Photostability is the process of exposing product to lighting conditions that simulate real-world environments. This can include direct sunlight, sunlight through a window, or simulated office lighting. The purpose of this testing is to demonstrate that unacceptable change to the drug formulation has not occurred. The testing can also be used to examine the lightfastness of inks used in packaging, as well as help provide a measure of the photodegredation of cosmetics.
The importance of this stability study can be seen in the way products are stored. For example, pills are often kept in transparent or semi-transparent containers and these containers are kept on a window sill, or otherwise in sunlight. Photostability can be used to assess the length of time these products can be exposed to direct or indirect sunlight and still be safe and effective. Cosmetics are kept in clear containers under bright lights in order to advertise the slightest variation of hues; and, once they’re purchased, they’re worn out under the harsh sunlight. Photostability can demonstrate how quickly these products fade under a range of lighting conditions.
For pharmaceuticals, the primary guidance comes from ICH Q1B. ICH requires multiple levels of test and check. From raw product to immediate pack to marketing pack, each level requires a check for an acceptable amount of change and makes recommendations for formulation or packaging change. The goal of photostability testing is to find a light-stable formulation. For cosmetics, the primary guidance on stability studies comes from the Colipa Guidelines which provides a method for predicting shelf-life of cosmetic products.
- ICH Q1B – Stability Testing: Photostability Testing of New Drug Substances and Products
- Colipa Guidelines – Guidelines on Stability Testing of Cosmetic Products
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