Evaluating Hepatobiliary Transport with 18F-Labeled Bile Acids: The Effect of Radiolabel Position and Bile Acid Structure on Radiosynthesis and In Vitro and In Vivo Performance

An in vivo determination of bile acid hepatobiliary transport efciency 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-fuorine substitution on bile acid handling in vitro and in vivo can be of signifcance. Methods. A number of bile acid analogues were conceived for nucleophilic substitution with [18F]fuoride: cholic acid analogues of which the 3-, 7-, or 12-OH function is substituted with a fuorine atom (3β-[18F]FCA; 7β-[18F]FCA; 12β-[18F]FCA); a glycocholic and chenodeoxycholic acid analogue, substituted on the 3-position (3β-[18F]FGCA and 3β-[18F]FCDCA, resp.). Uptake by the bile acid transporters NTCP and OATP1B1 was evaluated with competition assays in transfected CHO and HEK cell lines and efux by BSEP in membrane vesicles. PET-scans with the tracers were performed in wild-type mice (β=3 per group): hepatobiliary transport was monitored and compared to a reference tracer, namely, 3β-[18F]FCA. Results. Compounds 3β-[18F]FCA, 3β-[18F]FGCA, and 3β-[18F]FCDCA were synthesized in moderate radiochemical yields (4–10% n.d.c.) and high radiochemical purity (>99%); 7β- [ 18F]FCA and 12β-[18F]FCA could not be synthesized and included further in this study. In vitro evaluation showed that 3β-FCA, 3β-FGCA, and 3β-FCDCA all had a low micromolar Ki-value for NTCP, OATP1B1, and BSEP. In vivo, 3β-[18F]FCA, 3β-[18F]FGCA, and 3β-[18F]FCDCA displayed hepatobiliary transport with varying efciency. A slight yet signifcant diference in uptake and efux rate was noticed between the 3β-[18F]FCA and 3β-[18F]FCA epimers. Conjugation of 3β-[18F]FCA with glycine had no signifcant efect in vivo. Compound 3β-[18F]FCDCA showed a signifcantly slower hepatic uptake and efux towards gallbladder and intestines. Conclusion. A set of 18F labeled bile acids was synthesized that are substrates of the bile acid transportersin vitro and in vivo and can serve as PET-biomarkers for hepatobiliary transport of bile acids.