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Middle East respiratory syndrome coronavirus (MERS-CoV) is the causative agent of a severe respiratory disease associated with more than 2468 human infections and over 851 deaths in 27 countries since 2012. There are no approved treatments for MERS-CoV infection although a combination of lopinavir, ritonavir and interferon beta (LPV/RTV-IFNb) is currently being evaluated in humans in the Kingdom of Saudi Arabia. Here, we show that remdesivir (RDV) and IFNb have superior antiviral activity to LPV and RTV in vitro. In mice, both prophylactic and therapeutic RDV improve pulmonary function and reduce lung viral loads and severe lung pathology. In contrast, prophylactic LPV/RTV-IFNb slightly reduces viral loads without impacting other disease parameters. Therapeutic LPV/RTV-IFNb improves pulmonary function but does not reduce virus replication or severe lung pathology. Thus, we provide in vivo evidence of the potential for RDV to treat MERS-CoV infections. Remdesivir (RDV) is a broad-spectrum antiviral drug with activity against MERS coronavirus, but in vivo efficacy has not been evaluated. Here, the authors show that RDV has superior anti-MERS activity in vitro and in vivo compared to combination therapy with lopinavir, ritonavir and interferon beta and reduces severe lung pathology. [ABSTRACT FROM AUTHOR]

Antiviral drugs for managing infections with human coronaviruses are not yet approved, posing a serious challenge to current global efforts aimed at containing the outbreak of severe acute respiratory syndrome-coronavirus 2 (CoV-2). Remdesivir (RDV) is an investigational compound with a broad spectrum of antiviral activities against RNA viruses, including severe acute respiratory syndrome-CoV and Middle East respiratory syndrome (MERS-CoV).RDVis a nucleotide analog inhibitor of RNA-dependent RNA polymerases (RdRps). Here, we co-expressed the MERS-CoV nonstructural proteins nsp5, nsp7, nsp8, and nsp12 (RdRp) in insect cells as a part a polyprotein to study the mechanism of inhibition of MERS-CoV RdRp by RDV. We initially demonstrated that nsp8 and nsp12 form an active complex. The triphosphate form of the inhibitor (RDVTP) competes with its natural counterpart ATP. Of note, the selectivity value for RDV-TP obtained here with a steady-state approach suggests that it is more efficiently incorporated than ATP and two other nucleotide analogs. Once incorporated at position i, the inhibitor caused RNA synthesis arrest at position i + 3. Hence, the likely mechanism of action is delayed RNA chain termination. The additional three nucleotides may protect the inhibitor from excision by the viral 3'-5' exonuclease activity. Together, these results help to explain the high potency of RDV against RNA viruses in cell-based assays. [ABSTRACT FROM AUTHOR]

Effective treatments for coronavirus disease 2019 (COVID- 19) are urgently needed to control this current pandemic, caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Replication of SARS-CoV-2 depends on the viral RNA-dependent RNA polymerase (RdRp), which is the likely target of the investigational nucleotide analogue remdesivir (RDV). RDV shows broad-spectrum antiviral activity against RNA viruses, and previous studies with RdRps from Ebola virus and Middle East respiratory syndrome coronavirus (MERSCoV) have revealed that delayed chain termination is RDV's plausible mechanism of action. Here, we expressed and purified active SARS-CoV-2 RdRp composed of the nonstructural proteins nsp8 and nsp12. Enzyme kinetics indicated that this RdRp efficiently incorporates the active triphosphate form of RDV (RDV-TP) into RNA. Incorporation of RDV-TP at position i caused termination of RNA synthesis at position i+3. We obtained almost identical results with SARS-CoV, MERS-CoV, and SARS-CoV-2 RdRps. A unique property of RDV-TP is its high selectivity over incorporation of its natural nucleotide counterpart ATP. In this regard, the triphosphate forms of 2'-Cmethylated compounds, including sofosbuvir, approved for the management of hepatitisCvirus infection, and the broad-acting antivirals favipiravir and ribavirin, exhibited significant deficits. Furthermore, we provide evidence for the target specificity of RDV, as RDV-TP was less efficiently incorporated by the distantly related Lassa virus RdRp, and termination ofRNAsynthesis was not observed. These results collectively provide a unifying, refined mechanism of RDV-mediated RNA synthesis inhibition in coronaviruses and define this nucleotide analogue as a direct-acting antiviral. [ABSTRACT FROM AUTHOR]