Repurposing of US-FDA approved drugs against SARS-CoV-2 main protease (M(pro)) by using STD-NMR spectroscopy, in silico studies and antiviral assays

SARS-CoV-2 Main protease (M(pro)) is a well-known drug target against SARS-CoV-2 infection. Identification of M(pro) inhibitors is vigorously pursued due to its crucial role in viral replication. The present study was aimed to identify M(pro) inhibitors via repurposing of US-FDA approved drugs by STD-NMR spectroscopy. In this study, 156 drugs and natural compounds were evaluated against M(pro). Among them, 10 drugs were found to be interacting with M(pro), including diltiazem HCl (1), mefenamic acid (2), losartan potassium (3), mexiletine HCl (4), glaucine HBr (5), trimebutine maleate (6), flurbiprofen (7), amantadine HCl (8), dextromethorphan (9), and lobeline HCl (10) in STD-NMR spectroscopy. Their interactions were compared with three standards (Repurposed anti-viral drugs), dexamethasone, chloroquine phosphate, and remdesivir. Thermal stability of M(pro) and dissociation constant (K(d)) of six interacting drugs were also determined using DSF. RMSD plots in MD simulation studies showed the formation of stable protein-ligand complexes. They were further examined for their antiviral activity by plaque reduction assay against SARS-CoV-2, which showed 55–100% reduction in viral plaques. This study demonstrates the importance of drug repurposing against emerging and neglected diseases. This study also exhibits successful application of STD-NMR spectroscopy combined with plaque reduction assay in rapid identification of potential anti-viral agents.