Antiviral therapies that impede virus entry are attractive because they act on the first phase of the infectious cycle. Drugs that target pathways common to multiple viruses are particularly desirable when laboratory-based viral identification may be challenging, e.g., in an outbreak setting. We are interested in identifying drugs that block both Ebola virus (EBOV) and Lassa virus (LASV), two unrelated but highly pathogenic hemorrhagic fever viruses that have caused outbreaks in similar regions in Africa and share features of virus entry: use of cell surface attachment factors, macropinocytosis, endosomal receptors, and low pH to trigger fusion in late endosomes. Toward this goal, we directly compared the potency of eight drugs known to block EBOV entry with their potency as inhibitors of LASV entry. Five drugs (amodiaquine, apilimod, arbidol, niclosamide, and zoniporide) showed roughly equivalent degrees of inhibition of LASV and EBOV glycoprotein (GP)-bearing pseudoviruses; three (clomiphene, sertraline, and toremifene) were more potent against EBOV. We then focused on arbidol, which is licensed abroad as an anti-influenza drug and exhibits activity against a diverse array of clinically relevant viruses. We found that arbidol inhibits infection by authentic LASV, inhibits LASV GP-mediated cell-cell fusion and virus-cell fusion, and, reminiscent of its activity on influenza virus hemagglutinin, stabilizes LASV GP to low-pH exposure. Our findings suggest that arbidol inhibits LASV fusion, which may partly involve blocking conformational changes in LASV GP. We discuss our findings in terms of the potential to develop a drug cocktail that could inhibit both LASV and EBOV. IMPORTANCE Lassa and Ebola viruses continue to cause severe outbreaks in humans, yet there are only limited therapeutic options to treat the deadly hemorrhagic fever diseases they cause. Because of overlapping geographic occurrences and similarities in mode of entry into cells, we seek a practical drug or drug cocktail that could be used to treat infections by both viruses. Toward this goal, we directly compared eight drugs, approved or in clinical testing, for the ability to block entry mediated by the glycoproteins of both viruses. We identified five drugs with approximately equal potencies against both. Among these, we investigated the modes of action of arbidol, a drug licensed abroad to treat influenza infections. We found, as shown for influenza virus, that arbidol blocks fusion mediated by the Lassa virus glycoprotein. Our findings encourage the development of a combination of approved drugs to treat both Lassa and Ebola virus diseases. FIG 2 Comparative effects of arbidol on infection by MLV pseudoviruses bearing LASV or other viral glycoproteins: LASV, LCMV, and Junin GP (A) and LASV GP and influenza virus HA (B). MLV pseudoviruses bearing LASV GP, LCMV GP, Junin GP, or WSN influenza HA and NA were prepared as described in Materials and Methods. BSC-1 cells were pretreated with the indicated concentrations of arbidol and then processed an...

Ebolaviruses continue to inflict horrific disease and instill fear. The 2013–2016 outbreak in Western Africa caused unfathomable morbidity and mortality (over 11,000 deaths), and the second largest outbreak is on-going in the Democratic Republic of the Congo. The first stage of an Ebolavirus infection is entry, culminating in delivery of the viral genome into the cytoplasm to initiate replication. Among enveloped viruses, Ebolaviruses use a complex entry pathway: they bind to attachment factors on cell surfaces, are engulfed by macropinocytosis, and traffic through the endosomal system. En route , the receptor binding subunit of the glycoprotein (GP) is reduced from ~130 to ~19 kDa by cathepsins. This event allows cleaved GP (GP cl ) to bind to Niemann-Pick C1 (NPC1), its endosomal receptor. The virus then fuses with a late endosomal membrane, but how this occurs remains a subject of debate. An early, but standing, observation is that entry of particles bearing GP cl is inhibited by agents that raise endosomal pH or inhibit cysteine proteases, suggesting the need for an additional factor(s). Yet, some have concluded that NPC1 is sufficient to trigger the fusion activity of GP cl . Here, we re-examined this question using sensitive cell-cell and pseudovirus-cell fusion assays. We did not observe detectable GP cl -mediated fusion with NPC1 or its GP cl binding domain at any pH tested, while robust fusion was consistently observed with GP from lymphocytic choriomeningitis virus at low pH. Addition of proposed fusion-enhancing factors—cations (Ca ++ and K + ), a reducing agent, the anionic lipid Bis(Monoacylglycero)Phosphate, and a mixture of cathepsins B and L—did not induce detectable fusion. Our findings are in line with the earlier proposal that an additional factor is required to trigger the full fusion activity of GP cl after binding to NPC1. We discuss caveats to our study and what the missing factor(s) might be.