2'-Fluoro-6'-methylene Carbocyclic Adenosine and Its Phosphoramidate Prodrug: A Novel Anti-HBV Agent, Active Against Drug-resistant HBV Mutants

Chronic hepatitis B (CHB) is one of the major causes of morbidity and mortality worldwide. Currently, clinically approved nucleos(t)ide analogs (NAs) are very efficient in reducing the load of hepatitis B virus (HBV) with minimum side effects. However, the long-term administration of antiviral drugs promotes HBV for potential drug resistance. To overcome this problem, combination therapies are administered, but HBV progressively altered mutations remain a threat. Therefore, optimally designed NAs are urgently needed to treat drug-resistant HBV. Herein, 2'-fluoro-6'-methylene carbocyclic adenosine (FMCA) and its phosphoramidate (FMCAP) have been discovered, which may be utilized in combination therapies for curing drug-resistant chronic hepatitis B. In preclinical studies, these carbocyclic NAs demonstrated potential anti-HBV activity against adefovir, as well as lamivudine (LMV/LAM) drug-resistant mutants. In vitro, these molecules have demonstrated significant activity against LMV/entecavir (ETV) triple mutants (L180M + S202G + M204V). Also, preliminary studies of FMCA/FMCAP in chimeric mice and female Non-obese diabetic/severe combined immunodeficiency (NOD/SCID) mouse models having the LMV/ETV triple mutant have shown a high rate of reduction of HBV DNA levels compared to ETV. In this review, we have summarized preclinical studies of FMCA and its phosphoramidate prodrug (FMCAP).