Alendronate-loaded, biodegradable smart hydrogel: a promising injectable depot formulation for osteoporosis

Alendronate (ALN) is a BCS III bone resorption inhibitor, with very poor oral bioavailability. Our approach is to develop a minimally invasive thermogelling system for prolonged local delivery of ALN. For this, different chitosan-based thermogels were developed and characterised in terms of gelation time, injectability, pH, viscosity and thermoreversibility. Chitosan/beta-glycerophosphate (CS/(beta GP) hydrogel pursued temperature-dependent, thermoreversible gelation behaviour and was thus selected for drug loading. Increasing ALN concentration resulted in hydrogels with lower porosity and higher density. FTIR and DSC proved interaction between ALN, CS with beta GP. CS/beta GP hydrogel ensured controlled ALN release over 45-65 days depending on initial ALN loading. Freeze drying improved the shelf-life stability with minor impact on thermogelling character. In vivo injection of plain and ALN-loaded hydrogel in rats rapidly gelled 15 min post-injection. Based on histological examination, ALN-loaded thermogel showed less inflammatory response, faster proliferation and maturation of granulation tissue relative to plain thermogel. Hydrogels excised 21-days post-injection proved the biocompatibility and biodegradability of the system. The presented chitosan-based thermogel has significant positive attributes for site-specific, time-controlled, intra-articular delivery of ALN.