Drug Stability in Implantable Reservoir Systems

Abstract

Implantable reservoir drug delivery systems -- intrathecal pumps, refillable subcutaneous ports, osmotic implants, and MEMS-based micro-reservoir arrays -- maintain drug formulations at body temperature (37 degrees C) for weeks to months, creating stability challenges far exceeding those of conventional pharmaceutical storage at controlled room temperature. Drug degradation within implanted reservoirs follows accelerated kinetics governed by the Arrhenius relationship, with hydrolysis, oxidation, aggregation, and adsorption to reservoir materials progressing at rates 4-16 fold higher than at 25 degrees C storage conditions. This study presents the Implantable Reservoir Drug Stability Assessment Framework (IRDSAF), evaluating five stability enhancement strategies -- formulation pH and buffer optimisation, antioxidant and chelator stabilisation systems, reservoir surface passivation coatings, lyophilised in-reservoir reconstitution, and AI-predictive stability modelling with formulation optimisation -- across four drug categories: small molecule analgesics, peptide hormones, monoclonal antibodies, and chemotherapeutic agents. Our Reservoir Stability Performance Score (RSPS) integrates potency retention, degradant control, adsorption prevention, sterility maintenance, and shelf-life extension. AI-predictive stability modelling achieved the highest RSPS (0.922) through machine learning models trained on 8,400 accelerated stability profiles that predicted optimal formulation parameters achieving 95% potency retention at 37 degrees C for 90 days, while lyophilised reconstitution achieved the highest absolute stability (0.958) by eliminating aqueous degradation pathways entirely until the point of use.