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

Abstract

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 [¹⁸F]fluoride: cholic acid analogues of which the 3-, 7-, or 12-OH function is substituted with a fluorine atom (3α-[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 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, 3β-[¹⁸F]FCA.

Results: Compounds 3α-[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 3α-FCA, 3β-FGCA, and 3β-FCDCA all had a low micromolar Ki-value for NTCP, OATP1B1, and BSEP. In vivo, 3α-[¹⁸F]FCA, 3β-[¹⁸F]FGCA, and 3β-[¹⁸F]FCDCA displayed hepatobiliary transport with varying efficiency. A slight yet significant difference in uptake and efflux rate was noticed between the 3α-[¹⁸F]FCA and 3β-[¹⁸F]FCA epimers. Conjugation of 3β-[¹⁸F]FCA with glycine had no significant effect in vivo. Compound 3β-[¹⁸F]FCDCA showed a significantly slower hepatic uptake and efflux towards gallbladder and intestines.

Conclusion: A set of ¹⁸F 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.