Vol. 31, Issue 1, 37-45, January 2003

Acyl Glucuronidation and Glucosidation of a New and Selective Endothelin ET_A Receptor Antagonist in Human Liver Microsomes

Cuyue Tang, Jerome H. Hochman, Bennett Ma, Raju Subramanian, and Kamlesh P. Vyas

Department of Drug Metabolism, Merck Research Laboratories, West Point, Pennsylvania

Compound A [(+)-(5S,6R,7R)-2-isopropylamino-7-[4-methoxy-2-((2R)-3-methoxy-2-methylpropyl)-5-(3,4-methylenedioxyphenyl) cyclopenteno [1,2-b] pyridine 6-carboxylic acid] is a new and selective endothelin ET_A receptor antagonist. It underwent significant acyl glucuronidation and acyl glucosidation in human liver microsomes supplemented with UDP-glucuronic acid (UDPGA) and UDP-glucose (UDPG). These two conjugations were observed in a panel of human liver microsomal samples (n = 16) that gave rise to varying activities but with no significant correlation with each other in the native and activator-treated microsomal preparations (r²  0.4, p > 0.05). The lack of correlation may be explained by the involvement of multiple UDP-glucuronosyltransferases (UGTs; UGT1A1, 1A3, 1A9, 2B7 and 2B15) in the glucuronidation but essentially solely UGT2B7 in the glucosidation. Both reactions conformed to monophasic Michaelis-Menten kinetics in human liver microsomes. The glucuronidation reaction exhibited apparent K_m values (mean ± S.E.) for compound A and UDPGA of 8.4 ± 0.6 and 605 ± 35 µM, respectively, whereas the values for the glucosidation reaction were 10.2 ± 1.5 and 670 ± 120 µM, respectively. In both pooled human liver microsomes and expressed UGT2B7, UDPG and UDPGA competitively inhibited their counterpart conjugations with K_i values close to their K_m values, indicating a comparable affinity of the enzyme toward these two nucleotide sugars. We herein report a drug acyl glucoside formed in human liver microsomes at a considerable turnover rate and provide the evidence for a UGT isoform (UGT2B7) capable of transferring both glucuronic acid and glucose from UDPGA and UDPG to an aglycone.