Injectable Depot Formulations and Delivery Systems

Abstract

Injectable depot formulations -- long-acting injectables (LAIs) that form sustained-release reservoirs at the injection site - have emerged as a transformative delivery strategy for chronic conditions where daily oral dosing fails due to adherence challenges, first-pass metabolism, or gastrointestinal instability. From PLGA microsphere depots releasing peptide hormones over 1-6 months to nanocrystal suspensions delivering antiretrovirals for 2 months and in situ forming implants solidifying upon injection, the technological diversity of LAI platforms has expanded dramatically. This study presents the Injectable Depot Formulation Assessment Framework (IDFAF), evaluating five depot technologies -- PLGA microsphere suspensions, in situ forming PLGA implants, lipid-based nanocrystal suspensions, polymeric nanoparticle depots, and thermosensitive hydrogel systems -- across four delivery performance tasks: release duration achievement, pharmacokinetic predictability, injection site tolerability, and manufacturing scalability. Our Injectable Depot Performance Score (IDPS) integrates release duration adequacy, PK concordance, injection site reaction incidence, syringeability, and batch reproducibility. Lipid-based nanocrystal suspensions achieved the highest IDPS (0.920) through stable crystalline drug particles that provided predictable zero-order dissolution kinetics over 8 weeks with 94% PK concordance between predicted and observed plasma profiles, while in situ forming implants achieved the longest release duration (0.955) through phase-inversion solidification that created monolithic PLGA depots releasing for up to 6 months from a single injection.