4.2. Nitazoxanide (Alinia)
It is a nitro-thiazolyl-salicylamide derivative (Figure 7) with a broad-spectrum antimicrobial activity. The drug is effective against various helminthic, protozoal, bacterial, and viral infections. It was approved by the U.S. FDA in 2002 to be used orally for the treatment of diarrhea caused by Cryptosporidium parvum or Giardia lamblia [133,134]. It is also off-label used for cryptosporidiosis-associated diarrhea in HIV-infected patients. The drug has been reported as a broad-spectrum antiviral agent that inhibits the replication of several RNA and DNA viruses. Specifically, nitazoxanide has been found to affect influenza A and B viruses as well as neuraminidase inhibitors-resistant influenza viruses. It has also been found to inhibit the replication of coronavirus, dengue virus, HBV, HCV, HIV, Japanese encephalitis virus, norovirus, parainfluenza, rotavirus, respiratory syncytial virus, and yellow fever in cell culture assays [135]. Some of its activity is also potentially attributed to its active metabolite (tizoxanide), desacetyl-nitazoxanide [136]. Currently, nitazoxanide is being studied alone or in combination with hydroxychloroquine, ivermectin, ribavirin/ivermectin, or atazanavir/ritonavir for the prevention/treatment of COVID-19 in about 17 clinical trials (for details refer to clinicaltrials.gov).
The drug is associated with several mechanisms. The anti-protozoal activity demonstrated by this drug is promoted by the inhibition of pyruvate:ferredoxin oxidoreductase enzyme-dependent electron transfer reaction [137]. However, in the case of influenza, nitazoxanide and its metabolite inhibit the viral hemagglutinin maturation at the post-translational phase with no effect on the M2 protein or on the neuraminidase glycoprotein [138]. Furthermore, nitazoxanide modulates other targets and pathways in vitro including glutamate-gated chloride ion channels and glutathione-S-transferase in nematodes, respiration and other pathways in bacterial and cancer cells, and viral and host transcriptional factors [134]. In fact, nitazoxanide was shown to in vitro inhibit the replication of coronaviruses, including MERS-CoV in cells and the expression of the viral N protein [136,139]. The drug was also reported as a non-competitive inhibitor of thiol oxidoreductase ERp57 and thus demonstrated anti-paramyxovirus activity [140]. Moreover, nitazoxanide was reported to inhibit the production of pro-inflammatory cytokines in peripheral blood mononuclear cells and animal models [136]. In peripheral blood mononuclear cells exposed to influenza virus, nitazoxanide potentiated the release of INF-α and INF-β by fibroblasts [135]. In addition, nitazoxanide appears also to act as a bronchodilator in testing models by blocking the calcium-activated chloride channel TMEM16A [141].