Glucuronidation of the nonsteroidal antiestrogen EM-652 (SCH 57068), by human and monkey steroid conjugating UDP-glucuronosyltransferase enzymes

EM-652 (SCH 57068) is a new orally active antiestrogen that demonstrates pu re antagonistic effects in the mammary gland and endometrium. In vivo studi es have shown that EM-652 is primarily glucuronidated at the 7-hydroxy posi tion in rats and that the metabolite is present in the plasma of female mon keys and human subjects after EM-800 (SCH 57050) or EM-652 . HCl oral admin istration. Using hepatic microsomes from rat, monkey, and human, the format ion of two EM-652 monoglucuronides at positions 4' and 7 was demonstrated b y a liquid chromatographic tandem mass spectrometric method. Although no di fference in EM-652 conjugation was observed between male and female monkey livers, an interindividual variation of hepatic EM-652 glucuronidation was shown with female human donors. Using microsome preparations from human emb ryonic kidney 293 cells stably expressing each of the 12 human and 11 monke y UGT enzymes cloned to date, the two EM-652-monoglucuronides were detected after incubation with microsomes containing human UGT1A1, UGT1A3, UGT1A8, UGT1A9, and monkey monUGT1A01, monUGT1A03, and monUGT1A09. Despite human UG T1A1 and monkey monUGT1A09 favored formation of EM-652-7-glucuronide, other active UGT1A enzymes formed both 4'- and 7-glucuronide derivatives in equa l amounts. Kinetic analysis of EM-652 glucuronidation by these enzymes show ed Michaelis constant (K-m) values between 36 and 302 muM for EM-652-4'-glu curonide and 19 and 233 muM for EM-652-7-glucuronide. The present results d emonstrate the importance of UGT1A isoforms, mainly UGT1A1, for EM-652 meta bolism in humans.