In-vitro Release of Rapamycin from a Thermosensitive Polymer for the Inhibition of Vascular Smooth Muscle Cell Proliferation

Hemodialysis arteriovenous grafts are often plagued by stenosis at the anastomosis, which is due to the proliferation of vascular smooth muscle cells (SMCs). To prevent the stenosis, we have been developing a strategy for the sustained perivascular delivery of an antiproliferative agent, rapamycin, using an injectable biodegradable polymer, ReGel(®). In this study we examined the in-vitro kinetics of rapamycin released from ReGel and its efficacy for inhibiting the proliferation of human and porcine venous and arterial SMCs. To study the release from ReGel, rapamycin was mixed with ReGel and incubated at 37°C in a release medium. The release medium was periodically sampled and assayed for rapamycin concentration by UV. Cellular uptake and release of rapamycin were examined by incubating SMCs with rapamycin for various durations. Intracellular drug was extracted and measured by HPLC. Antiproliferative effects and cytotoxicity of stock rapamycin and that released from ReGel were examined using cell counting and lactate dehydrogenase (LDH)-release assay, respectively. Rapamycin exhibited a sustained-release pattern from ReGel for 52 days. The kinetics of rapamycin transport through the cell membrane was compatible with a passive diffusion mechanism. Rapamycin released from ReGel exhibited antiproliferative activity similar to the free drug. Our results support the concept of sustained delivery of rapamycin using ReGel as a promising strategy to inhibit SMC proliferation for the prevention of hemodialysis arteriovenous graft stenosis.