3 beta-Isoobeticholic acid efficiently activates the farnesoid X receptor (FXR) due to its epimerization to 3 alpha-epimer by hepatic metabolism
Abstract
Bile acids (BAs) are important signaling molecules acting via the farnesoid X nuclear receptor (FXR) and the membrane G protein-coupled bile acid receptor 1 (GPBAR1). Besides deconjugation of BAs, the oxidoreductive enzymes of colonic bacteria and hepatocytes enable the conversion of BAs into their epimers or dehydrogenated forms.
Obeticholic acid (OCA) is the first-in-class BA-derived FXR agonist approved for the treatment of primary biliary cholangitis. Herein, a library of OCA derivatives, including 7-keto, 6-ethylidene derivatives and 3 beta-epimers, was synthetized and investigated in terms of interactions with FXR and GPBAR1 in transaction assays and evaluated for FXR target genes expression in human hepatocytes and C57BL/6 mice.
The derivatives were further subjected to cell-free analysis employing in silico molecular docking and a TR-FRET assay. The conversion of the 3 beta-hydroxy epimer and its pharmacokinetics in mice were studied using LC-MS.
We found that only the 3 beta-hydroxy epimer of OCA (3 beta-isoOCA) possesses significant activity to FXR in hepatic cells and mice. However, in a cell-free assay, 3 beta-isoOCA had about 9-times lower affinity to FXR than did OCA.
We observed that 3 beta-isoOCA readily epimerizes to OCA in hepatocytes and murine liver. This conversion was significantly inhibited by the hydroxy-Delta(5)-steroid dehydrogenase inhibitor trilostane.
In addition, we found that 3,7-dehydroobeticholic acid is a potent GPBAR1 agonist. We conclude that 3 beta-isoOCA significantly activates FXR due to its epimerization to the more active OCA by hepatic metabolism.
Other modifications as well as epimerization on the C3/C7 positions and the introduction of 6-ethylidene in the CDCA scaffold abrogate FXR agonism and alleviate GPBAR1 activation.