Strategies for overcoming imatinib resistance in chronic myeloid leukemia.

Imatinib was the first treatment for chronic myeloid leukemia (CML) that specifically targeted the causative BCR-ABL oncoprotein, and represented a major therapeutic advance in this disease; however, some patients develop resistance or intolerance. Resistance can be classified as BCR-ABL-dependent (e.g., mutation in the BCR-ABL gene) or BCR-ABL-independent (alternative pathways of disease progression, e.g., SRC-family tyrosine kinases). The investigation of therapeutic options post-imatinib failure resulted in the development and regulatory approval of dasatinib, a BCR-ABL and SRC-family kinase inhibitor. Dasatinib is active across all phases of CML and Philadelphia chromosome-positive acute lymphoblastic leukemia, and demonstrates activity in almost all imatinib-resistant mutations. Other therapeutic options are also under investigation, with nilotinib being the most clinically advanced. Nilotinib is an analog of imatinib with similar multiple kinase targets, but without inhibition of SRC, and reduced in vitro activity against BCR-ABL P-loop mutations compared with dasatinib. Similar to dasatinib, nilotinib has no activity against T315I mutations. The availability of dasatinib and development of other tyrosine kinase inhibitors provide positive prospects for patients with imatinib-resistant or -intolerant CML. Here, we discuss several of these new strategies for treating patients after imatinib failure.