Development of an injectable two-phase drug delivery system for sequential release of antiresorptive and osteogenic drugs

Unlike controlled release systems that deliver a single drug, dual or multidrug delivery systems with distinct release profiles are more likely to promote timely and effective tissue regeneration as they provide both temporally and concentration‐dependent release of different molecules to mimic natural biological events. In this study, an injectable and biodegradable delivery system was developed to sequentially release an antiresorptive drug (clodronate) followed by an osteogenic agent (simvastatin) to treat bone disease. The injectable delivery system comprised simvastatin‐loaded gelatin microspheres suspended in a viscous solution of carboxymethylcellulose (CMC) containing clodronate. Several factors (CMC concentration, glutaraldehyde concentration, simvastatin loading, and gelatin microsphere processing conditions) were investigated for their effects on drug release. Clodronate release was not affected by CMC concentration, with complete delivery within 12 hr, and simvastatin release could be modulated by cross‐linking of the gelatin microspheres, loading, and washing conditions. Burst release of simvastatin was reduced from 70% to 6% in conjunction with sustained release for up to 3 weeks. The combined system showed early release of the antiresorptive clodronate sequentially followed by sustained delivery of the osteogenic simvastatin. This robust and flexible two‐phase delivery system may prove useful for applications in which multiple drug delivery is desired. © 2011 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 2012.