Signal transduction therapy in haematological malignancies: identification and targeting of tyrosine kinases

Tyrosine kinases play key roles in cell proliferation, survival and differentiation. Their aberrant activation, caused either by the formation of fusion genes by chromosome translocation or by intragenic changes, such as point mutations or internal duplications, is of major importance in the development of many haematological malignancies. An understanding of the mechanisms by which BCR-ABL contributes to the pathogenesis of chronic myeloid leukaemia led to the development of imatinib, the first of several tyrosine kinase inhibitors to enter clinical trials. Although the development of resistance has been problematic, particularly in aggressive disease, the development of novel inhibitors and combination with other forms of therapy shows promise.

Bcr-Abl resistance screening predicts a limited spectrum of point mutations to be associated with clinical resistance to the Abl kinase inhibitor nilotinib (AMN107)

In advanced-phase chronic myeloid leukemia (CML), resistance to imatinib mesylate is associated with point mutations in the BCR-ABL kinase domain. A new generation of potent ABL kinase inhibitors is undergoing clinical evaluation. It is important to generate specific resistance profiles for each of these compounds, which could translate into combinatorial and sequential treatment strategies. Having characterized nilotinib (AMN107) against a large panel of imatinib mesylate–resistant Bcr-Abl mutants, we investigated which mutants might arise under nilotinib therapy using a cell-based resistance screen. In contrast to imatinib mesylate, resistance to nilotinib was associated with a limited spectrum of Bcr-Abl kinase mutations. Among these were mutations affecting the P-loop and T315I. Rarely emerging resistant colonies at a concentration of 400 nM nilotinib exclusively expressed the T315I mutation. With the exception of T315I, all of the mutations that were identified were effectively suppressed when the nilotinib concentration was increased to 2000 nM, which falls within the peak-trough range in plasma levels (3.6-1.7 μM) measured in patients treated with 400 mg twice daily. Our findings suggest that nilotinib might be superior to imatinib mesylate in terms of the development of resistance. However, our study indicates that clinical resistance to nilotinib may be associated with the predominant emergence of T315I.

The Second Generation of BCR-ABL Tyrosine Kinase Inhibitors

Imatinib was developed as the first molecularly targeted therapy to specifically inhibit the BCR-ABL kinase in Philadelphia chromosome (Ph)-positive chronic myeloid leukemia (CML). Because of the excellent hematologic and cytogenetic responses, imatinib has moved toward first-line treatment for newly diagnosed CML. However, the emergence of resistance to imatinib remains a major problem in the treatment of Ph-positive leukemia. Several mechanisms of imatinib resistance have been identified, including BCR-ABL gene amplification that leads to overexpression of the BCR-ABL protein, point mutations in the BCR-ABL kinase domain that interfere with imatinib binding, and point mutations outside of the kinase domain that allosterically inhibit imatinib binding to BCR-ABL. The need for alternative or additional treatment for imatinib-resistant BCR-ABL-positive leukemia has guided the way to the design of a second generation of targeted therapies, which has resulted mainly in the development of novel small-molecule inhibitors such as AMN107, dasatinib, NS-187, and ON012380. The major goal of these efforts is to create new compounds that are more potent than imatinib and/or more effective against imatinib-resistant BCR-ABL clones. In this review, we discuss the next generation of BCR-ABL kinase inhibitors for overcoming imatinib resistance.

Advances in the structural biology, design and clinical development of Bcr-Abl kinase inhibitors for the treatment of chronic myeloid leukaemia

The constitutively activated Abl tyrosine kinase domain of the chimeric Bcr-Abl oncoprotein is responsible for the transformation of haematopoietic stem cells and the symptoms of chronic myeloid leukaemia (CML). Imatinib targets the tyrosine kinase activity of Bcr-Abl and is a first-line therapy for this malignancy. Although highly effective in chronic phase CML, patients who have progressed to the advanced phase of the disease frequently fail to respond to imatinib or develop resistance to therapy and relapse. This is often due to the emergence of clones expressing mutant forms of Bcr-Abl, which exhibit a decreased sensitivity towards inhibition by imatinib. Considerable progress has recently been made in understanding the structural biology of Abl and the molecular basis for resistance, facilitating the discovery and development of second generation drugs designed to combat mutant forms of Bcr-Abl. The first of these compounds to enter clinical development were BMS-354825 (BristolMyersSquibb) and AMN107 (Novartis Pharma) and, from Phase I results, both of these promise a breakthrough in the treatment of imatinib-resistant CML. Recent advances with these and other promising classes of new CML drugs are reviewed.