Effect of processing methods on drug release profiles of anti-restenotic self-assembled monolayers

Abstract

The use of anti-restenotic self-assembled monolayers (ARSAMs) has been previously demonstrated for delivering drugs from stents without polymeric carriers. ARSAMs have been prepared by coating an anti-restenotic drug (paclitaxel – PAT) on –COOH terminated phosphonic acid self-assembled monolayers (SAMs) coated Co–Cr alloy specimens. This study investigates the effect of different processing methods on the percentage of drug release from ARSAMs. The different methods that were used in this study to process ARSAMs include room temperature (RT) treatment, heat treatment (HT), cold treatment (CT) and quenching. The changes in polymorphism, chemical structure, morphology, and distribution of PAT on SAMs coated specimens were investigated using differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and atomic force microscopy (AFM), respectively. DSC showed dihydrate, dehydrated dihydrate, semi-crystalline, and mixed (amorphous and dihydrate) forms of PAT for RT, HT, CT, and quenched specimens, respectively. FTIR showed that the chemical structure of PAT was unaltered in all the specimens processed by various methods employed in this study. SEM showed a mixture of spherical, ovoid, and bean-shaped morphologies of PAT on RT, HT, and CT while particle-like and needle-shaped morphologies of PAT were observed on quenched specimens. AFM showed PAT was uniformly distributed on RT, HT and CT specimens while particle-like PAT was well distributed and needle-shaped PAT was sparsely distributed on quenched specimens. CT specimens showed greater density of PAT crystals when compared to other methods. Thus, this study demonstrated that processing methods have significant influence on the polymorphism, morphology, and distribution of PAT on SAMs coated Co–Cr alloy specimens. The in vitro drug elution studies for up to 56 days showed sustained release for all the different groups of specimens. CT showed lesser percentage of drug release when compared to that of other methods on the first day. The treatment at high temperatures (HT-100°C or HT-140°C) improved the stability of PAT on alloy surfaces and showed lesser percentage of drug release when compared to that of RT or HT-70°C at different time points. Large data scatter was observed for the release profiles of quenched specimens. No other major differences in the percentage of drug release were observed for the specimens prepared by different processing methods. These factors should be taken into consideration when drug delivery platforms are developed for stents or other medical devices.