Beatrice Semmling

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

The recently introduced vessel-simulating flow-through cell offers new possibilities to examine the release from drug-eluting stents in vitro. In comparison with standard dissolution methods, the additional compartment allows for the examination of distribution processes and creates dissolution conditions which simulate the physiological situation at the site of implantation. It was shown previously that these conditions have a distinct influence on the release rate from the stent coating. In this work, different preparation techniques were developed to examine the spatial distribution within the compartment simulating the vessel wall. These methods allowed for the examination of diffusion depth and the distribution resulting in the innermost layer of the compartment simulating the vessel wall. Furthermore, the in vitro release and distribution examined experimentally were modelled mathematically using finite element (FE) methods to gain further insight into the release and distribution behaviour. The FE modelling employing the experimentally determined diffusion coefficients yielded a good general description of the experimental data. The results of the modelling also provided important indications that inhomogeneous coating layer thicknesses around the strut may result from the coating process which influence release and distribution behaviour. Taken together, the vessel-simulating flow-through cell in combination with FE modelling represents a unique method to analyse drug release and distribution from drug-eluting stents in vitro with particular opportunities regarding the examination of spatial distributions within the vessel-simulating compartment.

Magnesium used in bioabsorbable stents controls smooth muscle cell proliferation and stimulates endothelial cells in vitro.

Magnesium-based bioabsorbable cardiovascular stents have been developed to overcome limitations of permanent metallic stents, such as late stent thrombosis. During stent degradation, endothelial and smooth muscle cells will be exposed to locally high magnesium concentrations with yet unknown physiological consequences. Here, we investigated the effects of elevated magnesium concentrations on human coronary artery endothelial and smooth muscle cell (HCAEC, HCASMC) growth and gene expression. In the course of 24 h after incubation with magnesium chloride solutions (1 or 10 mM) intracellular magnesium level in HCASMC raised from 0.55 ± 0.25 mM (1 mM) to 1.38 ± 0.95 mM (10 mM), while no increase was detected in HCAEC. Accordingly, a DNA microarray-based study identified 69 magnesium regulated transcripts in HCAEC, but 2172 magnesium regulated transcripts in HCASMC. Notably, a significant regulation of various growth factors and extracellular matrix components was observed. In contrast, viability and proliferation of HCAEC were increased at concentrations of up to 25 mM magnesium chloride, while in HCASMC viability and proliferation appeared to be unaffected. Taken together, our data indicate that magnesium halts smooth muscle cell proliferation and stimulates endothelial cell proliferation, which might translate into a beneficial effect in the setting of stent associated vascular injury.

In Vitro Dissolution Testing of Drug-Eluting Stents

Drug-eluting stents (DES) have revolutionized the treatment of coronary artery blockage by tremendously reducing the rate of in-stent restenosis and the necessity of repeat revascularization compared to bare-metal stents. They are also gaining increasing importance in other medical fields such as the treatment of certain localized tumors and in glaucoma therapy. DES generally contain most potent drugs, e.g. immunosuppressants or cytostatics, which are supposed to be released in a well controlled manner over time spans which are chosen according to disease progression. Typically, this means that fairly small amounts of drug are released over long periods of time. Therefore, quantification of in vivo plasma levels is often not feasible. Due to this limitation and the fact that tissue levels cannot be determined in humans, in vitro dissolution testing is one of the most powerful tools to gain insight into the release behaviour of DES. This article focuses on the methods for in vitro dissolution testing of DES which are available up to date and highlights the specific characteristics of drug release from stents arising from the composition and the in vivo localization of the dosage form.

Long-term stable hydrogels for biorelevant dissolution testing of drug-eluting stents

Abstract Background: For the purpose of biorelevant dissolution testing hydrogels have recently been used to investigate release and distribution behavior of drugs released from specialized dosage forms such as

Impact of different tissue-simulating hydrogel compartments on in vitro release and distribution from drug-eluting stents

In vitro drug release testing is an appropriate approach to identify critical parameters helping to predict drug release from drug-eluting stents (DES) prior to studying drug release behavior under in vivo conditions. Drug release and distribution from DES coated with a fluorescent model substance were studied in vitro using the vessel-simulating flow-through cell equipped with different long-term stable hydrogel compartments composed of agarose, polyacrylamide or poly(vinyl alcohol). The obtained experimental results were compared with the results of finite-element modeling obtained using experimentally determined diffusion coefficients and partition coefficients. In spite of differences regarding these parameters, experimental and mathematical data yielded only minor differences between the different gels regarding the release and distribution behavior and reasonable agreement between the modeling and the experiment was obtained. In an attempt to further elucidate the dosage form behavior, the diffusion coefficients in the gel as well as in the stent coating were systematically varied in the finite-element model. Changes in the diffusivity in the stent coating mainly impacted on the initial concentrations. Slower diffusion inside the hydrogel yielded a retarded elution from the stent coating and a higher model substance accumulation in the gel compartment at late time points.

Biorelevant Dissolution Testing of Drug-Eluting Stents: Experiences with a Modified Flow-Through Cell Setup

In vitro dissolution testing of drug-eluting stents (DES) poses a special challenge in terms of apparatus design due to the v ery specialized local treatment of the vessel wall in the immediate vicinity of the blood flowing through the vessel and the stent lumen. A vessel-simulating flow-through cell was designed to emulate the placement of a DES in vivo and the flow through the lumen in a simplified in vitro setup, which also allows for the examination of distribution processes. The method is based on the compendial flow-through cell apparatus (USP 4), which was modified by the addition of a hydrogel compartment that represents the vessel wall and the adaptation of the flow rate to the blood flow rate in the coronary vessels. A comparison of the dissolution and release results obtained with the vessel-simulating flow-through cell with standard paddle (USP 2) and flow-through cell (USP 4) apparatus methods shows that release from the coating was decelerated by embedding in the hydrogel in the adapted apparatus. Further experiments with both hydrophilic and hydrophobic fluorescent model compounds coated onto the stents were performed to investigate the effects of different method parameters and variations in the coating composition. While release and distribution of hydrophilic fluorescein sodium were dependent on the flow rate and implantation technique, release kinetics of hydrophobic triamterene were influenced by the coating thickness and the model substance content of the coating. These results illustrate the importance of choosing the correct apparatus design and test parameters adapted to biorelevant conditions for specialized dosage forms such as DES.

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.

In vitro estimation of drug loss during the implantation procedure of drug-eluting stents

Drug-eluting stents are drug/device combinatory products designed to physically re-establish the blood flow in arteriosclerotic blood vessels and deliver anti-proliferative drugs to the stented vessel section to prevent neointimal hyperplasia. Drug that is released during the implantation procedure is washed away with the blood flow. It was our aim to estimate this drug loss using stents coated with fluorescent model substances in an in vitro setup. Stents mounted on balloon catheters were introduced into a polymethacrylate coronary artery model (adapted from ASTM standard F 2394-07 intended for measurement of securement of stents on delivery systems) via a guiding catheter. The system was perfused by phosphate buffered saline pH 7.4 at a flow-rate adapted to the blood flow-rate in coronary arteries. The perfusate was collected in fractions and model substance content was determined fluorimetrically. The hydrophilic model substance fluorescein sodium was released very fast resulting in a loss of approximately 64 % of the drug load within the first minute and up to 82 % after 5 minutes. The hydrophobic model substance triamterene was released much slower from the coating. A total loss of 5 % was detected after 5 minutes. Furthermore, commercially available sirolimus-eluting Cypher Select + stents were tested. Release into the media was not detected. Obviously, the drug-free top coating effectively prevented drug release during the simulated passage to the site of implantation. An in vitro test system to estimate drug loss during the implantation procedure of drug-eluting stents was successfully established. First results using fluorescent model substances indicate that vast losses have to be expected when using fast releasing delivery systems. Therefore, the model may also prove valuable for the in vitro testing of drug-eluting balloons. Further adaptations, such as the inclusion of biorelevant media, may be suitable to further improve the predictability of in vivo performance.

Examination of steroid release from screw-in pacing leads.

Although constant progress in pacing lead design and life time of pacemaker battery has contributed to the clinical success of this treatment option, the mechanism of drug release from steroid-eluting pacing leads is not completely understood. Besides, drug concentrations of steroids released into the heart-tissue remain unclear. Therefore, three different dissolution test methods were examined: a non-standardized (reagent tube), a compendial USP (paddle) and a hydrogel method. The experimental results indicate that dissolution from pacing leads is governed by a diffusion-controlled mechanism and occurs over a prolonged time. Furthermore, the integration of a hydrogel compartment seemed to have an impact on drug release leading to a decrease in release rate. Nevertheless, the experimental results emphasize different experimental parameters in dissolution testing may have a distinct influence on drug dissolution.

Development of a dual extrusion printing technique for an acid- and thermo-labile drug

ABSTRACT Over the last years fused deposition modeling has been increasingly considered as a game‐changing technique for the preparation of individualized pharmaceutical products. Until now investigations have mainly focused on dosage forms loaded with very stable drugs or model substances. Going beyond this early stage of research, developers will also have to deal with more challenging active substances. In this work different printing designs for tablets containing the acid‐ and thermo‐labile drug pantoprazole sodium were tested. Initial dual extrusion printing of a cellulose acetate phthalate coat and a tablet core of polyethylene glycol 6000 with 10% (m/m) pantoprazole sodium resulted in thermal degradation of pantoprazole at cellulose acetate phthalate printing temperatures of 141 °C. Therefore, different tablet designs were developed. The sectioning of the design of the tablet coat in a gastro‐resistant cellulose acetate phthalate bottom part and an upper nearly insoluble polycaprolactone part printed at only 58 °C was suitable to prevent visible signs of thermal degradation. Dissolution testing indicated also no drug loss during dual extrusion printing. However, printed enteric tablets with shell thicknesses of 0.4 to 0.5 mm were not completely gastro‐resistant. Drug release at intestinal pH values was delayed compared to uncoated cores. In conclusion, 3D‐printing of gastro‐resistant tablets containing thermo‐ and acid‐labile drugs seems in principle possible. However, it remains an unsolved challenge to meet United States Pharmacopeia requirements. Graphical abstract Figure. No Caption available.