Imatinib mesylate is now the first-choice treatment for all newly diagnosed CML patients, but despite the impressive percentage of responding patients, some CML cases show primary resistance or relapse after an initial response. Although some clinical and biological findings have been found to be associated with a lower probability of response to imatinib, at present no precise markers allowing to predict outcome for individual patients exist. The most common mechanisms of resistance to imatinib include BCR-ABL kinase domain mutations, BCR-ABL amplification and overexpression, and clonal evolution with activation of additional transformation pathways. These mechanisms are caused by the genomic instability, which characterises the Ph-positive clone. Several approaches to overcome resistance have been proposed, including novel tyrosine kinase inhibitors (TKIs) that have now reached the clinical or pre-clinical phase of evaluation. Clinical trials with these novel TKIs have shown good rates of hematologic and cytogenetic responses that appear also durable in time, but still cases of resistance are also observed, supporting the notion that genomic instability represents the major determinant affecting the final outcome of the CML patients when treated with TKIs. Understanding exactly the mechanisms leading to genomic instability of the Ph-positive cells represents therefore the real challenge for the near future.
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