Darifenacin acid

Darifenacin acid is a degradation byproduct of the anticholinergic drug darifenacin, characterized by a benzhydryl piperidine core substituted with a 4-hydroxyphenyl ring and a carboxylic acid moiety at the terminal position of the side chain. Its structure retains the tricyclic scaffold of the parent compound but features an oxidized terminal methyl group converted to a carboxylic acid, differentiating it through enhanced polarity and altered hydrogen bonding capacity. This impurity arises via oxidative hydrolysis under acidic conditions, reflecting a key pathway in stability studies. It serves as a critical HPLC reference standard for quantifying degradation products in darifenacin formulations.