U.S. patent application number 12/429803 was filed with the patent office on 2009-08-20 for implantable device for promoting repair of a body lumen.
Invention is credited to David R. Holmes, JR., Robert S. Schwartz, Robert A. Van Tassel.
Application Number | 20090210050 12/429803 |
Document ID | / |
Family ID | 34713326 |
Filed Date | 2009-08-20 |
United States Patent
Application |
20090210050 |
Kind Code |
A1 |
Van Tassel; Robert A. ; et
al. |
August 20, 2009 |
Implantable Device For Promoting Repair Of A Body Lumen
Abstract
An implantable stent having surface features adapted to promote
an organized growth pattern of infiltrating cells when implanted in
a tubular organ is provided. The surface features comprise
depressions, pores, projections, pleats, channels or grooves in the
stent body and are designed to increase turbulence or stagnation in
the flow of a liquid, such as blood through the stent, and/or to
promote the growth of infiltrating cells in an organized pattern.
Alternatively, the invention stent can be populated with living
cells prior to implant and can be heatable from an external source
of energy, thereby inducing production of therapeutic bioactive
agents from ingrowing cells. The invention also provides an
implantable heatable stent for transcutaneously monitoring the flow
of fluid through a lumen into which the stent is implanted by
measuring the rate at which the heated stent cools in response to
blood flow when the source of heat is removed.
Inventors: |
Van Tassel; Robert A.;
(Excelsior, MN) ; Holmes, JR.; David R.;
(Rochester, MN) ; Schwartz; Robert S.; (Rochester,
MN) |
Correspondence
Address: |
INSKEEP INTELLECTUAL PROPERTY GROUP, INC
2281 W. 190TH STREET, SUITE 200
TORRANCE
CA
90504
US
|
Family ID: |
34713326 |
Appl. No.: |
12/429803 |
Filed: |
April 24, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11056829 |
Feb 10, 2005 |
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12429803 |
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09382275 |
Aug 25, 1999 |
7235096 |
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11056829 |
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09139804 |
Aug 25, 1998 |
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09382275 |
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Current U.S.
Class: |
623/1.39 ;
623/1.41 |
Current CPC
Class: |
A61F 2/82 20130101; A61F
2/91 20130101; A61B 5/0031 20130101; A61F 2/0077 20130101; A61L
31/14 20130101; A61B 5/028 20130101; A61B 5/6876 20130101; A61B
5/6862 20130101 |
Class at
Publication: |
623/1.39 ;
623/1.41 |
International
Class: |
A61F 2/06 20060101
A61F002/06 |
Claims
1. An implantable stent comprising a tubular stent body having a
plurality of interconnected microholes distributed throughout said
stent body substantially uniformally along the entire length of
said stent body, said plurality of microholes being sufficiently
small so as to promote an organized growth pattern of infiltrating
cells throughout said stent body, and said stent body being
otherwise substantially free of holes larger than said
microholes.
2. The stent according to claim 1 wherein the organized growth
pattern is angiogenesis.
3. The stent according to claim 1 wherein the stent is
diametrically adjustable.
4. An active stent comprising a stent according to claim 1 and
further comprising live cells growing in said interconnected
microholes.
5. The active stent according to claim 4 wherein the live cells are
selected from the group consisting of endothelial cells, smooth
muscle cells, leukocytes, monocytes, epithelial cells,
polymorphonuclear leukocytes, lymphocytes, basophils, fibroblasts,
stem cells, epithelial cells and eosinophils.
6. The active stent according to claim 5 wherein the live cells are
smooth muscle cells, epithelial cells, or endothelial cells.
7. A method for treating a tubular body organ in a subject in need
thereof said method comprising: promoting the ingrowth of living
cells in a stent having a plurality of interconnected microholes
distributed within said stent body substantially uniformally along
the entire length of said stent body, said plurality of microholes
being sufficiently small in size so as to promote ingrowth of the
cells, and said stent body being otherwise substantially free of
holes larger than said microholes, and, implanting the stent into
the tubular organ of the subject prior to or following the
promoting of the ingrowth of the living cells so as to treat the
tubular organ.
8. The method according to claim 7 wherein the living cells are
donor or autologous cells.
9. The method according to claim 8 wherein the living cells are
autologous.
10. The method according to claim 7 wherein the treatment further
comprises promoting or inhibiting angiogenesis within the stent
body.
11. The method according to claim 7 wherein the body organ is a
blood vessel.
12. The method according to claim 7 wherein the treating comprises
holding the cells in a specific pattern or stimulating the growth
of the cells into an organized growth pattern.
13. The method according to claim 12 wherein the organized growth
pattern develops into an organized cellular structure within the
stent body.
14. The method according to claim 7 wherein the living cells are
endothelial cells, smooth muscle cells, leukocytes, monocytes,
polymorphonuclear leukocytes, lymphocytes, basophils, fibroblasts,
stem cells, epithelial cells or eosinophils.
15. The stent according to claim 1, wherein said stent body is
penetrated with said microholes.
16. The stent according to claim 1, wherein said stent body is
formed from a three dimensional non-woven matrix.
17. The stent according to claim 1, wherein said microholes extend
throughout said stent body so as to promote cell growth outward
into said stent tube and into attachment with cells at either end
of said stent.
18. The method according to claim 7, wherein said stent body is
penetrated with said microholes.
19. The method according to claim 7, wherein said stent body is
formed from a three dimensional non-woven matrix.
20. The method according to claim 7, wherein after the implanting
of said stent, said ingrowth of living cells is promoted such that
said cells grow outward into said stent tube and into attachment
with cells at either end of said stent.
Description
RELATED APPLICATIONS
[0001] This application is a Continuation-In-Part application of
U.S. patent application Ser. No. 09/139,084, filed Aug. 25, 1998,
which is incorporated herein by reference in its entirety.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] This invention relates generally to an implantable medical
device; and more particularly to an implantable stent.
[0004] 2. Discussion of the Prior Art
[0005] Damage to the endothelial and medial layers of a blood
vessel, such as often occurs in the course of balloon angioplasty
and stent procedures, has been found to stimulate neointimal
proliferation, leading to restenosis of atherosclerotic
vessels.
[0006] The normal endothelium, which lines blood vessels, is
uniquely and completely compatible with blood. Endothelial cells
initiate metabolic processes, like the secretion of prostacylin and
endothelium-derived relaxing factor (EDRF), which actively
discourage platelet deposition and thrombus formation in vessel
walls. However, damaged arterial surfaces within the vascular
system are highly susceptible to thrombus formation. Abnormal
platelet deposition, resulting in thrombosis, is more likely to
occur in vessels in which endothelial, medial and adventitial
damage has occurred. While systemic drugs have been used to prevent
coagulation and to inhibit platelet aggregation, a need exists for
a means by which a damaged vessel can be treated directly to
prevent thrombus formation and subsequent intimal smooth muscle
cell proliferation.
[0007] Current treatment regimes for stenosis or occluded vessels
include mechanical interventions. However, these techniques also
serve to exacerbate the injury, precipitating new smooth muscle
cell proliferation and neointimal growth. For example, stenotic
arteries are often treated with balloon angioplasty, which involves
the mechanical dilation of a vessel with an inflatable catheter.
The effectiveness of this procedure is limited in some patients
because the treatment itself damages the vessel, thereby inducing
proliferation of smooth muscle cells and reocclusion or restenosis
of the vessel. It has been estimated that approximately 30 to 40
percent of patients treated by balloon angioplasty and/or stents
may experience restenosis within one year of the procedure.
[0008] To overcome these problems, numerous approaches have been
taken to providing stents useful in the repair of damaged
vasculature. In one aspect, the stent itself reduces restenosis in
a mechanical way by providing a larger lumen. For example, some
stents gradually enlarge over time. To prevent damage to the lumen
wall during implantation of the stent, many stents are implanted in
a contracted form mounted on a partially expanded balloon of a
balloon catheter and then expanded in situ to contact the lumen
wall. U.S. Pat. No. 5,059,211 discloses an expandable stent for
supporting the interior wall of a coronary artery wherein the stent
body is made of a porous bioabsorbable material. To aid in avoiding
damage to vasculature during implant of such stents, U.S. Pat. No.
5,662,960 discloses a friction-reducing coating of commingled
hydrogel suitable for application to polymeric plastic, rubber or
metallic substrates that can be applied to the surface of a
stent.
[0009] A number of agents that affect cell proliferation have been
tested as pharmacological treatments for stenosis and restenosis in
an attempt to slow or inhibit proliferation of smooth muscle cells.
These compositions have included heparin, coumarin, aspirin, fish
oils, calcium antagonists, steroids, prostacyclin, ultraviolet
irradiation, and others. Such agents may be systemically applied or
may be delivered on a more local basis using a drug delivery
catheter or a drug eluting stent. In particular, biodegradable
polymer matrices containing a pharmaceutical may be implanted at a
treatment site. As the polymer degrades, a medicament is released
directly at the treatment site. The rate at which the drug is
delivered is dependent upon the rate at which the polymer matrix is
resorbed by the body. U.S. Pat. No. 5,342,348 to Kaplan and U.S.
Pat. No. 5,419,760 to Norciso are exemplary of this technology.
U.S. Pat. No. 5,766,710 discloses a stent formed of composite
biodegradable polymers of different melting temperatures.
[0010] Porous stents formed from porous polymers or sintered metal
particles or fibers have also been used for release of therapeutic
drugs within a damaged vessel, as disclosed in U.S. Pat. No.
5,843,172. However, tissue surrounding a porous stent tends to
infiltrate the pores. In certain applications, pores that promote
tissue ingrowth are considered to be counterproductive because the
growth of neointima can occlude the artery, or other body lumen,
into which the stent is being placed.
[0011] Delivery of drugs to the damaged arterial wall components
has also been explored by using latticed intravascular stents that
have been seeded with sheep endothelial cells engineered to secrete
a therapeutic protein, such as t-PA (D. A. Dichek et al.,
Circulation, 80, 1347-1353, 1989). However, endothelium is known to
be capable of promoting both coagulation and thrombolysis.
[0012] Another approach to controlling the healing of a damaged
artery or vein is to induce apoptosis in neointimal cells to reduce
the size of a stenotic lesion. U.S. Pat. No. 5,776,905 to Gibbons
et al., which is incorporated herein by reference in its entirety,
describes induction of apoptosis by administering anti-sense
oligonucleotides that counteract the anti-apoptotic gene, bcl-x,
which is expressed at high levels by neointimal cells. These
anti-sense oligonucleotides are intended to block expression of the
anti-apoptotic gene bcl-x so that the neointimal cells are induced
to undergo programmed cell death.
[0013] Under certain conditions, the body naturally produces
another drug that has an influence on cell apoptosis among its many
effects. As is explained in U.S. Pat. No. 5,759,836 to Amin et al.,
which is incorporated herein by reference in its entirety, nitric
oxide (NO) is produced by an inducible enzyme, nitric oxide
synthase, which belongs to a family of proteins beneficial to
arterial homeostasis.
[0014] However, the effect of nitric oxide in the regulation of
apoptosis is complex. A pro-apoptotic effect seems to be linked to
pathophysiological conditions wherein high amounts of NO are
produced by the inducible nitric oxide synthase. By contrast, an
anti-apoptotic effect results from the continuous, low level
release of endothelial NO, which inhibits apoptosis and is believed
to contribute to the anti-atherosclerotic function of NO. Dimmeler
in "Nitric Oxide and Apoptosis: Another Paradigm For The
Double-Edged Role of Nitric Oxide" (Nitric Oxide 1(4): 275-281,
1997) discusses the pro- and anti-apoptotic effects of nitric
oxide.
[0015] In many instances it is desirable to prevent neointimal
proliferation that leads to stenosis or restenosis. U.S. Pat. No.
5,766,584 to Edelman et al. describes a method for inhibiting
vascular smooth muscle cell proliferation following injury to the
endothelial cell lining by creating a matrix containing endothelial
cells and surgically wrapping the matrix about the tunica
adventitia. The matrix, and especially the endothelial cells
attached to the matrix, secrete products that diffuse into
surrounding tissue, but do not migrate to the endothelial cell
lining of the injured blood vessel.
[0016] In the treatment of heart disease it is also important to
determine the overall effectiveness of the heart as a pump and the
ability of the blood vessels to carry blood to other organs. If
blood flow to an organ is significantly restricted, the organ can
be damaged, and if the flow is stopped, death may occur.
Consequently, the measure of the flow of blood within a blood
vessel has been used as an indicator of the condition of the blood
vessel and the pumping action of the heart. By monitoring the blood
flow of a patient, the early detection of a heart condition, or of
restenosis, is possible, and preventative measures may be taken to
address any problems. If the blood vessel becomes seriously
clogged, angioplasty or a by-pass operation may be performed that
uses a graft to circumvent the damaged vessel.
[0017] In overseeing the condition of a patient's blood vessel, a
number of blood flow measurements may be needed, over time, to
effectively monitor the patient's condition. One known method of
monitoring the flow of blood in a vessel involves the percutaneous
application of an instrument to measure the flow. Such methods are
termed "invasive" because the body must be pierced to obtain the
blood flow measurement. Clearly, invasive techniques to measure
blood flow have a disadvantage in that the measurement must be
taken under controlled conditions. For example, it is difficult, if
not impossible, to monitor blood flow during periods of increased
exercise.
[0018] Despite the progress of the art in providing implantable
stents useful for treating a damaged body lumen, there is a need
for new and better stents, particularly for stents that are adapted
to promote growth of infiltrating cells into organized cellular
structures, such as take place during angiogenesis and/or
neovascularization, to aid in repair of a damaged body lumen. It is
also apparent that a device that non-invasively measures the flow
of blood in a blood vessel is desirable.
SUMMARY OF THE INVENTION
[0019] The present invention is based upon the discovery that
neointimal proliferation can be promoted and turned to healing
effect if the infiltrating cells can be forced to assume an
organized growth pattern or by subjecting the cells to increased
stress, such as temperature or fluid shear stress. Thus, contrary
to present belief, in many instances in which it is desirable to
encourage regrowth of a damaged blood vessel or other body lumen,
such as a tubular organ, the natural phenomenon of neointimal
proliferation occurring at a site of damage can be transformed from
a cause of failure to a cause of healing.
[0020] Therefore, according to the present invention, there are
provided stent(s) comprising a tubular stent body and having
surface features sized and/or arranged to promote an organized
growth pattern of infiltrating cells. For example, a film of cells
covering at least the interior surface of the stent body may
encourage ingrowth of infiltrating cells.
[0021] In many instances, the organized growth pattern develops
into an organized cellular structure within the stent body to aid
in repair of a damaged body lumen. For example, in one embodiment,
the surface features are selected to promote angiogenesis when the
stent is implanted intravascularly. The surface features for
promoting organized cell growth can comprise a plurality of
depressions in the surface of at least a portion of the stent body,
preferably arranged in a regular pattern on at least the interior
surface of the stent body, such as a waffle weave. In other
embodiments, the surface features comprise a plurality of pleats,
ridges, channels or pores in the stent body wherein at least some
of the pores run between the interior and exterior sides of the
stent body (i.e., penetrate the stent body) and are sized to
promote the organized cell growth.
[0022] In typical embodiments, the invention stent body is formed
from a biocompatible polymer or a biocompatible metal with the
surface features stamped or molded into the surface. For example,
the invention stent body can be formed of a porous biocompatible
material, such as a porous matrix of sintered metal fibers or a
polymer wherein the pores are sized to promote the organization of
ingrowing cells therein. Preferably, the invention stent is
diametrically expandable for implant mounted upon such a device as
a balloon catheter.
[0023] In other embodiments according to the present invention,
there are provided stents having a surface feature that creates or
enhances a condition of turbulence in a fluid flowing through the
tubular stent body such that ingrowing cells are subjected to
increased fluid shear stress by action of the turbulence, and/or
the surface features create stagnant flow through the stent body
sufficient to cause clotting of blood, thereby promoting
angiogenesis and/or neovascularization within the stent body when
the stent is implanted intravascularly.
[0024] For example, in one embodiment, at least a portion of the
stent body is covered by a biocompatible substance that expands or
thickens in an aqueous environment to assume a three-dimensional
form that promotes turbulence within the stent body. The
liquid-expandable substance can be applied to the stent body in a
pattern, for example, a pattern of dots, lines or curvilinear
markings. In one embodiment, the biocompatible substance is a
biocompatible hydrogel, or a mixture thereof. Biocompatible
hydrogels useful in manufacture of the invention stents are those
that provide an interpenetrating polymer network (IPN) structure,
which upon expansion in an aqueous environment, is characterized by
the presence of interconnecting pores. If the stent body is itself
formed of a fibrous mesh, there is communication between cells
external to the stent (i.e., in the vessel or lumen wall) via the
holes or pores in the stent body and those growing within the
interconnecting pores of the hydrogel layer. Presently preferred
hydrogels for use in fabrication of the invention stents are
biodegradable hydrogels consisting of hydrophobic biodegradable
polymers, (e.g., polylactide) and hydrophilic natural polymers
(e.g., dextran) with an interpenetrating polymer network
structure.
[0025] The stent body is designed to promote infiltration and
population of the stent by living cells, when the stent is cultured
in a cell-rich medium or when the stent is implanted into a blood
vessel or other tubular body lumen in a living subject, such as a
mammal. Further the surface features in the stent body are selected
to cause the living cells that infiltrate and populate the stent to
undergo cell growth in a specific pattern determined by the placing
and dimensions of the surface features of the stent body. One
example of such pre-determined cell growth pattern is angiogenesis
and/or neovascularization.
[0026] The invention stent optionally further comprises a
transcutaneously energized heating mechanism attached to the stent
body. The heating mechanism, which can be energized remotely (i.e.,
transcutaneously), is adapted to controllably heat cells within and
surrounding the stent in the lumen wall to a temperature sufficient
to cause infiltrating cells, or cells seeded thereon prior to
transplant, to increase production of one or more bioactive agents,
such as one or more anti-proliferative, anti-restenotic, apoptotic,
or angiogenesis-stimulating agents. In one embodiment according to
the present invention wherein the invention stent is implanted in a
blood vessel, the heating mechanism includes from one to about six
temperature sensors and is adapted to control the heating of the
cells to an elevated temperature in the range of from 38.degree. C.
to about 49.degree. C. However, in certain body lumens, such as the
urinary tract, tracheobronchial tree, and the like, a temperature
of 49.degree. C. would cause damage. Therefore, those of skill in
the art will be able to adjust the allowable maximum temperature to
the body lumen being treated.
[0027] Upon application of external energy to the implanted stent,
its temperature can be elevated to promote the production of
beneficial molecules, such as nitric oxide, to effect a cessation
of neointimal hyperplasia within the cells in the lumen wall and/or
cells growing within the stent. Alternatively, the stent can be
populated before implant with cells engineered to express a
bioactive agent that promotes a healing bodily process, such as
angiogenesis and/or neovascularization. Suitable bioactive agents
that can be obtained from such genetically engineered cells include
several growth factors, e.g., platelet derived growth factor-A
(PDGF-A), transforming growth factor (TGF), nuclear
factor-.kappa..beta. (NF-.kappa..beta.), an inducible
redox-controlled transcription factor. In these studies, low levels
of thermal therapy inhibited smooth muscle cell proliferation after
balloon injury through suppression of growth factors PDGF-A and
NF-.kappa..beta.. If such cells are placed under the control of a
heat sensitive promoter, such as a heat shock protein promoter, the
heating mechanism can be used to switch on or off the production of
the bioactive agent upon application or withdrawal of external
energy to the implanted stent. Thus, the invention stent can be
used in a number of different applications wherein it is desirable
to chronically release a therapeutic substance from an implant, on
demand, for example to cells within the wall of a damaged body
lumen or tubular organ.
[0028] Accordingly, in another embodiment according to the present
invention, there are provided methods for treating a tubular body
organ in a subject in need thereof. The invention treatment method
comprises promoting the ingrowth of living cells on a stent having
surface features sized to promote ingrowth and/or orderly
development of the cells, and implanting the stent into the tubular
organ of the subject prior to or following the promoting of the
ingrowth of the living cells so as to treat the tubular body organ.
The living cells can be donor or autologous cells. The living cells
can be provided by a donor or the cells can be autologous. The
invention treatment method is particularly useful for promoting or
inhibiting angiogenesis within the stent body.
[0029] In another embodiment, the invention stents are adapted for
measuring the flow of a fluid through the stent body. In this
embodiment, the invention stent comprises a tubular stent body and
a transcutaneously energized heating mechanism attached to the
stent body that includes at least two to about six temperature
sensors attached at spaced locations along the length thereof, and
a telemetering device for transcutaneously transmitting the output
of the temperature sensors to an external monitor that records the
output. Methods are provided for using the output from the
temperature sensors to obtain the flow of a fluid, such as blood,
through the stent body.
[0030] Thus, it is an object of the present invention to provide an
implantable stent that is adapted to promote angiogenesis within a
blood vessel or other tubular lumen into which the stent is
implanted.
[0031] It is a further object of the present invention to provide
an implantable stent that is adapted to enhance or stimulate
neointimal infiltration, but with organization of the infiltrating
cells so as to result in neovascularization.
[0032] It is a further object of the present invention to provide
an implantable stent that is adapted to promote ingrowth of living
cells, when cultured in a cell-rich in vitro environment or when
implanted within a tubular body lumen, such as a blood vessel.
[0033] It is a further object of the present invention to provide a
stent that creates stagnant flow and/or enhances shear turbulence
in blood flowing therethrough when implanted into a blood vessel or
other tubular body lumen (as compared with that applied by a
similarly composed stent, but lacking the surface features of the
invention stent).
[0034] It is a further object of the present invention to provide a
living stent populated with living cells growing throughout pores
and/or other surface features designed to promote growth of the
cells into an organized cellular structure when the cell is
implanted into a tubular body lumen or organ.
[0035] It is a further object of the present invention to provide
such a living stent wherein the living cells are genetically
engineered to produce a therapeutic bioactive agent, such as one
selected to inhibit or promote angiogenesis or proliferation of
intima within the implanted stent.
[0036] It is a further object of the present invention to provide a
stent wherein there is attached or affixed thereto a mechanism for
controlling heating of the stent in response to a transcutaneously
applied energy source.
DESCRIPTION OF THE DRAWINGS
[0037] The foregoing features, objects and advantages of the
invention will become apparent to those skilled in the art from the
following detailed description, especially when considered in
conjunction with the accompanying drawings in which like numerals
in the several views refer to corresponding parts.
[0038] FIG. 1 is a greatly enlarged cross-sectional view through an
artery showing a stent positioned therein, the stent including
elements and/or circuitry for measuring and transmitting
temperature information from the stent;
[0039] FIG. 2 is a greatly enlarged cross-sectional view of a
preferred material from which a stent like that shown in FIG. 1 may
be formed;
[0040] FIG. 3 is a schematic block diagram of the system used with
the stent of FIG. 1;
[0041] FIG. 4 is a bar chart graph showing the percentage increase
in cell production of heat shock protein and inducible nitric oxide
synthase resulting from the hyperthermia.
[0042] FIG. 5 is a graph showing the results of experiments
conducted using the invention heatable stent to measure the rate of
flow of blood through the stent. The temperatures shown (.degree.
C.) are the average for four data points for three equidistant
temperature sensors on the stent, with "distal stent" representing
the sensor distal to the heating element and "proximal stent"
representing the sensor proximal to the heating element. Flow rate
was measured with flow from the distal to the proximal sensor (Flow
Distal) and from the proximal to the distal sensor (Flow Proximal).
-x-=distal stent-flow distal; .cndot.=mid stent-flow distal;
.box-solid.=mid stent-flow distal; .box-solid.=mid stent-flow
proximal; =distal stent-flow proximal; .tangle-solidup.=proximal
stent-flow distal.
DETAILED DESCRIPTION OF THE INVENTION
[0043] According to the present invention, there are provided
stents comprising a tubular stent body having surface features
adapted to promote an organized growth pattern of infiltrating
cells, such as takes place during angiogenesis and/or
neovascularization. For example, in one embodiment the surface
features comprise a plurality of depressions in the surface of at
least a portion of the stent body, for example the interior surface
of the stent body. It is presently preferred that the surface
depressions have an average volume per depression in the range from
about 10 .mu.m to about 100 .mu.m. The surface depressions are
generally arranged in an orderly pattern, such as a waffle weave
pattern than can be readily stamped into the material from which
the stent body is fabricated.
[0044] In one embodiment according to the present invention, the
invention stent has surface features comprising pores in the stent
body having an average diameter in the range from about 30 microns
to about 65 microns. Generally the invention stent has a slightly
greater inner diameter than that of the lumen into which it is
placed such that a layer of ingrowing cells will cause the
effective inner diameter to match the inner diameter of the vessel
or lumen into which it is placed. Cells growing in the stent (e.g.
in pores contained in the stent) will extend outward into the lumen
of the stent and grow into attachment with cells in the lumen at
either end of the stent, forming a continuous live cellular contact
for fluid flow within the lumen of the stent. Since there would
then be no contact with a foreign object in the vessel, thrombosis
and immune response, which would tend to close the lumen of the
stent with fibers and collagen, is reduced. Generally, the overall
porosity of the invention stent is in the range from about 50% to
about 85%, for example, at least about 70%.
[0045] It has been discovered that when the stent body is
penetrated with pores having such an average diameter, the pores
will be readily populated with living cells if the stent is
cultured in a cell-rich medium (e.g., 6-10.times.10.sup.4
endothelial cells in 0.8 ml culture medium) under cell-culturing
conditions, as is known in the art. Such a cell culturing procedure
is described, for example, in D. A. Dichek et al., supra, which is
incorporated herein by reference in its entirety. Alternatively, if
an invention stent having such pores is implanted into a body
lumen; for example intravascularly, the implanted stent will
readily be infiltrated by cells from the surrounding cellular
environment so as to create an organized cellular structure similar
to that of the surrounding bodily environment. For example, the
type of organized structure formed within the stent may be dictated
by the biological environment surrounding the implanted stent (e.g.
whether a blood vessel or a urethra). Alternatively, the type of
organized structure formed will correlate with the type of cells
seeded into the stent or that infiltrate the stent from the implant
site.
[0046] Surprisingly, it has been discovered that pores in the size
range from about 30 microns to about 65 microns are particularly
effective for promoting the growth and organization of infiltrating
cells, such as cells of the vascular intima, into organized
cellular structures, such as takes place during angiogenesis and
neovascularization.
[0047] In another embodiment according to the present invention,
the invention stent has surface features selected to organize the
infiltrating cells into a longitudinal growth pattern. To promote
this type of organized cell growth, the surface features of the
invention stent can comprise a plurality of longitudinal pleats,
grooves, channels, and the like, in the stent body (i.e., running
along the axis of the tubular stent body). The pleats, grooves, or
channels are preferably spaced and sized to create turbulence in
flow of blood through the stent and/or to cause longitudinal
alignment of cells that infiltrate the pleats, grooves, and/or
channels. To encourage ingrowth of cells and cellular alignment,
the pleats, grooves, or channels generally have an average height
or depth in the range from about 10 .mu.m to about 100 .mu.m and an
average distance from center to center in the range from about 10
.mu.m to about 100 .mu.m.
[0048] Alternatively, the surface features can be selected to
create turbulence in a fluid, such as blood, flowing through the
tubular stent body. For example, an undulating or uneven inner
stent surface will enhance turbulence within the stent. The
turbulence created by the surface features is intended to apply
increased fluid shear stress on infiltrating cells (as compared
with that applied by a similarly composed stent, but lacking the
surface features of the invention stent) when the stent is
implanted in the vasculature of a living body. Although the effect
of fluid shear upon cell growth within a stent is not completely
understood, it is believed that higher shear forces upon neointimal
and endothelia slow or stop the cell growth. The elevated shear may
force cells to mature earlier, the increased shear force being a
mechanical and fluid dynamic stimulus to maturation. Preferably the
fluid shear stress is created in the longitudinal direction
relative to the stent.
[0049] In another embodiment according to the present invention,
the invention stent has a tapered inner diameter for restricting
fluid flow in a nozzle like manner, thereby tending to control cell
growth by exerting increased fluid shear on the ingrowing
cells.
[0050] It has also been discovered that angiogenesis and
neovascularization are enhanced when blood flow through an
implanted stent is slowed down sufficiently to promote clot
formation, as clot formation is an initial step in the process
leading to formation of new vasculature. Therefore, the surface
features on the interior surface of the invention stent body can
efficaciously be selected to promote stagnation of blood flow
through the stent. There is evidence that smooth muscle cells
migrate from sites distant to colonize a resorbing thrombus, using
it as a bioabsorbable proliferation matrix in which to migrate and
replicate. Typically, the thrombus is colonized at progressively
deeper levels until the neointimal healing is complete R. S.
Schwartz et al., "Biomimicry, vascular restenosis and coronary
stents," Semin Interv Cardiol 3(3-4):151-6, 1998. Of course,
formation of a thrombus can lead to downstream embolism. Therefore,
care must be taken that the stagnation of flow is controlled in
such a way as to avoid production of an embolism, for example, by
adjustment of (i.e., by increasing) the fluid shear stress on the
blood cells within the stent.
[0051] To aid in the creation of turbulence within the stent body
that exerts fluid shear stress in a longitudinal direction on
infiltrating cells, the surface features on the stent body can
comprise an array of upstanding projections that promote or enhance
shear turbulence in blood flow along at least a portion of the
surface of the stent body (as compared with that applied by a
similarly composed stent, but lacking the surface features of the
invention stent). Preferably the array covers at least the interior
surface of the stent body. The projections generally have an
average height of from about 10 .mu.m to about 100 .mu.m. In one
embodiment, the projections comprise an orderly array of hooks,
such as is used in Velcro.RTM. fasteners, or stalks having a
diameter to height ratio of from about 10:1 to about 100:1.
Generally such stalks have a flow impeding feature, such as a
bulbous tip. The orderly array can have a uniform spacing of from
about 10 .mu.m to about 200 .mu.m from center to center. Methods
for fabricating a flexible backing having an array or such
projections are disclosed in U.S. Pat. No. 5,879,604, which is
incorporated herein by reference in its entirety.
[0052] In another embodiment according to the present invention,
the surface features on the invention stent comprise a layer of a
biocompatible substance that expands or thickens in an aqueous
environment to assume a three-dimensional form, wherein the layer
covers at least a portion of the surface of the stent body. For
example, the biocompatible substance can be or comprise one or more
hydrogels, such that the hydrogel layer expands as it absorbs water
upon contact with an aqueous environment to create a porous three
dimensional layer. Alternatively, the three dimensional form can
comprise an array of upstanding projections, such as described
above. In this case, it is preferred that the surfaces of the stent
be relatively smooth (e.g., with the projections lying recumbent
against the surface of the stent body or in an undeveloped state)
until such time as the stent is implanted and/or comes into contact
with an aqueous environment. For example, the projections can be
formed from dots of a substance that expands upon contact with
water, such as dots of hydrogel or calcium hydroxyapatite crystals
upon at least the interior surface of the stent body that expand
upon contact with an aqueous environment, thereby forming
projections into the interior void of the stent body. Such
projections aid in slowing the flow of fluid through the stent
body. In another embodiment according to the present invention, the
surface features on the invention stent comprise a pattern of
hydrogel markings on at least a portion of the surface of the stent
body, such as a pattern of dots, lines, curvilinear tracings, or a
mixture thereof. Preferably the markings are distributed over at
least the interior surface of the stent body, but the pattern of
markings can also cover the exterior surface of the stent body.
[0053] The stent body can be formed of any suitable substance, such
as is known in the art, that can be adapted (e.g., molded, stamped,
woven, etc.) to contain the surface features described herein. For
example, the stent body can be formed from a biocompatible metal,
such as stainless steel, tantalum, nitinol, elgiloy, and the like,
and suitable combinations thereof.
[0054] Preferred metal stents are formed of a material comprising
metallic fibers uniformly laid to form a three-dimensional
non-woven matrix and sintered to form a labyrinth structure
exhibiting high porosity, typically in a range from about 50
percent to about 85 percent, preferably at least about 70 percent.
The metal fibers typically have a diameter in the range from about
1 micron to 25 microns. The average effective pore size is in the
stent body such that cellular ingrowth into the pores and
interstices is enhanced, for example having an average diameter in
the range from about 30 microns to about 65 microns. A material
having these desired properties that can be used in manufacture of
the invention stent is available from the Bekaeart Corporation of
Marietta, Ga., and sold under the trademark, BEKIPOR.RTM. filter
medium.
[0055] Alternatively, the stent body can be formed of a
biocompatible non porous polymer or a polymer made porous by
incorporating dissolvable salt particles prior to curing thereof
and then dissolving away the salt particles to leave voids and
interstices therein. The polymer may be biostable or bioabsorbable,
such as a number of medical grade plastics, including but not
limited to, high-density polyethylene, polypropylene, polyurethane,
polysulfone, nylon and polytetra-fluoroethylene. A porous polymer
stent body can be made having pores with an average diameter in the
range from about 30 microns to about 65 microns, by procedures
known in the art. For example, polymer granules can be ground down
to obtain small particles of about 100 microns in diameter, mixed
with salt, and compressed into a compact form, for example using a
jack, a plate and a die. The compressed forms are then placed in a
pressure vessel and subjected to a gas, such as carbon dioxide, at
high pressure of about 800 pounds per square inch until the gas
dissolves into the polymer, the pressure is released rapidly, and
the polymer particles expand and fuse together, to yield a porous
polymer. Finally, the salt is leached out of the polymer to obtain
a polymer having up to about 85 percent porosity.
[0056] Autologous cells naturally invade the invention stent,
particularly the surface features thereof, following placement in a
body lumen of a host subject and spontaneously generate an
organized cellular structure that varies depending upon the
cellular makeup of the bodily lumen into which the stent is
implanted. Alternatively, endothelial or other suitable cells may
be made to invade the stent in a cell culture lab to create a
living stent prior to implant, using methods known in the art. For
example, a living stent can be obtained according to the invention
wherein the stent is populated with live cells selected from
endothelial cells, smooth muscle cells, leukocytes, monocytes,
epithelial cells, polymorphonuclear leukocytes, lymphocytes,
basophils, fibroblasts, stem cells, epithelial cells, eosinophils,
and the like, and combinations of any two or more thereof. In the
invention living stent, such cells actually live within the surface
features of the stent, such as the pores, grooves, channels, etc.,
and are not merely a surface coating, as may be the case when a
metal wire braided stent is used, or other stent lacking suitable
surface features as disclosed herein.
[0057] To enhance in vitro invasion of selected live cells, the
stent may first be coated with a suitable component, such as a
protein like fibronectin, elastin, mucopolysaccharide, or other
suitable extracellular matrix protein. The thus-treated stent is
placed in a cell culture dish and the selected living cells are
allowed to form a coating on non-porous stents and to invade the
interior of a porous stent material. Once the stent is populated
with living cells, it is ready for implant.
[0058] Without limitation, in overall size a typical intravascular
stent may have an outer diameter in a range of from about 2.0 mm to
about 6.0 mm and a wall thickness in a range from about 0.1 mm to
about 12 mm, for example about 0.1 mm to about 1.0 mm. The
particular size, of course, depends on the anatomy where the stent
is to be implanted.
[0059] In another embodiment, the invention stent is diametrically
adjustable, being designed to be remotely introduced into a body
cavity by the use of a catheter type of delivery system. Any of a
variety of techniques or designs, as is known in the art, can be
used for making the invention stent diametrically expandable. For
example, such designs are disclosed for example in U.S. Pat. No.
5,059,211, which discloses an expandable stent made of a porous
polymeric material. Alternatively, the stent body can be made of an
expanded metal or plastic device having a fenestrated side wall to
facilitate expansion thereof, as shown in FIG. 1. In yet another
embodiment, the stent may instead have a tubular configuration that
is pleated longitudinally prior to implant so as to exhibit a
reduced outside diameter to facilitate routing and placement
thereof, but which may later be expanded to a diameter equal to or
only slighter greater than the diameter of the blood vessel, body
lumen, or tubular organ at the treatment site. The stent may also
have a rolled or braided construction known in the art which can be
expanded from a lesser diameter to a larger diameter.
[0060] The diametrically expandable stent is designed to be
implanted in a contracted form, for example, mounted on a partially
expanded balloon of a balloon catheter and then expanded in situ to
contact the lumen wall. Although any appropriate ratio between the
collapsed and expanded diameters of the invention stent can be
employed, depending upon the body lumen into which the stent is to
be placed, generally in the diametrically adjustable stent, the
expanded diameter is at least about 1.5 times the size of the
collapsed diameter. Optionally, the invention stent can be coated
with a friction-reducing coating, for example of commingled
hydrogel, to reduce friction during implant, as disclosed in U.S.
Pat. No. 5,662,960.
[0061] Referring to FIG. 1, there is illustrated a greatly enlarged
cross-sectional view, through an arterial blood vessel 10. Formed
within the blood vessel is a stenotic lesion 12 that has been
subjected to balloon angioplasty for establishing greater patency
to the artery. In carrying out the balloon angioplasty procedure,
the blood vessel has been damaged, and a stent 14 constructed of a
material capable of supporting cellular growth thereon, has been
implanted into the lumen of the blood vessel and expanded to abut
the inner layer of the injured blood vessel.
[0062] Stent 14 is preferably a balloon expandable device made of
expandable metal or braided wire, but also may be designed as a
self-expanding structure. It may also be fabricated from a
composition of metallic fibers, uniformly laid to form a
three-dimensional, non-woven structure, such as is shown in FIG.
2.
[0063] In accordance with the present invention, the invention
stent may be used as part of a stent system which comprises, in
addition to the invention stent, an energy source for
transcutaneously transmitting heating energy to the stent to raise
the temperature of the implanted stent to a temperature above body
temperature. The energy source is external to the subject and
delivers electromagnetic energy to the stent in the form of radio
frequency energy, microwave energy, a magnetic field, and the like.
The percutaneously delivered electromagnetic energy is transformed
to heat energy in the stent body itself, for example through
induction of Eddy currents or dielectric heating. Optionally, but
preferably, delivery of energy to the stent, and consequently
heating of the stent, is controlled by from one to about six heat
sensors attached to the stent body that communicate percutaneously
with the energy source to regulate the heating of the stent to a
safe level. Preferably the energy source can transmit sufficient
energy to the implanted stent to stimulate the live cells therein
to increase production of one or more bioactive agents, such as are
effective to modify vascular structure in the hematologic system.
For example, if the ingrowing cells produce heparin, a coating of
heparin will be formed on the stent surface that modifies platelet
function.
[0064] For example, where a metal stent is employed, the energy
source for transcutaneously transmitting heating energy to the
invention stent can comprise a source of high frequency AC current,
shown here as generator 15, for externally applying an alternating
electromagnetic field that is transcutaneously transmitted from
generator 15 to the implanted stent 14 so as to induce Eddy
currents therein, thereby causing the temperature of the stent to
rise above normal body temperature. To avoid the need for
telemetry, if the stent is made of a suitable metal alloy
exhibiting a Curie point at a desired maximum temperature of about
49.degree. C. or less, no control need be maintained over the
externally applied magnetic field because the heating of the stent
will not increase above the point corresponding to the Curie point.
Similarly, in the case of a polymer stent, the source of
transcutaneously applied heating energy can comprise a source of
microwave energy, or another form of high frequency dielectric
heating known in the art, for transcutaneoulsy generating heat in
the polymer stent.
[0065] In another embodiment according to the present invention,
the invention stent used in the stent system as disclosed herein
further comprises a thermostat/heat regulator for monitoring the
temperature of the implanted stent and regulating the temperature
therein to avoid over-heating of the stent and cells living therein
to a temperature where cell necrosis occurs, as described above.
For example, FIG. 1 shows the thermostat/heat regulator as an
electronic sensor and telemetering device comprising antenna coil
16, which is wrapped about the surface of the stent 14, the antenna
coil being connected to a hybrid integrated circuit chip 18, which
is also mounted on the surface of the stent. When the source of
high frequency energy used in the invention stent system to
transcutaneously transmit energy to the invention stent is a radio
frequency generator, a portion of the RF energy used in heating the
stent 14 is picked up by the antenna coil 16 and converted to a DC
voltage for powering the electronics comprising the hybrid circuit.
Alternatively, a metal stent body may itself act as an antenna and
transfer energy to a temperature sensor sufficient to activate the
sensor and transmit temperature readings to a transcutaneous
monitor, and the like.
[0066] FIG. 3 is a schematic diagram of a representative hybrid
circuit and it shows an AC/DC converter 19 for producing a DC
voltage for powering the microprocessor 20. A temperature sensor,
such as a thermistor bead 22, is applied to the microprocessor and
more particularly to an on-chip A/D converter 24 to produce a
binary signal train proportional to the difference between stent
temperature and body temperature.
[0067] A program for controlling the conversion of the analog
output from the temperature sensor 22 to a digital representation
is stored in a ROM memory 26 in the hybrid circuit 18 and the data
may be transmitted to an external monitor/controller 28 by means of
a telemetry link 30 of conventional design known in the art. The
monitor/controller will then operate to increase or decrease the
energy being transcutaneously delivered to the stent by the high
frequency AC generator such that the stent temperature can be
maintained at a predetermined set-point value previously programmed
into the RAM memory 32 of the hybrid circuit 18.
[0068] The temperature sensor can be a passive heat sensor, such as
a temperature sensitive crystal, affixed to the stent and when an
interrogation frequency is applied, via an external power source,
such as a generator, the crystal will resonate at a frequency that
varies with temperature.
[0069] Our experiments have shown that elevated temperatures in the
range of from about 38.degree. C. to about 49.degree. C. will
induce production of positive enzymes and bioactive agents as gene
products in certain cells located in and near the stent. For
example, heat shock proteins and NOS can be generated in smooth
muscle cells. At these temperatures, the forming neointimal cells
in the surface features of the invention stent exhibit an
upregulation of useful proliferation-inhibitory products as
neointima forms in the surface features (i.e. pores) of the stent
and in the vessel wall contacted by the stent. However,
temperatures in excess of about 49.degree. C. may result in cell
necrosis and terminate production of beneficial gene products. In
addition, nitric oxide synthase, the enzyme known to trigger
production of nitric oxide in endothelial cells of the vasculature,
among others, has been found to be a by-product of hyperthermia and
NO has been shown to be shown to produce apoptosis inhibiting
proliferation of smooth muscle cells.
[0070] Experiments we have conducted have demonstrated that cyclic,
low level heat treatment reduced proliferation of cells following
vascular injury in an organ culture model of porcine coronary
arteries. While the exact mechanism whereby intimal hyperplasia is
reduced is not clear, it is known to be related to smooth muscle
cell proliferation, which, in turn, is controlled by several growth
factors, e.g., platelet derived growth factor-A (PDGF-A),
transforming growth factor (TGF), nuclear factor-.kappa..beta.
(NF-.kappa..beta.), an inducible redox-controlled transcription
factor. In these studies, low levels of thermal therapy inhibited
smooth muscle cell proliferation after balloon injury through
suppression of growth factors PDGF-A and NF-.kappa..beta..
[0071] The graph of FIG. 4 illustrates the up-regulation in a heat
shock protein, HSP70, and inducible nitric oxide synthase resulting
from an increase in the cell temperature from 37.degree. C. to
43.degree. C. Also shown is the corresponding increase in apoptosis
in smooth muscle cells.
[0072] Accordingly, in another embodiment of the present invention,
there are provided methods for treatment of a tubular body organ in
a subject in need thereof. The invention treatment method comprises
promoting the ingrowth of living cells in a stent having surface
features sized and/or arranged to promote ingrowth of the cells,
and implanting the stent into the tubular organ of the subject
prior to or following the promoting of the ingrowth of the living
cells so as to treat the tubular body organ. The invention stent
used in the treatment method holds the cells in a specific pattern
or stimulates the growth of the cells into an organized growth
pattern. Preferably, the organized growth pattern develops into an
organized cellular structure within the stent body, such as takes
place during angiogenesis and/or neovascularization. The living
cells can be either donor or autologous cells.
[0073] The stent of the present invention can be implanted using
any surgical technique known in the art as is dictated by the
particular tubular body organ to be treated. However, it is
presently preferred to implant the invention living stent by
placing the device in an unexpanded form over a deflated balloon on
the distal end of an intravascular catheter. The catheter is routed
through the vascular system until the stent is positioned adjacent
to target tissue where the balloon is then inflated to expand the
stent against the wall of the blood vessel. Once the stent is
lodged in place, the balloon is again deflated and the placement
catheter is withdrawn from the body.
[0074] The invention treatment method can be used to stimulate the
growth and/or repair of numerous tubular body organs, including,
but not limited to blood vessels, trachea, ureters, urethrea, the
common bile duct, the bronchi, and the like. So long as the body
lumen has not suffered a circumferential lesion that completely
destroys or disrupts the integrity of the lumen, the invention
stent can be used to repair most types of injuries in a tubular
body lumen, including tears, splits, and the like.
[0075] In another embodiment of the invention treatment method,
wherein the stent further comprises a transcutaneously energized
heating mechanism, the invention treatment method further comprises
transcutaneously applying energy to the stent, thereby heating the
stent to a temperature above normal body temperature sufficient to
cause the living cells to express one or more bioactive agents.
[0076] The invention treatment method can be self-administered. For
example, after the stent has been placed into the body lumen,
either percutaneously or surgically, the subject can place the
energy source on or next to the outer body surface proximal to the
stent so as to place the stent in the energy field. For example, if
the stent has been placed into a coronary artery, the subject would
hold the energy source against the surface of the chest. If the
stent comprises a thermostat/heat regulator as described herein, or
as known in the art, the sensor in the implanted stent will
regulate the energy field produced by the energy source as needed
to modulate the temperature of the stent and surrounding tissue to
the desired temperature range (i.e. above body temperature, but
below the temperature at which necrosis will occur).
[0077] The treatment can comprise operating the energy source with
the stent in the energy field for a single period of time, or at
repeated short intervals, for example about 20 to 30 minutes per
day. The treatment can be continued in this manner for as long as
desired, for example, over a period of weeks or even months.
[0078] The living cells ingrowing in the stent in the invention
treatment method, which produce beneficial bioactive agents can be
autologous cells of the subject into which the stent is implanted,
cells seeded into the stent prior to implant that naturally produce
the desired bioactive agent, or cells that are genetically modified
to produce a desired bioactive agent. Living cells that naturally
produce one or more bioactive agents useful in practice of the
invention methods include endothelial cells, smooth muscle cells,
leukocytes, monocytes, polymorphonuclear leukocytes, lymphocytes,
basophils, fibroblasts, stem cells, epithelial cells, eosinophils,
and the like, and suitable combinations thereof. Such cells can be
either donor or autologous cells.
[0079] Alternatively, the cells used in the invention treatment
method can be engineered to express and release a bioactive agent
in response to heating above body temperature such that the
recombinant gene products are delivered to a site implanted with an
invention stent. For example, a heat sensitive gene promoter can be
operatively associated with a gene that encodes such a bioactive
agent or a protein that regulates production of a bioactive agent
to regulate expression of the gene product. Heat sensitive gene
promoters suitable for use in the invention method include the E.
Coli and Drosophila heat shock promoters, and the like. Heating
(even to low temperatures) can be made to either turn on, or turn
off, the recombinant gene when the temperature is elevated,
depending upon the selection of the transcription regulatory
region, e.g., the promoter and other regulatory elements, as is
known in the art. The temperature elevation may be achieved, as
indicated above, utilizing an external energy source to
transcutaneously (i.e., non-invasively or potentially invasively)
heat the stent material and proximal cells.
[0080] The recombinant promoter/gene combination DNA can be
transfected into the cells of interest near the implant site, or
alternatively, may be eluted from the stent or implant device to
transfect, locally, proximal cells. Cells may also be externally
transfected with the heat sensitive promoter and gene, and then
implanted with the stent device, so that heating the device
following implant will activate (or inhibit) the gene product
directly. Heating can be done chronically over time, being
available to the biologic site of interest as long as the
recombinant cells survive at the implant site.
[0081] Optionally, the cells can be obtained from a donor or from
the host subject to be treated, modified as above, and then
reintroduced into the subject to be treated. In a presently
preferred embodiment, the transplanted cells are "autologous" with
respect to the subject, meaning that the donor and recipient of the
cells are one and the same.
[0082] Genetically modified cells are cultivated under growth
conditions (as opposed to protein expression conditions) until a
desired density is achieved. Stably transfected mammalian cells may
be prepared by transfecting cells with an expression vector having
a selectable marker gene (such as, for example, the gene for
thymidine kinase, dihydrofolate reductase, neomycin resistance, and
the like), and growing the transfected cells under conditions
selective for cells expressing the marker gene. To prepare
transient transfectants, mammalian cells are transfected with a
reporter gene (such as the E. coli .beta.-galactosidase gene) to
monitor transfection efficiency. Selectable marker genes are
typically not included in the transient transfections because the
transfectants are typically not grown under selective conditions,
and are usually analyzed within a few days after transfection.
[0083] Genes that encode useful bioactive agents that are not
normally transported outside the cell can be used in the invention
if such genes are "functionally appended" to a signal sequence that
can "transport" the encoded product across the cell membrane. A
variety of such signal sequences are known and can be used by those
skilled in the art without undue experimentation.
[0084] Gene transfer vectors (also referred to as "expression
vectors") contemplated for use herein are recombinant nucleic acid
molecules that are used to transport nucleic acid into host cells
for expression and/or replication thereof. Expression vectors may
be either circular or linear, and are capable of incorporating a
variety of nucleic acid constructs therein. Expression vectors
typically come in the form of a plasmid that, upon introduction
into an appropriate host cell, results in expression of the
inserted nucleic acid.
[0085] Suitable expression vectors for use herein are well known to
those of skill in the art and include a recombinant DNA or RNA
construct(s), such as plasmids, phage, recombinant virus or other
vectors that, upon introduction into an appropriate host cell,
result(s) in expression of the inserted DNA. Appropriate expression
vectors are well known to those of skill in the art and include
those that are replicable in eukaryotic cells and/or prokaryotic
cells and those that remain episomal or those which integrate into
the host cell genome. Expression vectors typically further contain
other functionally important nucleic acid sequences encoding
antibiotic resistance proteins, and the like.
[0086] The amount of exogenous nucleic acid introduced into a host
organism, cell or cellular system can be varied by those of skill
in the art. For example, when a viral vector is employed to achieve
gene transfer, the amount of nucleic acid introduced can be
increased by increasing the amount of plaque forming units (PFU) of
the viral vector.
[0087] As used herein, the phrase "operatively associated with"
refers to the functional relationship of DNA with regulatory and
effector sequences of nucleotides, such as promoters, enhancers,
transcriptional and translational stop sites, and other signal
sequences. For example, operative linkage of DNA to a promoter
refers to the physical and functional relationship between the DNA
and promoter such that the transcription of such DNA is initiated
from the promoter by an RNA polymerase that specifically
recognizes, binds to, and transcribes the DNA.
[0088] Preferably, the transcription regulatory region may further
comprise a binding site for ubiquitous transcription factor(s).
Such binding sites are preferably positioned between the promoter
and the regulatory element. Suitable ubiquitous transcription
factors for use herein are well-known in the art and include, for
example, Sp1.
[0089] Exemplary eukaryotic expression vectors include eukaryotic
constructs, such as the pSV-2 gpt system (Mulligan et al., (1979)
Nature, 277:108-114); pBlueSkript.RTM. (Stratagene, La Jolla,
Calif.), the expression cloning vector described by Genetics
Institute (Science, (1985) 228:810-815), and the like. Each of
these plasmid vectors is capable of promoting expression of the
gene product of interest.
[0090] Suitable means for introducing (transducing) expression
vectors containing heterologous nucleic acid constructs into host
cells to produce transduced recombinant cells (i.e., cells
containing recombinant heterologous nucleic acid) are well-known in
the art (see, for review, Friedmann, Science, 244:1275-1281, 1989;
Mulligan, Science, 260:926-932. 1993, each of which are
incorporated herein by reference in their entirety). Exemplary
methods of transduction include, e.g., infection employing viral
vectors (see, e.g., U.S. Pat. Nos. 4,405,712 and 4,650,764),
calcium phosphate transfection (U.S. Pat. Nos. 4,399,216 and
4,634,665), dextran sulfate transfection, electroporation,
lipofection (see, e.g., U.S. Pat. Nos. 4,394,448 and 4,619,794),
cytofection, particle bead bombardment, and the like. The
transduced nucleic acid can optionally include sequences which
allow for its extrachromosomal (i.e., episomal) maintenance, or the
transduced nucleic acid can be donor nucleic acid that integrates
into the genome of the host.
[0091] Bioactive agents suitable for delivery according to the
invention methods include those which the mammalian body utilizes
to stimulate angiogenesis, including those which regulate capillary
formation in wounds and attract smooth muscle to coat and support
the capillaries. Examples of such bioactive agents include vascular
endothelial growth factor (VEGF), fibroblast growth factors (FGFs),
particularly FGF-1, angiopoietin 1, thrombin, and the like.
Additional examples of bioactive agents suitable for delivery
according to the invention methods include anti-proliferative,
anti-restenotic or apoptotic agents, such as platelet-derived
growth factor-A (PDGF-A), transforming growth factor beta
(TGF-.beta.), nuclear factor-.kappa..beta. (NF-.kappa..beta.), an
inducible redox-controlled transcription factor, and the like.
[0092] In another embodiment according to the present invention,
there are provided temperature-sensing stents for measuring the
flow of a liquid, such as blood, through the stent. The invention
temperature sensitive stent is based upon the principle that a
liquid (e.g., blood) flowing through stent is a cooling medium and
that the amount of cooling of a stent that has been heated above
body temperature is directly proportional to the flow rate of the
liquid flowing through the stent. When the stent is implanted in a
blood vessel, the invention temperature-sensitive stent can be used
to measure and monitor the flow of blood in the blood vessel in a
non-invasive manner.
[0093] The invention temperature-sensitive stent comprises a
tubular stent body having attached thereto a heating mechanism that
includes one to about six temperature sensors, with the temperature
sensors attached at discrete spaced locations along the length
thereof, each adapted for sensing the temperature at the discrete
location, and a telemetering device for transcutaneously conveying
the temperature sensed by each sensor to a monitor. Optionally, the
monitor can transform the message from the telemetering device to a
visible display, or record the message in some other readable
format. The monitor generally is in communication with the energy
source so that temperature information from the sensors is used to
turn the energy source on and off to modulate and/or control the
temperature of the invention stent.
[0094] Generally the stent comprises from two to about six with the
temperature sensors spaced out along the length of the stent body.
For example, the stent may comprise three heat sensors equally
spaced along the length of the stent body. It is preferred that the
temperature sensors have sufficient sensitivity to detect a
temperature difference as small as 0.1.degree. C. from one end of
the stent to the other end. When the temperature sensor is a
thermocouple or thermopile, temperature differences as small as
0.1.degree. C. can be detected.
[0095] The invention temperature-sensing stent may further comprise
surface features in the stent body adapted to promote an organized
growth pattern of infiltrating cells as described herein.
[0096] In another embodiment, methods are provided for using the
invention temperature-sensing stent to measure flow of a fluid
through a body lumen into which the stent is implanted, for example
blood flow through a blood vessel. The invention method for
measuring flow of a fluid comprises implanting an invention stent
temperature-sensitive stent, as described herein, into a body lumen
having a flow of fluid therethrough, energizing the implanted stent
transcutaneously to raise the temperature thereof above body
temperature, monitoring transcutaneously the output from one or
more of the temperature sensors upon cessation of the energizing to
determine the cooling rate at the sensors, and obtaining the flow
rate of the fluid from the cooling rate at the one or more sensors.
To be useful in measuring the flow rate of fluid through the
implanted stent, the stent body is generally sufficiently to raise
the temperature of the stent about 2 to 12 degrees Centigrade above
body temperature.
[0097] Determination of the fluid flow rate from the temperature
information (e.g., the cooling rate of the fluid flowing through
the stent) provided by the temperature sensors via the telemetry in
the stent involves application of one or more mathematical
algorithms, such as are well known in the art. Such algorithms
generally take into account such parameters as the heat capacity
properties of the fluid, the interior cross-sectional area of the
stent body, the length of the stent, the distance between the
relevant temperature sensors, the difference between temperatures
sensed at any two locations along the length of the stent, the
difference between the temperature sensed at any discrete location
and body temperature, the neointimal thickness/area, and the like.
When the fluid whose flow rate is to be determined is blood, the
heat capacity of the blood may vary by patient when such factors as
hematocrit, and the like, are taken into account. Equation I below
can be used to obtain a fluid flow rate based on such parameters as
follows:
dT/dx=(.rho..sub.oP)A/Q (I)
wherein T=temperature; x=distance of fluid flow;
.rho..sub.o=specific heat of fluid; P=power in to heat the stent;
Q=flow rate; and A=cross-sectional area of the stent.
[0098] The invention will now be described in greater detail by
reference to the following non-limiting examples.
Example 1
Temperature vs. Variable Flow Rates
[0099] The concept of flow measurement by the temperature sensitive
stent is based upon the principle that a liquid (e.g. blood)
flowing through a stent is a cooling medium and that the amount of
cooling of a stent that has been heated above body temperature is
directly proportional to the flow rate of the liquid through the
stent. This is expressed by Equation I above. To validate the use
of the heated stent as a measure of flow rate, experimental data
was obtained through the bench testing as follows.
[0100] A GR2 type configuration stent was created using 38 AWG
Nichrome resistance wire. 30 AWG Type J thermocouples were attached
to the stent in three locations described as distal (furthest away
from stent heating leads), mid and proximal. The stent was deployed
in a simulated blood vessel made of silicone and submerged in a
37.degree. C. distilled water bath. The water bath temperature was
held constant during the testing. While a constant voltage of 11 V
was applied to the stent leads, 37.degree. C. distilled water was
pumped via a peristaltic pump through the deployed stent/vessel
assembly at flow rates of 10, 20, 30, 40, 50, 60, 70, 80 ml/min
while temperature data was collected from each of the three
thermocouples. The direction of the flow was then reversed and data
was again collected for these flow rates. Temperature measurements
were recorded for a total of three minutes. Four data points for
each stent location were collected per minute. The data shown in
Tables 1 and 2 below is the average of these four data points. The
first three data points at each location were thrown out (time for
stent temp to ramp to 11 V.apprxeq.40 sec).
[0101] FIG. 5 is a graph showing the average temperature plotted
against flow rate (ml/min) for each of the three thermocouples.
TABLE-US-00001 TABLE 1 Flow Proximal to Distal Flow Rate Distal
Stent - Flow Mid Stent - Flow Proximal Stent - Flow (ml/ Distal
Average Distal Average Distal Average min) Temp (.degree. C.) Temp
(.degree. C.) Temp (.degree. C.) 10 56.66 51.36 45.59 20 50.35
46.12 42.76 30 47.36 44.68 41.84 40 44.33 42.50 40.92 50 43.67
41.80 40.40 60 43.06 41.25 40.07 70 42.47 40.73 39.79 80 41.64
40.47 39.31
TABLE-US-00002 TABLE 2 Flow Distal to Proximal Flow Rate Distal
Stent - Flow Mid Stent - Flow Proximal Stent - Flow (ml/ Proximal
Average Proximal Average Proximal Average min) Temp (.degree. C.)
Temp (.degree. C.) Temp (.degree. C.) 10 45.66 49.94 52.36 20 43.63
46.29 48.93 30 42.34 44.42 46.71 40 41.27 42.80 44.35 50 40.86
42.87 44.59 60 40.49 42.41 43.76 70 40.14 42.08 42.91 80 39.99
41.67 42.45
The values obtained from theoretical calculations using Equation I
above correlated well with values obtained by these empirical
tests.
[0102] This invention has been described herein in considerable
detail in order to comply with the patent statutes and to provide
those skilled in the art with the information needed to apply the
novel principles and to construct and use such specialized
components as are required. However, it is to be understood that
the invention can be carried out by specifically different
equipment and devices, and that various modifications, both as to
the equipment, operating procedures and end use, can be
accomplished without departing from the scope of the invention
itself as defined by the appended claims.
* * * * *