New insights into small-molecule inhibitors of Bcr-Abl

Chronic myelogenous leukemia (CML) is a myeloproliferative disease associated with a defined genetic abnormality, the Bcr-Abl fusion gene on the Philadelphia chromosome that expresses the constitutively activated tyrosine kinase (TK) Bcr-Abl. This enzyme leads to the malignant transformation of primitive hematopoietic cells and to the consequent disease. The central role of Bcr-Abl in the pathogenesis of CML culminated in the discovery of imatinib (an ATP-competitive inhibitor), which is currently the frontline therapy for CML. Unfortunately, the initial enthusiasm generated by its high response rate has been dampened by the development of resistance, especially in the advanced phases of CML. To overcome imatinib resistance, several second-generation ATP-competitive inhibitors endowed with increased potency against imatinib-resistant mutants have been developed: the dual Src/Abl inhibitor dasatinib and the Abl inhibitor nilotinib have been recently approved by US-FDA for the treatment of imatinib-resistant CML, and many other compounds are currently in clinical trial. Although second-generation TK inhibitors have shown to be clinically effective against most of the imatinib-resistant mutants, to date poor results have been obtained in the treatment of the Bcr-Abl T315I mutant. In this review we will report the most interesting second-generation Abl and dual Src/Abl inhibitors recently entered in clinical trial, but also the new ATP-competitive and uncompetitive inhibitors published in the last few years, focusing on their chemical structure, mechanism of action, and structure-activity relationship.