Modelling drug release from polymer-free coronary stents with microporous surfaces

Controlling release from encapsulated drug-loaded devices: insights from modeling the dissolution front propagation

Dissolution of drug from its solid form to a dissolved form is an important consideration in the design and optimization of drug delivery devices, particularly owing to the abundance of emerging compounds that are extremely poorly soluble. When the solid dosage form is encapsulated, for example by the porous walls of an implant, the impact of the encapsulant drug transport properties is a further confounding issue. In such a case, dissolution and diffusion work in tandem to control the release of drug. However, the interplay between these two competing processes in the context of drug delivery is not as well understood as it is for other mass transfer problems, particularly for practical controlled-release considerations such as an encapsulant layer around the drug delivery device. To address this gap, this work presents a mathematical model that describes controlled release from a drug-loaded device surrounded by a passive porous layer. A solution for the drug concentration distribution is derived using the method of eigenfunction expansion. The model is able to track the dissolution front propagation, and predict the drug release curve during the dissolution process. The utility of the model is demonstrated through comparison against experimental data representing drug release from a cylindrical drug-loaded orthopedic fixation pin, where the model is shown to capture the data very well. Analysis presented here reveals how the various geometrical and physicochemical parameters influence drug dissolution and, ultimately, the drug release profile. It is found that the non-dimensional initial concentration plays a key role in determining whether the problem is diffusion-limited or dissolution-limited, whereas the nature of the problem is largely independent of other parameters including diffusion coefficient and encapsulant thickness. We expect the model will prove to be a useful tool for those designing encapsulated drug delivery devices, in terms of optimizing the design of the device to achieve a desired drug release profile.

Microporous surface as a new solution for stent surface modification: A review

The introduction of coronary stents into clinical practice has reduced repeated patient visits compared with balloon angioplasty alone. Also, drug-eluting stents substantially reduced the restenosis incidence. Therefore, later complications related to the implantation of a stent coated with a cytostatic-containing polymer became more relevant. The mechanism of late stent complications is multifactorial. It is mainly due to the body's response to the prolonged indwelling of the drug carrier polymer on the coronary stent's surface. There is a trend towards the return of polymer-free drug coating technologies, which are implemented through certain modifications of stent surfaces for better drug retention and proper drug distribution. It is mainly achieved using drug depots in various reservoirs: grooves, nanoparticles in the matrix compound, micropores, through and blind micro reservoirs, etc. New promising technologies for crystallizing cytostatic drugs or depositing them in specially designed reservoirs show good preclinical and clinical results, comparable or even superior to approved coronary stents. Micropores as carriers for antiproliferative agents on the stent surface are a promising direction to rejecting the use of polymers in stents.

Investigation of the drug release time from the biodegrading coating of an everolimus eluting stent

This case-study examines the release time of the everolimus drug from an experimental biodegrading coating of a Rontis corp. drug eluting stent (DES). The controlled drug release is achieved by the degradation of the coating, which consists of a mixture of polylactic co-glycolic acid (PLGA) and everolimus (55:45). In our analysis, we used the outcome of another study, which contains the geometry of an in-silico deployed Rontis corp. stent in a 3D reconstructed coney arterial segment. Using this geometry as input, the everolimus release was simulated using a computational model that includes: i) modeling of the blood flow dynamics, ii) modeling of PLGA degradation, and iii) modeling of the everolimus advection and diffusion towards both the lumen and the arterial wall. The results show the rapid release of everolimus. This is justified due to the high porosity of the coating, which is caused by the initial high concentration of everolimus in the coating.Clinical Relevance - The methodology presented in this work is an additional step towards predicting accurately drug release from DES. Also, the results of our work prove that high drug concentration in the coating causes its rapid release, which could be used as input in the design of new DES.

Numerical analysis of paclitaxel-eluting coronary stents: Mechanics and drug release properties

Since theoretical models provide data that cannot be otherwise gathered, numerical methods applied to medical devices analysis have emerged as fundamental tool in preclinical development. Large efforts were done to study mechanical and drug-eluting properties in stents but often the coating modelling is neglected. This work presents a finite element framework to calculate mechanical loads and drug distribution in three commercial drug-eluting stents (Palmaz-Schatz, Palmaz Genesis and Multi Link Vision), to check coatings strength and drug distribution maps in biological tissues. The promising copolymer poly(methylmethacrylate-co-n-butylmethacrylate), loaded with paclitaxel, is analyzed. Results demonstrated that the coating undergoes localized plastic phenomena, and calculated stresses are lower than the ultimate stress, ensuring coating integrity. Computed drug concentration depends on stent geometry and its values are in all cases lower than the toxicity level for this drug.

Recent Advances in Manufacturing Innovative Stents

Cardiovascular diseases are the most distributed cause of death worldwide. Stenting of arteries as a percutaneous transluminal angioplasty procedure became a promising minimally invasive therapy based on re-opening narrowed arteries by stent insertion. In order to improve and optimize this method, many research groups are focusing on designing new or improving existent stents. Since the beginning of the stent development in 1986, starting with bare-metal stents (BMS), these devices have been continuously enhanced by applying new materials, developing stent coatings based on inorganic and organic compounds including drugs, nanoparticles or biological components such as genes and cells, as well as adapting stent designs with different fabrication technologies. Drug eluting stents (DES) have been developed to overcome the main shortcomings of BMS or coated stents. Coatings are mainly applied to control biocompatibility, degradation rate, protein adsorption, and allow adequate endothelialization in order to ensure better clinical outcome of BMS, reducing restenosis and thrombosis. As coating materials (i) organic polymers: polyurethanes, poly(ε-caprolactone), styrene-b-isobutylene-b-styrene, polyhydroxybutyrates, poly(lactide-co-glycolide), and phosphoryl choline; (ii) biological components: vascular endothelial growth factor (VEGF) and anti-CD34 antibody and (iii) inorganic coatings: noble metals, wide class of oxides, nitrides, silicide and carbide, hydroxyapatite, diamond-like carbon, and others are used. DES were developed to reduce the tissue hyperplasia and in-stent restenosis utilizing antiproliferative substances like paclitaxel, limus (siro-, zotaro-, evero-, bio-, amphi-, tacro-limus), ABT-578, tyrphostin AGL-2043, genes, etc. The innovative solutions aim at overcoming the main limitations of the stent technology, such as in-stent restenosis and stent thrombosis, while maintaining the prime requirements on biocompatibility, biodegradability, and mechanical behavior. This paper provides an overview of the existing stent types, their functionality, materials, and manufacturing conditions demonstrating the still huge potential for the development of promising stent solutions.