Stability of the Cobalamin Moiety in Buffered Aqueous Solutions of Hydroxocobalamin

Abstract

The degradation of hydroxocobalamin has been studied kinetically in various buffer systems. Degradation was first order with respect to the substrate in all cases. The most stable solution studied was composed of a pH 4.3, 0.05 M acetate buffer made isotonic with sodium chloride. The apparent heat of activation has been found in the above system to be 26.95 Kcal./mole. Extrapolations to 25 and 30° show hydroxocobalamin to be sufficiently stable to permit formulation of injectable solutions under practical conditions. Even when the data are constrained to give a poor stability picture at 25 and 30°, the predicted stability remains excellent. At worst, a solution of hydroxocobalamin in pH 4.3 acetate buffer will retain at least 90 per cent of claimed cobalamin at 30° for 170 weeks if a modest 20 per cent overage is included in the solution. The poorer stability in other buffer systems indicates the degradative reaction(s) to be subject to general base catalysis. The influence of some amines on the reaction rate tend to support this inference. The degradation of hydroxocobalamin has been studied kinetically in various buffer systems. Degradation was first order with respect to the substrate in all cases. The most stable solution studied was composed of a pH 4.3, 0.05 M acetate buffer made isotonic with sodium chloride. The apparent heat of activation has been found in the above system to be 26.95 Kcal./mole. Extrapolations to 25 and 30° show hydroxocobalamin to be sufficiently stable to permit formulation of injectable solutions under practical conditions. Even when the data are constrained to give a poor stability picture at 25 and 30°, the predicted stability remains excellent. At worst, a solution of hydroxocobalamin in pH 4.3 acetate buffer will retain at least 90 per cent of claimed cobalamin at 30° for 170 weeks if a modest 20 per cent overage is included in the solution. The poorer stability in other buffer systems indicates the degradative reaction(s) to be subject to general base catalysis. The influence of some amines on the reaction rate tend to support this inference.