The objective of this research project was to identify and characterize probe substrates for specific hepatic transport proteins. Identification of specific probe substrates for basolateral and canalicular transport proteins is necessary to elucidate mechanisms of hepatobiliary drug transport, phenotype for interindividual differences in transport protein function, and identify potential drug-drug interactions at the hepatic transport level. Three probe substrates were selected for investigation: fexofenadine, 99mTechnetium mebrofenin (99mTc-MEB), and 99mTechnetium sestamibi (99mTc-MIBI). Although fexofenadine has been touted as a P-gp specific probe substrate, pharmacokinetic modeling/siumulation of clinical data, sandwich-cultured human hepatocyte experiments and perfused liver studies in rats and mice demonstrated that fexofenadine is not a suitable probe for a specific transport protein due to compensatory efflux pathways. The hepatic uptake and excretion of 99mTc-MEB and 99mTc-MIBI have not been investigated fully despite their use as non-invasive probes to assess transport function. Therefore, suspended and sandwich-cultured rat and human hepatocytes were used to fully characterize the mechanisms of hepatic transport of 99mTc-MEB and 99mTc-MIBI. Then to validate the use of 99mTc-MEB and/or 99mTc-MIBI as probe substrates to predict the hepatic clearance of the anticancer agent, sorafenib studies were conducted to confirm similar mechanisms of hepatic uptake. Lastly, the pharmacokinetics and hepatic exposure of 99mTc-MIBI and 99mTc-MEB were compared in a patient with hepatocellular carcinoma and Child's Pugh B cirrhosis vs. healthy human volunteers. Pharmacokinetic models were constructed to describe the distribution and elimination of 99mTc-MEB and 99mTc-MIBI, to compare alterations in key rate constants representing hepatic uptake and efflux mechanisms secondary to disease. In conclusion, 99mTc-MEB and 99mTc-MIBI may be very useful phenotypic probes that are sensitive to changes in hepatic function associated with liver disease. The hepatic exposure to 99mTc-MIBI was decreased in the patient with hepatocellular carcinoma and cirrhosis compared to healthy volunteers, whereas the hepatic exposure of 99mTc-MEB was similar but the hepatic exposure profile was notably different with a lower maximum but more prolonged exposure. Collectively, the results of this research negate the use of fexofenadine as a probe for hepatic transport, but support the use of 99mTc-MEB and 99mTc-MIBI as probe substrates, which add the unique ability to quantify liver concentrations
