Photochemical and oxidative degradation of the solid-state tretinoin tocoferil

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 and UV light below 480 nm and was degraded most remarkably at around 420 nm, showing a wavelength-dependency. The semi-logarithmic plots of apparent degradation rate constant against the reciprocal of illuminance showed a good linear relationship in the Arrhenius-type fashion, and the photostability under ordinary illumination conditions could be predicted from the data obtained under the accelerated illumination conditions. The rate of oxidative degradation was slightly accelerated with the rise of temperature. Thermodynamic parameter was calculated from the Arrhenius plot. The degradation rate constant rapidly increased in proportion to partial pressure of oxygen below 20 kPa.