11): 80-6).
In cultured SMCs from human saphenous vein, MMP-2 and MMP-9
transcript and protein levels increased when exposed to uniaxial
stationary strain, but decreased when exposed to uniaxial cyclic
strain (Asanuma K, Magid R, Johnson C, Nerem R M, and Galis Z S.
Uniaxial strain upregulates matrix-degrading enzymes produced by
human vascular smooth muscle cells. Am J Physiol Heart Circ
Physiol. 2003; 284(5): H1778-84). Cyclic strain of fibroblasts has
been shown to increase MT-1 MMP levels (Tyagi S C, Lewis K, Pikes
D, Marcello A, Mujumdar V S, Smiley L M, and Moore C K.
Stretch-induced membrane type matrix metalloproteinase and tissue
plasminogen activator in cardiac fibroblast cells. J Cell Physiol.
1998; 176(2): 374-82)[166] and decrease TIMP-1 levels (Yamaoka A,
Matsuo T, Shiraga F, and Ohtsuki H. Timp-1 production by human
scleral fibroblast decreases in response to cyclic mechanical
stretching. Opthalmic Research. 2001; 33(2): 98-101). In addition,
SMC migration was shown to be regulated by shear stress induced EC
signaling (Bassiouny H S, Song R H, Kocharyan H, Kins E, and Glagov
S. Low flow enhances platelet activation after acute experimental
arterial injury. Journal of Vascular Surgery. 1998; 27(5): 910-8;
Nakazawa T, Yasuhara H, Shigematsu K, and Shigematsu H. Smooth
muscle cell migration induced by shear-loaded platelets and
endothelial cells. Enhanced platelet-derived growth factor
production by shear-loaded platelets. Int Angiol. 2000; 19(2):
142-6; Powell R J, Carruth J A, Basson M D, Bloodgood R, and Sumpio
B E. Matrix-specific effect of endothelial control of smooth muscle
cell migration. Journal of Vascular Surgery. 1996; 24(1): 51-7; and
Shigematsu K, Yasuhara H, Shigematsu H, and Muto T. Direct and
indirect effects of pulsatile shear stress on the smooth muscle
cell. Int Angiol. 2000; 19(1): 39-46). Mechanical forces can
influence SMC de-adhesion and migration by directly regulating the
above factors. SMC migration was shown to be regulated by shear
stress induced EC signaling (Garanich J S, Pahakis M, and Tarbell J
M. Shear stress inhibits smooth muscle cell migration via nitric
oxide-mediated downregulation of matrix metalloproteinase-2
activity. Am J Physiol Heart Circ Physiol. 2005; 288(5): H2244-52;
Bassiouny H S, Song R H, Kocharyan H, Kins E, and Glagov S. Low
flow enhances platelet activation after acute experimental arterial
injury. Journal of Vascular Surgery. 1998; 27(5): 910-8; Nakazawa
T, Yasuhara H, Shigematsu K, and Shigematsu H. Smooth muscle cell
migration induced by shear-loaded platelets and endothelial cells.
Enhanced platelet-derived growth factor production by shear-loaded
platelets. Int Angiol. 2000; 19(2): 142-6; Powell R J, Carruth J A,
Basson M D, Bloodgood R, and Sumpio B E. Matrix-specific effect of
endothelial control of smooth muscle cell migration. Journal of
Vascular Surgery. 1996; 24(1): 51-7; Shigematsu K, Yasuhara H,
Shigematsu H, and Muto T. Direct and indirect effects of pulsatile
shear stress on the smooth muscle cell. Int Angiol. 2000; 19(1):
39-46; and Sho M, Sho E, Singh.TM., Komatsu M, Sugita A, Xu C,
Nanjo H, Zarins C K, and Masuda H. Subnormal shear stress-induced
intimal thickening requires medial smooth muscle cell proliferation
and migration. Exp Mol Pathol. 2002; 72(2): 150-60).
Proliferation
[0018] Several growth factors have been implicated as key
components in the hyperplastic response of vein grafts.
Transforming growth factor beta (TGF-.beta.) appears to be of
particular importance. For example, Wolf et al. demonstrated that
systemic administration of antibodies against TGF-.beta.
significantly reduced the development of IH in a rat model (Wolf Y
G, Rasmussen L M, and Ruoslahti E. Antibodies against transforming
growth factor-beta 1 suppress intimal hyperplasia in a rat model. J
Clin Invest. 1994; 93(3): 1172-8). Platelet derived growth factor
(PDGF) and basic fibroblast growth factor (bFGF) also appear to be
primary factors involved in IH associated SMC proliferation. For
example, PDGF causes a dose dependent proliferation response in
cultured SMCs (Uzui H, Lee J D, Shimizu H, Tsutani H, and Ueda T.
The role of protein-tyrosine phosphorylation and gelatinase
production in the migration and proliferation of smooth muscle
cells. Atherosclerosis. 2000; 149(1): 51-9), while TGF-.beta.
inhibits proliferation (Mii S, Ware J A, and Kent K C. Transforming
growth factor-beta inhibits human vascular smooth muscle cell
growth and migration. Surgery. 1993; 114(2): 464-70). bFGF released
from dead and damaged cells of autologous vein grafts promotes SMC
proliferation (Qian H, Zhang B, and Zhao H. [gene expression of
bfgf and intimal hyperplasia of autologous vein grafts in rats].
Zhonghua Yi Xue Za Zhi. 1996; 76(11): 826-8). mRNA levels of PDGF
transcripts as well as numbers of proliferating cells were found to
be highest in the neointima of porcine vein grafts (Francis S E,
Hunter S, Holt C M, Gadsdon P A, Rogers S, Duff G W, Newby A C, and
Angelini G D. Release of platelet-derived growth factor activity
from pig venous arterial grafts. J Thorac Cardiovasc Surg. 1994;
108(3): 540-8). While growth factors clearly play a role in IH,
MMPs have also been shown to be critical for the development of
arterial lesions by regulating SMC proliferation (Southgate K M,
Davies M, Booth R F, and Newby A C. Involvement of
extracellular-matrix-degrading metalloproteinases in rabbit aortic
smooth-muscle cell proliferation. Biochem J. 1992; 288 (Pt 1):
93-9; Cho A and Reidy M A. Matrix metalloproteinase-9 is necessary
for the regulation of smooth muscle cell replication and migration
after arterial injury. Circ Res. 2002; 91(9): 845-51), while TIMPs
have been shown to promote apoptosis of SMC (Annabi B, Shedid D,
Ghosn P, Kenigsberg R L, Desrosiers R R, Bojanowski M W, Beaulieu
E, Nassif E, Moumdjian R, and Beliveau R. Differential regulation
of matrix metalloproteinase activities in abdominal aortic
aneurysms. J Vasc Surg. 2002; 35(3): 539-46).
[0019] IH has been shown to be associated with increases in SMC
proliferation and both increases and decreases in apoptosis. It may
seem counter-intuitive that an increase in intimal apoptosis is
associated with IH, a condition associated with increased cell
numbers. However, it must be kept in mind that increases in cell
number is but a singular event in the balance that regulates IH.
That is, though there may be an absolute increase in apoptosis, a
greater increase in cell proliferation would result in a net
increase in cell number. For these reasons, it is important to
evaluate both sides of the balance (i.e., both promoting and
inhibiting factors) when assessing proliferation.
[0020] Proliferating cell nuclear antigen (PCNA) and terminal
deoxynucleotidyl transferase-mediated dUTP-biotin in situ nick end
labeling (TUNEL) have been used to label proliferating and
apoptotic cells, respectively, within intact AVGs, both in vivo
(Nishibe T, Miyazaki K, Kudo F, Flores J, Nagato M, Kumada T, and
Yasuda K. Induction of angiotensin converting enzyme in neointima
after intravascular stent placement. Int Angiol. 2002; 21(3):
250-5), and in vitro (Zuckerbraun B S, McCloskey C A, Mahidhara R
S, Kim P K, Taylor B S, and Tzeng E. Overexpression of mutated
ikappabalpha inhibits vascular smooth muscle cell proliferation and
intimal hyperplasia formation. J Vasc Surg. 2003; 38(4): 812-9).
Cell proliferation and apoptosis are simultaneous processes that
occur within the adventitia and media of the vein during the first
week following grafting, however this balance is thereafter
disrupted with proliferation rates increasing over rates of
apoptosis (Nishibe T, Miyazaki K, Kudo F, Flores J, Nagato M,
Kumada T, and Yasuda K. Induction of angiotensin converting enzyme
in neointima after intravascular stent placement. Int Angiol. 2002;
21(3): 250-5). The level of proliferation within the media and
neointima of stenosed aortocoronary bypass grafts excised upon
re-operation has been shown to be significantly higher than
non-stenosed controls (Hilker M, Buerke M, Lehr H A, Oelert H, and
Hake U. Bypass graft disease: Analysis of proliferative activity in
human aorto-coronary bypass grafts. 2002; 5 Suppl 4: S331-41).
[0021] Increased wall stress has been associated with AVG IH, and
this may be a direct result of a mechanical regulation of SMC
proliferation, and apoptosis. For example, venous SMCs have been
shown to increase their proliferation compared to arterial SMCs
when exposed to arterial levels of cyclic stretch (Predel H G, Yang
Z, von_Segesser L, Turina M, Buhler F R, and Luscher T F
Implications of pulsatile stretch on growth of saphenous vein and
mammary artery smooth muscle. Lancet. 1992; 340(8824): 878-9 and
Dethlefsen S M, Shepro D, and D'Amore P A. Comparison of the
effects of mechanical stimulation on venous and arterial smooth
muscle cells in vitro. J Vasc Res. 1996; 33(5): 405-13). Liu et al.
showed via bromodeoxyuridine staining and TUNEL analysis that
mechanical stretch due to arterial hemodynamics induces cell death,
which possibly mediates subsequent cell proliferation in a rat AVG
model (Liu B, Itoh H, Louie O, Kubota K, and Kent K C. The
signaling protein rho is necessary for vascular smooth muscle
migration and survival but not for proliferation. Surgery. 2002;
132(2): 317-25). Predel et al. showed that pulsatile stretch
stimulates SMC proliferation in saphenous veins, but not internal
mammary arteries, and may contribute to venous bypass graft disease
(Predel H G, Yang Z, von_Segesser L, Turina M, Buhler F R, and
Luscher T F Implications of pulsatile stretch on growth of
saphenous vein and mammary artery smooth muscle. Lancet. 1992;
340(8824): 878-9). When veins are transposed to the arterial
circulation they undergo an increase of luminal shear stress in
addition to intramural stress. Indeed it has been shown that a
combination of increased shear stress and cyclic stretch imposed on
cultured SMCs activates PDGF receptor alpha (Hu Y, Bock G, Wick G,
and Xu Q. Activation of pdgf receptor alpha in vascular smooth
muscle cells by mechanical stress. Faseb J. 1998; 12(12):
1135-42)[192].
Remodeling
[0022] Vascular remodeling typically refers to a change in the
morphology or microstructure of a blood vessel in response to
changes in the biomechanical environment. It is believed that this
occurs as an attempt by the tissue to restore biomechanical
homeostasis (i.e., to return to normal levels of shear and wall
stress). In the case of AVGs, IH is a pathological form of
remodeling that includes increased intimal thickness caused by SMC
migration and proliferation, increased intimal apoptosis, sclerosis
of the intima and media due to increased ECM deposition, and
hypertrophy of the medial and adventitial SMCs.
[0023] Vascular cells produce the ECM components such as collagen
and elastin. The phenotypic modulation of SMCs associated with vein
grafting has been shown to alter ECM synthesis characterized by
increasing collagen type I and elastin production. Veins used as
arterial bypass grafts undergo an alteration of their ECM
components, which can result in a loss of lumenal area and eventual
occlusion. An alteration in matrix synthesis directly leads to
increased collagen content in the hyperplastic neointima during the
first week after injury resulting from balloon angioplasty. In
addition, AVGs that undergo this hyperplastic remodeling exhibit
decreased compliance as compared to fresh veins, which can
contribute to their failure.
SUMMARY
[0024] Developing a reliable means to prevent the early events of
the IH process would contribute to improvements in the outcome of
arterial bypass procedures. Therefore, provided herein is a method
of mechanically conditioning an arterial vein graft, or any tubular
tissue (living cellular structure), typically, but not exclusively,
in autologous, allogeneic xenogeneic transplantation procedures. To
this end, provided herein is a method of wrapping a tubular tissue,
including, without limitation, a vein, artery, urethra, intestine,
esophagus, trachea, bronchi, ureter and fallopian tube. The tubular
tissue is wrapped with a restrictive fiber matrix of a bioerodible
(also referred to as biodegradable or bioresorbable) polymer about
a circumference of the tubular tissue. In one non-limiting
embodiment, the matrix is deposited onto tubular tissue by
electrospinning. In one particular non-limiting embodiment, the
tubular tissue is a vein, such as a saphenous vein, that is used,
for instance, in an arterial bypass procedure, such as a coronary
arterial bypass procedure.
[0025] The biodegradation rate of the polymer matrix may be
manipulated, optimized or otherwise adjusted so that the matrix
degrades over a useful time period. For instance, in the case of a
coronary artery bypass, it is desirable that the matrix dissolves
over 12 hours or more so as to prevent substantial sudden stress on
the graft. The polymer degrades over a desired period of time so
that the mechanical support offered by the polymer matrix is
gradually reduced over that period and the vein would be exposed to
gradually increasing levels of CWS.
[0026] This new approach would have two potential applications. In
the first non-limiting application, the matrix can be used as a
peri-surgical tool for the modification of vein segments intended
for use as an AVG. The modification of the vein or other tubular
anatomical structure would be performed by treating the vein at
bedside, immediately after removal from the body and just prior to
grafting, for example and without limitation, the arterial bypass
surgery. In one non-limiting example, after the saphenous vein is
harvested, and while the surgeon is exposing the surgical site, the
polymer wrap would be electrospun onto the vein just prior to it
being used for the bypass procedure.
[0027] In a second non-limiting embodiment, the polymer matrix can
be used as a new vehicle for the delivery of support to AVGs. While
modification of the mechanical environment of a vein graft over
time could itself improve AVG patency, delivery of active agents
and biological (cellular) support to AVGs may prove desirable in
many instances. By tuning an electrospun polymer wrap, in which
active agents and/or biologicals are incorporated, to degrade at a
desired rate, the rate of delivery of these support modalities
could be controlled.
[0028] According to one embodiment a tubular tissue graft device is
provided. The device comprises a tubular tissue and a restrictive
fiber matrix of a bioerodible polymer about a circumference of the
tubular tissue. The matrix is typically contiguous or essentially
contiguous about a circumference of at least a portion (part) of
the tubular tissue. In one embodiment, the tubular tissue is
obtained from a vein (is venous), for example and without
limitation, the venous tubular tissue is obtained from a portion of
a saphenous vein. In other embodiments, the tubular tissue is
chosen from (obtained from an organ/tissue chosen from) one or more
of an artery, urethra, intestine, esophagus, ureter, trachea,
bronchi, and fallopian tube. The matrix of the device typically
bioerodes in situ (when implanted) over a time period ranging from
12 hours to two weeks, meaning the supportive nature of the matrix
is degraded over that time period, not necessarily that the matrix
completely erodes.
[0029] In one embodiment, the device is prepared by electrospinning
the polymer fibers onto the tubular tissue. The polymer fibers can
comprise any useful bioerodible polymer composition. In one
embodiment, shown below, the fibers comprise a polymer comprising
ester and urethane linkages, including for example and without
limitation a poly(ester urethane)urea. In other embodiments, the
fibers comprise a polymer chosen from one or more of: a polymer
derived from an alpha-hydroxy acid, a polylactide, a
poly(lactide-co-glycolide), a poly(L-lactide-co-caprolactone), a
polyglycolic acid, a poly(dl-lactide-co-glycolide), a
poly(l-lactide-co-dl-lactide), a polymer comprising a lactone
monomer, a polycaprolactone, polymer comprising carbonate linkages,
a polycarbonate, polyglyconate, poly(glycolide-co-trimethylene
carbonate), a poly(glycolide-co-trimethylene
carbonate-co-dioxanone), a polymer comprising urethane linkages, a
polyurethane, a poly(ester urethane) urea, a poly(ester urethane)
urea elastomer, a polymer comprising ester linkages, a
polyalkanoate, a polyhydroxybutyrate, a polyhydroxyvalerate, a
polydioxanone, a polygalactin, a natural polymer, chitosan,
collagen, elastin, alginate, cellulose, hyaluronic acid and
gelatin. In one embodiment, the polymer composition comprises a
poly(ester urethane)urea with from about 25% wt. to about 75% wt.
collagen. This polymer also may comprise elastin, for example and
without limitation from about 25% wt. to about 75% wt. of a mixture
of collagen and elastin, which are, according to one embodiment, in
approximately (about) equal amounts.
[0030] In yet another embodiment, one or both of a cell and a
therapeutic agent (e.g., drug, cytokine, chemoattractant,
antibiotic, anti-inflammatory, etc.) is associated with (attached
to, absorbed into, adsorbed to, grown into, linked to, etc.) the
matrix. In one embodiment, cells are associated with the matrix,
for example and without limitation, one or more of cells chosen
from stem cells, progenitor (precursor) cells, smooth muscle cells,
skeletal myoblasts, myocardial cells, endothelial cells,
endothelial progenitor cells, bone-marrow derived mesenchymal cells
and genetically modified cells are associated with the matrix. In
another embodiment, a growth factor is associated with the matrix,
for example and without limitation, a growth factor chosen from one
or more of basic fibroblast growth factor (bFGF), acidic fibroblast
growth factor (aFGF), vascular endothelial growth factor (VEGF),
hepatocyte growth factor (HGF), insulin-like growth factors (IGF),
transforming growth factor-beta pleiotrophin protein, midkine
protein and IGF-1. In another embodiment, a drug is associated with
the matrix. In certain non-limiting embodiments, the drug is chosen
from one or more of a non-steroidal anti-inflammatory drug, an
antibiotic, an anticlotting factor, an immunosuppressant, a
glucocorticoid, a drug acting on an immunophilin, an interferon, a
TNF binding proteins, a taxane, a statin, and a nitric oxide donor.
In others, the drug is chosen from one or more of an NSAID,
salicylic acid, indomethacin, sodium indomethacin trihydrate,
salicylamide, naproxen, colchicine, fenoprofen, sulindac,
diflunisal, diclofenac, indoprofen sodium salicylamide,
antiinflammatory cytokines, antiinflammatory proteins, steroidal
anti-inflammatory agents, heparin, Pebac, enoxaprin, aspirin,
hirudin, plavix, bivalirudin, prasugrel, idraparinux, warfarin,
coumadin, clopidogrel, PPACK, GGACK, tissue plasminogen activator,
urokinase, streptokinase, a glucocorticoid, hydrocortisone,
betamethisone, dexamethasone, flumethasone, isoflupredone,
methylpred-nisolone, prednisone, prednisolone, triamcinolone
acetonide, an antiangiogenic, fluorouracil, paclitaxel,
doxorubicin, cisplatin, methotrexate, cyclophosphamide, etoposide,
pegaptanib, lucentis, tryptophanyl-tRNA synthetase, retaane, CA4P,
AdPEDF, VEGF-TRAP-EYE, AG-103958, Avastin, JSM6427, TG100801, ATG3,
OT-551, endostatin, thalidomide, becacizumab, neovastat, an
antiproliferative, sirolimus, paclitaxel, perillyl alcohol,
farnesyl transferase inhibitors, FPTIII, L744, antiproliferative
factor, Van 10/4, doxorubicin, 5-FU, Daunomycin, Mitomycin,
dexamethasone, azathioprine, chlorambucil, cyclophosphamide,
methotrexate, mofetil, vasoactive intestinal polypeptide, an
antibody, a drug acting on immunophilins, cyclosporine,
zotarolimus, everolimus, tacrolimus, sirolimus, an interferon, a
TNF binding protein, a taxane, paclitaxel, docetaxel, a statin,
atorvastatin, lovastatin, simvastatin, pravastatin, fluvastatin,
rosuvastatin a nitric oxide donor or precursor, Angeli's Salt,
L-Arginine, Free Base, Diethylamine NONOate, Diethylamine
NONOate/AM, Glyco-SNAP-1, Glyco-SNAP-2,
(.+-.)-S-Nitroso-N-acetylpenicillamine, S-Nitrosoglutathione,
NOC-5, NOC-7, NOC-9, NOC-12, NOC-18, NOR-1, NOR-3, SIN-1,
Hydrochloride, Sodium Nitroprusside, Dihydrate, Spermine NONOate,
Streptozotocin, an antibiotic, acyclovir, afloxacin, ampicillin,
amphotericin B, atovaquone, azithromycin, ciprofloxacin,
clarithromycin, clindamycin, clofazimine, dapsone, diclazaril,
doxycycline, erythromycin, ethambutol, fluconazole,
fluoroquinolones, foscarnet, ganciclovir, gentamicin,
iatroconazole, isoniazid, ketoconazole, levofloxacin, lincomycin,
miconazole, neomycin, norfloxacin, ofloxacin, paromomycin,
penicillin, pentamidine, polymixin B, pyrazinamide, pyrimethamine,
rifabutin, rifampin, sparfloxacin, streptomycin, sulfadiazine,
tetracycline, tobramycin, trifluorouridine, trimethoprim sulphate,
Zn-pyrithione, and silver salts such as chloride, bromide, iodide
and periodate.
[0031] Also provided herein is a method of preparing a tubular
graft comprising depositing a fiber matrix of a bioerodible polymer
about a perimeter (outside surface, circumference) of a tubular
tissue to produce a tubular tissue graft device. The matrix is
typically contiguous or essentially contiguous about a
circumference of at least a portion (part) of the tubular tissue.
In one embodiment, the matrix is deposited by electrospinning. As
above, the matrix typically bioerodes in situ over a time period
ranging from 12 hours to two weeks.
[0032] In one embodiment, the tubular tissue is obtained from a
vein, for example and without limitation, the venous tubular tissue
is obtained from a portion of a saphenous vein. In other
embodiments, the tubular tissue is chosen from (obtained from an
organ/tissue chosen from) one or more of an artery, urethra,
intestine, esophagus, ureter, trachea, bronchi, and fallopian
tube.
[0033] The polymer fibers can comprise any useful bioerodible and
biocompatible polymer composition. In one embodiment, shown below,
the fibers comprise a polymer comprising ester and urethane
linkages, including for example and without limitation a poly(ester
urethane)urea. In other embodiments, the fibers comprise a polymer
chosen from one or more of: a polymer derived from an alpha-hydroxy
acid, a polylactide, a poly(lactide-co-glycolide), a
poly(L-lactide-co-caprolactone), a polyglycolic acid, a
poly(dl-lactide-co-glycolide), a poly(l-lactide-co-dl-lactide), a
polymer comprising a lactone monomer, a polycaprolactone, polymer
comprising carbonate linkages, a polycarbonate, polyglyconate,
poly(glycolide-co-trimethylene carbonate), a
poly(glycolide-co-trimethylene carbonate-co-dioxanone), a polymer
comprising urethane linkages, a polyurethane, a poly(ester
urethane) urea, a poly(ester urethane) urea elastomer, a polymer
comprising ester linkages, a polyalkanoate, a polyhydroxybutyrate,
a polyhydroxyvalerate, a polydioxanone, a polygalactin, a natural
polymer, chitosan, collagen, elastin, alginate, cellulose,
hyaluronic acid and gelatin. In one embodiment, the polymer
composition comprises a poly(ester urethane)urea with from about
25% wt. to about 75% wt. collagen, including increments
therebetween. This polymer also may comprise elastin, for example
and without limitation from about 25% wt. to about 75% wt. of a
mixture of collagen and elastin, which are, according to one
embodiment, in approximately (about) equal amounts.
[0034] In another embodiment, the method comprises associating one
or both of a cell and a therapeutic agent (e.g., drug, cytokine,
chemoattractant, antibiotic, anti-inflammatory, etc.) is associated
with (attached to, absorbed into, adsorbed to, grown into, linked
to, etc.) the matrix. In one embodiment, cells are associated with
the matrix, for example and without limitation, one or more of
cells chosen from stem cells, progenitor (precursor) cells, smooth
muscle cells, skeletal myoblasts, myocardial cells, endothelial
cells, endothelial progenitor cells, bone-marrow derived
mesenchymal cells and genetically modified cells are associated
with the matrix. In another embodiment, a growth factor is
associated with the matrix, for example and without limitation, a
growth factor chosen from one or more of basic fibroblast growth
factor (bFGF), acidic fibroblast growth factor (aFGF), vascular
endothelial growth factor (VEGF), hepatocyte growth factor (HGF),
insulin-like growth factors (IGF), transforming growth factor-beta
pleiotrophin protein, midkine protein and IGF-1 is associated with
the matrix. In certain non-limiting embodiments, the drug is chosen
from one or more of a non-steroidal anti-inflammatory drug, an
antibiotic, an anticlotting factor, an immunosuppressant, a
glucocorticoid, a drug acting on an immunophilin, an interferon, a
TNF binding proteins, a taxane, a statin, and a nitric oxide donor.
In others, the drug is chosen from one or more of an NSAID,
salicylic acid, indomethacin, sodium indomethacin trihydrate,
salicylamide, naproxen, colchicine, fenoprofen, sulindac,
diflunisal, diclofenac, indoprofen sodium salicylamide,
antiinflammatory cytokines, antiinflammatory proteins, steroidal
anti-inflammatory agents, heparin, Pebac, enoxaprin, aspirin,
hirudin, plavix, bivalirudin, prasugrel, idraparinux, warfarin,
coumadin, clopidogrel, PPACK, GGACK, tissue plasminogen activator,
urokinase, streptokinase, a glucocorticoid, hydrocortisone,
betamethisone, dexamethasone, flumethasone, isoflupredone,
methylpred-nisolone, prednisone, prednisolone, triamcinolone
acetonide, an antiangiogenic, fluorouracil, paclitaxel,
doxorubicin, cisplatin, methotrexate, cyclophosphamide, etoposide,
pegaptanib, lucentis, tryptophanyl-tRNA synthetase, retaane, CA4P,
AdPEDF, VEGF-TRAP-EYE, AG-103958, Avastin, JSM6427, TG100801, ATG3,
OT-551, endostatin, thalidomide, becacizumab, neovastat, an
antiproliferative, sirolimus, paclitaxel, perillyl alcohol,
farnesyl transferase inhibitors, FPTIII, L744, antiproliferative
factor, Van 10/4, doxorubicin, 5-FU, Daunomycin, Mitomycin,
dexamethasone, azathioprine, chlorambucil, cyclophosphamide,
methotrexate, mofetil, vasoactive intestinal polypeptide, an
antibody, a drug acting on immunophilins, cyclosporine,
zotarolimus, everolimus, tacrolimus, sirolimus, an interferon, a
TNF binding protein, a taxane, paclitaxel, docetaxel, a statin,
atorvastatin, lovastatin, simvastatin, pravastatin, fluvastatin,
rosuvastatin a nitric oxide donor or precursor, Angeli's Salt,
L-Arginine, Free Base, Diethylamine NONOate, Diethylamine
NONOate/AM, Glyco-SNAP-1, Glyco-SNAP-2,
(.+-.)-S-Nitroso-N-acetylpenicillamine, S-Nitrosoglutathione,
NOC-5, NOC-7, NOC-9, NOC-12, NOC-18, NOR-1, NOR-3, SIN-1,
Hydrochloride, Sodium Nitroprusside, Dihydrate, Spermine NONOate,
Streptozotocin, an antibiotic, acyclovir, afloxacin, ampicillin,
amphotericin B, atovaquone, azithromycin, ciprofloxacin,
clarithromycin, clindamycin, clofazimine, dapsone, diclazaril,
doxycycline, erythromycin, ethambutol, fluconazole,
fluoroquinolones, foscarnet, ganciclovir, gentamicin,
iatroconazole, isoniazid, ketoconazole, levofloxacin, lincomycin,
miconazole, neomycin, norfloxacin, ofloxacin, paromomycin,
penicillin, pentamidine, polymixin B, pyrazinamide, pyrimethamine,
rifabutin, rifampin, sparfloxacin, streptomycin, sulfadiazine,
tetracycline, tobramycin, trifluorouridine, trimethoprim sulphate,
Zn-pyrithione, and silver salts such as chloride, bromide, iodide
and periodate.
[0035] In yet another embodiment, a cardiac bypass method is
provided comprising bypassing a coronary artery with a tubular
tissue graft device comprising a vein and a contiguous restrictive
fiber matrix of a bioerodible polymer about a circumference of the
vein. The contiguous bioerodible polymer matrix is any matrix as
described above and throughout this disclosure, and may include
additional therapeutic agents as described above.
BRIEF DESCRIPTION OF THE DRAWINGS
[0036] FIG. 1: Schematic of intimal hyperplasia progression. Please
note: IEL, internal elastic lamina; SMCs, smooth muscle cells.
Image adapted from Robbins Pathologic Basis of Disease, 1999 (Kumar
V, Fausto N, and Abbas A. Robbins & coltran pathologic basis of
disease. Saunders. 2004).
[0037] FIG. 2: Schematic showing the localization of Tenascin-C
(TN-C), thrombospondin-1,2 (TSP), secreted protein acidic and rich
in cysteine (SPARC), mitogen inducible gene 2 (Mig-2) and integrin
linked kinase (ILK). Please note: ECM, extracellular matrix;
.alpha. and .beta., integrins.
[0038] FIG. 3: Schematic of one of closed-loop perfusion/organ
culture system. The loop is composed of a Biomedicus centrifugal
pump that provides pulsatile pressure and flow (A), a heat
exchanger (D), a tissue-housing chamber (C), proximal (B1) and
distal (B2) pressure transducers, a variable resistance valve (E),
flow probe (F), collection reservoir (G), and vessel bypass (H).
Components not shown include, adventitial bath loop, He--Ne laser
micrometer, and data acquisition system. See, Labadie (1996) et al.
for more detail (Labadie, R. F., J. F. Antaki, J. L. Williams, S.
Katyal, J. Ligush, S. C. Watkins, S. M. Pham, and H. S. Borovetz,
"Pulsatile perfusion system for ex vivo investigation of
biochemical pathways in intact vascular tissue", American Journal
of Physiology, 1996. 270(2 Pt 2): p. H760-8).
[0039] FIG. 4: Pressure vs. diameter response of a porcine internal
jugular vein segment.
[0040] FIG. 5: The top three panels show representative scanning
electron micrography images of the lumen of baseline control
(BASE), "venous" 48 hour perfused control (venous), and "arterial"
48 hour perfused (arterial) porcine internal jugular vein segments.
Note the cobblestone appearance of an intact endothelial cell
layer. The second row of panels show representative microstructure
and live nuclei via H&E staining of each group (200.times.
magnification). The third row of panels show representative live
(green in original) and dead (red in original) cells within each
tissue group (200.times. magnification). Note that there does not
appear to be an increased level of necrosis in perfused tissue when
compared to BASE control tissue. The bottom three panels show
representative TUNEL assay images of tissue from the same 48 hour
perfusion experiment (400.times. magnification under immersion
oil). Note that there does not appear to be an increased level of
apoptosis in perfused tissue when compared to BASE. In all panels
the arrow designates the vessel lumen.
[0041] FIG. 6: Schematic depicting the VEN vs. ART ex vivo
perfusion experiments.
[0042] FIG. 7: Schematic depicting the ART vs. cART ex vivo
perfusion experiments.
[0043] FIG. 8: Schematic depicting the ART vs. wART ex vivo
perfusion experiments.
[0044] FIG. 9: Schematic showing a cross-sectional view of the
vein/wrap complex.
[0045] FIG. 10: Schematic of post perfusion venous segment
processing for endpoint analysis. Lengths given represent unloaded
vessel resting lengths.
[0046] FIG. 11: Normalized outer diameter response of PIJVs for
both sham and spun PIJVs. Both spun (wART) and sham control PIJVs
were perfused under ART conditions of 120/80 mmHg pressure and 100
ml/min mean flowrate. Note that the normalized diameter of the spun
veins (N=7) is dramatically reduced when compared to sham controls
(N=5). Pressurized outer diameter (ODp) was normalized to
unpressurized outer diameter (ODup) and data is shown as
mean.+-.standard error of the mean.
[0047] FIG. 12: CWS vs. time results from 24 hour ex vivo
perfusions of electrospun polymer wrapped PIJV segments for each
combination in Table 1. The lower dashed horizontal line indicates
the mean CWS level measured in an unwrapped vein under venous
conditions (CWSo.about.25 KPa), and the middle dashed horizontal
line indicates the mean CWS in a coronary artery (.about.120 KPa)
(Labadie R F, et al. Pulsatile perfusion system for ex vivo
investigation of biochemical pathways in intact vascular tissue. Am
J Physiol. 1996; 270(2 Pt 2): H760-8). The upper dashed line
represents the mean CWS measured in an unwrapped vein (sham
control) under ART conditions. In the legend, ET stands for
electrospinning time. All CWS values were normalized to CWS.sub.o.
The data are presented as mean.+-.standard error of the mean.
[0048] FIG. 13: Representative vasomotor challenge results obtained
using epinephrine (EPI) and sodium nitroprusside (SNP) to stimulate
both a spun and a sham control PIJV segment. Please note that SNP
was administered immediately upon observing a natural relaxation of
the tissue post-stimulation with EPI. That is, SNP was administered
at different times for the sham and spun PIJVs, depending on when
the natural relaxation of the tissue (post stimulation with EPI)
was observed. Outer diameter measurements of each PIJV segment over
the duration of the experiments were normalized to the baseline
outer diameter which was measured prior to administration of the
first dose of EPI.
[0049] FIG. 14: Results from vasomotor challenge experiments (N=4).
There appears to be no significant difference in the level of
contraction or dilation between the sham control and spun PIJVs.
The data are presented as mean.+-.standard error of the mean.
[0050] FIG. 15: Results from the compliance and .beta.-stiffness
calculations for both sham (A & C) and spun (B & D) PIJVs
over 24 hours. The data are presented as mean.+-.standard error of
the mean.
[0051] FIG. 16: H&E (A,B) and Masson's trichrome images (C,D)
for both before perfusion and after wrapping procedure (A,C) and
after 24 hours of ex vivo perfusion (B,D). Note the uniform
thickness of the polymer wrap prior to perfusion, and the absence
of the polymer wrap in the post-perfusion images. The single-headed
arrow indicates the vessel lumen. The double-headed arrow in (A)
and (C) indicates the thickness of the polymer wrap, which was not
detectable in (B) or (D).
[0052] FIG. 17: Representative birefringence images of vein
sections stained with picrosirius red (original in color). The
experimental conditions are defined as: Venous (VEN) conditions of
20 mmHg pressure and 20 ml/min flowrate; pulsatile arterial (ART)
conditions of 120/80 mmHg pressure and 100 ml/min mean flowrate;
and wrapped arterial (wART) conditions where the wrapped vein
segments were perfused under ART conditions for 24 hours ex vivo.
The arrow indicates the vessel lumen.
[0053] FIG. 18: Movat's pentachrome staining of vein tissue
sections (original in color). In each image collagen stains yellow,
elastin and nuclei stain black, and muscle stains red. The red
staining in the adventitial side of the wART sections is unspecific
staining of culture media proteins that become entrapped within the
polymer during ex vivo perfusion experiments. The arrow indicates
the vessel lumen.
[0054] FIG. 19: (A) shows a low magnification SEM image of the PIJV
segment with the electrospun polymer deposited onto its adventitial
surface. (B) is an SEM image (taken at 500.times. magnification) of
the adventitial surface of the PIJV after the polymer wrap was
applied. Note the high porosity of the polymer wrap. (C) is an SEM
image (taken at 500.times. magnification) showing the attachment of
the polymer wrap to the vein. (D) is an SEM image (taken at
500.times. magnification of the luminal surface of the vein and
shows a continuous endothelium layer which appears to have remained
intact.
[0055] FIG. 20: Quantified Live/Dead.TM. results to assess the
level of necrosis in PIJVs after electrospinning, and after 18 and
92 hours of post-electrospinning static culture. The data shown was
for a single experiment, and the error bars result from the 10
fields of view that were analyzed per PIJV segment. The data are
presented as mean.+-.standard error of the mean.
[0056] FIG. 21: Representative immunohistochemistry images from the
fluorescent based TUNEL analysis (originals in color). The top two
panels are from a 24-hour VEN (A) vs. ART (B) experiment. The next
two panels are from a 24-hour ART (C) vs. cART (D) experiment. The
third row of panels are from a 72-hour ART (E) vs. cART (F)
experiment. The bottom two panels are from a 24-hour ART (G) vs.
wART (H) experiment. The arrows indicate apoptotic cells. L
indicates the PIJV lumen.
[0057] FIG. 22: Quantified immunohistochemistry results from
fluorescent based TUNEL analysis to assess the percentage of
apoptotic cells within PIJVs from all the ex vivo vascular
perfusion experiments. The data are presented as mean.+-.standard
error of the mean.
[0058] FIG. 23: Representative immunohistochemistry images from the
HRP/ABC based PCNA analysis (originals in color). The top two
panels are from a 24-hour VEN (A) vs. ART (B) experiment. The next
two panels are from a 24-hour ART (C) vs. cART (D) experiment. The
third row of panels are from a 72-hour ART (E) vs. cART (F)
experiment. The bottom two panels are from a 24-hour ART (G) vs.
wART (H) experiment. The arrows indicate proliferating cells. L
indicates the PIJV lumen.
[0059] FIG. 24: Quantified immunohistochemistry results from
HRP/ABC based PCNA expression analysis to assess the percentage of
proliferating cells within PIJVs from all the ex vivo vascular
perfusion experiments. The data are presented as mean.+-.standard
error of the mean.
[0060] FIG. 25: Representative immunohistochemistry images from the
HRP/ABC based Golgi complex analysis (originals in color). The top
two panels are from a 24-hour VEN (A) vs. ART (B) experiment. The
next two panels are from a 24-hour ART (C) vs. cART (D) experiment.
The third row of panels are from a 72-hour ART (E) vs. cART (F)
experiment. The bottom two panels are from a 24-hour ART (G) vs.
wART (H) experiment. The arrows indicate positively stained cells.
L indicates the PIJV lumen.
[0061] FIG. 26: Quantified immunohistochemistry results from
HRP/ABC based Golgi complex expression analysis to assess the
percentage cells staining positive for Golgi complex within PIJVs
from all the ex vivo vascular perfusion experiments. The data are
presented as mean.+-.standard error of the mean.
[0062] FIG. 27: Left: wrapped PIJV segment during the
electrospinning process. Middle: wrapped PIJV implanted as a
carotid interposition graft as proposed here. Right: unwrapped PIJV
graft. Note that the wrapped PIJV (B) does not expand under
arterial pressure as does the unwrapped vein (C).
[0063] FIG. 28: Fluoroscopic angiography images from both spun and
sham AVGs.
[0064] FIG. 29: Representative Movats pentachrome staining images
that were used for morphometric measurements of IH (originals in
color). The imtimal to medial thickness ratio was calculated using
the above equation.
[0065] FIG. 30: Summary of quantified results from morphometric
measurements of IH. P<0.05 was considered statistically
significant. Note only a trend towards statistical significance was
observed.
[0066] FIG. 31: Low magnification (30.times.) SEM images from two
in vivo experiments where the AVGs were not occluded. A and B were
from an experiment where the grafts were fully patent. C and D are
from an experiment where the grafts were only partially occluded.
These images show the anastomotic interface between the vein graft
and the carotid artery.
DETAILED DESCRIPTION
[0067] Provided herein is a method of mechanically conditioning an
arterial vein graft, or any tubular tissue, typically, but not
exclusively, in autologous, allogeneic xenogeneic transplantation
procedures. To this end, provided herein is a method of wrapping
tubular tissue, including, without limitation, a vein, artery,
urethra, intestine, trachea, esophagus, ureter and fallopian tube
(meaning that any portion of those tissue sources for the graft,
and not implying that the entire stated anatomical structure is
used for the graft purposes, though use of the entire structure or
substantially the entire structure is one option. Thus, when the
tubular tissue is said to be a vein, such as a saphenous vein, this
does not mean that the entire saphenous vein has to be used). The
structure is wrapped with a restrictive fiber matrix of a
bioerodible polymer about a circumference of the tubular tissue. As
described herein, a "fiber" an elongated, slender, elongated,
thread-like and/or filamentous structure. A "matrix" is any two- or
three-dimensional arrangement of elements (e.g., fibers), either
ordered (e.g., in a woven or non-woven mesh) or randomly-arranged
(as is typical with a mat of fibers typically produced by
electrospinning).
[0068] The matrix typically is substantially or essentially
contiguous about a circumference of a tubular tissue, meaning that
the matrix forms a continuous, supportive ring on a surface and
about a circumference of a portion, but not necessarily over the
entire surface (e.g., length) of the tubular tissue. The matrix is
"restrictive," meaning that the matrix is in substantial contact
with the outer surface of the tubular tissue and restricts, hinders
and/or prevents substantial circumferential expansion of the
tubular tissue when grafted. The degree of restriction by the
matrix typically is such that under typical arterial pressures, the
tubular tissue is prevented from distending to substantially a
maximum distension diameter for that tissue (see, e.g., FIG. 4).
The matrix can be elastic, so long as it is restrictive. Where the
matrix is bioerodible, the restrictive nature of the matrix
declines over time as the matrix erodes.
[0069] In one non-limiting embodiment, the matrix is deposited onto
a tubular tissue, such as a tubular anatomical structure or organ
by electrospinning. In one particular non-limiting embodiment, the
anatomical structure is a vein, such as a saphenous vein, that is
used, for instance, in an arterial bypass procedure, such as a
coronary arterial bypass procedure.
[0070] Although any useful method of depositing fine fibers onto a
surface of a tubular tissue could be employed, electrospinning is a
useful method of depositing substantially uniform fibers onto such
a surface. Electrospinning permits fabrication of scaffolds that
resemble the scale and fibrous nature of the native extracellular
matrix (ECM). The ECM is composed of fibers, pores, and other
surface features at the sub-micron and nanometer size scale. Such
features directly impact cellular interactions with synthetic
materials such as migration and orientation. Electrospinning also
permits fabrication of oriented fibers to result in scaffolds with
inherent anisotropy. These aligned scaffolds can influence cellular
growth, morphology and ECM production. For example, Xu et al. found
smooth muscle cell (SMC) alignment with
poly(L-lactide-co-.epsilon.-caprolactone) fibers (Xu C. Y., Inai
R., Kotaki M., Ramakrishna S., "Aligned biodegradable nanofibrous
structure: a potential for blood vessel engineering", Biomaterials
2004 (25) 877-86.) and Lee et al. submitted aligned
non-biodegradable polyurethane to mechanical stimulation and found
cells cultured on aligned scaffolds produced more ECM than those on
randomly organized scaffolds (Lee C. H., Shin H. J., Cho I. H.,
Kang Y. M. Kim I. A., Park K. D., Shin, J. W., "Nanofiber alignment
and direction of mechanical strain affect the ECM production of
human ACL fibroblast", Biomaterials 2005 (26) 1261-1270).
[0071] Generally, the process of electrospinning involves placing a
polymer-containing fluid (e.g, a polymer solution, a polymer
suspension, or a polymer melt) in a reservoir equipped with a small
orifice, such as a needle or pipette tip and a metering pump. One
electrode of a high voltage source is also placed in electrical
contact with the polymer-containing fluid or orifice, while the
other electrode is placed in electrical contact with a target
(typically a collector screen or rotating mandrel). During
electrospinning, the polymer-containing fluid is charged by the
application of high voltage to the solution or orifice (e.g., about
3-15 kV) and then forced through the small orifice by the metering
pump that provides steady flow. While the polymer-containing fluid
at the orifice normally would have a hemispherical shape due to
surface tension, the application of the high voltage causes the
otherwise hemispherically shaped polymer-containing fluid at the
orifice to elongate to form a conical shape known as a Taylor cone.
With sufficiently high voltage applied to the polymer-containing
fluid and/or orifice, the repulsive electrostatic force of the
charged polymer-containing fluid overcomes the surface tension and
a charged jet of fluid is ejected from the tip of the Taylor cone
and accelerated towards the target, which typically is biased
between -2 to -10 kV. Optionally, a focusing ring with an applied
bias (e.g., 1-10 kV) can be used to direct the trajectory of the
charged jet of polymer-containing fluid. As the charged jet of
fluid travels towards the biased target, it undergoes a complicated
whipping and bending motion. If the fluid is a polymer solution or
suspension, the solvent typically evaporates during mid-flight,
leaving behind a polymer fiber on the biased target. If the fluid
is a polymer melt, the molten polymer cools and solidifies in
mid-flight and is collected as a polymer fiber on the biased
target. As the polymer fibers accumulate on the biased target, a
non-woven, porous mesh (matrix) is formed on the biased target.
[0072] The properties of the electrospun elastomeric matrices can
be tailored by varying the electrospinning conditions. For example,
when the biased target is relatively close to the orifice, the
resulting electrospun mesh tends to contain unevenly thick fibers,
such that some areas of the fiber have a "bead-like" appearance.
However, as the biased target is moved further away from the
orifice, the fibers of the non-woven mesh tend to be more uniform
in thickness. Moreover, the biased target can be moved relative to
the orifice. In certain embodiments, the biased target is moved
back and forth in a regular, periodic fashion, such that fibers of
the non-woven mesh are substantially parallel to each other. When
this is the case, the resulting non-woven mesh may have a higher
resistance to strain in the direction parallel to the fibers,
compared to the direction perpendicular to the fibers. In other
embodiments, the biased target is moved randomly relative to the
orifice, so that the resistance to strain in the plane of the
non-woven mesh is isotropic. The target can also be a rotating
mandrel. In this case, the properties of the non-woven mesh may be
changed by varying the speed of rotation. The properties of the
electrospun elastomeric scaffold may also be varied by changing the
magnitude of the voltages applied to the electrospinning system. In
one particularly preferred embodiment, the electrospinning
apparatus includes an orifice biased to 12 kV, a target biased to
-7 kV, and a focusing ring biased to 3 kV. Moreover, a useful
orifice diameter is 0.047'' (I.D.) and a useful target distance is
about 23 cm. A useful range of high-voltage to be applied to a
polymer suspension or melt is from 0.5-30 kV, more preferably 5-25
kV, even more preferably 10-15 kV.
[0073] Electrospinning may be performed using two or more nozzles,
wherein each nozzle is a source of a different polymer solution.
The nozzles may be biased with different biases or the same bias in
order to tailor the physical and chemical properties of the
resulting non-woven polymeric mesh. Additionally, many different
targets may be used. In addition to a flat, plate-like target, a
mandrel may be used as a target.
[0074] When the electrospinning is to be performed using a polymer
suspension, the concentration of the polymeric component in the
suspension can also be varied to modify the physical properties of
the elastomeric scaffold. For example, when the polymeric component
is present at relatively low concentration, the resulting fibers of
the electrospun non-woven mesh have a smaller diameter than when
the polymeric component is present at relatively high
concentration. Without any intention to be limited by this theory,
it is believed that lower concentration solutions have a lower
viscosity, leading to faster flow through the orifice to produce
thinner fibers. One skilled in the art can adjust polymer
concentrations to obtain fibers of desired characteristics. Useful
ranges of concentrations for the polymer component include from
about 1% wt. to about 15% wt., from about 4% wt. to about 10% wt.
and from about 6% wt. to about 8% wt.
[0075] In use, the mandrel is placed inside a tubular tissue, such
as a vein, and polymer fibers are deposited about the circumference
of at least a portion of the tissue by rotation of the mandrel. The
mandrel can be reciprocated longitudinally between the spinneret
and collector to increase the coverage of the tubular tissue.
[0076] Thickness of the matrix can be controlled by either
adjusting the viscosity of the polymer composition to be deposited
and/or adjusting duration of the electrospinning Use of more
viscous polymer composition may result in thicker fibers, requiring
less time to deposit a matrix of a desired thickness. Use of a less
viscous polymer composition may result in thinner fibers, requiring
increased deposition time to deposit a matrix of a desired
thickness. The thickness of the matrix and fibers within the matrix
affects the speed of bioerosion of the matrix. These parameters are
optimized, depending on the end-use of the matrix, to achieve a
desired or optimal physiological effect.
[0077] The biodegradation rate of the polymer matrix may be
manipulated, optimized or otherwise adjusted so that the matrix
degrades over a useful time period. For instance, in the case of a
coronary artery bypass, it is desirable that the matrix dissolves
over 12 hours or more so as to prevent substantial sudden stress on
the graft. The polymer degrades over a desired period of time so
that the mechanical support offered by the polymer matrix is
gradually reduced over that period and the vein would be exposed to
gradually increasing levels of CWS.
[0078] This new approach would have two potential applications. In
the first non-limiting application, the matrix can be used as a
peri-surgical tool for the modification of vein segments intended
for use as an AVG. The modification of a vein or other tubular
tissue or anatomical structure may be performed at bedside,
immediately after removal from the body and just prior to grafting,
for example and without limitation, during arterial bypass surgery.
In one non-limiting example, after the saphenous vein is harvested,
and while the surgeon is exposing the surgical (graft) site, the
polymer wrap would be electrospun onto the vein just prior to it
being used for the bypass procedure.
[0079] In a second non-limiting embodiment, the polymer matrix can
be used as a vehicle for the delivery of support to AVGs. While
modification of the mechanical environment of a vein graft over
time could itself improve AVG patency, delivery of active agents
and biological (cellular) support to AVGs may prove desirable in
many instances. By tuning an electrospun polymer wrap, in which
active agents and/or biologicals are incorporated, to degrade at a
desired rate, the rate of delivery of these support modalities
could be controlled.
[0080] Previous approaches to perivascular placement of a wrap to
deliver support to AVGs had rate-limiting barriers to clinical
translation, and the approach presented herein, using an
electrospun biodegradable polymer, addresses these limitations.
[0081] The use of an external sheath around vein grafts was first
described by Parsonnet et al. They showed that the sheath prevented
dilatation, that it was well accepted by the host tissue, and that
there was no difference in the tensile strength between supported
and non-supported vessels (Parsonnet V, Lari A A, and Shah I H. New
stent for support of veins in arterial grafts. Arch Surg. 1963; 87:
696-702). Karayannacos et al. showed reduced thrombosis and
sub-endothelial proliferation in AVGs with both loose and tight
fitting Dacron mesh sheaths compared with unsupported control
grafts (Karayannacos P E, Hostetler J R, Bond M G, Kakos G S,
Williams R A, Kilman J W, and Vasko J S. Late failure in vein
grafts: Mediating factors in subendothelial fibromuscular
hyperplasia. Ann Surg. 1978; 187(2): 183-8). Mehta et al.
demonstrated that placement of an external, macroporous,
nonrestrictive, polyester stent reduces neointima formation in
porcine vein grafts (Mehta D, George S J, Jeremy J Y, Izzat M B,
Southgate K M, Bryan A J, Newby A C, and Angelini G D. External
stenting reduces long-term medial and neointimal thickening and
platelet derived growth factor expression in a pig model of
arteriovenous bypass grafting. Nat Med. 1998; 4(2): 235-9). More
recently, polytetrofluoroethylene sheaths were used to permanently
restrict AVGs from expansion under arterial pressure and this led
to reduced IH formation in a pig model (Liu S Q, Moore M M,
Glucksberg M R, Mockros L F, Grotberg J B, and Mok A P. Partial
prevention of monocyte and granulocyte activation in experimental
vein grafts by using a biomechanical engineering approach. J
Biomech. 1999; 32(11): 1165-75).
[0082] Clinical translation of permanent mechanical support to AVGs
has not yet been reported, most likely due to the unfavorable
inflammatory response to biodurable synthetic materials in vascular
applications (Bunt T J. Synthetic vascular graft infections. I.
Graft infections. Surgery. 1983; 93(6): 733-46 and Edwards W H,
Jr., Martin R S, 3rd, Jenkins J M, Edwards W H, Sr., and Mulherin J
L, Jr. Primary graft infections. J Vasc Surg. 1987; 6(3): 235-9).
This limitation motivated Vijayan et al. and Jeremy et al. to use a
polyglactin based biodegradable sheath to reduce IH in AVGs (Jeremy
J Y, Bulbulia R, Johnson J L, Gadsdon P, Vijayan V, Shukla N, Smith
F C, and Angelini G D. A bioabsorbable (polyglactin),
nonrestrictive, external sheath inhibits porcine saphenous vein
graft thickening. J Thorac Cardiovasc Surg. 2004; 127(6): 1766-72;
Vijayan V, Shukla N, Johnson J L, Gadsdon P, Angelini G D, Smith F
C, Baird R, and Jeremy J Y. Long-term reduction of medial and
intimal thickening in porcine saphenous vein grafts with a
polyglactin biodegradable external sheath. J Vasc Surg. 2004;
40(5): 1011-9; and Vijayan V, Smith F C, Angelini G D, Bulbulia R
A, and Jeremy J Y. External supports and the prevention of
neointima formation in vein grafts. Eur J Vasc Endovasc Surg. 2002;
24(1): 13-22). The noted beneficial effects included enhanced
neo-vasa-vasorum development over unwrapped controls (Vijayan V,
Shukla N, Johnson J L, Gadsdon P, Angelini G D, Smith F C, Baird R,
and Jeremy J Y. Long-term reduction of medial and intimal
thickening in porcine saphenous vein grafts with a polyglactin
biodegradable external sheath. J Vasc Surg. 2004; 40(5): 1011-9).
However, these biodegradable sheaths were loose-fitting and allowed
the AVGs to expand to their maximum diameters under arterial
pressure, and thus did not offer mechanical support against the
increased level of CWS. Prior to the approach used by Vijayan et
al. (Vijayan V, Shukla N, Johnson J L, Gadsdon P, Angelini G D,
Smith F C, Baird R, and Jeremy J Y. Long-term reduction of medial
and intimal thickening in porcine saphenous vein grafts with a
polyglactin biodegradable external sheath. J Vasc Surg. 2004;
40(5): 1011-9 and Vijayan V, Smith F C, Angelini G D, Bulbulia R A,
and Jeremy J Y. External supports and the prevention of neointima
formation in vein grafts. Eur J Vasc Endovasc Surg. 2002; 24(1):
13-22) and Jeremy et al. (Jeremy J Y, Bulbulia R, Johnson J L,
Gadsdon P, Vijayan V, Shukla N, Smith F C, and Angelini G D. A
bioabsorbable (polyglactin), nonrestrictive, external sheath
inhibits porcine saphenous vein graft thickening. J Thorac
Cardiovasc Surg. 2004; 127(6): 1766-72), Huynh et al. used a
temporary external collagen tube support to reduce IH formation in
rabbit vein grafts. These collagen tubes were also non-restrictive,
and no mention of the degradation kinetics was reported (Huynh T T,
Iaccarino G, Davies M G, Safi H J, Koch W J, and Hagen P O.
External support modulates g protein expression and receptor
coupling in experimental vein grafts. Surgery. 1999; 126(2):
127-34). It has been reported that electrospun cross-linked
collagen degrades very rapidly in an aqueous solution (Rho K S,
Jeong L, Lee G, Seo B M, Park Y J, Hong S D, Roh S, Cho J J, Park W
H, and Min B M. Electrospinning of collagen nanofibers: Effects on
the behavior of normal human keratinocytes and early-stage wound
healing. Biomaterials. 2006; 27(8): 1452-61) and hence the
structural support offered to AVGs by sheaths made of collagen
alone may be too temporary to be effective over the long-term. An
external AVG sheath developed by Liao et al. was designed to
degrade at a desired rate in order to transfer CWS to an AVG
gradually over time. Poly lactic-co glycolic acid sheets were
prefabricated into tubes by wrapping around a Teflon rod, and
therefore are not customizable to each AVG (Liao S W, Lu X, Putnam
A J, and Kassab G S. A novel time-varying poly lactic-co glycolic
acid external sheath for vein grafts designed under physiological
loading. Tissue Eng. 2007; 13(12): 2855-62). That is, as with
previous approaches the Liao et al. approach allows expansion of an
AVG under arterial pressure before delivering any mechanical
support. The degradation kinetics and resulting CWS vs. time
profile in the sheaths, not in the mid-AVG-wall as described here,
were reported. Our approach addresses the two major limitations
associated with the previous work described above, specifically
with respect to biodurable and/or non-restrictive external
sheaths.
[0083] Delivery of mechanical support to AVGs is but one
possibility for an adventitial wrap. Other applications could be as
a vehicle for the local delivery of biochemicals, drugs, genes, or
cells. Kanjickal et al. used a poly(ethylene glycol) hydrogel for
sustained local delivery of cyclosporine to AVGs, and successfully
reduced anastomotic IH development (Kanjickal D, Lopina S,
Evancho-Chapman M M, Schmidt S, Donovan D, and Springhetti S.
Polymeric sustained local drug delivery system for the prevention
of vascular intimal hyperplasia. J Biomed Mater Res A. 2004; 68(3):
489-95). In another study, Cagiannos et al. used a
polytetrafluoroethylene sheath to locally deliver rapamycin
(sirolimus) to AVGs, and effectively reduced anastomotic IH in a
pig model (Cagiannos C, Abul-Khoudoud O R, DeRijk W, Shell D Ht,
Jennings L K, Tolley E A, Handorf C R, and Fabian T C.
Rapamycin-coated expanded polytetrafluoroethylene bypass grafts
exhibit decreased anastomotic neointimal hyperplasia in a porcine
model. J Vasc Surg. 2005; 42(5): 980-8). More recently, Kohler et
al. used a biodegradable mesh to deliver paclitaxel to effectively
reduce IH at the graft-vein anastomosis in a sheep model of
dialysis access (Kohler T R, Toleikis P M, Gravett D M, and Avelar
R L. Inhibition of neointimal hyperplasia in a sheep model of
dialysis access failure with the bioabsorbable vascular wrap
paclitaxel-eluting mesh. J Vasc Surg. 2007; 45(5): 1029-1037;
discussion 1037-8). Such activities could theoretically be
incorporated using the electrospun polymer wrap technique, with the
potential to control the delivery rate to some extent by tuning the
degradation rate of the electrospun polymer wrap.
[0084] To our knowledge, delivery of cells via a biodegradable AVG
wrap/sheath has not been previously reported and hence this
possible future application of the adventitial wrap would be novel.
The polymer that was used here has been characterized (Stankus J J,
Guan J, and Wagner W R. Fabrication of biodegradable elastomeric
scaffolds with sub-micron morphologies. J Biomed Mater Res A. 2004;
70(4): 603-14), and successfully micro-integrated with viable SMCs
(Stankus J J, Guan J, Fujimoto K, and Wagner W R. Microintegrating
smooth muscle cells into a biodegradable, elastomeric fiber matrix.
Biomaterials. 2006; 27(5): 735-44), and would lend itself to this
potential future application.
[0085] A biodegradeable polymer is "biocompatible" in that the
polymer and degradation products thereof are substantially
non-toxic, including non-carcinogenic and non-immunogenic, and are
cleared or otherwise degraded in a biological system, such as an
organism (patient) without substantial toxic effect. Non-limiting
examples of degradation mechanisms within a biological system
include chemical reactions, hydrolysis reactions, and enzymatic
cleavage. Biodegradable polymers include natural polymers,
synthetic polymers, and blends of natural and synthetic polymers.
For example and without limitation, natural polymers include
chitosan, collagen, elastin, alginate, cellulose, polyalkanoates,
hyaluronic acid, or gelatin. Natural polymers can be obtained from
natural sources or can be prepared by synthetic methods (including
by recombinant methods) in their use in the context of the
technologies described herein. Non-limiting examples of synthetic
polymers include: homopolymers, heteropolymers, co-polymers and
block polymers or co-polymers.
[0086] As used herein, the term "polymer composition" is a
composition comprising one or more polymers. As a class, "polymers"
includes homopolymers, heteropolymers, co-polymers, block polymers,
block co-polymers and can be both natural and synthetic.
Homopolymers contain one type of building block, or monomer,
whereas co-polymers contain more than one type of monomer. For
example and without limitation, polymers comprising monomers
derived from alpha-hydroxy acids including polylactide,
poly(lactide-co-glycolide), poly(L-lactide-co-caprolactone),
polyglycolic acid, poly(dl-lactide-co-glycolide),
poly(l-lactide-co-dl-lactide); monomers derived from esters
including polyhydroxybutyrate, polyhydroxyvalerate, polydioxanone
and polygalactin; monomers derived from lactones including
polycaprolactone; monomers derived from carbonates including
polycarbonate, polyglyconate, poly(glycolide-co-trimethylene
carbonate), poly(glycolide-co-trimethylene carbonate-co-dioxanone);
monomers joined through urethane linkages, including polyurethane,
poly(ester urethane) urea elastomer.
[0087] According to a non-limiting embodiment, the polymer
composition comprises one or both of a collagen and an elastin.
Collagen is a common ECM component and typically is degraded in
vivo at a rate faster than many synthetic bioerodable polymers.
Therefore, manipulation of collagen content in the polymer
composition can be used as a method of modifying bierosion rates in
vivo. Collagen may be present in the polymer composition in any
useful range, including, without limitation, from about 2% wt. to
about 95% wt., but more typically in the range of from about 25%
wt. to about 75% wt., inclusive of all ranges and points
therebetween, including from about 40% wt. to about 75%, including
about 75% wt. and about 42.3% wt. Elastin may be incorporated into
the polymer composition in order to provide increased elasticity.
Use of elastin can permit slight circumferential expansion of the
restrictive matrix in order to assist the tubular tissue, such as a
vein, adapt to its new function, such as an arterial use. Elastin
may be present in the polymer composition in any useful range,
including without limitation, from about 2% wt. to about 50% wt.,
inclusive of all ranges and points therebetween, including from
about 40% wt. and about 42.3% wt., inclusive of all integers and
all points therebetween and equivalents thereof. In one
non-limiting embodiment, collagen and elastin are present in
approximately equal amounts in the polymer composition. In another
embodiment, the sum of the collagen and elastin content in the
polymer composition is in any useful range, including, without
limitation, from about 2% wt. to about 95% wt., but more typically
in the range of from about 25% wt. to about 75% wt., inclusive of
all ranges and points therebetween, including from about 40% wt. to
about 75%, including about 75% wt. and about 42.3% wt.
[0088] All ranges or numerical values stated herein, whether or not
preceded by the term "about" unless stated otherwise are considered
to be preceded by the term "about" to account for variations in
precision of measurement and functionally equivalent ranges. For
example, collagen may be stated as being present in a polymer
composition at 10% wt., but, due to measurement variation, may be
literally present at 10% wt..+-.0.05% wt., 0.10% wt. or 1.0% wt.,
and is likely to function in the same manner at such weight
percentages.
[0089] As used herein, the terms "comprising," "comprise" or
"comprised," and variations thereof, are meant to be open ended.
The terms "a" and "an" are intended to refer to one or more.
[0090] As used herein, the term "patient" or "subject" refers to
members of the animal kingdom including but not limited to human
beings.
[0091] A polymer "comprises" or is "derived from" a stated monomer
if that monomer is incorporated into the polymer. Thus, the
incorporated monomer that the polymer comprises is not the same as
the monomer prior to incorporation into a polymer, in that at the
very least, certain terminal groups are incorporated into the
polymer backbone. A polymer is said to comprise a specific type of
linkage if that linkage is present in the polymer.
[0092] The biodegradable polymers described herein are said to be
bioerodible. By "bioerodible", it is meant that the polymer, once
implanted and placed in contact with bodily fluids and tissues,
will degrade either partially or completely through chemical
reactions with the bodily fluids and/or tissues, typically and
often preferably over a time period of hours, days, weeks or
months. Non-limiting examples of such chemical reactions include
acid/base reactions, hydrolysis reactions, and enzymatic cleavage.
In certain embodiments, the polymers contain labile chemical
moieties, examples of which include esters, anhydrides,
polyanhydrides, or amides. Alternatively, the polymers may contain
peptides or biomacromolecules as building blocks which are
susceptible chemical reactions once placed in situ. For example,
the polymer may contain the peptide sequence
alanine-alanine-lysine, which confers enzymatic lability to the
polymer. In another embodiment, the polymer may include an
extracellular matrix protein as a building block, such as
collagen.
[0093] The polymer or polymers typically will be selected so that
it degrades in situ over a time period to optimize mechanical
conditioning of the tissue. Non-limiting examples of useful in situ
degradation rates include between 12 hours and 2 weeks, and
increments of 1, 2, 3, 6, 12, 24 and/or 48 hours therebetween.
[0094] The biodegradable polymers useful herein also can be
elastomeric. Generally, any elastomeric polymer that has properties
similar to that of the soft tissue to be replaced or repaired is
appropriate. For example, in certain embodiments, the polymers used
to make the wrap are highly distensible. Non-limiting examples of
suitable polymers include those that have a breaking strain of from
100% to 1700%, more preferably between 200% and 800%, and even more
preferably between 325% and 600%. In particularly preferred
embodiments, the breaking strain of the polymer is between 5% and
50%, more preferably between 10% and 40%, and even more preferably
between 20% and 30%. Further, it is often useful to select polymers
with tensile strengths of from 10 kPa-30 MPa, more preferably from
5-25 MPa, and even more preferably beween 8 and 20 MPa. In certain
embodiments, the initial modulus is between 10 kPa to 100 MPa, more
preferably between 10 and 90 MPa, and even more preferably between
20 and 70 MPa.
[0095] In certain embodiments, the polymers used herein also
release therapeutic agents when they degrade within the patient's
body. For example, the individual building blocks of the polymers
may be chosen such that the building blocks themselves provide a
therapeutic benefit when released in situ through the degradation
process. In one particularly preferred embodiment, one of the
polymer building blocks is putrescine, which has been implicated as
a substance that causes cell growth and cell differentiation.
[0096] In one embodiment, the fibers comprise a biodegradable
poly(ester urethane) urea elastomer (PEUU). An example of such a
PEUU is an elastomeric polymer made from polycaprolactonediol (MW
2000) and 1,4-diisocyanatobutane, with a diamine, such as
putrescine as the chain extender. A suitable PEUU polymer may be
made by a two-step polymerization process whereby
polycaprolactonediol (MW 2000), 1,4-diisocyanatobutane, and
putrescine are combined in a 2:1:1 molar ratio. In the first
polymerization step, a 15 wt % solution of 1,4-diisocyanatobutane
in DMSO is stirred continuously with a 25 wt % solution of diol in
DMSO. In the second step, stannous octoate is added and the mixture
is allowed to react at 75.degree. C. for 3 hours, with the addition
of triethylamine to aid dissolution. The elastomeric polymer may
also be a poly(ether ester urethane) urea elastomer (PEEUU). For
example, the PEEUU may be made by reacting
polycaprolactone-b-polyethylene glycol-b-polycaprolactone triblock
copolymers with 1,4-diisocyanatobutane and putrescine. In a
preferred embodiment, PEEUU is obtained by a two-step reaction
using a 2:1:1 reactant stoichiometry of
1,4-diisocyanatobutane:triblock copolymer:putrescine. In the first
polymerization step, a 15 wt % solution of 1,4-diisocyanatobutane
in DMSO is stirred continuously with a 25 wt % solution of triblock
compolymer diol in DMSO. In the second step, stannous octoate is
added and the mixture is allowed to react at 75.degree. C. for 3
hours. The reaction mixture is then cooled to room temperature and
allowed to continue for 18 h. The PEEUU polymer solution is then
precipitated with distilled water and the wet polymer is immersed
in isopropanol for 3 days to remove unreacted monomer and dried
under vacuum.
[0097] In other embodiments, at least one therapeutic agent is
added to the bioerodible fibers. Useful therapeutic agents include
any substance that can be coated on, attached, absorbed, adsorbed,
embedded or otherwise associated with the bioerodible fibers that
would provide a therapeutic benefit to a patient. Therapeutic agent
may be blended with the polymer while the polymer is being
processed. For example, the therapeutic agent may be dissolved in a
solvent (e.g., DMSO) and added to the polymer blend during
processing. In another embodiment, the therapeutic agent is mixed
with a carrier polymer (for example and without limitation, a
polyethylene glycol hydrogel or polylactic-glycolic acid
microparticles) which is subsequently processed with the
elastomeric polymer. By blending the therapeutic agent with a
carrier polymer or the elastomeric polymer itself, the rate of
release of the therapeutic agent may be controlled by the rate of
polymer degradation. In one embodiment, a bioerodible hydrogel
comprising an active agent or cells is applied to the bioerodible
fibers after they are applied to a surface of a tubular tissue.
[0098] As used herein, "biodegradable", "bioresorbable" and
"bioerodible" are synonymous. Also, the descriptor "tubular" does
not refer specifically to a geometrically perfect tube having a
constant diameter and a circular cross-section. It also embraces
tissues having non-circular and varying cross sections, and can
have a variable diameter, and thus any shape having a contiguous
wall surrounding a lumen (that is, they are hollow), and two
openings into the lumen such that a liquid, solid or gas can travel
from one opening to the other. As indicated herein, specific
non-limiting, but illustrative examples of tubular tissues include
arterial, urethral, intestinal, esophageal, ureter, tracheal,
bronchial, and fallopian tube tissue.
[0099] Additionally, other active agents that may be incorporated
into the bioerodible fibers include, without limitation,
anti-inflammatories, such as, without limitation, NSAIDs
(non-steroidal anti-inflammatory drugs) such as salicylic acid,
indomethacin, sodium indomethacin trihydrate, salicylamide,
naproxen, colchicine, fenoprofen, sulindac, diflunisal, diclofenac,
indoprofen sodium salicylamide, antiinflammatory cytokines, and
antiinflammatory proteins or steroidal anti-inflammatory agents);
antibiotics; anticlotting factors such as heparin, Pebac,
enoxaprin, aspirin, hirudin, plavix, bivalirudin, prasugrel,
idraparinux, warfarin, coumadin, clopidogrel, PPACK, GGACK, tissue
plasminogen activator, urokinase, and streptokinase; growth
factors. Other active agents include, without limitation: (1)
immunosuppressants; glucocorticoids such as hydrocortisone,
betamethisone, dexamethasone, flumethasone, isoflupredone,
methylpred-nisolone, prednisone, prednisolone, and triamcinolone
acetonide; (2) antiangiogenics such as fluorouracil, paclitaxel,
doxorubicin, cisplatin, methotrexate, cyclophosphamide, etoposide,
pegaptanib, lucentis, tryptophanyl-tRNA synthetase, retaane, CA4P,
AdPEDF, VEGF-TRAP-EYE, AG-103958, Avastin, JSM6427, TG100801, ATG3,
OT-551, endostatin, thalidomide, becacizumab, neovastat; (3)
antiproliferatives such as sirolimus, paclitaxel, perillyl alcohol,
farnesyl transferase inhibitors, FPTIII, L744, antiproliferative
factor, Van 10/4, doxorubicin, 5-FU, Daunomycin, Mitomycin,
dexamethasone, azathioprine, chlorambucil, cyclophosphamide,
methotrexate, mofetil, vasoactive intestinal polypeptide, and
PACAP; (4) antibodies; drugs acting on immunophilins, such as
cyclosporine, zotarolimus, everolimus, tacrolimus and sirolimus
(rapamycin), interferons, TNF binding proteins; (5) taxanes, such
as paclitaxel and docetaxel; statins, such as atorvastatin,
lovastatin, simvastatin, pravastatin, fluvastatin and rosuvastatin;
(6) nitric oxide donors or precursors, such as, without limitation,
Angeli's Salt, L-Arginine, Free Base, Diethylamine NONOate,
Diethylamine NONOate/AM, Glyco-SNAP-1, Glyco-SNAP-2,
(.+-.)-S-Nitroso-N-acetylpenicillamine, S-Nitrosoglutathione,
NOC-5, NOC-7, NOC-9, NOC-12, NOC-18, NOR-1, NOR-3, SIN-1,
Hydrochloride, Sodium Nitroprusside, Dihydrate, Spermine NONOate,
Streptozotocin; and (7) antibiotics, such as, without limitation:
acyclovir, afloxacin, ampicillin, amphotericin B, atovaquone,
azithromycin, ciprofloxacin, clarithromycin, clindamycin,
clofazimine, dapsone, diclazaril, doxycycline, erythromycin,
ethambutol, fluconazole, fluoroquinolones, foscarnet, ganciclovir,
gentamicin, iatroconazole, isoniazid, ketoconazole, levofloxacin,
lincomycin, miconazole, neomycin, norfloxacin, ofloxacin,
paromomycin, penicillin, pentamidine, polymixin B, pyrazinamide,
pyrimethamine, rifabutin, rifampin, sparfloxacin, streptomycin,
sulfadiazine, tetracycline, tobramycin, trifluorouridine,
trimethoprim sulphate, Zn-pyrithione, and silver salts such as
chloride, bromide, iodide and periodate.
[0100] Cells may be microintegrated within the restrictive,
bioerodible matrix using a variety of methods. For example, the
matrix may be submersed in an appropriate growth medium for the
cells of interest, and then directly exposed to the cells. The
cells are allowed to proliferate on the surface and interstices of
the matrix. The matrix is then removed from the growth medium,
washed if necessary, and implanted. But because electrospun
non-woven fabrics often have pore sizes that are relatively small
(e.g., compared to the pore sizes of non-woven fabrics fabricated
by other methods such as salt leaching or thermally induced phase
separation), culturing cells on the surface of the scaffold is
usually used when microintegration of cells only near the surface
of the elastomeric scaffold is desired.
[0101] In another embodiment, the cells of interest are dissolved
into an appropriate solution (e.g., a growth medium or buffer) and
then sprayed onto a restrictive, bioerodible matrix while the
matrix is being formed by electrospinning. This method is
particularly suitable when a highly cellularized tissue engineered
construct is desired. In one embodiment, pressure spraying (i.e.,
spraying cells from a nozzle under pressure) is used to deposit the
cells. In another, the cells are electrosprayed onto the non-woven
mesh during electrospinning. As described herein, electrospraying
involves subjecting a cell-containing solution with an appropriate
viscosity and concentration to an electric field sufficient to
produce a spray of small charged droplets of solution that contain
cells. In one experiment (not shown), cell viability was examineed
for smooth muscle cells (SMCs) sprayed under different conditions.
These different conditions include spraying alone, spraying onto a
target charged at -15 kV, spraying onto a target charged at -15 kV
with PEUU electro spinning, electrospraying at 10 kV onto a target
charged at -15 kV, and electrospraying at 10 kV onto a target
charged at -15 kV with PEUU electrospinning A significant reduction
in SMC viability resulted from spraying cells through the nozzle.
Without any intent to be bound by theory, it is believed that the
physical forces of the pressurized spray in combination with the
exposure of cells to processing solvents may have caused this
result since viability was lost both from spraying alone and even
more so by spraying during electrospun PEUU (e-PEUU) fabrication.
Decreased viability from cell aerosol spraying has been reported by
others and found to depend largely on nozzle diameter, spray
pressure, and solution viscosity (Veazey W. S., Anusavice K. J.,
Moore K., "Mammalian cell delivery via aerosol deposition", J.
Biomed. Mater. Res. 2005 (72B)334-8.). Therefore, cells were also
sprayed from media supplemented with gelatin to increase viscosity
and help protect the cells from mechanical and chemical stresses.
Viability was recovered, yet the mechanical integrity of the PEUU
matrices was disrupted because of gelation within the fiber
network.
[0102] In contrast to pressurized spraying, electrospraying cells
did not significantly affect cell viability or proliferation. This
is consistent with reports by others that cells can survive
exposure to high voltage electric fields (see, e.g., Nedovic V. A.,
Obradovic B., Poncelet D., Goosen M. F. A., Leskosek-Cukalovic O.,
Bugarski B., "Cell immobiliation by electrostatic droplet
generation", Landbauforsch Volk 2002, (241) 11-17; Temple M. D.,
Bashari E., Lu J., Zong W. X., Thompson C. B., Pinto N. J., Monohar
S. K., King R. C. Y., MacDiarmid A. G., "Electrostatic
transportation of living cells through air", Abstracts of Papers,
223 ACS National Meeting, Orlando, Fla., Apr. 7-11, 2002). Even in
the presence of PEUU electrospinning, SMC viability was not
reduced, perhaps because the positively charged electrospinning and
electrospraying streams repelled each other and avoided exposing
cells to solvent prior to deposition. Also, due to the relatively
large electrospinning distance of 23 cm, PEUU fibers were likely
free of solvent by the time they were deposited. Electrospraying
from media supplemented with gelatin resulted in a greater number
of viable cells compared to electrospraying from media without
gelatin. However, the use of gelatin leads to reduced construct
mechanical properties. Accordingly, in many cases electrospraying
from media alone maybe a preferred cellular incorporation
method.
[0103] The cells that may be incorporated on or into the bioerodibe
matrix include stem cells, progenitor (precursor) cells, smooth
muscle cells, skeletal myoblasts, myocardial cells, endothelial
cells, endothelial progenitor cells, bone-marrow derived
mesenchymal cells and genetically modified cells. In certain
embodiments, the genetically modified cells are capable of
expressing a therapeutic substance, such as a growth factor.
Examples of suitable growth factors include angiogenic or
neurotrophic factor, which optionally may be obtained using
recombinant techniques. Non-limiting examples of growth factors
include basic fibroblast growth factor (bFGF), acidic fibroblast
growth factor (aFGF), vascular endothelial growth factor (VEGF),
hepatocyte growth factor (HGF), insulin-like growth factors (IGF),
transforming growth factor-beta pleiotrophin protein, midkine
protein. In one preferred embodiment, the growth factor is
IGF-1.
EXAMPLES
[0104] The autogenous saphenous vein remains the graft of choice
for both coronary (500,000 annually) and peripheral (80,000
annually) arterial bypass procedures. Failure of AVGs remains a
major problem, and patients with failed grafts will die or require
re-operation. IH accounts for 20% to 40% of all AVG failures. It is
believed that IH is triggered by abrupt exposure of AVGs to the
harsh new biomechanical environment of the arterial circulation and
the elevated levels of CWS associated with the arterial system
(140-fold increase compared to native venous conditions). The
working hypothesis herein is that the IH response may be reduced or
eliminated by more gradually exposing AVGs to arterial levels of
CWS. That is, if an AVG is given an ample opportunity to adapt and
remodel to the stresses of its new environment, cellular injury may
be reduced, thus limiting the initiating mechanisms of IH. Clearly,
developing a reliable means to prevent the early events of the IH
process would contribute significantly to improvements in the
clinical outcome of arterial bypass procedures. Therefore, the
long-term goal of this work is to develop a new mechanical
conditioning paradigm, in the form of a peri-adventitially placed,
biodegradable polymer wrap, to safely and functionally
"arterialize" AVGs in situ. The polymer wrap is tuned so that as it
degrades over a desired period of time, the mechanical support
offered by it is reduced and the vein is exposed to gradually
increasing levels of CWS in situ.
[0105] Several of the molecular signals outlined herein, and the
rationale for selecting them as endpoints for this study, are
summarized in Table 1.
TABLE-US-00001 TABLE 1 Summary of and rationale for the chosen
endpoints in this study. Proposed endpoints in this study Role in
IH Rationale supported by the literature Golgi Complex Phenotypic
modulation Increased quantities in synthetic vs. Protein Synthesis
contractile SMCs.sup.a. PCNA Proliferation Increased cell
proliferation in abruptly- exposed AVGs.sup.b. TUNEL Apoptosis
Altered apoptosis in abruptly-exposed AVGs.sup.c Compliance
Clinical Performance Important predictor of AVG patency.sup.d.
Compliance decreases in abruptly exposed arterialized AVGs, thereby
increasing compliance mismatch.sup.e Important predictor of AVG
patency.sup.d. Stiffness Clinical Performance Stiffness increases
in abruptly exposed arterialized AVGs and could contribute to
reduced clinical performance.sup.f .sup.aMorisaki N, et al. Cell
cycle-dependent inhibition of DNA synthesis by prostaglandin i2 in
cultured rabbit aortic smooth muscle cells. Atherosclerosis. 1988;
71(2-3): 165-71; Campbell GR, et al. Arterial smooth muscle. A
multifunctional mesenchymal cell. Arch Pathol Lab Med. 1988;
112(10): 977-86; and Nagai R, et al. Identification of two types of
smooth muscle myosin heavy chain isoforms by cdna cloning and
immunoblot analysis. The Journal of Biological Chemistry. 1989;
264(17): 9734-7. .sup.bNishibe T, et al. Induction of angiotensin
converting enzyme in neointima after intravascular stent placement.
Int Angiol. 2002; 21(3): 250-5 and Zuckerbraun BS, et al.
Overexpression of mutated ikappabalpha inhibits vascular smooth
muscle cell proliferation and intimal hyperplasia formation. J Vasc
Surg. 2003; 38(4): 812-9. .sup.cWang GJ, et al. Regulation of vein
graft hyperplasia by survivin, an inhibitor of apoptosis protein.
Arterioscler Thromb Vasc Biol. 2005; 25(10): 2081-7 and Wang AY, et
al. Expression of apoptosis-related proteins and structural
features of cell death in explanted aortocoronary saphenous vein
bypass grafts. Cardiovasc Surg. 2001; 9(4): 319-28 .sup.dDavies AH,
et al. Prevention of malalignment during non-reversed femorodistal
bypass. Ann R Coll Surg Engl. 1992; 74(6): 434-5 .sup.eJacot JG, et
al. Early adaptation of human lower extremity vein grafts: Wall
stiffness changes accompany geometric remodeling. J Vasc Surg.
2004; 39(3): 547-55 .sup.fTai NR, et al. Compliance properties of
conduits used in vascular reconstruction. Br J Surg. 2000; 87(11):
1516-24 and Jacot JG, J Vasc Surg. 2004; 39(3): 547-55
Example 1
Fabrication of PEUU Structures
[0106] By syringe pump into a stainless-steel capillary suspended
13-cm vertically over a 4.5'' diameter aluminum mandrel 5-% wt.
PEUU solution in hexafluoroisopropanol (HFIP) was fed at 1.0 mL/h
PEUU was charged with +12 kV and the aluminum target with -7 kV
using high voltage generators (Gamma High Voltage Research).
Aligned PEUU fibers were formed by electrospinning onto the target
rotating at speeds ranging from 0.0 to 13.8 m/s. Scaffolds were
allowed to dry overnight at room temperature and then placed under
vacuum for 48 h at 30.degree. C. A portion of each sample was
mounted into a standard X-ray diffraction holder for analysis so
that the fiber orientation was parallel to the X-ray beam. The
samples were run on a PANalytical X'Pert Pro diffractometer using
copper radiation. PEUU number average and weight average molecular
weight were 228,700 and 87,600, respectively, resulting in a
polydispersity index of 2.61. DSC demonstrated a glass transition
temperature of -54.6.degree. C. and a melt temperature of the PEUU
soft segment at 41.0.degree. C.
Electrospun Tubular Constructs for Blood Vessel Tissue
Engineering
[0107] This example describes one method of producing a highly
cellularized blood vessel construct that is capable of also
providing substantial elastomeric mechanical support. The method
involves a micro-integrated approach wherein a meshwork of
submicron elastomeric fibers is built into a vessel wall with or
without the cellular placement process. Cellularity can be
developed through in vitro culture methods or in vivo. These
methods are applicable to the coating of tubular tissues as
described herein.
[0108] This example provides a method to luminally surface seed
small diameter electrospun polyurethane conduits that may be used
for coating tubular tissues as described herein. Electrospinning
technology is used to incorporate cells during scaffold fabrication
to better encourage tissue development.
[0109] Poly (ester urethane) urea was synthesized from
poly(.epsilon.-caprolactone)diol and 1,4-diisocyanatobutane with
putrescine chain extension. PEUU was dissolved at 6% wt. in
hexafluoroisopropanol and electrospun. Electrospinning conditions
included a solution volumetric flowrate of 1.0 mL/hr, a distance
between nozzle and target of 13.5 cm, and voltages of +12 kV to the
nozzle and -3 kV to the target. The target used for fabrication of
small diameter tubes for implantation was a Type 316 stainless
steel mandrel of 1.3 mm diameter that was rotating at 250 rpm.
[0110] The mandrel was also translating along its axis 8 cm on a
linear stage at a speed of approximately 8 cm/s to produce a more
uniform conduit thickness. Samples were electrospun for 15 min to
produce porous tubular constructs with wall thicknesses on the
order of 150 to 200 .mu.m. For endothelialization studies a 4.7 mm
stainless mandrel was instead utilized with the same process
conditions.
[0111] PEUU at 6% wt. in HFIP was electrospun onto a negatively
charged rotating mandrel at 250 rpm to produce a tubular construct.
The electrospun tubes possessed 1.3 mm inner diameters, lengths up
to 8 cm and wall thicknesses of 150-200 .mu.m. Fiber sizes
approximately in the range of 1000 .mu.m were. In addition, these
constructs were suturable and retained their lumens.
[0112] After fabrication, the mandrel was dipped in 70% ethanol in
order to more easily remove it from the steel mandrel. The conduit
was then rinsed in deionized water multiple times, blotted dry and
then dried under vacuum at room temperature 24 to 48 h. Conduits
were then examined for their gross structure with a dissecting
microscope or their fibrous morphologies with scanning electron
microscopy. In order to view an uninterrupted fibrous
cross-section, samples were dipped in liquid N.sub.2 for 1 min and
then fractured before sputter-coating for SEM.
[0113] PEUU conduits (4.7 mm) were positioned inside a custom
designed rotational vacuum seeding device and seeded with
20.times.10.sup.6 muscle derived stem cells (MDSCs). More
specifically, the electrospun conduit was placed on metal stubs and
a light vacuum was applied to the exterior of the conduit.
Subcultured MSDCs were then perfused through the lumen of the
conduit and forced into the fibrous lumen side wall of the tube by
vacuum. Constructs were cultured under static conditions in Petri
dishes for 24 h. After 24 h of static culture, cells were viable,
adhered to the lumen and formed a monolayer.
[0114] Porous 1.3 mm inner diameter tubular electrospun scaffolds
were implanted as interposition grafts in the abdominal aorta of
rats. Constructs were suturable and easily retained their lumens in
vivo. Female Lewis rats weighing 250-300 g were anesthetized with
1% isofluorane and 2.5 2.5 mg/100 g ketamine. A mid-abdominal
incision was performed and the retroperitoneal cavity exposed. The
descending aorta below renal level was dissected, clamped
proximally and distally sectioned to make a 1 cm gap. The
electrospun conduit was then implanted in an end-to-end manner
using 10.0 prolene sutures. Intravenous heparin was administered
before clamping with 200 Units/kg. The abdominal wall was closed in
two layers with 2.0 Vycril sutures. Rats were able to recover from
the surgeries with limb function. Rats were sacrificed at 2 wks and
sample explants fixed in 10% neutral buffered formalin at room
temperature. At 2 wks after implantation, grafts remained patent
and functional. Samples were then embedded in paraffin and
sectioned before staining with Hematoxylin and Eosin or Masson's
Trichrome. Hematoxylin and eosin staining demonstrated external
capsule formation around the explanted grafts. Masson's Trichrome
staining indicated the capsule was composed of aligned collagen
together with the presence of newly developed capillary vessels.
Cell and tissue in-growth was observed throughout the constructs
with the presence of collagen development. Cells were also
demonstrated to have formed a monolayer in locations around the
construct lumens.
[0115] Whereas the previous example provided in vivo approach, a
biodegradable and cytocompatible, elastomeric poly (ester urethane)
urea was electro spun into small diameter tubes appropriate for
implantation in a rat model.
[0116] Like the previous example, this example provides methods for
fabricating a highly cellularized blood vessel construct that also
provides substantial elastomeric mechanical support. However, the
previous model was an in vivo approach in a biodegradable and
cytocompatible, elastomeric poly (ester urethane) urea was electro
spun into small diameter tubes appropriate for implantation in a
rat model. This example provides an in vitro approach, wherein SMCs
were seeded into electrospun nanofibers concurrently with scaffold
fabrication using a microintegration technique.
[0117] Vascular smooth muscle cells (SMCs) isolated from rat aortas
were expanded on tissue culture polystyrene (TCPS) culture plates
under Dulbecco's Modified Eagle Medium (DMEM) supplemented with 10%
fetal bovine serum and 1% penicillin-streptomycin. Microintegration
was performed similar to described previously with some
modifications to allow for a smaller diameter
electrospraying/electro spinning mandrel.
[0118] 7.5.times.10.sup.6 SMCs/mL were subcultured in medium and
fed at 0.1 mL/min into a sterile Type 316 stainless steel capillary
charged at 8.5 kV and located 4.5 cm from the target. 6% wt. PEUU
or 6% wt. PEUU/collagen (75/25) in HFIP was fed at 1.5 mL/min into
a capillary charged at 12 kV and located 23 cm from the target. The
target consisted of a sterile stainless steel mandrel (4.7 mm
diameter) charged at -3 kV and rotating at 250 rpm while
translating 8-cm along its axis at 1.6 mm/s A fabrication time of
30 min was used to produce each microintegrated conduit. After
fabrication the conduit and mandrel were gently placed with aseptic
technique into a roller bottle and cultured statically for 16 h.
After 16 h, samples were gently removed from the mandrel for
culture. Samples were then cut into 15 mm lengths and sutured to
metal stubs and perfused media with pulsatile flow for 3 days in
device substantially as shown in FIG. 3.
[0119] At timepoints of 1 day and 4 days after fabrication, samples
were characterized. A MTT mitochondrial assay was used to measure
cell viability. For histological investigation, samples were fixed
in 10% neutral buffered formalin at room temperature. Samples were
then embedded in paraffin, sectioned and stained with hematoxylin
and eosin. Samples were analyzed for their biomechanical properties
immediately after fabrication. Properties measured included ring
strength, dynamic compliance, and burst pressure. In order to
measure ring strength, stainless steel staples were inserted into 5
mm long tubular sections and then into the grips of a uniaxial
tensile tester (A TS). Using a 10 lb load cell and a displacement
rate of 10.05 mm/min samples were strained until break.
[0120] For dynamic compliance and burst strength, 15 mm long
tubular samples were mounted in a flow loop driven by a centrifugal
pump (Biomedicus) and submerged in PBS at 37.degree. C. The
pressure was monitored and recorded at 30 Hz using a standard
in-line strain-gage pressure transducer and a PC acquisition board.
The vessel construct was perfused with a pulsatile flow (110/70
mmHg, 1.2 Hz) and the dynamic compliance, C, was measured by
recording the external diameter of the sample with a He--Ne laser
micrometer (Beta Lasermike) Compliance was calculated as:
C = ( D max - D min ) D min ( P max - P min ) ##EQU00001##
for each pulse (D=maximum or minimum diameter, P=maximum or minimum
pressure). A porcine mammary artery was used as a control for
comparison with microintegrated PEUU in compliance studies. For
measuring burst pressure, the sample outlet was sealed and flow was
increased until tube rupture. The maximum pressure before rupture
was taken as the burst pressure.
[0121] In order to extend the technology of cellular
microintegration to small diameter tubes, a 4.7 mm diameter
stainless steel mandrel was used in the place of the previously
employed 19 mm diameter mandrel for sheet microintegration. In
order to microintegrate highly cellular and defect free tubular
constructs, it was useful to slightly decrease electrospraying
distance 0.5 cm and lower the mandrel negative charge from -10 kV
to -3 kV from previous methods. During fabrication, PEUU appeared
pink and glistening on the mandrel indicative of uniform cellular
electrospray. After removal from the mandrel, samples of either
PEUU or PEUU/collagen (75/25) were found to be mechanically robust
in that they were suturable and could retain their lumens after
compression.
[0122] Cell placement and viability in the SMC micro integrated
constructs was investigated initially and again after 4 days of
static or perfusion culture. After perfusion, samples were gently
removed from the stubs and then sectioned into representative
slices for MTT and histology. MTT results indicated viable cells 1
day after fabrication. Furthermore, cells remained viable at day 4
with either static or perfusion culture with cell number values
reported slightly higher for perfusion culture. Samples were fixed
and stained with hematoxylin and eosin staining. H&E staining
showed uniform initial cell integration within the tubular
construct.
[0123] Ring strength, burst pressure, and suture retention strength
were assessed in the micro integrated constructs after fabrication.
Small tube sections (rings) were mechanically robust and flexible
with maximum stress and strain values of 6.3 MPa and 170%
respectively. The ring samples did not break cleanly in each case
and seemed to pull apart or delaminate past the ultimate stress
value. In order to calculate the dynamic compliance of the
microintegrated constructs, samples were exposed to pulsatile flow
and the pressure/diameter relationship was evaluated. This
relationship was compared with a porcine mammary artery (pMA)
exposed to the same pulsatile flow. The mechanical response of both
the pMA and microintegrated PEUU was very similar with values
falling for both samples falling between one another. Compliance
values were 1.02.+-.0.33.times.10.sup.-3 mmHg for pMA and
0.71.+-.0.13.times.10.sup.-3 mmHg.sup.-1 for SMC microintegrated
PEUU. Burst pressure values for all samples were greater than 1500
mmHg. The burst pressure values were approximations due to the
porous nature of the microintegrated tubes.
[0124] This method produced highly cellularized elastomeric
scaffolds. Cells were viable after fabrication and proliferated
under perfusion culture. In order to extend this technology to
micro integrate cells into small diameter tubular constructs as a
blood vessel prototype, it was advantageous to modify some process
variables. For example, in order to target and electro spray cells
onto the smaller diameter mandrel it was useful to decrease the
distance between electro spray nozzle and mandrel. Also, it was
useful to avoid a large negative bias on the mandrel. Using a high
negative charge to the rotating mandrel target resulted in polymer
protrusion defects, or "spikes" in the tube which could disrupt
conduit integrity and cell viability. Therefore, it was useful to
decrease mandrel charge to result in homogenously cellular and
fibrous tubular conduits. These constructs were then cultured under
a perfusion bioreactor to encourage better exchange of nutrients,
waste, and oxygen to the cells in the tube interior. H&E and
MTT results indicated viable cells present within the constructs
after fabrication and perfusion culture.
Example 2
Changes in Mechanical Properties Due to Vein Graft
Arterialization
[0125] In the arterial pressure range an AVG is essentially a rigid
tube due to the degree of its over distension (Stooker W, Gok M,
Sipkema P, Niessen H W, Baidoshvili A, Westerhof N, Jansen E K,
Wildevuur C R, and Eijsman L Pressure-diameter relationship in the
human greater saphenous vein. Ann Thorac Surg. 2003; 76(5):
1533-8). To confirm this we performed a pressure ramping
experiment. The results of this experiment are shown in FIG. 4. It
can be seen that the vein reaches maximum distension at
approximately 30 mmHg Consequently, at arterial levels of pressure
a vein is very stiff, and we hope to counteract this phenomenon by
providing temporary external structural support with a
biodegradable adventitial wrap.
[0126] The degree of AVG distension is directly related to vein
properties such as compliance, which, in turn, is related to
patency rates according to Davies et al. (Davies A H, Magee T R,
Baird R N, and Horrocks M. Prevention of malalignment during
non-reversed femorodistal bypass. Ann R Coll Surg Engl. 1992;
74(6): 434-5 and Davies A H, Magee T R, Baird R N, Sheffield E, and
Horrocks M. Pre-bypass morphological changes in vein grafts. Eur J
Vasc Surg. 1993; 7(6): 642-7), who reported lower patency rates of
less compliant AVGs in peripheral bypass surgery. This reduced
patency has been largely attributed to compliance mismatch between
the AVG and the native artery to which it is grafted (Bandyk D F
and Mills J L. The failing graft: An evolving concept. Semin Vasc
Surg. 1993; 6(2): 75-7; Bassiouny H S, White S, Glagov S, Choi E,
Giddens D P, and Zarins C K. Anastomotic intimal hyperplasia:
Mechanical injury or flow induced. J Vasc Surg. 1992; 15(4):
708-16; discussion 716-7; and Berkowitz H D, Fox A D, and Deaton D
H. Reversed vein graft stenosis: Early diagnosis and management. J
Vasc Surg. 1992; 15(1): 130-41; discussion 141-2). Veins are
inherently less compliant than arteries (Tai N R, Salacinski H J,
Edwards A, Hamilton G, and Seifalian A M. Compliance properties of
conduits used in vascular reconstruction. Br J Surg. 2000; 87(11):
1516-24) and become even less compliant upon abruptly exposed
arterialization (Jacot J G, Abdullah I, Belkin M, Gerhard-Herman M,
Gaccione P, Polak J F, Donaldson M C, Whittemore A D, and Conte M
S. Early adaptation of human lower extremity vein grafts: Wall
stiffness changes accompany geometric remodeling. J Vasc Surg.
2004; 39(3): 547-55). It appears as though change in AVG compliance
is an important predictor of AVG failure.
Example 3
AVG Coated with a Restrictive Polymer Matrix
[0127] The data provided herein cover two distinct areas of ongoing
research: i) investigation of the mechanopathobiological response
of intact vein segments to arterial hemodynamics and ii)
development of a biodegradable electrospun polymer for use as an
adventitial wrap.
[0128] An ex vivo vascular perfusion apparatus was developed to
study the responses of intact vascular segments and grafts to
realistic, well-controlled biomechanical and metabolic conditions.
FIG. 3 shows such a device. This device permits ex vivo exposure of
porcine internal jugular vein segments to precisely controlled
hemodynamics and dissolved gases (pH, pO.sub.2, pCO.sub.2) to
simulate various conditions, including the venous and realistic AVG
environment. Achieving these controlled conditions is accomplished
using two independent perfusion/organ culture systems (shown
schematically in FIG. 3). The closed loop perfusion design allows
the circulation of sterile perfusate (tissue culture Media 199
supplemented with 1% fetal bovine serum, 0.5 g/liter Cefoxitin). A
second roller pump circulates an adventitial bath (DMEM with 1%
fetal bovine serum and 0.5 g/liter Cefoxitin) around the specimen,
which is mounted in a sealed chamber.
[0129] To simulate native venous hemodynamics and biomechanics, the
roller pump and flow resistors of the perfusion loop are set to
provide nonpulsatile flow of 20 ml/min and pressure of 20 mmHg To
simulate AVG hemodynamics, the pump and flow resistors are set to
provide a pulsatile pressure waveform of 120/80 mmHg and a mean
perfusate flow of 100 ml/min. The "AVG conditioning" regimen will
begin first by setting the perfusion system to provide arterial
conditions as decribed above. The circumferential wall stress in a
perfused vein segment will be controlled via the application of a
tuned biodegradable perivascular electrospun polymer wrap. That is,
the midvein-wall circumferential wall stress vs. time profile will
involve the gradual imposition from venous (approximately 25 KPa)
levels to arterial (approximately 140 KPa peak) levels, increasing
linearly over a 24 or 192 hour period. Achieving this desired
degradation rate would make in vivo mechanical conditioning of AVGs
a possible treatment alternative perhaps improving patency rates in
all AVGs.
[0130] To further validate ex vivo perfusion capabilities, tissue
viability analysis of vein segments perfused under venous vs.
arterial conditions was performed and the results to baseline level
of tissue viability was compared. Scanning electron micrography,
H&E staining, Live/Dead.TM. staining, and TUNEL analyses were
performed after 48 hours of ex vivo perfusion (see FIG. 5).
Scanning electron micrography and H&E staining indicated that
the morphologic integrity of the tissue was intact after harvesting
and after 48 hours of perfusion. Live/dead and TUNEL analyses
showed no significant necrosis or apoptosis, respectively, in
either the venous or arterial conditions when compared to baseline
at 48 hours. Similar observations were made for perfusions lasting
14 days using an earlier generation of this system (Ligush, J., R.
F. Labadie, S. A. Berceli, J. B. Ochoa, and H. S. Borovetz,
Evaluation of endothelium derived nitric oxide mediated
vasodilation utilizing ex vivo perfusion of an intact vessel. The
Journal of Surgical Research, 1992. 52(5): p. 416-21). These
experiments demonstrate the ability to perform the proposed ex vivo
porcine internal jugular vein perfusions, with maintenance of
sterile conditions and tissue viability.
[0131] Several sets of ex vivo vascular perfusion experiments were
performed. Initially, one set of experiments (N=6 animals per set)
was performed to establish the acute hyperplastic response of PIJVs
abruptly exposed to arterial biomechanical conditions, and to
compare this response to PIJVs exposed to native venous conditions.
FIG. 6 is a schematic showing this experimental design, which is
also described in detail below. We then attempted to attenuate this
acute hyperplastic response by gradually exposing porcine internal
jugular vein segments (PIJVs) to desired CWS profiles via manual
adjustment of validated ex vivo vascular perfusion system (EVPS)
pressure. FIG. 7 is a schematic showing this experimental design
which is also described in detail below. These experiments were
directly related to establishing a CWS profile necessary to achieve
a reduced acute hyperplastic response by freshly-excised vein
segments perfused ex vivo under incrementally-imposed compared to
abruptly-exposed arterial conditions. Using these results, we also
wanted to tune the degradation rate of an adventitial biodegradable
polymer wrap so as to achieve the same CWS profiles, and then to
use this wrap to attenuate the acute hyperplastic response in PIJVs
compared to unwrapped controls. FIG. 8 is a schematic showing this
experimental design, which is also described in detail below. Each
of the experiments described above was "paired" to account for
animal-to-animal variability, and generally, proceeded as follows.
Bilateral PIJVs were surgically harvested from juvenile pigs and
tied into separate, independent EVPSs (see below). Vascular
perfusion experiments were carried out for 24 or 72 hours since the
majority of the endpoints under investigation have been
successfully detected within a few hours of these time points (see
references in Table 1, above). At the conclusion of each
experiment, the tissue was processed (see below) for biological
assays to assess the endpoints outlined in Table 1.
Tissue Harvest and Transport
[0132] The porcine internal jugular vein (PIJV) was chosen as a
model because of its similarity in inner diameter and wall
thickness to the human greater saphenous vein, and because this
tissue has previously been used to investigate the pathologic
response of veins exposed to arterial hemodynamic conditions. The
surgical harvest procedure was performed in the manner of a
saphenectomy for bypass. Briefly, the anesthetized animal was
placed in supine position, cervical incisions were made
bilaterally, and dissection was done in layers to the vascular
fascia of the neck. Each PIJV was identified and dissected proximal
to the jugular confluence and distal to the jugular foramen. All
tributaries were identified and carefully ligated to avoid leakage.
After the desired length (6-8 cm) was exposed, the segment was
cannulated on each end with duck billed vessel cannulae. Just prior
to explant, a custom-designed vascular clamp (Ligush J, Labadie R
F, Berceli S A, Ochoa J B, and Borovetz H S. Evaluation of
endothelium-derived nitric oxide mediated vasodilation utilizing ex
vivo perfusion of an intact vessel. J Surg Res. 1992; 52(5):
416-21) was attached onto the ends of the cannulae to maintain the
in vivo length of the vessel following removal. The vessel was then
cut on either side between the clamped cannulae and the ligations
Immediately after removal, the vessels were placed in a sterile
transport box (containing lactated ringers solution supplemented
with heparin (500 units/liter), papaverine (60 mg/liter), and
Cefoxitin (1.0 g/liter). The time between tissue harvest and
mounting into the perfusion system described below was always less
than one hour.
Perivascular Placement of Electrospun Biodegradable Polymer
Wrap
[0133] The biodegradable polymer composite used to form the
adventitial wrap was based on the poly(ester urethane)urea (PEUU)
material developed by Guan et al. (Guan J, Sacks M S, Beckman E J,
and Wagner W R. Synthesis, characterization, and cytocompatibility
of elastomeric, biodegradable poly(ester-urethane)ureas based on
poly(caprolactone) and putrescine. J Biomed Mater Res. 2002; 61(3):
493-503) and further characterized in electrospun format by Stankus
et al. (Stankus J J, Guan J, and Wagner W R. Fabrication of
biodegradable elastomeric scaffolds with sub-micron morphologies. J
Biomed Mater Res A. 2004; 70(4): 603-14 and Stankus J J, Guan J,
Fujimoto K, and Wagner W R. Microintegrating smooth muscle cells
into a biodegradable, elastomeric fiber matrix. Biomaterials. 2006;
27(5): 735-44). This polymer undergoes hydrolytic degradation in
vitro into non-cytotoxic degradation products and has been shown to
degrade to near completion in vivo at approximately 3 months
(Fujimoto K L, Guan J, Oshima H, Sakai T, and Wagner W R. In vivo
evaluation of a porous, elastic, biodegradable patch for
reconstructive cardiac procedures. Ann Thorac Surg. 2007; 83(2):
648-54 and Fujimoto K L, Tobita K, Merryman W D, Guan J, Momoi N,
Stolz D B, Sacks M S, Keller B B, and Wagner W R. An elastic,
biodegradable cardiac patch induces contractile smooth muscle and
improves cardiac remodeling and function in subacute myocardial
infarction. J Am Coll Cardiol. 2007; 49(23): 2292-300). To control
the degradation rate of the wrap, a composite of PEUU, collagen,
and elastin proteins was utilized, with protein addition used to
hasten mass loss.
[0134] PEUU was synthesized from poly(.epsilon.-caprolactone)diol
and 1,4-diisocyanatobutane with putrescine chain extension. PEUU,
collagen, and elastin were combined in solution in
1,1,1,3,3,3-hexafluoro-2-propanol (HFIP), and then electrospun onto
a PIJV segment using a procedure explained in detail elsewhere
(Stankus J J, Guan J, and Wagner W R. Fabrication of biodegradable
elastomeric scaffolds with sub-micron morphologies. J Biomed Mater
Res A. 2004; 70(4): 603-14). Briefly, electrospinning conditions
included a mixture solution volumetric flowrate of 0.28 .mu.L/s, a
distance between nozzle and target of 17 cm, and electrical charges
of +12 kV to the nozzle and -3 kV to the target. The target used
for fabrication of spun AVGs for implantation was a Type 316
stainless steel mandrel of 3 mm diameter that was carefully
inserted into the AVG lumen to avoid endothelial injury. The
mandrel and coaxial vein were rotated together at 250 rpm, and
translated axially on a linear stage at a speed of approximately 8
cm/s over 10 cm to produce a more uniform coating thickness.
[0135] There were three parameters used to tune the mechanical
properties and degradation rate of the polymer: 1) the final
polymer concentration in a mixture solution; 2) the
PEUU:collagen:elastin ratio in the mixture solution; and 3) the
wrap thickness, which was proportional to electrospinning time. A
summary of all tested combinations of these parameters is shown in
Table 2.
TABLE-US-00002 TABLE 2 Summary of polymer tuning parameter
combinations. Electrospinning Final PEUU:collagen:elastin Time
Concentration Combination (%) (min) (%) A 14.3:42.3:42.3 20 6 B
25:75:0 15 6 C 50:50:0 15 6 D 50:50:0 20 12
Ex Vivo Perfusion Conditions
[0136] Vein segments were mounted in our well established,
validated ex vivo vascular perfusion/organ culture system (EVPS,
see, e.g., Labadie R F, Antaki J F, Williams J L, Katyal S, Ligush
J, Watkins S C, Pham S M, and Borovetz H S. Pulsatile perfusion
system for ex vivo investigation of biochemical pathways in intact
vascular tissue. Am J Physiol. 1996; 270(2 Pt 2): H760-8; Severyn D
A, Muluk S C, and Vorp D A. The influence of hemodynamics and wall
biomechanics on the thrombogenicity of vein segments perfused in
vitro. J Surg Res. 2004; 121(1): 31-7 and Muluk S C, Vorp D A,
Severyn D A, Gleixner S, Johnson P C, and Webster M W. Enhancement
of tissue factor expression by vein segments exposed to coronary
arterial hemodynamics Journal of Vascular Surgery: Official
Publication, the Society For Vascular Surgery [and] International
Society For Cardiovascular Surgery, North American Chapter. 1998;
27(3): 521-7). Briefly, the closed loop perfusion design allows the
circulation of sterile perfusate (tissue culture Media 199
supplemented with 1% fetal bovine serum and 1.0 g/liter cefoxitin)
through the vascular segment as well as circulation of an
adventitial bath (DMEM with 1% fetal bovine serum and 1.0 g/liter
cefoxitin) within a sealed chamber. Both the perfusate and bathing
media were maintained at 37.degree. C. and physiologic levels of
dissolved gasses. Experiments utilized one of two simulated
hemodynamic conditions (Severyn D A, Muluk S C, and Vorp D A. The
influence of hemodynamics and wall biomechanics on the
thrombogenicity of vein segments perfused in vitro. J Surg Res.
2004; 121(1): 31-7 and Muluk S C, Vorp D A, Severyn D A, Gleixner
S, Johnson P C, and Webster M W. Enhancement of tissue factor
expression by vein segments exposed to coronary arterial
hemodynamics. Journal of Vascular Surgery: Official Publication,
the Society For Vascular Surgery [and] International Society For
Cardiovascular Surgery, North American Chapter. 1998; 27(3):
521-7)--either native venous (VEN) or arterial (ART) conditions. To
simulate VEN hemodynamics the perfusion loop was set to provide
nonpulsatile flow of 20 ml/min and pressure of 20 mmHg To simulate
ART hemodynamics, the system was set to provide a pulsatile
pressure waveform of 120/80 mmHg with a mean perfusate flow of 100
ml/min Separate experiments were performed to examine unwrapped
veins under VEN or ART conditions, and wrapped veins under ART
conditions (wART). Each perfusion experiment lasted for 24 hours
with intraluminal pressure, outer diameter and flowrate being
measured every hour. Vein segments were then analyzed either
histologically or via immunohistochemistry as described below.
[0137] VEN vs. ART Experiments
[0138] FIG. 6 is a schematic depicting the first set of ex vivo
experiments that were performed. In these experiments we evaluated
the beneficial effects of a biodegradable electrospun polymer wrap
on PIJVs the abrupt exposure of PIJVs to ART conditions vs. PIJVs
exposed to VEN conditions for 24 hours.
[0139] ART vs. cART Experiments
[0140] FIG. 7 is a schematic depicting the second set of ex vivo
experiments that were performed. In these experiments we evaluated
the effects of a mechanical conditioning paradigm (cART conditions)
on PIJVs vs. PIJVs abruptly exposed to ART conditions for 24 and 72
hours.
[0141] ART vs. wART Experiments
[0142] FIG. 8 is a schematic depicting the third set of ex vivo
experiments that were performed. We evaluated the beneficial
effects a tuned biodegradable polymer wrap on PIJVs to ART
conditions (wART conditions) vs. unwrapped PIJVs exposed to ART
conditions for 24 hours.
[0143] CWS Calculation in a Compound Cylinder
[0144] Since it is believed that an abrupt exposure of AVGs to
arterial levels of CWS may contribute to their failure modalities,
we believe that one potential application of the electrospun
biodegradable polymer wrap would be to gradually expose AVGs to
arterial levels of CWS. Previous attempts to limit CWS using an
external sheath have not been fully successful because they were
either biodurable and/or loose fitting. To demonstrate how the wrap
may modulate CWS, and how the wrap may be tuned to achieve desired
results, we examined the CWS-over-time profile for each of the wrap
combinations given in Table 1 and compared these to unwrapped vein
segments exposed to venous or arterial conditions. This was
achieved using the data collected from ex vivo perfusion
experiments and a mathematical model for CWS.
[0145] For biomechanical modeling purposes, consider the schematic
in FIG. 9 showing an idealized cross section of the vein/wrap
complex. The outer layer of the bi-layer compound tube is taken as
the electrospun polymer wrap and the concentric inner layer is the
vein segment.
[0146] The following assumptions were then made (Vorp D A, Raghavan
M L, Borovetz H S, Greisler H P, and Webster M W. Modeling the
transmural stress distribution during healing of bioresorbable
vascular prostheses. Ann Biomed Eng. 1995; 23(2): 178-88): [0147]
i) There is no slipping or detachment between layers [0148] ii)
Compatibility of deformation across the interface is maintained
[0149] iii) There is only a small deformation under mean arterial
pressure [0150] iv) The system is under a state of plane stress
[0151] v) Both layers are incompressible, isotropic, homogeneous
and linearly elastic materials [0152] vi) Each separate layer may
be generalized as a single, thick-walled cylinder subjected to
internal and external pressure
[0153] The mathematical model developed by Vorp et al. (Id.) was
adapted for the model represented by FIG. 9. In short, we used the
classic Lame solution for radial and circumferential wall stresses
(.sigma..sub.r and .sigma..sub..theta., respectively), and radial
deformation (u.sub.r) at any radius, r, in an open-ended,
thick-walled cylinder under the action of internal and external
pressures (Id.). For the inner (vein) layer shown in FIG. 9, we
obtain (Id.):
.sigma. r , V = a 2 P i - b 2 P 2 b 2 - a 2 - ( P i - P 2 ) a 2 b 2
( b 2 - a 2 ) r 2 .sigma. .theta. , V = a 2 P i - b 2 P 2 b 2 - a 2
+ ( P i - P 2 ) a 2 b 2 ( b 2 - a 2 ) r 2 u r , V = 1 - V V E V ( a
2 P i - b 2 P 2 ) r b 2 - a 2 + 1 + V V E V ( P i - P 2 ) a 2 b 2 (
b 2 - a 2 ) r } for a .ltoreq. r .ltoreq. b ( 15 a ) ( 15 b ) ( 16
) ##EQU00002##
where the "V" subscript refers to quantities with respect to the
vein, and a and b are the inner and outer radii, respectively, of
the vein layer. P.sub.i is the internal pressure, and P.sub.2 is
the interfacial pressure acting between the two layers of the
concentric cylinder resulting from their difference in mechanical
properties. V is the Poisson's ratio and E is the Young's modulus
of elasticity. For the outer (wrap) layer shown in FIG. 9, we
have:
.sigma. r , W = b 2 P 2 - c 2 P o c 2 - b 2 - ( P 2 - P o ) b 2 c 2
( c 2 - b 2 ) r 2 .sigma. .theta. , W = b 2 P 2 - c 2 P o c 2 - b 2
+ ( P 2 - P o ) b 2 c 2 ( c 2 - b 2 ) r 2 u r , W = 1 - V W E W ( b
2 P 2 - c 2 P o ) r c 2 - b 2 + 1 + V W E W ( P 2 - P o ) b 2 c 2 (
c 2 - b 2 ) r } for b .ltoreq. r .ltoreq. c ( 17 a ) ( 15 b ) ( 1 8
) ##EQU00003##
[0154] where the "W" subscript refers the quantities to the region
occupied by the wrap, and b and c are the inner and outer radii,
respectively, of the wrap layer. P.sub.o is the external pressure.
With compatibility of deformations across the interface between the
layers, it must be that:
u.sub.r,V=u.sub.r,W at r=b (19)
[0155] Substituting (16) and (18) into (19), letting
v.sub.W=v.sub.V=v=0.5 (both materials assumed to be
incompressible), setting P.sub.o=0 (i.e., atmospheric pressure),
and solving for P.sub.2 we obtain:
P 2 = a 2 P i ( 1 - v ) E W b ( c 2 - b 2 ) + ( 1 + v ) E W ( c 2 -
b 2 ) ba 2 P i b 2 ( 1 - v ) E W b ( c 2 - b 2 ) + ( 1 - v ) a 3 E
V ( b 2 - a 2 ) + ( 1 + v ) E W ( c 2 - b 2 ) ba 2 + c 2 E v b ( b
2 - a 2 ) ( 20 ) ##EQU00004##
[0156] Recall that P.sub.i and outer diameter (i.e., c) were
measured in our ex vivo perfusion experiments. Therefore we had to
estimate the inner (r=a) and interfacial (r=b) radii for each set
of measured P.sub.i and c. Since we utilized the assumption that
both the vein and wrap are incompressible materials, which requires
the volume of each cylinder to be constant at any state of
deformation, it must be that:
[.pi.(r.sub.o.sup.2-r.sub.i.sup.2)L].sub.u=[.pi.(r.sub.o.sup.2-r.sub.i.s-
up.2)L].sub.p (21)
[0157] where r.sub.o and r.sub.i are the outer and inner radii,
respectively, and L is the length of each cylinder, and the
subscripts u and p refer to the unpressurized and pressurized
states, respectively. Applying equation (21) to the geometry of the
"wrap" cylinder in FIG. 9, yields:
b p = c p 2 L p - ( c u 2 - b u 2 ) L u L p ( 22 ) ##EQU00005##
[0158] Therefore for any measured c.sub.p and L.sub.p, a value of
b.sub.p can be calculated. Similarly, considering only the "vein"
cylinder in FIG. 9 and utilizing equation (22) for b.sub.p, we
find:
a p = ( c p 2 L p ( c u 2 - b u 2 ) L u L p ) L p - ( b u 2 - a u 2
) L u L p ( 23 ) ##EQU00006##
[0159] Substituting equations (20), (22) and (23) into equation
(15b), and evaluating at the mean arterial pressure and at
r = a p + b p 2 , ##EQU00007##
we can calculate the mid-wall CWS in the polymer wrapped vein. We
assumed that E.sub.W=7.5 MPa (Stankus J J, Guan J, and Wagner W R.
Fabrication of biodegradable elastomeric scaffolds with sub-micron
morphologies. J Biomed Mater Res A. 2004; 70(4): 603-14), and
E.sub.V=600 KPa (Wesly R L, Vaishnav R N, Fuchs J C, Patel D J, and
Greenfield J C. Static linear and nonlinear elastic properties of
normal and arterialized venous tissue in dog and man Circulation
Research (Online) 1975; 37(4): 509-20) in our calculations.
Vasomotor Challenge Experiments
[0160] To assess the effects of the electrospinning process on
tissue viability we examined PIJV segments with ("spun") and
without ("sham") the polymer wrap in place, as well as untreated
freshly excised ("control") tissue. For the sham PIJV segments
without the electrospun polymer wrap, we mimicked the
electrospinning process up to the point of actually placing the
polymer wrap (i.e., including the insertion of the mandrel and
rotating/translating the vein within the electrical field). Tissue
functionality was assessed using an ex vivo vasomotor challenge as
previously described (Labadie R F, Antaki J F, Williams J L, Katyal
S, Ligush J, Watkins S C, Pham S M, and Borovetz H S. Pulsatile
perfusion system for ex vivo investigation of biochemical pathways
in intact vascular tissue. Am J Physiol. 1996; 270(2 Pt 2): H760-8
and Ligush J, Labadie R F, Berceli S A, Ochoa J B, and Borovetz H
S. Evaluation of endothelium-derived nitric oxide mediated
vasodilation utilizing ex vivo perfusion of an intact vessel. J
Surg Res. 1992; 52(5): 416-21). In short, vessel segments were
cannulated, placed under a constant intraluminal pressure of 20
mmHg, and exposed to incremental doses of epinephrine (EPI).
Throughout the experiment, outer vessel diameter (D) was
continuously measured with a laser micrometer (Labadie R F, Antaki
J F, Williams J L, Katyal S, Ligush J, Watkins S C, Pham S M, and
Borovetz H S. Pulsatile perfusion system for ex vivo investigation
of biochemical pathways in intact vascular tissue. Am J Physiol.
1996; 270(2 Pt 2): H760-8; Brant A M, Rodgers G J, and Borovetz H
S. Measurement in vitro of pulsatile arterial diameter using a
helium-neon laser. J Appl Physiol. 1987; 62(2): 679-83; and Ligush
J, Labadie R F, Berceli S A, Ochoa J B, and Borovetz H S.
Evaluation of endothelium-derived nitric oxide mediated
vasodilation utilizing ex vivo perfusion of an intact vessel. J
Surg Res. 1992; 52(5): 416-21). The baseline diameter
(D.sub.baseline) was measured before injection of the first dose of
EPI. EPI was subsequently injected to final concentrations of
2.times.10.sup.5, 2.times.10.sup.4, and 2.times.10.sup.3 mg/ml at
1, 4.5, and 10 minutes, respectively. Following observation of
maximal vasoconstriction with each dose, each subsequent dose was
administered. After administration of the maximal dose of EPI, and
observation of maximal level of constriction (D.sub.constricted), a
2 ml bolus of 25 mg/ml sodium nitroprusside (SNP) was injected to
give a final concentration of 0.125 mg/ml. When full dilation was
observed, D.sub.dilated was recorded. The level of constriction in
response to EPI was calculated as:
% Constriction = D baseline - D constricted D constricted * 100 (
24 ) ##EQU00008##
[0161] Similarly, the level of dilation in response to SNP was
calculated as:
% Dilation = D dilated - D constricted D constricted * 100 ( 25 )
##EQU00009##
Compliance and .beta.-Stiffness Measurements
[0162] Hourly measurements of outer diameter (OD) and intraluminal
pulsatile pressure (P) were made during the ART vs. wART 24-hour
perfusion experiments (N=6) described above. These measurements
were used to calculate the compliance (C) and .beta.-stiffness
(.beta.) of both spun and sham control PIJVs. Using a sampling
frequency of 150 Hz, the hourly measurements were made for 5
seconds so that approximately 5 complete "cardiac cycles" of data
were collected. The acquired signals were then filtered and
plotted. Using the maximum (OD.sub.s and P.sub.s) and minimum
(OD.sub.d and P.sub.d) values for each cycle, equation 26 was used
to calculate C and equation 27 was used to calculate .beta.
(Hayashi K. Experimental approaches on measuring the mechanical
properties and constitutive laws of arterial walls. J Biomech Eng.
1993; 115(4B): 481-8). The 5 values were averaged and single values
of C and .beta. were calculated every hour.
C = ( OD s - OD d / OD d ) P s - P d ( 26 ) .beta. = ln ( P s - P d
) ( OD s - OD d / OD d ) ( 27 ) ##EQU00010##
Post-Perfusion Tissue Processing
[0163] We will evaluate endpoints from experiments of 1 day, and 3
days duration. We have established that maintenance of tissue
viability is achievable for this perfusion duration when utilizing
freshly excised vascular segments (Ligush J, Labadie R F, Berceli S
A, Ochoa J B, and Borovetz H S. Evaluation of endothelium-derived
nitric oxide mediated vasodilation utilizing ex vivo perfusion of
an intact vessel. J Surg Res. 1992; 52(5): 416-21). The
hyperplastic response of the veins will be quantified by measuring
the various carefully chosen endpoints summarized in Section 1.2.
These endpoints can be grouped into three categories based on the
required tissue processing: i) histology (including
micro/ultrastructure); ii) RNA analysis; and iii) protein analysis.
All vein segments were segmented and processed according to FIG.
10.
Biological Analyses
[0164] The biological endpoints proposed for above can be
characterized as either histological or molecular-based with
respect to the type of assay required. The histological endpoints
included evaluation of microstructure, apoptosis, proliferation, a
SMC phenotype marker, and a cell-adhesion marker. The protein and
gene expression endpoints required isolation of protein and RNA and
are classified as molecular.
[0165] The samples dedicated for histological analysis (FIG. 10)
were taken from the freezer and immediately embedded in Tissue
Freezing Medium.TM. (Triangle Biomedical Sciences, Durham, N.C.)
and frozen at -65.degree. C. Five-micron cross-sections were cut
using a cryotome and placed on positively charged, glass microscope
slides. Slides were stored at -80.degree. C. until they could be
processed for histological or immunohistochemical assays.
Histology
[0166] Following removal of the veins from the ex vivo perfusion
system, they were fixed in 4% paraformaldehyde for 4 hours at
4.degree. C. followed by 30% sucrose at 4.degree. C. overnight 5 mm
tissue rings were cut, washed with PBS, embedded in Tissue Freezing
Medium.TM. (Triangle Biomedical Sciences, Durham, N.C.), and cut
into 5 .mu.m sections. The tissue sections were either stained with
hematoxylin and eosin (H&E), Masson's trichrome (MTC),
picrosirius red, or Movat's pentachrome stains. Stained tissue
sections were then visualized using an Olympus Provis light
microscope (Olympus, Center Valley, Pa., USA) and compared
qualitatively.
Scanning Electron Microscopy
[0167] The electrospun wrapped PIJVs were examined under scanning
electron microscopy (SEM). In short, tissue segments designated for
SEM were fixed in ultrapure 2.5% gluteraldehyde, dehydrated through
a graded series of ethanol solutions (30-100%), critical point
dried (Emscope, CPD 750, Ashford, Kent, UK), then overcoated with
vaporized carbon (Cressington Freeze Fracture Device, Cressington,
Cranberry, Pa., USA). The tissue was visualized using a JEOL
JEM-6335F field emission gun SEM (JEOL, Peabody, M A, USA).
Necrosis
[0168] To assess the effects of the electrospinning process on
tissue viability we examined spun and sham PIJV segments, as well
as untreated freshly excised ("control") tissue. Tissue necrosis
was examined using Live/Dead.TM. staining (Molecular Probes,
Carlsbad, Calif., USA) of cryosections, according to manufacturer's
instructions. Each segment (control, sham control and spun)
intended for Live/Dead.TM. staining was cut in half and placed in
static culture within a Petri-dish under standard incubator
conditions. One-half of each segment was assessed after 18 hours of
culture, the other after 92 hours. 5 mm rings were cut from each
sample and embedded in cryomatrix (TBS, Durham, N.C.) then frozen.
Five 8 .mu.m sections were cut from each ring and imaged under
20.times. magnification using an epifluorescent microscope (Nikon,
Model E800, Melville, N.Y., USA). Two images were taken per section
so that a total of 10 fields of view were quantified per PIJV
segment. Scion Image (Version Beta 4.02, NIH, Bethesda, Md.) was
used to count the total number of cells in a field of view. To
determine the percentage of live cells in a field of view, dead
cells were counted manually, divided by the total number of cells,
and multiplied by 100%. The percentage of dead cells was subtracted
from 100% to calculate the percentage of live cells.
Apoptosis
[0169] Apoptosis was assessed using the In Situ Cell Death Kit,
fluorescein (TUNEL) (Roche Applied Science, Indianapolis, Ind.).
This assay uses the TUNEL technology which identifies the genomic
DNA cleavage component of apoptosis. Briefly, cross-sections were
dried at 37.degree. C. for 20 minutes, fixed in 4% paraformaldehyde
for 20 minutes, and rehydrated in phosphate buffered saline (PBS)
for 30 minutes. Samples were then incubated at room temperature for
10 minutes each in 10 .mu.g/ml Proteinase K followed by a freshly
prepared solution of 0.1% Triton X-100 and 0.1% sodium citrate for
permeabilization of membranes. DNA strand breaks were identified by
incubation at 37.degree. C. for one hour with Terminal
deoxynucleotidyl transferase and fluorescein labeled dUTP (both
provided in the kit from Roche). Nuclei were counterstained with
Hoechst 33258. A small set of samples was treated with 100 U/ml of
DNase I to serve as positive controls each time the assay was
performed to ensure efficacy. All sample preparation parameters
including incubation times, temperatures, and reagent
concentrations were optimized using DNase I treated positive
controls. Negative controls were incubated with labeled dUTP
without the transferase enzyme.
[0170] Quantification of the percent of TUNEL positive cells was
performed using a manual counting procedure. Positive cells from
each of 5 FOVs (field of views) from a given 5 .mu.m cross-section
were averaged to define the mean percent positive cells for that
cross-section. The mean percent TUNEL positive cells from one
segment (FIG. 10) was determined.
Proliferation
[0171] Proliferation was assessed by the expression of
proliferating cell nuclear antigen (PCNA) determined by
immunohistochemistry. Five-micron cross-sections were dried, fixed,
and permeabilized as described for the TUNEL assay. Nonspecific
binding of antibodies was blocked by incubating the samples for 15
minutes with 1% horse serum in PBS. Following this, the samples
were incubated with a primary mouse monoclonal antibody against
human PCNA (Dako Cytomation, Clone PC10, Denmark) overnight at
4.degree. C. in a moist chamber to prevent sample drying. Unbound
primary antibody was removed by subsequent washes in PBS. Next,
cross-sections were incubated with a universal (anti-mouse and
anti-rabbit) biotinylated secondary antibody which was part of the
Vectastain Elite.TM. horse-radish perxidase and avidin-biotin
detection system (Vector Labs, Cat. # PK-6200, Burlingame, Calif.)
for 60 minutes at 37.degree. C. in a moist chamber and then rinsed
3 times with PBS. Incubation with the Vectastain.TM. reagent was
then performed for 30 minutes at room temperature. To detect
positively stained cells, a diaminobenzidine (DAB) substrate
(Vector Labs, Cat. # SK-4100, Burlingame, Calif.) was used. The
enzymatic reaction caused PCNA positive cells to stain brown which
was visualized via microscope (100.times. magnification) until the
desired level of staining was achieved. The reaction was then
stopped by placing the slides into deioinized water. For nuclear
visualization, cells were counter-stained with Hematoxylin (Vector
Labs, Cat. # H-3401, Burlingame, Calif.) according to
manufacturer's instructions. Quantification of the percent PCNA
positive cells was performed using the same methodology as for
TUNEL.
SMC Phenotype
[0172] To detect a synthetic SMC phenotype, we used a mouse
monoclonal antibody raised against human Golgi complex (Abcam, Cat.
# ab14487, Cambridge, Mass.). The same procedure as described above
(PCNA) was used to quantify the mean percentage of Golgi complex
positive cells per segment of vein.
Statistics
[0173] For the vasomotor challenge data, and the
immunohistochemistry image quantification data a paired student's
t-test for means was performed, and P<0.05 was considered
statistically significant. Unless otherwise stated all data is
presented as mean.+-.standard error of the mean.
Results
CWS Profiles
[0174] The structural support offered to a vein by the wrap is
evident when we examine the outer diameter profiles in FIG. 11. It
was shown that a vein with a wrap does not expand to the same
degree under ART conditions as a vein without a wrap. The reduction
in diameter effectively reduced the CWS in the vein wall vs.
unwrapped controls under the same level of arterial pressure as
described below.
[0175] The CWS-over time profile for the polymer solution
combinations of Table 3.1 were quite variable (FIG. 12). In one
case (combination B), the wrap degraded too quickly and resulted in
a rapid increase in CWS under ART conditions. Other combinations (C
and D) did not degrade quickly enough and resulted in no
appreciable increase in CWS over a 24-hour period. Combination A
degraded at a rate which resulted in a nearly linear variation over
the 24-hour period between VEN and ART levels of CWS. This
combination was repeated (N=7) and the effect was found to be
repeatable.
Vasomotor Challenge Results
[0176] The results of a typical vasomotor challenge experiment are
shown in FIG. 13. The sham PIJV segment responded in a predictable
dose-dependent manner to stimulation with EPI, while the spun PIJV
exhibited a single contraction commencing with the lowest dose of
EPI. Vasodilation in response to SNP was similar for both the
control and spun PIJVs, each resulting in a larger outer diameter
than that at baseline, suggesting a certain level of basal tone in
both the sham and spun PIJVs. Overall, there was no significant
difference in the level of contraction (FIG. 14A) or dilation (FIG.
14B) between sham and spun PIJV segments.
Compliance and .beta.-Stiffness
[0177] In FIGS. 15A and 15C, we see that PIJVs are very stiff (and
hence much less compliant) when exposed to arterial levels of
pressure. Under the same hemodynamic conditions, the tuned polymer
wrap that was spun onto the adventitial surface of the PIJVs
offered structural support which is evident by the decreased
stiffness (FIG. 15B) and increased compliance (FIG. 15D). Please
note that due to technical issues, the pressure and diameter
measurements for one of the sham controls were not possible and
thus there was one less data set (N=5) than in the spun group
(N=6).
Biological Analyses
Histology
[0178] Histologic images were consistent with the SEM images in
that they also showed the polymer wrap to be well attached to the
adventitial surface of the vein and that it can be electrospun with
an approximately uniform thickness (FIGS. 16A and 19C). Further,
the polymer degraded nearly completely following the 24 hour
perfusion period (FIGS. 16B and 16D).
[0179] FIG. 17 shows representative birefringence images of vein
sections stained with picrosirius red. In each image, the color
range from red to green indicates a range of collagen fiber
organization with red being most organized and green being less
organized. The granulated appearance of the staining indicates the
natural crimped collagen fiber state, whereas stretched fibers
appear striated rather than granulated. These results suggest that
the polymer wrap reduces the level of collagen fiber stretching
(including greater organization and reduced crimping) when compared
to a control PIJV segment perfused ex vivo under ART conditions for
24 hours.
[0180] Representative images of Movat's pentachrome stained tissue
section are shown in FIG. 18. The internal elastic lamina appears
disrupted in the PIJVs perfused under ART conditions when compared
to both VEN and wART conditions. As with the picrosirius red
staining, this data suggests that the polymer wrap was successful
in reducing the level of stretch within the vein wall when exposed
to ART conditions.
SEM
[0181] The electrospun adventitial wrap exhibited high porosity and
tight adherence to the adventitial surface of the veins (FIGS.
19A-C), which suggests that the wrap would provide structural
support to an AVG without inhibiting adventitial nutrient and gas
diffusion into the tissue. Another important observation was that
the electrospinning process did not appear to damage the
endothelial layer, which remained continuous (FIG. 19D).
Necrosis
[0182] There was no significant difference in tissue viability
between each experimental group for each timepoint (FIG. 20).
Apoptosis
[0183] FIG. 21 shows representative paired fluorescent
immunohistochemistry images of TUNEL staining from all four ex vivo
vascular perfusion experiments described above. FIG. 22 shows the
quantified TUNEL analysis results from these experiments. It can be
seen that there is a statistically significant increase in
apoptotic cells within PIJVs abruptly exposed to ART conditions vs.
VEN controls. However, the mechanical conditioning paradigm imposed
via cART conditions (for both 24 and 72 hours) and via the
biodegradable electrospun polymer wrap (wART conditions)
statistically significantly reduced the number of apoptotic cells
within PIJVs vs. ART control conditions.
Proliferation
[0184] FIG. 23 shows representative paired HRP/ABC based
immunohistochemistry images of PCNA staining from all four ex vivo
vascular perfusion experiments described above. FIG. 24 shows the
quantified PCNA analysis results from these experiments. It can be
seen that there is a statistically significant decrease in
proliferating cells within PIJVs abruptly exposed to ART conditions
vs. VEN controls. However, the mechanical conditioning paradigm
imposed via cART conditions (24 hours) and via the biodegradable
electrospun polymer wrap (wART conditions) statistically
significantly inhibited the decrease in the number of proliferating
cells within PIJVs vs. ART control conditions. The number of
proliferating cells within PIJVs exposed to cART conditions for 72
hours was not statistically significantly different than ART
controls.
SMC Phenotype
[0185] FIG. 25 shows representative paired HRP/ABC based
immunohistochemistry images of Golgi complex staining from all four
ex vivo vascular perfusion experiments described in above. FIG. 26
shows the quantified Golgi complex analysis results from these
experiments. It can be seen that there is a statistically
significant increase in the number of cells staining positive for
Golgi complex within PIJVs abruptly exposed to ART conditions vs.
VEN controls. The mechanical conditioning paradigm imposed via cART
conditions (for both 24 and 72 hours) and via the biodegradable
electrospun polymer wrap (wART conditions) suggests only a trend
towards statistically significantly inhibiting the increase in the
number of cells positively stained for Golgi complex within PIJVs
vs. ART control conditions.
Discussion
[0186] The work presented in this chapter shows, that a
biodegradable electrospun polymer wrap can be uniformly (FIG. 16)
and safely (FIGS. 13 and 14) electrospun onto vein segments, and
that the wrap can be tuned to completely degrade (FIG. 16) such
that CWS is applied to an AVG at a desired rate (FIG. 12). Having
control over the biodegradation rate of an adventitially placed
electrospun polymer wrap could lend itself to three potentially
beneficial support modalities for attenuating IH in AVGs. As shown
here, biomechanical support can be delivered at a desired rate.
Consequently, delivery of both biochemical (drugs), and biological
(cellular) support might theoretically be achieved using the same
approach (Stankus J J, Guan J, Fujimoto K, and Wagner W R.
Microintegrating smooth muscle cells into a biodegradable,
elastomeric fiber matrix. Biomaterials. 2006; 27(5): 735-44 and
Stankus J J, Soletti L, Kazuro F, Hong Y, and Vorp D A. Fabrication
of cell microintegrated blood vessel constructs through
electrohydrodynamic atomization. 2007; Accepted). The potentially
beneficial effects of the polymer wrap on AVG microstructure were
observed from the picrosirius-red and Movat's pentachrome staining
(FIGS. 17 and 18, respectively). The polymer wrap seems to provide
structural support to AVGs resulting in a more naturally crimped
configuration of the collagen fibers (FIG. 17), as well as less
damage to the internal elastic lamina (FIG. 18). Maintaining
integrity of the structural proteins that comprise the AVG wall may
help to minimize the detrimental mechanical triggers received by
the vascular ECs and SMCs and hence could help to attenuate IH in
AVGs. We also assessed the level of necrosis via Live/Dead.TM.
staining in the electrospun PIJVs and showed no appreciable
increase in necrosis due to electrospinning over sham and static
controls (FIG. 19). This data in addition to the vasomotor
challenge data (FIGS. 13 and 14) is more evidence to show that
tissue viability is not affected by electrospinning.
[0187] The immunohistochemistry results suggest that gradual vs.
abrupt exposure of AVGs to arterial levels of CWS may be
beneficial. The balance between apoptosis and proliferation, as
seen in FIGS. 22 and 24 respectively, was shown to be disrupted due
to abrupt exposure of PIJVs to ART conditions over VEN controls.
The observed increase in apoptosis and reduction in proliferation
in PIJVs perfused under ART conditions suggests that there is an
immediate shift in cellular function due to the altered
biomechanical environment of the vein. This shift in cellular
function within veins was shown to be inhibited by more gradual
imposition of arterial levels of CWS via cART and wART ex vivo
perfusion conditions. In addition, as expected the level of Golgi
complex expression in PIJVs exposed to ART conditions was increased
over VEN controls (FIG. 26), suggesting a modulation in SMC
phenotype to a more synthetic state. This observed shift in
cellular function was not statistically significantly inhibited by
gradual exposure to ART levels of CWS via cART or wART conditions.
An observed trend towards inhibition, however, of this shift was
shown in FIG. 26. Additional experiments are required to determine
if this trend becomes statistically significant.
[0188] The observed alteration in SMC phenotype that resulted from
exposing PIJVs to ART conditions agrees with previously reported
data (Simosa H F, Wang G, Sui X, Peterson T, Narra V, Altieri D C,
and Conte M S. Survivin expression is up-regulated in vascular
injury and identifies a distinct cellular phenotype. J Vasc Surg.
2005; 41(4): 682-90; Zhang W D, Bai H Z, Sawa Y, Yamakawa T, Kadoba
K, Taniguchi K, Masuda J, Ogata J, Shirakura R, and Matsuda H.
Association of smooth muscle cell phenotypic modulation with
extracellular matrix alterations during neointima formation in
rabbit vein grafts. J Vasc Surg. 1999; 30(1): 169-83; and Wolff R
A, Malinowski R L, Heaton N S, Hullett D A, and Hoch J R.
Transforming growth factor-beta1 antisense treatment of rat vein
grafts reduces the accumulation of collagen and increases the
accumulation of h-caldesmon. J Vasc Surg. 2006; 43(5): 1028-36).
The concept of more gradual imposition of arterial levels of CWS to
AVGs has not previously been reported but could result in a means
to retard or inhibit SMC phenotypic modulation which could
consequently reduce the hyperplastic response. The reduction in
apoptosis in PIJVs exposed to ART vs. VEN conditions also agrees
with published results (Liu B, Itoh H, Louie O, Kubota K, and Kent
K C. The signaling protein rho is necessary for vascular smooth
muscle migration and survival but not for proliferation. Surgery.
2002; 132(2): 317-25; Pintucci G, Saunders P C, Gulkarov I, Sharony
R, Kadian-Dodov D L, Bohmann K, Baumann F G, Galloway A C, and
Mignatti P. Anti-proliferative and anti-inflammatory effects of
topical mapk inhibition in arterialized vein grafts. Faseb J. 2006;
20(2): 398-400; Alcocer F, Whitley D, Salazar J, Jordan W, and
Bland K I. Mutual exclusion of apoptosis and hsp70 in human vein
intimal hyperplasia in vitro. J Surg Res. 2001; 96(1): 75-80; Igase
M, Okura T, Kitami Y, and Hiwada K. Apoptosis and bcl-xs in the
intimal thickening of balloon-injured carotid arteries. 1999;
96(6): 605-12; Kamenz J, Seibold W, Wohlfrom M, Hanke S, Heise N,
Lenz C, and Hanke H. Incidence of intimal proliferation and
apoptosis following balloon angioplasty in an atherosclerotic
rabbit model. Cardiovasc Res. 2000; 45(3): 766-76; and Wang G J,
Sui X X, Simosa H F, Jain M K, Altieri D C, and Conte M S.
Regulation of vein graft hyperplasia by survivin, an inhibitor of
apoptosis protein. Arterioscler Thromb Vasc Biol. 2005; 25(10):
2081-7). However, the reduction in proliferation in ART perfused
PIJVs vs. VEN, cART, and wART groups was inconsistent with some
published data (Nishibe T, Miyazaki K, Kudo F, Flores J, Nagato M,
Kumada T, and Yasuda K. Induction of angiotensin converting enzyme
in neointima after intravascular stent placement. Int Angiol. 2002;
21(3): 250-5; Predel H G, Yang Z, von_Segesser L, Turina M, Buhler
F R, and Luscher T F Implications of pulsatile stretch on growth of
saphenous vein and mammary artery smooth muscle. Lancet. 1992;
340(8824): 878-9 and Dethlefsen S M, Shepro D, and D'Amore P A.
Comparison of the effects of mechanical stimulation on venous and
arterial smooth muscle cells in vitro. J Vasc Res. 1996; 33(5):
405-13). Liu et al. suggested however that mechanical stretch due
to arterial hemodynamics induces cell death, which possibly
mediates subsequent cell proliferation (Liu B, Itoh H, Louie O,
Kubota K, and Kent K C. The signaling protein rho is necessary for
vascular smooth muscle migration and survival but not for
proliferation. Surgery. 2002; 132(2): 317-25). The short-term
timepoints studied in this dissertation may not have been long
enough to see a rise in proliferation after the initial increase in
apoptosis in the ART perfused PIJVs.
[0189] Several limitations of this chapter should be noted.
Although the Live/Dead.TM. assay is widely used to evaluate
necrosis in living cells and tissues, it arguably was not ideally
suited for our application. This was due to the limited distance
the reagents were able to diffuse through the thickness of vascular
tissue. It was observed that the staining occurred predominantly in
the intimal and adventitial layers of the vein wall, while the
media was largely devoid of signal. It is true that the adverse
effect of the electrospinning process would be in the area of
contact between the polymer wrap and the vein wall (i.e., the
adventitia), as well as in the area of contact between the mandrel
and the vein wall (i.e., the lumen). The Live/Dead.TM. assay
appeared to work well in both of these areas and showed no
appreciable increase in the level of necrosis when compared to
control tissue. Additionally, the vasomotor challenge data
indicated that the spun PIJV was able to contract with the same
intensity as the sham control which demonstrated the viability of
the SMCs comprising the medial layer of the tissue. Finally, we
would have ideally compared the vasomotor responses of the sham and
spun PIJVs to a baseline control response--that is, with a freshly
excised PIJV segment. However, obtaining a third segment of PIJV
for immediate testing was not feasible since we could only harvest
two PIJV segments per animal. We feel that the choice of a sham
control over a baseline control was acceptable in that we wanted to
assess the differences associated only with electrospinning.
Conclusion
[0190] We showed here that a tunable polymer wrap can be applied to
vein segments without compromising viability or function, and
demonstrated one potential application; i.e., gradually imposing
the mid-wall CWS in wrapped veins exposed to arterial levels of
pressure. The gradual imposition of arterial levels of CWS, rather
than abrupt exposure, may be an important new means to reduce the
hyperplastic response of AVGs, promoting instead safe
arterialization.
[0191] Incorporation of either pharmaceuticals or cells into an
adventitial polymer wrap represents a possible future application,
and may further enhance the patency of AVGs. To our knowledge,
controlled delivery of cellular support via a biodegradable AVG
wrap/sheath has not been previously reported and hence this
possible future application of the adventitial wrap would be novel.
The polymer that was used in this report has been characterized,
and successfully micro-integrated with viable SMCs, and would lend
itself to this possible future application.
Example 4
In Vivo Arterial Vein Grafting
[0192] Eight (n=8) "proof of concept" carotid interposition vein
graft experiments were performed. We wanted to evaluate the
mitigating effect of the electrospun PEUU adventitial wrap on the
acute and chronic hyperplasic response of vein segments implanted
as carotid interposition grafts in a preclinical model. For this,
we used a unilateral autologous carotid interposition graft
protocol for pigs. Pigs were divided into two groups: a "spun" AVG
group and a "sham control" AVG group. Each animal served as its own
vein graft donor. In brief, PIJVs were harvested as described in
Example 2 and were either spun with the same wrap composition and
thickness as described in that Example using the electrospinning
process described therein, or designated as sham controls. Again,
for the sham PIJV segments without the electrospun polymer wrap, we
mimicked the electrospinning process up to the point of actually
placing the polymer wrap (i.e., including the insertion of the
mandrel and rotating/translating the vein within the electrical
field). The AVGs were then implanted as carotid interposition
grafts (as described in below) for 30 days (or upon observing
irreversible complications), an implant duration sufficient to
allow IH to be grossly apparent in the sham control group (Angelini
G D, Bryan A J, Williams H M, Morgan R, and Newby A C. Distention
promotes platelet and leukocyte adhesion and reduces short-term
patency in pig arteriovenous bypass grafts. J Thorac Cardiovasc
Surg. 1990; 99(3): 433-9; Vijayan V, Shukla N, Johnson J L, Gadsdon
P, Angelini G D, Smith F C, Baird R, and Jeremy J Y. Long-term
reduction of medial and intimal thickening in porcine saphenous
vein grafts with a polyglactin biodegradable external sheath. J
Vasc Surg. 2004; 40(5): 1011-9 and Jeremy J Y, Dashwood M R, Timm
M, Izzat M B, Mehta D, Bryan A J, and Angelini G D. Nitric oxide
synthase and adenylyl and guanylyl cyclase activity in porcine
interposition vein grafts. Ann Thorac Surg. 1997; 63(2): 470-6) to
which the spun group was compared. In addition to evaluating
patency via angiography, the explanted AVGs were processed for
histological evaluation of IH. Please note that the quantified
endpoints of the in vivo studies were strictly histological in
nature.
Methods
Unilateral Porcine Carotid Interposition Grafting
[0193] Animals were brought into the facility 7-10 days prior to
the day of the experiment, and kept NPO 12 hours prior to surgery.
Prior to surgery, animals were anesthetized with Acepromazine, 0.15
mg/kg IM, and Ketamine, 15.0 mg/kg, IM combination, intubated and
maintained at a surgical plane of anesthesia with Isoflurane (1-3%
in oxygen). Once each animal was clipped and prepped for the
procedures it was moved into the surgical suite and placed on
positive pressure ventilation and instrumented with monitoring
equipment (ECG). Pulse oximetry and blood pressure were monitored
throughout the surgical procedure. After the induction of
anesthesia, aseptic surgery was performed.
[0194] Unilateral cervical incision was made to expose the common
carotid artery. The animal was then heparinized (300 UI/Kg), and
the artery clamped proximally and distally using atraumatic
vascular clamps. The segment between clamps was excised (.about.6
cm). Each pig served as its own graft donor. A fresh unilateral IJV
harvest was performed on the pig as described above. The harvested
IJV was then either spun (as described above and in Stankus et al.
[47]) or designated as the sham control. The vein segment was then
implanted as a unilateral carotid interposition graft (end to end)
using interrupted 7-0 prolene sutures.
[0195] Post-operatively, animals were recovered and housed in an
intensive care unit. Following the surgical procedure and cessation
of inhalation anesthesia, the animal were extubated when it
exhibited a swallowing reflex and the protective cough reflexes are
functional. The animals were continually monitored for 24 hours,
and the following parameters were recorded every hour: pulse rate,
strength of pulse, capillary refill time, respiratory rate, urinary
output, and defecation. Body temperature was determined and
recorded every 2 hours. The animal was kept warm and dry to prevent
hypothermia. Buprenorphine hydrochloride (0.005-0.01 mg/kg, IM,
q12h) was administered at regular intervals for 4 days for pain and
continued to be administered for pain management if signs of pain
were exhibited. Acute pain in animals is expressed by guarding,
vocalization, mutilation, restlessness, recumbency for an unusual
length of time, depression (reluctance to move or difficulty in
rising), or abnormal appearance (head down, tucked abdomen,
hunched). Skin staples/sutures were removed 10 days post-op. All
animals were monitored daily by a trained staff of Veterinarians,
Registered Veterinary Technicians, and animal care personnel.
[0196] An anti-coagulation regimen was used to battle acute AVG
failure via thrombosis. Oral doses of aspirin (325 mg/day) and
Plavix (75 mg/day) were both started 3 days pre-operatively. The
Aspirin was administered daily for the entire 30 day post-operative
period, and Plavix was administered daily for only 14 days
post-operatively.
[0197] After a 30-day survival time (or upon observing irreversible
complications), the animals were euthanized. The pigs were deeply
anesthetized with Acepromazine, 0.15 mg/kg IM, and Ketamine, 30.0
mg/kg, IM combination, and the animals were then euthanized by
injection of an overdose of intravenous potassium chloride to
induce cardiac arrest. Vital signs were monitored to effect.
Fluoroscopic Angiography
[0198] After euthanasia and just prior to graft explant,
fluoroscopic angiography was performed to assess graft patency. The
carotid artery was clamped approximately 3 cm upstream of the
proximal graft anastomosis, and contrast medium was infused into
the carotid artery immediately distal to the clamp. Angiograms were
recorded (Model OEC 9800 Plus, General Electric Inc.) to verify
flow through the entire graft segment. If flow could not be
established through a graft (ie. due to occlusion), angiography was
not performed.
Post-Explant Tissue Processing
[0199] The grafts were extracted and 1/2 the tissue was immediately
fixed in 4% paraformaldehyde and analyzed histologically as
described in below. The other 1/2 of the tissue was fixed in
ultrapure 2.5% gluteraldehyde for SEM analysis as described in
Section above.
Histological Measurements of IH
[0200] Morphometric analysis was performed on sections from the
central region of the explanted grafts. Using standard Movat's
pentachrome staining techniques, intimal and medial thicknesses
were measured. The intimal to medial thickness ratios were
calculated from these measurements. Measurements were made from 4
fields of view and averaged to yield a sinlge value for each AVG
section.
Scanning Electron Microscopy
[0201] The same procedure as described above was used to process
and image the explanted AVGs from the in vivo experiments.
Statistics
[0202] An unpaired student's t-test was performed on the intimal to
medial thickness ratio data. P<0.05 was considered statistically
significant. Unless otherwise indicated, data are presented as
mean.+-.standard error of the mean.
Results
[0203] The adventitial polymer wrap had an immediately apparent
effect of maintaining the AVG at a diameter consistent with that
for the native vein (compare FIG. 27 middle and right) under
arterial pressure. In addition, the wrapped AVGs exhibited
pulsatile radial excursions (i.e., compliance) similar to the
native carotid artery, whereas the un-wrapped AVG appeared to be a
rigid tube with no detectable pulsations. That is, upon
establishing flow through the control grafts, it was observed that
unlike the native carotid arteries and spun veins, the sham control
veins did not change in diameter in response to the pulsatile
pressure.
[0204] Out of the 8 in vivo experiments that were performed, only 1
experiment was completely successful. That is, the AVGs from both
the spun and sham pigs were 100% patent after 30 days. Angiography
images of these AVGs can be seen in FIG. 28. The rest of the
experiments were deemed unsuccessfull due to one of 3 reasons: 1)
partial occlusion of one or both the spun and sham AVGs due to IH
or thrombosis; 2) post-operative complications leading to the death
of one animal in the spun group; and 3) infection resulting in the
need to euthanize one animal in the spun group after 1 week
post-op. However, with the 2 patent AVGs and the AVGs that were
only partially occluded (sham, N=6; spun, N=4) we performed
morphometric measurements to assess IH development for comparison
between the two groups. Representative images of Movat's
pentachrome stainging that were used in the morphpometric analysis
are shown in FIG. 29, which also shows a sample measurement. The
quantified results can be seen in FIG. 30. There seems to be only a
trend towards statistical significance between the intimal to
medial thickness ratios of the spun vs. sham control groups.
[0205] SEM images were taken of the AVGs from one completely
successful experiment (FIGS. 31A and 31B) as well as from another
experiment where the AVGs were not completely occluded (FIGS. 31C
and 31D). The anastomotic interface between the vein graft and
artery, evidenced by the suture line, can be seen in each
image.
Discussion
[0206] Although we observed only a trend towards a statistically
significant difference in the intimal to medial thickness ratio
between the spun and sham groups, it is likely that this difference
would become statistically significant if the number of experiments
was increased. The quantified morphometric results as well as the
qualitative SEM results suggest that the electrospun biodegradable
polymer wrap does offer a favorable effect to AVGs. However,
further investigation is necessary to determine if these effects
are in fact consistently beneficial. In addition to the inherent
variability associated with mechanopathobiological data, there was
also variability introduced into our results by having 3 different
surgeons, of varying experience, perform the surgeries. It is also
true that there is a "learning curve" associated with creating
anastomoses using a two layered AVG (spun group) instead of the
normal one layered AVG (sham group). As with any new surgical
procedure, as the comfort level of the surgeon performing the
surgery increases, the success rate of the surgery consequently
increases.
[0207] Previous studies that have attempted to use an external
sheath to reduce AVG IH, described above, focused on the delivery
of mechanical (as described in this dissertation) and biochemical
support to AVGs in various animal models. Clinical translation of
these previous approaches was not achieved due to two main
limitations. Specifically, they all used either
loose-fitting/biodegradable or loose-fitting/biodurable sheaths. In
this work, we desired to address these limitations by developing a
means to safely "wrap" an AVG with a tight-fitting and
biodegradable polymer.
[0208] There are limitations to the work presented here. The fact
that the sham controls were not paired to the spun AVGs (i.e., from
the same pig) provides us with less statistical power in the study.
However, the unpaired experimental design that was used was deemed
necessary in order to avoid post-operative complications in the
animals. We felt it was safer to perform unilateral surgeries
instead of bilateral so that the venous blood return from the brain
would not be excessively altered. Another limitation stems from the
varying experience of the surgeons who performed the procedures. It
is likely that the results would be more statistically significant
if the patency rate of the AVGs was increased. If the procedures
were all performed by the most experienced surgeon, the electrospun
biodegradable polymer wrap may have significantly reduced IH in the
AVGs over sham controls. A third limitation is that the 30-day
duration of the implants was too short. Longer term experiments,
perhaps as long as 6 months, are required to determine if the
efficacy of our approach in reducing AVG IH is sustained over
time.
* * * * *