ABSTRACT
INTRODUCTION Many pharmaceutical drug substances and drug products are photosensitive and will undergo photodegradation upon exposure to outdoor or indoor photon sources. The topic of photostability has been dealt with by the International Conference on Harmonization (ICH) as a special case of stability (ICH Topic Q1A), and guidelines for photostability testing are presented as an annex (ICH Topic Q1B) to the parent stability guideline. Photostability testing, however, presents many unique difficulties when compared with thermal and humidity testing outlined by ICH Q1A. Issues such as the choice of photon source, irradiation homogeneity, containers/sample holders, sample thickness, and orientation are just a few of the parameters that are critical to meaningful evaluation of the photostability of a drug substance or product. Furthermore, careful consideration should be given to the type of analytical testing performed as part of the control strategy for evaluation of photostability postexposure. Sample presentation, the manner in which the samples are prepared, packaged, and aligned relative to the photolysis source for photostability testing, can have a significant effect on the outcome of the photostability experiment. This chapter will provide some practical information about the critical parameters involved in photostability testing with respect to “presentation” of the sample to the photon source.
