Low Selectivity Indices of Ivermectin and Macrocyclic Lactones on SARS-CoV-2 Replication In Vitro

Ivermectin was first approved for human use as an endectocide in the 1980s. It remains one of the most important global health medicines in history and has recently been shown to exert in vitro activity against SARS-CoV-2. However, the macrocyclic lactone family of compounds has not previously been evaluated for activity against SARS-CoV-2. The present study aims at comparing their anti-viral activity in relevant human pulmonary cell lines in vitro. Here, in vitro antiviral activity of the avermectins (ivermectin and selamectin) and milbemycins (moxidectin and milbemycin oxime) were assessed against a clinical isolate from a CHU Montpellier patient infected with SARS-CoV-2 in 2020. Ivermectin, like the other macrocyclic lactones moxidectin, milbemycin oxime and selamectin, reduced SARS-CoV-2 replication in vitro (EC50 of 2–5 μM). Immunofluorescence assays with ivermectin and moxidectin showed a reduction in the number of infected and polynuclear cells, suggesting a drug action on viral cell fusion. However, cellular toxicity of the avermectins and milbemycins during infection showed a very low selectivity index of <10. Thus, none of these agents appears suitable for human use for its anti-SARS-CoV-2 activity per se, due to low selectivity index.