In vitro activity: Entecavir-triphosphate is a highly potent inhibitor of wild-type HBV Pol and is 100- to 300-fold more potent than lamivudine-triphosphate against 3TC-resistant HBV Pol. Entecavir inhibits the replication of 3TC-resistant HBV, but 20- to 30-fold higher concentrations are required. Entecavir results in an impressive reduction of serum viral DNA with covalently closed circular DNA and hepatitis B viral core antigen negativity in liver biopsy specimens. Entecavir has potent activity (EC50, 0.1 nM) against HIV in a unique single-cycle, single-cell-based pseudovirus assay (24) with CD4+ lymphocytes using a green fluorescent protein reporter fluorescence-activated cell sorter assay as the endpoint.

Kinase Assay: BMS-200475 has a EC50 of 3.75 nM against HBV. It is incorporated into the protein primer of HBV and subsequently inhibits the priming step of the reverse transcriptase. The antiviral activity of BMS-200475 is significantly less against the other RNA and DNA viruses. Entecavir is more readily phosphorylated to its active metabolites than other deoxyguanosine analogs (penciclovir, ganciclovir, lobucavir, and aciclovir) or lamivudine. The intracellular half-life of entecavir is 15 h.

Cell Assay: BMS 200475 is prepared in phosphate-buffered saline (PBS) and diluted with appropriate medium containing 2% fetal bovine serum. HepG2 2.2.15 cells are plated at a density of 5×105 cells per well on 12-well Biocoat collagen-coated plates and are maintained in a confluent state for 2 to 3 days before being overlaid with 1 mL of medium spiked with BMS 200475. Quantification of HBV was performed on day 10.