Applications of Bile Acid Transporters in Drug Delivery and Enterohepatic Circulation Assessment

Abstract

The objective of this dissertation was to investigate the application of modified bile acids for the assessment of bile acid enterohepatic circulation and for prodrug development. Bile Acid Malabsorption (BAM) results in excess fecal bile acid levels and chronic, intermittent diarrhea. Although BAM may account for up to 30% of patients diagnosed as irritable bowel syndrome (IBS-D), current methods of diagnosis are limited, and BAM is thought to be underdiagnosed. To address this gap, two trifluorinated bile acids were developed for use in fluorine magnetic resonance imaging (MRI) of bile acid enterohepatic circulation. CA-lys-TFA was synthesized by conjugating trifluoroacetyl lysine to cholic acid. CA-lys-TFA was a potent inhibitor and substrate of both the apical sodium dependent bile acid transporter and the Na+/taurocholate cotransporting polypeptide. Stability results were favorable in most conditions tested, but in choloylglycine hydrolase (CGH), the compound showed deconjugation. A preliminary pharmacokinetic analysis showed that accumulation of CA-lys-TFA in the gallbladder was maximal within 4 to 7 h. In vivo mouse imaging after oral dosing showed reproducible, clear <super>19</super>F signal from the gallbladder. This represents the first report of in vivo imaging of an orally absorbed drug using <super>19</super>F MRI. To improve upon CA-lys-TFA, CA-sar-TFMA was synthesized as potentially more stable against deconjugation in the intestine. CA-sar-TFMA was a potent inhibitor and substrate of ASBT and NTCP. Contrary to CA-lys-TFA, CA-sar-TFMA showed no degradation when exposed to CGH, and was successfully imaged in the mouse gallbladder. Both CA-lys-TFA and CA-sar-TFMA showed significantly lower concentrations in the gallbladders of Asbt-deficient mice (Slc10A2-/-), which have impaired bile acid transport relative to their WT littermates. These findings suggest that fluorinated non-radioactive bile acid analogues have potential for use in MRI to measure in vivo bile acid transport and diagnose bile acid malabsorption and other conditions associated with impaired bile acid transport. Additionally, two bile acid prodrugs of floxuridine were developed to target the liver through NTCP uptake. Finally, a model of substrate translocation in the presence of an inhibitor was derived to determine the impact of microrate constants on K_m, K_i, and V_max.