Evaluating Hepatobiliary Transport With F-18-Labeled Bile Acids: The Effect Of Radiolabel Position And Bile Acid Structure On Radiosynthesis And In Vitro And In Vivo Performance

Introduction. An in vivo determination of bile acid hepatobiliary transport efficiency can be of use in liver disease and preclinical drug development. Given the increased interest in bile acid Positron Emission Tomography- (PET-) imaging, a further understanding of the impact of 18-fluorine substitution on bile acid handling in vitro and in vivo can be of significance. Methods. A number of bile acid analogues were conceived for nucleophilic substitution with [F-18]fluoride: cholic acid analogues of which the 3-, 7-, or 12-OH function is substituted with a fluorine atom (3 alpha-[F-18]FCA; 7 beta-[F-18]FCA; 12 beta-[F-18]FCA); a glycocholic and chenodeoxycholic acid analogue, substituted on the 3-position (3 beta-[F-18]FGCA and 3 beta-[F-18]FCDCA, resp.). Uptake by the bile acid transporters NTCP and OATP1B1 was evaluated with competition assays in transfected CHO and HEK cell lines and efflux by BSEP in membrane vesicles. PET-scans with the tracers were performed in wild-type mice (n = 3 per group): hepatobiliary transport was monitored and compared to a reference tracer, namely, 3 beta-[F-18]FCA. Results. Compounds 3 alpha-[F-18]FCA, 3 beta-[F-18]FGCA, and 3 beta-[F-18]FCDCA were synthesized in moderate radiochemical yields (4-10% n.d.c.) and high radiochemical purity (> 99%); 7 beta-[F-18]FCA and 12 beta-[F-18]FCA could not be synthesized and included further in this study. In vitro evaluation showed that 3 alpha-FCA, 3 beta-FGCA, and 3 beta-FCDCA all had a low micromolar Ki-value for NTCP, OATP1B1, and BSEP. In vivo, 3 alpha-[F-18]FCA, 3 beta-[F-18]FGCA, and 3 beta-[F-18]FCDCA displayed hepatobiliary transport with varying efficiency. A slight yet significant difference in uptake and efflux rate was noticed between the 3 alpha-[F-18]FCA and 3 beta-[F-18]FCA epimers. Conjugation of 3 beta-[F-18]FCA with glycine had no significant effect in vivo. Compound 3 beta-[F-18]FCDCA showed a significantly slower hepatic uptake and efflux towards gallbladder and intestines. Conclusion. A set of F-18 labeled bile acids was synthesized that are substrates of the bile acid transporters in vitro and in vivo and can serve as PET-biomarkers for hepatobiliary transport of bile acids.