Evaluation of the in vitro drug release from resorbable biocompatible coatings for vascular stents

The objective of this work was to prepare a biocompatible and degradable polymer coating for vascular stents that would release a restenosis inhibitor for a sustained period of time. To do so, two film processing techniques were compared using poly(lactic acid) (PLA) and poly(lactic-co-glycolic acid) (PLAGA) polymers as well as the effect of the polymer composition, film thickness and drug loading on the in vitro release of dexamethasone from a stent model. Stainless steel square plates (SSSP) were either dip- or spray-coated with different polymer proportions of PLA of two different molecular weights (25,000 and 120,000) and PLAGA with a molecular weight of 48,000 with specific drug content and film thickness. The release study was carried out over four months by placing the coated SSSP in a phosphate buffer solution (PBS) pH 7.4. The surface topography of the coated stents was investigated throughout the study by atomic force microscopy (AFM) and differential scanning calorimetry (DSC). Analyses were performed on samples before and after the release study. Dip coating produced a much slower release profile than the spray-coated films. None of the dip-coated SSSP samples achieved a similar to50% release in 100 days while most of the spray-coated films released 100% of their load in similar to120 days. The polymer composition affected the release profile with the dip-coated film with blended homopolymer films releasing more drug than films composed entirely of one homopolymer; however, this difference was not seen with the spray coated films. Drug loading and film thickness did not show a difference in drug release with the spray-coated films. DSC and AFM results showed progression, with time, in the polymer film degradation and erosion. This study demonstrated that it is feasible to coat a stent model with a biocompatible material and that this coating can release a restenosis inhibitor over time. The release from the biocompatible matrix can be modified by using different processing techniques and the release can be engineered to a desired release profile by blending different molecular weight homopolymers in dip-coated films. On the other hand, spray coating with different molecular weight fractions of the same homopolymer did not produce significant release profiles.