Synthesis and biological evaluation of glutathione-responsive 2-alkoxycarbonyl allyl niclosamide prodrugs as anticancer agents

Reprogrammed mitochondrial metabolism is recognized as an important target for anticancer therapy. Niclosamide, an FDA approved anthelmintic agent with mitochondrial uncoupling activity, has shown promise as an anticancer agent. However, off target mitochondrial toxicity has rendered the utility of this agent at clinically effective doses. Here, we synthesize a variety of prodrugs on the niclosamide template based on the Baylis-Hillman (BH) reaction, with the hypothesis that niclosamide will be released upon the reaction with cellular nucleophiles, including thiols. Consistent with this hypothesis, the BH-prodrugs release the parent niclosamide in the presence of cysteine and glutathione, and the cancer cell proliferation inhibition properties of the lead candidates are retained when compared to the parent niclosamide. Mitochondrial respiration assays illustrate that niclosamide acutely uncouples the mitochondria, and the lead BH-prodrug 6b does not, providing evidence of this prodrug strategy in mitigating off target toxicities. In a dose escalation study, the lead candidate 6b is generally well tolerated in healthy mice as evidenced by zero mortality, normal grooming pattern, and normal weight gains. Finally, 6b exhibits 54% volume tumor growth inhibition properties in a syngraft model of breast cancer in mice. The studies herein provide a novel methodology for the application of BH prodrugs on the niclosamide template and the anticancer applications.