Some factors affecting the photodecomposition of nifedipine

Nifedipine, like most nitrophenyldihydropyridine derivatives, undergoes rapid photochemical oxidation to the corresponding nitrophenylpyridine when exposed to light. This is accompanied by a remarkable diminution of the pharmacological activity. In this report, the stability of the drug in different organic solvents and in the presence of an antioxidant was investigated. The UV-spectral properties of the oxidized and reduced forms of the drug were similar in acetonitrile, ethanol and chloroform. In cyclohexane the oxidized form showed absorption characteristics different from those in other solvents. Kinetic parameters for the disappearance of the reduced form or the appearance of the oxidized form as a function of bisulfite concentration were determined and the half-lives calculated. The degradation of two structural analogues of nifedipine was also studied. The results have shown that an electron donating substituent in the position of the nitro group imparts high light resistance to this group of substances.