Topiroxostat

Uric acid synthesis depends on the action of xanthine oxidase activity in the conversion of hypoxanthine to xanthine, followed by the conversion of xanthine to uric acid. Xanthine oxidase consists of a molybdenum ion as cofactor in the active center that has different redox states upon substrate binding. When a substrate such as hypoxanthine or xanthine binds, xanthine oxidase hydroxylates it and molybdenum ion is reduced from hexavalent, Mo(VI), to tetravalent form, Mo(IV). Molybdenum ion is reoxidized into hexavalent state once the hydroxylated substrate, xanthine or uric acid, dissociates from the active site. Topiroxostat is shown to interact with multiple amino acid residues of the solvent channel and additionally forms a reaction intermediate by covalent binding with molybdenum (IV) ion via an oxygen atom. It also forms hydrogen bonds with molybdenum (VI) ion, suggesting that it has multiple inhibition modes to xanthine oxidase. Enhanced binding interactions to xanthine oxidase achieves delayed dissociation of topiroxostat from the enzyme. 2-hydroxy-topiroxostat, the metabolite formed by primary hydroxylation of topiroxostat by xanthine oxidase, also causes time and concentration-dependent inhibition of the enzyme. Topiroxostat is shown to inhibit ATP-binding cassette transporter G2 (ABCG2) in vitro, which is a membrane protein responsible for recovering uric acid in the kidneys and secreting uric acid from the intestines.