Comparative Stability Evaluation of 4-Phenylbutyric Acid-Loaded Nanocarrier Formulations for Topical Arsenical Medical Countermeasure Delivery

Background/Objective: Effective topical delivery of 4-phenylbutyric acid (4-PBA) for arsenical vesicant-induced skin injury requires nanocarrier systems that maintain physicochemical and chemical stability during extended storage. This study systematically evaluated the six-month stability of five 4-PBA-loaded micro/nanoparticulate formulations—chitosan nanoparticles (N31, N35), emulsomes (E2), microsponges (MSs), and PLGA nanoparticles (P1)—to identify lead candidates suitable for field deployment and foam integration. Methods: Formulations were subjected to ICH-accelerated stability testing at 25 °C/60% RH and 40 °C/75% RH, with monthly evaluation of particle size, polydispersity index, zeta potential, drug content by HPLC, and chemical/thermal stability by FTIR and DSC. Results: N31 demonstrated superior colloidal stability, maintaining particle size within acceptable limits at both conditions despite progressive surface charge neutralization. E2 showed consistent drug content retention and preserved chemical integrity, though moderate vesicle fusion occurred. MS underwent complete physical degradation at 40 °C within the first month, while P1 exhibited hydrolytic degradation with substantial drug loss. N35 showed severe aggregation indicating colloidal instability. Conclusions: N31 and E2 emerged as lead candidates: N31 is recommended for field deployment where environmental control is limited, while E2 is suitable for controlled storage settings prioritizing drug loading capacity.