The number and the wavelength of incident photons affect the photodegradation rate of drugs. It is not easy to study the effect of light quantitatively because the wavelength dependence of degradation varies among drug substances and because light sources have different spectral distributions. In many instances, only qualitative data on the photodegradation of drugs have been reported.
As shown in Fig. 88, the amount of photodegraded nifedipine was proportional to the number of incident photons.403 Maximum photodegradation of nifedipine in tablets occurred at 420 nm (Fig. 89).404 On the other hand, the relationship between the discoloration rate of sulfisomidine in tablets irradiated by a mercury lamp versus ultraviolet light intensity was complex.405 The values of L, a, and b determined for the discoloration of the tablet depended on the energy of the mercury lamp.406 Photodegradation of menatetrenon yielded linear plots of log k versus the reciprocal of the illumination intensity, as shown in Fig. 90.407
Photodegradation of drug substances strongly depends on the spectral properties of the drug substance and the spectral distribution of the light source. Discoloration of sulpyrine is significant in the presence of a mercury lamp, which is a good source of UV energy; however, little discoloration occurs in the presence of a fluorescent lamp, which radiates mainly visible light.408
300 400 500
300 400 500
Figure 89. Wavelength dependence of photodegradation of nifedipine tablets at 15°C. Light intensity: 1.23 x 108 erg/cm2. (Reproduced from Ref. 404 with permission.)
1/illumination intensity (xlO"3 Ix"1)
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