Quantification of drug transport function across the multiple resistance-associated protein 2 (Mrp2) in rat livers

New Pharmacokinetic Parameters of Imaging Substrates Quantified from Rat Liver Compartments

Hepatobiliary imaging is increasingly used by pharmacologists to quantify liver concentrations of transporter-dependent drugs. However, liver imaging does not quantify concentrations in extracellular space, hepatocytes, and bile canaliculi. Our study compared the compartmental distribution of two hepatobiliary substrates gadobenate dimeglumine [BOPTA; 0.08 liver extraction ratio (ER)] and mebrofenin (MEB; 0.93 ER) in a model of perfused rat liver. A gamma counter placed over livers measured liver concentrations. Livers were preperfused with gadopentetate dimeglumine to measure extracellular concentrations. Concentrations coming from bile canaliculi and hepatocytes were calculated. Transporter activities were assessed by concentration ratios between compartments and pharmacokinetic parameters that describe the accumulation and decay profiles of hepatocyte concentrations. The high liver concentrations of MEB relied mainly on hepatocyte and bile canaliculi concentrations. In contrast, the three compartments contributed to the low liver concentrations obtained during BOPTA perfusion. Nonlinear regression analysis of substrate accumulation in hepatocytes revealed that cellular efflux is measurable ∼4 minutes after the start of perfusion. The hepatocyte-to-extracellular concentration ratio measured at this time point was much higher during MEB perfusion. BOPTA transport by multidrug resistance associated protein 2 induced an aquaporin-mediated water transport, whereas MEB transport did not. BOPTA clearance from hepatocytes to bile canaliculi was higher than MEB clearance. MEB did not efflux back to sinusoids, whereas BOPTA basolateral efflux contributed to the decrease in hepatocyte concentrations. In conclusion, our ex vivo model quantifies substrate compartmental distribution and transport across hepatocyte membranes and provides an additional understanding of substrate distribution in the liver. SIGNIFICANCE STATEMENT: When transporter-dependent drugs target hepatocytes, cellular concentrations are important to investigate. Low concentrations on cellular targets impair drug therapeutic effects, whereas excessive hepatocyte concentrations may induce cellular toxicity. With a gamma counter placed over rat perfused livers, we measured substrate concentrations in the extracellular space, hepatocytes, and bile canaliculi. Transport across hepatocyte membranes was calculated. The study provides an additional understanding of substrate distribution in the liver.

Validation of Pharmacological Protocols for Targeted Inhibition of Canalicular MRP2 Activity in Hepatocytes Using [99mTc]mebrofenin Imaging in Rats

The multidrug resistance-associated protein 2 (MRP2) mediates the biliary excretion of drugs and metabolites. [^99mTc]mebrofenin may be employed as a probe for hepatic MRP2 activity because its biliary excretion is predominantly mediated by this transporter. As the liver uptake of [^99mTc]mebrofenin depends on organic anion-transporting polypeptide (OATP) activity, a safe protocol for targeted inhibition of hepatic MRP2 is needed to study the intrinsic role of each transporter system. Diltiazem (DTZ) and cyclosporin A (CsA) were first confirmed to be potent MRP2 inhibitors in vitro. Dynamic acquisitions were performed in rats (n = 5-6 per group) to assess the kinetics of [^99mTc]mebrofenin in the liver, intestine and heart-blood pool after increasing doses of inhibitors. Their impact on hepatic blood flow was assessed using Doppler ultrasound (n = 4). DTZ (s.c., 10 mg/kg) and low-dose CsA (i.v., 0.01 mg/kg) selectively decreased the transfer of [^99mTc]mebrofenin from the liver to the bile (k₃). Higher doses of DTZ and CsA did not further decrease k₃ but dose-dependently decreased the uptake (k₁) and backflux (k₂) rate constants between blood and liver. High dose of DTZ (i.v., 3 mg/kg) but not CsA (i.v., 5 mg/kg) significantly decreased the blood flow in the portal vein and hepatic artery. Targeted pharmacological inhibition of hepatic MRP2 activity can be achieved in vivo without impacting OATP activity and liver blood flow. Clinical studies are warranted to validate [^99mTc]mebrofenin in combination with low-dose CsA as a novel substrate/inhibitor pair to untangle the role of OATP and MRP2 activity in liver diseases.

Hepatocyte Concentrations of Imaging Compounds Associated with Transporter Inhibition: Evidence in Perfused Rat Livers

In the liver, several approaches are used to investigate and predict the complex issue of drug-induced transporter inhibition. These approaches include in vitro assays and pharmacokinetic models that predict how inhibitors modify the systemic and liver concentrations of the victim drugs. Imaging is another approach that shows how inhibitors might alter liver concentrations stronger than systemic concentrations. In perfused rat livers associated with a gamma counter that measures liver concentrations continuously, we previously showed how fluxes across transporters generate the hepatocyte concentrations of two clinical imaging compounds, one with a low extraction ratio [gadobenate dimeglumine (BOPTA)] and one with a high extraction ratio [mebrofenin (MEB)]. BOPTA and MEB are transported by rat organic anion transporting polypeptide and multiple resistance-associated protein 2, which are both inhibited by rifampicin. The aim of the study is to measure how rifampicin modifies the hepatocyte concentrations and membrane clearances of BOPTA and MEB and to determine whether these compounds might be used to investigate transporter-mediated drug-drug interactions in clinical studies. We show that rifampicin coperfusion greatly decreases BOPTA hepatocyte concentrations, but increases those of MEB. Rifampicin strongly decreases BOPTA hepatic clearance. In contrast, rifampicin decreases moderately MEB hepatic clearance and blocks the biliary intrinsic clearance, increasing MEB hepatocyte concentrations. In conclusion, low concentrations prevent the quantification of BOPTA biliary intrinsic clearance, while MEB is a promising imaging probe substrate to evidence transporter-mediated drug-drug interactions when inhibitors act on influx and efflux transporters.

Isolated Perfused Rat Livers to Quantify the Pharmacokinetics and Concentrations of Gd-BOPTA

With recent advances in liver imaging, the estimation of liver concentrations is now possible following the injection of hepatobiliary contrast agents and radiotracers. However, how these images are generated remains partially unknown. Most experiments that would be helpful to increase this understanding cannot be performed in vivo. For these reasons, we investigated the liver distribution of the magnetic resonance (MR) contrast agent gadobenate dimeglumine (Gd-BOPTA, MultiHance®, Bracco Imaging) in isolated perfused rat livers (IPRLs). In IPRL, we developed a new set up that quantifies simultaneously the Gd-BOPTA compartment concentrations and the transfer rates between these compartments. Concentrations were measured either by MR signal intensity or by count rates when the contrast agent was labelled by [¹⁵³Gd]. With this experimental model, we show how the Gd-BOPTA hepatocyte concentrations are modified by temperature and liver flow rates. We define new pharmacokinetic parameters to quantify the canalicular transport of Gd-BOPTA. Finally, we present how transfer rates generate Gd-BOPTA concentrations in rat liver compartments. These findings better explain how liver imaging with hepatobiliary radiotracers and contrast agents is generated and improve the image interpretation by clinicians.

Hepatocyte Concentrations of Indocyanine Green Reflect Transfer Rates Across Membrane Transporters

Perioperative imaging with indocyanine green (ICG) is developing to increase safety in dissecting anatomical structures during hepatobiliary surgery. Images obtained with the fluorescence camera rely on concentrations measured in liver regions of interest. However, how ICG sinusoidal uptake and hepatocyte elimination rates generate ICG hepatocyte concentrations is largely unknown. To investigate such issue and better understand the role of membrane transporters in generating ICG hepatocyte concentrations, we perfused ICG in livers isolated from normal livers. Whether the well-known transporter inhibitor rifampicin modifies hepatocyte ICG concentrations was also studied. The dye has a very high and constant extraction ratio (96%) into hepatocytes. This persistent high extraction ratio generates a huge uphill concentration gradient across the sinusoidal membrane: from 5 μM (sinusoids) to 1600 μM (liver). When inside hepatocytes, ICG has low hepatocyte elimination (7 nmol/min.) and liver concentrations do not decrease much over time. Moreover, the tiny hepatocyte ICG efflux is mainly due to ICG return back to sinusoids (90%). Rifampicin slightly inhibits ICG uptake into hepatocytes and when inside hepatocytes blocks ICG efflux into bile canaliculi. In contrast, it increases ICG efflux back to sinusoids with significant decrease in ICG liver concentrations. Imaging with ICG in the perioperative period reflects the high hepatocyte concentrations and relies on the high extraction ratio across hepatocyte sinusoidal membrane. Although ICG concentrations are low in bile ducts, they are adequate for a good visualization and avoid bile duct injury.