Topiroxostat is an inhibitor of xanthine oxidoreductase (IC50 = 5.3 nM), an enzyme that converts xanthine to urate. Formulations containing topiroxostat are effective in vivo, lowering urate levels in the serum of patients with hyperuricemia. In rats, but not primates, topiroxostat treatment results in the formation of xanthine crystals, resulting in nephritis. Topiroxostat also inhibits ATP-binding cassette transporter G2 (ABCG2; IC50 = 0.18 μM), a high-capacity urate transporter that functions in both renal and extrarenal urate secretion.




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