Antiviral activity of nucleoside analogues against norovirus

Background: Norovirus (NoV) is the leading cause of epidemic gastroenteritis worldwide. The lack of a cell culture has significantly hampered the development of effective therapies against human NoV. Clinically approved nucleoside and non-nucleoside analogues have been used successfully against RNA viruses. Methods: In this study, we evaluated the efficacy of four nucleoside analogues (2 '-C-MeC, 2 '-F-2 '-C-MeC, beta-DN(4)-hydroxycytidine [NHC] and lamivudine) on Norwalk virus (NV) RNA levels and protein expression in NV repliconharbouring cells (HG23 cells), and their efficacy in blocking murine norovirus (MNV) replication in RAW 264.7 cells. Results: 2 '-C-MeC and 2 '-F-2 '-C-MeC reduced MNV RNA levels and infectivity in RAW 264.7 cells in a concentration- and time-dependent manner. The median effective concentrations (EC50) of 2 '-C-MeC and 2 '-F-2 '-C-MeC were 6.9 mu M and 12.7 mu M, respectively. 2 '-C-MeC, 2 '-F-2 '-C-MeC and NHC reduced NV RNA levels and protein expression in HG23 cells. For the NV replicon, the EC50 of 2'-C-MeC (1.3 mu M) was comparable to the antiviral activity of NHC (1.5 mu M) and twofold more potent than 2 '-F-2 '-C-MeC (3.2 mu M). The combination of 2 '-C-MeC/ ribavirin resulted in modest synergistic activity, whereas NHC/ribavirin was antagonistic for NV replication in HG23 cells. Conclusions: The antiviral activity of 2'-C-MeC against strains of two different NoV genogroups and the low EC50 suggest that this nucleoside analogue may be effective against the more prevalent GII NoVs. In the absence of a vaccine, antiviral agents could be an effective intervention to control the spread of human NoV in populations at a high risk for NoV disease.