Lux is not just lux: Method for determining the photostability of drug products

Photostability is a critical factor influencing the quality and safety of drug products. This study presents a novel method to assess the photostability of drug products under indoor LED lighting, complementing certain aspects of but also challenging the limitations within the existing ICH guideline. In particular, the focus on actual indoor lighting is emphasized, contrasting with the guideline's use of only daylight (UV + visible), and the inappropriate use of the illuminance unit lux is critically analysed. Instead, an LED-array-based tool is presented, which provides a deeper understanding of the photostability by determining both its wavelength dependency and threshold. For the specific GLP-1 analogue test drug product used in this study, wavelengths above 590 nm (yellow-red) are safe, while exposure to shorter wavelengths (blue-green) can lead to photodegradation. Finally, practical solutions and pathways are discussed for minimizing photodegradation, including the optimization of lighting conditions and packaging designs, ultimately ensuring the development of safer drug product.