Stability of Pilocarpine Ophthalmic Formulations

The stability of various dosage forms of pilocarpine ophthalmic formulations available on the Japanese market was studied. For opthalmic solutions, hydrolysis was found to be a main route of degradation, and the stability depended largely on the pH and the concentration of general bases. Nonlinear regression analysis allowed estimation of the rate constants of degradation, including reversible hydrolysis to pilocarpic acid and reversible epimerization to isopilocarpine. It was also found that an ointment formulation was stable, while a conjunctival insert preparation was susceptible to epimerization and hydrolysis. The factors affecting the stability of pilocarpine preparations are discussed.