Photostability testing of drug substances and drug products in UK pharmaceutical laboratories.

Results of a survey on photostability testing of drug substances and products by UK pharmaceutical laboratories are presented. The results indicate diverse practices in the form of presentation of the product and particularly in exposure to light (as measured in lx days), although there was more consistency in analytical methods and in the analytical criteria used to classify products as stable or unstable for those laboratories adopting this type of classification. A majority of laboratories use daylight in addition to artificial light sources for tests on drug substances and products. Artificial daylight fluorescent tubes and xenon lamps are the most widely used sources of artificial light and both should provide a reasonable simulation of natural light. All laboratories intend their photostability tests to represent light exposure which exceeds that expected to occur in practice but the tests actually applied vary widely in severity as shown in the wide range in light exposure (8-4500 klx days). Therefore the classification of products as stable or unstable needs to be considered carefully in relation to the severity of the test used. Testing procedures for drug substances are broadly similar to those used for drug products. It is concluded that the variation in testing procedure is the result of differing perceptions regarding product exposure in practical usage and the absence of regulatory guidelines.