Mechanism of action against SARS-CoV-2
The antiviral activity of niclosamide against SARS-CoV-2 is complex and involves multiple cellular processes as illustrated in Figure 1. SARS-CoV-2 uses the angiotensin converting enzyme-2 (ACE-2) as a cellular entry receptor in permissive cells of the respiratory tract and the spike proteins initiate the merging of the viral envelope with the host cell cytomembrane (Zhou et al., 2020). Following receptor binding and conformational changes in the spike protein, cathepsin L mediates proteolysis within endosomes leading to viral entry into host cells (Beniac, Andonov, Grudeski, & Booth, 2006). The protonophoric activity of niclosamide that causes endosomal neutralization can also interfere with viral entry and prevent viral genome release into the cytosol thus further limiting SARS-CoV-2 replication (Jurgeit et al., 2012). Garret et al., (2021) recently demonstrated that the total lipid profile is amplified during SARS-CoV-2 infection in VeroE6 cells and treatment with niclosamide led to a reduction in lipids available for virus production. Additionally, in primary human lung cells and intestinal organoids niclosamide enhances autophagy, thus further attenuating SARS-CoV-2 replication (Nils C. Gassen et al., 2021).
Syncytia formation in SARS-CoV-2 infected pneumocytes has been observed in COVID-19 lungs. To identify inhibitors of spike-driven syncytia formation, a high-content microscopy-based screening of more than 3,000 compounds was conducted (L. Braga et al., 2021). The screen identified efficacious drugs that inhibited viral replication, with one of the most potent being niclosamide. Niclosamide also has potent bronchodilatory effects, inhibits excessive mucus production and down-regulates the release of pro-inflammatory cytokines such as IL-8 by inhibiting TMEM16A (Cabrita, Benedetto, Schreiber, & Kunzelmann, 2019). Due to its effects on intracellular calcium levels, niclosamide can inhibit other cytokines and could therefore play an important role in controlling the cytokine storm and acute respiratory distress syndrome (ARDS) in acutely ill COVID-19 patients. The above studies show several plausible mechanisms of action of niclosamide against COVID-19, including prevention of viral entry, prevention of viral replication via autophagy inhibition and finally, inhibition of spike-driven syncytia formation. In conclusion, these studies have confirmed potent, multi-faceted and pleiotropic activity of niclosamide against SARS-CoV-2, targeting multiple aspects of the viral life cycle.