Quantitative analysis of degradation products in pilocarpine hydrochloride ophthalmic formulations.

The presence of isopilocarpine, an epimer of pilocarpine, and of pilocarpinic acid, a hydrolytic degradation product of pilocarpine, was established and all three substances were assayed in various commercial ophthalmic formulations of pilocarpine hydrochloride by 13C-Fourier transform spectroscopy. Assay was based upon integrated intensities of selected resonances from any formulation calibrated against the intensity of tetramethylammonium bromide, used as a common external reference. The normalized intensities were then related to those of a reference solution of pilocarpine hydrochloride, thereby eliminating any factor arising from variability of 13C-relaxation times. The 13C-resonance for the N-methyl group, being common to all products, provides a convenient basis for the assay of the total alkaloid content whereas the C-8 resonances are best suited for assaying residual pilocarpine and its degradation products. This procedure, estimated as accurate to +/- 5%, constitutes the first comprehensive analytical method to differentiate between pilocarpine and its degradation products.