An antineoplastic agent that inhibits the Bcr-Abl fusion protein tyrosine kinase, an abnormal enzyme produced by chronic myeloid leukemia cells that contain the Philadelphia chromosome. Imatinib also inhibits the receptor tyrosine kinases for platelet-derived growth factor (PDGF) and stem cell factor (SCF)/c-kit; the SCF/c-kit receptor tyrosine kinase is activated in gastrointestinal stromal tumor (GIST). This agent inhibits proliferation and induces apoptosis in cells that overexpress these oncoproteins.