Erythromycin, retapamulin, pyridoxine, folic acid, and ivermectin inhibit cytopathic effect, papain-like protease, and M^PRO enzymes of SARS-CoV-2

Background: Although tremendous success has been achieved in the development and deployment of effective COVID-19 vaccines, developing effective therapeutics for the treatment of those who do come down with the disease has been with limited success. To repurpose existing drugs for COVID-19, we previously showed, qualitatively, that erythromycin, retapamulin, pyridoxine, folic acid, and ivermectin inhibit SARS-COV-2-induced cytopathic effect (CPE) in Vero cells.Aim: This study aimed to quantitatively explore the inhibition of SARS-CoV-2-induced CPE by erythromycin, retapamulin, pyridoxine, folic acid, and ivermectin and to determine the effect of these drugs on SARS-CoV-2 papain-like protease and 3CL protease (M-PRO) enzymes.Methods: Neutral red (3-amino-7-dimethylamino-2-methyl-phenazine hydrochloride) cell viability assay was used to quantify CPE after infecting pre-treated Vero cells with clinical SARS-Cov-2 isolates. Furthermore, SensoLyte (R) 520 SARS-CoV-2 papain-like protease and SensoLyte (R) 520 SARS-CoV-2 M-PRO activity assay kits were used to evaluate the inhibitory activity of the drugs on the respective enzymes.Results: Erythromycin, retapamulin, pyridoxine, folic acid, and ivermectin dose-dependently inhibit SARS-CoV-2-induced CPE in Vero cells, with inhibitory concentration-50 (IC50) values of 3.27 mu M, 4.23 mu M, 9.29 mu M, 3.19 mu M, and 84.31 mu M, respectively. Furthermore, erythromycin, retapamulin, pyridoxine, folic acid, and ivermectin dose-dependently inhibited SARS-CoV-2 papain-like protease with IC50 values of 0.94 mu M, 0.88 mu M, 1.14 mu M, 1.07 mu M, and 1.51 mu M, respectively, and inhibited the main protease (M-PRO) with IC50 values of 1.35 mu M, 1.25 mu M, 7.36 mu M, 1.15 mu M, and 2.44 mu M, respectively.Conclusion: The IC50 for all the drugs, except ivermectin, was at the clinically achievable plasma concentration in humans, 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.