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Hydrolysis and epimerization kinetics of pilocarpine in aqueous solution.

The kinetics of the hydroxide-ion-catalyzed hydrolysis and epimerization of pilocarpine in aqueous solution were investigated utilizing pH-stat titrimetry and NMR spectroscopy. The mechanism of epimerization involves formation of a carbanion stabilized by resonance with the enolate hybrid. Both hydrolysis and epimerization follow pseudo-first-order kinetics, and the appropriate rate constants and energies of activation were calculated. Epimerization was found to occur to a greater extent than previously assumed and must be considered as a major pathway of degradation and inactivation of pilocarpine. The rate of hydroxide-ion-catalyzed epimerization increases more rapidly with temperature than does the rate of hydrolysis, a fact that should be considered if ophthalmic solutions of pilocarpine are sterilized by heat. It is suggested that isopilocarpine may not be a genuine jaborandi alkaloid but an artifact produced by epimerization of pilocarpine during drying, storage, and extraction of the plant material.