BCR-ABL is an unregulated tyrosine kinase expressed as a consequence of a reciprocal chromosomal translocation that is common in chronic myelogenous and acute lymphocytic leukemia. BCR-ABL induces transformation of hematopoetic stem cells through tyrosine phosphorylation of multiple substrates. The src-family kinases (SFKs), Lyn and Hck, are highly activated by BCR-ABL in leukemic cells and recent studies suggest that they are substrates and essential mediators of BCR-ABL signal transduction and transformation. In cells selected for resistance to the BCR-ABL inhibitor, imatinib mesylate, Lyn kinase is overexpressed and its activation is not dependent on or regulated by BCR-ABL, suggesting that autonomous regulation of SFKs may play a role in imatinib resistant. In this report, activation of Lyn and Hck was compared in CML specimens derived from imatinib responsive and resistant patients that did not express a mutant BCR-ABL protein as their primary mediator of resistance. In imatinib sensitive cell lines and specimens derived from imatinib responsive CML patients imatinib effectively reduced activation of both BCR-ABL and SFKs. However, in multiple specimens from resistant patients, imatinib reduced BCR-ABL kinase activation but failed to reduce SFK activation. The dual ABL/SRC inhibitor, BMS-354825, blocked activation of both BCR-ABL and SFKs expressed in leukemic cells and correlated with clinical responsiveness to this agent. Animal models demonstrated that loss of imatinib-mediated inhibition of Lyn kinase activation significantly impaired its anti-tumor activity which was recovered by treatment with BMS-354825. Direct silencing of Lyn or Hck reduced CML cell survival in imatinib resistant patient specimens and cell models, suggesting a direct role for these kinases in cell survival. Our results show that SFK activation is mediated by BCR-ABL in imatinib responsive cells but these kinases escape control by BCR-ABL in CML patients that develop imatinib resistance in the absence of BCR-ABL point mutations. This form of resistance can effectively be overcome by BMS-354825 through its dual SRC and ABL kinase inhibitory activities. Dual specificity kinase inhibitors may be indicated for the treatment and prevention of imatinib resistance in CML when it is associated with constitutively activated src-family kinases.

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