Current Cancer Drug Targets - Volume 13, Issue 7, 2013

Over the past decades the prognosis of patients with Chronic Myeloid Leukemia (CML) has radically changed due to groundbreaking scientific and translational studies that have revealed the biologic basis of such a hematologic malignancy. These studies have led to the rapid development of many BCR-ABL specific tyrosine kinase inhibitors (TKIs), such as Imatinib, Nilotinib and Dasatinib, which have improved 10-years survival to more than 80%. Although these therapies represent a landmark step in the race for the cure of CML, they did not change the progression in advanced phase of disease. Therefore unravel the molecular mechanisms and the biological basis of CML, especially during the advanced stage, is of seminal importance as this would result in the design of more effective and less toxic therapies. In such a scenario, several novel drugs designed to specifically target biological features of CML cells are currently in clinical trials with promising results that would provide not only improve the therapeutic armamentarium but also to overcome drug resistance of this tumor. Here we review recent advances in biology of CML and their therapeutic implications.

The insensitivity of Chronic Myeloid Leukaemia (CML) stem cells to Tyrosine Kinase Inhibitor (TKI) treatment is now believed to be the main reason for disease persistence experienced in patients. It has been shown that autophagy, an evolutionarily conserved catabolic process that involves degradation of unnecessary or harmful cellular components via lysosomes, is induced following TKI treatment in CML cells. Of clinical importance, autophagy inhibition, using the anti-malarial drug hydroxychloroquine (HCQ), sensitised CML cells, including primitive CML stem cells, to TKI treatment. In this review we discuss the role of autophagy in the maintenance and survival of stem cells in more detail, with a focus on its role in survival of CML stem cells and the possibility to inhibit this pathway as a way to eliminate persistent CML stem cells in vitro and in patients.

Although the imatinib based therapy of chronic myeloid leukemia (CML) represents a triumph of medicine, not all patients with CML benefit from this drug due to the development of resistance and intolerance. The interruption of imatinib treatment is often followed by clinical relapse, suggesting a failure in the killing of residual leukaemic stem cells. There is need to identify alternative selective molecular targets for this disease and develop more effective therapeutic approaches. Alternative pre-mRNA splicing (AS) is an epigenetic process that greatly diversifies the repertoire of the transcriptome. AS orchestrates interactions between various types of proteins and between proteins and nucleic acids. Changes caused by individual splicing events in the cells are small, however, “splicing programs” typically react to these individual changes with considerable effects in cell proliferation, cell survival, and apoptosis. Current evidence suggests a pivotal role of AS in leukemias, particularly in myelodisplastic syndrome (MDS) and chronic lymphocyte leukemia (CLL). From these studies and studies in other malignances, it is clear that splicing abnormalities play a significant role in malignant transformation. Evaluation of AS events in CML can be used to identify novel disease markers and drugsensitive targets to overcome the limits of the small molecule inhibitors currently used for treating patients with CML. The use of aberrant splice variants as disease markers has been reported, however, little is known about the use of splicing abnormalities as drug targets in CML. Herein we discuss potential therapeutic approaches that can be used to target splicing abnormalities in CML.

Tyrosin Kinase inhibitors (TKI), have dramatically changed the natural history of chronic myeloid leukemia (CML) leading to an impressive increase in overall survival rates and allowing many CML patients to achieve a close-tonormal life expectancy. Unfortunately, there is growing evidence that these drugs are not curative, about 30-35% of the patients who receive imatinib become resistant or discontinue the drug because of side effects; moreover, 15% of all patients become resistant to all TKIs, a condition which represents the biggest challenge in CML treatment. Recent progresses in CML stem cell biology have identified new agents and therapeutic strategies that can be used to target the CML stem cell compartment. These studies have opened new perspectives and have highlighted key strategies for treating, and possibly curing, CML in the upcoming years.

The prevalence of chronic myeloid leukemia (CML) is expected to double in the next 15 years. The introduction of imatinib significantly changed the prognosis of CML, challenging the concept of a fatal disease. Nowdays, imatinib, nilotinib and dasatinib are registered for first-line treatment of CML patients in chronic phase (CP). Considering elderly patients, the most extensively studied TKI is imatinib, that induces a rate of cytogenetic and molecular responses comparable between the younger and the elderly patients. Once a CCgR with imatinib is achieved, the probability to be alive and disease free at 8 years is more than 80%. These results confirm that imatinib has to be considered the first-line treatment for the elderly and that the CCgR is the guide parameter for treatment modulation and the most solid marker of long term outcome. Nevertheless, older patients tolerate imatinib worse in comparison to the younger, and this causes a higher rate of therapy discontinuation and less adherence to chronic treatment. Thus, the toxic profile of each TKI is one of the most important factors driving the choice of the best drug. Another important factor is the potency of the TKI. Since nilotinib and dasatinib are more potent than imatinib in inducing cytogenetic and molecular responses, they could be preferred for increasing the proportion of patients who can achieve deeper molecular responses, allowing treatment discontinuation. This approach is intriguing, but it is still experimental. Another therapeutic strategy could be the identification of the minimal effective dose of TKI in order to maintain the CCgR, but also this approach is under clinical investigation.

Tyrosine kinase inhibitors induce sustained disease remissions in chronic myeloid leukemia by exploiting the addiction of this type of leukemia to the activity of the fusion oncogene BCR-ABL. However, these agents fail to eradicate CML stem cells which are ultimately responsible for disease relapses upon treatment discontinuation. Evidence that the immune system can effectively reject CML stem cells potentially leading to patient cure is provided by the experience with patients receiving allogeneic bone marrow transplantations. Compelling evidence indicates that more modern, antigen-specific immunotherapeutic approaches are also feasible and hold strong potential to be clinically effective. Amongst these, particularly promising is the use of autologous dendritic cells pulsed with antigens or direct application of in vitro transcribed RNA encoding for leukemia-associated antigens, since this approach allows to circumvent HLA-restriction of the leukemia-associated T cell epitopes that have been eventually identified. Combining these strategies with monoclonal antibodies, such as anti-CTLA-4 or anti-PD-1, may help to obtain even stronger immune responses and better clinical results. This narrative review addresses this topic by focusing in particular on the cell-based immunotherapeutic strategies for CML and on the issue of the leukemia-associated antigens to be selected for targeting.

The introduction of tyrosine kinase inhibitors (TKIs) has reduced the indications for allogeneic stem cell transplants (SCT) in patients with chronic myeloid leukemia (CML) from 1500/year in 2000 to approximately 500/year in 2010. The recently updated indications by the European Leukemia Network include advanced-phase CML and nonresponders to second-line TKIs. Changes in transplant programs, over the past decade, have resulted in a significant reduction in transplant related mortality and this has expanded the transplant option to older patients, who may be eligible for SCT. Transplantation for blastic phase CML remains a challenge due to the high rate of post-transplant relapses. Thus, in view of this issue, several studies of TKIs after allogeneic SCT have been undertaken with encouraging results. In conclusion, allogeneic SCT remains a therapeutic option for selected patients with CML and is currently being integrated with the use of TKIs both before and after transplantation.

Chronic myeloid leukemia (CML) is a hematological disease accounting for about 15-20% of all adult leukemias. The clinical and biologic advances achieved in such a malignancy, represent one of the best successes obtained by translational medicine. Indeed, identification of the fusion oncogene BCR-ABL has allowed using of small molecule inhibitors of its tyrosine kinase activity which, in turn, have literally revolutionized the treatment of CML. Importantly the successfully clinical management was also realized on appropriate diagnosis, disease monitoring as well as early identification of such mutations causing drug resistance. Notably the recent availability of refined laboratory equipments represented by the Next Generation Sequencing (NGS) and genomic analyses has further contributed to gain ground towards the cure of this tumor. These issues are discussed here together with an overview on how patients treated with tyrosine kinase inhibitors should be monitored.

Acute leukemias are characterized by recurring chromosomal and genetic abnormalities that disrupt normal development and drive aberrant cell proliferation and survival. Identification of these abnormalities plays important role in diagnosis, risk assessment and patient classification. Until the last decade methods to detect these aberrations have included genome wide approaches, such as conventional cytogenetics, but with a low sensitivity (5-10%), or gene candidate approaches, such as fluorescent in situ hybridization, having a greater sensitivity but being limited to only known regions of the genome. Single nucleotide polymorphism (SNP) technology is a screening method that has revolutionized our way to find genetic alterations, enabling linkage and association studies between SNP genotype and disease as well as the identification of alterations in DNA content on a whole genome scale. The adoption of this approach for the study of lymphoid and myeloid leukemias contributed to the identification of novel genetic alterations, such as losses/gains/uniparental disomy not visible by cytogenetics and implicated in pathogenesis, improving risk assessment and patient classification and in some cases working as targets for tailored therapies. In this review, we reported recent advances obtained in the knowledge of the genomic complexity of chronic myeloid leukemia and acute leukemias thanks to the use of high-throughput technologies, such as SNP array.