In vitro activity: Telbivudine shows potent, selective, and specific antiviral activity against HBV and other hepadna viruses. Telbivudine is phosphorylated by intracellular thymidine kinases to the active triphosphate form, which has an intracellular half-life of 14 hours. Telbivudine 5′-triphosphate inhibits HBV DNA polymerase (reverse transcriptase) by competing with the natural substrate, dTTP. Telbivudine significantly increases the production of tumour necrosis factor-alpha and interleukin-12 in MHV-3-induced macrophages. Telbivudine significantly elevates serum levels of interferon-gamma. Telbivudine treatment enhances the ability of T cells to undergo proliferation and secrete cytokines but does not affect cytotoxicity of infected hepatocytes. Telbivudine treatment suppresses programmed death ligand 1 expression on T cells. Telbivudine remains active as shown by respective fold-changes of 0.5 (N236T) and 1.0 (A181V and A194T). Telbivudine is not active against HBV strains bearing lamivudine mutations L180M/M204V/I but remains active against the M204V single mutant in vitro, potentially explaining the difference in resistance profiles between telbivudine and lamivudine.