Species- and disposition model-dependent metabolism of raloxifene in gut and liver: Role of UGT1A10

Caco-2 cell lysate, and intestinal and liver microsomes derived from female humans and rats were used to compare and contrast the metabolism and disposition of raloxifene. In Caco-2 cell lysate, raloxifene 6-beta-glucuronide (M1) was the main metabolite, although raloxifene 4'-beta-glucuronide (M2) was formed in comparable abundance (58% versus 42%). In rat liver and intestinal microsomes, M1 represented about 76 to 86% of glucuronidated metabolites. In contrast, raloxifene 4'-beta-glucuronide ( M2) was the predominant metabolite in expressed UGT1A10 (96%) and human intestinal (92%) microsomes. Intrinsic clearance for M2 (CLint, M2) in human intestinal microsomes was 33- to 72-fold higher than in rat microsomes, whereas intrinsic clearance for M1 (CLint, M1) was 3- to 4-fold lower. Taken together, total intrinsic clearance (CLint, M1 + CLint, M2) in human intestinal microsomes was 3- to 6-fold higher than that in rat intestinal microsomes, but was similar in liver microsomes. In addition, intrinsic clearance in small intestinal microsomes was 2- to similar to 5-fold higher than that in hepatic microsomes, regardless of species. To account for the difference in species- and disposition model-dependent intestinal metabolism, we probed the presence of various UGT1A isoforms in Caco-2 cells using real-time reverse transcriptase-polymerase chain reaction and, as expected, detected no UGT1A10. In conclusion, the lack of UGT1A10 may explain why Caco-2 cell and rat intestinal microsomes metabolized raloxifene differently from human intestinal microsomes. The presence of human intestinal UGT1A10 and the higher overall intrinsic clearance value in the human intestine as the result of UGT1A10 expression could explain why raloxifene has much lower bioavailability in humans (2%) than in rats (39%).