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 M, 4.23 M, 9.29 M, 3.19 M and 84.31 M respectively. Furthermore, Erythromycin, Retapamulin, Pyridoxine, Folic acid and Ivermectin dose dependently inhibit SARS-CoV-2 Papain-like Protease with IC50 of 0.94 M, 0.88 M, 1.14 M, 1.07 M, 1.51 M respectively and the main protease(MPRO) with IC50 of 1.35 M, 1.25 M, 7.36 M, 1.15 M and 2.44 M 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.