A STAT3 signaling pathway inhibitor. Niclosamide potently inhibits the activation and transcriptional function of STAT3 and induces cell growth inhibition, apoptosis, and cell cycle arrest of cancer cells with constitutively active STAT3. Also reversibly inhibits mTORC1 signaling and stimulates autophagy in vitro; displays antineoplastic effects in acute myelogenous leukemia (AML) stem cells.