The subject invention pertains to the treatment of tumors and cancerous tissues and the prevention of tumorigenesis and malignant transformation through the modulation of STAT3 intracellular signaling. The subject invention concerns pharmaceutical compositions containing one or more withacnistin compounds, or a pharmaceutically acceptable salt or derivative thereof. In one embodiment, the subject invention concerns a composition comprising a mixture of withacnistin, 3-methoxy-2,3-dihydrowithacnistin, and 3-ethoxy-2,3-dihydrowithacnistin, or a salt or derivative of any of the foregoing. Another aspect of the invention concerns methods of inhibiting the growth of a tumor by administering one or more withacnistin compounds, or a pharmaceutically acceptable salt or derivative thereof, to a patient, wherein the tumor is characterized by the constitutive activation of the STAT3 intracellular signaling pathway. The present invention further pertains to methods of moderating the STAT3 signaling pathway in vitro or in vivo using one or more withacnistin compounds, or a pharmaceutically acceptable salt or derivative thereof.
