Bioresorbable microporous stents deliver recombinant adenovirus gene transfer vectors to the arterial wall

Polymeric Biodegradable Stent Insertion in the Esophagus

Esophageal stent insertion has been used as a well-accepted and effective alternative to manage and improve the quality of life for patients diagnosed with esophageal diseases and disorders. Current stents are either permanent or temporary and are fabricated from either metal or plastic. The partially covered self-expanding metal stent (SEMS) has a firm anchoring effect and prevent stent migration, however, the hyperplastic tissue reaction cause stent restenosis and make it difficult to remove. A fully covered SEMS and self-expanding plastic stent (SEPS) reduced reactive hyperplasia but has a high migration rate. The main advantage that polymeric biodegradable stents (BDSs) have over metal or plastic stents is that removal is not require and reduce the need for repeated stent insertion. But the slightly lower radial force of BDS may be its main shortcoming and a post-implant problem. Thus, strengthening support of BDS is a content of the research in the future. BDSs are often temporarily effective in esophageal stricture to relieve dysphagia. In the future, it can be expect that biodegradable drug-eluting stents (DES) will be available to treat benign esophageal stricture, perforations or leaks with additional use as palliative modalities for treating malignant esophageal stricture, as the bridge to surgery or to maintain luminal patency during neoadjuvant chemoradiation.

Dexamethasone-eluting vascular stents

Percutaneous transluminal angioplasty (PTA) with stenting is widely used in the treatment of vascular disorders, but restenosis remains a significant problem. Drug-eluting stents (DES) have been developed as an attempt to reduce the intimal response leading to restenosis. Drugs used in DES include mainly immunosuppressive and anti-proliferative compounds. Glucocorticoids are also an interesting possibility for those purposes because they have anti-proliferative effects in vascular smooth muscle cells and down-regulate the production of cytokines and growth factors driving inflammation and fibrosis. In this MiniReview, feasibility and safety of drug-eluting metal and biodegradable vascular stents are discussed with special emphasis on dexamethasone-eluting stents.

Recent advances in drug eluting stents

One of the most common medical interventions to reopen an occluded vessel is the implantation of a coronary stent. While this method of treatment is effective initially, restenosis, or the re-narrowing of the artery frequently occurs largely due to neointimal hyperplasia of smooth muscle cells. Drug eluting stents were developed in order to provide local, site-specific, controlled release of drugs that can inhibit neointima formation. By implementing a controlled release delivery system it may be possible to control the time release of the pharmacological factors and thus be able to bypass some of the critical events associated with stent hyperplasia and prevent the need for subsequent intervention. However, since the advent of first-generation drug eluting stents, long-term adverse effects have raised concerns regarding their safety. These limitations in safety and efficacy have triggered considerable research in developing biodegradable stents and more potent drug delivery systems. In this review, we shed light on the current state-of-the-art in drug eluting stents, problems related to them and highlight some of the ongoing research in this area.

Microfabrication and nanotechnology in stent design

Intravascular stents were first introduced in the 1980s as an adjunct to primary angioplasty for management of early complications, including arterial dissection, or treatment of an inadequate technical outcome due to early elastic recoil of the atherosclerotic lesion. Despite the beneficial effects of stenting, persistent high rates of restenosis motivated the design of drug-eluting stents for delivery of agents to limit the proliferative and other inflammatory responses within the vascular wall that contribute to the development of a restenotic lesion. These strategies have yielded a significant reduction in the incidence of restenosis, but challenges remain, including incomplete repair of the endothelium at the site of vascular wall injury that may be associated with a late risk of thrombosis. A failure of vessel wall healing has been attributed primarily to the use of polymeric stent coatings, but the effects of the eluted drug and other material properties or design features of the stent cannot be excluded. Improvements in stent microfabrication, as well as the introduction of alternative materials may help to address those limitations that inhibit stent performance. This review describes the application of novel microfabrication processes and the evolution of new nanotechnologies that hold significant promise in eliminating existing shortcomings of current stent platforms.

A new wave in treatment of vascular occlusive disease: biodegradable stents--clinical experience and scientific principles

Stent-based therapies in percutaneous vascular intervention are associated with significant long-term complications related to in-stent restenosis. A growing body of literature demonstrates the feasibility of biodegradable materials for endovascular stents, which may, in theory, circumvent many of the immunologic and inflammatory response issues seen with long-term metallic stent failure in coronary and peripheral applications. This review describes the history of endovascular stents and the challenges encountered with metallic, drug-eluting, and biodegradable stents. A review of the basic engineering principles of biodegradable stents is provided, along with a discussion of the cellular mechanisms of restenosis.

Immobilization of plasmid DNA on an anti-DNA antibody modified coronary stent for intravascular site-specific gene therapy

BACKGROUND

The aim of the present study was to investigate the incorporation of plasmid DNA (pDNA) onto a coronary stent by chemo-immunoconjugation for achieving site-specific gene delivery.

METHODS

Anti-DNA immunoglobulin M antibody was chemically linked onto collagen-coated stent by using N-succinimidyl-3-(2-pyridyldithiol)-propionate as cross-linker. pDNA was tethered on the antibody-immobilized stent by highly specific antigen-antibody affinity interaction. Radioactive-labeled antibody and pDNA were used to evaluate binding capacity and stability. A reporter plasmid pEGFP was tethered on the antibody-immobilized stents that was assessed in cell culture and in rabbit carotid model.

RESULTS

The amount of antibody chemically linked on the stents was 15-fold higher than that of the control and its retention time was also significantly longer. The pEGFP-tethered stents had no detrimental effects on cell growth. In cell culture studies, numerous green fluorescent protein (GFP)-transfected cells were only found on the stent, which demonstrated high localization and efficiency of gene delivery. The overall GFP transfection efficiency in treated rabbit carotid arteries was 2.8 +/- 0.7% of the total cells. However, the rate of neointima transfection was 7.0 +/- 0.8% of total cells in this region. Importantly, no distal spreading of the vector was detected by polymerase chain reaction, either in distal organs or in the downstream segments of the stented arteries.

CONCLUSIONS

For the first time, our group reports the successful use of anti-DNA antibody-immobilized metal stent as plasmid gene delivery system that possess high efficiency and site-specificity in vitro and in vivo.

Intravascular bioresorbable polymeric stents: a potential alternative to current drug eluting metal stents

Stent implantation following angioplasty is the standard treatment of coronary artery disease necessitating interventional procedures. The use of stents as a platform for local drug delivery is a popular strategy to achieve local pharmacological treatment to the diseased artery. Drug eluting stents (DES) are now largely preferred to bare metal stents when stent implantation is necessary. Lately, there have been several reports questioning the long-term safety of DES. An alternative to these drug eluting metal stents are bioresorbable polymeric stents (BPS) because of the many advantages of bioresorbable material. However, the fundamental differences in polymeric and metallic materials make the development of such an alternative a significant challenge. This review discusses the different advantages of BPS and the many constrains and requirements of such devices. An up to date commented review of published data concerning BPS is presented. Considerations are given on using BPS as local drug delivery systems as well as on evaluating BPS performances.

Delivery of large biopharmaceuticals from cardiovascular stents: a review

This review focuses on new and emerging large-molecule bioactive agents delivered from stent surfaces in drug-eluting stents (DESs) to inhibit vascular restenosis in the context of interventional cardiology. New therapeutic agents representing proteins, nucleic acids (small interfering RNAs and large DNA plasmids), viral delivery vectors, and even engineered cell therapies require specific delivery designs distinct from traditional smaller-molecule approaches on DESs. While small molecules are currently the clinical standard for coronary stenting, extension of the DESs to other lesion types, peripheral vasculature, and nonvasculature therapies will seek to deliver an increasingly sophisticated armada of drug types. This review describes many of the larger-molecule and biopharmaceutical approaches reported recently for stent-based delivery with the challenges associated with formulating and delivering these drug classes compared to the current small-molecule drugs. It also includes perspectives on possible future applications that may improve safety and efficacy and facilitate diversification of the DESs to other clinical applications.

DRUG-ELUTING BIORESORBABLE STENTS FOR VARIOUS APPLICATIONS

A stent is a medical device designed to serve as a temporary or permanent internal scaffold to maintain or increase the lumen of a body conduit. Metallic coronary stents were first introduced to prevent arterial dissections and to eliminate vessel recoil and intimal hyperplasia associated with percutaneous transluminal coronary angioplasty. The stent application range has expanded as more experience was gained, and encouraging results have been obtained in the treatment of vascular diseases. Stents are currently used for support of additional body conduits, including the urethra, trachea, and esophagus. The rationale for bioresorbable stents is the support of a body conduit only during its healing process. The stent mass and strength decrease with time, and the mechanical load is gradually transferred to the surrounding tissue. Bioresorbable stents also enable longer term delivery of drugs to the conduit wall from an internal reservoir and abolish the need for a second surgery to remove the device. The present review describes recent advances in bioresorbable stents, focusing on drug-eluting bioresorbable stents for various applications. Controlled release of an active agent from a stent can be used to enhance healing of the surrounding tissues, to increase the implant's biocompatibility, as well as to help cure certain diseases. Because a lot of research in this field has been done by us, examples for these functions are described based mainly on developments in our laboratories.

Cardiovascular gene delivery: The good road is awaiting

Atherosclerotic cardiovascular disease is a leading cause of death worldwide. Despite recent improvements in medical, operative, and endovascular treatments, the number of interventions performed annually continues to increase. Unfortunately, the durability of these interventions is limited acutely by thrombotic complications and later by myointimal hyperplasia followed by progression of atherosclerotic disease over time. Despite improving medical management of patients with atherosclerotic disease, these complications appear to be persisting. Cardiovascular gene therapy has the potential to make significant clinical inroads to limit these complications. This article will review the technical aspects of cardiovascular gene therapy; its application for promoting a functional endothelium, smooth muscle cell growth inhibition, therapeutic angiogenesis, tissue engineered vascular conduits, and discuss the current status of various applicable clinical trials.

Role of stent design and coatings on restenosis and thrombosis

More than 15 years have passed since stent technology was introduced by Sigwart et al. [U. Sigwart, J. Puel, V. Mirkovitch, F. Joffe, et al. Intravascular stents to prevent occlusion and restenosis after transluminal angioplasty. N. Engl. J. Med. 316 (1987) 701-706.] among interventional cardiologists. Recently drug eluting stents have assumed dominance in the interventional world as positive trial results revealed their efficacy for preventing restenosis. Stent design, delivery-vehicle materials, and drug properties affect the function of these stents. Stainless steel stents with tubular and multicellular design have proven superior to coil or hybrid stent models. This chapter describes stents which have subtle influences of modular design, metal coverage, strut thickness, strut shape, surface smoothness, and coating materials like an alloy composition.

PLA stereocopolymers as sources of bioresorbable stents: Preliminary investigation in rabbit

The aim of the present work was to evaluate whether the degradation of PLA-based bioresorbable stents can be modulated via the configuration of repeating units as it is the case in other applications like osteosynthesis. The first obstacle was finding a stent design that could allow implantation in the aorta of a rabbit taken as a model of a human coronary artery. In the absence of guidelines other than those tentatively proposed in patents, several simple designs were considered that allowed us to evaluate the fate of the stents made of poly(lactic acid) stereocopolymers with L/(L + D) ratio of 0.92 (PLA92) and 0.50 (PLA50) up to 6 months post in vivo implantation. Our findings show the feasibility of bioresorbable stenting using PLA stereocopolymers and that PLA50 degraded faster than PLA92. Therefore, using stereocopolymers appears as a means to vary the degradation rate and adapt it to the artery remodelling process that is very much dependent on the release of the stenting stress protection.

Site specific gene delivery in the cardiovascular system

Gene therapy holds great promise for treating both genetic and acquired disorders. However, progress toward effective human gene therapy has been thwarted by a number of problems including vector toxicity, poor targeting of diseased tissues, and host immune and inflammatory activity to name but a few of the challenges. Gene therapy for cardiovascular disease has been the subject of many fewer clinical trials than other disorders such as cancer or cystic fibrosis. Nevertheless, the challenges are comparable. The present paper reports a review of investigations related to our hypothesis that site specific cardiovascular gene therapy represents an approach that can lead to both optimizing efficacy and reducing the impact of gene vector-related systemic adverse effects. We report experimental studies demonstrating proof of principle in three areas: gene therapy for heart valve disease, gene delivery stents, and gene therapy to treat cardiac arrhythmias. Heart valve disease is the second most common indication for open heart surgery and is now only treatable by surgical removal or repair of the diseased heart valve. Our investigations demonstrate that gene vectors can be immobilized on the surface of prosthetic heart valve leaflets thereby enabling a therapeutic genetic modification of host cells around the valve annulus and on the leaflet. Other animal studies have shown that vascular stents used to relieve arterial obstruction can also be used as gene delivery systems to provide therapeutic vector constructs that can both locally prevent post stenting reobstruction, known as in-stent restenosis, and treat the underlying vascular disease. Cardiac arrhythmias are the cause of sudden death due to heart disease and affect millions of others on a chronic basis. Our group has successfully investigated in animal studies localized gene therapy using an ion channel mutation to treat atrial arrhythmias.

Curcumin impregnation improves the mechanical properties and reduces the inflammatory response associated with poly(L-lactic acid) fiber

We investigated poly(L-lactic acid) (PLLA) fibers and coils, simulating stents and the influence of impregnation with curcumin, a non-steroidal anti-inflammatory drug, intended to reduce the pro-inflammatory property of these implants. Fibers obtained by melt extrusion of 137 kDa PLLA resin containing 10% curcumin (C-PLLA) exhibited a stable curcumin release rate for periods up to 36 days. Curcumin increased the fiber tensile strength at break and decreased embrittlement vs. controls in 36 day 37 degrees C saline incubation. A mouse peritoneal phagocyte model was employed to test the anti-inflammatory properties of C-PLLA fibers in vitro. Myeloperoxidase and non-specific esterase activity assays were performed for adherent cells (polymorphonuclear leukocytes (PMN) and macrophages (MPhi), respectively). PMN and MPhi adhesion to C-PLLA fibers were significantly reduced compared to control PLLA fibers (2.6 +/- 0.91) x 10(5) vs. (5.6 +/- 0.67) x 10(5) PMN/cm2 and (3.9 +/- 0.23) x 10(3) vs. (9.1 +/- 0.7) x 10(3) MPhi/cm2 (P < 0.05), respectively. In addition, superoxide release in the phagocyte pool contacting C-PLLA fibers was 97% less than that for PLLA controls. A fresh human whole blood recirculation system was employed to analyze cell adhesion under flow conditions, employing scanning electron microscopy (SEM). Reduced adhesion of cells on C-PLLA fiber coils vs. controls was observed. These in vitro studies demonstrate that bulk curcumin impregnation can reduce the inflammatory response to bioresorbable PLLA fibers, whilst improving mechanical properties, thereby suggesting curcumin loading may benefit PLLA-based implants.

Mechanical properties andin vitro degradation of bioresorbable fibers and expandable fiber-based stents

Bioresorbable polymeric support devices (stents) are being developed in order to improve the biocompatibility and drug reservoir capacity of metal stents, as well as to offer a temporary alternative to permanent metallic stents. These temporary devices may be utilized for coronary, urethral, tracheal, and other applications. The present study focuses on the mechanical properties of bioresorbable fibers as well as stents developed from these fibers. Fibers made of poly(L-lactide) (PLLA), polydioxanone (PDS), and poly(glycolide-co-epsilon-caprolactone) (PGACL) were studied in vitro. These fibers combine a relatively high initial strength and modulus together with sufficient ductility and flexibility, and were therefore chosen for use in stents. The effect of degradation on the tensile mechanical properties and morphology of these fibers was examined. The expandable stents developed from these fibers demonstrated excellent initial radial compression strength. The PLLA stents exhibited excellent in vitro degradation resistance and can therefore support body conduits such as blood vessels for prolonged periods of time. PDS and PGACL stents can afford good support for 5 and 2 weeks, respectively, and can therefore be utilized for short-term applications. The degradation resistance of the stents correlates with the profile of mechanical property deterioration of the corresponding bioresorbable fibers.

Protein-loaded bioresorbable fibers and expandable stents: Mechanical properties and protein release

There is an increasing interest in bioresorbable polymeric stents for coronary, urethral and tracheal applications. These stents can support body conduits during their healing process and release biologically active agents from an internal reservoir to the surrounding tissue. A removal operation is not needed. Bioresorbable poly(L-lactic acid) fibers were prepared through melt spinning accompanied by a postpreparation drawing process. Novel expandable bioresorbable stents were developed from these fibers. Bioresorbable microspheres containing albumin were prepared and attached to the stents, to serve as a protein reservoir coating. The controlled release of albumin from the microsphere-loaded stent was studied. The fibers combine high strength and modulus, together with good ductility and flexibility. An increase in draw ratio increases the tensile strength and modulus and decreases the ultimate strain. The stents demonstrated excellent initial radial compression strength and good in vitro degradation resistivity, which makes them applicable for supporting blood vessels for at least 20 weeks. Microspheres bound to these stents enable effective protein loading, without reducing the stent's mechanical properties. The protein release from the microsphere-loaded stent occurs by diffusion, is determined mainly by the initial molecular weight of the bioresorbable polymer and its erosion rate, and is strongly affected by the microsphere structure.

Molecular responses of vascular smooth muscle cells and phagocytes to curcumin-eluting bioresorbable stent materials

A major complication of coronary stenting is restenosis, often accompanied by inflammatory reactions and smooth muscle cell proliferation. Curcumin has been shown to possess anti-inflammatory and anti-proliferative properties, thus we hypothesize that locally released curcumin by coronary stent would diminish in-stent restenosis. As a first test of this hypothesis, curcumin-eluting PLLA films (C-PLLA) were produced and the anti-inflammatory and anti-proliferative properties were then tested using peritoneal phagocytes and human coronary artery smooth muscle cell (hCASMCs) culture systems. We find that the addition of curcumin reduced phagocyte accumulation and activation on C-PLLA films. On the other hand, C-PLLA significantly reduced the proliferation, but not the adhesion, of hCASMCs. The molecular responses of hCASMCs to C-PLLA were further assessed by cDNA microarray analysis. Curcumin up-regulated genes related to apoptosis and enhanced the expression of anti-proliferative and anti-inflammatory factors, and of antioxidants. Equally important, C-PLLA inhibited the cell cycle progression of adherent hCASMCs. The results suggest that curcumin regulates gene expression and cell function through the protein kinase (PK) and mitogen-activated protein kinase (MAPK) pathways. These results support the use of curcumin to inhibit in-stent restenosis.

[Prevention of in-stent restenosis: towards an in situ treatment?]

The use of intracoronary stents represent a major breakthrough in the armamentarium of interventional cardiology. Stents reduce significantly the incidence of recurrent stenosis (in-stent restenosis) via an improved post-procedure luminal diameter and an abrogation of the constrictive remodeling of the arterial wall. However, stent-related arterial injury results in intense proliferative and inflammatory responses and severe intimal hyperplasia, which, in 20% to 40% of the patients, may end up with clinically significant in-stent restenosis. Efficient prevention of in-stent restenosis has yet to be found. Systemic treatments have failed because they don't take into account the specific physiopathology and, most importantly, the focal nature of in-stent intimal hyperplasia. Hence, local prevention appears to be a straightforward approach to the unsolved issue of in-stent restenosis. In situ beta- or gamma-irradiation (brachytherapy) has received much attention as a curative treatment of in-stent restenosis but is not indicated for prevention. In contrast, drug-releasing stents have been tested in experimental models and have already provided very promising results in randomized clinical trials. Most of clinical studies have been performed with the antiproliferative agents sirolimus and paclitaxel, but other agents are under scrutiny. In addition, important research is carried out, in which the efficacy of antiproliferative genes is investigated. Clearly, drug-releasing stents are on the verge of profoundly modifying our practice of interventional cardiology. However, several questions remain unanswered as regard to the long term efficacy/toxicity and the cost-effectiveness of this new approach.

Bioresorbable polymeric stents: current status and future promise

Metal stents and, more recently, polymer-coated metal stents are used to stabilize dissections, eliminate vessel recoil, and guide remodeling after balloon angioplasty and other treatments for arterial disease. Bioresorbable polymeric stents are being developed to improve the biocompatibility and the drug reservoir capacity of metal stents, and to offer a transient alternative to the permanent metallic stent implant. Following a brief review of metal stent technology, the emerging class of expandable, bioresorbable polymeric stents is described, with emphasis on developments in the authors' laboratory.

In vitro study of drug-loaded bioresorbable films and support structures

Bioresorbable films can serve simultaneously as anatomic support structures and as drug delivery platforms. In the present study, bioresorbable poly(L-lactic acid) (PLLA) films containing dexamethasone were prepared by solution processing methods. Their in vitro studies focused on the mechanical properties with respect to morphology and degradation and erosion processes. Novel expandable support devices (stents) developed from these films were studied. Such a stent would support conduits, such as the neonatal trachea to treat tracheal malacia, until the airway matures, and would then be totally resorbed, obviating the need for a removal operation. The PLLA films showed good initial mechanical properties. They can accommodate drug incorporation on the film surface and also in the bulk. Water incubation of the films results in a decrease in their tensile mechanical properties, due to chain scission and morphological changes. These changes can vary from degradation and small changes in morphological features to erosion, leading to a microporous structure, depending on the polymer. The cumulative release of dexamethasone from the films is linear. The rate of release is determined by the film's structure (drug location/dispersion). The stents demonstrated good mechanical properties. The initial radial compression strength of the stent is determined mainly by the polymer structure. Drug incorporation has a minor effect on the initial stent strength. Exposure to radial compression stress results in elastic reversible deformation or a sudden brittle fracture, depending on the polymer. A 20-week in vitro study of the stents showed that they are applicable for supporting body conduits, such as the trachea.

Drug-eluting stents: potential applications for peripheral arterial occlusive disease

Many different approaches have been evaluated to prevent restenosis in stents after vascular implantation. Currently, drug-eluting stents are extremely promising in suppressing neointimal hyperplasia. Various animal studies and randomized trials in humans have shown excellent results in terms of safety and efficacy during intermediate-term follow-up. This article will give an overview of experimental and clinical data of the different agents in published and ongoing trials.

Biodegradable nanoparticles for drug and gene delivery to cells and tissue

Biodegradable nanoparticles formulated from poly (D,L-lactide-co-glycolide) (PLGA) have been extensively investigated for sustained and targeted/localized delivery of different agents including plasmid DNA, proteins and peptides and low molecular weight compounds. Research about the mechanism of intracellular uptake of nanoparticles, their trafficking and sorting into different intracellular compartments, and the mechanism of enhanced therapeutic efficacy of nanoparticle-encapsulated agent at cellular level is more recent and is the primary focus of the review. Recent studies in our laboratory demonstrated rapid escape of PLGA nanoparticles from the endo-lysosomal compartment into cytosol following their uptake. Based on the above mechanism, various potential applications of nanoparticles for delivery of therapeutic agents to the cells and tissue are discussed.

Endovascular restorative neurosurgery: a novel concept for molecular and cellular therapy of the nervous system

The amalgam of molecular biology and neurosurgery offers immense promise for neurorestoration and the management of neurodegenerative deficiencies, developmental disorders, neoplasms, stroke, and trauma. This article summarizes present strategies for and impediments to gene therapy and stem cell therapy of the central nervous system and advances the concept of a potential new approach, namely endovascular restorative neurosurgery. The objectives of gene transfer to the central nervous system are efficient transfection of host cells, selective sustained expression of the transgene, and lack of toxicity or immune excitation. The requisite elements of this process are the identification of candidate diseases, the construction of vehicles for gene transfer, regulated expression, and physical delivery. In the selection of target disorders, the underlying genetic events to be overcome, as well as their spatial and temporal distributions, must be considered. These factors determine the requirements for the physical dispersal of the transgene, the duration of transgene expression, and the quantity of transgene product needed to abrogate the disease phenotype. Vehicles for conveying the transgene to the central nervous system include viral vectors (retroviruses, lentiviruses, adenoviruses, adeno-associated viruses, and herpes simplex virus), liposomes, and genetically engineered cells, including neural stem cells. Delivery of the transgene into the brain presents several challenges, including limited and potentially risky access through the cranium, sensitivity to volumetric changes, restricted diffusion, and the blood-brain barrier. Genetic or cellular therapeutic agents may be injected directly into the brain parenchyma (via stereotaxy or craniotomy), into the cerebrospinal fluid (in the ventricles or cisterns), or into the bloodstream (intravenously or intra-arterially). The advantages of the endovascular route include the potential for widespread distribution, the ability to deliver large volumes, limited perturbation of neural tissue, and the feasibility of repeated administration.

Coronary artery stents: evaluating new designs for contemporary percutaneous intervention

Intracoronary stents have markedly improved the short- and long-term safety and efficacy of percutaneous coronary intervention by improving acute gains in luminal dimensions, decreasing abrupt vessel occlusion, and reducing restenosis. At present, nearly 90% of all coronary interventions involve stenting. A variety of advances in stent technology and design have expanded the clinical application of stenting to include complex coronary lesions, multivessel disease, and small-diameter vessels. In addition, the development of stents as drug delivery systems for antithrombotic or antiproliferative agents has the potential to expand the role of coronary stenting, and early clinical experience appears promising. The purpose of this review is to describe recent developments in stent design, examine the results of clinical trials of contemporary stents, and present future directions for investigation of new stent technologies.

Development of a platform to evaluate and limit in-stent restenosis

The objective of this work was to develop a platform to evaluate and deliver putative therapeutic agents for in-stent restenosis. Arterial stenting is applied in more than 60% of balloon angioplasties for treating cardiovascular disease. However, stented arteries encounter accelerated rates of restenosis. No prior platform has allowed evaluation or local management of in-stent restenosis without perturbing the very system being examined. A stainless steel, balloon-expandable stent was modified to serve as an ablumenal drug delivery platform. Several combinations of bioerodible polymer microspheres and gels were evaluated for channel retention under in vitro flow and in vivo conditions. A stent-anchored hybrid system prevented material embolization under all conditions. Unlike prior platforms, these stents do not alter local inflammation or in-stent plaque formation relative to conventional Palmaz-Schatz stents after in vivo deployment. The system also proved sensitive enough to detect plaque reduction with an antirestenotic agent. We conclude that a platform to evaluate and deliver therapeutic agents for in-stent restenosis has been achieved.

Transforming growth factor-beta: a promising target for anti-stenosis therapy

Transforming growth factor-beta (TGF-beta) is the general name for a family of cytokines which have widespread effects on many aspects of growth and development. The TGF-beta isoforms are produced by most cell types and exert a wide range of effects in a context-dependent autocrine, paracrine or endocrine fashion via interactions with distinct receptors on the cell surface. TGF-beta is involved in the wound healing process and, thus plays a significant role in the formation of a restenotic lesion after percutaneous transluminal coronary angioplasty (PTCA) or stenting. Perhaps because of its wide-ranging effects, TGF-beta is usually released from cells in a latent form, and its activation and signaling are complex. Manipulation of the TGF-beta1, TGF-beta2, and TGF-beta3 isoforms by inhibiting their expression, activation, or signaling reduces scarring and fibrosis in animal models. However, to date, few have reached clinical trial. This review summarizes current knowledge on the activation and signaling of TGF-beta, and focuses on the anti-TGF-beta strategies which may lead to clinical applications in the prevention of restenosis following PTCA or stenting.

Drug-coated stents

Drug-coated stents appear to be the most promising approach among all interventional strategies to prevent restenosis. These stents both suppress geometric remodeling and inhibit neointimal hyperplasia with a therapeutic agent. Animal studies and recent randomised clinical trials with sirolimus-eluting stents have achieved excellent results in the prevention of restenosis. These stents also have a good safety record and demonstrate a durable clinical benefit for patients at long-term follow-up. This article summarises experimental and clinical experiences with local drug delivery via a stent coating in the prevention of restenosis after coronary angioplasty, outlining the clinician's view of current trends.

Development of high-performance stent: gelatinous photogel-coated stent that permits drug delivery and gene transfer

Hydrogel-coated metallic stents may provide supplementary functions such as local drug delivery and gene transfer in addition to mechanical dilation function. To this end, we used a photoreactive material consisting of gelatin macromer (multiple styrene-derivatized gelatin) and carboxylated camphorquinone (photo-initiator). A few minutes of visible light irradiation of a stent after dip-coating of an aqueous solution of the photoreactive material resulted in the formation of a homogeneously crosslinked gelatinous layer on the entire exterior surface of the stent. As the metal stent, gold stents under development were used. Rhodamine-conjugated albumin as a model drug or adenoviral vector expressing bacterial beta-galactosidase (AdLacZ) as a model gene were photo-immobilized in the gelatinous gel layer. In vitro experiments using hybrid tubular tissue, which is a self-shrinkaged, vascular smooth muscle cell-incorporated type-I collagen gel, as a vascular model, showed that the immobilized dye-derivatized albumin was released on and permeated into tissues, as observed by confocal laser microscopy, and that the cells transfected with immobilized AdLacZ produced beta-galactosidase up to almost 3 weeks, as observed by x-gal staining. In preliminary in vivo experiments these drug- or adenovirus-immobilized stents were implanted in rabbit common carotid arteries. Within 3 weeks of implantation, drug permeation and gene expression in the vascular tissues were observed, indicating that the gelatinous photogel effectively serves as a matrix or coating for a bioactive stent,which permits drug release as well as gene transfer. This intraluminal approach has the potential to realize drug and gene therapy in atherosclerotic plaque.

Biodegradable Polyglycolide Endovascular Coils Promote Wall Thickening and Drug Delivery in a Rat Aneurysm Model

OBJECTIVE

We designed biodegradable polyglycolide coils (BPCs) and compared the histopathological response to the coils with that to platinum Guglielmi detachable coils (GDCs), after insertion into ligated common carotid arteries (CCAs) of adult rats. BPCs were also tested for use in local drug delivery.

METHODS

Segments (4-mm) of unmodified BPCs, unmodified GDCs, or BPCs coated with Type I bovine collagen and recombinant human vascular endothelial growth factor-165 (500 μg/ml) were inserted into ligated CCAs of adult rats for 14 days, and specimens were compared with contralateral CCA control specimens.

RESULTS

Arterial segments with BPCs exhibited substantially increased wall thickening, compared with GDCs (0.33 mm versus 0.10 mm, P &lt; 0.005), which reduced the luminal diameter by 40%, relative to untreated contralateral control specimens (P &lt; 0.05, n = 6). Arterial segments with BPCs also exhibited a marked reduction (P &lt; 0.05, n = 6) in luminal area (0.72 ± 0.93 mm2), with marked cellular proliferation within the coil diameter, indicating coil integration. Arterial segments with collagen/recombinant human vascular endothelial growth factor-coated BPCs also exhibited a marked 2.9-fold increase (P &lt; 0.005, n = 5) in wall thickness (0.29 ± 0.11 mm) and a 34% reduction in luminal diameter, compared with contralateral control vessels. There was marked proliferation of cells within the coil lumen of vessels treated with BPCs with collagen/recombinant human vascular endothelial growth factor.

CONCLUSION

In this feasibility study, BPCs enhanced the vascular response of CCA segments, compared with GDCs, and were also suitable for local protein delivery to the vessel lumen, under conditions of stasis and arterial pressurization of vascular cells.

Biodegradation of the copolymeric polylactide stent. Long-term follow-up in a rabbit aorta model

The behavior of biodegradable polylactide as a stent material has not yet been fully established in small vessels such as arteries with a diameter <3 mm. The aim of this study was to investigate the long-term effect of a copolymeric polylactide (PLA96) stent. Appropriately sized spiral PLA96 stents were implanted into the infrarenal aortas of 20 rabbits. Intraoperative systemic heparinization (150 IU/kg), perioperative subcutaneous enoxaheparin sodium (10 mg), ticlopidine (250 mg/day) for 1 month, and acetosalicylic acid (12.5 mg/day) were continuously administered. Animals were euthanized according to a fixed timetable for up to 34 months for histologic and scanning-electron-microscopic assessment. Endothelialization was complete within 1 month. In 2 of the 3 aortas sampled 3 months after implantation, a mild inflammatory reaction was visible, with no sign of granulomatous or foreign-body reaction in the vessel wall. Instead, in 1 sample examined at the same time point, neointimal chondroid metaplasia was detected. After 6 months, inflammatory reaction declined in the vessel wall. Hydrolyzation of the stent was histologically evident at 12 months, with mild foreign-body reaction detectable in 2 of 5 aortas sampled at this time point. The stent disintegrated without fragmentation by 24 months, as it was gradually replaced by fibrosis. The vessel lumen remained patent at all time points. We conclude that the PLA96 stent degraded with minimal tissue response within 24 months. PLA96 may thus be a promising stent core material for small vessels in the future, although further investigation is needed to establish its final biocompatibility.

Cell cycle in vasculoproliferative diseases: potential interventions and routes of delivery

Atherosclerosis and restenosis of epicardial vessels are among the greatest challenges facing the clinical cardiologist, and phenotypic modulation and proliferation of smooth muscle cells are major components of the vasculoproliferative response. Proliferation is regulated by the interplay of regulatory proteins at checkpoints in the cell cycle that alter cellular growth. Activation of the cell cycle and the genetic control of its progression are final common pathways in this process. Investigators have postulated that cell-cycle inhibition using drugs and genetic or physical methods has the potential to reverse or prevent the vasculoproliferative process. The current challenge is to translate in vitro data demonstrating the efficacy of cell-cycle inhibition to clinical trials. At present, the steps that must be taken to meet this goal are (1) to design methods of delivery of these agents to specific sites, (2) to identify appropriate cellular targets to elicit cell-cycle arrest, and (3) to improve the therapeutic ratio by minimizing potential side effects. This review discusses current concepts of the cell cycle, target-regulating mechanisms, and possible interventions in vasculoproliferative diseases. We also discuss ongoing clinical trials that use antiproliferative agents in the hope of limiting the course of these diseases, as well as the promise that antiproliferative therapy holds in the coming decade.

Local drug delivery: an emerging approach in the treatment of restenosis

Very limited success has been demonstrated with systemic pharmacological treatment to reduce the incidence of restenosis following angioplasty in patients. The lack of success of many of the pharmacotherapeutic agents in reducing the restenosis rates post-angioplasty and following stent implementation is believed to arise from inadequate concentrations of the agents at the lesion site. This has led to the development of various local delivery devices that would ideally deliver and retain adequate amounts of drug to the vessel wall for sufficient periods of time to ensure a therapeutic effect without inducing further injury or compromising blood flow. Local dosing would avoid systemic toxicity, and the use of modified balloon catheters or coated stents might enable percutaneous approaches.

Topic review: surface modifications enhancing biological activity of guglielmi detachable coils in treating intracranial aneurysms

BACKGROUND

Endovascular therapy with Guglielmi detachable coils is an accepted treatment option for patients with intracranial aneurysms. However, an emerging technology in the realm of endovascular tools is the use of traditional Guglielmi detachable coils with biologically active substances complexed to the coil surface to enhance aneurysm occlusion.

METHODS

We review the literature and current trends in modified Guglielmi detachable coils. Surface modifications with extracellular matrix proteins, growth factors, ion impregnation, and genetically altered cells have been used in animal studies to improve the cellular response of Guglielmi detachable coils. Similarly, coronary artery stents have been modified in several different ways to maintain vessel patency, contrary to the goal of endovascular therapy. We comparatively reviewed this literature to add insight into the evolution of the research on modified Guglielmi detachable coils.

CONCLUSIONS

Guglielmi detachable coil modifications have the potential to enhance aneurysm obliteration with directed cellular responses. This may allow aneurysm occlusion with coils in less time than untreated coils, thus decreasing the risks of aneurysm enlargement and hemorrhage.