Nucleosides for the treatment of respiratory RNA virus infections

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

The characterization of the broad antiviral spectrum of GS-5734 was further expanded to another (þ) ssRNA virus family: Coronaviridae. It was shown that GS-5734 inhibits SARS-CoV and MERS-CoV replication in multiple in vitro systems, including primary human airway epithelial cell cultures with sub-micromolar EC 50 values. 169 GS-5734 was also effective against other human and bat CoV. In a mouse model of SARS-CoV infection, prophylactic and early therapeutic administration of GS-5734 reduced lung viral load and improved clinical signs of disease as well as respiratory function. Although there is limited data to confirm the proposed mechanism of action of GS-5734 against each virus, it is generally assumed that the molecule targets the RdRp function of the viral polymerase. In the case of CoV, this is supported by the identification of two mutations (F476L and V553L) within the predicted fingers subdomain of the RdRp protein nsp12 from murine hepatitis virus. 170 These mutations emerged over 23 passages and confer 4-to 6-fold resistance to GS-5734, combined with overall reduced replication fitness. At this point, the precise mechanism of action of GS-5734 against CoV remains elusive. It is possible that GS-5734 triphosphate is not excised by the proofreading activity of nsp14 because of lack of immediate chain termination, as observed for RSV polymerase. In this case, could the resistance mutations identified in nsp12 alter the chain termination profile of GS-5734, and make it more susceptible to excision? Such studies are needed, not only to understand how GS-5734 works but also to design new molecules against CoV polymerases.