Niclosamide and ivermectin modulate caspase-1 activity and proinflammatory cytokine secretion in a monocytic cell line

The COVID-19 pandemic has led to an unprecedented demand for new and repurposed therapeutics to ameliorate the morbidity and mortality associated with SARS-CoV-2 infection. However, there is still a paucity of information relating to successful antiviral compounds. The repurposing of immune modulators, such as dexamethasone and tocilizumab, has shown significant improvement in survival rates. Repurposing of small molecules that may have antiviral and immunomodulatory potential may have significant impact on the pandemic. Niclosamide and ivermectin are being investigated for repurposing as potential treatments for COVID-19 patients. Both niclosamide and ivermectin have been proposed and studied based upon possible immunomodulatory and antiviral activity. To improve their posology, there are also ongoing efforts to nano-formulate these drugs, but a much greater understanding of their mechanisms of action is required to rationalise their plausibility as candidates. We have previously shown that niclosamide can affect responses to immune stimulation in ex vivo cells from healthy rats exposed via a long-acting injectable formulation. The current study aimed to further understand the effects of niclosamide and ivermectin on inflammasome activity in human cells due to the involvement of inflammasomes in the hyperinflammation and coagulation observed in severely ill COVID-19 patients. Caspase-1 activity and proinflammatory cytokine secretion in THP1 cells exposed to physiologically-relevant concentrations of niclosamide and ivermectin were measured as markers of inflammasome activity. Exposure to both niclosamide and ivermectin led to lower caspase-1 activity compared to untreated cells as well as resulting in lower secretion of IL-1β, IL-18, and TNF-α if treated prior to LPS induction. These data in their own right should not be interpreted as being a conclusive indicator of the utility of these drugs in COVID-19. However, the data presented suggests a putative mechanism for the proposed immune modulation. Substantive further work is still needed to determine the precise mechanism(s) that underpin these findings and whether the observations are relevant in vivo.