Clinical Antiviral Drug Arbidol Inhibits Infection by SARS-CoV-2 and Variants through Direct Binding to the Spike Protein

Arbidol (ARB) is a broad-spectrum antiviral drug approved in Russia and China for the treatment of influenza. ARB was tested in patients as a drug candidate for the treatment at the early onset of COVID-19 caused by the novel severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Despite promising clinical results and multiple ongoing trials, preclinical data are lacking and the molecular mechanism of action of ARB against SARS-CoV-2 remains unknown. Here, we demonstrate that ARB binds to the spike viral fusion glycoprotein of the SARS-CoV-2 Wuhan strain as well as its more virulent variants from the United Kingdom (strain B.1.1.7) and South Africa (strain B.1.351). We pinpoint the ARB binding site on the S protein to the S2 membrane fusion domain and use an infection assay with Moloney murine leukemia virus (MLV) pseudoviruses (PVs) pseudotyped with the S proteins of the Wuhan strain and the new variants to show that this interaction is sufficient for the viral cell entry inhibition by ARB. Finally, our experiments reveal that the ARB interaction leads to a significant destabilization and eventual lysosomal degradation of the S protein in cells. Collectively, our results identify ARB as the first clinically approved small molecule drug binder of the SARS-CoV-2 S protein and place ARB among the more promising drug candidates for COVID-19.