Evaluation of Photostability of Drugs and Design for Photostabilization of the Drugs

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(1)Effect of the particle size on the photostability of nifedipine powder and its tablet was investingated using HPLC method and Fourier-transformed infrared reflection-absorption spectroscopy (FT-IR-RAS) under the non-destructive condition.The nifedipine content on the surface of the tablet was determined based on the absorption at 1682 cm^<-1> attributable to the C=O stretch vibration in FT-IR-RAS spectra before and after irradiation by fluorescent lamp. The apparent photodegradation constant of nifedipine powder increased with decrease of the particle size, while that of its tablet was approximately constant irrespective of particle size.(2)The photostability of tretinoin tocoferil was investigated under irradiation with three kinds of lamps, i.e., a cool white fluorescent lamp, a UV-A fluorescent lamp and a D65 fluorescent lamp. A combination of the cool white fluorescent lamp and the UV-A fluorescent lamp, and the D65 lamp having relative spectral power distribution similar to that of direct daylight, correspond to options 2 and 1 in ICH Guidelines, respectively. The photodegradation apparently followed second-order kinetics under these light sources and the degradation rate constant under exposure by the D65 lamp was larger than that by the cool white fluorescent lamp. The drug was susceptible to degradation by visible light below 480 nm and was degraded most remarkably at around 420 nm, showing a wavelength- dependency.The rate of oxidative degradation was slightly accelerated with the rise of temperature. The degradation rate constant rapidly increased in proportion to partial pressure of oxygen below 20kPa.