Effect of Ritonavir on 99m Technetium–Mebrofenin Disposition in Humans: A Semi‐PBPK Modeling and In Vitro Approach to Predict Transporter‐Mediated DDIs

A semiphysiologically based pharmacokinetic (semi‐PBPK) model was developed to describe a unique blood, liver, and bile clinical data set for the hepatobiliary imaging agent 99m Technetium–mebrofenin ( 99m Tc–mebrofenin), and to simulate sites/mechanisms of a 99m Tc–mebrofenin–ritonavir drug–drug interaction (DDI). The transport inhibitor ritonavir (multiple‐dose: 2 × 300 mg) significantly increased systemic 99m Tc–mebrofenin exposure as compared with control (4,464 ± 1,861 vs. 1,970 ± 311 nCi min/ml; mean ± SD), without affecting overall hepatic exposure or biliary recovery. A novel extrahepatic distribution compartment was required to characterize 99m Tc–mebrofenin disposition. Ritonavir inhibited 99m Tc–mebrofenin accumulation in human sandwich‐cultured hepatocytes (SCH) (half maximal inhibitory concentration (IC 50 ) = 3.46 ± 1.53 µmol/l). Despite ritonavir accumulation in hepatocytes, intracellular binding was extensive (97. 6%), which limited interactions with multidrug resistance protein 2 (MRP2)‐mediated biliary excretion. These in vitro data supported conclusions from modeling/simulation that ritonavir inhibited 99m Tc–mebrofenin hepatic uptake, but not biliary excretion, at clinically relevant concentrations. This integrated approach, utilizing modeling, clinical, and in vitro data, emphasizes the importance of hepatic and extrahepatic distribution, assessment of inhibitory potential in relevant in vitro systems, and intracellular unbound concentrations to assess transporter‐mediated hepatic DDIs. CPT: Pharmacometrics & Systems Pharmacology (2013) 2, e20; doi: 10.1038/psp.2012.21 ; advance online publication 2 January 2013