U.S. patent application number 11/941596 was filed with the patent office on 2008-02-28 for medical devices, drug coatings and methods for maintaining the drug coatings thereon.
Invention is credited to Gerard H. Llanos, Mark B. Roller, Angelo George Scopelianos.
Application Number | 20080051885 11/941596 |
Document ID | / |
Family ID | 46328144 |
Filed Date | 2008-02-28 |
United States Patent
Application |
20080051885 |
Kind Code |
A1 |
Llanos; Gerard H. ; et
al. |
February 28, 2008 |
Medical Devices, Drug Coatings and Methods for Maintaining the Drug
Coatings Thereon
Abstract
Medical devices, and in particular implantable medical devices,
may be coated to minimize or substantially eliminate a biological
organism's reaction to the introduction of the medical device to
the organism. The medical devices may be coated with any number of
biocompatible materials. Therapeutic drugs, agents or compounds may
be mixed with the biocompatible materials and affixed to at least a
portion of the medical device. These therapeutic drugs, agents or
compounds may also further reduce a biological organism's reaction
to the introduction of the medical device to the organism. Various
materials and coating methodologies may be utilized to maintain the
drugs, agents or compounds on the medical device until delivered
and positioned.
Inventors: |
Llanos; Gerard H.;
(Stewartsville, NJ) ; Roller; Mark B.; (North
Brunswick, NJ) ; Scopelianos; Angelo George;
(Whitehouse Station, NJ) |
Correspondence
Address: |
WOODCOCK WASHBURN LLP
CIRA CENTRE, 12TH FLOOR
2929 ARCH STREET
PHILADELPHIA
PA
19104-2891
US
|
Family ID: |
46328144 |
Appl. No.: |
11/941596 |
Filed: |
November 16, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11782918 |
Jul 25, 2007 |
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11941596 |
Nov 16, 2007 |
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11437572 |
May 19, 2006 |
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11782918 |
Jul 25, 2007 |
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10636435 |
Aug 7, 2003 |
7056550 |
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11437572 |
May 19, 2006 |
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09962496 |
Sep 25, 2001 |
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10636435 |
Aug 7, 2003 |
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09675882 |
Sep 29, 2000 |
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09962496 |
Sep 25, 2001 |
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Current U.S.
Class: |
623/1.42 ;
427/2.25 |
Current CPC
Class: |
B05D 3/0254 20130101;
A61F 2/915 20130101; A61F 2230/0067 20130101; A61L 2300/00
20130101; A61F 2230/005 20130101; A61L 2300/606 20130101; C08L
27/12 20130101; A61L 31/16 20130101; A61L 2300/416 20130101; A61L
31/10 20130101; Y10T 428/3154 20150401; C08L 27/16 20130101; C08L
27/14 20130101; A61F 2230/0054 20130101; A61L 31/10 20130101; A61L
2300/41 20130101; A61F 2/91 20130101; A61L 31/10 20130101; A61F
2250/0098 20130101; A61F 2250/0067 20130101; A61L 31/10
20130101 |
Class at
Publication: |
623/001.42 ;
427/002.25 |
International
Class: |
A61F 2/82 20060101
A61F002/82 |
Claims
1. A device for intraluminal implantation in a blood vessel, said
device comprising a balloon-expandable stent and a coating, said
coating comprising a biocompatible polyfluoro copolymer that
comprises about eighty-five weight percent vinylidinefluoride
copolymerized with about fifteen weight percent
hexafluoropropylene, and from about 12% to about 20% by weight of a
pharmaceutical agent, wherein said pharmaceutical agent and said
copolymer are selected such that said agent releases from said
coating for at least 800 hours following intraluminal implantation
in said blood vessel.
2. The device of claim 1 wherein said polyfluoro copolymer consists
essentially of 85.5 weight percent vinylidinefluoride and 14.5
weight percent hexafluropropylene.
3. A method for preparing a device according to claim 1, said
method comprising the steps of: combining said pharmaceutical agent
with said polyfluoro copolymer to provide a coating; applying said
coating to said balloon-expandable stent; and heating the
balloon-expandable stent comprising said coating to a maximum
temperature of no greater than about 125.degree. C.
4. A method according to claim 3, wherein said balloon-expandable
stent comprising said coating is heated to a maximum temperature of
no greater than about 60.degree. C.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation application of copending
U.S. Ser. No. 11/782,918, filed Jul. 25, 2007, which in turn is a
continuation of copending U.S. Ser. No. 11/437,572, filed May 19,
2006, which is in turn a continuation of U.S. Ser. No. 10/636,435
filed Aug. 7, 2003, now U.S. Pat. No. 7,056,550, which is a
continuation application of U.S. Ser. No. 09/962,496 filed Sep. 25,
2001, which is a continuation-in-part application of U.S.
application Ser. No. 09/675,882, filed Sep. 29, 2000.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to the local administration of
drug/drug combinations for the prevention and treatment of vascular
disease, and more particularly to intraluminal medical devices for
the local delivery of drug/drug combinations for the prevention and
treatment of vascular disease caused by injury and methods for
maintaining the drug/drug combinations on the intraluminal medical
devices. The present invention also relates to medical devices
having drugs, agents or compounds affixed thereto to minimize or
substantially eliminate a biological organism's reaction to the
introduction of the medical device to the organism.
[0004] 2. Discussion of the Related Art
[0005] Many individuals suffer from circulatory disease caused by a
progressive blockage of the blood vessels that perfuse the heart
and other major organs with nutrients. More severe blockage of
blood vessels in such individuals often leads to hypertension,
ischemic injury, stroke, or myocardial infarction. Atherosclerotic
lesions, which limit or obstruct coronary blood flow, are the major
cause of ischemic heart disease. Percutaneous transluminal coronary
angioplasty is a medical procedure whose purpose is to increase
blood flow through an artery. Percutaneous transluminal coronary
angioplasty is the predominant treatment for coronary vessel
stenosis. The increasing use of this procedure is attributable to
its relatively high success rate and its minimal invasiveness
compared with coronary bypass surgery. A limitation associated with
percutaneous transluminal coronary angioplasty is the abrupt
closure of the vessel which may occur immediately after the
procedure and restenosis which occurs gradually following the
procedure. Additionally, restenosis is a chronic problem in
patients who have undergone saphenous vein bypass grafting. The
mechanism of acute occlusion appears to involve several factors and
may result from vascular recoil with resultant closure of the
artery and/or deposition of blood platelets and fibrin along the
damaged length of the newly opened blood vessel.
[0006] Restenosis after percutaneous transluminal coronary
angioplasty is a more gradual process initiated by vascular injury.
Multiple processes, including thrombosis, inflammation, growth
factor and cytokine release, cell proliferation, cell migration and
extracellular matrix synthesis each contribute to the restenotic
process.
[0007] While the exact mechanism of restenosis is not completely
understood, the general aspects of the restenosis process have been
identified. In the normal arterial wall, smooth muscle cells
proliferate at a low rate, approximately less than 0.1 percent per
day. Smooth muscle cells in the vessel walls exist in a contractile
phenotype characterized by eighty to ninety percent of the cell
cytoplasmic volume occupied with the contractile apparatus.
Endoplasmic reticulum, Golgi, and free ribosomes are few and are
located in the perinuclear region. Extracellular matrix surrounds
the smooth muscle cells and is rich in heparin-like
glycosylaminoglycans which are believed to be responsible for
maintaining smooth muscle cells in the contractile phenotypic state
(Campbell and Campbell, 1985).
[0008] Upon pressure expansion of an intracoronary balloon catheter
during angioplasty, smooth muscle cells within the vessel wall
become injured, initiating a thrombotic and inflammatory response.
Cell derived growth factors such as platelet derived growth factor,
basic fibroblast growth factor, epidermal growth factor, thrombin,
etc., released from platelets, invading macrophages and/or
leukocytes, or directly from the smooth muscle cells provoke a
proliferative and migratory response in medial smooth muscle cells.
These cells undergo a change from the contractile phenotype to a
synthetic phenotype characterized by only a few contractile
filament bundles, extensive rough endoplasmic reticulum, Golgi and
free ribosomes. Proliferation/migration usually begins within one
to two days post-injury and peaks several days thereafter (Campbell
and Campbell, 1987; Clowes and Schwartz, 1985).
[0009] Daughter cells migrate to the intimal layer of arterial
smooth muscle and continue to proliferate and secrete significant
amounts of extracellular matrix proteins. Proliferation, migration
and extracellular matrix synthesis continue until the damaged
endothelial layer is repaired at which time proliferation slows
within the intima, usually within seven to fourteen days
post-injury. The newly formed tissue is called neointima. The
further vascular narrowing that occurs over the next three to six
months is due primarily to negative or constrictive remodeling.
[0010] Simultaneous with local proliferation and migration,
inflammatory cells adhere to the site of vascular injury. Within
three to seven days post-injury, inflammatory cells have migrated
to the deeper layers of the vessel wall. In animal models employing
either balloon injury or stent implantation, inflammatory cells may
persist at the site of vascular injury for at least thirty days
(Tanaka et al., 1993; Edelman et al., 1998). Inflammatory cells
therefore are present and may contribute to both the acute and
chronic phases of restenosis.
[0011] Numerous agents have been examined for presumed
anti-proliferative actions in restenosis and have shown some
activity in experimental animal models. Some of the agents which
have been shown to successfully reduce the extent of intimal
hyperplasia in animal models include: heparin and heparin fragments
(Clowes, A. W. and Karnovsky M., Nature 265: 25-26,1977; Guyton, J.
R. et al., Circ. Res., 46: 625-634,1980; Clowes, A. W. and Clowes,
M. M., Lab. Invest. 52: 611-616,1985; Clowes, A. W. and Clowes, M.
M., Circ. Res. 58: 839-845, 1986; Majesky et al., Circ. Res. 61:
296-300,1987; Snow et al., Am. J. Pathol. 137: 313-330, 1990;
Okada, T. et al., Neurosurgery 25: 92-98, 1989), colchicine
(Currier, J. W. et al., Circ. 80: 11-66, 1989), taxol (Sollot, S.
J. et al., J. Clin. Invest. 95: 1869-1876, 1995), angiotensin
converting enzyme (ACE) inhibitors (Powell, J. S. et al., Science,
245: 186-188, 1989), angiopeptin (Lundergan, C. F. et al. Am. J.
Cardiol. 17(Suppl. B):132B-136B, 1991), cyclosporin A (Jonasson, L.
et al., Proc. Natl., Acad. Sci., 85: 2303, 1988), goat-anti-rabbit
PDGF antibody (Ferns, G. A. A., et al., Science 253: 1129-1132,
1991), terbinafine (Nemecek, G. M. et al., J. Pharmacol. Exp.
Thera. 248: 1167-1174, 1989), trapidil (Liu, M. W. et al., Circ.
81: 1089-1093, 1990), tranilast (Fukuyama, J. et al., Eur. J.
Pharmacol. 318: 327-332, 1996), interferon-gamma (Hansson, G. K.
and Holm, J., Circ. 84: 1266-1272, 1991), rapamycin (Marx, S. O. et
al., Circ. Res. 76: 412-417, 1995), steroids (Colburn, M. D. et
al., J. Vasc. Surg. 15: 510-518, 1992), see also Berk, B. C. et
al., J. Am. Coll. Cardiol. 17: 111B-117B, 1991), ionizing radiation
(Weinberger, J. et al., Int. J. Rad. Onc. Biol. Phys. 36: 767-775,
1996), fusion toxins (Farb, A. et al., Circ. Res. 80: 542-550,
1997) antisense oligionucleotides (Simons, M. et al., Nature 359:
67-70, 1992) and gene vectors (Chang, M. W. et al., J. Clin.
Invest. 96: 2260-2268, 1995). Anti-proliferative action on smooth
muscle cells in vitro has been demonstrated for many of these
agents, including heparin and heparin conjugates, taxol, tranilast,
colchicine, ACE inhibitors, fusion toxins, antisense
oligionucleotides, rapamycin and ionizing radiation. Thus, agents
with diverse mechanisms of smooth muscle cell inhibition may have
therapeutic utility in reducing intimal hyperplasia.
[0012] However, in contrast to animal models, attempts in human
angioplasty patients to prevent restenosis by systemic
pharmacologic means have thus far been unsuccessful. Neither
aspirin-dipyridamole, ticlopidine, anti-coagulant therapy (acute
heparin, chronic warfarin, hirudin or hirulog), thromboxane
receptor antagonism nor steroids have been effective in preventing
restenosis, although platelet inhibitors have been effective in
preventing acute reocclusion after angioplasty (Mak and Topol,
1997; Lang et al., 1991; Popma et al., 1991). The platelet GP
II.sub.b/III.sub.a receptor, antagonist, Reopro.RTM. is still under
study but Reopro.RTM. has not shown definitive results for the
reduction in restenosis following angioplasty and stenting. Other
agents, which have also been unsuccessful in the prevention of
restenosis, include the calcium channel antagonists, prostacyclin
mimetics, angiotensin converting enzyme inhibitors, serotonin
receptor antagonists, and anti-proliferative agents. These agents
must be given systemically, however, and attainment of a
therapeutically effective dose may not be possible;
anti-proliferative (or anti-restenosis) concentrations may exceed
the known toxic concentrations of these agents so that levels
sufficient to produce smooth muscle inhibition may not be reached
(Mak and Topol, 1997; Lang et al., 1991; Popma et al., 1991).
[0013] Additional clinical trials in which the effectiveness for
preventing restenosis utilizing dietary fish oil supplements or
cholesterol lowering agents has been examined showing either
conflicting or negative results so that no pharmacological agents
are as yet clinically available to prevent post-angioplasty
restenosis (Mak and Topol, 1997; Franklin and Faxon, 1993: Serruys,
P. W. et al., 1993). Recent observations suggest that the
antilipid/antioxident agent, probucol, may be useful in preventing
restenosis but this work requires confirmation (Tardif et al.,
1997; Yokoi, et al., 1997). Probucol is presently not approved for
use in the United States and a thirty-day pretreatment period would
preclude its use in emergency angioplasty. Additionally, the
application of ionizing radiation has shown significant promise in
reducing or preventing restenosis after angioplasty in patients
with stents (Teirstein et al., 1997). Currently, however, the most
effective treatments for restenosis are repeat angioplasty,
atherectomy or coronary artery bypass grafting, because no
therapeutic agents currently have Food and Drug Administration
approval for use for the prevention of post-angioplasty
restenosis.
[0014] Unlike systemic pharmacologic therapy, stents have proven
useful in significantly reducing restenosis. Typically, stents are
balloon-expandable slotted metal tubes (usually, but not limited
to, stainless steel), which, when expanded within the lumen of an
angioplastied coronary artery, provide structural support through
rigid scaffolding to the arterial wall. This support is helpful in
maintaining vessel lumen patency. In two randomized clinical
trials, stents increased angiographic success after percutaneous
transluminal coronary angioplasty, by increasing minimal lumen
diameter and reducing, but not eliminating, the incidence of
restenosis at six months (Serruys et al., 1994; Fischman et al.,
1994).
[0015] Additionally, the heparin coating of stents appears to have
the added benefit of producing a reduction in sub-acute thrombosis
after stent implantation (Serruys et al., 1996). Thus, sustained
mechanical expansion of a stenosed coronary artery with a stent has
been shown to provide some measure of restenosis prevention, and
the coating of stents with heparin has demonstrated both the
feasibility and the clinical usefulness of delivering drugs
locally, at the site of injured tissue.
[0016] As stated above, the use of heparin coated stents
demonstrates the feasibility and clinical usefulness of local drug
delivery; however, the manner in which the particular drug or drug
combination is affixed to the local delivery device will play a
role in the efficacy of this type of treatment. For example, the
processes and materials utilized to affix the drug/drug
combinations to the local delivery device should not interfere with
the operations of the drug/drug combinations. In addition, the
processes and materials utilized should be biocompatible and
maintain the drug/drug combinations on the local device through
delivery and over a given period of time. For example, removal of
the drug/drug combination during delivery of the local delivery
device may potentially cause failure of the device.
[0017] Accordingly, there exists a need for drug/drug combinations
and associated local delivery devices for the prevention and
treatment of vascular injury causing intimal thickening which is
either biologically induced, for example atherosclerosis, or
mechanically induced, for example, through percutaneous
transluminal coronary angioplasty. In addition, there exists a need
for maintaining the drug/drug combinations on the local delivery
device through delivery and positioning as well as ensuring that
the drug/drug combination is released in therapeutic dosages over a
given period of time.
[0018] A variety of stent coatings and compositions have been
proposed for the prevention and treatment of injury causing intimal
thickening. The coatings may be capable themselves of reducing the
stimulus the stent provides to the injured lumen wall, thus
reducing the tendency towards thrombosis or restenosis.
Alternately, the coating may deliver a pharmaceutical/therapeutic
agent or drug to the lumen that reduces smooth muscle tissue
proliferation or restenosis. The mechanism for delivery of the
agent is through diffusion of the agent through either a bulk
polymer or through pores that are created in the polymer structure,
or by erosion of a biodegradable coating.
[0019] Both bioabsorbable and biostable compositions have been
reported as coatings for stents. They generally have been polymeric
coatings that either encapsulate a pharmaceutical/therapeutic agent
or drug, e.g. rapamycin, taxol etc., or bind such an agent to the
surface, e.g. heparin-coated stents. These coatings are applied to
the stent in a number of ways, including, though not limited to,
dip, spray, or spin coating processes.
[0020] One class of biostable materials that has been reported as
coatings for stents is polyfluoro homopolymers.
Polytetrafluoroethylene (PTFE) homopolymers have been used as
implants for many years. These homopolymers are not soluble in any
solvent at reasonable temperatures and therefore are difficult to
coat onto small medical devices while maintaining important
features of the devices (e.g. slots in stents).
[0021] Stents with coatings made from polyvinylidenefluoride
homopolymers and containing pharmaceutical/therapeutic agents or
drugs for release have been suggested. However, like most
crystalline polyfluoro homopolymers, they are difficult to apply as
high quality films onto surfaces without subjecting them to
relatively high temperatures, that correspond to the melting
temperature of the polymer.
[0022] It would be advantageous to develop coatings for implantable
medical devices that will reduce thrombosis, restenosis, or other
adverse reactions, that may include, but do not require, the use of
pharmaceutical or therapeutic agents or drugs to achieve such
affects, and that possess physical and mechanical properties
effective for use in such devices even when such coated devices are
subjected to relatively low maximum temperatures.
SUMMARY OF THE INVENTION
[0023] The drug/drug combination therapies, drug/drug combination
carriers and associated local delivery devices of the present
invention provide a means for overcoming the difficulties
associated with the methods and devices currently in use, as
briefly described above. In addition, the methods for maintaining
the drug/drug combinations and drug/drug combination carriers on
the local delivery device ensure that the drug/drug combination
therapies reach the target site.
[0024] In accordance with one aspect, the present invention is
directed to a medical device for implantation into a treatment site
of a living organism. The device comprises a biocompatible vehicle
affixed to at least a portion of the medical device, and at least
one agent in therapeutic dosages incorporated into the
biocompatible vehicle for the treatment of reactions by the living
organism caused by the medical device or the implantation
thereof.
[0025] In accordance with another aspect, the present invention is
directed to a medical device for implantation into a treatment site
of a living organism. The device comprises a biocompatible vehicle
affixed to at least a portion of the medical device, at least one
agent in therapeutic dosages incorporated into the biocompatible
vehicle for the treatment of reactions by the living organism
caused by the medical device or the implantation thereof, and a
material for preventing the at least one agent from separating from
the medical device prior to and during implantation of the medical
device at the treatment site, the material being affixed to at
least one of the medical devices or a delivery system for the
medical device.
[0026] In accordance with another aspect, the present invention is
directed to a medical device for implantation into a treatment site
of a living organism. The device comprises a stent, a biocompatible
vehicle affixed to at least a portion of the stent, and at least
one agent in therapeutic dosages incorporated into the
biocompatible vehicle for the treatment of reactions by the living
organism caused by the medical device or the implantation
thereof.
[0027] In accordance with another aspect, the present invention is
directed to a medical device for implantation into a treatment site
of a living organism. The device comprises a stent having a
substantially tubular member having open ends, and a first diameter
for insertion into a lumen of a vessel and a second diameter for
anchoring in the lumen of the vessel, a biocompatible vehicle
affixed to at least a portion of the stent, at least one agent in
therapeutic dosages incorporated into the biocompatible vehicle for
the treatment of reactions by the living organism caused by the
medical device or the implantation thereof, and a material for
preventing the at least one agent from separating from the medical
device prior to and during implantation of the medical device at
the treatment site, the material being affixed to at least one of
the medical devices or a delivery system for the medical
device.
[0028] In accordance with another aspect, the present invention is
directed to a local drug delivery device. The device comprises a
stent having a substantially tubular member having open ends, and a
first diameter for insertion into a lumen of a vessel and a second
diameter for anchoring in the lumen of a vessel, a biocompatible
polymeric vehicle affixed to at least a portion of the stent, and
rapamycin, in therapeutic dosages, incorporated into the
biocompatible polymeric vehicle.
[0029] In accordance with another aspect, the present invention is
directed to a method of coating a medical device with a therapeutic
agent. The method comprises the steps of creating a polymer
utilizing vinylidene fluoride and hexafluoropropylene in a batch
emulsion polymerization process, priming the medical device with
the polymer utilizing a dip coating process, creating a polymer and
therapeutic agent mixture, applying the polymer and therapeutic
agent mixture on the primer layer utilizing a spin coating process,
and drying the medical device in a vacuum oven for approximately
sixteen hours at a temperature in the range of fifty to sixty
degrees centigrade.
[0030] In accordance with another aspect, the present invention is
directed to a medical device for implantation into a treatment site
of a living organism. The medical device comprises a biocompatible
vehicle affixed to at least a portion of the medical device, and at
least one agent incorporated into the biocompatible vehicle. The at
least one agent being designed to react with one or more chemicals
produced by the living organism to treat reactions by the living
organism caused by the medical device or the implantation
thereof.
[0031] In accordance with another aspect, the present invention is
directed to a medical device for implantation into the vasculature
of a living organism. The medical device comprises a self-expanding
stent, a biocompatible vehicle affixed to at least a portion of the
stent, and rapamycin, in therapeutic dosages, incorporated into the
biocompatible vehicle for the prevention of restenosis.
[0032] In accordance with another aspect, the present invention is
directed to a method of coating a medical device with a therapeutic
agent. The method comprises the steps of creating a polymer
utilizing vinylidene fluoride and hexafluoropropylene, adding one
or more therapeutic agents to the polymer to create a polymer and
therapeutic agent mixture, and applying the polymer and therapeutic
agent mixture to the medical device.
[0033] In accordance with another aspect, the present invention is
directed to a medical device for implantation into a treatment site
of a living organism. The medical device comprises a biocompatible
vehicle affixed to at least a portion of the medical device, at
least one agent in therapeutic dosages incorporated into the
biocompatible vehicle for the treatment of disease proximate the
implantation site.
[0034] In accordance with another aspect, the present invention is
directed to a medical device for implantation into a treatment site
of a living organism. The medical device comprises a biocompatible
vehicle affixed to at least a portion of the medical device, at
least one agent in therapeutic dosages incorporated into the
biocompatible vehicle for the treatment of disease remote from the
implantation site.
[0035] The medical devices, drug coatings and methods for
maintaining the drug coatings or vehicles thereon of the present
invention utilizes a combination of materials to treat disease, and
reactions by living organisms due to the implantation of medical
devices for the treatment of disease or other conditions. The local
delivery of drugs, agents or compounds generally substantially
reduces the potential toxicity of the drugs, agents or compounds
when compared to systemic delivery while increasing their
efficacy.
[0036] Drugs, agents or compounds may be affixed to any number of
medical devices to treat various diseases. The drugs, agents or
compounds may also be affixed to minimize or substantially
eliminate the biological organism's reaction to the introduction of
the medical device utilized to treat a separate condition. For
example, stents may be introduced to open coronary arteries or
other body lumens such as biliary ducts. The introduction of these
stents cause a smooth muscle cell proliferation effect as well as
inflammation. Accordingly, the stents may be coated with drugs,
agents or compounds to combat these reactions.
[0037] The drugs, agents or compounds will vary depending upon the
type of medical device, the reaction to the introduction of the
medical device and/or the disease sought to be treated. The type of
coating or vehicle utilized to immobilize the drugs, agents or
compounds to the medical device may also vary depending on a number
of factors, including the type of medical device, the type of drug,
agent or compound and the rate of release thereof.
[0038] In order to be effective, the drugs, agents or compounds
should preferably remain on the medical devices during delivery and
implantation. Accordingly, various coating techniques for creating
strong bonds between the drugs, agents or compounds may be
utilized. In addition, various materials may be utilized as surface
modifications to prevent the drugs, agents or compounds from coming
off prematurely.
BRIEF DESCRIPTION OF THE DRAWINGS
[0039] The foregoing and other features and advantages of the
invention will be apparent from the following, more particular
description of preferred embodiments of the invention, as
illustrated in the accompanying drawings.
[0040] FIG. 1 is a view along the length of a stent (ends not
shown) prior to expansion showing the exterior surface of the stent
and the characteristic banding pattern.
[0041] FIG. 2 is a view along the length of the stent of FIG. 1
having reservoirs in accordance with the present invention.
[0042] FIG. 3 indicates the fraction of drug released as a function
of time from coatings of the present invention over which no
topcoat has been disposed.
[0043] FIG. 4 indicates the fraction of drug released as a function
of time from coatings of the present invention including a topcoat
disposed thereon.
[0044] FIG. 5 indicates the fraction of drug released as a function
of time from coatings of the present invention over which no
topcoat has been disposed.
[0045] FIG. 6 indicates in vivo stent release kinetics of rapamycin
from poly(VDF/HFP).
[0046] FIG. 7 is a cross-sectional view of a band of the stent of
FIG. 1 having drug coatings thereon in accordance with a first
exemplary embodiment of the invention.
[0047] FIG. 8 is a cross-sectional view of a band of the stent of
FIG. 1 having drug coatings thereon in accordance with a second
exemplary embodiment of the invention.
[0048] FIG. 9 is a cross-sectional view of a band of the stent of
FIG. 1 having drug coatings thereon in accordance with a third
exemplary embodiment of the present invention.
[0049] FIG. 10 is a perspective view of an exemplary stent in its
compressed state which may be utilized in conjunction with the
present invention.
[0050] FIG. 11 is a sectional, flat view of the stent shown in FIG.
10.
[0051] FIG. 12 is a perspective view of the stent shown in FIG. 10
but showing it in its expanded state.
[0052] FIG. 13 is an enlarged sectional view of the stent shown in
FIG. 12.
[0053] FIG. 14 is an enlarged view of section of the stent shown in
FIG. 11.
[0054] FIG. 15 is a view similar to that of FIG. 11 but showing an
alternate embodiment of the stent.
[0055] FIG. 16 is a perspective view of the stent of FIG. 10 having
a plurality of markers attached to the ends thereof in accordance
with the present invention.
[0056] FIG. 17 is a cross-sectional view of a marker in accordance
with the present invention.
[0057] FIG. 18 is an enlarged perspective view of an end of the
stent with the markers forming a substantially straight line in
accordance with the present invention.
[0058] FIG. 19 is a simplified partial cross-sectional view of a
stent delivery apparatus having a stent loaded therein, which can
be used with a stent made in accordance with the present
invention.
[0059] FIG. 20 is a view similar to that of FIG. 19 but showing an
enlarged view of the distal end of the apparatus.
[0060] FIG. 21 is a perspective view of an end of the stent with
the markers in a partially expanded form as it emerges from the
delivery apparatus in accordance with the present invention.
[0061] FIG. 22 is a cross-sectional view of a balloon having a
lubricious coating affixed thereto in accordance with the present
invention.
[0062] FIG. 23 is a cross-sectional view of a band of the stent in
FIG. 1 having a lubricious coating affixed thereto in accordance
with the present invention.
[0063] FIG. 24 is a cross-sectional view of a self-expanding stent
in a delivery device having a lubricious coating in accordance with
the present invention.
[0064] FIG. 25 is a cross-sectional view of a band of the stent in
FIG. 1 having a modified polymer coating in accordance with the
present invention.
[0065] FIG. 26 illustrates an exemplary balloon-expandable stent
having an alternative arrangement of "N" and "J" links between sets
of strut members, represented on a flat, two-dimensional plan view
in accordance with the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0066] The drug/drug combinations and delivery devices of the
present invention may be utilized to effectively prevent and treat
vascular disease, and in particular, vascular disease caused by
injury. Various medical treatment devices utilized in the treatment
of vascular disease may ultimately induce further complications.
For example, balloon angioplasty is a procedure utilized to
increase blood flow through an artery and is the predominant
treatment for coronary vessel stenosis. However, as stated above,
the procedure typically causes a certain degree of damage to the
vessel wall, thereby potentially exacerbating the problem at a
point later in time. Although other procedures and diseases may
cause similar injury, exemplary embodiments of the present
invention will be described with respect to the treatment of
restenosis and related complications following percutaneous
transluminal coronary angioplasty and other similar arterial/venous
procedures.
[0067] While exemplary embodiments of the invention will be
described with respect to the treatment of restenosis and related
complications following percutaneous transluminal coronary
angioplasty, it is important to note that the local delivery of
drug/drug combinations may be utilized to treat a wide variety of
conditions utilizing any number of medical devices, or to enhance
the function and/or life of the device. For example, intraocular
lenses, placed to restore vision after cataract surgery is often
compromised by the formation of a secondary cataract. The latter is
often a result of cellular overgrowth on the lens surface and can
be potentially minimized by combining a drug or drugs with the
device. Other medical devices which often fail due to tissue
in-growth or accumulation of proteinaceous material in, on and
around the device, such as shunts for hydrocephalus, dialysis
grafts, colostomy bag attachment devices, ear drainage tubes, leads
for pace makers and implantable defibrillators can also benefit
from the device-drug combination approach.
[0068] Devices which serve to improve the structure and function of
tissue or organ may also show benefits when combined with the
appropriate agent or agents. For example, improved osteointegration
of orthopedic devices to enhance stabilization of the implanted
device could potentially be achieved by combining it with agents
such as bone-morphogenic protein. Similarly other surgical devices,
sutures, staples, anastomosis devices, vertebral disks, bone pins,
suture anchors, hemostatic barriers, clamps, screws, plates, clips,
vascular implants, tissue adhesives and sealants, tissue scaffolds,
various types of dressings, bone substitutes, intraluminal devices,
and vascular supports could also provide enhanced patient benefit
using this drug-device combination approach. Essentially, any type
of medical device may be coated in some fashion with a drug or drug
combination which enhances treatment over use of the singular use
of the device or pharmaceutical agent.
[0069] In addition to various medical devices, the coatings on
these devices may be used to deliver therapeutic and pharmaceutic
agents including: antiproliferative/antimitotic agents including
natural products such as vinca alkaloids (i.e. vinblastine,
vincristine, and vinorelbine), paclitaxel, epidipodophyllotoxins
(i.e. etoposide, teniposide), antibiotics (dactinomycin
(actinomycin D) daunorubicin, doxorubicin and idarubicin),
anthracyclines, mitoxantrone, bleomycins, plicamycin (mithramycin)
and mitomycin, enzymes (L-asparaginase which systemically
metabolizes L-asparagine and deprives cells which do not have the
capacity to synthesize their own asparagine); antiplatelet agents
such as G(GP)II.sub.bIII.sub.a inhibitors and vitronectin receptor
antagonists; antiproliferative/antimitotic alkylating agents such
as nitrogen mustards (mechlorethamine, cyclophosphamide and
analogs, melphalan, chlorambucil), ethylenimines and
methylmelamines (hexamethylmelamine and thiotepa), alkyl
sulfonates-busulfan, nirtosoureas (carmustine (BCNU) and analogs,
streptozocin), trazenes--dacarbazinine (DTIC);
antiproliferative/antimitotic antimetabolites such as folic acid
analogs (methotrexate), pyrimidine analogs (fluorouracil,
floxuridine, and cytarabine), purine analogs and related inhibitors
(mercaptopurine, thioguanine, pentostatin and
2-chlorodeoxyadenosine {cladribine}); platinum coordination
complexes (cisplatin, carboplatin), procarbazine, hydroxyurea,
mitotane, aminoglutethimide; hormones (i.e. estrogen);
anticoagulants (heparin, synthetic heparin salts and other
inhibitors of thrombin); fibrinolytic agents (such as tissue
plasminogen activator, streptokinase and urokinase), aspirin,
dipyridamole, ticlopidine, clopidogrel, abciximab; antimigratory;
antisecretory (breveldin); antiinflammatory: such as adrenocortical
steroids (cortisol, cortisone, fludrocortisone, prednisone,
prednisolone, 6.alpha.-methylprednisolone, triamcinolone,
betamethasone, and dexamethasone), non-steroidal agents (salicylic
acid derivatives i.e. aspirin; para-aminophenol derivatives i.e.
acetominophen; indole and indene acetic acids (indomethacin,
sulindac, and etodalac), heteroaryl acetic acids (tolmetin,
diclofenac, and ketorolac), arylpropionic acids (ibuprofen and
derivatives), anthranilic acids (mefenamic acid, and meclofenamic
acid), enolic acids (piroxicam, tenoxicam, phenylbutazone, and
oxyphenthatrazone), nabumetone, gold compounds (auranofin,
aurothioglucose, gold sodium thiomalate); immunosuppressives:
(cyclosporine, tacrolimus (FK-506), sirolimus (rapamycin),
azathioprine, mycophenolate mofetil); angiogenic agents: vascular
endothelial growth factor (VEGF), fibroblast growth factor (FGF);
angiotensin receptor blocker; nitric oxide donors; anti-sense
oligionucleotides and combinations thereof; cell cycle inhibitors,
mTOR inhibitors, and growth factor signal transduction kinase
inhibitors.
[0070] As stated previously, the implantation of a coronary stent
in conjunction with balloon angioplasty is highly effective in
treating acute vessel closure and may reduce the risk of
restenosis. Intravascular ultrasound studies (Mintz et al., 1996)
suggest that coronary stenting effectively prevents vessel
constriction and that most of the late luminal loss after stent
implantation is due to plaque growth, probably related to
neointimal hyperplasia. The late luminal loss after coronary
stenting is almost two times higher than that observed after
conventional balloon angioplasty. Thus, inasmuch as stents prevent
at least a portion of the restenosis process, a combination of
drugs, agents or compounds which prevents smooth muscle cell
proliferation, reduces inflammation and reduces coagulation or
prevents smooth muscle cell proliferation by multiple mechanisms,
reduces inflammation and reduces coagulation combined with a stent
may provide the most efficacious treatment for post-angioplasty
restenosis. The systemic use of drugs, agents or compounds in
combination with the local delivery of the same or different
drug/drug combinations may also provide a beneficial treatment
option.
[0071] The local delivery of drug/drug combinations from a stent
has the following advantages; namely, the prevention of vessel
recoil and remodeling through the scaffolding action of the stent
and the prevention of multiple components of neointimal hyperplasia
or restenosis as well as a reduction in inflammation and
thrombosis. This local administration of drugs, agents or compounds
to stented coronary arteries may also have additional therapeutic
benefit. For example, higher tissue concentrations of the drugs,
agents or compounds may be achieved utilizing local delivery,
rather than systemic administration. In addition, reduced systemic
toxicity may be achieved utilizing local delivery rather than
systemic administration while maintaining higher tissue
concentrations. Also in utilizing local delivery from a stent
rather than systemic administration, a single procedure may suffice
with better patient compliance. An additional benefit of
combination drug, agent, and/or compound therapy may be to reduce
the dose of each of the therapeutic drugs, agents or compounds,
thereby limiting their toxicity, while still achieving a reduction
in restenosis, inflammation and thrombosis. Local stent-based
therapy is therefore a means of improving the therapeutic ratio
(efficacy/toxicity) of anti-restenosis, anti-inflammatory,
anti-thrombotic drugs, agents or compounds.
[0072] There are a multiplicity of different stents that may be
utilized following percutaneous transluminal coronary angioplasty.
Although any number of stents may be utilized in accordance with
the present invention, for simplicity, a limited number of stents
will be described in exemplary embodiments of the present
invention. The skilled artisan will recognize that any number of
stents may be utilized in connection with the present invention. In
addition, as stated above, other medical devices may be
utilized.
[0073] A stent is commonly used as a tubular structure left inside
the lumen of a duct to relieve an obstruction. Commonly, stents are
inserted into the lumen in a non-expanded form and are then
expanded autonomously, or with the aid of a second device in situ.
A typical method of expansion occurs through the use of a
catheter-mounted angioplasty balloon which is inflated within the
stenosed vessel or body passageway in order to shear and disrupt
the obstructions associated with the wall components of the vessel
and to obtain an enlarged lumen.
[0074] FIG. 1 illustrates an exemplary stent 100 which may be
utilized in accordance with an exemplary embodiment of the present
invention. The expandable cylindrical stent 100 comprises a
fenestrated structure for placement in a blood vessel, duct or
lumen to hold the vessel, duct or lumen open, more particularly for
protecting a segment of artery from restenosis after angioplasty.
The stent 100 may be expanded circumferentially and maintained in
an expanded configuration, that is circumferentially or radially
rigid. The stent 100 is axially flexible and when flexed at a band,
the stent 100 avoids any externally-protruding component parts.
[0075] The stent 100 generally comprises first and second ends with
an intermediate section therebetween. The stent 100 has a
longitudinal axis and comprises a plurality of longitudinally
disposed bands 102, wherein each band 102 defines a generally
continuous wave along a line segment parallel to the longitudinal
axis. A plurality of circumferentially arranged links 104 maintain
the bands 102 in a substantially tubular structure. Essentially,
each longitudinally disposed band 102 is connected at a plurality
of periodic locations, by a short circumferentially arranged link
104 to an adjacent band 102. The wave associated with each of the
bands 102 has approximately the same fundamental spatial frequency
in the intermediate section, and the bands 102 are so disposed that
the wave associated with them are generally aligned so as to be
generally in phase with one another. As illustrated in the figure,
each longitudinally arranged band 102 undulates through
approximately two cycles before there is a link to an adjacent band
102.
[0076] The stent 100 may be fabricated utilizing any number of
methods. For example, the stent 100 may be fabricated from a hollow
or formed stainless steel tube that may be machined using lasers,
electric discharge milling, chemical etching or other means. The
stent 100 is inserted into the body and placed at the desired site
in an unexpanded form. In one exemplary embodiment, expansion may
be effected in a blood vessel by a balloon catheter, where the
final diameter of the stent 100 is a function of the diameter of
the balloon catheter used.
[0077] It should be appreciated that a stent 100 in accordance with
the present invention may be embodied in a shape-memory material,
including, for example, an appropriate alloy of nickel and titanium
or stainless steel. Structures formed from stainless steel may be
made self-expanding by configuring the stainless steel in a
predetermined manner, for example, by twisting it into a braided
configuration. In this embodiment after the stent 100 has been
formed it may be compressed so as to occupy a space sufficiently
small as to permit its insertion in a blood vessel or other tissue
by insertion means, wherein the insertion means include a suitable
catheter, or flexible rod. On emerging from the catheter, the stent
100 may be configured to expand into the desired configuration
where the expansion is automatic or triggered by a change in
pressure, temperature or electrical stimulation.
[0078] FIG. 2 illustrates an exemplary embodiment of the present
invention utilizing the stent 100 illustrated in FIG. 1. As
illustrated, the stent 100 may be modified to comprise one or more
reservoirs 106. Each of the reservoirs 106 may be opened or closed
as desired. These reservoirs 106 may be specifically designed to
hold the drug/drug combinations to be delivered. Regardless of the
design of the stent 100, it is preferable to have the drug/drug
combination dosage applied with enough specificity and a sufficient
concentration to provide an effective dosage in the lesion area. In
this regard, the reservoir size in the bands 102 is preferably
sized to adequately apply the drug/drug combination dosage at the
desired location and in the desired amount.
[0079] In an alternate exemplary embodiment, the entire inner and
outer surface of the stent 100 may be coated with drug/drug
combinations in therapeutic dosage amounts. A detailed description
of a drug for treating restenosis, as well as exemplary coating
techniques, is described below. It is, however, important to note
that the coating techniques may vary depending on the drug/drug
combinations. Also, the coating techniques may vary depending on
the material comprising the stent or other intraluminal medical
device.
[0080] FIG. 26 illustrates another exemplary embodiment of a
balloon-expandable stent. FIG. 26 illustrates the stent 900 in its
crimped, pre-deployed state as it would appear if it were cut
longitudinally and then laid out into a flat, two-dimensional
configuration. The stent 900 has curved end struts 902 and diagonal
struts 904 with each set of strut members 906 connected by sets of
flexible links 908, 910 or 912. In this exemplary embodiment, three
different types of flexible links are used. A set of "N" links 910
comprising six circumferentially spaced "N" links 914 and a set of
inverted "N" links 912 comprising six circumferentially spaced
inverted "N" links 916 each connect to adjacent sets of strut
members 906 at the ends of the stent 900. A set of inverted "J"
links 918 comprising six circumferentially spaced inverted "J"
links 908 are used to connect the adjacent sets of strut members
906 in the center of the stent 900. The shape of the "N" links 914
and inverted "N" links 916 facilitate the links' ability to
lengthen and shorten as the stent bends around a curve during
delivery into the human body. This ability to lengthen and shorten
helps to prevent the sets of strut members from being pushed or
pulled off the balloon during delivery into the body and is
particularly applicable to short stents which tend to have
relatively poor stent retention onto an inflatable balloon. The
stent 900 with its greater strength at its central region would
advantageously be used for comparatively short stenoses that have a
tough, calcified central section. It should also be understood that
a regular "J" link could be used for the stent 900 in place of the
inverted "J" link 908. Other exemplary embodiments of balloon
expandable stents may be found in U.S. Pat. No. 6,190,403 B1 issued
on Feb. 20, 2001 and which is incorporated by reference herein.
[0081] Rapamycin is a macrocyclic triene antibiotic produced by
Streptomyces hygroscopicus as disclosed in U.S. Pat. No. 3,929,992.
It has been found that rapamycin among other things inhibits the
proliferation of vascular smooth muscle cells in vivo. Accordingly,
rapamycin may be utilized in treating intimal smooth muscle cell
hyperplasia, restenosis, and vascular occlusion in a mammal,
particularly following either biologically or mechanically mediated
vascular injury, or under conditions that would predispose a mammal
to suffering such a vascular injury. Rapamycin functions to inhibit
smooth muscle cell proliferation and does not interfere with the
re-endothelialization of the vessel walls.
[0082] Rapamycin reduces vascular hyperplasia by antagonizing
smooth muscle proliferation in response to mitogenic signals that
are released during an angioplasty induced injury. Inhibition of
growth factor and cytokine mediated smooth muscle proliferation at
the late G1 phase of the cell cycle is believed to be the dominant
mechanism of action of rapamycin. However, rapamycin is also known
to prevent T-cell proliferation and differentiation when
administered systemically. This is the basis for its
immunosuppresive activity and its ability to prevent graft
rejection.
[0083] As used herein, rapamycin includes rapamycin and all
analogs, derivatives and congeners that find FKBP12, and other
immunophilins, and possesses the same pharmacologic properties as
rapamycin.
[0084] Although the anti-proliferative effects of rapamycin may be
achieved through systemic use, superior results may be achieved
through the local delivery of the compound. Essentially, rapamycin
works in the tissues, which are in proximity to the compound, and
has diminished effect as the distance from the delivery device
increases. In order to take advantage of this effect, one would
want the rapamycin in direct contact with the lumen walls.
Accordingly, in a preferred embodiment, the rapamycin is
incorporated onto the surface of the stent or portions thereof.
Essentially, the rapamycin is preferably incorporated into the
stent 100, illustrated in FIG. 1, where the stent 100 makes contact
with the lumen wall.
[0085] Rapamycin may be incorporated onto or affixed to the stent
in a number of ways. In the exemplary embodiment, the rapamycin is
directly incorporated into a polymeric matrix and sprayed onto the
outer surface of the stent. The rapamycin elutes from the polymeric
matrix over time and enters the surrounding tissue. The rapamycin
preferably remains on the stent for at least three days up to
approximately six months, and more preferably between seven and
thirty days.
[0086] Any number of non-erodible polymers may be utilized in
conjunction with the rapamycin. In one exemplary embodiment, the
polymeric matrix comprises two layers. The base layer comprises a
solution of poly(ethylene-co-vinylacetate) and
polybutylmethacrylate. The rapamycin is incorporated into this base
layer. The outer layer comprises only polybutylmethacrylate and
acts as a diffusion barrier to prevent the rapamycin from eluting
too quickly. The thickness of the outer layer or top coat
determines the rate at which the rapamycin elutes from the matrix.
Essentially, the rapamycin elutes from the matrix by diffusion
through the polymer matrix. Polymers are permeable, thereby
allowing solids, liquids and gases to escape therefrom. The total
thickness of the polymeric matrix is in the range from about one
micron to about twenty microns or greater. It is important to note
that primer layers and metal surface treatments may be utilized
before the polymeric matrix is affixed to the medical device. For
example, acid cleaning, alkaline (base) cleaning, salinization and
parylene deposition may be used as part of the overall process
described below.
[0087] The poly(ethylene-co-vinylacetate), polybutylmethacrylate
and rapamycin solution may be incorporated into or onto the stent
in a number of ways. For example, the solution may be sprayed onto
the stent or the stent may be dipped into the solution. Other
methods include spin coating and RF-plasma polymerization. In one
exemplary embodiment, the solution is sprayed onto the stent and
then allowed to dry. In another exemplary embodiment, the solution
may be electrically charged to one polarity and the stent
electrically changed to the opposite polarity. In this manner, the
solution and stent will be attracted to one another. In using this
type of spraying process, waste may be reduced and more precise
control over the thickness of the coat may be achieved.
[0088] In another exemplary embodiment, the rapamycin or other
therapeutic agent may be incorporated into a film-forming
polyfluoro copolymer comprising an amount of a first moiety
selected from the group consisting of polymerized
vinylidenefluoride and polymerized tetrafluoroethylene, and an
amount of a second moiety other than the first moiety and which is
copolymerized with the first moiety, thereby producing the
polyfluoro copolymer, the second moiety being capable of providing
toughness or elastomeric properties to the polyfluoro copolymer,
wherein the relative amounts of the first moiety and the second
moiety are effective to provide the coating and film produced
therefrom with properties effective for use in treating implantable
medical devices.
[0089] The present invention provides polymeric coatings comprising
a polyfluoro copolymer and implantable medical devices, for
example, stents coated with a film of the polymeric coating in
amounts effective to reduce thrombosis and/or restenosis when such
stents are used in, for example, angioplasty procedures. As used
herein, polyfluoro copolymers means those copolymers comprising an
amount of a first moiety selected from the group consisting of
polymerized vinylidenefluoride and polymerized tetrafluoroethylene,
and an amount of a second moiety other than the first moiety and
which is copolymerized with the first moiety to produce the
polyfluoro copolymer, the second moiety being capable of providing
toughness or elastomeric properties to the polyfluoro copolymer,
wherein the relative amounts of the first moiety and the second
moiety are effective to provide coatings and film made from such
polyfluoro copolymers with properties effective for use in coating
implantable medical devices.
[0090] The coatings may comprise pharmaceutical or therapeutic
agents for reducing restenosis, inflammation and/or thrombosis, and
stents coated with such coatings may provide sustained release of
the agents. Films prepared from certain polyfluoro copolymer
coatings of the present invention provide the physical and
mechanical properties required of conventional coated medical
devices, even where maximum temperature, to which the device
coatings and films are exposed, are limited to relatively low
temperatures. This is particularly important when using the
coating/film to deliver pharmaceutical/therapeutic agents or drugs
that are heat sensitive, or when applying the coating onto
temperature-sensitive devices such as catheters. When maximum
exposure temperature is not an issue, for example, where
heat-stable agents such as itraconazole are incorporated into the
coatings, higher melting thermoplastic polyfluoro copolymers may be
used and, if very high elongation and adhesion is required,
elastomers may be used. If desired or required, the polyfluoro
elastomers may be crosslinked by standard methods described in,
e.g., Modern Fluoropolymers, (J. Shires ed.) John Wiley & Sons,
New York, 1997, pp. 77-87.
[0091] The present invention comprises polyfluoro copolymers that
provide improved biocompatible coatings or vehicles for medical
devices. These coatings provide inert biocompatible surfaces to be
in contact with body tissue of a mammal, for example, a human,
sufficient to reduce restenosis, or thrombosis, or other
undesirable reactions. While many reported coatings made from
polyfluoro homopolymers are insoluble and/or require high heat, for
example, greater than about one hundred twenty-five degrees
centigrade, to obtain films with adequate physical and mechanical
properties for use on implantable devices, for example, stents, or
are not particularly tough or elastomeric, films prepared from the
polyfluoro copolymers of the present invention provide adequate
adhesion, toughness or elasticity, and resistance to cracking when
formed on medical devices. In certain exemplary embodiments, this
is the case even where the devices are subjected to relatively low
maximum temperatures.
[0092] The polyfluoro copolymers used for coatings according to the
present invention are preferably film-forming polymers that have
molecular weight high enough so as not to be waxy or tacky. The
polymers and films formed therefrom should preferably adhere to the
stent and not be readily deformable after deposition on the stent
as to be able to be displaced by hemodynamic stresses. The polymer
molecular weight should preferably be high enough to provide
sufficient toughness so that films comprising the polymers will not
be rubbed off during handling or deployment of the stent. In
certain exemplary embodiments the coating will not crack where
expansion of the stent or other medical devices occurs.
[0093] Coatings of the present invention comprise polyfluoro
copolymers, as defined hereinabove. The second moiety polymerized
with the first moiety to prepare the polyfluoro copolymer may be
selected from those polymerized, biocompatible monomers that would
provide biocompatible polymers acceptable for implantation in a
mammal, while maintaining sufficient elastomeric film properties
for use on medical devices claimed herein. Such monomers include,
without limitation, hexafluoropropylene (HFP), tetrafluoroethylene
(TFE), vinylidenefluoride, 1-hydropentafluoropropylene,
perfluoro(methyl vinyl ether), chlorotrifluoroethylene (CTFE),
pentafluoropropene, trifluoroethylene, hexafluoroacetone and
hexafluoroisobutylene.
[0094] Polyfluoro copolymers used in the present invention
typically comprise vinylidinefluoride copolymerized with
hexafluoropropylene, in the weight ratio in the range of from about
fifty to about ninety-two weight percent vinylidinefluoride to
about fifty to about eight weight percent HFP. Preferably,
polyfluoro copolymers used in the present invention comprise from
about fifty to about eighty-five weight percent vinylidinefluoride
copolymerized with from about fifty to about fifteen weight percent
HFP. More preferably, the polyfluoro copolymers will comprise from
about fifty-five to about seventy weight percent vinylidineflyoride
copolymerized with from about forty-five to about thirty weight
percent HFP. Even more preferably, polyfluoro copolymers comprise
from about fifty-five to about sixty-five weight percent
vinylidinefluoride copolymerized with from about forty-five to
about thirty-five weight percent HFP. Such polyfluoro copolymers
are soluble, in varying degrees, in solvents such as
dimethylacetamide (DMAc), tetrahydrofuran, dimethyl formamide,
dimethyl sulfoxide and n-methyl pyrrolidone. Some are soluble in
methylethylketone (MEK), acetone, methanol and other solvents
commonly used in applying coatings to conventional implantable
medical devices.
[0095] Conventional polyfluoro homopolymers are crystalline and
difficult to apply as high quality films onto metal surfaces
without exposing the coatings to relatively high temperatures that
correspond to the melting temperature (Tm) of the polymer. The
elevated temperature serves to provide films prepared from such
PVDF homopolymer coatings that exhibit sufficient adhesion of the
film to the device, while preferably maintaining sufficient
flexibility to resist film cracking upon expansion/contraction of
the coated medical device. Certain films and coatings according to
the present invention provide these same physical and mechanical
properties, or essentially the same properties, even when the
maximum temperatures to which the coatings and films are exposed is
less than about a maximum predetermined temperature. This is
particularly important when the coatings/films comprise
pharmaceutical or therapeutic agents or drugs that are heat
sensitive, for example, subject to chemical or physical degradation
or other heat-induced negative affects, or when coating heat
sensitive substrates of medical devices, for example, subject to
heat-induced compositional or structural degradation.
[0096] Depending on the particular device upon which the coatings
and films of the present invention are to be applied and the
particular use/result required of the device, polyfluoro copolymers
used to prepare such devices may be crystalline, semi-crystalline
or amorphous.
[0097] Where devices have no restrictions or limitations with
respect to exposure of same to elevated temperatures, crystalline
polyfluoro copolymers may be employed. Crystalline polyfluoro
copolymers tend to resist the tendency to flow under applied stress
or gravity when exposed to temperatures above their glass
transition (Tg) temperatures. Crystalline polyfluoro copolymers
provide tougher coatings and films than their fully amorphous
counterparts. In addition, crystalline polymers are more lubricious
and more easily handled through crimping and transfer processes
used to mount self-expanding stents, for example, nitinol
stents.
[0098] Semi-crystalline and amorphous polyfluoro copolymers are
advantageous where exposure to elevated temperatures is an issue,
for example, where heat-sensitive pharmaceutical or therapeutic
agents are incorporated into the coatings and films, or where
device design, structure and/or use preclude exposure to such
elevated temperatures. Semi-crystalline polyfluoro copolymer
elastomers comprising relatively high levels, for example, from
about thirty to about forty-five weight percent of the second
moiety, for example, HFP, copolymerized with the first moiety, for
example, VDF, have the advantage of reduced coefficient of friction
and self-blocking relative to amorphous polyfluoro copolymer
elastomers. Such characteristics may be of significant value when
processing, packaging and delivering medical devices coated with
such polyfluoro copolymers. In addition, such polyfluoro copolymer
elastomers comprising such relatively high content of the second
moiety serves to control the solubility of certain agents, for
example, rapamycin, in the polymer and therefore controls
permeability of the agent through the matrix.
[0099] Polyfluoro copolymers utilized in the present inventions may
be prepared by various known polymerization methods. For example,
high pressure, free-radical, semi-continuous emulsion
polymerization techniques such as those disclosed in
Fluoroelastomers-dependence of relaxation phenomena on
compositions, POLYMER 30, 2180, 1989, by Ajroldi, et al., may be
employed to prepare amorphous polyfluoro copolymers, some of which
may be elastomers. In addition, free-radical batch emulsion
polymerization techniques disclosed herein may be used to obtain
polymers that are semi-crystalline, even where relatively high
levels of the second moiety are included.
[0100] As described above, stents may comprise a wide variety of
materials and a wide variety of geometrics. Stents may be made of
biocomptible materials, including biostable and bioabsorbable
materials. Suitable biocompatible metals include, but are not
limited to, stainless steel, tantalum, titanium alloys (including
nitinol), and cobalt alloys (including cobalt-chromium nickel
alloys). Suitable nonmetallic biocompatible materials include, but
are not limited to, polyamides, polyolefins (i.e. polypropylene,
polyethylene etc.), nonabsorbable polyesters (i.e. polyethylene
terephthalate), and bioabsorbable aliphatic polyesters (i.e.
homopolymers and copolymers of lactic acid, glycolic acid, lactide,
glycolide, para-dioxanone, trimethylene carbonate,
.epsilon.-caprolactone, and blends thereof).
[0101] The film-forming biocompatible polymer coatings generally
are applied to the stent in order to reduce local turbulence in
blood flow through the stent, as well as adverse tissue reactions.
The coatings and films formed therefrom also may be used to
administer a pharmaceutically active material to the site of the
stent placement. Generally, the amount of polymer coating to be
applied to the stent will vary depending on, among other possible
parameters, the particular polyfluoro copolymer used to prepare the
coating, the stent design and the desired effect of the coating.
Generally, the coated stent will comprise from about 0.1 to about
fifteen weight percent of the coating, preferably from about 0.4 to
about ten weight percent. The polyfluoro copolymer coatings may be
applied in one or more coating steps, depending on the amount of
polyfluoro copolymer to be applied. Different polyfluoro copolymers
may be used for different layers in the stent coating. In fact, in
certain exemplary embodiments, it is highly advantageous to use a
diluted first coating solution comprising a polyfluoro copolymer as
a primer to promote adhesion of a subsequent polyfluoro copolymer
coating layer that may include pharmaceutically active materials.
The individual coatings may be prepared from different polyfluoro
copolymers.
[0102] Additionally, a top coating may be applied to delay release
of the pharmaceutical agent, or they could be used as the matrix
for the delivery of a different pharmaceutically active material.
Layering of coatings may be used to stage release of the drug or to
control release of different agents placed in different layers.
[0103] Blends of polyfluoro copolymers may also be used to control
the release rate of different agents or to provide a desirable
balance of coating properties, i.e. elasticity, toughness, etc.,
and drug delivery characteristics, for example, release profile.
Polyfluoro copolymers with different solubilities in solvents may
be used to build up different polymer layers that may be used to
deliver different drugs or to control the release profile of a
drug. For example, polyfluoro copolymers comprising 85.5/14.5
(wt/wt) of poly(vinylidinefluoride/HFP) and 60.6/39.4 (wt/wt) of
poly(vinylidinefluoride /HFP) are both soluble in DMAc. However,
only the 60.6/39.4 PVDF polyfluoro copolymer is soluble in
methanol. So, a first layer of the 85.5/14.5 PVDF polyfluoro
copolymer comprising a drug could be over coated with a topcoat of
the 60.6/39.4 PVDF polyfluoro copolymer made with the methanol
solvent. The top coating may be used to delay the drug delivery of
the drug contained in the first layer. Alternately, the second
layer could comprise a different drug to provide for sequential
drug delivery. Multiple layers of different drugs could be provided
by alternating layers of first one polyfluoro copolymer, then the
other. As will be readily appreciated by those skilled in the art,
numerous layering approaches may be used to provide the desired
drug delivery.
[0104] Coatings may be formulated by mixing one or more therapeutic
agents with the coating polyfluoro copolymers in a coating mixture.
The therapeutic agent may be present as a liquid, a finely divided
solid, or any other appropriate physical form. Optionally, the
coating mixture may include one or more additives, for example,
nontoxic auxiliary substances such as diluents, carriers,
excipients, stabilizers or the like. Other suitable additives may
be formulated with the polymer and pharmaceutically active agent or
compound. For example, a hydrophilic polymer may be added to a
biocompatible hydrophobic coating to modify the release profile, or
a hydrophobic polymer may be added to a hydrophilic coating to
modify the release profile. One example would be adding a
hydrophilic polymer selected from the group consisting of
polyethylene oxide, polyvinyl pyrrolidone, polyethylene glycol,
carboxylmethyl cellulose, and hydroxymethyl cellulose to a
polyfluoro copolymer coating to modify the release profile.
Appropriate relative amounts may be determined by monitoring the in
vitro and/or in vivo release profiles for the therapeutic
agents.
[0105] The best conditions for the coating application are when the
polyfluoro copolymer and pharmaceutic agent have a common solvent.
This provides a wet coating that is a true solution. Less
desirable, yet still usable, are coatings that contain the
pharmaceutical agent as a solid dispersion in a solution of the
polymer in solvent. Under the dispersion conditions, care must be
taken to ensure that the particle size of the dispersed
pharmaceutical powder, both the primary powder size and its
aggregates and agglomerates, is small enough not to cause an
irregular coating surface or to clog the slots of the stent that
need to remain essentially free of coating. In cases where a
dispersion is applied to the stent and the smoothness of the
coating film surface requires improvement, or to be ensured that
all particles of the drug are fully encapsulated in the polymer, or
in cases where the release rate of the drug is to be slowed, a
clear (polyfluoro copolymer only) topcoat of the same polyfluoro
copolymer used to provide sustained release of the drug or another
polyfluoro copolymer that further restricts the diffusion of the
drug out of the coating may be applied. The topcoat may be applied
by dip coating with mandrel to clear the slots. This method is
disclosed in U.S. Pat. No. 6,153,252. Other methods for applying
the topcoat include spin coating and spray coating. Dip coating of
the topcoat can be problematic if the drug is very soluble in the
coating solvent, which swells the polyfluoro copolymer, and the
clear coating solution acts as a zero concentration sink and
redissolves previously deposited drug. The time spent in the dip
bath may need to be limited so that the drug is not extracted out
into the drug-free bath. Drying should be rapid so that the
previously deposited drug does not completely diffuse into the
topcoat.
[0106] The amount of therapeutic agent will be dependent upon the
particular drug employed and medical condition being treated.
Typically, the amount of drug represents about 0.001 percent to
about seventy percent, more typically about 0.001 percent to about
sixty percent.
[0107] The quantity and type of polyfluoro copolymers employed in
the coating film comprising the pharmaceutic agent will vary
depending on the release profile desired and the amount of drug
employed. The product may contain blends of the same or different
polyfluoro copolymers having different molecular weights to provide
the desired release profile or consistency to a given
formulation.
[0108] Polyfluoro copolymers may release dispersed drug by
diffusion. This can result in prolonged delivery (over, say
approximately one to two-thousand hours, preferably two to
eight-hundred hours) of effective amounts (0.001 .mu.g/cm.sup.2-min
to 1000 .mu.g/cm.sup.2-min) of the drug. The dosage may be tailored
to the subject being treated, the severity of the affliction, the
judgment of the prescribing physician, and the like.
[0109] Individual formulations of drugs and polyfluoro copolymers
may be tested in appropriate in vitro and in vivo models to achieve
the desired drug release profiles. For example, a drug could be
formulated with a polyfluoro copolymer, or blend of polyfluoro
copolymers, coated onto a stent and placed in an agitated or
circulating fluid system, for example, twenty-five percent ethanol
in water. Samples of the circulating fluid could be taken to
determine the release profile (such as by HPLC, UV analysis or use
of radiotagged molecules). The release of a pharmaceutical compound
from a stent coating into the interior wall of a lumen could be
modeled in appropriate animal system. The drug release profile
could then be monitored by appropriate means such as, by taking
samples at specific times and assaying the samples for drug
concentration (using HPLC to detect drug concentration). Thrombus
formation can be modeled in animal models using the In-platelet
imaging methods described by Hanson and Harker, Proc. Natl. Acad.
Sci. USA 85:3184-3188 (1988). Following this or similar procedures,
those skilled in the art will be able to formulate a variety of
stent coating formulations.
[0110] While not a requirement of the present invention, the
coatings and films may be crosslinked once applied to the medical
devices. Crosslinking may be affected by any of the known
crosslinking mechanisms, such as chemical, heat or light. In
addition, crosslinking initiators and promoters may be used where
applicable and appropriate. In those exemplary embodiments
utilizing crosslinked films comprising pharmaceutical agents,
curing may affect the rate at which the drug diffuses from the
coating. Crosslinked polyfluoro copolymers films and coatings of
the present invention also may be used without drug to modify the
surface of implantable medical devices.
EXAMPLES
Example 1
[0111] A PVDF homopolymer (Solef.RTM. 1008 from Solvay Advanced
Polymers, Houston, Tex., Tm about 175.degree. C.) and polyfluoro
copolymers of poly(vinylidenefluoride/HFP), 92/8 and 91/9 weight
percent vinylidenefluoride/HFP as determined by F.sup.19 NMR,
respectively (eg: Solef.RTM. 11010 and 11008, Solvay Advanced
Polymers, Houston, Tex., Tm about 159 degrees C. and 160 degrees
C., respectively) were examined as potential coatings for stents.
These polymers are soluble in solvents such as, but not limited to,
DMAc, N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO),
N-methylpyrrolidone (NMP), tetrahydrofuran (THF) and acetone.
Polymer coatings were prepared by dissolving the polymers in
acetone, at five weight percent as a primer, or by dissolving the
polymer in 50/50 DMAc/acetone, at thirty weight percent as a
topcoat. Coatings that were applied to the stents by dipping and
dried at 60 degrees C. in air for several hours, followed by 60
degrees C. for three hours in a <100 mm Hg vacuum, resulted in
white foamy films. As applied, these films adhered poorly to the
stent and flaked off, indicating they were too brittle. When stents
coated in this manner were heated above 175 degrees C., i.e. above
the melting temperature of the polymer, a clear, adherent film was
formed. Since coatings require high temperatures, for example,
above the melting temperature of the polymer, to achieve high
quality films. As mentioned above, the high temperature heat
treatment is unacceptable for the majority of drug compounds due to
their thermal sensitivity.
Example 2
[0112] A polyfluoro copolymer (Solef.RTM. 21508) comprising 85.5
weight percent vinylidenefluoride copolymerized with 14.5 weight
percent HFP, as determined by F.sup.19 NMR, was evaluated. This
copolymer is less crystalline than the polyfluoro homopolymer and
copolymers described in Example 1. It also has a lower melting
point reported to be about 133 degrees C. Once again, a coating
comprising about twenty weight percent of the polyfluoro copolymer
was applied from a polymer solution in 50/50 DMAc/MEK. After drying
(in air) at 60 degrees C. for several hours, followed by 60 degrees
C. for three hours in a <100 mtorr Hg vacuum, clear adherent
films were obtained. This eliminated the need for a high
temperature heat treatment to achieve high quality films. Coatings
were smoother and more adherent than those of Example 1. Some
coated stents that underwent expansion show some degree of adhesion
loss and "tenting" as the film pulls away from the metal. Where
necessary, modification of coatings containing such copolymers may
be made, e.g. by addition of plasticizers or the like to the
coating compositions. Films prepared from such coatings may be used
to coat stents or other medical devices, particularly where those
devices are not susceptible to expansion to the degree of the
stents.
[0113] The coating process above was repeated, this time with a
coating comprising the 85.5/14.6 (wt/wt) (vinylidenefluoride/HFP)
and about thirty weight percent of rapamycin (Wyeth-Ayerst
Laboratories, Philadelphia, Pa.), based on total weight of coating
solids. Clear films that would occasionally crack or peel upon
expansion of the coated stents resulted. It is believed that
inclusion of plasticizers and the like in the coating composition
will result in coatings and films for use on stents and other
medical devices that are not susceptible to such cracking and
peeling.
Example 3
[0114] Polyfluoro copolymers of still higher HFP content were then
examined. This series of polymers were not semicrystalline, but
rather are marketed as elastomers. One such copolymer is
Fluorel.TM. FC2261Q (from Dyneon, a 3M-Hoechst Enterprise, Oakdale,
Minn.), a 60.6/39.4 (wt/wt) copolymer of vinylidenefluoride/HFP.
Although this copolymer has a Tg well below room temperature (Tg
about minus twenty degrees C.) it is not tacky at room temperature
or even at sixty degrees C. This polymer has no detectable
crystallinity when measured by Differential Scanning Calorimetry
(DSC) or by wide angle X-ray diffraction. Films formed on stents as
described above were non-tacky, clear, and expanded without
incident when the stents were expanded.
[0115] The coating process above was repeated, this time with
coatings comprising the 60.6/39.4 (wt/wt) (vinylidenefluoride/HFP)
and about nine, thirty and fifty weight percent of rapamycin
(Wyeth-Ayerst Laboratories, Philadelphia, Pa.), based on total
weight of coating solids, respectively. Coatings comprising about
nine and thirty weight percent rapamycin provided white, adherent,
tough films that expanded without incident on the stent. Inclusion
of fifty percent drug, in the same manner, resulted in some loss of
adhesion upon expansion.
[0116] Changes in the comonomer composition of the polyfluoro
copolymer also can affect the nature of the solid state coating,
once dried. For example, the semicrystalline copolymer, Solef.RTM.
21508, containing 85.5 percent vinylidenefluoride polymerized with
14.5 percent by weight HFP forms homogeneous solutions with about
30 percent rapamycin (drug weight divided by total solids weight,
for example, drug plus copolymer) in DMAc and 50/50 DMAc/MEK. When
the film is dried (60 degrees C./16 hours followed by 60 degrees
C./3 hours in vacuum of 100 mm Hg) a clear coating, indicating a
solid solution of the drug in the polymer, is obtained. Conversely,
when an amorphous copolymer, Fluorel.TM. FC2261Q, of PDVF/HFP at
60.6/39.5 (wt/wt) forms a similar thirty percent solution of
rapamycin in DMAc/MEK and is similarly dried, a white film,
indicating phase separation of the drug and the polymer, is
obtained. This second drug containing film is much slower to
release the drug into an in vitro test solution of twenty-five
percent ethanol in water than is the former clear film of
crystalline Solef.RTM. 21508. X-ray analysis of both films
indicates that the drug is present in a non-crystalline form. Poor
or very low solubility of the drug in the high HFP containing
copolymer results in slow permeation of the drug through the thin
coating film. Permeability is the product of diffusion rate of the
diffusing species (in this case the drug) through the film (the
copolymer) and the solubility of the drug in the film.
Example 4
In vitro Release Results of Rapamycin from Coating
[0117] FIG. 3 is a plot of data for the 85.5/14.5
vinylidenefluoride/HFP polyfluoro copolymer, indicating fraction of
drug released as a function of time, with no topcoat. FIG. 4 is a
plot of data for the same polyfluoro copolymer over which a topcoat
has been disposed, indicating that most effect on release rate is
with a clear topcoat. As shown therein, TC150 refers to a device
comprising one hundred fifty micrograms of topcoat, TC235 refers to
two hundred thirty-five micrograms of topcoat, etc. The stents
before topcoating had an average of seven hundred fifty micrograms
of coating containing thirty percent rapamycin. FIG. 5 is a plot
for the 60.6/39.4 vinylidenefluoride/HFP polyfluoro copolymer,
indicating fraction of drug released as a function of time, showing
significant control of release rate from the coating without the
use of a topcoat. Release is controlled by loading of drug in the
film.
Example 5
In vivo Stent Release Kinetics of Rapamycin from Poly(VDF/HFP)
[0118] Nine New Zealand white rabbits (2.5-3.0 kg) on a normal diet
were given aspirin twenty-four hours prior to surgery, again just
prior to surgery and for the remainder of the study. At the time of
surgery, animals were premedicated with Acepromazine (0.1-0.2
mg/kg) and anesthetized with a Ketamine/Xylazine mixture (40 mg/kg
and 5 mg/kg, respectively). Animals were given a single
intraprocedural dose of heparin (150 IU/kg, i.v.)
[0119] Arteriectomy of the right common carotid artery was
performed and a 5 F catheter introducer (Cordis, Inc.) placed in
the vessel and anchored with ligatures. Iodine contrast agent was
injected to visualize the right common carotid artery,
brachlocephalic trunk and aortic arch. A steerable guide wire
(0.014 inch/180 cm, Cordis, Inc.) was inserted via the introducer
and advanced sequentially into each iliac artery to a location
where the artery possesses a diameter closest to 2 mm using the
angiographic mapping done previously. Two stents coated with a film
made of poly(VDF/HFP): (60.6/39.4) with thirty percent rapamycin
were deployed in each animal where feasible, one in each iliac
artery, using 3.0 mm balloon and inflation to 8-10 ATM for thirty
seconds followed after a one minute interval by a second inflation
to 8-10 ATM for thirty seconds. Follow-up angiographs visualizing
both iliac arteries are obtained to confirm correct deployment
position of the stent.
[0120] At the end of procedure, the carotid artery was ligated and
the skin is closed with 3/0 vicryl suture using a one layered
interrupted closure. Animals were given butoropanol (0.4 mg/kg,
s.c.) and gentamycin (4 mg/kg, i.m.). Following recovery, the
animals were returned to their cages and allowed free access to
food and water.
[0121] Due to early deaths and surgical difficulties, two animals
were not used in this analysis. Stented vessels were removed from
the remaining seven animals at the following time points: one
vessel (one animal) at ten minutes post implant; six vessels (three
animals) between forty minutes and two hours post-implant (average,
1.2 hours); two vessels (two animals) at three days post implant;
and two vessels (one animal) at seven days post-implant. In one
animal at two hours, the stent was retrieved from the aorta rather
than the iliac artery. Upon removal, arteries were carefully
trimmed at both the proximal and distal ends of the stent. Vessels
were then carefully dissected free of the stent, flushed to remove
any residual blood, and both stent and vessel frozen immediately,
wrapped separately in foil, labeled and kept frozen at minus eighty
degrees C. When all samples had been collected, vessels and stents
were frozen, transported and subsequently analyzed for rapamycin in
tissue and results are illustrated in FIG. 4.
Example 6
Purifying the Polymer
[0122] The Fluorel.TM. FC2261Q copolymer was dissolved in MEK at
about ten weight percent and was washed in a 50/50 mixture of
ethanol/water at a 14:1 of ethanol/water to the MEK solution ratio.
The polymer precipitated out and was separated from the solvent
phase by centrifugation. The polymer again was dissolved in MEK and
the washing procedure repeated. The polymer was dried after each
washing step at sixty degrees C. in a vacuum oven (<200 mtorr)
over night.
Example 7
In vivo Testing of Coated Stents in Porcine Coronary Arteries
[0123] CrossFlex.RTM. stents (available from Cordis, a Johnson
& Johnson Company) were coated with the "as received"
Fluorel.TM. FC2261Q PVDF copolymer and with the purified polyfluoro
copolymer of Example 6, using the dip and wipe approach. The coated
stents were sterilized using ethylene oxide and a standard cycle.
The coated stents and bare metal stents (controls) were implanted
in porcine coronary arteries, where they remained for twenty-eight
days. P Angiography was performed on the pigs at implantation and
at twenty-eight days. Angiography indicated that the control
uncoated stent exhibited about twenty-one percent restenosis. The
polyfluoro copolymer "as received" exhibited about twenty-six
percent restenosis(equivalent to the control) and the washed
copolymer exhibited about 12.5 percent restenosis.
[0124] Histology results reported neointimal area at twenty-eight
days to be 2.89.+-.0.2, 3.57.+-.0.4 and 2.75.+-.0.3, respectively,
for the bare metal control, the unpurified copolymer and the
purified copolymer.
[0125] Since rapamycin acts by entering the surrounding tissue, it
is preferably only affixed to the surface of the stent making
contact with one tissue. Typically, only the outer surface of the
stent makes contact with the tissue. Accordingly, in one exemplary
embodiment, only the outer surface of the stent is coated with
rapamycin.
[0126] The circulatory system, under normal conditions, has to be
self-sealing, otherwise continued blood loss from an injury would
be life threatening. Typically, all but the most catastrophic
bleeding is rapidly stopped though a process known as hemostasis.
Hemostasis occurs through a progression of steps. At high rates of
flow, hemostasis is a combination of events involving platelet
aggregation and fibrin formation. Platelet aggregation leads to a
reduction in the blood flow due to the formation of a cellular plug
while a cascade of biochemical steps leads to the formation of a
fibrin clot.
[0127] Fibrin clots, as stated above, form in response to injury.
There are certain circumstances where blood clotting or clotting in
a specific area may pose a health risk. For example, during
percutaneous transluminal coronary angioplasty, the endothelial
cells of the arterial walls are typically injured, thereby exposing
the sub-endothelial cells. Platelets adhere to these exposed cells.
The aggregating platelets and the damaged tissue initiate further
biochemical process resulting in blood coagulation. Platelet and
fibrin blood clots may prevent the normal flow of blood to critical
areas. Accordingly, there is a need to control blood clotting in
various medical procedures. Compounds that do not allow blood to
clot are called anti-coagulants. Essentially, an anti-coagulant is
an inhibitor of thrombin formation or function. These compounds
include drugs such as heparin and hirudin. As used herein, heparin
includes all direct or indirect inhibitors of thrombin or Factor
Xa.
[0128] In addition to being an effective anti-coagulant, heparin
has also been demonstrated to inhibit smooth muscle cell growth in
vivo. Thus, heparin may be effectively utilized in conjunction with
rapamycin in the treatment of vascular disease. Essentially, the
combination of rapamycin and heparin may inhibit smooth muscle cell
growth via two different mechanisms in addition to the heparin
acting as an anti-coagulant.
[0129] Because of its multifunctional chemistry, heparin may be
immobilized or affixed to a stent in a number of ways. For example,
heparin may be immobilized onto a variety of surfaces by various
methods, including the photolink methods set forth in U.S. Pat.
Nos. 3,959,078 and 4,722,906 to Guire et al. and U.S. Pat. Nos.
5,229,172; 5,308,641; 5,350,800 and 5,415,938 to Cahalan et al.
Heparinized surfaces have also been achieved by controlled release
from a polymer matrix, for example, silicone rubber, as set forth
in U.S. Pat. Nos. 5,837,313; 6,099,562 and 6,120,536 to Ding et
al.
[0130] In one exemplary embodiment, heparin may be immobilized onto
the stent as briefly described below. The surface onto which the
heparin is to be affixed is cleaned with ammonium peroxidisulfate.
Once cleaned, alternating layers of polyethylenimine and dextran
sulfate are deposited thereon. Preferably, four layers of the
polyethylenimine and dextran sulfate are deposited with a final
layer of polyethylenimine. Aldehyde-end terminated heparin is then
immobilized to this final layer and stabilized with sodium
cyanoborohydride. This process is set forth in U.S. Pat. Nos.
4,613,665; 4,810,784 to Larm and U.S. Pat. No. 5,049,403 to Larm et
al.
[0131] Unlike rapamycin, heparin acts on circulating proteins in
the blood and heparin need only make contact with blood to be
effective. Accordingly, if used in conjunction with a medical
device, such as a stent, it would preferably be only on the side
that comes into contact with the blood. For example, if heparin
were to be administered via a stent, it would only have to be on
the inner surface of the stent to be effective.
[0132] In an exemplary embodiment of the invention, a stent may be
utilized in combination with rapamycin and heparin to treat
vascular disease. In this exemplary embodiment, the heparin is
immobilized to the inner surface of the stent so that it is in
contact with the blood and the rapamycin is immobilized to the
outer surface of the stent so that it is in contact with the
surrounding tissue. FIG. 7 illustrates a cross-section of a band
102 of the stent 100 illustrated in FIG. 1. As illustrated, the
band 102 is coated with heparin 108 on its inner surface 110 and
with rapamycin 112 on its outer surface 114.
[0133] In an alternate exemplary embodiment, the stent may comprise
a heparin layer immobilized on its inner surface, and rapamycin and
heparin on its outer surface. Utilizing current coating techniques,
heparin tends to form a stronger bond with the surface it is
immobilized to then does rapamycin. Accordingly, it may be possible
to first immobilize the rapamycin to the outer surface of the stent
and then immobilize a layer of heparin to the rapamycin layer. In
this embodiment, the rapamycin may be more securely affixed to the
stent while still effectively eluting from its polymeric matrix,
through the heparin and into the surrounding tissue. FIG. 8
illustrates a cross-section of a band 102 of the stent 100
illustrated in FIG. 1. As illustrated, the band 102 is coated with
heparin 108 on its inner surface 110 and with rapamycin 112 and
heparin 108 on its outer surface 114.
[0134] There are a number of possible ways to immobilize, i.e.,
entrapment or covalent linkage with an erodible bond, the heparin
layer to the rapamycin layer. For example, heparin may be
introduced into the top layer of the polymeric matrix. In other
embodiments, different forms of heparin may be directly immobilized
onto the top coat of the polymeric matrix, for example, as
illustrated in FIG. 9. As illustrated, a hydrophobic heparin layer
116 may be immobilized onto the top coat layer 118 of the rapamycin
layer 112. A hydrophobic form of heparin is utilized because
rapamycin and heparin coatings represent incompatible coating
application technologies. Rapamycin is an organic solvent-based
coating and heparin, in its native form, is a water-based
coating.
[0135] As stated above, a rapamycin coating may be applied to
stents by a dip, spray or spin coating method, and/or any
combination of these methods. Various polymers may be utilized. For
example, as described above, poly(ethylene-co-vinyl acetate) and
polybutyl methacrylate blends may be utilized. Other polymers may
also be utilized, but not limited to, for example, polyvinylidene
fluoride-co-hexafluoropropylene and polyethylbutyl
methacrylate-co-hexyl methacrylate. Also as described above,
barrier or top coatings may also be applied to modulate the
dissolution of rapamycin from the polymer matrix. In the exemplary
embodiment described above, a thin layer of heparin is applied to
the surface of the polymeric matrix. Because these polymer systems
are hydrophobic and incompatible with the hydrophilic heparin,
appropriate surface modifications may be required.
[0136] The application of heparin to the surface of the polymeric
matrix may be performed in various ways and utilizing various
biocompatible materials. For example, in one embodiment, in water
or alcoholic solutions, polyethylene imine may be applied on the
stents, with care not to degrade the rapamycin (e.g., pH <7, low
temperature), followed by the application of sodium heparinate in
aqueous or alcoholic solutions. As an extension of this surface
modification, covalent heparin may be linked on polyethylene imine
using amide-type chemistry (using a carbondiimide activator, e.g.
EDC) or reductive amination chemistry (using CBAS-heparin and
sodium cyanoborohydride for coupling). In another exemplary
embodiment, heparin may be photolinked on the surface, if it is
appropriately grafted with photo initiator moieties. Upon
application of this modified heparin formulation on the covalent
stent surface, light exposure causes cross-linking and
immobilization of the heparin on the coating surface. In yet
another exemplary embodiment, heparin may be complexed with
hydrophobic quaternary ammonium salts, rendering the molecule
soluble in organic solvents (e.g. benzalkonium heparinate,
troidodecylmethylammonium heparinate). Such a formulation of
heparin may be compatible with the hydrophobic rapamycin coating,
and may be applied directly on the coating surface, or in the
rapamycin/hydrophobic polymer formulation.
[0137] It is important to note that the stent, as described above,
may be formed from any number of materials, including various
metals, polymeric materials and ceramic materials. Accordingly,
various technologies may be utilized to immobilize the various
drugs, agent, compound combinations thereon. Specifically, in
addition to the polymeric matricies, described above, biopolymers
may be utilized. Biopolymers may be generally classified as natural
polymers, while the above-described polymers may be described as
synthetic polymers. Exemplary biopolymers, which may be utilized
include agarose, alginate, gelatin, collagen and elastin. In
addition, the drugs, agents or compounds may be utilized in
conjunction with other percutaneously delivered medical devices
such as grafts and perfusion balloons.
[0138] In addition to utilizing an anti-proliferative and
anti-coagulant, anti-inflammatories may also be utilized in
combination therewith. One example of such a combination would be
the addition of an anti-inflammatory corticosteroid such as
dexamethasone with an anti-proliferative, such as rapamycin,
cladribine, vincristine, taxol, or a nitric oxide donor and an
anti-coagulant, such as heparin. Such combination therapies might
result in a better therapeutic effect, i.e., less proliferation as
well as less inflammation, a stimulus for proliferation, than would
occur with either agent alone. The delivery of a stent comprising
an anti-proliferative, anti-coagulant, and an anti-inflammatory to
an injured vessel would provide the added therapeutic benefit of
limiting the degree of local smooth muscle cell proliferation,
reducing a stimulus for proliferation, i.e., inflammation and
reducing the effects of coagulation thus enhancing the
restenosis-limiting action of the stent.
[0139] In other exemplary embodiments of the inventions, growth
factor inhibitor or cytokine signal transduction inhibitor, such as
the ras inhibitor, R115777 or P38 kinase inhibitor RWJ67657, or a
tyrosine kinase inhibitor, such as tyrphostin, might be combined
with an anti-proliferative agent such as taxol, vincristine or
rapamycin so that proliferation of smooth muscle cells could be
inhibited by different mechanisms. Alternatively, an
anti-proliferative agent such as taxol, vincristine or rapamycin
could be combined with an inhibitor of extracellular matrix
synthesis such as halofuginone. In the above cases, agents acting
by different mechanisms could act synergistically to reduce smooth
muscle cell proliferation and vascular hyperplasia. This invention
is also intended to cover other combinations of two or more such
drug agents. As mentioned above, such drugs, agents or compounds
could be administered systemically, delivered locally via drug
delivery catheter, or formulated for delivery from the surface of a
stent, or given as a combination of systemic and local therapy.
[0140] In addition to anti-proliferatives, anti-inflammatories and
anti-coagulants, other drugs, agents or compounds may be utilized
in conjunction with the medical devices. For example,
immunosuppressants may be utilized alone or in combination with
these other drugs, agents or compounds. Also gene therapy delivery
mechanisms such as modified genes (nucleic acids including
recombinant DNA) in viral vectors and non-viral gene vectors such
as plasmids may also be introduced locally via a medical device. In
addition, the present invention may be utilized with cell based
therapy.
[0141] In addition to all of the drugs, agents, compounds and
modified genes described above, chemical agents that are not
ordinarily therapeutically or biologically active may also be
utilized in conjunction with the present invention. These chemical
agents, commonly referred to as pro-drugs, are agents that become
biologically active upon their introduction into the living
organism by one or more mechanisms. These mechanisms include the
addition of compounds supplied by the organism or the cleavage of
compounds from the agents caused by another agent supplied by the
organism. Typically, pro-drugs are more absorbable by the organism.
In addition, pro-drugs may also provide some additional measure of
time release.
[0142] The coatings and drugs, agents or compounds described above
may be utilized in combination with any number of medical devices,
and in particular, with implantable medical devices such as stents
and stent-grafts. Other devices such as vena cava filters and
anastomosis devices may be used with coatings having drugs, agents
or compounds therein. The exemplary stent illustrated in FIGS. 1
and 2 is a balloon expandable stent. Balloon expandable stents may
be utilized in any number of vessels or conduits, and are
particularly well suited for use in coronary arteries.
Self-expanding stents, on the other hand, are particularly well
suited for use in vessels where crush recovery is a critical
factor, for example, in the carotid artery. Accordingly, it is
important to note that any of the drugs, agents or compounds, as
well as the coatings described above, may be utilized in
combination with self-expanding stents such as those described
below.
[0143] There is illustrated in FIGS. 10 and 11, a stent 200, which
may be utilized in connection with the present invention. FIGS. 10
and 11 illustrate the exemplary stent 200 in its unexpanded or
compressed state. The stent 200 is preferably made from a
superelastic alloy such as Nitinol. Most preferably, the stent 200
is made from an alloy comprising from about fifty percent (as used
herein these percentages refer to weight percentages) Ni to about
sixty percent Ni, and more preferably about 55.8 percent Ni, with
the remainder of the alloy being Ti. Preferably, the stent 200 is
designed such that it is superelastic at body temperature, and
preferably has an Af in the range from about twenty-four degrees C.
to about thirty-seven degrees C. The superelastic design of the
stent 200 makes it crush recoverable which, as discussed above,
makes it useful as a stent or frame for any number of vascular
devices in different applications.
[0144] Stent 200 is a tubular member having front and back open
ends 202 and 204 and a longitudinal axis 206 extending
therebetween. The tubular member has a first smaller diameter,
FIGS. 10 and 11, for insertion into a patient and navigation
through the vessels, and a second larger diameter, FIGS. 12 and 13,
for deployment into the target area of a vessel. The tubular member
is made from a plurality of adjacent hoops 208, FIG. 10 showing
hoops 208(a)-208(d), extending between the front and back ends 202
and 204. The hoops 208 include a plurality of longitudinal struts
210 and a plurality of loops 212 connecting adjacent struts,
wherein adjacent struts are connected at opposite ends so as to
form a substantially S or Z shape pattern. The loops 212 are
curved, substantially semi-circular with symmetrical sections about
their centers 214.
[0145] Stent 200 further includes a plurality of bridges 216 which
connect adjacent hoops 208 and which can best be described in
detail by referring to FIG. 14. Each bridge 216 has two ends 218
and 220. The bridges 216 have one end attached to one strut and/or
loop, and another end attached to a strut and/or loop on an
adjacent hoop. The bridges 216 connect adjacent struts together at
bridge to loop connection points 222 and 224. For example, bridge
end 218 is connected to loop 214(a) at bridge to loop connection
point 222, and bridge end 220 is connected to loop 214(b) at bridge
to loop connection point 224. Each bridge to loop connection point
has a center 226. The bridge to loop connection points are
separated angularly with respect to the longitudinal axis. That is,
the connection points are not immediately opposite each other.
Essentially, one could not draw a straight line between the
connection points, wherein such line would be parallel to the
longitudinal axis of the stent.
[0146] The above described geometry helps to better distribute
strain throughout the stent, prevents metal to metal contact when
the stent is bent, and minimizes the opening size between the
struts, loops and bridges. The number of and nature of the design
of the struts, loops and bridges are important factors when
determining the working properties and fatigue life properties of
the stent. It was previously thought that in order to improve the
rigidity of the stent, that struts should be large, and therefore
there should be fewer struts per hoop. However, it has now been
discovered that stents having smaller struts and more struts per
hoop actually improve the construction of the stent and provide
greater rigidity. Preferably, each hoop has between twenty-four to
thirty-six or more struts. It has been determined that a stent
having a ratio of number of struts per hoop to strut length L (in
inches) which is greater than four hundred has increased rigidity
over prior art stents, which typically have a ratio of under two
hundred. The length of a strut is measured in its compressed state
parallel to the longitudinal axis 206 of the stent 200 as
illustrated in FIG. 10.
[0147] As seen from a comparison of FIGS. 10 and 12, the geometry
of the stent 200 changes quite significantly as the stent 200 is
deployed from its un-expanded state to its expanded state. As a
stent undergoes diametric change, the strut angle and strain levels
in the loops and bridges are affected. Preferably, all of the stent
features will strain in a predictable manor so that the stent is
reliable and uniform in strength. In addition, it is preferable to
minimize the maximum strain experienced by struts loops and
bridges, since Nitinol properties are more generally limited by
strain rather than by stress. As will be discussed in greater
detail below, the stent sits in the delivery system in its
un-expanded state as shown in FIGS. 19 and 20. As the stent is
deployed, it is allowed to expand towards its expanded state, as
shown in FIG. 12, which preferably has a diameter which is the same
or larger than the diameter of the target vessel. Nitinol stents
made from wire deploy in much the same manner, and are dependent
upon the same design constraints, as laser cut stents. Stainless
steel stents deploy similarly in terms of geometric changes as they
are assisted by forces from balloons or other devices.
[0148] In trying to minimize the maximum strain experienced by
features of the stent, the present invention utilizes structural
geometries which distribute strain to areas of the stent which are
less susceptible to failure than others. For example, one of the
most vulnerable areas of the stent is the inside radius of the
connecting loops. The connecting loops undergo the most deformation
of all the stent features. The inside radius of the loop would
normally be the area with the highest level of strain on the stent.
This area is also critical in that it is usually the smallest
radius on the stent. Stress concentrations are generally controlled
or minimized by maintaining the largest radii possible. Similarly,
we want to minimize local strain concentrations on the bridge and
bridge connection points. One way to accomplish this is to utilize
the largest possible radii while maintaining feature widths which
are consistent with applied forces. Another consideration is to
minimize the maximum open area of the stent. Efficient utilization
of the original tube from which the stent is cut increases stent
strength and its ability to trap embolic material.
[0149] Many of these design objectives have been accomplished by an
exemplary embodiment of the present invention, illustrated in FIGS.
10, 11 and 14. As seen from these figures, the most compact designs
which maintain the largest radii at the loop to bridge connections
are non-symmetric with respect to the centerline of the strut
connecting loop. That is, loop to bridge connection point centers
226 are offset from the center 214 of the loops 212 to which they
are attached. This feature is particularly advantageous for stents
having large expansion ratios, which in turn requires them to have
extreme bending requirements where large elastic strains are
required. Nitinol can withstand extremely large amounts of elastic
strain deformation, so the above features are well suited to stents
made from this alloy. This feature allows for maximum utilization
of Ni--Ti or other material properties to enhance radial strength,
to improve stent strength uniformity, to improve fatigue life by
minimizing local strain levels, to allow for smaller open areas
which enhance entrapment of embolic material, and to improve stent
apposition in irregular vessel wall shapes and curves.
[0150] As seen in FIG. 14, stent 200 comprises strut connecting
loops 212 having a width W1, as measured at the center 214 parallel
to axis 206, which are greater than the strut widths W2, as
measured perpendicular to axis 206 itself. In fact, it is
preferable that the thickness of the loops vary so that they are
thickest near their centers. This increases strain deformation at
the strut and reduces the maximum strain levels at the extreme
radii of the loop. This reduces the risk of stent failure and
allows one to maximize radial strength properties. This feature is
particularly advantageous for stents having large expansion ratios,
which in turn requires them to have extreme bending requirements
where large elastic strains are required. Nitinol can withstand
extremely large amounts of elastic strain deformation, so the above
features are well suited to stents made from this alloy. As stated
above, this feature allows for maximum utilization of Ni--Ti or
other material properties to enhance radial strength, to improve
stent strength uniformity, to improve fatigue life by minimizing
local strain levels, to allow for smaller open areas which enhance
entrapment of embolic material, and to improve stent apposition in
irregular vessel wall shapes and curves.
[0151] As mentioned above, bridge geometry changes as a stent is
deployed from its compressed state to its expanded state and
vise-versa. As a stent undergoes diametric change, strut angle and
loop strain is affected. Since the bridges are connected to either
the loops, struts or both, they are affected. Twisting of one end
of the stent with respect to the other, while loaded in the stent
delivery system, should be avoided. Local torque delivered to the
bridge ends displaces the bridge geometry. If the bridge design is
duplicated around the stent perimeter, this displacement causes
rotational shifting of the two loops being connected by the
bridges. If the bridge design is duplicated throughout the stent,
as in the present invention, this shift will occur down the length
of the stent. This is a cumulative effect as one considers rotation
of one end with respect to the other upon deployment. A stent
delivery system, such as the one described below, will deploy the
distal end first, then allow the proximal end to expand. It would
be undesirable to allow the distal end to anchor into the vessel
wall while holding the stent fixed in rotation, then release the
proximal end. This could cause the stent to twist or whip in
rotation to equilibrium after it is at least partially deployed
within the vessel. Such whipping action may cause damage to the
vessel.
[0152] However, one exemplary embodiment of the present invention,
as illustrated in FIGS. 10 and 11, reduces the chance of such
events happening when deploying the stent. By mirroring the bridge
geometry longitudinally down the stent, the rotational shift of the
Z-sections or S-sections may be made to alternate and will minimize
large rotational changes between any two points on a given stent
during deployment or constraint. That is, the bridges 216
connecting loop 208(b) to loop 208(c) are angled upwardly from left
to right, while the bridges connecting loop 208(c) to loop 208(d)
are angled downwardly from left to right. This alternating pattern
is repeated down the length of the stent 200. This alternating
pattern of bridge slopes improves the torsional characteristics of
the stent so as to minimize any twisting or rotation of the stent
with respect to any two hoops. This alternating bridge slope is
particularly beneficial if the stent starts to twist in vivo. As
the stent twists, the diameter of the stent will change.
Alternating bridge slopes tend to minimize this effect. The
diameter of a stent having bridges which are all sloped in the same
direction will tend to grow if twisted in one direction and shrink
if twisted in the other direction. With alternating bridge slopes
this effect is minimized and localized.
[0153] The feature is particularly advantageous for stents having
large expansion ratios, which in turn requires them to have extreme
bending requirements where large elastic strains are required.
Nitinol, as stated above, can withstand extremely large amounts of
elastic strain deformation, so the above features are well suited
to stents made from this alloy. This feature allows for maximum
utilization of Ni--Ti or other material properties to enhance
radial strength, to improve stent strength uniformity, to improve
fatigue life by minimizing local strain levels, to allow for
smaller open areas which enhance entrapment of embolic material,
and to improve stent apposition in irregular vessel wall shapes and
curves.
[0154] Preferably, stents are laser cut from small diameter tubing.
For prior art stents, this manufacturing process led to designs
with geometric features, such as struts, loops and bridges, having
axial widths W2, W1 and W3, respectively, which are larger than the
tube wall thickness T (illustrated in FIG. 12). When the stent is
compressed, most of the bending occurs in the plane that is created
if one were to cut longitudinally down the stent and flatten it
out. However, for the individual bridges, loops and struts, which
have widths greater than their thickness, there is a greater
resistance to this in-plane bending than to out-of-plane bending.
Because of this, the bridges and struts tend to twist, so that the
stent as a whole may bend more easily. This twisting is a buckling
condition which is unpredictable and can cause potentially high
strain.
[0155] However, this problem has been solved in an exemplary
embodiment of the present invention, as illustrated in FIGS. 10-14.
As seen from these figures, the widths of the struts, hoops and
bridges are equal to or less than the wall thickness of the tube.
Therefore, substantially all bending and, therefore, all strains
are "out-of-plane." This minimizes twisting of the stent which
minimizes or eliminates buckling and unpredictable strain
conditions. This feature is particularly advantageous for stents
having large expansion ratios, which in turn requires them to have
extreme bending requirements where large elastic strains are
required. Nitinol, as stated above, can withstand extremely large
amounts of elastic strain deformation, so the above features are
well suited to stents made from this alloy. This feature allows for
maximum utilization of Ni--Ti or other material properties to
enhance radial strength, to improve stent strength uniformity, to
improve fatigue life by minimizing local strain levels, to allow
for smaller open areas which enhance entrapment of embolic
material, and to improve stent apposition in irregular vessel wall
shapes and curves.
[0156] An alternate exemplary embodiment of a stent that may be
utilized in conjunction with the present invention is illustrated
in FIG. 15. FIG. 15 shows stent 300 which is similar to stent 200
illustrated in FIGS. 10-14. Stent 300 is made from a plurality of
adjacent hoops 302, FIG. 15 showing hoops 302(a)-302(d). The hoops
302 include a plurality of longitudinal struts 304 and a plurality
of loops 306 connecting adjacent struts, wherein adjacent struts
are connected at opposite ends so as to form a substantially S or Z
shape pattern. Stent 300 further includes a plurality of bridges
308 which connect adjacent hoops 302. As seen from the figure,
bridges 308 are non-linear and curve between adjacent hoops. Having
curved bridges allows the bridges to curve around the loops and
struts so that the hoops can be placed closer together which in
turn, minimizes the maximum open area of the stent and increases
its radial strength as well. This can best be explained by
referring to FIG. 13. The above described stent geometry attempts
to minimize the largest circle which could be inscribed between the
bridges, loops and struts, when the stent is expanded. Minimizing
the size of this theoretical circle, greatly improves the stent
because it is then better suited to trap embolic material once it
is inserted into the patient.
[0157] As mentioned above, it is preferred that the stent of the
present invention be made from a superelastic alloy and most
preferably made of an alloy material having greater than 50.5
atomic percentage Nickel and the balance Titanium. Greater than
50.5 atomic percentage Nickel allows for an alloy in which the
temperature at which the martensite phase transforms completely to
the austenite phase (the Af temperature) is below human body
temperature, and preferably is about twenty-four degrees C. to
about thirty-seven degrees C., so that austenite is the only stable
phase at body temperature.
[0158] In manufacturing the Nitinol stent, the material is first in
the form of a tube. Nitinol tubing is commercially available from a
number of suppliers including Nitinol Devices and Components,
Fremont Calif. The tubular member is then loaded into a machine
which will cut the predetermined pattern of the stent into the
tube, as discussed above and as shown in the figures. Machines for
cutting patterns in tubular devices to make stents or the like are
well known to those of ordinary skill in the art and are
commercially available. Such machines typically hold the metal tube
between the open ends while a cutting laser, preferably under
microprocessor control, cuts the pattern. The pattern dimensions
and styles, laser positioning requirements, and other information
are programmed into a microprocessor which controls all aspects of
the process. After the stent pattern is cut, the stent is treated
and polished using any number of methods or combination of methods
well known to those skilled in the art. Lastly, the stent is then
cooled until it is completely martensitic, crimped down to its
un-expanded diameter and then loaded into the sheath of the
delivery apparatus.
[0159] As stated in previous sections of this application, markers
having a radiopacity greater than that of the superelastic alloys
may be utilized to facilitate more precise placement of the stent
within the vasculature. In addition, markers may be utilized to
determine when and if a stent is fully deployed. For example, by
determining the spacing between the markers, one can determine if
the deployed stent has achieved its maximum diameter and adjusted
accordingly utilizing a tacking process. FIG. 16 illustrates an
exemplary embodiment of the stent 200 illustrated in FIGS. 10-14
having at least one marker on each end thereof. In a preferred
embodiment, a stent having thirty-six struts per hoop can
accommodate six markers 800. Each marker 800 comprises a marker
housing 802 and a marker insert 804. The marker insert 804 may be
formed from any suitable biocompatible material having a high
radiopacity under X-ray fluoroscopy. In other words, the marker
inserts 804 should preferably have a radiopacity higher than that
of the material comprising the stent 200. The addition of the
marker housings 802 to the stent necessitates that the lengths of
the struts in the last two hoops at each end of the stent 200 be
longer than the strut lengths in the body of the stent to increase
the fatigue life at the stent ends. The marker housings 802 are
preferably cut from the same tube as the stent as briefly described
above. Accordingly, the housings 802 are integral to the stent 200.
Having the housings 802 integral to the stent 200 serves to ensure
that the markers 800 do not interfere with the operation of the
stent
[0160] FIG. 17 is a cross-sectional view of a marker housing 802.
The housing 802 may be elliptical when observed from the outer
surface as illustrated in FIG. 16. As a result of the laser cutting
process, the hole 806 in the marker housing 802 is conical in the
radial direction with the outer surface 808 having a diameter
larger than the diameter of the inner surface 810, as illustrated
in FIG. 17. The conical tapering in the marker housing 802 is
beneficial in providing an interference fit between the marker
insert 804 and the marker housing 802 to prevent the marker insert
804 from being dislodged once the stent 200 is deployed. A detailed
description of the process of locking the marker insert 804 into
the marker housing 802 is given below.
[0161] As set forth above, the marker inserts 804 may be made from
any suitable material having a radiopacity higher than the
superelastic material forming the stent or other medical device.
For example, the marker insert 804 may be formed from niobium,
tungsten, gold, platinum or tantalum. In the preferred embodiment,
tantalum is utilized because of its closeness to nickel-titanium in
the galvanic series and thus would minimize galvanic corrosion. In
addition, the surface area ratio of the tantalum marker inserts 804
to the nickel-titanium is optimized to provide the largest possible
tantalum marker insert, easy to see, while minimizing the galvanic
corrosion potential. For example, it has been determined that up to
nine marker inserts 804 having a diameter of 0.010 inches could be
placed at the end of the stent 200; however, these marker inserts
804 would be less visible under X-ray fluoroscopy. On the other
hand, three to four marker inserts 804 having a diameter of 0.025
inches could be accommodated on the stent 200; however, galvanic
corrosion resistance would be compromised. Accordingly, in the
preferred embodiment, six tantalum markers having a diameter of
0.020 inches are utilized on each end of the stent 200 for a total
of twelve markers 800.
[0162] The tantalum markers 804 may be manufactured and loaded into
the housing utilizing a variety of known techniques. In the
exemplary embodiment, the tantalum markers 804 are punched out from
an annealed ribbon stock and are shaped to have the same curvature
as the radius of the marker housing 802 as illustrated in FIG. 17.
Once the tantalum marker insert 804 is loaded into the marker
housing 802, a coining process is used to properly seat the marker
insert 804 below the surface of the housing 802. The coining punch
is also shaped to maintain the same radius of curvature as the
marker housing 802. As illustrated in FIG. 17, the coining process
deforms the marker housing 802 material to lock in the marker
insert 804.
[0163] As stated above, the hole 806 in the marker housing 802 is
conical in the radial direction with the outer surface 808 having a
diameter larger than the diameter of the inner surface 810 as
illustrated in FIG. 17. The inside and outside diameters vary
depending on the radius of the tube from which the stent is cut.
The marker inserts 804, as stated above, are formed by punching a
tantalum disk from annealed ribbon stock and shaping it to have the
same radius of curvature as the marker housing 802. It is important
to note that the marker inserts 804, prior to positioning in the
marker housing 804, have straight edges. In other words, they are
not angled to match the hole 806. The diameter of the marker insert
804 is between the inside and outside diameter of the marker
housing 802. Once the marker insert 804 is loaded into the marker
housing, a coining process is used to properly seat the marker
insert 804 below the surface of the housing 802. In the preferred
embodiment, the thickness of the marker insert 804 is less than or
equal to the thickness of the tubing and thus the thickness or
height of the hole 806. Accordingly, by applying the proper
pressure during the coining process and using a coining tool that
is larger than the marker insert 804, the marker insert 804 may be
seated in the marker housing 802 in such a way that it is locked
into position by a radially oriented protrusion 812. Essentially,
the applied pressure, and size and shape of the housing tool forces
the marker insert 804 to form the protrusion 812 in the marker
housing 802. The coining tool is also shaped to maintain the same
radius of curvature as the marker housing. As illustrated in FIG.
17, the protrusion 812 prevents the marker insert 804 from becoming
dislodged from the marker housing.
[0164] It is important to note that the marker inserts 804 are
positioned in and locked into the marker housing 802 when the stent
200 is in its unexpanded state. This is due to the fact that it is
desirable that the tube's natural curvature be utilized. If the
stent were in its expanded state, the coining process would change
the curvature due to the pressure or force exerted by the coining
tool.
[0165] As illustrated in FIG. 18, the marker inserts 804 form a
substantially solid line that clearly defines the ends of the stent
in the stent delivery system when seen under fluoroscopic
equipment. As the stent 200 is deployed from the stent delivery
system, the markers 800 move away from each other and flower open
as the stent 200 expands as illustrated in FIG. 16. The change in
the marker grouping provides the physician or other health care
provider with the ability to determine when the stent 200 has been
fully deployed from the stent delivery system.
[0166] It is important to note that the markers 800 may be
positioned at other locations on the stent 200.
[0167] It is believed that many of the advantages of the present
invention can be better understood through a brief description of a
delivery apparatus for the stent, as shown in FIGS. 19 and 20.
FIGS. 19 and 20 show a self-expanding stent delivery apparatus 10
for a stent made in accordance with the present invention.
Apparatus 10 comprises inner and outer coaxial tubes. The inner
tube is called the shaft 12 and the outer tube is called the sheath
14. Shaft 12 has proximal and distal ends. The proximal end of the
shaft 12 terminates at a luer lock hub 16. Preferably, shaft 12 has
a proximal portion 18 which is made from a relatively stiff
material such as stainless steel, Nitinol, or any other suitable
material, and a distal portion 20 which may be made from a
polyethylene, polyimide, Pellethane, Pebax, Vestamid, Cristamid,
Grillamid or any other suitable material known to those of ordinary
skill in the art. The two portions are joined together by any
number of means known to those of ordinary skill in the art. The
stainless steel proximal end gives the shaft the necessary rigidity
or stiffness it needs to effectively push out the stent, while the
polymeric distal portion provides the necessary flexibility to
navigate tortuous vessels.
[0168] The distal portion 20 of the shaft 12 has a distal tip 22
attached thereto. The distal tip 22 has a proximal end 24 whose
diameter is substantially the same as the outer diameter of the
sheath 14. The distal tip 22 tapers to a smaller diameter from its
proximal end to its distal end, wherein the distal end 26 of the
distal tip 22 has a diameter smaller than the inner diameter of the
sheath 14. Also attached to the distal portion 20 of shaft 12 is a
stop 28 which is proximal to the distal tip 22. Stop 28 may be made
from any number of materials known in the art, including stainless
steel, and is even more preferably made from a highly radiopaque
material such as platinum, gold or tantalum. The diameter of stop
28 is substantially the same as the inner diameter of sheath 14,
and would actually make frictional contact with the inner surface
of the sheath. Stop 28 helps to push the stent out of the sheath
during deployment, and helps keep the stent from migrating
proximally into the sheath 14.
[0169] A stent bed 30 is defined as being that portion of the shaft
between the distal tip 22 and the stop 28. The stent bed 30 and the
stent 200 are coaxial so that the distal portion 20 of shaft 12
comprising the stent bed 30 is located within the lumen of the
stent 200. However, the stent bed 30 does not make any contact with
stent 200 itself. Lastly, shaft 12 has a guidewire lumen 32
extending along its length from its proximal end and exiting
through its distal tip 22. This allows the shaft 12 to receive a
guidewire much in the same way that an ordinary balloon angioplasty
catheter receives a guidewire. Such guidewires are well known in
art and help guide catheters and other medical devices through the
vasculature of the body.
[0170] Sheath 14 is preferably a polymeric catheter and has a
proximal end terminating at a sheath hub 40. Sheath 14 also has a
distal end which terminates at the proximal end 24 of distal tip 22
of the shaft 12, when the stent is in its fully un-deployed
position as shown in the figures. The distal end of sheath 14
includes a radiopaque marker band 34 disposed along its outer
surface. As will be explained below, the stent is fully deployed
from the delivery apparatus when the marker band 34 is lined up
with radiopaque stop 28, thus indicating to the physician that it
is now safe to remove the apparatus 10 from the body. Sheath 14
preferably comprises an outer polymeric layer and an inner
polymeric layer. Positioned between outer and inner layers is a
braided reinforcing layer. Braided reinforcing layer is preferably
made from stainless steel. The use of braided reinforcing layers in
other types of medical devices can be found in U.S. Pat. No.
3,585,707 issued to Stevens on Jun. 22, 1971, U.S. Pat. No.
5,045,072 issued to Castillo et al. on Sep. 3, 1991, and U.S. Pat.
No. 5,254,107 issued to Soltesz on Oct. 19, 1993.
[0171] FIGS. 19 and 20 illustrate the stent 200 as being in its
fully un-deployed position. This is the position the stent is in
when the apparatus 10 is inserted into the vasculature and its
distal end is navigated to a target site. Stent 200 is disposed
around stent bed 30 and at the distal end of sheath 14. The distal
tip 22 of the shaft 12 is distal to the distal end of the sheath
14, and the proximal end of the shaft 12 is proximal to the
proximal end of the sheath 14. The stent 200 is in a compressed
state and makes frictional contact with the inner surface 36 of the
sheath 14.
[0172] When being inserted into a patient, sheath 14 and shaft 12
are locked together at their proximal ends by a Tuohy Borst valve
38. This prevents any sliding movement between the shaft and sheath
which could result in a premature deployment or partial deployment
of the stent 200. When the stent 200 reaches its target site and is
ready for deployment, the Tuohy Borst valve 38 is opened so that
that the sheath 14 and shaft 12 are no longer locked together.
[0173] The method under which the apparatus 10 deploys the stent
200 is readily apparent. The apparatus 10 is first inserted into
the vessel until the radiopaque stent markers 800 (front 202 and
back 204 ends, see FIG. 16) are proximal and distal to the target
lesion. Once this has occurred the physician would open the Tuohy
Borst valve 38. The physician would then grasp hub 16 of shaft 12
so as to hold it in place. Thereafter, the physician would grasp
the proximal end of the sheath 14 and slide it proximal, relative
to the shaft 12. Stop 28 prevents the stent 200 from sliding back
with the sheath 14, so that as the sheath 14 is moved back, the
stent 200 is pushed out of the distal end of the sheath 14. As
stent 200 is being deployed the radiopaque stent markers 800 move
apart once they come out of the distal end of sheath 14. Stent
deployment is complete when the marker 34 on the outer sheath 14
passes the stop 28 on the inner shaft 12. The apparatus 10 can now
be withdrawn through the stent 200 and removed from the
patient.
[0174] FIG. 21 illustrates the stent 200 in a partially deployed
state. As illustrated, as the stent 200 expands from the delivery
device 10, the markers 800 move apart from one another and expand
in a flower like manner.
[0175] It is important to note that any of the above-described
medical devices may be coated with coatings that comprise drugs,
agents or compounds or simply with coatings that contain no drugs,
agents or compounds. In addition, the entire medical device may be
coated or only a portion of the device may be coated. The coating
may be uniform or non-uniform. The coating may be discontinuous.
However, the markers on the stent are preferably coated in a manner
so as to prevent coating buildup which may interfere with the
operation of the device.
[0176] In a preferred exemplary embodiment, the self-expanding
stents, described above, may be coated with a rapamycin containing
polymer. In this embodiment, the polymeric coated stent comprises
rapamycin in an amount ranging from about fifty to one-thousand
micrograms per square centimeter surface area of the vessel that is
spanned by the stent. The rapamycin is mixed with the
polyvinylidenefluoride-hexafluoropropylene polymer (described
above) in the ratio of drug to polymer of about thirty/seventy. The
polymer is made by a batch process using the two monomers,
vinylidene fluoride and hexafluoropropylene under high pressure by
an emulsion polymerization process. In an alternate exemplary
embodiment, the polymer may be made utilizing a batch dispersion
process. The polymeric coating weight itself is in the range from
about two-hundred to about one thousand seven hundred micrograms
per square centimeter surface area of the vessel that is spanned by
the stent.
[0177] The coated stent comprises a base coat, commonly referred to
as a primer layer. The primer layer typically improves the adhesion
of the coating layer that comprises the rapamycin. The primer also
facilitates uniform wetting of the surface thereby enabling the
production of a uniform rapamycin containing coating. The primer
layer may be applied using any of the above-described techniques.
It is preferably applied utilizing a dip coating process. The
primer coating is in the range from about one to about ten percent
of the total weight of the coating. The next layer applied is the
rapamycin containing layer. The rapamycin containing layer is
applied by a spin coating process and subsequently dried in a
vacuum oven for approximately sixteen hours at a temperature in the
range from about fifty to sixty degrees centigrade. After drying or
curing, the stent is mounted onto a stent delivery catheter using a
process similar to the uncoated stent. The mounted stent is then
packaged and sterilized in any number of ways. In one exemplary
embodiment, the stent is sterilized using ethylene oxide.
[0178] As described above, various drugs, agents or compounds may
be locally delivered via medical devices. For example, rapamycin
and heparin may be delivered by a stent to reduce restenosis,
inflammation, and coagulation. Various techniques for immobilizing
the drugs, agents or compounds are discussed above, however,
maintaining the drugs, agents or compounds on the medical devices
during delivery and positioning is critical to the success of the
procedure or treatment. For example, removal of the drug, agent or
compound coating during delivery of the stent can potentially cause
failure of the device. For a self-expanding stent, the retraction
of the restraining sheath may cause the drugs, agents or compounds
to rub off the stent. For a balloon expandable stent, the expansion
of the balloon may cause the drugs, agents or compounds to simply
delaminate from the stent through contact with the balloon or via
expansion. Therefore, prevention of this potential problem is
important to have a successful therapeutic medical device, such as
a stent.
[0179] There are a number of approaches that may be utilized to
substantially reduce the above-described concern. In one exemplary
embodiment, a lubricant or mold release agent may be utilized. The
lubricant or mold release agent may comprise any suitable
biocompatible lubricious coating. An exemplary lubricious coating
may comprise silicone. In this exemplary embodiment, a solution of
the silicone base coating may be introduced onto the balloon
surface, onto the polymeric matrix, and/or onto the inner surface
of the sheath of a self-expanding stent delivery apparatus and
allowed to air cure. Alternately, the silicone based coating may be
incorporated into the polymeric matrix. It is important to note,
however, that any number of lubricious materials may be utilized,
with the basic requirements being that the material be
biocompatible, that the material not interfere with the
actions/effectiveness of the drugs, agents or compounds and that
the material not interfere with the materials utilized to
immobilize the drugs, agents or compounds on the medical device. It
is also important to note that one or more, or all of the
above-described approaches may be utilized in combination.
[0180] Referring now to FIG. 22, there is illustrated a balloon 400
of a balloon catheter that may be utilized to expand a stent in
situ. As illustrated, the balloon 400 comprises a lubricious
coating 402. The lubricious coating 402 functions to minimize or
substantially eliminate the adhesion between the balloon 400 and
the coating on the medical device. In the exemplary embodiment
described above, the lubricious coating 402 would minimize or
substantially eliminate the adhesion between the balloon 400 and
the heparin or rapamycin coating. The lubricious coating 402 may be
attached to and maintained on the balloon 400 in any number of ways
including but not limited to dipping, spraying, brushing or spin
coating of the coating material from a solution or suspension
followed by curing or solvent removal step as needed.
[0181] Materials such as synthetic waxes, e.g. diethyleneglycol
monostearate, hydrogenated castor oil, oleic acid, stearic acid,
zinc stearate, calcium stearate, ethylenebis (stearamide), natural
products such as paraffin wax, spermaceti wax, carnuba wax, sodium
alginate, ascorbic acid and flour, fluorinated compounds such as
perfluoroalkanes, perfluorofatty acids and alcohol, synthetic
polymers such as silicones e.g. polydimethylsiloxane,
polytetrafluoroethylene, polyfluoroethers, polyalkylglycol e.g.
polyethylene glycol waxes, and inorganic materials such as talc,
kaolin, mica, and silica may be used to prepare these coatings.
Vapor deposition polymerization e.g. parylene-C deposition, or
RF-plasma polymerization of perfluoroalkenes and perfluoroalkanes
can also be used to prepare these lubricious coatings.
[0182] FIG. 23 illustrates a cross-section of a band 102 of the
stent 100 illustrated in FIG. 1. In this exemplary embodiment, the
lubricious coating 500 is immobilized onto the outer surface of the
polymeric coating. As described above, the drugs, agents or
compounds may be incorporated into a polymeric matrix. The stent
band 102 illustrated in FIG. 23 comprises a base coat 502
comprising a polymer and rapamycin and a top coat 504 or diffusion
layer 504 also comprising a polymer. The lubricious coating 500 is
affixed to the top coat 502 by any suitable means, including but
not limited to spraying, brushing, dipping or spin coating of the
coating material from a solution or suspension with or without the
polymers used to create the top coat, followed by curing or solvent
removal step as needed. Vapor deposition polymerization and
RF-plasma polymerization may also be used to affix those lubricious
coating materials that lend themselves to this deposition method,
to the top coating. In an alternate exemplary embodiment, the
lubricious coating may be directly incorporated into the polymeric
matrix.
[0183] If a self-expanding stent is utilized, the lubricious
coating may be affixed to the inner surface of the restraining
sheath. FIG. 24 illustrates a self-expanding stent 200 (FIG. 10)
within the lumen of a delivery apparatus sheath 14. As illustrated,
a lubricious coating 600 is affixed to the inner surfaces of the
sheath 14. Accordingly, upon deployment of the stent 200, the
lubricious coating 600 preferably minimizes or substantially
eliminates the adhesion between the sheath 14 and the drug, agent
or compound coated stent 200.
[0184] In an alternate approach, physical and/or chemical
cross-linking methods may be applied to improve the bond strength
between the polymeric coating containing the drugs, agents or
compounds and the surface of the medical device or between the
polymeric coating containing the drugs, agents or compounds and a
primer. Alternately, other primers applied by either traditional
coating methods such as dip, spray or spin coating, or by RF-plasma
polymerization may also be used to improve bond strength. For
example, as shown in FIG. 25, the bond strength can be improved by
first depositing a primer layer 700 such as vapor polymerized
parylene-C on the device surface, and then placing a second layer
702 which comprises a polymer that is similar in chemical
composition to the one or more of the polymers that make up the
drug-containing matrix 704, e.g., polyethylene-co-vinyl acetate or
polybutyl methacrylate but has been modified to contain
cross-linking moieties. This secondary layer 702 is then
cross-linked to the primer after exposure to ultra-violet light. It
should be noted that anyone familiar with the art would recognize
that a similar outcome could be achieved using cross-linking agents
that are activated by heat with or without the presence of an
activating agent. The drug-containing matrix 704 is then layered
onto the secondary layer 702 using a solvent that swells, in part
or wholly, the secondary layer 702. This promotes the entrainment
of polymer chains from the matrix into the secondary layer 702 and
conversely from the secondary layer 702 into the drug-containing
matrix 704. Upon removal of the solvent from the coated layers, an
interpenetrating or interlocking network of the polymer chains is
formed between the layers thereby increasing the adhesion strength
between them. A top coat 706 is used as described above.
[0185] A related difficultyoccurs in medical devices such as
stents. In the drug-coated stents crimped state, some struts come
into contact with each other and when the stent is expanded, the
motion causes the polymeric coating comprising the drugs, agents or
compounds to stick and stretch. This action may potentially cause
the coating to separate from the stent in certain areas. The
predominant mechanism of the coating self-adhesion is believed to
be due to mechanical forces. When the polymer comes in contact with
itself, its chains can tangle causing the mechanical bond, similar
to hook and loop fasteners such as Velcro.RTM.. Certain polymers do
not bond with each other, for example, fluoropolymers. For other
polymers, however, powders may be utilized. In other words, a
powder may be applied to the one or more polymers incorporating the
drugs, agents or other compounds on the surfaces of the medical
device to reduce the mechanical bond. Any suitable biocompatible
material which does not interfere with the drugs, agents, compounds
or materials utilized to immobilize the drugs, agents or compounds
onto the medical device may be utilized. For example, a dusting
with a water soluble powder may reduce the tackiness of the
coatings surface and this will prevent the polymer from sticking to
itself thereby reducing the potential for delamination. The powder
should be water-soluble so that it does not present an emboli risk.
The powder may comprise an anti-oxidant, such as vitamin C, or it
may comprise an anti-coagulant, such as aspirin or heparin. An
advantage of utilizing an anti-oxidant may be in the fact that the
anti-oxidant may preserve the other drugs, agents or compounds over
longer periods of time.
[0186] It is important to note that crystalline polymers are
generally not sticky or tacky. Accordingly, if crystalline polymers
are utilized rather than amorphous polymers, then additional
materials may not be necessary. It is also important to note that
polymeric coatings without drugs, agents, and/or compounds may
improve the operating characteristics of the medical device. For
example, the mechanical properties of the medical device may be
improved by a polymeric coating, with or without drugs, agents
and/or compounds. A coated stent may have improved flexibility and
increased durability. In addition, the polymeric coating may
substantially reduce or eliminate galvanic corrosion between the
different metals comprising the medical device.
[0187] Any of the above-described medical devices may be utilized
for the local delivery of drugs, agents and/or compounds to other
areas, not immediately around the device itself. In order to avoid
the potential complications associated with systemic drug delivery,
the medical devices of the present invention may be utilized to
deliver therapeutic agents to areas adjacent to the medical device.
For example, a rapamycin coated stent may deliver the rapamycin to
the tissues surrounding the stent as well as areas upstream of the
stent and downstream of the stent. The degree of tissue penetration
depends on a number of factors, including the drug, agent or
compound, the concentrations of the drug and the release rate of
the agent.
[0188] The drug, agent and/or compound/carrier or vehicle
compositions described above may be formulated in a number of ways.
For example, they may be formulated utilizing additional components
or constituents, including a variety of excipient agents and/or
formulary components to affect manufacturability, coating
integrity, sterilizability, drug stability, and drug release rate.
Within exemplary embodiments of the present invention, excipient
agents and/or formulary components may be added to achieve both
fast-release and sustained-release drug elution profiles. Such
excipient agents may include salts and/or inorganic compounds such
as acids/bases or buffer components, anti-oxidants, surfactants,
polypeptides, proteins, carbohydrates including sucrose, glucose or
dextrose, chelating agents such as EDTA, glutathione or other
excipients or agents.
[0189] Although shown and described is what is believed to be the
most practical and preferred embodiments, it is apparent that
departures from specific designs and methods described and shown
will suggest themselves to those skilled in the art and may be used
without departing from the spirit and scope of the invention. The
present invention is not restricted to the particular constructions
described and illustrated, but should be constructed to cohere with
all modifications that may fall within the scope of the appended
claims.
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