Drugs and hepatic transporters: A review

The liver is the primary organ for the metabolic degradation of xenobiotics. Transmembrane transport proteins from the ABC and the SLC families mediate the uptake of endogenous compounds and xenobiotics into the hepatocyte as well as their elimination from the cells. Multiple processes are involved. The uptake of xenobiotics in hepatocytes is mediated by organic anion transporting polypeptides (OATPs) and by organic anion and cation transporters (OATs and OCTs). The elimination of drugs and metabolites from the liver cell back to the bloodstream is accomplished mainly by multidrug resistance-associated protein 3 (MRP3) and MRP4, while the elimination towards the biliary canaliculi is mediated by several different transporters (MRP2, BCRP, MDR1 and MATE1). Since bile acids and their salts are toxic detergents for hepatocytes, they have to be eliminated efficiently. This task is accomplished by the bile salt export pump BSEP. Two further transporters, MDR3 and ATP8B1 are involved in the proper constitution of bile. All these transporters can be influenced, mainly inhibited by a number of drugs, but also by metabolites from endogenous compounds such as estrogens. Additionally, rare monogenetic diseases exist which can be explained by absence of function or dysfunction of specific hepatic transporters, such as progressive familial intrahepatic cholestasis type 2 by genetic modifications in BSEP that lead to a loss of transporter function. Functional impairment of other transporters by genetics or by drugs also leads to liver injury, a potentially life-threatening disease that is still not fully understood. Hence, the interplay between drugs and hepatic transporters is multiple, and the knowledge of this interplay helps in understanding the etiology and molecular mechanisms behind some forms of (drug-induced) liver injury.