Nucleosides for the treatment of respiratory RNA virus infections

Jordan, Paul C; Stevens, Sarah K; Deval, Jerome

2 0 -Deoxy-2 0 -fluoro nucleosides for influenza Fluorinated nucleosides are well known for their antiviral and anticancer properties (for review 126 ). In particular, 2 0 -deoxy-2 0 -fluoro guanosine (2 0 FdG) was at one time considered a potential candidate for influenza treatment ( Figure 5 ). In vitro, 2 0 FdG inhibits influenza A virus replication with an EC 50 of about 20 mM, without causing apparent cell toxicity. 127 In ferrets, treatment with 2 0 FdG at 20 mg/kg starting 1 h postinfection significantly reduced H3N2 influenza A virus titers in nasal washes, associated with reduction in fever and inflammation. 128 Although time-of-addition experiments suggested that the molecule inhibits an early step of virus replication, more direct evidence for the mechanism of action came from enzyme inhibition studies. 129 In cell-free transcription experiments, 2 0 FdG triphosphate inhibited influenza A virus RNA polymerase activity by competing with natural GTP. The inhibition of the enzyme was caused by the incorporation of 2 0 FdG monophosphate into the viral RNA. 129 More recently, the related nucleoside analog 2 0 -deoxy-2 0 -fluoro cytidine (2 0 FdC) was evaluated against the highly pathogenic H5N1 and the pandemic H1N1 strains. 130 When administered intraperitoneally, 2 0 FdC significantly enhanced survival of BALB/c mice infected with a lethal dose of either H5N1 or H1N1 viruses. Although these studies show compelling evidence of in vivo efficacy in preclinical species, 2 0 FdG and 2 0 FdC are not suitable candidates for clinical development. One of the main limitations of these molecules is their lack of specificity for influenza virus polymerase. The ability of a nucleotide to inhibit distant molecular targets is not detrimental per se. As such, 2 0 -deoxy-2 0 -fluoro nucleotides and their derivatives interact with the RNA polymerase of HCV. [131] [132] [133] [134] But the substitution of the 2 0 -hydroxy by a fluoro group also makes the resulting nucleotides broad substrates for viral 135 and human 136 DNA polymerases. In the latter study, the authors have shown that the monophosphate form of both 2 0 FdC and 2 0 FdG can be incorporated into DNA by human DNA polymerase alpha and gamma. This might explain the changes in cell cycle distribution and cytostatic effect caused by prolonged in vitro incubation with 2 0 FdC. 133 Paradoxically, the same molecule was well tolerated when administered intravenously to rats and woodchucks for up to 90 days. 137 One hypothesis for this discrepancy is that a low organ exposure of the phosphorylated metabolite(s) of 2 0 FdC could limit the toxic effect on dividing cells in these animals.