Nucleosides for the treatment of respiratory RNA virus infections

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

The recent Ebola virus outbreak of 2013-2016 in West Africa triggered increased efforts to identify new antivirals targeting filoviruses. As a result, the development of a new series of C-linked nucleoside analogs with anti-Ebola properties was soon reported. 162 In a cellbased infectious assay, the 1 0 -cyano C-linked adenosine derivative (GS-441524, or compound 4) was moderately active against Ebola replication with EC 50 values around 1.5 mM, whereas the 1 0 -methyl and -ethynyl counterparts were completely inactive. GS-441524 is also a broad-spectrum inhibitor of a variety of RNA viruses from four families (Filoviridae, Flaviviridae, Paramyxoviridae, and Pneumoviridae), including HCV and RSV. 163, 164 However, the addition of a 2 0 -C-methyl group, as in the case of the GS-6620, 165, 166 significantly reduces the antiviral spectrum to HCV only. The relatively weak antiviral activity of GS-441524 across all viruses (0.5-50 mM EC 50 ) was attributed to its inefficient intracellular phosphorylation, which could be improved by adding a monophosphate prodrug to the parent nucleoside. The resulting compound, GS-5734 ( Figure 5) inhibits the Zaire and Sudan species of Ebola virus and Marburg virus with EC 50 values ranging from 0.01 to 0.20 mM, and exhibits moderate cytotoxicity (CC 50 ¼ 2 to >20 mM) in multiple human cell types. GS-5734 exhibits the same broad antiviral spectrum as its parent molecule. 163 The triphosphate form of GS-5734 is recognized as substrate by RSV polymerase, but its incorporation into RNA does not lead to immediate chain termination. 162 The favorable in vitro data led to further evaluation of GS-5734 in a macaque lethal model of Ebola virus disease. Complete protection was achieved when GS-5734 was administered at a daily intravenous dose of 10 mg/kg, beginning on Day 3 post-infection. 162 Following Phase I safety testing in healthy human volunteers, GS-5724 was first given as a 14-day course for compassionate use to an Ebola-infected nurse who had survived the disease and developed a recurrence in the central nervous system. 167 Soon after, a neonate who had congenital Ebola virus infection received three different experimental therapies, including a 12-day treatment with GS-5734. 168 In both cases, patients cleared the virus and survived the infection.