In vitro characterization of hepatic flavopiridol metabolism using human liver microsomes and recombinant UGT enzymes.
OBJECTIVE
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 microl/min/mg) and UGT1A9 (1.5 microl/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.
Ramírez
Jacqueline
J
Department of Medicine, University of Chicago, Illinois 60637, USA.
Iyer
Lalitha
L
Journault
Kim
K
Bélanger
Patrick
P
Innocenti
Federico
F
Ratain
Mark J
MJ
Guillemette
Chantal
C
D000970:Antineoplastic Agents / Q000378:metabolism*; D005419:Flavonoids / Q000378:metabolism*; D008401:Gas Chromatography-Mass Spectrometry; D005965:Glucuronates / Q000378:metabolism; D020719:Glucuronides / Q000378:metabolism; D014453:Glucuronosyltransferase / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D066298:In Vitro Techniques; D007527:Isoenzymes / Q000235:genetics / Q000378:metabolism; D008862:Microsomes, Liver / Q000201:enzymology / Q000378:metabolism*; D010880:Piperidines / Q000378:metabolism*; D015742:Propofol / Q000378:metabolism; D011994:Recombinant Proteins / Q000378:metabolism; D014407:Tumor Cells, Cultured
C471033:5-O-glucopyranuronosylflavopiridol; C471032:7-O-glucopyranuronosylflavopiridol; D000970:Antineoplastic Agents; D005419:Flavonoids; D005965:Glucuronates; D020719:Glucuronides; D007527:Isoenzymes; D010880:Piperidines; D011994:Recombinant Proteins; C077990:alvocidib; D014453:Glucuronosyltransferase; D015742:Propofol