Synthesis and Biological Evaluation of Glutathione-Responsive 2-Alkoxycarbonyl Allyl Niclosamide Prodrugs as Anticancer Agents

INTRODUCTION: 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 can limit its utility at clinically effective doses. METHODS: This study synthesized a variety of prodrugs based on the niclosamide template via the Baylis-Hillman (BH) reaction, with the hypothesis that niclosamide would be released upon reaction with cellular nucleophiles, including thiols. RESULTS AND DISCUSSION: Consistent with this hypothesis, BH-prodrugs release the parent niclosamide in the presence of cysteine and glutathione. The lead candidates exhibit cancer cell proliferation inhibitory properties comparable to those of the parent compound, niclosamide. Mitochondrial respiration assays demonstrated that niclosamide acutely uncouples mitochondria, whereas the lead BH-prodrug 6b does not, providing evidence for this prodrug strategy in mitigating off-target toxicities. In a dose-escalation study, lead compound 6b was well tolerated in healthy mice up to 30 mg/kg, with no mortality and no abnormal changes in grooming behavior or body weight. In a 67NR syngeneic breast cancer model, 6b produced 54% tumor growth inhibition by volume, compared with 19% for niclosamide. CONCLUSION: The studies herein provide a novel methodology for the application of BH prodrugs on the niclosamide template and anticancer applications.