Drug-eluting stent coatings

Coatings for Cardiovascular Stents-An Up-to-Date Review

Cardiovascular diseases (CVDs) increasingly burden health systems and patients worldwide, necessitating the improved awareness of current treatment possibilities and the development of more efficient therapeutic strategies. When plaque deposits narrow the arteries, the standard of care implies the insertion of a stent at the lesion site. The most promising development in cardiovascular stents has been the release of medications from these stents. However, the use of drug-eluting stents (DESs) is still challenged by in-stent restenosis occurrence. DESs' long-term clinical success depends on several parameters, including the degradability of the polymers, drug release profiles, stent platforms, coating polymers, and the metals and their alloys that are employed as metal frames in the stents. Thus, it is critical to investigate new approaches to optimize the most suitable DESs to solve problems with the inflammatory response, delayed endothelialization, and sub-acute stent thrombosis. As certain advancements have been reported in the literature, this review aims to present the latest updates in the coatings field for cardiovascular stents. Specifically, there are described various organic (e.g., synthetic and natural polymer-based coatings, stents coated directly with drugs, and coatings containing endothelial cells) and inorganic (e.g., metallic and nonmetallic materials) stent coating options, aiming to create an updated framework that would serve as an inception point for future research.

Computational simulation of the flow dynamic field in a porous ureteric stent

Ureteric stents are employed clinically to manage urinary obstructions or other pathological conditions. Stents made of porous and biodegradable materials have gained increasing interest, because of their excellent biocompatibility and the potential for overcoming the so-called ‘forgotten stent syndrome’. However, there is very limited characterisation of their flow dynamic performance. In this study, a CFD model of the occluded and unoccluded urinary tract was developed to investigate the urinary flow dynamics in the presence of a porous ureteric stent. With increasing the permeability of the porous material (i.e., from 10⁻¹⁸ to 10⁻¹⁰ m²) both the total mass flow rate through the ureter and the average fluid velocity within the stent increased. In the unoccluded ureter, the total mass flow rate increased of 7.7% when a porous stent with permeability of 10⁻¹⁰ m² was employed instead of an unporous stent. Drainage performance further improved in the presence of a ureteral occlusion, with the porous stent resulting in 10.2% greater mass flow rate compared to the unporous stent. Findings from this study provide fundamental insights into the flow performance of porous ureteric stents, with potential utility in the development pipeline of these medical devices.

Recent advance in treatment of atherosclerosis: Key targets and plaque-positioned delivery strategies

Atherosclerosis, a chronic inflammatory disease of vascular wall, is a progressive pathophysiological process with lipids oxidation/depositing initiation and innate/adaptive immune responses. The coordination of multi systems covering oxidative stress, dysfunctional endothelium, diseased lipid uptake, cell apoptosis, thrombotic and pro-inflammatory responding as well as switched SMCs contributes to plaque growth. In this circumstance, inevitably, targeting these processes is considered to be effective for treating atherosclerosis. Arriving, retention and working of payload candidates mediated by targets in lesion direct ultimate therapeutic outcomes. Accumulating a series of scientific studies and clinical practice in the past decades, lesion homing delivery strategies including stent/balloon/nanoparticle-based transportation worked as the potent promotor to ensure a therapeutic effect. The objective of this review is to achieve a very brief summary about the effective therapeutic methods cooperating specifical targets and positioning-delivery strategies in atherosclerosis for better outcomes.

Emerging Encapsulation Technologies for Long-Term Reliability of Microfabricated Implantable Devices

The development of reliable long-term encapsulation technologies for implantable biomedical devices is of paramount importance for the safe and stable operation of implants in the body over a period of several decades. Conventional technologies based on titanium or ceramic packaging, however, are not suitable for encapsulating microfabricated devices due to their limited scalability, incompatibility with microfabrication processes, and difficulties with miniaturization. A variety of emerging materials have been proposed for encapsulation of microfabricated implants, including thin-film inorganic coatings of Al2O3, HfO2, SiO2, SiC, and diamond, as well as organic polymers of polyimide, parylene, liquid crystal polymer, silicone elastomer, SU-8, and cyclic olefin copolymer. While none of these materials have yet been proven to be as hermetic as conventional metal packages nor widely used in regulatory approved devices for chronic implantation, a number of studies have demonstrated promising outcomes on their long-term encapsulation performance through a multitude of fabrication and testing methodologies. The present review article aims to provide a comprehensive, up-to-date overview of the long-term encapsulation performance of these emerging materials with a specific focus on publications that have quantitatively estimated the lifetime of encapsulation technologies in aqueous environments.

Non-polymer drug-eluting coronary stents

Cardiovascular complications are leading causes of most fatalities. Coronary artery disease and surgical failures contribute to the death of the majority of patients. Advanced research in the field of medical devices like stents has efficiently resolved these problems. Clinically, drug-eluting stents have proven their efficacy and safety compared to bare metal stents, which have problems of in-stent restenosis. However, drug-loaded stents coated with polymers have shown adverse effects related to the stability and deterioration of the polymer coating over time. This results in late stent thrombosis and immunogenicity. These reasons laid the foundation for the development of non-polymeric drug-eluting stents. This review focuses on non-polymer drug-eluting stents loaded with different drugs like anti-inflammatory agents, anti-thrombotic, anti-platelet agents, immune suppressants and others. Surface modification techniques on stents like crystalline coating; microporous, macroporous, and nanoporous coatings; and chemically modified self-assembled monolayers are described in detail. There is also an update on clinically approved products and those under development.

One-year outcomes of a BioMime™ Sirolimus-Eluting Coronary Stent System with a biodegradable polymer in all-comers coronary artery disease patients: The meriT-3 study

OBJECTIVES

The aim of the merit-3 study was to determine the safety and performance of the BioMime Sirolimus-Eluting Coronary Stent System (SES) in all-comer patients with coronary artery disease (CAD) in one-year clinical follow-up period.

METHODS

The meriT-3 was a multi-centre, observational, post-marketing study conducted in 1161 patients with CAD who were implanted with BioMime SES at 15 sites in India. The primary endpoint was major adverse cardiac event (MACE) at one year defined as the composite of cardiac death, myocardial infarction (MI) and target lesion revascularization (TLR). Clinical follow-up was performed at 1, 6, and 12 months. Major adverse cardiac event occurred at 30 days and subsequently at 6 months and at long-term follow-up of 1 year was analyzed.

RESULTS

MACE observed at 1 and 6 months follow-up was 16 (1.38%) and 21 (1.83%) respectively. Cumulative 1 year MACE was 26 (2.35%) with 16 (1.39%) all cause death, 4 (0.35%) MI and 6 (0.52%) TLR. In addition, ST was observed in 1 (0.09%) patient.

CONCLUSIONS

The present study suggests that the BioMime SES is safe and effective in a "real-world", all-comers CAD patients, indicating low rates of MACE.

CTRI ACKNOWLEDGEMENT NO

REF/2016/07/011808.

Enhancing Stent Effectiveness with Nanofeatures

Drug-eluting stents are an effective therapy for symptomatic arterial obstructions, substantially reducing the incidence of restenosis by suppressing the migration and proliferation of vascular smooth muscle cells into the intima. However, current drug-eluting stents also inhibit the growth of endothelial cells, which are required to cover the vascular stent to reduce an excessive inflammatory response. As a result, the endothelial lining of the lumen is not regenerated. Since the loss of this homeostatic monolayer increases the risk of thrombosis, patients with drug-eluting stents require long-term antithrombotic therapy. Thus, there is a need for improved devices with enhanced effectiveness and physiological compatibility towards endothelial cells. Current developments in nanomaterials may enhance the function of commercially available vascular devices. In particular, modified design schemes might incorporate nanopatterns or nanoparticle-eluting features that reduce restenosis and enhance re-endothelialization. The intent of this review is to discuss emerging nanotechnologies that will improve the performance of vascular stents.

Electrochemical etching of micro-pores in medical grade cobalt-chromium alloy as reservoirs for drug eluting stents

Drug eluting stents (DES) have shown efficacy in reducing restenosis after angioplasty followed by application of a coronary stent. However, polymer matrices typically used for immobilizing drugs on the stent surface may cause irritation and have limited drug loading capacity. In contrast, drug loading into micro- or nanopores created within the stent material could avoid these problems. We present a technology based on electrochemically induced pitting corrosion to form pores in medical grade steel, followed by loading with rapamycin. This process is applied to pore formation and drug loading in coronary stents consisting of L605 medical steel. Sustained release of the drug over 28 days at rates comparable to established DES was demonstrated. This technology is capable of creating pores with well-defined pore size and filling of these pores by a drug employing a crystallization process thus completely avoiding polymer matrices to immobilize drugs. Electrochemically induced pitting corrosion provides a generic means to introduce micro-pores suitable as drug reservoirs into medical grade steel without the need for any further matrix material. Further research will expand these findings to other materials and types of implants that could benefit from the additional function of drug release and/or improved implant/tissue integration.

Some design considerations for polymer-free drug-eluting stents: a mathematical approach

In this paper we provide the first model of drug elution from polymer-free arterial drug-eluting stents. The generalised model is capable of predicting drug release from a number of polymer-free systems including those that exhibit nanoporous, nanotubular and smooth surfaces. We derive analytical solutions which allow us to easily determine the important parameters that control drug release. Drug release profiles are provided, and we offer design recommendations so that the release profile may be tailored to achieve the desired outcome. The models presented here are not specific to drug-eluting stents and may also be applied to other biomedical implants that use nanoporous surfaces to release a drug.

Nonvascular drug-eluting stent coated with sodium caprate-incorporated polyurethane for the efficient penetration of paclitaxel into tumor tissue

To increase the therapeutic potency of nonvascular drug-eluting stents, sodium caprate was employed as a drug-penetration enhancer. A polytetrafluoroethylene-covered drug-eluting stent was coated with a mixture containing sodium caprate, paclitaxel, and polyurethane via the rolling coating technique. The coated stent has a smooth membrane surface with a 40-µm membrane thickness. Paclitaxel was released from the coated stent for two months. In the multilayered cell sheet model, sodium caprate in the polyurethane membrane (PUSC10) showed the possibility of enhancing the paclitaxel tissue penetration. The amount of penetrated paclitaxel for the sodium caprate-containing polyurethane membrane (PUSC10) was two times higher than that of sodium caprate-free polyurethane membrane. Additionally, the potential of sodium caprate was confirmed by a tumor-bearing small animal model. PUSC10 incorporated with Nile red (as a model fluorescence dye for visualization of drug penetration; PUSC10–Nile red) or PUSC10 incorporated with paclitaxel (PUSC10–paclitaxel) membrane was implanted at tumor sites in Balb/c mice. In the case of PUSC10–Nile red, the tissue penetration depth of Nile red was significantly increased from 30 µm (without sodium caprate) to 1060 µm (with sodium caprate). After seven days, an almost four times higher therapeutic area of PUSC10–paclitaxel was observed compared to that of polyurethane–paclitaxel (without sodium caprate) by a terminal deoxynucleotidyl transferase dUTP nick end labeling assay. The results indicate that sodium caprate improves the penetration and therapeutic efficiencies of drugs in drug-eluting stents, and thus, it has potential for local stent therapy.

Dual drug-eluting stents coated with multilayers of hydrophobic heparin and sirolimus

Polymer coatings for stents are considered one of the key factors that lead to adverse cardiac events after coronary arterial stenting. This study presents a dual drug-eluting stent (DES) that is coated with multilayers of Duraflo heparin and sirolimus but containing no other organic polymers. The hydrophobic Duraflo heparin coating was used to improve the hemocompatibility of the stent and serve as a drug reservoir for the controlled release of sirolimus, thus avoiding inflammatory reactions induced by the conventional polymers. The Duraflo heparin and sirolimus were coated layer-by-layer onto the stent surface using a homemade spray-coating device. The drug loading amount can be easily controlled by adjusting the numbers of layers applied and the concentration of the drug solution, indicating the developed coating process is reproducible and well-controlled. After balloon expansion, the coating did not crack or peel off, which demonstrates that the sirolimus/Duraflo heparin coating layers tightly adhere to the stent surface. The activated partial thromboplastin time (APTT) assay showed that the Duraflo heparin coating significantly prolonged the APTT from 27.3 ± 0.3 s to 69.7 ± 6.2 s, demonstrating the anticoagulant ability of the coated stents. The dual DES exhibited a nearly linear sustained-release profile of Duraflo heparin and an initial burst release followed by a slow release of sirolimus. Less than 15% of heparin was released from the DES within 14 days, indicating the stent can maintain its antithrombotic surface for a long time. Because of the layer-by-layer structure, the most outer layer of Duraflo heparin coating may act as a diffusion barrier to retard sirolimus release from the stent. These results confirm that the dual DESs enable simultaneous delivery of antithrombotic and antiproliferative drugs and have potential for the treatment of coronary artery disease.

Fabrication of a novel polymer-free nanostructured drug-eluting coating for cardiovascular stents

Angioplasty with stents is the most important method for the treatment of coronary artery disease (CAD). However, the drug-eluting stents (DES) that are widely used have the increased risks of inflammatory reactions and late stent thrombosis (LST) because of the persistence of the polymer coatings. To improve the biosafety, a novel polymer-free-composite drug-eluting coating composed of magnetic mesoporous silica nanoparticles (MMSNs) and carbon nanotubes (CNTs) was constructed using the electrophoretic deposition (EPD) method in this study. A crack-free two-layered coating with impressive network nanotopologies was successfully obtained by regulating the composition and structures. This nanostructured coating exhibits excellent mechanical flexibility and blood compatibility in vitro, and the drug-loading and release performance is satisfactory as well. The in vivo study shows that this composite coating has the obvious advantage of rapid endothelialization because of its unique 3D nanostructured topology in comparison with the commercial polymer-coated DES. This study aims to provide new ideas and reliable data to design novel functional coatings that could accelerate the re-endothelialization process and avoid inflammatory reactions, thus improving the in vivo biosafety of DES.