Activation of tyrosine kinases in cancer

Receptor and nonreceptor tyrosine kinases (TKs) have emerged as clinically useful drug target molecules for treating certain types of cancer. Epidermal growth factor receptor (EGFR)-TK is a transmembrane receptor TK that is overexpressed or aberrantly activated in the most common solid tumors, including non-small cell lung cancer and cancers of the breast, prostate, and colon. Activation of the EGFR-TK enzyme results in autophosphorylation, which drives signal transduction pathways leading to tumor growth and malignant progression. Randomized clinical trials of the EGFR-TK inhibitor gefitinib have demonstrated clinical benefits in patients with advanced non-small cell lung cancer whose disease had previously progressed on platinum-and docetaxel-based chemotherapy regimens. Bcr-Abl is a constitutively activated nonreceptor TK enzyme found in the cytoplasm of Philadelphia chromosome-positive leukemia cells. STI571 (imatinib mesylate) inhibits the Bcr-Abl TK, blocks the growth of these leukemia cells, and induces apoptosis. STI571 also inhibits other TKs, including the receptor TK c-kit, which is expressed in gastrointestinal stromal tumors. As TK inhibitors become available for clinical use, new challenges include predicting which patients are most likely to respond to these targeted TK inhibitors. Additional clinical trials are needed to develop the full potential of receptor and nonreceptor TK inhibitors for cancer treatment.