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

Abstract

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 (IC50) = 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.

Figures

Figure 1

99mTc–mebrofenin (a) blood concentration vs. time curves; (b) liver scintigraphy vs. time curves by treatment group (red shaded area represents mean ± SD of control); (c) liver-to-blood ratio vs. time curves; mean data by treatment group. Data are presented as group means (±SD in (a) and (b)); red = control group, open/dashed = 200 mg ritonavir group, closed/solid = 2 × 300 mg ritonavir group.

Figure 2

Semi-PBPK model scheme (a) representing 99mTc–mebrofenin disposition in humans (Q denotes blood flow (ml/min), subscripts represent tissue or vascular compartments, as follows: liver (h), bile (bile), extrahepatic (“other”) tissue (o); clearance values are designated as CLuptake for influx from blood into tissue, CLefflux for efflux from tissue to blood, and CLbiliary for excretion from liver into bile). Simulations based on the semi-PBPK model and observed blood, liver, and bile curves for 99mTc–mebrofenin in subjects with quantitative scintigraphy data for (b) subject 8 (control), and (c) subject 17 (2 × 300 mg ritonavir). Circles represent observed blood samples, solid lines represent simulated blood concentrations, + symbols represent observed liver concentrations from attenuation-corrected (quantitative) scintigraphy data of the liver region of interest, dashed lines represent simulated liver concentrations, squares represent observed mass recovered in bile (corrected for gallbladder ejection fraction), and dotted lines represent simulated biliary excretion data. PBPK, physiologically based pharmacokinetic.

Figure 3

Sensitivity analysis of parameter estimates determined from the semiphysiologically based pharmacokinetic model (Figure 2; Table 2). Parameters were altered 10-fold in either direction of the values estimated for mean control data (Table 2), and the fold-change in the predicted study end points (99mTc–mebrofenin central compartment (blood) exposure, hepatic exposure, and biliary excretion) were examined. Solid lines represent influx clearance (CLuptake) from blood into tissue, dashed lines represent efflux clearance (CLefflux) from tissue to blood, and gray lines represent biliary excretion from liver into bile (CLbiliary). AUC, blood concentration–time curve.

Figure 4

Simulations based on the 99mTc–mebrofenin semiphysiologically based pharmacokinetic model scheme (Figure 2) for 99mTc–mebrofenin (a) blood, and (b) liver concentration–time data, and (c) cumulative % dose excreted in bile resulting from parameters describing mean control data (Table 2) and the following changes in transport-mediated processes, alone and in combination. Mean control data (red); 1/3 CLuptake,h + 3x CLefflux,h (blue); 1/2 CLuptake,h and CLuptake,o + 2x CLefflux,h and CLefflux,o (dashed black); 1/2 CLbiliary (solid black); mean control data without extrahepatic compartment (gray).

Figure 5

Human sandwich-cultured hepatocytes (SCH) were incubated for 10 min at 37 °C with 0.5 µCi/ml 99mTc–mebrofenin in standard (cells + bile) or Ca+-free (cells) HBSS (using B-CLEAR technology) alone and with various concentrations of ritonavir in SCH from three separate donors, in triplicate. (a) 99mTc–mebrofenin accumulation and biliary excretion index (BEI) in cells + bile (closed bars) and cells (open bars) in the presence of increasing concentrations of extracellular ritonavir. 99mTc–mebrofenin BEI values (above the bars) represent mean data. (b) Ritonavir accumulation in hepatocytes (Ca2+-free HBSS) when coadministered with 99mTc–mebrofenin, represented as intracellular total concentration (Ccell,total), as described in Methods section. 99mTc–mebrofenin and ritonavir accumulation data are represented as mean ± SEM of three separate donors in triplicate (5 µmol/l data are mean ± range of two donors, in triplicate).