Human organic anion transporter 2 is an entecavir, but not tenofovir, transporter.

Entecavir (ETV) and tenofovir (TFV) are essential nucleoside analogues in current hepatitis B virus (HBV) treatments. Since these drugs target the HBV polymerase that is localized within human hepatocytes, determining of their cellular uptake process is an important step in fully understanding their pharmacological actions. However, the human hepatic transporters responsible for their uptake have remained unidentified. Therefore, this study aimed at identifying the primary ETV and TFV uptake transporter(s) in human hepatocytes. In transport assays, temperature-sensitive ETV and TFV uptake by human hepatocytes were observed, and their uptake were strongly inhibited by bromosulfophthalein, which is an inhibitor of organic anion transporters/organic anion transporting polypeptides (OATs/OATPs). Given these results, ETV and TFV uptake activities in several human OAT/OATP expression systems were examined. The results showed that, among the transporters tested, only OAT2 possessed ETV transport activity. On the other hand, none of the transporters showed any TFV uptake activity. To summarize, our results identify that human OAT2 is an ETV transporter, thereby suggesting that it plays an important part in the mechanisms underlying ETV antiviral activity. Furthermore, although the hepatic TFV transporters remain unknown, our results have, at least, clarified that these two anti-HBV drugs have different hepatocyte entry routes.