Abstract

In humans, orally administered 4-(5-pyridin-4-yl-1H-[1,2,4]triazol-3-yl) pyridine-2-carbonitrile (FYX-051) is excreted mainly as triazole N₁- and N₂-glucuronides in urine. It is important to determine the enzyme(s) that catalyze the metabolism of a new drug to estimate individual differences and/or drug-drug interactions. Therefore, the characterization and mechanism of these glucuronidations were investigated using human liver microsomes (HLMs), human intestinal microsomes (HIMs), and recombinant human UDP-glucuronosyltransferase (UGT) isoforms to determine the UGT isoform(s) responsible for FYX-051 N₁- and N₂-glucuronidation. FYX-051 was metabolized to its N₁- and N₂-glucuronide forms by HLMs, and their K_m values were 64.1 and 72.7 μM, respectively; however, FYX-051 was scarcely metabolized to its glucuronides by HIMs. Furthermore, among the recombinant human UGT isoforms, UGT1A1, UGT1A7, and UGT1A9 catalyzed the N₁- and N₂-glucuronidation of FYX-051. To estimate their contribution to FYX-051 glucuronidation, inhibition analysis with pooled HLMs was performed. Mefenamic acid, a UGT1A9 inhibitor, decreased FYX-051 N₁- and N₂-glucuronosyltransferase activities, whereas bilirubin, a UGT1A1 inhibitor, did not affect these activities. Furthermore, in the experiment using microsomes from eight human livers, the N₁- and N₂-glucuronidation activity of FYX-051 was found to significantly correlate with the glucuronidation activity of propofol, a specific substrate of UGT1A9 (N₁: r² = 0.868, p < 0.01; N₂: r² = 0.775, p < 0.01). These results strongly suggested that the N₁- and N₂-glucuronidation of FYX-051 is catalyzed mainly by UGT1A9 in human livers.