Drug Stability in Liposomal Suspensions: Hydrolysis of Indomethacin, Cyclocytidine, and p-Nitrophenyl Acetate

Abstract

First-order rate constants (kL) for hydrolysis of p-nitro-phenyl acetate, cationic cyclocytidine, and anionic indomethacin in the presence of buffered liposomal suspensions of positive, negative, and neutral charge were compared to those determined in the corresponding buffers (kB) using the ratio, Rk=kL/kB. Association between the reactants and the liposomes was evaluated by comparing assays for concentration in the filtrates (CF) with the total concentration in the liposomal suspension (CT) using RC=CF/CT. Liposomes did not influence cyclocytidine hydrolysis rates and no association was observed (Rk≃RC≃1). In contrast, indomethacin showed ~80% reduction in hydrolysis rate and ~80% liposome association value (Rk≃0.2≃RC). In neutral and negatively charged liposomal suspensions, p-nitrophenyl acetate displayed ~30% decrease in kB (Rk≃0.7) together with ~90% liposomal association (RC≃0.1). However, hydrolysis was greatly accelerated in positively charged liposomal suspensions. Loss was described by a biexponential equation where α is the fast and β is the slow pre-exponential coefficient and α/β/kB=39:6:1. The observed relationships between hydrolysis rates and reactant-liposome associations are reconciled in terms of the hydrophilicity of the reactants and the potential influence of the liposomes on the expected transition states for the hydrolysis reactions.