Multidrug resistance protein 4 (MRP4) is capable of transporting acyclic nucleotide phosphonates, but little is known about its role in lamivudine (LAM) and entecavir (ETV) transport. In the present study, the involvement of MRP4 in the transport of LAM and ETV was investigated through in vitro experiments. The cytotoxicity of three antiviral drugs and their activities against HBV as characterized in HepG2.4D14 [wild-type hepatitis B virus (HBV)] and HepG2.A64 (ETV-resistant HBV) cells. LAM, ETV and tenofovir (TFV) demonstrated a 50% effective concentration against HBV of 4.14 +/- 0.03, 0.13 +/- 0.02 and 3.24 +/- 0.01 mu M in HepG2.4D14 cells and of 5.94 +/- 0.20, 6.28 +/- 0.07 and 11.43 +/- 0.09 mu M in HepG2.A64 cells, respectively. After administering 3-([(3-(2-[7-chloro-2-quinolinyl]ethyl)phenyl]-[(3-dimethylamino-3-oxoporphyl)-thio)-methyl]-thio) propanoic acid (MK571), the intracellular concentrations of all three drugs were much lower than the extracellular drug concentrations in these two cell types, whereas the intracellular drug concentrations in wild-type cells were higher than those in ETV-resistant cells. Furthermore, the intracellular levels of LAM, ETV and TFV were enhanced and the extracellular concentrations were reduced by addition of MK571. Thus, MRP4 is mainly responsible for the efflux of LAM and ETV in hepatocyte cultures. These results may contribute to enhancing antiviral efficacy.