Patent Application
20230002709
The present invention is generally in the field of tissue engineered grafts, particularly those that are seeded with cells and are designed to be absorbed and replaced by patient's own tissues, and methods of making.
Surgical treatment of many complex congenital cardiac anomalies involves implantation of synthetic scaffolds of materials such as GORE-TEX® and DACRON®. A common example of such an application is the total cavopulmonary connection (TCPC) for single ventricle anomalies. In some instances, the use of these synthetic grafts as scaffolds is complicated by progressive obstruction, susceptibility to infection, and risk of thromboembolic complications. In all instances, a significant limitation to cardiovascular reconstruction is lack of growth potential of synthetic implants. For TCPC, this may lead to sub-optimal management strategies including either postponement of completion of the Fontan circulation because of patient size, or over-sizing of scaffolds, which results in sub-optimal flow characteristics such as expiratory phase back-flow and regions of flow stagnation.
Tissue-engineered vascular grafts (TEVGs) offer the potential to overcome these problems by providing a biodegradable scaffold in which autologous cells proliferate and mature into a physiologically functional blood vessel as scaffold polymers degrade (Shin'oka, et al. N. Engl. J. Med. 344(7), 532-533 (2001); Hibino, et al. J Thorac Cardiovasc Surg. 139(2), 431-436.e432 (2010 Roh J D, et al. Biomaterials. 29(10), 1454-1463 (2008); Hibino, et al. FASEB J. 25(8), 2731-2739 (2011); Hibino, et al. FASEB J. 25(12), 4253-4263 (2011); Kurobe, et al. Tissue Eng Part C Methods 21(1), 88-93 (2015)).
Clinical trials in humans confirmed the growth capacity of the TEVG and demonstrated no graft related deaths or graft failures (Shin′oka, et al. J Thorac Cardiovasc Surg. 129(6), 1330-1338 (2005)). However, the results of this study also demonstrated that stenosis was the primary graft-related complication, effecting nearly 25% of graft recipients, with 16% of recipients developing critical stenosis (>75% decrease in luminal diameter) (Hibino, et al. J Thorac Cardiovasc Surg. 139(2), 431-436.e432 (2010)). Despite promising results using TEVGs for the treatment of patients with congenital heart diseases, the high incidence of graft stenosis in clinical applications hinders wide spread use of this technology (Fernandez, et al. Current opinion in chemical engineering 3, 83-90 (2014); Mcallister, et al. Lancet 373(9673), 1440-1446 (2009); Wystrychowski, et al. J Vasc Surg 60(5), 1353-1357 (2014)).
Moreover, before routine clinical use of the TEVGs can be recommended, the assembly of the TEVG must be optimized to personalize the grafts and minimize the time required to make the graft and improve its overall utility (Patterson, et al. Regenerative Medicine 7(3), 409-419 (2012)).
U.S. Pat. No. 9,090,863 and U.S. Publication No. US 2018/0353649 describe closed disposable seeding systems (CDSS) for seeding scaffolds and grafts with cells. The CDSS includes seeding chambers for housing and seeding the scaffolds and grafts with cells. The seeding of scaffolds and grafts using the CDSS of U.S. Pat. No. 9,090,863 and U.S. Publication No. US 2018/0353649 provide a non-uniform cell seeding density along the length of the scaffolds and grafts.
There remains a need for improved seeded scaffolds and grafts having a uniform cell density along the length of the scaffolds and grafts. There remains a need for scaffolds and grafts inducing reversible stenosis following implantation.
Therefore, it is the object of the present invention to provide improved seeding chambers for use with the CDSS for uniform cell seeding along the length of the scaffolds and grafts to reduce the rates of or prevent stenosis following implantation.
It is another object of the present invention to provide a system with improved seeding chambers for seeding cells on scaffolds and grafts to reduce the rates of, prevent, or reverse stenosis following implantation.
It is yet another object of the present invention to provide scaffolds and grafts with structural parameters inducing reversible stenosis following implantation.
It is yet another object of the present invention to provide a method for seeding cells on scaffolds and grafts to reduce the rates of, prevent, or reverse stenosis following implantation.
It was discovered during clinical trials that scaffolds and grafts having non-uniform cell seeding density along the length of the scaffold or graft lead to an increased incidence of graft stenosis following implantation. Improved seeding chambers for use in a closed disposable seeding system have been developed which provide a uniform density of cells seeded along the length of the scaffold or graft. The cell seeding chambers, termed “flip” chamber and “capacitor” chamber, have a cap with lateral ports. The flip chamber typically has a variable width along its length. The flip chamber may be operated by flipping the seeding chamber about half-way (180°) during the seeding. The capacitor chamber typically has a uniform width along its length and a narrow gap between the scaffold and the chamber wall.
The cell seeding chambers generally include a housing, a cap with one or more lateral ports, and a base. The cap typically has an opening for a suction rod insertable into the housing, and a mandrel positioned over the suction rod.
The cap may include one or more lateral tubes, each connecting to a lateral port. The lateral tubes typically include a lateral inlet tube connecting to a lateral inlet port, a lateral outlet tube connecting to a lateral outlet port, and a lateral went tube connected to a lateral vent port. The cap typically has a superior smooth surface and an inferior threaded surface. The threaded surface secures the cap onto the housing. The top portion of the cap is generally flat, and the cap may serve as a base if the chamber is inverted.
The mandrel is typically a perforated, porous mandrel. Alternatively or additionally, the mandrel may have protrusions of any suitable arrangements, such as axially arranged protrusions running in parallel to one another, spiraling down, arranged in a diamond pattern, in circles, in zig-zags, and/or waves.
The housing of the cell seeding chamber may have a uniform or a variable width, along its length. The cell seeding chamber may have a gap between about 1 mm and about 30 mm, more preferably between about 3 mm and 20 mm, between the suction rod or the mandrel and the housing for the flip chamber. The cell seeding chamber may have a gap between about 1 mm and about 10 mm, more preferably between about 1 mm and 5 mm between the suction rod or the mandrel and the housing for the capacitor. The housing may have a region with the greatest width positioned between about 30% and 60% of the length of the housing. For example, the region with the greatest width may be positioned at about 50% (half way) along the length of the housing. The housing typically has a threaded portion for receiving the threaded surface of the cap. The ‘Flip’ seeding chamber eliminates the variability of the distribution of MNCs on the scaffold. The central premise behind the device is to vary the cross section of the seeding chamber along its length. The shape of the chamber resembles that of the inverse of an hour glass, with a bulge (greatest cross section) approximately 40% of the way down the height of the device. Both the top and the bottom of the device narrow to accommodate the mandrel with the scaffold mounted to it, along with a very minimal volume for part clearance and excess liquid. The effect of the change in cross section is that it effectively slows the speed at which the MNCs are drawn through the least seeded portions of the scaffold.
Also provided are methods for uniformly seeding a graft or scaffold with cells. In some aspects, the method includes the steps of a) connecting a seeding chamber to a closed disposable seeding system containing a vessel with fluid having blood or enriched cells, b) inserting a graft or scaffold over the mandrel of the seeding chamber, c) filling the seeding chamber with the fluid to about half way of graft or scaffold by gravity flow and bleeding air, d) filing the seeding chamber with the fluid to cover the graft or scaffold, and e) introducing negative pressure to draw all the fluid through the graft or scaffold.
In other aspects, the method includes the steps of: a) connecting a seeding chamber to a closed disposable seeding system containing a vessel with fluid having blood or enriched cells, b) inserting a graft or scaffold over the mandrel of the seeding chamber, c) filling the seeding chamber with the fluid, d) applying negative pressure to lower mandrel outlet and emptying the chamber to about half way, e) inverting the chamber, and f) applying negative pressure and emptying the chamber through an upper outlet port.
Typically, the seeding chamber is connected to the closed disposable seeding system via the lateral inlet port and the lateral outlet port. The negative pressure may be provided by a syringe, a pump, or a vacuum source. Typically, the graft or the scaffold is uniformly seeded using a minimal volume of a cell-containing fluid, such as a volume between about 10 ml and 200 ml, preferably between 10 ml and 150 ml, of blood, bone marrow aspirate, or mononuclear cell (MNC)-enriched fluid.
The term “Tissue-Engineered Vascular Graft (TEVG)” refers to a vascular graft or scaffold that is designed for insertion into the body for use in the repair or augmentation of one or more vessels, such as arteries and veins.
The term “biocompatible” refers to a material that the body generally accepts without a major immune response, which is capable of implantation in biological systems, for example, tissue implantation, without causing excessive fibrosis or rejection reactions.
The term “biodegradable” refers to the ability of a substance or material to break down when exposed to water, enzymes or in an in vivo environment.
As used herein, “stenosis” refers to a reduction in lumen diameter of 25% or more relative to lumen diameter of scaffold at implantation.
The term “mandrel” refers to a cylindrical device or tube, e.g., a metal bar, that serves as a core around which material, e.g., a matrix scaffold for seeding and growing cells, may be cast, molded, forged, bent, or otherwise shaped. The mandrel is typically open on at least one end. The mandrel may also contain holes or perforations along its longitudinal axis (i.e., along its length).
As used herein, the term “porous” refers to having one or more openings, pores, perforations or holes that may be filled or perfused by a liquid and/or a gas, or that allows for the flow of a liquid and/or gas therethrough.
As used herein, the term “spontaneous” in the context of reversal of stenosis refers to an action occurring without an additional invasive or non-invasive procedure, such as without a surgical intervention or a surgical correction.
As used herein, the term “reversal of stenosis” refers to reduction of stenosis by at least about 70%, 75%, 80%, 85%, 90%, 95% or by 100% relative to a an implanted graft without seeded cells and stenosed.
As used herein, the term “substantially” refers to a measure of about 80%, about 85%, about 90%, about 95%, or about 98%.
A. Closed Disposable Seeding Systems
Systems and methods thereof for seeding TEVGs with cells are known in the art. An exemplary system is described in U.S. Pat. No. 9,090,863 and in US 2018/0353649 (
Typically, systems for seeding of cells into tissue engineered vascular grafts include means for creating a vacuum that is connected to a patient for extracting biological material from the patient, into a chamber or scaffold. The scaffold acts as an incubator, allowing at least a portion of the biological material to contact and interact with the scaffold. The scaffold is in fluid communication with a filter/switch combination that allows selectable fluids to transfer from the biological material, back into the patient. An exemplary biological materials that are transferred back into the patient include serum, red bloods, platelets, white blood cells and combinations. Extracting selected biological materials from the fluid that contacts the scaffold reduces the time required for seeding the scaffold with a desired cell types, and increases healing rates in the patient.
The component parts of an exemplary system, described in c, are illustrated in
The components are interconnected and used as described in U.S. Pat. No. 9,090,863 and US 2018/0353649.
For example, the system includes a vessel for containing a cellular isolate, e.g., a cellular isolate fluid, such as a container (3). Exemplary containers include a media bag or any flexible or rigid container capable of being sterilized and/or that is hermetically sealed (e.g., Gibco-BFL 1 L media bag). In certain embodiments, the cellular isolate fluid container (3) is formed from of a biocompatible, rigid material capable of being sterilized, such as TEFLON®, polycarbonate, polyvinyl chloride (PVC), or stainless steel. The container (3) can have any suitable volume for containing the cellular isolate fluid, typically less than 250 ml. In a preferred embodiment, the container (3) has at least one port adapted for the sterile filling and/or dispensing of a fluid, for example, a bone marrow aspirate. For example, a bone marrow aspirate (e.g., 5 ml/kg body weight) is aseptically collected and passed, e.g., injected, into container (3) via port (1) or port (2).
Typically, the container (3) has at least one inlet and one outlet. In some embodiments, the container (3) includes a port having one or more valves to allow for the one-way flow of a fluid or gas. For example, using the embodiment shown in
Examples of fluid which may be used in the system include, but are not limited to, sterilizing fluid, contrast media fluid, biological fluid, fluid containing cells, blood, serum, bone marrow aspirate, or fluid containing a culture medium. It is to be understood that during testing, seeding, and culturing in a preferred embodiment, the fluid may be kept at human body temperature, and may be composed of a fluid which approximates the viscosity of human blood. One illustrative example of a solution which approximates the viscosity of blood is saline with glycerol.
The fluid in container (3) is passed from the container through fluid line (51) of
Fluid line (51), as well as all other fluid lines in the system (e.g., lines 52, 53, 54, 55, 56, 57, 58a-58d, and 59), may be made of any type of medical grade, sterilizable, durable tubing suitable for transporting the fluid or gas in use. For example, the fluid line can be flexible or rigid plastic.
The system also includes a flow channel (7) including at least one inlet, at least one outlet and at least one filter including at least one filter medium (e.g., disposed in a filter housing) between. In a preferred embodiment, the filter is disposed at an angle that is approximately perpendicular to the direction of flow through the flow channel (7), although in some embodiments, the filter can be disposed at an angle approximately parallel to the direction of flow, e.g., tangential flow filtration. In preferred embodiments, the filter is adapted to allow flow through in at least two directions, for example, where the first and second directions are approximately opposite, e.g., wherein a fluid can be passed in a first direction from the upstream surface of the filter through the downstream surface, and a fluid can be passed in a second direction from the downstream surface of the filter though the upstream surface. In an example of this embodiment, a cellular isolate fluid is passed in a first direction through a filter having a suitable pore size (or mesh size), wherein the filter medium is at an angle that is approximately perpendicular to the direction of flow such that the filter retains cells and/or biological material that is too large to pass through the filter. A second fluid is subsequently passed in a second direction through the filter which can wash the retained cells and/or biological material off of the filter medium. Filters that can be employed for use in the flow channel are well known in the art and include, for example, Pall Corporation. In other embodiments, the filter (e.g., at least one filter medium) has a porosity suitable to retain cells, e.g., bone marrow-derived mononuclear cells. In certain embodiments, the filter includes a matrix that is designed to reversibly bind and retain the cells of interest based upon, for example, ligand-receptor interactions. In other embodiments, multiple filters can be assembled in series or in parallel for use in the system as described herein. It is contemplated that the system and method can have any number of desired flowpaths and shutoffs. The liquid or gas can be fed through the system by positive pressure or negative pressure, such as via a syringe, pump, or vacuum source.
In certain embodiments, the system includes a means for containing a collection fluid. In certain embodiments the means is a container (13), such as a media bag or any flexible or rigid container capable of being sterilized and/or that is hermetically sealed (e.g., GIBCO-BFL 1 L media bag). In certain embodiments, collection container (13) is formed from a biocompatible, rigid material capable of being sterilized such as Teflon, polycarbonate, acrylic, PVC, or stainless steel. Container (13) can have any suitable volume for containing the collection fluid.
In a preferred embodiment, the container (13) has at least one port adapted for the sterile filling and/or dispensing of a fluid, for example, a bone marrow aspirate filtrate or flow through. For example, a bone marrow aspirate (e.g., 5 cc/kg body weight) is aseptically collected and passed, e.g., injected, into container (3) and subsequently passed (e.g., through optional pre-filter (4) and via fluid flow lines (51) and (52) through flow channel (7) including a filter. The filter retains cells and/or the biological material of interest, allowing the filtrate to flow through fluid lines (53) and (54) and inlet port (11) into container (13). In another preferred embodiment, the container (13) has at least one flow port, e.g., a bi-directional flow port; typically, however, the container (13) has at least two ports. In certain embodiments, container (13) includes an inlet port and/or an outlet port.
In the embodiment illustrated in
In certain embodiments (e.g., after elution fluid is passed through the flow channel (7) and cells are passed into seeding container (18) as noted in more detail below), the collection fluid or filtrate contained in container (13) is passed through fluid lines (59) and (57) into seeding container (18) (in
In a preferred embodiment, the fluid is directed away from container (13) by a vacuum. In other embodiments, a fluid pump is used (e.g., Masterflex L/S Digital Drive peristaltic pump manufactured by Cole-Palmer, although one skilled in the art could select from a variety of commercially available pumps).
In certain embodiments, the system includes a means for containing an elution fluid. In certain embodiments, the means is a container (9), such as a media bag, syringe, or any flexible or rigid container capable of being sterilized and/or that is hermetically sealed (e.g., a Gibco-BFL 1 L media bag).
In accordance with the embodiment illustrated in
In one embodiment, container (9) is a syringe filled with a sterile elution or wash fluid. The elution or wash fluid is passed through flow channel 7 and into seeding container (18) through valve (8), fluid lines 53, 52, 56 and 57 in
In certain embodiments, the system includes a means for containing a cell seeding assembly. In certain embodiments, the means is a seeding container (18), such as a media bag or any flexible or rigid container capable of being sterilized and/or that is hermetically sealed. For example, a GIBCO-BFL 1 L media bag could be used. In certain embodiments, container 18 may be composed of any biocompatible, rigid material capable of being sterilized such as Teflon, polycarbonate, acrylic, PVC, or stainless steel. Seeding container (18) can have any suitable volume.
As illustrated in
The seeding container 18 includes an inlet port 16 and outlet port 24, which allows for the perfusion and/or circulation of fluid into and through the container. Inlet port 16 and outlet port 24 are also used to attach container 18 to fluid lines 57 and 58a, respectively. Fluid line 58a connects seeding container 18 to one or more residual seeded cell fluid containers 35a and 35b, while maintaining a closed system. It is to be understood that although only one seeding container 18 is shown in
The means for containing residual seeded cell fluid is at least one residual seeded cell fluid container 35a, 35b, such as a media bag or any flexible or rigid container capable of being sterilized and/or that is hermetically sealed. For example, a Gibco-BFL 1 L media bag could be used. In certain embodiments, containers 35a, 35b may be composed of any biocompatible, rigid material capable of being sterilized, such as TEFLON®, polycarbonate, PVC, or stainless steel.
In a preferred embodiment, fluid is drawn out of container 18 into a residual seeded cell fluid container 35a via port 25 and fluid line 58a through the use of vacuum assembly having a vacuum source, e.g., a pump, and a regulator 28, wherein the negative pressure from the pump is conveyed through fluid lines connected to residual seeded cell fluid container 35a, 35b and seeding container 18.
In certain embodiments, seeding container (18) houses a seeding assembly (100) containing a porous tube (20) and a scaffold (21), e.g., cell or tissue scaffold or graft, such as a vascular graft scaffolding. The porous tube (20) may include any suitable rigid material, such as TEFLON, PVC, polycarbonate, plastic, metal, e.g., stainless steel, which may be made fluid permeable. One or more retaining elements such as clips (60), O-rings, or grommets may also be placed on tube, e.g., at both ends of scaffolding (21), to hold the scaffolding in place on the tube during seeding, culturing, storing, shipping, or treatment.
In certain embodiments, the system is disposable. The closed disposable system allows for a procedure for the construction of tissue engineered graft, e.g., a vascular graft, that can be performed rapidly while achieving similar seeding efficiency as compared to previously described methods (Matsumura, et al., Biomaterials 2003; 24:2303-8; and FDA IDE 14127), which are incorporated herein by reference in their entirety. In addition, the use of the system allows one to construct the tissue engineered graft, e.g., vascular graft, at the point of care (i.e., in the operating room precluding the need for scaffold transport.
The seeding system may use seeding chamber 700 (flip seeding chamber) or seeding chamber 800 (capacitor seeding chamber) instead of the seeding container 18.
1. Seeding Chambers
The seeding chambers generally include a housing having a width and a length, a cap with one or more lateral ports, and a base. The cap typically includes a suction rod insertable into the housing, and a mandrel positioned over the suction rod. Typically, the cap is flat on its upper surface. The cap generally does not include valves, ports, or attachments on the upper surface.
In an exemplary embodiment, the seeding chamber base has a radius of between 10 and 100 mm, for example, between about 35 mm and 80 mm In an exemplary embodiment, the top of the seeding chamber has a diameter of between 10 and 100 mm, between about 25 mm and 60 mm, for example, 38.1 mm. In an exemplary embodiment, the seeding chamber base has a height of between 20 and 1000 mm, between about 120 mm and 200 mm, for example, 180 mm Typically, the inner diameter of the seeding chamber aperture is between 5 and 90 mm, for example, 25.86 mm A typical thickness for the wall of the seeding chamber is between 0.5 and 10 mm, for example, approximately 2 mm. When the seeding chamber has a threaded top, the height of the threaded section is typically approximately 10% of the total length of the chamber, for example, 20.55 mm. The mandrel, seeding chamber, and one or more scaffold clips are typically sized corresponding to the desired size of the vascular graft, seeding chamber, and are sized to fit together within the seeding chamber.
a. Flip Seeding Chamber
The ‘Flip’ seeding chamber has a variable cross section diameter along its length.
The shape of the chamber resembles that of the inverse of an hour glass, with greatest cross section approximately 30% to 60%, preferably about 40%, of the way down the height of the device. Both the top and the bottom of the device narrow to accommodate the mandrel with the scaffold mounted to it, along with a very minimal volume for part clearance and excess liquid. Typically, the cross section diameter of the flip chamber at its narrowest portions is between about 22 and 25 mm, inclusive. Typically, the cross section diameter of the flip chamber at its widest portion is between about 60 and 80 mm, inclusive.
The effect of the change in cross section diameter is that it effectively slows the speed at which the MNCs are drawn through the least seeded portions of the scaffold.
In addition, the mandrel for this design has two vacuum nubs—one to draw negative pressure in the upright orientation, and one to draw negative pressure in the inverted orientation.
An exemplary flip seeding chamber is presented in
The housing 740 has a variable cross-sectional diameter along its length. The upper 10% of the housing includes threats to receive the threaded portion of the cap 710. The bottom section of housing 740 is connected to the base 730. The housing 740 may include a lower mandrel outlets 732 and 734. O-ring grooves 760 may also be present for positioning of o-rings to seal scaffold to mandrel.
The cap 710 is connected to the suction rod 720. A porous mandrel 750 is attached to the suction rod 720. The attachment may be by any means, including clamp, screw or luer connector, pressure fitting, friction fitting, or coupling.
Typically, the flip seeding chamber is configured to stably hold in the vertical position when positioned on its base, or when positioned on its cap. This is shown in
b. Capacitor Seeding Chamber
The capacitor seeding chamber is typically a hollow tube having a consistent cross-sectional diameter along the length of the tube. Typically, the cross section diameter of the capacitor chamber is between about 10 and 20 mm, inclusive. The capacitor seeding chamber is very narrow as compared to the assembled mandrel/scaffold assembly. It is designed to hold a minimum volume of fluid. The minimum and the maximum gap between the scaffold and the wall of the chamber may be about 1 mm and about 5 mm.
An exemplary capacitor seeding chamber is presented in
The housing 840 has a constant cross-sectional diameter along its length. The upper 10% of the housing includes threads to receive the threaded portion of the cap 810. An O-ring 860 may be positioned between the cap 810 and the housing 840. The bottom section of housing 840 is connected to the base 830.
The cap 810 is connected to the suction rod 820. A porous mandrel 850 is attached to the suction rod 820. The attachment may be by any means, including clamp, screw or luer connector, pressure fitting, friction fitting, coupling or the like.
2. Mandrel
Mandrels are typically sized to fit over suction rod. The mandrels may be secured to the suction rod or to the cap with any suitable attachments. Exemplary attachments include clips, hooks, and slots accepting mandrel width. The mandrel may be attached to the suction rod by molding or gluing.
The mandrel may have any shape suitable to receive a vascular graft or scaffold. The mandrel is typically a perforated, porous mandrel. Alternatively or additionally, the mandrel may have protrusions of any suitable arrangements, such as axially arranged protrusions running in parallel to one another, spiraling down, arranged in a diamond pattern, in circles, in zig-zags, and/or waves.
Exemplary dimensions for the mandrel are provided in
B. Fluid Volume, Cell Density, and Seeding Time
Seeding of the graft or the scaffold typically includes contacting the graft or scaffold with fluid containing cells. The fluid may be blood, bone marrow aspirate, or cell extract from the blood or bone marrow. The fluid may be of a volume between about 10 ml and 200 ml, such as between about 25 ml, about 50 ml, about 75 ml, about 100 ml, about 125 ml, about 150 ml, about 175 ml, and about 200 ml. The fluid typically has a white blood cell (WBC) concentration has between about 105 cells/ml and 108 cells/ml, preferably between about 106 and 108 cells/ml, more preferably between about 106 and 107 cells/ml.
After seeding, the graft or scaffold typically has a cell density between about 0.1×103 cells/mm2 and 105 cells/mm2, inclusive, along the length of the scaffold, preferably between about 0.1×103 cells/mm2 and 104 cells/mm2, inclusive, along the length of the scaffold, most preferably between 1×103 cells/mm2 and 104 cells/mm2, inclusive, along the length of the scaffold.
Typically the graft is contacted with fluid for a period of a period of time between about 1 min and 15 min, preferably between about 1 min and 10 min, most preferably between about 1 min and 7 min. In most preferred embodiments, seeding is complete is about 15 min, about 7 min, or about 1 min.
C. Grafts or Scaffolds with Reversible Stenosis
Typically, the graft or the scaffold is positioned between the mandrel and the housing. The graft or the scaffold is generally polymeric and porous, made of biodegradable polymer. The graft or the scaffold may have an inner surface structure and an outer surface structure for inducing spontaneous reversal of stenosis. The graft or the scaffold may have an average pore size on its inner surface and that is different from the average pore size on its outer surface. For example, the average pore size on the inner surface may be between about 35 μm and 50 μm, preferably between about 38 μm and 50 μm, most preferably between about 38 μm and 45 μm. The average pore size on the outer surface may be between about 25 μm and 45 μm, preferably between about 27 μm and 43 μm, most preferably between about 30 μm and 43 μm. The surface porosity may be between about 0.6 and 0.95, preferably between about 0.7 and 0.9, most preferably between about 0.8 and 0.9, of the surface area of the inner surface and/or the outer surface.
Typically, the graft or the scaffold is formed from knitted polymeric fibers. In some aspects, the fibers may be organized in fiber bundles. The fiber bundles may be knitted, such as knitted in a weft pattern. The fiber diameter may be between about 5 nm and 30 nm. Typically, the knitted pattern forms polymer fiber layers arranged axially and polymer fiber peaks arranged circumferentially. The separation between the layers may be between about 0.5 mm and 2 mm, preferably between about 0.5 mm and 1.5 mm, most preferably about 1 mm. The separation between the peaks may be between about 0.5 mm and 2 mm, preferably between about 0.5 mm and 1.5 mm, most preferably about 1 mm.
The grafts or the scaffolds may include a fibrous polymer coating. Typically, the TEVG or the scaffolds have an inner diameter between about 14 mm and 22 mm, thickness between 0.1 mm and 3 mm, and a length between about 5 cm and 15 cm. The TEVG or the scaffold are typically biodegradable and are substantially degraded within about six months following implantation. For example, by six months following implantation, the graft has been reduced by greater than about 80% by weight, greater than about 80% by surface area, or greater than about 80% by thickness, of the scaffold.
The polymeric vascular grafts or scaffolds typically include an effective amount of viable cells to reduce or prevent post-operative stenosis of the graft relative to the graft without the cells or with less cells. Typically, the graft has attached thereto an amount of viable cells at a cell density between about 0.1×103 cells/mm2 and 10×104 cells/mm2, inclusive, along the length of the scaffold, preferably between about 0.1×103 cells/mm2 and 10×103 cells/mm2, inclusive, along the length of the scaffold, most preferably between 1×103 cells/mm2 and 10×103 cells/mm2, inclusive, along the length of the scaffold.
Preferred cells are obtained from the patient's bone marrow. In preferred embodiments, the vascular graft is seeded with viable autologous cells. In a particular embodiment, the cells are human bone marrow mononuclear cells. The polymeric vascular graft or scaffold can include one or more additional agents selected from the group consisting of anti-neointima agents, chemotherapeutic agents, steroidal and non-steroidal anti-inflammatories, conventional immunotherapeutic agents, immune-suppressants, cytokines, chemokines, and growth factors.
1. Polymers
TEGV scaffolds can be formed of one or more polymers. In some embodiments, the polymers are biodegradable. In other embodiments, the polymers are non-biodegradable. In some embodiments TEVG are formed from a mixture of more than a single polymer. When biodegradable polymers are used, mixtures of biodegradable and non-biodegradable polymers can be used, for example, to provide long-lasting TEVG implants, as desired.
In certain embodiments, TEVGs are a three-dimensional matrix formed of polymeric (homopolymer and/or copolymer) fibers that are assembled in a woven or non-woven mesh, in random or aligned configurations. Preferably, the nanofiber materials are FDA approved biodegradable nanofiber materials.
The fibers in the scaffold matrix can be of any desired size, but generally are between about 1.5 mm and 1 nm. In certain embodiments, the fibers are nanoscale (i.e., from about 1 nm to about 1000 nm) and/or microscale (from about 1 μm to about 1000 μm).
Polymers useful for creating a scaffold for use in formation of TEVGs may be inorganic (e.g., siloxane, sulfur chains, black phosphorus, boron-nitrogen, silicones) or organic (meaning containing carbon). Organic polymers may be natural (e.g., polysaccharides, such as starch, cellulose, pectin, seaweed gums, vegetable gums; polypeptides, such as casein, albumin, globulin, keratin, collagen, nucleic acid, and hydrocarbons), synthetic (such as thermoplastics, unvulcanized elastomers, nylon, polyvinyl chloride, linear polyethylene, polystyrene, polypropylene, polyurethane, acrylate resins); thermosetting (e.g., vulcanized elastomers, crosslinked polyethylene, phenolics, alkyds, polyesters), and semisynthetic (e.g., cellulosics, such as rayon, methylcellulose, cellulose acetate; and modified starches)). In addition, useful scaffolds may include hydrogels formed from water soluble or water insoluble cellulose compounds. As would be readily understood by the skilled artisan, the particular type and composition of scaffold will vary depending upon the desired application. However, it is generally preferred that the polymeric material in the scaffold be biocompatible (i.e., will not elicit an unwanted immune reaction). In certain embodiments, the scaffold is biodegradable. Exemplary degradable polymers include poly(lactic acid-glycolic acid), poly(lactic acid), poly(glycolic acid), poly(orthoesters), poly(phosphazenes), polycaprolactones, or polyamides. In a preferred embodiment, the polymer is poly(lactic acid-glycolic acid). In one embodiment, the TEVGs are formed from a biodegradable tubular scaffold fabricated from a polyglycolic acid-fiber tube. The tube can be coated with a copolymer such as a 50:50 poly lactic acid (PLA) and poly-caprolactone copolymer.
In one embodiment, the grafts are formed from a felt or sheet like material of the polymer that can be formed into a tubular scaffold. For example, the device could be fabricated as a nonwoven, woven or knitted structure from extruded polymeric fibers. Typically, the polymeric sheet is formed using any textile construction, including, but not limited to, weaves, knits, braids or filament windings. Any suitable method, such as electrospinning, can be used to fabricate the nonwoven or woven polymeric textile.
The polymers and fabrication methods selected to fabricate the polymeric vascular grafts are suitable to produce grafts with biomechanical properties suitable for use as vascular scaffolds. Biomechanical properties that are important for vascular graft function include initial burst pressure, suture retention strength and elasticity. In one embodiment, the initial burst pressure of the polymeric vascular graft is between about 1,500 mmHg and about 50,000 mmHg, preferably between about 2,000 mmHg and about 10,000 mmHg. In another embodiment, the polymeric vascular grafts possess suture retention strengths between about 1 N and about 5 N, preferably between about 2 N and about 4 N. In another embodiment, the intrinsic elasticity of the vascular grafts is between about 10 MPa and about 50 MPa, preferably between about 15 MPa and about 40 MPa. In another embodiment, the initial tensile strength of the vascular grafts is between about 1 MPa and about 10 MPa, preferably between about 3 MPa and about 6 MPa.
2. Cells
In certain embodiments, the TEVGs scaffolds include one or more types of cells. In some embodiments, one or more types of cells are included within the lumen of the TEVG, within porous spaces throughout the walls of the TEVG, on the exterior and surface of the TEVG, or combinations. Typically, cells are attached to the surface of the TEVG, either directly, or through one or more accessory substances. In some embodiments, the cells are autologous cells, derived from one or more tissues of the intended recipient of the cell-seeded TEVG. In other embodiments, the cells are exogenous to the intended recipient of the graft. The cells can be un-differentiated cells, such as pluripotent stem cells, or differentiated cells. In preferred embodiments, the cells are viable human cells. Exemplary cell types for inclusion in TEVGs include white blood cells (WBC), such as monocytes lymphocytes, neutrophils, basophils, eosinophils; fibroblasts; myofibroblasts fibroblast cells; smooth muscle cells; bone marrow progenitor cells; red blood cells; embryonic stem cells; and combinations. White blood cells are made in bone marrow. In a particular embodiment, autologous bone marrow mononuclear cells (BM-MNCs) are included within the TEVG.
Cells for use with the TEVGs can be obtained from multiple sources. Methods for isolating and optionally manipulating one or more cell types from mixtures of cells are known in the art. In an exemplary embodiment, bone marrow is collected from the one or more long bones (e.g., femurs and/or tibias) of a subject (e.g., the intended recipient of the TEVG) and mononuclear cells are isolated using the density centrifugation method (Lee, et al. J Vis Exp. (88), (2014); Udelsman, et al. Tissue Eng Part C Methods. 17(7), 731-736 (2011)).
a. Seeding Dose
Typically, the seeding chambers provide the maximum uniform seeding density along the length of the graft or scaffold with the minimum volume of cell-containing fluid (blood, bone marrow aspirate, or mononuclear cell (MNC)-enriched fraction).
Typically, the amount of cells seeded into the TEVG is directly proportional to post-operative graft patency. Therefore, in preferred embodiments, TEVGs are seeded with a sufficient amount of cells effective to enhance the patency, or reduce the rate of post-operative stenosis of the TEVG. Methods for manually seeding TEVGs with cells are known in the art (Udelsman, et al. Tissue Eng Part C Methods. 17(7), 731-736 (2011)).
The optimal number of cells seeded can vary according to the cell type, and the size and shape of the TEVG, as well as the intended use. TEVGs are typically contacted with an amount of cells between 1.0×106 cells and 500×106 cells, inclusive, preferably between 3.0×106 cells and 250×106 cells.
When seeded, the resulting seeding density is uniform along the length of the scaffold. The seeding density may be between about 0.1×103 cells/mm2 and 10×104 cells/mm2 along the length of the scaffold. In preferred embodiments, TEVG are seeded at a density of between about 0.1×103 cells/mm2 and 10×103 cells/mm2, inclusive, along the length of the scaffold preferably between 1×103 cells/mm2 and 10×103 cells/mm2, inclusive, along the length of the scaffold.
In some embodiments, TEVGs are seeded with cells in solution having a concentration of between about 0.1×104 cells/ml and 10×107 cells/ml, inclusive. Typical volumes of cell solutions range between 1 ml and 100 ml, inclusive, preferably between 10 ml and 100 ml, such as 10 ml, 50 ml, 75 ml, or 100 ml.
Preferably, TEVGs are seeded with cells in an amount sufficient to yield a cell density of between 0.1×103 cells/mm2 and 10.0×104 cells/mm2, preferably between 1.0×103 cells/mm2 and 10.0×103 cells/mm2. Therefore, seeding chambers allow the grafts or scaffolds to be seeded using the minimum amount of bone marrow harvested from the patient and achieve uniform cell density along the length of the graft or scaffold.
When autologous bone marrow mononuclear cells (BM-MNCs) cells are prepared for seeding, the cells from 5 ml/kg of bone marrow are typically provide a sufficient seeding dose. From a clinical perspective, up to 20 ml/kg of bone marrow can be harvested from an individual without incurring significant adverse effects and is routinely used for harvesting bone marrow for bone marrow transplantations.
3. Additional Active Agents
It has been established that TEVG seeded with bone marrow-derived mononuclear cells reduce and prevent the incidence of post-operative stenosis via a paracrine effect. Advantages of cell seeding include the release signals in response to the body's feedback mechanism, unlike the drug-eluting scaffolds which release the drug regardless of feedback. Therefore, in some embodiments, cell-seeded TEVG are used in combination with one or more non-cell based synthetic or non-synthetic compounds that replicate the paracrine effect of seeded cells.
The TEVGs can include additional active agents, for example, that enhance the adhesion of cells to the vascular graft, or reduce the incidence of post-operative stenosis of the graft following insertion. Active gents include, but are not limited to, anti-neointima agents, chemotherapeutic agents, steroidal and non-steroidal anti-inflammatories, conventional immunotherapeutic agents, immune-suppressants, cytokines, chemokines, and growth factors.
Use of growth factors to stimulate bone marrow growth represents an additional strategy for increasing the yield of BM-MNC. Therefore, in some embodiments, the TEVGS include growth factors. Exemplary growth factors for incorporating into TEVGS include growth factors released in the physiological response to tissue injury, which stimulate the deposition of extracellular matrix, such as Platelet-Derived Growth Factor (PDGF), a potent chemotactic agent, and Transforming Growth Factor beta (TGF-β).
A. Seeding Chambers
The one or more of the component parts of the seeding system are custom designed such that the assembly is optimally sized to accommodate the dimensions of the TEVG. Preferably, the system, or parts of the system are fabricated using 3D printing of suitable materials. In an exemplary embodiment, one or more of the component parts of the seeding chamber of the system illustrated in
Suitable materials for forming the different components of the seeding systems include polymers and metals, including, but not limited to, stainless steel, iridium, platinum, gold, tungsten, tantalum, palladium, silver, niobium, zirconium, aluminum, copper, indium, ruthenium, molybdenum, niobium, tin, cobalt, nickel, zinc, iron, gallium, manganese, chromium, titanium, aluminum, vanadium, and carbon, as well as combinations, alloys, and/or laminations thereof.
B. Mandrel
Mandrels for use in the formation of TEVGs can be fabricated using means known in the art. Exemplary methods for the fabrication of mandrels include 3D printing. In some embodiments, the final mandrel design is converted to a suitable computer-readable format for 3D fabrication. An exemplary computer-readable format is STL format. Suitable materials for 3D printing of mandrel models include polymers and metals and carbon, as well as combinations, alloys, and/or laminations thereof. In some embodiments, the mandrel is made of a liquefiable material, thereby allowing the release of the mandrel from the graft in an easy fashion. The use of liquefiable mandrels also allows for forming complex shapes of the graft.
C. Methods of Making TEVGs
Methods of fabricating TEVGs based on a template structure, such as a custom-designed mandrel, are provided. TEVGs can be fabricated using any appropriate method, such as electrospinning, stamping, templating, molding, weaving and combinations melt processing, solvent processing, leaching, foaming, extrusion, injection molding, compression molding, blow molding, spray drying, extrusion coating, and spinning of fibers with subsequent processing into woven, non-woven, or knitted constructs.
In some embodiments, TEVGs are fabricated to include pores in the graft. Pores can be derived by any suitable method, including salt leaching, sublimation, solvent evaporation, spray drying, foaming, processing of the materials into fibers and subsequent processing into woven or non-woven devices. In a preferred embodiment, the fiber matrix of the scaffold includes pores of a suitable size to allow cells to adhere and grow and/or differentiate. Since the diameter of a cell is approximately 10 μm to 20 μm, pore sizes within this range are desired in certain embodiments. Preferably, the pores of the device are between 5 and 500 μm, more preferably between 5 and 250 μm, more preferably between 5 and 100 μm, in diameter. In certain embodiments, the polymeric scaffolds are generated or fabricated in order to more closely mimic the structure and composition of the natural extracellular matrix in order to promote growth and differentiation of the seeded cell and to facilitate transplantation and/or implantation of the scaffold or cells grown on the same.
In a preferred embodiment, TEVGs are fabricated by electrospinning of a stock solution containing one or more polymers. Typically, one or more polymers used to fabricate TEVGs are biodegradable polymers.
D. Methods of Seeding TEVGs
Typically, the seeding chamber, such as the chamber 700 or 800, is connected to a closed disposable seeding system containing a fluid with cells.
Typically, the number of cells used to contact the graft or scaffold may be proportional to the surface area of the graft, wherein the number of cells is between about 1.0×104 cells/mm2 graft and 1.0×106 cells/mm2 graft, inclusive, preferably between 0.7×105 cells/mm2 graft and 7.0×105 cells/mm2 graft, inclusive. In some embodiments, the polymeric vascular grafts or scaffolds are contacted with an amount of cells between 0.5×106 cells and 500×106 cells, inclusive, preferably between 1.0×106 cells and 100×106 cells. In preferred embodiments, the graft is contacted with the cells for less than 3 hours, preferably less than 2 hours, such as about 30 min, about 20 min, about 15 min, about 7 min, about 5 min, or about 1 min. The contacting is typically carried out within a sterile, closed seeding chamber.
Methods for increasing the patency of a polymeric vascular graft or scaffold, include the steps of administering an effective amount of viable cells onto the graft or scaffold to reduce the infiltration of macrophages to the graft, to promote the recruitment of host cells to the graft or to reduce or prevent platelet activation are also provided.
Methods of reducing or reducing or preventing post-operative stenosis in a subject have been developed. The subject can be a subject at risk of or has restenosis or other vascular proliferation disorder. For example, in some embodiments, the subject has undergone, is undergoing, or will undergo vascular trauma, angioplasty, vascular surgery, or transplantation arteriopathy. The methods reduce neointima formation, stenosis or restenosis, reduce or prevent thrombosis, or any combination thereof in a subject relative to an untreated control subject.
Restenosis means the recurrence of a treated coronary artery stenosis over time. Restenosis is most commonly defined as luminal renarrowing of greater than 50% (binary angiographic restenosis), either within the stent (in-stent restenosis) or within the stent and including 5 mm proximal or distal to the stent margin (in-segment restenosis) on follow-up angiography (typically 6 or 9 months later). Restenosis may manifests itself clinically over the 1- to 6-month period following a PCI (Percutaneous Coronary Intervention).
Customizable systems and compositions for seeding of cells into a vascular graft or scaffold are described in U.S. Pat. No. 9,090,863 and US Application Publication No. US 2018/0353649.
Typically, the seeding chambers are used for fast and efficient seeding of the grafts and scaffolds. The seeded scaffolds are then used in cardiovascular surgeries to repair or replace damaged vessels.
A. Methods of Using the Seeding Chambers
1. Flip Seeding Chamber
Methods of using the flip seeding chamber include the following steps. The chamber is filled, a negative pressure is applied, the chamber has 50% of the volume drained (at which point the bottom ˜50% of the scaffold is saturated with mononuclear cells (MNCs)), the negative pressure is interrupted, the device is inverted, and the negative pressure is re-introduced, allowing the remaining 50% of the volume to be drawn through the “upper” half of the scaffold, which then also becomes saturated with MNCs. Therefore, the bottom and top halves of the scaffold saturate sequentially, resulting in a completely saturated scaffold with minimal MNC loss (high seeding efficiency).
The negative pressure may be provided by a syringe, pump, or vacuum source.
2. Capacitor Seeding Chamber
Methods of using the capacitor seeding chamber include the following steps. The mandrel and scaffold assembly are carefully placed into the seeding chamber. The seeding chamber is very narrow as compared to the assembled mandrel/scaffold assembly. It is designed to hold a minimum volume of MNC solution. The cell seeding chamber may have a gap between about 1 mm and about 10 mm, more preferably between about 1 mm and 5 mm between the between the suction rod or the mandrel and the housing.
After the mandrel and chamber are secured together, the MNCs are introduced into the device. The fluid quickly fills the chamber and “overflows” into an IV bag located above the chamber. After the entire volume of MNCs have been introduced into both the seeding chamber and overflow (capacitor) IV bag, a negative pressure is applied. The MNC fluid begins to pass through the scaffold, and the entire scaffold remains submerged in MNCs as the fluid level is lowered in the IV bag. It is not until the very end of the seeding process that the fluid is completely drained from the IV bag and the fluid level eventually falls below the top of the seeding device. There is only a very small amount of time in which the fluid is drained from the top of the scaffold exposing the scaffold to filtered air while the bottom of the scaffold remains submerged. Thus, the top and bottom of the scaffold remained submerged throughout the majority of the seeding process. Only for a small fraction of the MNC solution is the top exposed while the bottom is submerged. Therefore, the seeding gradient is minimized along the length of the scaffold.
The negative pressure may be provided by a syringe, pump, or vacuum source.
B. Methods of Using Grafts or Scaffolds
It has been established that the cell-seeding dose on tissue engineered vascular graft (TEVG) is an effect-dependent variable for improving the performance and utility of the graft, regardless of cell incubation time. Typically, TEVGs are seeded with cells prior to implanting into a subject. Typically, the cells are autologous cells from the intended recipient, and the methods of seeding can include the step of harvesting the cells from the recipient. One or more cell types can be isolated from a mixture of cells using any techniques known in the art. Therefore, the methods can also include the step of isolating or purifying the cells prior to application (i.e., seeding).
Seeding of TEVG with cells is carried out using a kit or device, such as the closed disposable seeding system. The closed disposable seeding system may include any one of flip seeding chamber or capacitor seeding chamber for seeding the cells onto the graft or scaffold. Preferably, the kit or device enables sterile and efficient seeding of TEVG with a controllable amount of cells.
1. Methods of Use of TEVGs Seeded with Cells
TEVGs seeded with cells can reduce or prevent the rate of post-operative stenosis of the TEVG, relative to the rate of stenosis in the equivalent TEVG in the absence of cells. Therefore, TEVGs can be seeded with an effective amount of cells to reduce or prevent one or more of the immune processes associated with development of post-operative stenosis, including inflammation.
Tissue repair has four distinct stages, including: a) clotting/coagulation; b) inflammation; c) fibroblast migration/proliferation; and d) a final remodeling phase where normal tissue architecture is restored. In the earliest stages after tissue damage, epithelial cells and/or endothelial cells release inflammatory mediators that initiate an antifibrinolytic-coagulation cascade that triggers clotting and development of a provisional extracellular matrix (ECM). Aggregation and subsequent degranulation of platelets promotes blood vessel dilation and increased permeability, allowing efficient recruitment of inflammatory cells such as neutrophils, macrophages, lymphocytes, and eosinophils to the damaged tissue. Neutrophils are the most abundant inflammatory cell at the earliest stages of wound healing, but are quickly replaced by macrophages after neutrophil degranulation. Activated macrophages and neutrophils debride the wound, eliminate any invading organisms and produce a variety of cytokines and chemokines that amplify the inflammatory response as well as trigger fibroblast proliferation and recruitment. Upon activation, fibroblasts transform into myofibroblasts that secrete α-smooth muscle actin and ECM components. Finally, in the remodeling phase epithelial/endothelial cells divide and migrate over the temporary matrix to regenerate the damaged tissue. Thus, healing and neotissue generation is a finely regulated process that balances the need to regenerate tissue and thicken blood vessel walls, without excessive thickening and stenosis or fibrosis.
a. Macrophages
It has been shown that the presence of circulating monocytes and infiltrating macrophages is critical for wound healing and neotissue development (Arras, et al., J Clin Invest, 101(1): 40-50 (1998)). However, the extent of macrophage infiltration at a site of tissue damage has also been correlated with proliferative dysregulation and neointima formation (Hibino, et al., FASEB J. 25(12):4253-63 (2011)). Further, numerous studies have indicated that macrophages and fibroblasts are the main effector cells involved in the pathogenesis of fibrosis (reviewed in Wynn, Nat Rev Immunol. 4(8):583-94 (2004)).
Following vascular damage, inflammatory monocyte cells (CD16-hi, CD64-hi and CD14-hi in humans; CD115+, CD11b+ and Ly6c-hi in mice) are recruited to the damaged tissue and differentiate into activated macrophages (Emr1-hi in humans; F4/80-hi in mice) upon exposure to local growth factors, pro-inflammatory cytokines and microbial compounds (Geissmann et al., Science 327: 656-661 (2010)). Excessive macrophage infiltration results in stenosis, whilst complete inhibition of macrophage infiltration prevents neotissue formation (Hibino, et al., FASEB J. 25(12):4253-63 (2011).
Two distinct states of polarized activation for macrophages have been defined: the classically activated (M1) macrophage phenotype and the alternatively activated (M2) macrophage phenotype (Gordon and Taylor, Nat. Rev. Immunol. 5: 953-964 (2005); Mantovani et al., Trends Immunol. 23: 549-555 (2002)). The role of the classically activated (M1) macrophage is an effector cell in TH1 cellular immune responses, whereas the alternatively activated (M2) macrophage appears to be involved in immunosuppression and wound healing/tissue repair. M1 and M2 macrophages have distinct chemokine and chemokine receptor profiles, with M1 secreting the TH1 cell-attracting chemokines CXCL9 and CXCL10, and with M2 macrophages expressing chemokines CCL17, CCL22 and CCL24.
The presence of M2 macrophages has been associated with neo-intima development and stenosis (Hibino, et al., FASEB J. 25(12):4253-63 (2011)). The correlation between the extent of macrophage infiltration, neotissue formation and stenosis at certain time points following tissue graft implantation provides means to prevent stenosis through modulation of macrophage activity.
Further, macrophages are typically located close to collagen-producing myofibroblast cells, and it has been shown that monocyte-derived macrophages critically perpetuate inflammatory responses after injury as a prerequisite for fibrosis (Wynn and Barron, Semin Liver Dis., 30(3):245-257 (2010)). Macrophages produce pro-fibrotic mediators that activate fibroblasts, including platelet-derived growth factor (PDGF), a potent chemotactic agent, and transforming growth factor beta (TGF-B). Specifically, a marked increase of the non-classical M2 (CD14+, CD16+) subset of macrophages has been correlated with pro-inflammatory cytokines and clinical progression in patients suffering from chronic liver disease. During fibrosis progression, monocyte-derived macrophages release cytokines perpetuating chronic inflammation as well as directly activate hepatic stellate cells (HSCs), resulting in their proliferation and trans-differentiation into collagen-producing myofibroblasts (Zimmermann, et al., PLOS One, 5(6):e11049 (2010)).
b. Platelets
Aggregated platelets assist the repair of blood vessels by secreting chemicals that attract fibroblasts from surrounding connective tissue into the wounded area to heal the wound or, in the case of dysregulated inflammatory responses, form scar tissue. In response to tissue injury, platelets become activated and release a multitude of growth factors which stimulate the deposition of extracellular matrix, such as platelet-derived growth factor (PDGF), a potent chemotactic agent, as well as transforming growth factor beta (TGF-β). Both of these growth factors have been shown to play a significant role in the repair and regeneration of connective tissues. PDGF functions as a primary mitogen and chemo-attractant which significantly augments the influx of fibroblasts and inflammatory cells, as well as stimulating cell proliferation and gene expression. PDGF enables leukocytes to firmly attach to the vessel wall and finally to transmigrate into the subendothelial tissue. However, the platelet-derived chemokines are also known to induce smooth muscle cell (SMC) proliferation and play a role in neointimal proliferation and organ fibrosis (Chandrasekar, et al., J Am College Cardiology, Vol 35, No. 3, pp. 555-562 (2000)). Increased expression of PDGF and its receptors is associated with scleroderma lung and skin tissue. Specifically, there is evidence for an autocrine PDGF-receptor mediated signaling loop in scleroderma lung and skin fibroblasts, implicating both TGF-β and PDGF pathways in chronic fibrosis in scleroderma (Trojanowska, Rheumatology; 47:v2-v4 (2008)). In addition, deregulation of PDGF signaling is associated with cardiovascular indications such as pulmonary hypertension, and atherosclerosis.
Media layer smooth muscle cell (SMC) proliferation and migration in response to injury-induced PDGF are essential events contributing to neointimal thickening (Fingerle, et al., Proc Natl Acad Sci., 86:8412 (1989); Clowes, et al., Circ. Res., 56:139-145 (1985)) which eventually leads to blood vessel narrowing and stenosis.
Other healing-associated growth factors released by platelets include basic fibroblast growth factor, insulin-like growth factor 1, platelet-derived epidermal growth factor, and vascular endothelial growth factor.
The TEVGs can be seeded with an effective amount of cells to create a pro-regenerative immune environment that enhances wound healing and prevents stenosis. The TEVG can also be seeded with an effective amount of cells to modulate platelet activity and function. Thus, the TEVG can also be seeded with an effective amount of cells to reduce or prevent the biological functions of platelets, such as platelet aggregation and the production/expression of platelet derived growth factor (PDGF).
Methods of using the tissue engineering vascular grafts seeded with cells to reduce post-operative stenosis of the graft include surgically implanting, or otherwise administering, the cell-seeded grafts to within a patient. Typically, the method of implanting includes attaching the graft to a section of an artery that is to be replaced or augmented. Methods of attaching vascular grafts are known in the art. The methods typically reduce or inhibit the infiltration of macrophage cells, or the conversion of macrophage cells from M1 to M2 phenotype, or both, compared to a control, such as an equivalent graft that is not seeded with cells, or seeded with fewer cells. In some embodiments, the methods reduce or inhibit proliferation of macrophage cells without reducing or inhibiting vascular neotissue development. A subject can have stenosis, restenosis or other vascular proliferation disorders, or be identified as being at risk for restenosis or other vascular proliferation disorders, for example subjects who have undergone, are undergoing, or will undergo a vascular trauma, angioplasty, surgery, or transplantation arteriopathy, etc. Any of the methods described can include the step of identifying a subject in need of treatment.
The present invention will be further understood by reference to the following non-limiting examples.
Materials and Methods
U.S. Pat. No. 9,090,863 describes how a closed, disposable tissue engineered vascular graft seeding device was used to seed mononuclear cells (MNCs) onto an implantable scaffold.
Every scaffold seeded using this method had a gradient (distribution) of MNCs along the longitudinal axis of the graft (meaning there were less cells seeded on the top of the graft than the bottom of the graft). With new concentrations and seeding volumes, seeded scaffolds reached a point of saturation towards the bottom 3 cm of the graft (
The entire graft (13 cm in length) save 5 mm from the top and the bottom graft was seeded with enough MNCs to meet release criteria, however, there were many more MNCs found in the lower sections of the graft than the upper sections. This example presents two seeding chambers—flip and capacitor, for seeding devices to eliminate this disparity.
The Flip Seeding Chamber
The ‘Flip’ seeding chamber eliminates the variability of the distribution of MNCs on the scaffold. The central premise behind the device is to vary the cross section of the seeding chamber along its length.
The effect of the change in cross section is that it effectively slows the speed at which the MNCs are drawn through the least seeded portions of the scaffold.
The chamber is filled, a vacuum applied, the chamber has 50% of the volume drained (at which point the bottom ˜50% of the scaffold is saturated with MNCs), the vacuum is interrupted, the device is inverted, and the vacuum is re-introduced, allowing the remaining 50% of the volume to be drawn through the “upper” half of the scaffold, which then also becomes saturated with MNCs. Via this method, the bottom and top halves of the scaffold saturate sequentially, resulting in a completely saturated scaffold with minimal MNC loss (high seeding efficiency).
In addition, the mandrel for this design has two vacuum nubs—one to draw negative pressure in the upright orientation, and one to draw negative pressure in the inverted orientation.
Method of Seeding a Graft with Flip Chamber
1. The scaffold is secured on the mandrel (upper portion of the device).
2. The mandrel (upper) is secured to the chamber (lower) part of the device by rotating it counter clockwise until it compresses a sealing o-ring
3. All necessary fixtures, tubes, and air vents are placed on the device
4. The chamber is filled with MNCs
5. Negative pressure is introduced to the lower mandrel outlet.
6. The chamber has 50% of the volume pulled through it.
7. The vacuum is interrupted.
8. The device is inverted
9. The vacuum is re-introduced
10. The remaining 50% of volume is drawn through the “upper” portion of the scaffold.
11. The device is disassembled and the scaffold is now ready for implantation.
Total seeding time was 7 min (7:01±0.03 min).
After seeding, the seeded scaffold was cut into 1 cm-wide rings along the length of the scaffold forming 12 rings. The number of cells in each ring was counted and the data are shown in
The Capacitor Seeding Chamber
The ‘Capacitor’ seeding chamber eliminates the variability of the distribution of MNCs on the scaffold. The central premise behind the device is to reduce the MNC gradient by exposing the entire scaffold to the MNC seeding process for as long as possible.
After the mandrel and chamber are secured together, the MNCs are introduced into the device. The liquid quickly fills the chamber and “overflows” into an IV bag located above the chamber.
After the entire volume of MNCs have been introduced into both the seeding chamber and overflow (capacitor) IV bag, the vacuum is applied. The MNC liquid begins to pass through the scaffold, and the entire scaffold remains submerged in MNCs as the liquid level is lowered in the IV bag. It is not until the very end of the seeding process that the liquid is completely drained from the IV bag and the fluid level eventually falls below the top of the seeding device. There is only a very small amount of time in which the liquid is drained from the top of the scaffold exposing the scaffold to filtered air while the bottom of the scaffold remains submerged. Thus, the top and bottom of the scaffold remained submerged throughout the majority of the seeding process. Only for a small fraction of the MNC solution is the top exposed while the bottom is submerged. Therefore, the seeding gradient is minimized along the length of the scaffold.
Method of Seeding a Graft with Capacitor Chamber
Total seeding time was 1 min 15 seconds.
After seeding, the seeded scaffold was cut into 1 cm-wide rings along the length of the scaffold forming 12 rings. The number of cells in each ring was counted and the data are shown in
Results
The results show uniform seeding of the scaffolds seeded with either flip seeding chamber (1) or capacitor seeding chamber (2) in
A computational model was developed to simulate neovessel formation. This computational model was initially formulated from data collected in prior studies of TEVG development in mouse models, and it successfully described and predicted neovessel formation over a 2-year period. Presented is the first analysis of data from a United States clinical trial as well as computational simulations that suggested that the early stenosis observed in a prior clinical trial may have reversed naturally without intervention. Presented are also experiments using an established large animal model to test the simulation-generated suggestion that a transient period of TEVG narrowing would develop and subsequently resolve spontaneously as part of the natural history of neovessel formation. Characterized are the evolving geometry, composition, and biomechanical properties of TEVGs up to 1.5 years post-implantation in a large animal model. The data validated the primary predictions of the computational model. Comparisons of in vivo observations to results from the computational model further enabled to refine values of the model parameters and thereby to glean increased insight into the mechanisms that underlie the transformation of TEVGs from scaffolds seeded with autologous cells into living neovessels capable of growth and remodeling.
Materials and Methods
TEVG
Scaffold Characterization
Scaffolds were characterized using Scanning Electron Microscopy (SEM). Samples of nonimplanted scaffold were cut along the axial direction to create 0.5 cm squares that were mounted on SEM stages with carbon tape. Samples were sputter coated with gold to 3-nm thickness under vacuum in argon gas and imaged on a Hitachi 54800 SEM at 5 kV and 10 mA. Images were analyzed with FIJI image analysis software. Pore size was calculated from 7 SEM images at 100×. Fiber diameter was calculated on average by at least 5 PGA fibers.
Release Criteria and Post Process Testing
Samples were obtained from cells (0.2 mL aliquots) and seeded scaffold (5×5 mm sections), and subject to release and post-process monitoring. Cell count and viability were performed using trypan blue exclusion and a hemocytometer. FACS was performed using FITC-CD45 and 7AAD to determine the number of leukocytes and cell viability. Seeding efficacy was determined by quantifying the number of cells in samples obtained from the pre-seeding and post-seeding solutions using a hemocytometer and then calculating the difference in the number of cells in the pre-seeding and post-seeding solutions divided by the number of cells in the pre-seeding solution.
Clinical Trial
Study Design
The primary objective of this pilot trial was to evaluate the safety of TEVGs as extracardiac modified Fontan conduits in patients with single ventricle cardiac anomalies. The secondary objective was to determine the growth potential of the TEVG by evaluating its change in length between 6 months and 3 years after implantation. The original design was to enroll six patients and monitor them with serial echocardiography and MRI over a three-year period.
Growth Analysis
The growth capacity of the TEVG was evaluated using serial MRI studies performed 6 months and 3 years after implantation. The growth capacity of the TEVG was estimated by comparing its change in length over time against the patient's Glenn shunt SVC measured from its first branch to the pulmonary artery anastomoses.
Safety Analysis
Patients were seen and evaluated by the study team during the initial hospitalization and following TEVG implantation at all scheduled follow up appointments (1, 6, 12, 24, and 36 months postoperatively), as well as at any unscheduled cardiology or cardiac surgery appointments or hospitalizations. In addition, the study nurse contacted the patient's parent or guardian monthly by phone to review their medical status using a standardized survey. All adverse events were recorded and their grade and attribution were determined by the study team and reviewed with the data safety monitoring board. The data were analyzed and compared to the incidence of graft-related complications in the initial pilot study performed at Tokyo Women's Hospital in Japan.
Angioplasty
Patients who developed stenosis defined as >50% reduction in luminal diameter of the TEVG were treated with angioplasty without stenting.
Computational Model
Constrained Mixture Framework
The α=1, . . . n structurally significant constituents are endowed with individual time-varying rates of production and degradation as well as material properties, but are constrained to deform with the graft as a whole. Hence, constituent-specific deformation gradients at a current growth and remodeling time s for material produced at an intermediate time τ are
F
n(τ)
α
=F(s)F−1(τ)Gα(τ),
where F is the deformation gradient for the bulk material (i.e., mixture) and Gα are constituent-specific “deposition stretches” at which new matrix is incorporated within external material, whether polymer or extracellular matrix. The (τ) denotes the natural configuration at which the individual constituents are stress-free. A simple rule of mixtures expression for the elastic energy that is stored in the graft upon deformation stems from the sum of the constituent-specific stored energies, namely W=ΣWα, which allows a classical continuum formulation of the wall mechanics, with the Cauchy stress t=2F(∂W/∂C) FT/det(F) where C=FT F is the right Cauchy-Green tensor. Growth and remodeling processes are slow and thus can be described via a series of quasi-static equilibrium states such that divt=0 satisfies Newton's second law of motion for a continuum body. Because each constituent can evolve, the stored energy density is taken to be
ρ(s)Wα(s)=ρα(0)Qα(s)Ŵα(Fn(0)α(s))+∫0smα(τ)qα(s,τ)
where ρ(s) is the overall mass density, ρα(0) the initial apparent mass density of constituent α, Ŵα(Fn(0)α(s))>0 stored energy density of the material that was initially present in the scaffold, mα(τ))>0 the mass density production rate, qα(s,τ)ϵ[0,1] the survival fraction of material produced at time τ that remains at time s, and Ŵα(Fn(τ)α(s))>0 the energy stored at time s in the cohort of constituent α that was produced at time τϵ[0, s] and deformed via Fn(τ)α. The deposition and degradation of cellular and matrix components are governed according to immunological and mechanical stimuli, with production and degradation rates increased for inflammation-driven turnover. A specific functional form is given in Results.
Computational Studies
The computational model was first used to predict the evolving normalized luminal diameter and wall thickness of an implanted clinical TEVG using values of the model parameters determined by fitting biomechanical and geometric data from mouse experiments, with appropriate modifications to geometric (diameter/thickness ratio, D/H=16 for the clinical scaffold vs. D/H=3 for the mouse scaffold) and microstructural (scaffold pore size rp=41.9 μm for clinical scaffold vs. 11.2 μm for mouse scaffold) properties of the simulated TEVG to account for differences in the clinical and mouse scaffolds. Since the inflammation-driven kinetics were based on murine data and the immune response is known to play a critical role in stenosis, the effects of four key inflammatory parameters (δ, β, Kih, and Kimax) on model outputs were investigated. Parametric studies were performed by varying the value of an individual inflammatory parameter while holding the other three parameters constant and vice versa. This allowed to isolate effects of each parameter on the evolving TEVG properties, including normalized diameter, wall thickness, diameter compliance, and inflammatory areal mass density. Diameter and thickness were normalized by the original values for the implanted TEVG scaffold; normalized diameter compliance was calculated as the change in diameter for a 50% increase in pressure from the homeostatic pressure normalized by the original graft diameter. The depicted parameters (see Table 1) were chosen to capture a broad range of potential physiologic outcomes based on pilot simulations.
. Inflammatory Parameters (inputs)
indicates data missing or illegible when filed
The key equation within the overall computational model of TEVG development that accounts for changes in mass via changes in the rates of production mα and removal qα of different constituents a, each of which can depend on the inflammatory burden (superscript i), due to the foreign body response, and mechanobiological responses (superscript m), due to deviations Δ in circumferential wall stress to and luminal wall shear stress τw from homeostatic target values. Four key model parameters and possible ranges therein were identified from prior experiments in immuno-competent and immuno-compromised mice to bound the possible inflammation-driven process of neovessel formation.
Large Animal Study
Study Design
The objective of this study was to test a computationally generated output, that is, to quantify the natural history of neotissue formation and thus neovessel development over 1 year in an established IVC interposition TEVG model. Seeded TEVGs were implanted in 24 lambs, and in vivo data were collected via serial angiography and intravascular ultrasound at 1 week, 6 weeks, 6 months, and 1 year. The 1-week time point was used for baseline anatomic information; it provided comparable data to the immediate post-operative period, but allowed the animal to recover from the initial surgical insult and decreased the risk associated with the prolonged anesthesia needed to perform the implantation surgery and an initial catheterization during the same period. Pilot data demonstrated significant stenosis at 6 weeks, consistent with computational predictions, and the later times were chosen to monitor graft performance over the long term. The primary endpoint was the narrowest cross-sectional area of the graft on intravascular ultrasound imaging at each time. No data were excluded from the study.
Large Animal Scaffold
TEVG scaffolds were provided by Gunze Ltd. (Tokyo, Japan) and were identical to those used in clinical trial: knitted PGA core with a 50:50 co-polymer sealant of PCLA.
Bone Marrow Aspiration, Assembly, and Implantation of the TEVG
Twenty-four juvenile lambs underwent bone marrow aspiration (5 mL/kg body weight) and implantation of an autologous cell-seeded TEVG as intrathoracic IVC interposition grafts. Animals were anesthetized using propofol (5 mg/kg) for induction and isoflurane (1-4%) or propofol (20-40 mg/kg/hr) for maintenance. Lambs were placed in the lateral recumbent position, and the area overlying the iliac crest was shaved and prepped in standard sterile fashion. A 2-mm incision was made and an aspiration needle was inserted into the bone. Heparinized syringes (100 U/mL) were used to aspirate 5 mL/kg of bone marrow.
Following aspiration, the bone marrow was processed using Ficoll density gradient separation to isolate the bone marrow-derived mononuclear cells as previously described. Briefly, bone marrow was filtered through 100-μm cell strainers to remove bone spicules and clots. A 1:1 dilution was achieved with phosphate buffered saline (PBS) and the bone marrow was layered onto Ficoll 1077 (Sigma-Aldrich, St. Louis, Mo.). The plasma and mononuclear cell layers were isolated after centrifugation. The mononuclear cell layer underwent two washes with PBS to yield a cell pellet that was diluted in 20 mL of PBS to seed the scaffold. The mononuclear cells were vacuum-seeded onto the scaffold, which was incubated in plasma until the time of implantation.
The scaffolds were implanted in the intrathoracic IVC as previously described. Lambs were placed in a left lateral recumbent position. Depending on each animal's anatomy, a right thoracotomy was made in the fifth or sixth intercostal space, and the thoracic IVC was dissected between the diaphragm and right atrium. A cavoatrial shunt was placed to maintain perfusion during cross-clamping of the IVC. The vessel was clamped and a 2-cm segment of a diameter-matched seeded scaffold was implanted, with end-to-end anastomoses performed using a running nonabsorbable monofilament suture. No native vessel was removed. Titanium vascular clips were applied to the suture tails to mark the anastomoses for postoperative imaging. The chest wall, overlying muscle, and skin layers were reapproximated with absorbable sutures.
Interventional Imaging
Postoperative catheterizations were performed at 1 week, 6 weeks, 6 months, and 1 year. Additional imaging was performed as needed based on the animals' clinical conditions. After sedation and intubation, lambs were placed in a left lateral recumbent position. The right internal jugular vein was cannulated and a 9-French sheath (Terumo, Somerset, N.J.) was inserted followed by an intravenous bolus of heparin (150 U/kg). A 5-French JR 2.5 catheter (Cook Medical, Bloomington, Ind.) was passed into the right internal jugular vein through the SVC and into the right atrium. Using an angled Glidewire (Terumo), the JR catheter was then passed through the TEVG into the intraabdominal IVC where a Rosen exchange guidewire (Cook Medical, Bloomington, Ind.) was placed. The JR catheter was then exchanged for a 5-French multi-track angiographic catheter (NuMed, Hopkinton, N.Y.) which was used to measure hemodynamic pressures in the intraabdominal IVC, intrathoracic IVC below and above the TEVG, and within the TEVG. A mean pressure gradient was calculated by subtracting the mean pressure above the TEVG from the mean pressure below the TEVG. A digital angiogram was then obtained by injecting ioversol 68% (Mallinckrodt Pharmaceuticals, Raleigh, N.C.) through the multi-track angiographic catheter positioned in the intraabdominal IVC. Diameters were measured at seven points: the intraabdominal IVC, low intrathoracic IVC (on the diaphragmatic side of the TEVG), proximal anastomosis (defined with respect to blood flow), midgraft, distal anastomosis, high intrathoracic IVC (on the atrial side of the TEVG), and the area of most severe narrowing. The proximal and distal anastomoses were identified by the aforementioned surgically-placed radiopaque clips. A 0.035-inch digital intravascular ultrasound catheter (Volcano, San Diego, Calif.) was advanced through the graft over the Rosen guidewire. This was used to obtain images at the same seven points measured during angiography. These images were analyzed using Volcano software to obtain a cross-sectional area as described previously.
Due to the requirement to size match grafts at implant, angiographic and IVUS data were normalized to their respective 1-week measurement and graft stenosis was thus represented as a fold change relative to the baseline midgraft size at 1 week, namely
Neotissue development within the TEVG was measured using intravascular ultrasound and reported as a percentage of the diameter of the TEVG. Graphical reconstructions of IVUS imaging data were performed using Rhino 3D (Seattle, Wash.).
Euthanasia
At the prescribed endpoint, animals were deeply sedated with ketamine (20 mg/kg) and diazepam (0.02-0.08 mg/kg), followed by induction of bilateral pneumothoraces and exsanguination. A complete veterinary necropsy was performed at the time of TEVG explanation. Animals were also euthanized if they developed critical stenosis, defined here as graft narrowing with systemic symptoms. Animals that were not euthanized for critical stenosis were euthanized at 6 months (n=5) and 12 months (n=2) post implantation. The remaining animals were survived for long-term follow up, with three euthanized at 18 months for late-term mechanical testing.
Neotissue Structural Characterization
Histology and Immunohistochemistry
TEVG explants were fixed with 4% formalin, dehydrated, and embedded in paraffin before 4-μm transverse sections of the midgraft were prepared and mounted on slides and heat fixed. Standard techniques were adopted for hematoxylin & eosin and Picro-Sirius Red staining. Immunohistochemistry was used to detect macrophage antigen CD68 (CD68, 1:500, Abcam), and alpha smooth muscle actin (aSMA, 1:2000, Dako). Samples underwent heat-induced antigen retrieval with Dako target retrieval solution (90° C., pH 6.0) followed by blocking endogenous peroxidase activity (0.3% H2O2 in H2O) and non-specific binding (3% normal goat serum in Background Sniper, BioCare Medical). After primary antibody incubation, sections were incubated sequentially in appropriate biotinylated secondary antibodies (1:1500, Vector) and streptavidin-horseradish peroxidase (Vector). DAB+ substrate chromogen (Vector) was used for color development. All samples were counterstained with Gill's hematoxylin (Vector) prior to dehydration and coverslipping.
Image Quantification
Photomicrographs of histological stains were quantified using ImageJ. For Picro-Sirius Red stained specimens, tiled 25× magnification images were used to capture the entire vessel area. Areas of interest were quantified using the Color Threshold command. For the immunohistochemically stained slides, four representative tiled images were taken at 100× around the vessel, each capturing the full width of a section of graft or native IVC. The images were assessed with use of the Color Deconvolution and Threshold commands to quantify positively stained area or count positive cells.
Biaxial Mechanical Testing
A composite specimen, including the TEVG and adjacent proximal and distal thoracic IVC, was excised at 18 months post-implantation in the lambs from right atrium to the diaphragm (59.3±16.2 mm), then cleaned of perivascular tissue by blunt dissection. The specimens were cannulated on custom acrylic cannulae, mounted within a custom computer-controlled testing device, and immersed in Hank's buffered physiologic solution at room temperature. Outer diameter was measured using a video camera and length was prescribed via a stepper motor; luminal pressure and axial force were measured using standard transducers. The specimen was equilibrated for 15 minutes under low flow at a pressure of 5 mmHg, then preconditioned with 6 cycles of pressurization (1 to 30 mmHg), both at a fixed value of the in vivo axial stretch. Acquisition of passive data consisted of cyclic pressure-diameter tests (1.5 to 30 mmHg) at three different fixed values of axial stretch (95%, 100%, and 105% of the in vivo value). Data from the unloading curve of each protocol were used for analysis.
Computational Model Validation
After gathering data on the evolving ovine TEVG, and noting differences between the predicted (based on parameters from the prior mouse studies) and measured evolving geometries, a non-intrusive optimization technique was used, the Surrogate Management Framework described in detail previously, to identify values of the four key inflammatory parameters to capture the evolving normalized luminal diameter and wall thickness of the lamb grafts throughout the first year of implantation as measured by intravascular ultrasound. As the intravascular ultrasound showed non-circular graft cross-sections, the hydraulic diameter of the TEVGs was calculated from area measurements for the fitting process. Bounds for the Surrogate Management Framework optimization were informed by the parametric studies. To validate the computational model, the measured compliance of the TEVG at 18 months with that predicted from the Growth and Remodeling model were compared.
Statistical Analysis
Statistical analyses were performed and graphs created using GraphPad Prism version 7.03 (GraphPad Software, Inc., La Jolla, Calif.). Comparison between the incidence of early stenosis in the Japanese vs United States clinical trials was performed via two-tailed Fisher's exact test. Serial measurements from the ovine study (angiography and IVUS) were first normalized to paired 1-week values to control for variable scaffold sizes used at implantation to ensure size matching to the native vessel or variable degrees of anastomotic narrowing as a result of the implantation procedure and are thus represented as a fold change relative to the respective 1-week measurement. Pressure measurement or normalized angiographic and IVUS values were analyzed using an ordinary oneway ANOVA with Tukey's post-hoc multiple comparison's test. Histomorphometric and micrographic data (wall thickness, aSMA+ area fraction, CD68+ cells/mm2, and collagen area fraction) were analyzed via ordinary one-way ANOVA with Tukey's post-hoc multiple comparison's test. For all statistical tests, a was restricted to 0.05 and p values <0.05 were considered statistically significant.
Ethical Compliance
Clinical Trial
Institutional review board approval was obtained from Yale University (HIC #0701002198 and Nationwide Children's Hospital (IRB12-00357). This clinical trial was performed under FDA IDE 14127 in compliance with good clinical practice guidelines.
Ovine Study
The Institutional Animal Care and Use Committee of Nationwide Children's Hospital (Columbus, Ohio) reviewed and approved the protocol (Ar13-00079). Representatives of the animal care staff monitored all animals intraoperatively and during their postoperative courses. Animal care was within the humane guidelines published by the Public Health Service, National Institutes of Health (Bethesda, Md.) in the Care and Use of Laboratory Animals (2011), as well as within USDA regulations set forth in the Animal Welfare Act.
Results
Design and Characterization of the TEVG
TEVGs were assembled by seeding autologous bone marrow-derived mononuclear cells onto a biodegradable tubular scaffold. The scaffolds (Gunze Ltd, Kyoto, Japan) were made from poly(glycolic acid) fibers (PGA) and a copolymer of caprolactone and lactide (PCLA) synthesized by ring opening polymerization with a 50:50 molar composition. The PGA fibers were knitted into a tube and coated on the inner and outer surface with the PCLA solution and then freeze dried under a vacuum, creating a matrix of knitted PGA fibers embedded within a porous sponge of PCLA. The scaffold was designed to degrade by hydrolysis over approximately 6 months. Scaffold dimensions measured either 16±0.5 mm or 18±0.5 mm on the inner diameter, 13±0.5 cm in length, and 0.7±0.1 mm in wall thickness (
/ml
/ml
cell/mL
indicates data missing or illegible when filed
Clinical Performance of the TEVG
The FDA-approved clinical trial evaluated the safety and growth capacity of the TEVG when used as a vascular conduit connecting the inferior vena cava (IVC) to the pulmonary artery in children with single ventricle cardiac anomalies undergoing a modified Fontan operation (
Growth capacity was assessed using serial magnetic resonance imaging (MRI) studies, performed 6 months and 3 years after implantation. Noting that the IVC is a capacitance vessel that changes its diameter on a moment-to-moment basis, the growth of the TEVG by comparing its change in length over time against that of an internal control was estimated: the superior vena cava (SVC) when anastomosed to the pulmonary artery (called a Glenn shunt, which is a component of the Fontan operation). The four TEVGs increased 2.5 mm (range 1.1 to 4.2 mm) in length between 6 months and 3 years after implantation whereas the Glenn shunt increased 1.5 mm (range 0.9-2.4 mm) in length during the same period. The average percent increase in length of the TEVG and Glenn shunt were both 7%.
Safety analyses demonstrated no graft-related deaths, catastrophic graft failures, or complications requiring graft replacement during the 3-year study. All four patients continue to do well 4-7 years after implantation. However, three of the four patients developed critical stenosis (>50% narrowing of the graft diameter) and were successfully treated with angioplasty 5-8 months after implantation. There were no additional graft-related complications. Enrollment was capped at 4 patients instead of the intended 6 due to this unexpectedly high incidence of early TEVG stenosis: 3 out of 4 patients (75%) in the United States trial developed stenosis and were treated with angioplasty, while only 1 out of 25 (4%) in the original Japanese trial developed stenosis and required angioplasty within three years after implantation (two-sided Fisher's exact test, p<0.01).
Building a Computational Model of Neovessel Development
A general constrained mixture theory was used for describing changes in mass and changes in the microstructure of soft tissues as the basis for the model. The computational model to simulate the evolving geometry, composition, and mechanical properties of TEVGs implanted in mice over time was previously developed. The model considered the degrading polymeric scaffold, organizing vascular neotissue, and newly produced collagendominated extracellular matrix to be separate structurally significant constituents. Specifically, datadriven constitutive relationships define the intrinsic material properties for each of the α=1, 2, . . . n constituents as well as their individual rates of production and removal: a neoHookean relation describes the mechanical behavior of the polymer, a Fung-exponential relation describes that of the neotissue, the rate of mass density production m) τ>0 depends on mechanobiological stimuli, such as intramural wall stress, and immunobiological stimuli, proportional to macrophage invasion, and removal is defined via a survival function q) s, τϵ[0,1] that tracks the percentage of the material produced at time τϵ[0, s] that remains at time s. For example, deviations in intramural stress from homeostatic conditions regulate mechano-mediated kinetics, as vascular cells typically promote mechanobiological homeostasis, while scaffold microstructure, as quantified from scanning electron microscopy, modulates inflammation-driven kinetics since a pore size sufficient for cellular infiltration is a critical regulator of macrophage activity and phenotype.
Previous studies of immuno-competent and immuno-compromised mice identified four key model parameters that control the deposition and degradation of inflammation-driven extracellular matrix (Table 1): δ modulates the onset and duration of the inflammatory response, β controls the skewness of the production function, Kih controls rates of inflammatory extracellular matrix production and degradation, and Kimax scales the inflammatory effects of extracellular matrix degradation, with production given by
m
infl
α(τ)=mhα,infl(1−exp)(−τ))Ki(τ)δβτβ-1exp(−δτ),),
Where mhα,infl is the basal rate of production of matrix constituent α driven by inflammation and Ki(τ)=Khi(rp(τ)/rn)+Kwi, is the gain on a gamma distribution function that describes the onset and resolution of the foreign body response to the degrading polymer, with rp(τ)/rn the pore size of the scaffold normalized by a critical pore size for cellular infiltration, and Kiw a basal gain on inflammation-driven extracellular matrix production. Degradation of the inflammatory extracellular matrix was governed by a first-order kinetic type decay,
q
α(s,τ)=exp(−∫τskhl(1+Kl(t)/Kmaxl)dt)|,
where kih is a rate-parameter and Kimax is again the maximum value of Ki(τ).
Parametric Simulations of Neotissue Formation
Myriad simulations of neovessel development (
In
TEVG Stenosis Reverses Spontaneously in a Large Animal Model
The predictions of the computational model were tested by performing a time-course study in an ovine intrathoracic IVC interposition graft model. The ovine IVC interposition graft is a validated model that is used as a surrogate for the Fontan operation since there are no large animal models with single ventricle cardiac anomalies and performance of a Fontan operation on an animal with a structurally normal heart is associated with excessive mortality (>80%). Size-matched TEVGs were implanted into 24 juvenile lambs (
Intravascular ultrasound suggested that early TEVG narrowing resulted from appositional growth of neotissue on the luminal surface of the scaffold, thus thickening the wall and narrowing the lumen. At 6 weeks, the TEVG stenosis was localized to the mid-distal segment of the graft (i.e., region closer to the heart) while no changes were appreciated at the proximal anastomosis. Quantitative intravascular ultrasound assessment confirmed that the luminal area decreased significantly from 1 to 6 weeks (−0.67±0.17-fold area change from 1 week midgraft area, one-way ANOVA with Tukey's multiple comparisons test: a=0.05, p<0.0001), the narrowing reversed by 6 months, and grafts remained patent at 1 year (
Hemodynamic data during each angiography was also collected. The mean pressure gradient across the graft followed the aforementioned morphometric changes. The gradient increased significantly from 1 to 6 weeks (0.5±0.5 vs. 11.8±5.5 mmHg, p<0.001), then decreased to near baseline by 6 months (1.3+2 8 mmHg, one-way ANOVA with Tukey's multiple comparisons test: a=0.05, p<0.001 vs. 6 weeks, p=0.8883 vs. 1 week) (
Computational Model Validation
Whereas the parametric studies were parameterized based on studies of a murine IVC-interposition TEVG that consisted of a similar scaffold design as used in the ovine and clinical grafts, it was tested whether the model could also describe the in vivo ovine data. These data fit well for all times (R2=0.83) using many model parameters from the murine experiments (e.g., mechanical properties of collagen and basal rates of collagen turnover), but generated ovine-specific values for the four key parameters that control the temporal inflammatory response: δ=0.32 days-1, β=3.96, Kih=5.13, and Kimax=72. These best-fit values of the parameters were identified using a Surrogate Management Framework optimization method, as in prior growth and remodeling studies of native blood vessels. Among the different computed metrics, note in particular the evolution of luminal diameter and wall thickness which dictate the presence or absence of stenosis (
Evolving Cellular Composition of Neovessel is Consistent with Computational Model Predictions
The changes in the ovine neotissue over time were characterized using histology and immunohistochemistry. Histological sections confirmed changes in the lumen and wall of the TEVG that were observed with in vivo imaging, and verified that the transient luminal narrowing was primarily due to scaffold thickening and partly due to neotissue formation on the luminal surface of the scaffold that appeared to resolve by 6 months after implantation. Quantitative histomorphometry demonstrated that wall thickening peaked 6 weeks after implantation, consistent with the computational predictions (
Computational Model Accurately Predicts Neovessel Biomechanics
The sum of the mechanical contributions of the polymeric scaffold and neotissue (which is primarily determined by the extracellular matrix component of the neotissue) determine the biomechanical properties of the TEVG. The scaffold degradation and neotissue formation was characterized using polarized light images of Picro-Sirius Red (PSR) stained sections from ovine TEVGs obtained 1 week, 6 weeks, 6 months, and 1 year after implantation, which revealed both the degradation of PGA fibers and the deposition and maturation of collagen fibers. The PGA fibers remained highly organized within the scaffold 1 week after implantation, but had thinned and begun to show evidence of early fragmentation at 6 weeks. Only rare thin individual fragments of the PGA fibers were visible by 6 months after implantation and beyond. In contrast, minimal staining was detected for fibrillar collagen at 1 week. The total amount of collagen in the neotissue peaked 6 weeks after implantation, with significant amounts of thin (green) fibers within the scaffold and along its luminal surface. Collagen density increased steadily over the first year as it compacted and matured (
In vitro biaxial mechanical testing of TEVGs excised 1.5 years post-implantation revealed a structural response (pressure-diameter) of the TEVG that was similar to model predictions (
Discussion
The first FDA-approved clinical trial evaluating the use of TEVGs in the repair of complex congenital cardiac anomalies confirmed that stenosis is the most prevalent graft-related complication, but formation of early TEVG stenosis occurred at a much higher incidence than previously observed.
To gain mechanistic insight into the complex immunological and mechanical processes underlying the formation of early TEVG stenosis, a computational model of neovessel development was used to study parametrically the relative contributions of multiple critical factors that control neotissue formation. This model predicted spontaneous resolution of stenosis, a finding that was reproducibly confirmed in an ovine IVC interposition graft model. Asymptomatic TEVG stenosis in the lambs could be monitored safely without acute graft failures or thrombosis over the entire 1 to 1.5-year study period. Based on the available data, symptoms and the mean pressure gradient across the graft could be primary criteria in assessing the clinical significance of TEVG stenosis rather than morphometric changes alone. Because most stenoses resolved naturally, appropriate monitoring rather than overly aggressive intervention may be the way forward.
Collectively, the results from the previous studies coupled with the human clinical trials, computational simulations, and ovine studies suggest that implantation of a cell-seeded PGA/PCLA scaffold as a TEVG within the vasculature sets into motion an inflammation-driven, mechanomediated evolution of a neovessel over time. The polymer initially incites a strong foreign body response and host inflammatory cells infiltrate the scaffold. At the same time, the scaffold partially shields infiltrating vascular cells and the neotissue they deposit from the hemodynamically imposed mechanical forces until the structural integrity of the polymer is lost. Mechanobiological processes thus appear to increase as scaffold integrity diminishes, and the low wall stresses due to overthickening of the graft cause degradation to outpace subsequent deposition, resulting in a progressive resolution of the initial inflammation-driven stenosis. The consequences of inflammation are not completely negative, however; indeed, some inflammation is fundamental to early host cell recruitment and neotissue formation. Infiltrating monocytes/macrophages in particular orchestrate early neotissue formation by inducing ingrowth of host endothelial cells and smooth muscle cells from the neighboring vessel wall. Overall TEVG functionality thus requires a balanced degradation of the polymeric scaffold (primarily by hydrolysis) and appropriate deposition of neotissue. It appears that it is critical to promote, but limit, inflammation while simultaneously optimizing the timing of load transferal from the initially stiff polymeric scaffold to the cell-matrix composite that constitutes the neotissue (
Similarly, in situations of high mechanical stimulus, these cells produce and remodel the extracellular matrix to increase stress shielding until mechanical homeostasis is attained. The mechanobiological contribution continues to build as the stress-shielding capability of the degrading polymeric scaffold diminishes. During the first 6 months after implantation, the orchestration of neotissue formation by macrophages occurs through paracrine signaling that drives cellular migration and extracellular matrix production, while after the scaffold degrades and loses its biomechanical integrity, kinetics are mediated by the ability of vascular cells to sense and respond to their local mechanical environment via the deposition and degradation of neotissue. Note that the loss of scaffold mechanical integrity precedes its disappearance, hence an overlap in inflammatory and mechanical contributions to neotissue turnover. Importantly, the exuberant production of neotissue during the inflammation-driven period results in neotissue wall stresses well below the normal homeostatic target, thus promoting a mechano-mediated degradation of neotissue that outpaces deposition, contributing to the natural resolution of the stenosis.
In conclusion, this study highlighted the utility of combining advanced computational modeling and model-driven pre-clinical experiments in translational research. A framework for creating robust computational models that can accurately predict changes in the geometry, composition, and biomechanics of the neotissue that ultimately determine graft performance was developed. Results suggested that the early stenosis observed in the clinical trial, which resulted in the study ending prematurely, would have resolved spontaneously without angioplasty. The computational simulations predicted that the scaffold could be modified to reduce the degree and duration of narrowing by altering the scaffold design (including fiber diameter, fiber alignment, porosity and pore size) to reduce the associated inflammation and stress shielding.
The application claims the benefit of and priority to U.S. Provisional Application No. 62/936,225 filed Nov. 15, 2019, which is hereby incorporated by reference in its entirety.
This invention was made with Government Support under grant Nos. HL098228, HL128602, HL128847, HL139996, GM068412 from the National Institutes of Health & W81XWH-18-1-0518 from the Department of Defense. The Government has certain rights in the invention.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US2020/060717 | 11/16/2020 | WO |
| Number | Date | Country | |
|---|---|---|---|
| 62936225 | Nov 2019 | US |
