Determination of diphenoxylate hydrochloride and atropine sulfate in combination drug formulations by liquid chromatography.

An isocratic, reversed-phase liquid chromatographic (LC) method was developed for simultaneous determination of diphenoxylate hydrochloride (I) and atropine sulfate (II) in pharmaceutical products. Analysis is conducted on a Spherisorb CN column (5 microns), with a mobile phase consisting of 47% 0.001M pentanesulfonic acid sodium salt monohydrate, 53% acetonitrile, and 0.1% phosphoric acid. The detection wavelength is 220 nm. Tablets are extracted with acetonitrile-water (50 + 50). Oral solutions are determined directly after dilution. The method is validated for linearity, precision, system reproducibility, and accuracy. Recoveries at 80-120% of label claim ranged from 98.7 to 101.0% and from 99.6 to 102.5% for I and II, respectively. Results were linear (r > 0.9999) in the ranges 125-375 micrograms/mL and 1.25-3.75 micrograms/mL for I and II, respectively. Assay and content uniformity results for each ingredient, in both innovator products and generic formulations, are reported. Diphenoxylic acid, the principle degradation product and major metabolite of I, is separated and can be determined in tablets and oral solutions.