The subject invention pertains to the treatment of tumors and cancerous tissues and the prevention of tumorigenesis and malignant transformation through the modulation of JAK/STAT3 intracellular signaling. The subject invention concerns pharmaceutical compositions containing cucurbitacin I, or a pharmaceutically acceptable salt or analog thereof. Another aspect of the invention concerns methods of inhibiting the growth of a tumor by administering a cucurbitacin I, or a pharmaceutically acceptable salt or analog thereof, to a patient, wherein the tumor is characterized by the constitutive activation of the JAK/STAT3 intracellular signaling pathway. The present invention further pertains to methods of moderating the JAK and/or STAT3 signaling pathways in vitro or in vivo using cucurbitacin I, or a pharmaceutically acceptable salt or analog thereof. Another aspect of the present invention concerns a method for screening candidate compounds for JAK and/or STAT3 inhibition and anti-tumor activity.