The dual orexin receptor antagonist daridorexant was approved in 2022 in the USA and EU for the treatment of insomnia. The purpose of this study was the identification of its metabolic pathways and the human cytochrome P450 (P450) enzymes involved in its biotransformation. With human liver microsomes, daridorexant underwent hydroxylation at the methyl group of the benzimidazole moiety, oxidative O-demethylation of the anisole to the corresponding phenol, and hydroxylation to a 4-hydroxy piperidinol derivative. While the chemical structures of the benzylic alcohol and the phenol proved to be products of standard P450 reactions, 1D and 2D NMR data of the latter hydroxylation product was incompatible with the initially postulated hydroxylation of the pyrrolidine ring and suggested the disappearance of the pyrrolidine ring and formation of a new 6-membered ring. Its formation is best explained by initial hydroxylation of the pyrrolidine ring in 5-position to yield a cyclic hemiaminal. Hydrolytic ring opening then results in an aldehyde that subsequently cyclizes onto one of the benzimidazole nitrogen atoms to yield the final 4-hydroxy piperidinol. The proposed mechanism was substantiated using an N-methylated analogue, which might hydrolyze to the open-chain aldehyde but cannot undergo the final cyclization step. CYP3A4 was the major P450 enzyme responsible for daridorexant metabolism, accounting for 89 % of metabolic turnover.
Keywords: CYP3A4 catalysis; P450 metabolism; daridorexant; rearrangement.
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