UV derivative spectrophotometric study of the photochemical degradation of nisoldipine

The photodecomposition of nisoldipine ((±)3-isobutyl-5-methyl-1,4-dihydro-2,6-dimethyl-4-(2-nitrophenyl)-pyridine-3,5-dicarboxylate), whereby its 4-(2-nitrosophenyl) pyridine analogue is obtained as the photolytic product, was investigated under daylight exposure by means of UV derivative spectrophotometry. The optimal instrumental parameters (120 nm/min scan speed; 2 nm slit width; Δ λ=10 nm and 5 s response time) for analogue derivative spectra were established for amplitudes 1 D 285 and 2 D 291 (measured to the baseline) of the nitroso analogue assay, as well as for 1 D 386 of the parent compound–nisoldipine assay. Using the first-order derivative spectrum, the minimum detectable amount of nitroso analogue in the presence of nisoldipine was equivalent to an impurity level of 5% and by the second-order derivative spectrum, the determination limit was equivalent to an impurity level of 2%. The degradation of nisoldipine followed within 30 days and the calculated maximal degradation rate was 1.6% per day for nisoldipine raw material, but significantly lower values of 0.19 and 0.15% per day were obtained for Nisoldin® tablets (10 and 5 mg, respectively).