Definition: Light sensitivity means a drug can be damaged by light. In peptides and proteins, light drives oxidation of certain amino acids, which can change the shape of the molecule and how well it works. That is why many labels for injectable peptides say to keep the pen, cartridge or vial in its original carton.
A 2007 review names the amino acids that are first to oxidize under light: tryptophan, tyrosine, phenylalanine and cysteine (Kerwin 2007). The damage can change the protein's structure at several levels. The authors say this could affect long-term stability, activity or immune reactions, though that link is not firmly proven. FDA guidance on protein drugs says most of them degrade on exposure to heat and light.
Light damage is one form of peptide degradation. Like the others, it does not always change how the liquid looks.
FDA guidance asks makers to test protein products in their packaging under in-use conditions, including light, and to put the resulting storage rules in the patient labeling. So the carton rule comes from the maker's own testing of that product. These label rules belong to the named products. Compounded drugs are not FDA approved, so their light stability has not been reviewed by FDA.
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