Simple rapid in vitro screening method for SARS-CoV-2 anti-virals that identifies potential cytomorbidity-associated false positives

The international SARS-CoV-2 pandemic has resulted in an urgent need to identify new anti-viral drugs for treatment of COVID-19 patients. The initial step to identifying potential candidates usually involves in vitro screening. Here we describe a simple rapid bioassay for drug screening using Vero E6 cells and inhibition of cytopathic effects (CPE) measured using crystal violet staining. The assay clearly illustrated the anti-viral activity of remdesivir, a drug known to inhibit SARS-CoV-2 replication. A key refinement involves a simple growth assay to identify drug concentrations that cause cellular stress or "cytomorbidity", as distinct from cytotoxicity or loss of viability. For instance, hydroxychloroquine shows anti-viral activity at concentrations that slow cell growth, arguing that its purported in vitro anti-viral activity arises from non-specific impairment of cellular activities.