Screening and evaluation of approved drugs as inhibitors of main protease of SARS-CoV-2

The COVID-19 pandemic caused by SARS-CoV-2 has emerged as a global catastrophe. The virus requires main protease for processing the viral polyproteins PP1A and PP1AB translated from the viral RNA. In search of a quick, safe and successful therapeutic agent; we screened various clinically approved drugs for the in-vitro inhibitory effect on 3CL^{Pro} which may be able to halt virus replication. The methods used includes protease activity assay, fluorescence quenching, surface plasmon resonance (SPR), Thermofluor® Assay, Size exclusion chromatography and in-silico docking studies. We found that Teicoplanin as most effective drug with IC_{50} ~ 1.5 μM. Additionally, through fluorescence quenching Stern-Volmer quenching constant (K_{SV}) for Teicoplanin was estimated as 2.5 × 10^{5} L·mol^{-1}, which suggests a relatively high affinity between Teicoplanin and 3CL^{Pro} protease. The SPR shows good interaction between Teicoplanin and 3CL^{Pro} with K_{D} ~ 1.6 μM. Our results provide critical insights into the mechanism of action of Teicoplanin as a potential therapeutic against COVID-19. We found that Teicoplanin is about 10-20 fold more potent in inhibiting protease activity than other drugs in use, such as lopinavir, hydroxychloroquine, chloroquine, azithromycin, atazanavir etc. Therefore, Teicoplanin emerged as the best inhibitor among all drug molecules we screened against 3CL^{Pro} of SARS-CoV-2.