Species- And Disposition Model-Dependent Metabolism of Raloxifene in Gut and Liver: Role of Ugt1a10

Abstract

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-␤-glucuronide (M1) was the main metabolite, although raloxifene 4-␤-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-␤-glucuronide (M2) was the predominant metabolite in expressed UGT1A10 (96%) and human intestinal (92%) microsomes. Intrinsic clearance for M2 (CL int, M2 ) in human intestinal microsomes was 33-to 72-fold higher than in rat microsomes, whereas intrinsic clearance for M1 (CL int, M1 ) was 3-to 4-fold lower. Taken together, total intrinsic clearance (CL int, M1 ؉ CL int, 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 ϳ5-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%).

New insights into the metabolism of tamoxifen and its role in the treatment and prevention of breast cancer

“…Only about 2% of administered raloxifene is bioavailable [1] but despite this, the drug is known to have a long biological half life of 27 hr. The reason for this disparity is that raloxifene is a polyphenolic drug that can be glucuronidated and sulfated by bacteria in the gut so the drug cannot be absorbed [107,108]. This phase II metabolism in turn controls enterohepatic recirculation and ultimately impairs the drug from reaching and interacting with receptors in the target.…”

Prominent but Reverse Stereoselectivity in Propranolol Glucuronidation by Human UDP-Glucuronosyltransferases 1A9 and 1A10

“…The role of intestinal first-pass glucuronidation may be significant, however, especially if the drug is a substrate to an extrahepatically expressed UGT isoform, such as UGT1A10, and if glucuronidation is an important metabolic route for the given compound. Raloxifene may be an interesting example in this respect (Jeong et al, 2005).…”

Influence of hepatic and intestinal efflux transporters and their genetic variants on the pharmacokinetics and pharmacodynamics of raloxifene in osteoporosis treatment

“…According to the estimations, the fraction of raloxifene absorbed is high, 60 %, but only 2% reaches the systemic circulation in unconjugated form due to the high metabolic turnover during absorption [7]. The rest represents raloxifene conjugated by UDP-glucuronosyltransferases (UGTs) to raloxifene-4'-β-glucuronide (M2), raloxifene-6-β-glucuronide (M1) [7,8] and raloxifene-6,4'-diglucuronide (M3) [7,9]. Although the glucuronides show little affinity for the estrogen receptors and for bone tissue, they are important because they can be readily reconverted back into active raloxifene in various organs by β-glucuronidases residing in the liver, lung, spleen, kidney, bone and uterus [10].…”

“…According to the estimations, the fraction of raloxifene absorbed is high, 60 %, but only 2% reaches the systemic circulation in unconjugated form due to the high metabolic turnover during absorption [7]. The rest represents raloxifene conjugated by UDP-glucuronosyltransferases (UGTs) to raloxifene-4'-β-glucuronide (M2), raloxifene-6-β-glucuronide (M1) [7,8] and raloxifene-6,4'-diglucuronide 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 5 shown that raloxifene is probably transported by Pgp [11,12] while the corresponding glucuronides utilize the MRPs and organic anion transporter(s) (OAT) for their transport [12].…”