Semi-synthetic Bile Acid FXR and TGR5 Agonists: Physicochemical Properties, Pharmacokinetic, and Metabolism in Rat

We report on the relationship between the structure-pharmacokinetics, metabolism, and therapeutic activity of semi-synthetic bile acid analogues including 6α-ethyl-3α,7α-dihydroxy-5β-cholan-24-oic acid (a selective FXR receptor agonist), 6α-ethyl-23(S)-methyl-3α,7α,12α-trihydroxy-β-cholan-24-oic acid (a specific TGR5 receptor agonist), and 6α-ethyl-3α,7α-dihydroxy-24-nor-5β-cholan-23-sulfate (a dual FXR/TGR5 agonist). We measured the main physicochemical properties of these molecules, including ionization constants, water solubility, lipophilicity, detergency, and protein binding. Biliary secretion and metabolism, plasma and hepatic concentrations were evaluated by HPLC-ES-MS/MS in bile fistula rat, and compared with natural analogues chenodeoxycholic, cholic and taurochenodexycholic acid intestinal bacteria metabolism was evaluated in terms of 7α-dehydroxylase substrate-specificity in anaerobic human stool culture. The semi-synthetic derivatives detergency, measured in terms of their critical micellar concentration was quite similar to the natural analogues. They were slightly more lipophilic than the corresponding natural analogues, evaluated by their 1-octanol water partition coefficient (LogP), due to the ethyl group in 6 position, which makes these molecules very stable toward bacterial 7-dehydroxylation. The hepatic metabolism and biliary secretion were different: 6α-ethyl-3α,7α-dihydroxy-5β-cholan-24-oic acid, as chenodeoxycholic acid, were efficiently conjugated with taurine in the liver and only in this form promptly and efficiently secreted in bile. 6α-ethyl-23(S)-methyl-3α,7α,12α-trihydroxy-5β-cholan-24-oic acid was poorly conjugated with taurine, due to the steric hindrance of the methyl at C23 (S) position, metabolized to the C23 (R) isomer and partly conjugated with taurine. Conversely, 6α-ethyl-3α,7α-dihydroxy-24-nor-5β-cholan-23-sulfate was secreted in bile unmodified and as 3-glucuronide. Therefore, minor structural modifications profoundly influence the metabolism and biodistribution in the target organs where these analogues exert therapeutic effects by interacting with FXR and/or TGR5 receptors.