Summary The most recent Ebola virus outbreak in West Africa – unprecedented in the number of cases and fatalities, geographic distribution, and number of nations affected – highlights the need for safe, effective, and readily available antiviral agents for treatment and prevention of acute Ebola virus (EBOV) disease (EVD) or sequelae1. No antiviral therapeutics have yet received regulatory approval or demonstrated clinical efficacy. Here we describe the discovery of a novel anti-EBOV small molecule antiviral, GS-5734, a monophosphoramidate prodrug of an adenosine analog. GS-5734 exhibits antiviral activity against multiple variants of EBOV in cell-based assays. The pharmacologically active nucleoside triphosphate (NTP) is efficiently formed in multiple human cell types incubated with GS-5734 in vitro, and the NTP acts as an alternate substrate and RNA-chain terminator in primer-extension assays utilizing a surrogate respiratory syncytial virus RNA polymerase. Intravenous administration of GS-5734 to nonhuman primates resulted in persistent NTP levels in peripheral blood mononuclear cells (half-life = 14 h) and distribution to sanctuary sites for viral replication including testes, eye, and brain. In a rhesus monkey model of EVD, once daily intravenous administration of 10 mg/kg GS-5734 for 12 days resulted in profound suppression of EBOV replication and protected 100% of EBOV-infected animals against lethal disease, ameliorating clinical disease signs and pathophysiological markers, even when treatments were initiated three days after virus exposure when systemic viral RNA was detected in two of six treated animals. These results provide the first substantive, post-exposure protection by a small-molecule antiviral compound against EBOV in nonhuman primates. The broad-spectrum antiviral activity of GS-5734 in vitro against other pathogenic RNA viruses – including filoviruses, arenaviruses, and coronaviruses – suggests the potential for expanded indications. GS-5734 is amenable to large-scale manufacturing, and clinical studies investigating the drug safety and pharmacokinetics are ongoing.

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Bone marrow stromal antigen 2 (BST-2/tetherin) is a cellular membrane protein that inhibits the release of HIV-1. We show for the first time, using infectious viruses, that BST-2 also inhibits egress of arenaviruses but has no effect on filovirus replication and spread. Specifically, infectious Lassa virus (LASV) release significantly decreased or increased in human cells in which BST-2 was either stably expressed or knocked down, respectively. In contrast, replication and spread of infectious Zaire ebolavirus (ZEBOV) and Lake Victoria marburgvirus (MARV) were not affected by these conditions. Replication of infectious Rift Valley fever virus (RVFV) and cowpox virus (

e We show that interferon-induced transmembrane protein 1 (IFITM-1), IFITM-2, and IFITM-3 exhibit a broad spectrum of antiviral activity against several members of the Bunyaviridae family, including Rift Valley fever virus (RVFV), La Crosse virus, Andes virus, and Hantaan virus, all of which can cause severe disease in humans and animals. We found that RVFV was restricted by IFITM-2 and -3 but not by IFITM-1, whereas the remaining viruses were equally restricted by all IFITMs. Indeed, at low doses of alpha interferon (IFN-␣), IFITM-2 and -3 mediated more than half of the antiviral activity of IFN-␣ against RVFV. IFITM-2 and -3 restricted RVFV infection mostly by preventing virus membrane fusion with endosomes, while they had no effect on virion attachment to cells, endocytosis, or viral replication kinetics. We found that large fractions of IFITM-2 and IFITM-3 occupy vesicular compartments that are distinct from the vesicles coated by IFITM-1. In addition, although overexpression of all IFITMs expanded vesicular and acidified compartments within cells, there were marked phenotypic differences among the vesicular compartments occupied by IFITMs. Collectively, our data provide new insights into the possible mechanisms by which the IFITM family members restrict distinct viruses.