Abstract

EM-652 (SCH 57068) is a new orally active antiestrogen that demonstrates pure antagonistic effects in the mammary gland and endometrium. In vivo studies have shown that EM-652 is primarily glucuronidated at the 7-hydroxy position in rats and that the metabolite is present in the plasma of female monkeys and human subjects after EM-800 (SCH 57050) or EM-652·HCl oral administration. Using hepatic microsomes from rat, monkey, and human, the formation of two EM-652 monoglucuronides at positions 4′ and 7 was demonstrated by a liquid chromatographic tandem mass spectrometric method. Although no difference 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 embryonic kidney 293 cells stably expressing each of the 12 human and 11 monkey 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 UGT1A1 and monkey monUGT1A09 favored formation of EM-652–7-glucuronide, other active UGT1A enzymes formed both 4′- and 7-glucuronide derivatives in equal amounts. Kinetic analysis of EM-652 glucuronidation by these enzymes showed Michaelis constant (K _m) values between 36 and 302 μM for EM-652–4′-glucuronide and 19 and 233 μM for EM-652–7-glucuronide. The present results demonstrate the importance of UGT1A isoforms, mainly UGT1A1, for EM-652 metabolism in humans.