[The stability of pilocarpine hydrochloride in eyedrops]

This study investigate the kinetics of the degradation of pilocarpine in aqueous solutions within the therapeutically relevant pH-value range from 2.9 to 6.5. A linear system of first order equations for the kinetics of complex pilocarpine degradation can be derived from the concentration course of decomposed products determined by HPLC. The kinetic parameters calculated from the results of isothermic short-term tests permit the prediction of a five-year durability of non-buffered pilocarpine eye drops, if their pH-value is in the maximum stability range of less than or equal to 4. Comparison of non-stereoselective photometric results with the hplc results shows the inappropriateness of the results of conventional spectrophotometric analysis in the presence of epimeric decomposition products for the interpretation of kinetic reaction and, accordingly, for evaluating stability. Contrary to previous studies, the results indicate a reversibility of epimerization in temperature-stressed isopilocarpine solutions.