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In Vitro Characterization of Hepatic Flavopiridol Metabolism Using Human Liver Microsomes and Recombinant UGT Enzymes

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Abstract

Purpose. To assess the contribution of drug metabolism to the variability on flavopiridol glucuronidation observed in cancer patients, and to determine the ability of all known human UDP-glucuronosyltransferase (UGT) isoforms to glucuronidate flavopiridol.

Methods. Inter-individual variation in flavopiridol glucuronidation was determined by HPLC using hepatic microsomes from 62 normal liver donors. Identification of enzymes capable of glucuronidating flavopiridol was determined by LC/MS using human embryonic kidney 293 (HEK293) cells stably expressing all sixteen known human UGTs.

Results. The major product of the flavopiridol glucuronidation reaction in human liver microsomes was FLAVO-7-G. High variability (coefficient of variation = 49%) was observed in the glucuronidation of flavopiridol by human liver microsomes. In vitro formation of FLAVO-7-G and FLAVO-5-G was mainly catalyzed by UGT1A9 and UGT1A4, respectively. Similar catalytic efficiencies (Vmax/Km) were observed for human liver microsomes (1.6 μl/min/mg) and UGT1A9 (1.5 μl/min/mg).

Conclusions. UGT1A9 is the major UGT involved in the hepatic glucuronidation of flavopiridol in humans. The data suggests that hepatic glucuronidation may be a major determinant of the variable systemic glucuronidation of flavopiridol in cancer patients. The large variability in flavopiridol glucuronidation may be due to differences in liver metabolism among individuals, as a result of genetic differences in UGT1A9.

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Ramírez, J., Iyer, L., Journault, K. et al. In Vitro Characterization of Hepatic Flavopiridol Metabolism Using Human Liver Microsomes and Recombinant UGT Enzymes. Pharm Res 19, 588–594 (2002). https://doi.org/10.1023/A:1015341726183

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