In Vitro and In Vivo Characterization of Novel Biodegradable Polymers for Application as Drug-Eluting Stent Coatings

We have used a series of in vitro and in vivo tests to assess the suitability of two new degradable polymers for application as coatings for drug-eluting stents. The first is a family of urethane-linked multi-block copolymers (MBCP) that comprise blocks of lactide, glycolide, epsilon-caprolactone and/or poly(ethylene glycol) chain-extended with 1,4-butanediisocyanate (SynBiosys polymers). The second is a family of maltodextrin (MD) modified with fatty acid sidechains to yield a hydrophobic polymer (Eureka() SOLO polymers). We coated stainless-steel stents with two representative urethane-linked MBCPs and one hydrophobic MD polymer alone or in combination with the anti-restenotic drug sirolimus. Urethane-linked MBCPs formed uniform coatings on the stent substrates, withstood crimping and expansion on balloon catheters, completely released sirolimus from the coating within 30 days, and degraded within 30-60 days in PBS. The hydrophobic MD polymer formed uniform coatings, exhibited somewhat slower release of sirolimus (approx. 85% within 30 days), degraded within 60 days in PBS when sirolimus was incorporated in the coating, but showed very slow degradation in the absence of drug. We implanted stents coated with urethane-linked MBCPs or hydrophobic MD polymers in a porcine coronary artery model and used histological analysis at 28- and 90-day end-points to assess the biological response to the materials. Measures of stenosis and inflammation for urethane-linked MBCP and hydrophobic MD polymer coatings were not statistically different from bare metal controls at 28 and 90 days, suggesting that the polymers show good vascular biocompatibility. Endothelialization was nearly complete at 28 days and complete at 90 days for all formulations. Urethane-linked MBCP polymer-only and drug-eluting coatings and hydrophobic MD drug-eluting coatings were nearly completely degraded within 90 days in vivo whereas roughly half of hydrophobic MD polymer-only coatings remained after 90 days. Taken together, our in vitro and in vivo results suggest that SynBiosys urethane-linked MBCP and Eureka SOLO hydrophobic MD polymer families possess the physical and chemical properties and vascular biocompatibility necessary for further investigation for use in the next generation of drug-eluting stents.