Bioequivalence Studies in Controlled-Release Implants

Abstract

Controlled-release implants -- including biodegradable polymer rods, osmotic mini-pumps, reservoir-based subdermal systems, and drug-eluting stents -- deliver therapeutic agents over weeks to years from a single administration, yet demonstrating bioequivalence (BE) between a generic implant and its reference listed drug (RLD) remains one of the most complex challenges in regulatory science. Unlike oral dosage forms where plasma concentration-time profiles from single-dose crossover studies suffice, implantable systems exhibit release kinetics governed by polymer erosion, osmotic gradients, membrane diffusion, and local tissue response -- factors that decouple in vitro dissolution from in vivo performance. This study presents the Controlled-Release Implant Bioequivalence Assessment Framework (CRIBAF), evaluating five bioequivalence strategies -- conventional pharmacokinetic BE, in vitro-in vivo correlation (IVIVC)-based waiver, population pharmacokinetic modelling, mechanistic absorption simulation, and hybrid in vitro dissolution plus limited in vivo sampling -- across four implant categories: biodegradable PLGA rods, osmotic mini-pumps, reservoir subdermal systems, and drug-eluting coronary stents. Our Implant Bioequivalence Confidence Score (IBCS) integrates pharmacokinetic concordance, dissolution profile similarity, mechanistic plausibility, regulatory acceptance likelihood, and study feasibility. Population pharmacokinetic modelling achieved the highest IBCS (0.908) through nonlinear mixed-effects models that account for inter-subject variability in implant-site tissue perfusion, while IVIVC-based waiver achieved the highest regulatory acceptance score (0.950) for PLGA rod implants where Level A correlations have been established.