While certain nucleoside/nucleotide reverse transcriptase inhibitors (NRTIs) are efficacious in treating HBV infection, their effects are yet to be optimized and the emergence of NRTI-resistant HBVs remains to be a challenging issue since life-long medication is needed, making it crucial to develop agents that more profoundly suppress wild-type and drug-resistant HBVs and have a long-acting feature for patients' greater quality of life.(1S,3S,5S,E)-3-(2-amino-6-oxo-1,6-dihydro-9H-purin-9-yl)-2-(fluoromethylene)-5-hydroxy-1-(hydroxymethyl)cyclopentane-1-carbonitrile (E-CFCP) was newly synthesized. E-CFCP's in vitro anti-HBV activity was evaluated. In vivo anti-HBV activity was examined using HBV-infected human-liver-chimeric mice (PXB-mice). E-CFCP's long-acting features and E-CFCP-triphosphate's interactions with HBV-reverse transcriptase (HBV-RT) were examined.E-CFCP potently blocked HBVWTD1 production (IC50qPCR_cell=1.8 nM) in HepG2.2.15 cells and HBVWTC2 (IC50SB_cell=0.7 nM), ETV-resistant HBVETV-RL180M/S202G/M204V (IC50SB_cell=77.5 nM), and ADV-resistant HBVADV-RA181T/N236T production (IC50SB_cell=14.1 nM) in Huh7 cells. E-CFCP profoundly inhibited intracellular HBV-DNA production down to below detection limit, but ETV and TAF failed to do so. E-CFCP also showed less toxicity than ETV and TAF. E-CFCP better penetrated hepatocytes and was better tri-phosphorylated and E-CFCP-triphosphate intracellularly persisted longer compared to ETV-triphosphate. Once-daily peroral E-CFCP administration over 2 weeks (0.02∼0.2 mg/kg/day) reduced HBVWTC2-viremia by 2-3 logs in PXB-mice without significant toxicities and the reduction persisted over 1-3 weeks following treatment cessation, suggesting once-weekly dosing capabilities. E-CFCP also reduced HBVETV-RL180M/S202G/M204V-viremia by 2 logs over 2 weeks, while ETV completely failed HBVETV-RL180M/S202G/M204V-viremia reduction. None of E-CFCP-treated mice had significant adverse changes. Van der Waals interactions of E-CFCP's 4'-cyano and polar interactions of the fluorine with HBV-RT are important for E-CFCP's anti-HBV potency and that both 4'-cyano and fluorine help retain E-CFCP-triphosphate's interactions of HBVETV-RL180M/S202G/M204V-RT.E-CFCP represents the first reported potential long-acting NRTI potent against HBVs.