DEGRADATION RATES OF CAPTOPRIL IN AQUEOUS-MEDIUM THROUGH BUFFER-CATALYSIS OXIDATION

Initial degradation rates of captopril (0.1 mg/mL) in acetate, citric, and phosphate buffer solutions with different buffer concentrations a t 80 degrees C (mu=0.5) were studied at pH 6.0. All degradation reacti ons of captopril solutions fitted an apparent first-order plot. The de gradation rates of captopril rose with increasing buffer concentration s. A mechanism involving the buffer-catalysis oxidation reaction of ca ptopril was proposed in this study. The low apparent first-order degra dation rates of captopril in citric buffer solutions might have been d ue to the chelating effect of the citric buffer, which reduced the met al-catalysis oxidation reaction of captopril. Therefore, using low con centrations of citric buffer to improve solution stability seems to be an appropriate approach in a liquid formulation development of captop ril. To select citric buffer at a low concentration for the dissolutio n medium might be the right choice for a sustained-release formulation dissolution study of captopril.