Buprenorphine long acting injectables: clinical needs, pharmacodynamic and pharmacokinetic basis, and design challenges for solid preformed implants.

With annual overdose deaths in the United States over 1 million, medication-assisted treatment with buprenorphine (BUP) remains the first-line, gold-standard therapy for opioid use disorder (OUD). Because OUD is a chronic, relapsing condition that requires long-term pharmacotherapy, long acting injectables (LAI) and implantable formulations offer important advantages over daily formulations for maintenance treatment. By comparing transmucosal BUP and LAI formulations' systemic exposure profiles and μ-opioid receptor (MOR) occupancy, converging data demonstrate that higher and more sustained BUP exposure with low variability is required to fully suppress withdrawal, cravings, and illicit opioid use. These findings indicated that currently marketed formulations may not adequately address the clinical challenges associated in the fentanyl/polysubstance era. Accordingly, this rationale-based review proposes a mechanistic framework supporting the development of next-generation BUP-PLGA solid biodegradable implants to maintain a conservative therapeutic benchmark (e.g. Css ≥ 5 ng/mL) for extended durations (e.g. 3-6 months) with low variability (e.g. no large burst release, major lag phase or phase inversion). However, progress in implant development has been hindered by limited mechanistic understanding of drug release. In PLGA-BUP systems, poor IVIVC is largely driven by the low and pH-dependent solubility of BUP, which can make dissolution rate-limiting in vivo and interact with the evolving PLGA acidic microenvironment (acidification, porosity formation, and autocatalytic degradation). Future research should be prioritized to determine directly whether polymer erosion coincides with drug release in PLGA depots, or whether residual, poorly soluble BUP persists locally and releases under dissolution-limited kinetics. Clarifying these mechanisms is not only essential to fulfill the regulatory and translational expectations of the FDA and NIDA, but also to deepen mechanistic understanding and accelerate the rational development of LAI formulations for poorly soluble drug.