7-Deaza-7-fluoro modification confers on 4'-cyano-nucleosides potent activity against entecavir/adefovir-resistant HBV variants and favorable safety.

We designed, synthesized and identified a novel nucleoside derivative, 4'-C-cyano-7-deaza-7-fluoro-2'-deoxyadenosine (CdFA), which exerts potent anti-HBV activity (IC50 ~26 nM) with favorable hepatocytotoxicity (CC50 ~56 μM). Southern blot analysis using wild-type HBV (HBVWT)-encoding-plasmid-transfected HepG2 cells revealed that CdFA efficiently suppresses the production of HBVWT (IC50 = 153.7 nM), entecavir (ETV)-resistant HBV carrying L180M/S202G/M204V substitutions (HBVETVR; IC50 = 373.2 nM), and adefovir dipivoxil (ADV)-resistant HBV carrying A181T/N236T substitutions (HBVADVR; IC50=192.6 nM), whereas ETV and ADV were less potent against HBVETVR and HBVADVR (IC50: >1,000 and 4,022.5 nM, respectively). Once-daily peroral administration of CdFA to human-liver-chimeric mice over 14 days (1 mg/kg/day) comparably blocked HBVWT and HBVETVR viremia by 0.7 and 1.2 logs, respectively, without significant changes in body-weight or serum human-albumin levels, although ETV only slightly suppressed HBVETVR viremia (CdFA vs ETV; p = 0.032). Molecular modeling suggested that ETV-TP has good nonpolar interactions with HBVWT reverse transcriptase (RTWT)'s Met204 and Asp205, while CdFA-TP fails to interact with Met204, in line with the relatively inferior activity against HBVWT of CdFA compared to ETV (IC50: 0.026 versus 0.003 nM). In contrast, the 4'-cyano of CdFA-TP forms good nonpolar contacts with RTWT's Leu180 and RTETVR's Met180, while ETV-TP loses interactions with RTETVR's Met180, explaining in part why ETV is less potent against HBVETVR than CdFA. The present results show that CdFA exerts potent activity against HBVWT, HBVETVR and HBVADVR with enhanced safety and that 7-deaza-7-fluoro modification confers potent activity against drug-resistant HBV variants and favorable safety, shedding light to further design more potent and safer anti-HBV nucleoside analogs.