Role of hepatic efflux transporters in regulating systemic and hepatocyte exposure to xenobiotics

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

To understand the transport function of drugs across the canalicular membrane of hepatocytes, it would be important to measure concentrations in hepatocytes and bile. However, these concentration gradients are rarely provided. The aim of the study is then to measure these concentrations and define parameters to quantify the canalicular transport of drugs through the multiple resistance associated-protein 2 (Mrp2) in entire rat livers. Besides drug bile excretion rates, we measured additional parameters to better define transport function across Mrp2: (1) Concentration gradients between hepatocyte and bile concentrations over time; and (2) a unique parameter (canalicular concentration ratio) that represents the slope of the non-linear regression curve between hepatocyte and bile concentrations. This information was obtained in isolated rat livers perfused with gadobenate dimeglumine (BOPTA) and mebrofenin (MEB), two hepatobiliary drugs used in clinical liver imaging. Interestingly, despite different transport characteristics including excretion rates into bile and hepatocyte clearance into bile, BOPTA and MEB have a similar canalicular concentration ratio. In contrast, the ratio was null when BOPTA was not excreted in bile in hepatocytes lacking Mrp2. The canalicular concentration ratio is more informative than bile excretion rates because it is independent of time, bile flows, and concentrations perfused in portal veins. It would be interesting to apply such information in human liver imaging where hepatobiliary compounds are increasingly investigated.

Role of multidrug resistance-associated protein 4 in the basolateral efflux of hepatically derived enalaprilat

Hepatic uptake and efflux transporters govern the systemic and hepatic exposure of many drugs and metabolites. Enalapril is a pharmacologically inactive prodrug of enalaprilat. Following oral administration, enalapril is converted to enalaprilat in hepatocytes and undergoes translocation into the systemic circulation to exert its pharmacologic effect by inhibiting angiotensin-converting enzyme. Although the transport proteins governing hepatic uptake of enalapril and the biliary excretion of enalapril and enalaprilat are well established, it remains unknown how hepatically derived enalaprilat translocates across the basolateral membrane into the systemic circulation. In this study, the role of ATP-binding cassette transporters in the hepatic basolateral efflux of enalaprilat was investigated using membrane vesicles. ATP-dependent uptake of enalaprilat into vesicles expressing multidrug resistance-associated protein (MRP) 4 was significantly greater (∼3.8-fold) than in control vesicles. In contrast, enalaprilat was not transported to a significant extent by MRP3, and enalapril was not transported by either MRP3 or MRP4. The functional importance of MRP4 in the basolateral excretion of derived enalaprilat was evaluated using a novel basolateral efflux protocol developed in human sandwich-cultured hepatocytes. Under normal culture conditions, the mean intrinsic basolateral efflux clearance (CLint ,basolateral) of enalaprilat was 0.026 ± 0.012 µl/min; enalaprilat CLint,basolateral was significantly reduced to 0.009 ± 0.009 µl/min by pretreatment with the pan-MRP inhibitor MK-571. Results suggest that hepatically derived enalaprilat is excreted across the hepatic basolateral membrane by MRP4. Changes in MRP4-mediated basolateral efflux may alter the systemic concentrations of this active metabolite, and potentially the efficacy of enalapril.

Quantitative targeted proteomics for membrane transporter proteins: method and application

Although global proteomics has shown promise for discovery of many new proteins, biomarkers, protein modifications, and polymorphisms, targeted proteomics is emerging in the proteomics research field as a complement to untargeted shotgun proteomics, particularly when a determined set of low-abundance functional proteins need to be measured. The function and expression of proteins related to drug absorption, distribution, metabolism, and excretion (ADME) such as cytochrome P450 enzymes and membrane transporters are of great interest in biopharmaceutical research. Since the variation in ADME-related protein expression is known to be a major complicating factor encountered during in vitro-in vivo and in vivo-in vivo extrapolations (IVIVE), the accurate quantification of the ADME proteins in complex biological systems becomes a fundamental element in establishing IVIVE for pharmacokinetic predictions. In this review, we provide an overview of relevant methodologies followed by a summary of recent applications encompassing mass spectrometry-based targeted quantifications of membrane transporters.

The role of organic anion transporters in diagnosing liver diseases by magnetic resonance imaging

The expression and transport functions of organic anion transporters are modified in liver diseases, and therefore the vascular clearances of endogenous and exogenous organic anions that are taken up by these transporters have been used to assess liver diseases in patients. More recently, liver imaging with hepatobiliary contrast agents, tracers, and dyes that cross hepatocytes through the organic anion transporting polypeptides (OATPs)-multidrug resistance-associated proteins (MRPs) pathway were developed to detect and characterize focal lesions and to assess the severity of diffuse liver diseases. This review focuses mainly on magnetic resonance imaging and highlights the growing interest in imaging the OATPs-MRP2 pathway to better understand liver diseases. Imaging provides noninvasive measurements of tissue concentrations that result from the interplay between influx and efflux membrane transport systems in normal or injured hepatocytes. Imaging with magnetic resonance hepatobiliary contrast agents improves the detection and the characterization of hepatic focal lesions. New developments of imaging to assess liver function and understand the hepatocellular concentrations of contrast agents are discussed.