Application of sandwich-cultured hepatocytes and the B-Clear assay for the IVIVE of hepatic clearance, drug-drug interactions and unbound intracellular drug concentrations

Speaker: Birk Poller, Ph.D  Lab Head, PK Sciences Department, Novartis, Basel

Summary

Sandwich-cultured hepatocytes (SCH) grown between two layers of collagen and matrigel are characterized by a polarized morphology and by the formation of biliary networks. In contrast to suspended hepatocytes, SCH express the relevant hepatic uptake and efflux transporters, which are involved in the hepatobiliary disposition of drugs and endogenous compounds. The B-Clear assay allows determining the biliary secretion process in vitro using SCH. This parameter has been incorporated in different in vitro-in vivo extrapolation (IVIVE) applications for hepatobiliary clearance and disposition: 1. Traditionally hepatic clearance predictions have been performed based on metabolic stability data from liver microsomes or hepatocytes. However, IVIVE for drugs that are actively excreted into bile requires a quantitative measure of the biliary clearance process, obtained from the B-Clear assay. Applying in vitro process clearances for biliary secretion, hepatic uptake and metabolism to the extended clearance model (ECM) provided successful predictions of the hepatic organ clearance in rat and human. 2. Mechanistic insights in the hepatic drug-drug interaction potential of statins were obtained from the in vitro based ECM. Thereby, incorporation of biliary secretion data has been demonstrated to be crucial for drugs that are secreted into bile. 3. Risk assessments for the inhibition of intrahepatic DDI or toxicity targets have previously been assessed using unbound systemic drug exposure under assumption of the “free drug hypothesis”. This concept, however, is unlikely valid as unbound intrahepatic drug concentrations are affected by active sinusoidal and biliary transport and by metabolism. Therefore, the ECM has been applied to determine the in vitro liver-to-blood partition coefficients (Kp,uu) for drugs that inhibit BSEP in vitro. Correlation of unbound intrahepatic drug concentrations, obtained from clinical unbound systemic concentrations and measured Kp,uu, with BSEP IC50 data allowed to estimate the risk for observed drug-induced cholestasis in clinics.