Examination of drug release and distribution from drug-eluting stents with a vessel-simulating flow-through cell

An Overview of In Vitro Drug Release Methods for Drug-Eluting Stents

The drug release profile of drug-eluting stents (DESs) is affected by a number of factors, including the formulation, design, and physicochemical properties of the utilized material. DES has been around for twenty years and despite its widespread clinical use, and efficacy in lowering the rate of target lesion restenosis, it still requires additional development to reduce side effects and provide long-term clinical stability. Unfortunately, for analyzing these implants, there is still no globally accepted in vitro test method. This is owing to the stent's complexity as well as the dynamic arterial compartments of the blood and vascular wall. The former is the source of numerous biological, chemical, and physical mechanisms that are more commonly observed in tissue, lumen, and DES. As a result, universalizing bio-relevant apparatus, suitable for liberation testing of such complex implants is difficult. This article aims to provide a comprehensive review of the methods used for in vitro release testing of DESs. Aspects related to the correlation of the release profiles in the cases of in vitro and in vivo are also addressed.

An Implantable Artificial Atherosclerotic Plaque as a Novel Approach for Drug Transport Studies on Drug-Eluting Stents

Atherosclerotic arteries are commonly treated using drug-eluting stents (DES). However, it remains unclear whether and how the properties of atherosclerotic plaque affect drug transport in the arterial wall. A limitation of the currently used atherosclerotic animal models to study arterial drug distribution is the unpredictability of plaque size, composition, and location. In the present study, the aim is to create an artificial atherosclerotic plaque-of reproducible and controllable complexity and implantable at specific locations-to enable systematic studies on transport phenomena of drugs in stented atherosclerosis-mimicking arteries. For this purpose, mixtures of relevant lipids at concentrations mimicking atherosclerotic plaque are incorporated in gelatin/alginate hydrogels. Lipid-free (control) and lipid-rich hydrogels (artificial plaque) are created, mounted on DES and successfully implanted in porcine coronary arteries ex-vivo. Matrix-assisted laser desorption ionization mass spectrometry imaging (MALDI-MSI) is used to measure local drug distribution in the arterial wall behind the prepared hydrogels, showing that the lipid-rich hydrogel significantly hampers drug transport as compared to the lipid-free hydrogel. This observation confirms the importance of studying drug transport phenomena in the presence of lipids and of having an experimental model in which lipids and other plaque constituents can be precisely controlled and systematically studied.

Swelling and Diffusion in Polymerized Ionic Liquids-Based Hydrogels

Hydrogels are one of the emerging classes of materials in current research. Besides their numerous applications in the medical sector as a drug delivery system or in tissue replacement, they are also suitable as irrigation components or as immobilization matrices in catalysis. For optimal application of these compounds, knowledge of the swelling properties and the diffusion mechanisms occurring in the gels is mandatory. This study is focused on hydrogels synthesized by radical polymerization of imidazolium-based ionic liquids. Both the swelling and diffusion behavior of these hydrogels were investigated via gravimetric swelling as well as sorption experiments implemented in water, ethanol, n-heptane, and tetrahydrofuran. In water and ethanol, strong swelling was observed while the transport mechanism deviated from Fickian-type behavior. By varying the counterion and the chain length of the cation, their influences on the processes were observed. The calculation of the diffusion coefficients delivered values in the range of 10⁻¹⁰ to 10⁻¹² m² s⁻¹. The gravimetric results were supported by apparent diffusion coefficients measured through diffusion-weighted magnetic resonance imaging. A visualization of the water diffusion front within the hydrogel should help to further elucidate the diffusion processes in the imidazolium-based hydrogels.

Assessing the influence of atherosclerosis on drug coated balloon therapy using computational modelling

Interventional therapies such as drug-eluting stents (DES) and drug-coated balloons (DCB) have significantly improved the clinical outcomes of patients with coronary occlusions in recent years. Despite this marked improvement, ischemic cardiovascular disease remains the most common cause of death worldwide. To address this, research efforts are focused on improving the safety and efficacy of the next generation of these devices. However, current experimental methods are unable to account for the influence of atherosclerotic lesions on drug uptake and retention. Therefore, in this study, we used an integrated approach utilizing both in vitro and in silico methods to assess the performance of DCB therapy. This approach was validated against existing in vivo results before being used to numerically estimate the effect of the atheroma. A bolus release of sirolimus was observed with our coating matrix. This, coupled with the rapid saturation of specific and non-specific binding sites observed in our study, indicated that increasing the therapeutic dose coated onto the balloons might not necessarily result in greater uptake and/or retention. Additionally, our findings alluded to an optimal exposure time, dependent on the coating matrix, for the DCBs to be expanded against the vessel. Moreover, our findings suggest that a biphasic drug release profile might be beneficial for establishing and maintaining the saturation of bindings sites within severely occluded vessels. Ultimately, we have demonstrated that computational methods may be capable of assessing the efficacy of DCB therapy as well as predict the influence of atherosclerotic lesions on said efficacy.

Stent coating by electrospinning with chitosan/poly-cyclodextrin based nanofibers loaded with simvastatin for restenosis prevention

The main cause of failure of angioplasty stenting is restenosis due to neointimal hyperplasia, a too high proliferation of smooth muscle cells (SMC). The local and sustained delivery of selective pleiotropic drugs to limit SMC proliferation seems to be the hopeful solution to minimize this post surgery complication. The aim of this study is to develop a stent covered by nanofibers (NFs) produced by electrospinning, loaded with simvastatin (SV), a drug commonly used for restenosis prevention. NFs were prepared from the electrospinning of a solution containing SV and a mixture of chitosan (cationic) and β-cyclodextrin (CD) polymer (anionic) which form together a polyelectrolyte complex that makes up the NFs matrix. First, the SV/CD interactions were studied by phase solubility diagram, DRX and DSC. The electrospinning process was then optimized to cover a self-expandable NiTiNOL stent and the mechanical resistance of the NFs sheath upon its introduction inside the delivery catheter was considered, using a crimper apparatus. The morphology, coating thicknesses and diameters of nanofibers were studied by scanning electron microscopy. The SV loading rates on the stents were controlled by the electrospinning time, and the presence of SV in the NFs was confirmed by FTIR. NFs stability in PBS pH 7.4 buffer could be improved after thermal post-treatment of NFs and in vitro release of SV in dynamic conditions demonstrated that the release profiles were influenced by the presence of CD polymer in NFs and by the thickness of the NFs sheath. Finally, a covered stent delivering 3 µg/mm² of SV within 6 h was obtained, whose efficiency will be investigated in a further in vivo study.

Evaluation of the suitability of a fluidized bed process for the coating of drug-eluting stents

Drug-eluting stents are often coated using single-stent coating techniques. In pharmaceutical industry, single-tablet coating is unthinkable. Instead large batches of tablets are coated in fluidized bed apparatuses or pan coaters. Therefore, it was the aim of this work to evaluate whether stents can be coated using a fluidized bed process. For this purpose stents were coated with the model fluorescent drug triamterene embedded in ammonium methacrylate copolymer. Different stent lengths as well as different coating yields were assessed and also a drug-free topcoat was evaluated. The coated stents were analysed regarded coating layer mass, drug content, surface structure, coating thickness and drug release. Furthermore, coating yield and stent defect rate were examined. Except for one stent configuration good results were obtained without optimization of process parameters which indicates the suitability of the method to coat large amounts of stents simultaneously in principle. Drug release was tuneable over a wide range of time spans and a wide range of drug loadings was produced. Further work will be necessary to transform the results of this study from a model stent to a clinically relevant product.

In vitro and in silico characterization of fibrous scaffolds comprising alternate colistin sulfate-loaded and heat-treated polyvinyl alcohol nanofibrous sheets

A multilayer mat for dispensing colistin sulfate through a body surface was prepared by electrospinning. The fabricated system comprised various polyvinyl alcohol fibrous layers prepared with or without the active ingredient. One of the electrospun layers contained water-soluble colistin sulfate and the other was prepared from the same polymer type and composition without the active drug and was finally heat-treated. The heat treatment modified the supramolecular structure and conferred the polymer nanofibre with the rate-controlling function. The microstructure of different layers was tracked by positron annihilation lifetime spectroscopy, and detailed morphological analysis of the fibre mats was performed using a scanning electron microscope. The drug-release profiles of various layer arrangements were studied in relation to their antimicrobial activity. The finite element method was applied to overcome the challenge of diffusion-controlled drug release from multilayer polymer scaffolds. The finite element method was first verified using analytical solutions for a simple arrangement (one drug-loaded swellable fibre and one rate-controlling nonswellable fibre) under perfect sink conditions and in a well-stirred finite volume. The effect of alternate layer arrangements on the drug-release profiles was also investigated to plan for controlled topical drug release from fibrous scaffolds. This design is expected to aid in increasing local effectiveness, thus reducing the systemic loading and the consequent side effects of colistin.

Towards the development of an in vitro model of atherosclerotic peripheral vessels for evaluating drug-coated endovascular technologies

Here, we review the in vitro models used to evaluate drug-coated endovascular technologies. The models are assessed in the context of representing the drug transport/uptake and mechanical properties of atherosclerotic peripheral vessels. Studies to date have incorporated a vessel-simulating hydrogel compartment to examine drug elution from endovascular devices. However, comparisons between in vitro models and atherosclerotic tissue are difficult because ex vivo data are limited in their applicability to diseased peripheral vessels. Furthermore, appropriate ex vivo mechanical properties are not incorporated into these models. Therefore, there is a need to characterise the drug transport/uptake properties of appropriate atherosclerotic tissue and incorporate existing ex vivo mechanical data into current in vitro models to more accurately represent drug behaviour in atherosclerotic peripheral vessels.

Drug diffusion and biological responses of arteries using a drug-eluting stent with nonuniform coating

The purpose of this study was to determine the effect of a nonuniform coating, abluminal-gradient coating (AGC), which leaves the abluminal surface of the curves and links parts of the stent free from the drug coating, on the diffusion direction of the drug and the biological responses of the artery to drug-eluting stent (DES) by comparing the AGC-sirolimus stent and the conventional full-surface coating (CFC) sirolimus stent. The study aimed to verify whether the AGC approach was appropriate for the development of a safer DES, minimizing the risks of stent thrombosis due to delayed endothelialization by the drug and distal embolization due to cracking of the coating layer on the hinge parts of the DES on stent expansion. In the in vitro local drug diffusion study, we used rhodamine B as a model drug, and rhodamine B released from the AGC stent diffused predominantly into the abluminal side of the alginate artery model. Conversely, rhodamine B released from the CFC stent quickly spread to the luminal side of the artery model, where endothelial cell regeneration is required. In the biological responses study, the luminal surface of the iliac artery implanted with the AGC-sirolimus stent in a rabbit iliac artery for 2 weeks was completely covered with endothelial-like cells. On the other hand, the luminal surface of the iliac artery implanted with the CFC-sirolimus stent for 2 weeks only showed partial coverage with endothelial-like cells. While thrombosis was observed in two of the three CFC-sirolimus stents, it was observed in only one of the three AGC-sirolimus stents. Taken together, these findings indicate that the designed nonuniform coating (AGC) is an appropriate approach to ensure a safer DES. However, the number of studies is limited and a larger study should be conducted to reach a statistically significant conclusion.

Modelling the Impact of Atherosclerosis on Drug Release and Distribution from Coronary Stents

Although drug-eluting stents (DES) are now widely used for the treatment of coronary heart disease, there remains considerable scope for the development of enhanced designs which address some of the limitations of existing devices. The drug release profile is a key element governing the overall performance of DES. The use of in vitro, in vivo, ex vivo, in silico and mathematical models has enhanced understanding of the factors which govern drug uptake and distribution from DES. Such work has identified the physical phenomena determining the transport of drug from the stent and through tissue, and has highlighted the importance of stent coatings and drug physical properties to this process. However, there is limited information regarding the precise role that the atherosclerotic lesion has in determining the uptake and distribution of drug. In this review, we start by discussing the various models that have been used in this research area, highlighting the different types of information they can provide. We then go on to describe more recent methods that incorporate the impact of atherosclerotic lesions.

Controlling drug delivery from coronary stents: are we aiming for the right targets?

In this review article, the currently employed or explored delivery concepts for local intravascular drug delivery with drug-eluting stents (DES) are discussed with a special emphasis on clinical evidence regarding the desired release profiles. Traditional concepts to control drug release from DES include diffusion through polymers, polymer degradation and erosion as well as dissolution of particulate drug. Published clinical studies do not always reveal fine mechanistic details. The long duration of release favored for DES and the short duration of release favored for drug-eluting balloons require further investigation in experimental studies and clinical trials.

Polymers for Cardiovascular Stent Coatings

Polymers have found widespread applications in cardiology, in particular in coronary vascular intervention as stent platforms (scaffolds) and coating matrices for drug-eluting stents. Apart from permanent polymers, current research is focussing on biodegradable polymers. Since they degrade once their function is fulfilled, their use might contribute to the reduction of adverse events like in-stent restenosis, late stent-thrombosis, and hypersensitivity reactions. After reviewing current literature concerning polymers used for cardiovascular applications, this review deals with parameters of tissue and blood cell functions which should be considered to evaluate biocompatibility of stent polymers in order to enhance physiological appropriate properties. The properties of the substrate on which vascular cells are placed can have a large impact on cell morphology, differentiation, motility, and fate. Finally, methods to assess these parameters under physiological conditions will be summarized.

New stent surface materials: the impact of polymer-dependent interactions of human endothelial cells, smooth muscle cells, and platelets

Despite the development of new coronary stent technologies, in-stent restenosis and stent thrombosis are still clinically relevant. Interactions of blood and tissue cells with the implanted material may represent an important cause of these side effects. We hypothesize material-dependent interaction of blood and tissue cells. The aim of this study is accordingly to investigate the impact of vascular endothelial cells, smooth muscle cells and platelets with various biodegradable polymers to identify a stent coating or platform material that demonstrates excellent endothelial-cell-supportive and non-thrombogenic properties. Human umbilical venous endothelial cells, human coronary arterial endothelial cells and human coronary arterial smooth muscle cells were cultivated on the surfaces of two established biostable polymers used for drug-eluting stents, namely poly(ethylene-co-vinylacetate) (PEVA) and poly(butyl methacrylate) (PBMA). We compared these polymers to new biodegradable polyesters poly(l-lactide) (PLLA), poly(3-hydroxybutyrate) (P(3HB)), poly(4-hydroxybutyrate) (P(4HB)) and a polymeric blend of PLLA/P(4HB) in a ratio of 78/22% (w/w). Biocompatibility tests were performed under static and dynamic conditions. Measurement of cell proliferation, viability, glycocalix width, eNOS and PECAM-1 mRNA expression revealed strong material dependency among the six polymer samples investigated. Only the polymeric blend of PLLA/P(4HB) achieved excellent endothelial markers of biocompatibility. Data show that PLLA and P(4HB) tend to a more thrombotic response, whereas the polymer blend is characterized by a lower thrombotic potential. These data demonstrate material-dependent endothelialization, smooth muscle cell growth and thrombogenicity. Although polymers such as PEVA and PBMA are already commonly used for vascular implants, they did not sufficiently meet the criteria for biocompatibility. The investigated biodegradable polymeric blend PLLA/P(4HB) evidently represents a promising material for vascular stents and stent coatings.

In vitro determination of drug transfer from drug-coated balloons

Drug-coated balloons are medical devices designed to locally deliver drug to diseased segments of the vessel wall. For these dosage forms, drug transfer to the vessel wall needs to be examined in detail, since drug released into the blood is cleared from the site. In order to examine drug transfer, a new in vitro setup was developed combining the estimation of drug loss during advancement to the site of application in a model coronary artery pathway with a hydrogel compartment representing, as a very simplified model, the vessel wall. The transfer of fluorescent model substances as well as the drug paclitaxel from coated balloons to the simulated vessel wall was evaluated using this method. The model was suitable to quantify the fractions transferred to the hydrogel and also to qualitatively assess distribution patterns in the hydrogel film. In the case of fluorescein sodium, rhodamin b and paclitaxel, vast amounts of the coated substance were lost during the simulated passage and only very small fractions of about 1% of the total load were transferred to the gel. This must be attributed to good water solubility of the fluorescent substances and the mechanical instability of the paclitaxel coating. Transfer of the hydrophobic model substance triamterene was however nearly unaffected by the preliminary tracking procedure with transferred fractions ranging from 8% to 14%. Analysis of model substance distribution yielded inhomogeneous distributions indicating that the coating was not evenly distributed on the balloon surface and that a great fraction of the coating liquid did not penetrate the folds of the balloon. This finding is contradictory to the generally accepted assumption of a drug depot inside the folds and emphasizes the necessity to thoroughly characterize in vitro performance of drug-coated balloons to support the very promising clinical data.

Development of hydrophobized alginate hydrogels for the vessel-simulating flow-through cell and their usage for biorelevant drug-eluting stent testing

The vessel-simulating flow-through cell (vFTC) has been used to examine release and distribution from drug-eluting stents in an in vitro model adapted to the stent placement in vivo. The aim of this study was to examine the effect of the admixture of different hydrophobic additives to the vessel wall simulating hydrogel compartment on release and distribution from model substance-coated stents. Four alginate-based gel formulations containing reversed-phase column microparticles LiChroprep® RP-18 or medium-chain triglycerides in form of preprocessed oil-in-water emulsions Lipofundin® MCT in different concentrations were successfully developed. Alginate and modified gels were characterized regarding the distribution coefficient for the fluorescent model substances, fluorescein and triamterene, and release as well as distribution of model substances from coated stents were investigated in the vFTC. Distribution coefficients for the hydrophobic model substance triamterene and the hydrophobized gel formulations were up to four times higher than for the reference gel. However, comparison of the obtained release profiles yielded no major differences in dissolution and distribution behavior for both fluorescent model substances (fluorescein, triamterene). Comparison of the test results with mathematically modeled data acquired using finite element methods demonstrated a good agreement between modeled data and experimental results indicating that gel hydrophobicity will only influence release in cases of fast releasing stent coatings.

Analysis of drug distribution from a simulated drug-eluting stent strut using an in vitro framework

The mechanisms of delivery of anti-proliferative drug from a drug-eluting stent are defined by transport forces in the coating, the lumen, and the arterial wall. Dynamic asymmetries in the localized flow about stent struts have previously been shown to contribute to significant heterogeneity in the spatial distribution of drug in in silico three-compartmental models of stent based drug delivery. A novel bench-top experiment has been created to confirm this phenomena. The experiment simulates drug release from a single stent strut, and then allows visualization of drug uptake into both lumen and tissue domains using optical techniques. Results confirm the existence of inhomogeneous and asymmetric arterial drug distributions, with this distribution shown to be sensitive to the flow field surrounding the strut.

In-vitro dissolution methods for controlled release parenterals and their applicability to drug-eluting stent testing

Objectives

Dissolution testing is a powerful tool for the characterization of dosage form performance in vitro under standardized conditions. In spite of the increasing number of parenterally administered medicinal products, currently there are no compendial dissolution test methods designed especially for these types of dosage forms. In addition to classical drug delivery systems, drug/device combination products, such as drug-eluting stents, are being used increasingly.

Key findings

This review describes the current methods that are used most often for in-vitro dissolution testing of parenteral dosage forms, i.e. the ‘sample and separate’ methods, the ‘dialysis’ methods, and the ‘flow-through’ methods, with a special emphasis on whether these methods can be used for drug-eluting stent testing. In the light of current regulatory requirements and with the exploding costs of preclinical and clinical development, test systems that include biorelevant parameters and are predictive of in-vivo performance are increasingly important. Published attempts to take biorelevant conditions into consideration in the design of dissolution test apparatus developed for parenteral dosage forms, including a method that was designed to emulate the embedding and flow-conditions at the site of stent implantation, have been outlined in this review.

Summary

In spite of the large quantity of highly potent controlled release parenteral products marketed today, there is still a lack of suitable methods for in vitro dissolution testing for these dosage forms especially with regard to biorelevant testing conditions. For dosage forms implanted into tissues it seems of major importance to reproduce the transport forces which are predominant in vivo (diffusive versus convective) in the in-vitro experimental setup.

Accelerated in vitro release testing of implantable PLGA microsphere/PVA hydrogel composite coatings

Dexamethasone loaded poly(lactic-co-glycolic acid) (PLGA) microsphere/PVA hydrogel composites have been investigated as an outer drug-eluting coating for implantable devices such as glucose sensors to counter negative tissue responses to implants. The objective of this study was to develop a discriminatory, accelerated in vitro release testing method for this drug-eluting coating using United States Pharmacopeia (USP) apparatus 4. Polymer degradation and drug release kinetics were investigated under "real-time" and accelerated conditions (i.e. extreme pH, hydro-alcoholic solutions and elevated temperatures). Compared to "real-time" conditions, the initial burst and lag phases were similar using hydro-alcoholic solutions and extreme pH conditions, while the secondary apparent zero-order release phase was slightly accelerated. Elevated temperatures resulted in a significant acceleration of dexamethasone release. The accelerated release data were able to predict "real-time" release when applying the Arrhenius equation. Microsphere batches with faster and slower release profiles were investigated under "real-time" and elevated temperature (60°C) conditions to determine the discriminatory ability of the method. The results demonstrated both the feasibility and the discriminatory ability of this USP apparatus 4 method for in vitro release testing of drug loaded PLGA microsphere/PVA hydrogel composites. This method may be appropriate for similar drug/device combination products and drug delivery systems.