Photochemical stabilities of some dihydropyridine calcium-channel blockers in powdered pharmaceutical tablets.

Photostabilities of some dihydropyridine calcium-channel blockers in pulverized pharmaceutical tablets were studied. Powdered tablets including amlodipine, nifedipine, or nilvadipine were exposed to D65 daylight lamp radiation according to an ICH guideline (ICH Q1B). The photodegradation of pharmaceutical components and their degradation products were monitored by HPLC using a reversed phase column with UV detection; their peak components were identified using MS analysis. Photochemical reactions involved in the photodegradation of these pharmaceuticals include aromatization of the dihydropyridine moiety and conversion to nitroso group from the nitro group in benzene rings. Chemical stability studies of these drugs indicate that nifedipine is the most photosensitive. The rate constant of nifedipine is indicated as seven times higher than those of the other two drugs.