Implant dynamics, inner structure, and their impact on drug release of in situ forming implants uncovered through CT imaging

Abstract

In situ forming implants (ISFIs) composed of biodegradable polymers and biocompatible solvents are generally designed for sustained drug release. In this study, a non-invasive computed tomography (CT) imaging approach is used to achieve real time imaging of ISFIs in vivo and in vitro using leuprolide acetate in situ forming implant as a model drug product. The process of implant formation, inner structure change and their impact on drug release were elucidated. Real-time drug distribution was unveiled by the CT contrast agent, iohexol, where it shows a core-shell structure of the deposition. The incorporation of leuprolide acetate (LA) led to a reduced extent of burst release, prolongated release profile, and extended implant size expansion. LA was found to interact with the solvent and slowed down the polymer phase inversion, thus significantly changed the drug distribution in the implant and reduced the drug release. The implant inner structure identified through SEM, implant size change, and polymer degradation along with the CT real time imaging all consistently support the implant formation differences and their implant on the drug release. Similar patterns of implant size expansion and iohexol distribution in the implants were observed both in vitro and in vivo for the implants with and without LA. The comprehensive understanding of the impact of implant formation on drug release through real time CT imaging facilitates the ISFI product development and evaluation.