Background Smooth muscle cell proliferation plays a major role in the genesis of restenosis after angioplasty or vascular injury.Local application of agents capable of modulating vascular responses, including smooth muscle cell proliferation, has been achieved, but difficulty in maintaining active levels locally has been a factor limiting the efficacy of such approaches. One strategy to maintain adequate levels is the local delivery of microspheres that release active agents over sustained time periods.Methods and Results We incorporated a colchicine analogue into biodegradable microspheres composed of a lactic acid/glycolic acid copolymer and characterized their drug release behavior as well as their effects on bovine aortic smooth muscle cells (BASMCs) in culture.Drug release was evaluated by spectrophotometric assay. Drug effects on DNA synthesis were measured by thymidine incorporation after addition of serum to subconfluent cells synchronized by serum withdrawal as well as in asynchronous cell populations. Polymeric microspheres incorporating 10% to 17% drug by weight and averaging 6 microns in size were found to release the colchicine analog in buffered saline solutions over more than several weeks. Drug-loaded particles inhibited DNA synthesis completely, with EC50values ranging from 0.001 to 0.005 g% (wt/wt). Morphological changes suggesting microtubule depolymerization were observed after drug particle treatment, with similar EC50values. Microspheres allowed to contact the cell surface demonstrated effects similar to those seen with microspheres suspended in the nutrient medium by porous polycarbonate filters, at EC50values approximately fivefold lower. In contrast, control microspheres composed only of polymer with no incorporated active drug demonstrated no observable toxicity to BASMCs and <40% inhibition of thymidine incorporation even in suspensions containing up to 0.5 g% particles.Conclusions Biodegradable microspheres were fashioned that release a colchicine analogue and inhibit DNA synthesis in smooth muscle cells.Drug-loaded polymeric particles are candidates for local delivery at sites of arterial injury to decrease restenosis. (Circulation. 1994;89:1929-1933.)