U.S. patent application number 11/590648 was filed with the patent office on 2007-06-07 for expandable biodegradable polymeric stents for combined mechanical support and pharmacological or radiation therapy.
This patent application is currently assigned to Texas Stent Technologies, Inc.. Invention is credited to Robert C. Eberhart, Shih-Horng Su.
Application Number | 20070129793 11/590648 |
Document ID | / |
Family ID | 26841294 |
Filed Date | 2007-06-07 |
United States Patent
Application |
20070129793 |
Kind Code |
A1 |
Su; Shih-Horng ; et
al. |
June 7, 2007 |
Expandable biodegradable polymeric stents for combined mechanical
support and pharmacological or radiation therapy
Abstract
An expandable biodegradable polymeric stent is fabricated with
biodegradable polymer fibers (Poly-L-lactic acid, PLLA) in a coil
shape that is constructed with both central and external or
internal peripheral lobes. It is delivered and expanded using a
conventional angioplasty balloon system. The disclosed stent can
serve as a temporary scaffold for coronary vessels after PTCA or
for peripheral endovascular stenting, or it can provide mechanical
palliation for strictures of ductile organs (trachea, esophagus,
bile and pancreatic ducts, ureter etc.). The disclosed stent also
serves as a unique device for specific local drug delivery.
Therapeutic agents (chemical compounds, protein enzyme and DNA
sequences) and cells can be loaded into the stent and gradually
released to target tissues. Local radiation therapy can also be
delivered by a specially adapted stent.
Inventors: |
Su; Shih-Horng; (Irvine,
CA) ; Eberhart; Robert C.; (Dallas, TX) |
Correspondence
Address: |
FISH & RICHARDSON PC
P.O. BOX 1022
MINNEAPOLIS
MN
55440-1022
US
|
Assignee: |
Texas Stent Technologies,
Inc.
|
Family ID: |
26841294 |
Appl. No.: |
11/590648 |
Filed: |
October 30, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10143676 |
May 10, 2002 |
7128755 |
|
|
11590648 |
Oct 30, 2006 |
|
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|
60295039 |
Jun 1, 2001 |
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Current U.S.
Class: |
623/1.49 |
Current CPC
Class: |
A61F 2210/0095 20130101;
A61F 2230/0017 20130101; A61F 2230/0021 20130101; A61F 2250/0067
20130101; A61F 2/86 20130101; A61F 2230/0023 20130101 |
Class at
Publication: |
623/001.49 |
International
Class: |
A61F 2/06 20060101
A61F002/06 |
Claims
1-20. (canceled)
21. An implantable vascular stent having a compressed configuration
and an expanded configuration, comprising: at least one fiber cord
defining a repeating structure orbiting and extending along a
central axis of the stent, the repeating structure having a
plurality of furled segments formed of a first length of the fiber
cord and each connecting the proximal ends of two adjacent cord
segments, each cord segment formed of a second length of the fiber
cord; wherein in the compressed configuration, the total length
between the distal ends of the two adjacent cord segments is less
than the sum of the first length and twice of the second length;
wherein the repeating structure is adapted to inelasticly expand
into a substantially helical shape about the central axis in the
expanded configuration such that the total length between the
distal ends of the two adjacent cord segments is approximately
equal to the sum of the first length and twice of the second
length.
22. The implantable vascular stent of claim 21 wherein the
repeating structure is adapted to expand into a substantially
helical shape, in part, by the unfurling of the furled
segments.
23. The implantable vascular stent of claim 22 wherein the fiber
cord has an elastic limit such that actuation stress forces
exceeding the elastic limit will set the repeating structure in the
substantially helical shape following removal of an expanding
stress forces.
24. The implantable vascular stent of claim 21 wherein the
repeating structure is adapted to expand into a substantially
helical shape, in part, by the deformation of the cord
segments.
25. The implantable vascular stent of claim 21 wherein the
repeating structure is adapted to be expanded by a removably
insertable inflatable membrane.
26. The implantable vascular stent of claim 21 wherein the
repeating structure is adapted to be expanded by the body heat of a
host.
27. The implantable vascular stent of claim 21, further comprising
a series of longitudinal rods extending along the repeating
structure parallel to the central axis and attached to one or more
of the cord segments.
28. The implantable vascular stent of claim 21 wherein the furled
segments extend inward from the repeating structure toward the
central axis of the stent.
29. The implantable vascular stent of claim 21 wherein the furled
segments extend outward away from the central axis of the stent
30. The implantable vascular stent of claim 21 wherein the fiber
cord is biodegradable within a host.
31. The implantable vascular stent of claim 21 wherein the fiber
cord is adapted to provide medication to an implant site of the
stent.
32. The implantable vascular stent of claim 21 wherein the fiber
cord comprises a polymer fiber.
33. An implantable vascular stent having a compressed configuration
and an expanded configuration, comprising: at least one fiber cord
defining a repeating structure orbiting and extending along a
central axis of the stent, the repeating structure having a
plurality of furled segments formed of a first length of the fiber
cord and each connecting the proximal ends of two adjacent cord
segments, cord segment formed of a second length of the fiber cord;
wherein in the compressed configuration, the total length between
the distal ends of the two adjacent cord segments is less than the
sum of the first length and twice of the second length; wherein the
repeating structure is adapted to be substantially irreversibly
expanded into a substantially helical shape about the central axis
in the expanded configuration such that the total length between
the distal ends of the two adjacent cord segments is approximately
equal to the sum of the first length and twice of the second
length.
34. An implantable vascular stent having a compressed configuration
and an expanded configuration, comprising: at least one fiber cord
defining a repeating structure orbiting and extending along a
central axis of the stent, the repeating structure having at least
one furled segment formed of a first of the fiber cord and
connecting the proximal ends of two adjacent cord segments, each
cord segment formed of a second length of the fiber cord; wherein
in the compressed configuration, the total length between the
distal ends of the two adjacent cord segments is less than the sum
of the first length and twice of the second length; wherein the
repeating structure is adapted to inelasticly expand into a
substantially helical shape about the central axis in the expanded
configuration such that the total length between the distal ends of
the two adjacent cord segments is approximately to the sum of the
first length and twice of the second length.
Description
TECHNICAL FIELD OF THE INVENTION
[0001] This invention generallyrelates to stents for implantation
into blood vessels or other organs, and more specifically to stents
that are absorbable over time and capable of local drug/gene
delivery for enhancing therapeutic effects.
BACKGROUND OF THE INVENTION
[0002] Intraluminal stents are commonly employed for treatment of
various vascular conditions such as arteriosclerosis, often as
coronary artery implants. A stent can be implanted at the site of a
vessel stricture or stenosis using a conventional balloon catheter
delivery system as used in angioplasty. Stents also maybe employed
in bodypassageways other than blood vessels to treat strictures or
prevent luminal occlusion. Such stents ordinarily consist of a
cylindrical network of very small metal wires. The stent is
inserted in a small-diameter configuration and then expanded to a
large-diameter final configuration against the walls of the blood
vessel or other body lumen. Such stent structures and implantation
techniques are well known.
[0003] Great efforts have been expended to modify metallic stents
to eliminate stress-induced and/or inflammation-induced restenosis,
and to effectively deliver therapeutic agents to lesion sites. Some
advancements in drug-coated metal stents have been made recently.
However, metallic stents still present a potential vessel injury
problem. Furthermore, the delivery of medicine to a lesion site
either by local or systemic means is unsatisfactory with current
stent and catheter technology. The present invention addresses
these problems.
SUMMARY OF THE INVENTION
[0004] In accordance with a principal object of the present
invention, luminal support and localized treatment of lesion sites
within body passageways is accomplished by the implantation of an
expandable biodegradable polymeric stent that includes therapeutic
agents. By virtue of its gradual absorption over time, the
inventive stent avoids residual stress, and permits local drug
delivery or local radiation treatment.
[0005] In its preferred implantation, the stent of the present
invention provides adequate mechanical support during and following
the interventional procedure, and, by being absorbed over
controllable periods, avoids chronic mechanical disturbance of the
vessel wall. The residual stress against the vessel wall is
eliminated after the stent is degraded. During the degradation
process, loaded therapeutic agents are released in a controlled
fashion, and effective concentrations at target lesions can be
maintained. Local radiation treatment can likewise be
maintained.
[0006] The stent of the present invention preferably has the
following features: (1) it has an all-polymer construction with
similar mechanical function to conventional metallic stents; (2) it
is constructed with fiber cords having both central and peripheral
lobes and is stabilized by longitudinal rods, thus presenting a low
profile during delivery and a large effective diameter following
expansion; (3) it is expandable with an expansion ratio that can be
customized to meet various needs; (4) it can be deployed at body
temperature with low inflation pressure (3 atm); (5) it is a
temporary implant; (6) it may be a local drug or gene delivery
device; (7 ) it may be a local radiation therapy device; and (8) it
can include fibers with various functions (mechanical support,
acute drug burst, long-term drug release, etc.), enabling a variety
of treatment options including multiple functions with a single
stent and using a single stent-implant procedure.
[0007] The present invention has a number of advantages over
conventional stents. Firstly, in contrast to metal stents, the
polymeric stent of the present invention is a temporary implant.
The temporary residence permits the residual stress against the
vessel wall to be resolved, a factor commonly leading to in-stent
restenosis in the case of metallic stents. Secondly, the inventive
stent is also capable of carrying therapeutic agents either
incorporated in the polymer bulk or coated on the polymer surface.
Thirdly, it is possible to control the operation of the inventive
stent by selection of the polymer composition, the polymer
molecular weight, fiber cord diameter and processing conditions,
thus controlling the degradation rate, drug release rate and period
of mechanical support. Fourthly, compared with tubular-shaped
polymeric stents, the inventive stent has superior expandability
and flexibility. Additionally, the inventive stent also has
advantages over the "zigzag" polymeric stent recently disclosed in
the prior art (Circulation, vol. 102, pp. 399-404, 2000), since it
is deployed at body temperature with low inflation pressure.
[0008] In addition to being biodegradable, the stent of the present
invention synergistically combines excellent mechanical support and
local drug delivery, for both short-term and long-term
applications. Current metallic stents are incapable of delivering
drugs without polymer coatings. Moreover, metallic stents are known
to be a stimulus for chronic vessel injury. Other current
approaches, such as the combination of a metallic stent and bolus
drug delivery by a porous angioplasty balloon, provide both
mechanical support and short-term drug delivery. However, other
than initial control of drug concentration at the lesion site, the
porous angioplasty balloon approach is limited in its application
and is incapable of performing certain desirable functions, such as
prolonged drug delivery and transient radiation therapy. The
biodegradable polymeric stent of the present invention provides
sufficient mechanical strength as well as controllable short-term
and long-term drug delivery while eliminating the stimulus for
chronic vessel wall injury.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] For a more complete understanding of the present invention
and the advantages thereof reference is now made to the following
description taken in conjunction with the accompanying Drawings in
which:
[0010] FIG. 1 is a schematic illustration of the three-dimensional
structure of an expandable stent according to the present
invention;
[0011] FIGS. 2A, 2B and 2C are schematic end views of the inventive
stent at three stages during deployment with an inflatable balloon
shown therein in phantom, FIG. 2A showing the stent in its furled
state, FIG. 2B showing the stent in a partially expanded state, and
FIG. 2C showing the stent in its fully expanded state;
[0012] FIG. 2D is a schematic perspective view of the inventive
stent in its fully expanded state; and
[0013] FIGS. 3A and 3B are schematic end views of an alternative
embodiment of the inventive stent.
DETAILED DESCRIPTION OF THE INVENTION
[0014] Referring to FIG. 1, a preferred embodiment of a stent
according to the present invention is shown and generally
designated by reference numeral 10. The stent 10 comprises a coiled
cord 50 of non-metallic material, preferably a polymer fiber or ply
of multiple polymer fibers, wherein the polymer preferably
comprises Poly-L-Lactic Acid ("PLLA"). The use of PLLA to construct
the stent 10 is advantageous because it is biodegradable. It
degrades away gradually within the body, the chemical products of
the degradation process being primarily carbon dioxide and water,
which are harmless to the host patient. Degradation occurs over a
period of about six months to three years, mainly depending on the
molecular weight of the polymer employed. PLLA is also advantageous
because it can be impregnated with drugs or other chemical agents
for local treatment of tissue at the stent implant site.
[0015] By way of example, the stent 10 of FIG. 1 is constructed
with twelve coil rotations of a single-fiber cord, each rotation
having one central lobe 20 and three peripheral lobes 30. The
twelve central lobes 20 form the backbone of the stent 10. Three
longitudinal rods 40 are attached on the exterior surface of the
central lobes 20, preferably using a viscous PLLA-chloroform
solution. The rods 40 may comprise the same single-fiber material
as the coil of lobes 20 and 30. Alternatively, the coil of lobes 20
and 30 and the rods 40 may comprise a multiple-fiber ply material.
For example, the coil of lobes 20 and 30 may be formed from a
double-fiber ply material, and each of the three rods 40 maybe
formed from a triple-fiber ply material for added rigidity. Also,
by way of example, the length of inventive stent is 15 mm and the
initial diameter is 1.9 mm. In this example, the final diameter,
after balloon expansion, can reach 3.24 mm. The length of the stent
can be increased by increasing the number of coil rotations. The
peripheral and central lobe diameters determine the final diameter
of the stent. To assure mechanical integrity, it is preferred that
the coiled lobes 20 and 30 of the stent 10 be formed from a single
cord that is continuous end-to-end.
[0016] The mechanical strength of the stent 10 can easily be varied
(1) by adjusting the coil density; or (2) by adjusting the fiber
ply. In practice, a stent with 15 coil rotations and a length of 15
mm will be stronger than a stent of the same length with 12
rotations. However, a stent in which the cord 50 is composed of a
multiple-fiber ply will have higher resistance to radial
compression. A double-fiber ply will have about twice the radial
compression resistance of a single-fiber construction, and
triple-fiber ply will have about three times the radial compression
resistance of a single-fiber construction. Additionally, the
diameter of the stent 10 can be adjusted (1) by adjusting the
diameter of central and peripheral lobes; or (2) by adjusting the
number of multiple peripheral lobes percentral lobe. The stent
diameter increases as the diameter of central and peripheral lobes
increases, and vise versa. It will also be appreciated that more
peripheral lobes with the same diameter results in a stent of
larger diameter in its fully expanded state.
[0017] The above-described design provides an excellent way to
maximize the expandability of a polymeric stent. The major
difference between metal and polymeric stent materials is that
metal is more malleable and generally has a greater tensile
strength. Thus, a metal wire can be deformed without affecting
mechanical strength. In contrast, a polymer fiber cord cannot
retain its original mechanical strength following permanent
deformation (bending, for example). Despite the lower mechanical
strength of polymeric materials relative to metals,- the polymeric
stent of the present invention has sufficient strength to retain
its shape in the expanded state, thereby stabilizing the vessel or
duct wall for the intended purposes as with a conventional metal
stent.
[0018] In accordance with an important concept of the invention, an
extra length of cord is provided by the peripheral lobes to
facilitate expansion from the furled state to the final
large-diameter state. If the desired final length of the stent in
the furled, multiple-lobe configuration is known, stents can be
prepared using the exact same initial length of cord. After
expansion, the final deployed length is achieved without damaging
cord. It will be appreciated that this approach to stent design and
fabrication provides a polymeric stent with excellent mechanical
strength and flexibility for effective implantation.
[0019] According to another important feature of the invention, the
longitudinal rods 40 provide support for the flexible coiled cord
50. Furthermore, the longitudinal rods 40 maintain the axial length
of the stent 10 constant as its radial dimension increases during
expansion. Solid wall tubular stents have the practical limitations
that they are relatively inflexible, making it difficult for them
to pass through sometimes tortuous vessel networks. This is because
their relatively rigid cylindrical structure reduces the freedom to
bend in all directions. In this invention, the integrity of the
stent 10 is maintained by the longitudinal rods 40, three in
embodiment of FIG. 1 preferably arranged at 120.degree. intervals.
Therefore, the expandable stent 10 has the inherent flexibility of
a coil design yet has sufficient rigidity for effective handling
due to the presence of the longitudinal rods 40. The advantages of
this design compared with currently available clinical metal models
will be readily apparent to the skilled practitioner.
[0020] It should be mentioned that the number of longitudinal
reinforcing rods can be selected based on the number of peripheral
lobes that design considerations dictate. Preferably, the
longitudinally aligned groups of peripheral lobes are equal in
number to the longitudinal reinforcing rods, which are alternately
positioned so that each rod is midway between its two neighboring
peripheral lobe groups. In FIG. 1, the preferred arrangement is
illustrated in which there are three longitudinal reinforcing rods
40 and three longitudinally aligned groups of peripheral lobes
30.
[0021] Prototypes of the inventive stent have been constructed
using a fixture and manually winding a fiber cord in a spiral
fashion along the fixture. The fixture employed included a central
cylindrical mandrel attached to a base at one end, and three
cylindrical side posts attached to the base and extending along and
parallel to the mandrel, the posts being circumferentially spaced
around the mandrel at 120.degree. intervals. The stent is
constructed by attaching one end of the cord to the free end of the
mandrel, then winding the cord around the mandrel, and successively
looping the cord around the posts moving downward toward the base
until twelve rotations of the mandrel have been completed.
Periodically during the winding process, each of three longitudinal
rods 40 are attached to the central lobes 20 in the manner depicted
in FIG. 1. Upon completion, the stent is slidably removed from the
mandrel and side posts. Design of an automated system is
contemplated for reducing the labor-intensive winding process used
to make the prototype stents.
[0022] The stent delivery and deployment system is based on
conventional balloon catheter delivery systems used currently in
clinical angioplasty. Therefore, the stent of the present invention
can be implanted in practice using much of the conventional
clinical deployment techniques used with metal stents.
[0023] FIGS. 2A-D illustrate the procedure of stent expansion and
the structure of an expanded stent. In FIG. 2A, the stent 10 is in
its small-diameter furled state, which enables the stent 10 to
readily travel through a vessel to a site where it is to be
deployed. A balloon 60, shown in phantom, is provided inside the
stent 10 to effect expansion. In this end view, the symmetrical
spacing of the three rods 40 with the three longitudinally aligned
groups of peripheral lobes 30 can be envisioned more clearly when
considered together with FIG. 1. In the small-diameter furled
state, it will be seen that the central lobes 20 viewed from the
end of the stent 10 are generally triangular in shape. Thus, the
term "small-diameter" is used herein to describe the relative size
of the stent 10 in the original furled state, the "diameter" in
this context being the effective diameter of a circle or imaginary
cylinder tangentially contacting the outer ends of the peripheral
lobes 30.
[0024] In FIG. 2B, the stent 10 is starting to expand under the
force of the expanding balloon 60, as indicated by the arrows. For
comparison, dashed lines are provided in FIG. 2B to show the
configuration of the stent 10 in its original furled state as
depicted in FIG. 2A.
[0025] In FIG. 2C, the stent 10 is shown in its large-diameter,
fully expanded state, in which the peripheral lobes 30 (shown in
FIGS. 2A and 2B) have disappeared, their cord lengths having merged
into the central lobe 20 of each of the twelve coils. Experimental
data reveals that the stent 10 expands uniformly under increasing
balloon pressure until it reaches its final diameter. The terms
"final diameter" and "large-diameter" are used to describe the
relative size of the stent 10 in its fully expanded state as
depicted in FIG. 2C, the "diameter" being the effective diameter of
a circle or imaginary cylinder tangentially contacting the outer
edges of the longitudinal rods 40. FIG. 2C is not drawn to an
accurate relative scale compared to FIG. 2A. In practice, it has
been found that sufficient cord length can be provided in the
peripheral lobes 30 to cause the effective diameter of the stent 10
to approximately double in size going from the original furled
state of FIG. 2A to the final fully expanded state of FIG. 2C.
[0026] FIG. 2D shows the stent 10 with the balloon removed in its
large-diameter state and also depicts the longitudinal rods 40 in
their 1200 spaced peripheral positions along the length of the
stent 10. The helical nature of the stent 10 in its fully expanded
state is evident in FIG. 2D. Though the central lobes 20 are
derived from a single cord of polymeric material that generally
defines a helix in the fully expanded state, each lobe 20 can be
viewed as one 360.degree. length of cord with a leading end and a
trailing end spaced apart by one-twelfth (in the case of a
twelve-lobe stent) of the length of the stent 10. For example, to
illustrate this concept, the first lobe 20a at the right end of the
stent 10 of FIG. 2D has a leading end 70 and a trailing end 80. The
trailing end 80 of the first lobe 20a corresponds to the leading
end of the second lobe 20b. The pattern continues through the
length of the stent 10, each lobe's trailing end corresponding to
the next successive lobe's leading end until the last lobe is
reached, whose trailing end (not shown in FIG. 2D) is the free end
of the cord 50 at the left end of the stent 10.
[0027] It will be appreciated from FIGS. 1 and 2A that the stent 10
in its original furled state has a more complex shape. From the
example shown in FIG. 2A, it will be appreciated that each central
lobe 20 has three peripheral lobes 30, a leading one of which being
defined by a portion of the cord 50 that adjoins the leading end of
the corresponding central lobe 20, a trailing one of which being
defined by a portion of the cord 50 that adjoins the trailing end
of the corresponding central lobe 20, and the last of the three
peripheral lobes 30 being defined by a portion of the cord 50 at an
intermediate point of the corresponding central lobe 20.
[0028] The stent 10 of the present invention can be adapted to a
broad range of inflation pressures from 3 to 10 atm (a maximum
pressure possibly even exceeding 10 atm). Experimental data has
shown that, using a double-fiber ply stent, full expansion occurs
at about 3 atm, and that the fully expanded diameter is stably
maintained at inflation pressures of up to 10 atm. In the
above-described example, the stent 10 has limited recoil about 4%
when in an unstressed condition. The collapsing pressure holds at
least up to 16 psi (i.e., greater than 1 atm), which is comparable
to conventional metal stents.
[0029] It will be appreciated that the preferred PLLA fibers
preferably used for the stent fabrication can be loaded with a
non-steroid type anti-inflammation agent, such as curcumin. The
curcumin-loaded fibers significantly reduce inflammation at the
stent implant site by reducing the adhesion of inflammatory cells.
Other drugs can be used with the expandable biodegradable polymer
stent of the present invention. The impregnated drugs can be
prepared in a way that controllably delivers the drug over a
predetermined time period.
[0030] FIGS. 3A and 3B show an alternate embodiment of the
inventive stent, generally designated by reference numeral 100. The
stent 100 has a furled state shown in FIG. 3A in which the fiber
coils are tightly furled and central lobes 120 (one shown) are
confined to a small diameter. There are three peripheral lobes per
coil, which are designated by numerals 130 and, in this embodiment,
are located inside the central lobes 120. As in the
previously-described embodiment of the stent 10 shown in FIG. 1,
there may be twelve coils, which are formed from a continuous cord
and extend longitudinally to define the body of the stent 100. Each
coil has a large central lobe 120 and three internally-disposed
peripheral lobes 130, shown in FIG. 3A. As in the previously
described stent 10, the stent 100 has longitudinally extending rods
140 that support the coil structure. When the stent 100 is expanded
as shown in FIG. 3B, the peripheral lobes merge into a single
large-diameter central lobe 120 for each of the twelve coils of the
stent 100. Using this construction of internal peripheral lobes
130, the ratio of the final expanded stent diameter to the initial
furled stent diameter can be greater than a factor of two.
[0031] Those skilled in the art will appreciate that the inventive
stent, in its disclosed embodiments or variations thereof, provides
mechanical and therapeutic advantages over conventional stents. In
addition, advantageous treatments will suggest themselves to the
skilled practitioner considering the foregoing description of the
invention. By virtue of the biodegradable polymeric nature of the
inventive stent, the same vessel site can be retreated at a later
time if needed, including staging procedures during growth of the
patient. Similarly, successive treatments of a tissue that is
changing size can be facilitated with the disclosed stent. It
should-also be noted that the inventive stent can be implanted at a
site of healthy tissue for diagnostic purposes or therapeutic
treatment of adjacent tissue.
[0032] Although preferred embodiments have been described and
illustrated, it should be understood that various changes,
substitutions and alterations can be made therein without departing
from the spirit and scope of the invention as defined by the
appended claims.
* * * * *