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First published on March 15, 2005; DOI: 10.1124/dmd.104.001883

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Yong Liu
Huimin Lin
Ming Hu
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Received for publication September 1, 2004.
Revised March 9, 2005.
Accepted for publication March 10, 2005.

Species- and Disposition Model-Dependent Disposition of Raloxifene in Gut and Liver: Role of UGT1A10

Eun-Ju Jeong 1, Yong Liu 1, Huimin Lin 1, Ming Hu 1*

1 Washington State University

* Address correspondence to: E-mail: mhu{at}uh.edu

Abstract

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 76-86% 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 33-72 folds higher than in rat microsomes, whereas intrinsic clearance for M1 (CLint, M1) was 3-4 folds lower. Taken together, total intrinsic clearance (CLint, M1 + CLint, M2) in human intestinal microsomes was 3-6 folds higher than rat intestinal microsomes, but was similar in liver microsomes. In addition, intrinsic clearance in small intestinal microsomes was 2~5 folds 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 RT-PCR, 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 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%).


Key words: drug interactions, extrahepatic drug metabolism, intestinal bioavailability, phase II drug metabolism, UDP glucuronyltransferases


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