Erythromycin, Retapamulin, Pyridoxine, Folic acid and Ivermectin dose dependently inhibit cytopathic effect, Papain-like Protease and MPRO of SARS-CoV-2

We previously showed that Erythromycin, Retapamulin, Pyridoxine, Folic acid and Ivermectin inhibit SARS-COV-2 induced cytopathic effect (CPE) in Vero cells. In this study and using validated quantitative neutral red assay, we show that the inhibition of CPE is concentration dependent with Inhibitory Concentration-50(IC50) of 3.27 μM, 4.23 μM, 9.29 μM, 3.19 μM and 84.31 μM respectively. Furthermore, Erythromycin, Retapamulin, Pyridoxine, Folic acid and Ivermectin dose dependently inhibit SARS-CoV-2 Papain-like Protease with IC50 of 0.94 μM, 0.88 μM, 1.14 μM, 1.07 μM, 1.51 μM respectively and the main protease(MPRO) with IC50 of 1.35 μM, 1.25 μM, 7.36 μM, 1.15 μM and 2.44 μM respectively. The IC50 for all the drugs, except ivermectin, are at the clinically achievable plasma concentration in human, which supports a possible role for the drugs in the management of COVID-19. The lack of inhibition of CPE by Ivermectin at clinical concentrations could be part of the explanation for its lack of effectiveness in clinical trials.