Xanthine oxidase is the rate-limiting enzyme critical for the synthesis of uric acid, and therefore xanthine oxidase inhibitors are considered as one of the promising therapies for hyperuricemia and gout. In our previous study, series of 2-(4-alkoxy-3-cyano)phenyl-6-oxo-1,6-dihydropyrimidine-5-carboxylic acids and 2-(4-alkoxy-3-cyano)phenyl-6-imino-1,6-dihydropyrimidine-5-carboxylic acids were synthesized that presented excellent in vitro xanthine oxidase inhibitory potency. Interestingly, molecular docking studies revealed that the interaction behavior of these compounds with xanthine oxidase was changed after the conversion from a hydroxy group to amine group. To further investigate the structure-activity relationships of these pyrimidine-containing xanthine oxidase inhibitors and explore the contribution of amino or hydroxy group on xanthine oxidase inhibitory potency, several 2-phenylpyrimidine derivatives with amino or hydroxy functional group were designed and synthesized. Thereafter, the structure-activity research and molecular modeling study proved that hydroxy and amino groups could be used as pharmacophore elements for the design of 2-phenylpyrimidines xanthine oxidase inhibitors. Particularly, the optimized compound, 2-(3-cyano-4-isopentoxy)phenylpyrimidine-4-ol, emerged the strongest xanthine oxidase inhibitor potency, with an IC value of 0.046 µM, which was approximately 120-fold more potent than that of allopurinol (IC = 5.462 µM). Additionally, Lineweaver-Burk plot analysis revealed that the optimized compound acted as a mixed-type inhibitor. Furthermore, the in vivo hypouricemic effect of the optimized compound was investigated in a hyperuricemia rat model induced by potassium oxonate, and the results showed that the optimized compound could effectively reduce serum uric acid levels at an oral dose of 30 mg/kg.
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