A new approach to evaluating photo-stability of nifedipine and its derivatives in solution by actinometry.

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Abstract
The photo-stability of nifedipine and its 4- or 1,4-substituted derivatives in methanol was studied. A ferrioxalate actinometer was used to measure cumulative number of photons as an index of light intensity. Under irradiation with a high-pressure mercury lamp, the photo-stability decreased in the order: 1-hydroxy-4-(m-nitrophenyl) compound, 4-(p-nitrophenyl)compound, 4-(m-nitrophenyl) compound, 1-methyl-4-(o-nitrophenyl) compound and nifedipine itself. The 4-phenyl compound and 4-(o-chlorophenyl) compound did not decompose in the range to 4 .times. 1021 quanta. Further, the photo-stability of nifedipine was quantitatively examined under sunlight, a fluorescent lamp and a high-pressure mercury lamp. The slopes of the linear plots of residual percent against cumulative number of photons irradiated through a UV-D33S colored glass filter were essentially the same for all three light sources. On the other hand, the slopes did not agree in the case of direct exposure without a filter. A good correlation between extent of photo-degradation and integrated illumination (lux .times. h) was not observed with or without the filter. By means of this actinometry with a UV-D33S colored glass filter, the 1-hydroxy-4-(m-nitrophenyl) compound and 4-(m-nitrophenyl) compound were quantitatively estimated to more photo-stable than nifedipine by factors of 270 and 76, respectively, under a given light source.

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