Formulation Intervention to Overcome Decreased Kinetic Solubility of a Low T_g Amorphous Drug

Abstract

This technical note investigated the loss of dissolution rate during accelerated stability studies with a dry blend capsule formulation containing an amorphous salt of drug NVS-1 (T_g 76°C). After 6 m at 40°C/75%RH, dissolution of NVS-1 was ≤40% of initial value. Scanning electron microscope characterization of the undissolved capsule contents from samples stored at 50°C/75%RH for 3 weeks showed agglomeration with a distinct “melt and fuse” morphology of particles. At elevated temperature and humidity conditions, undesired sintering among the amorphous drug particles was observed. Humidity plasticizes the drug as the stability temperature (T) gets closer to the glass transition temperature (T_g) of the amorphous salt (i.e., smaller T_g-T); a decreased viscosity favors viscoplastic deformation and sintering of drug particles. When moisture is adsorbed onto agglomerated drug particles, partial dissolution of the drug forms a viscous surface layer, further reducing the rate of dissolution media penetration into the bulk solid, hence the slower dissolution rate. Formulation intervention focused on the use of L-HPC and fumed silica as disintegrant and glidant and the removal of the hygroscopic crospovidone. Reformulation improved dissolution performance at short-term accelerated stability conditions of 50°C (± 75%RH); however, sintering to a lesser extent was still observed at high humidity, impacting the dissolution rate. We infer reducing the impact of moisture at high humidity conditions in a formulation with a 34% drug load is challenging. Future formulation efforts will focus on the addition of water scavengers, reducing drug load by ~50% to physically separate drug particles by water-insoluble excipients, and optimizing disintegrant levels.