New tyrosine kinase inhibitors in chronic myeloid leukemia

The deregulated activity of BCR-ABL tyrosine kinase originating from the t(9;22) chromosomal translocation has been shown to be necessary and sufficient for the transformed phenotype of chronic myeloid leukemia (CML) cells. This peculiarity has paved the way for the development of novel therapies specifically targeting the BCR-ABL gene product. The first BCR-ABL inhibitor to come into use in clinical practice, imatinib mesylate, is now the first-choice treatment for all newly diagnosed CML patients, but the initial striking efficacy of this drug has been overshadowed by the development of clinical resistance. The most common mechanisms of resistance include (i) BCR-ABL overexpression, and (ii) BCR-ABL kinase domain mutations disrupting critical contact points between imatinib and BCR-ABL or inducing a transition to a conformation to which imatinib is unable to bind. Several approaches to overcoming resistance have been studied both in vitro and in vivo. They include dose escalation of imatinib, the combination of imatinib with chemotherapeutic drugs, alternative BCR-ABL inhibitors, and inhibitors of kinases acting downstream of BCR-ABL such as Src kinases. Various novel tyrosine kinase inhibitors (TKI) have been synthesized and have now reached the pre-clinical or clinical phase. This review highlights the development of new TKI as specific molecularly targeted therapy and as the principal mechanisms for overcoming imatinib resistance.