ABL Mutations in Late Chronic Phase Chronic Myeloid Leukemia Patients With Up-Front Cytogenetic Resistance to Imatinib Are Associated With a Greater Likelihood of Progression to Blast Crisis and Shorter Survival: A Study by the GIMEMA Working Party on Chronic Myeloid Leukemia

Advances in treatment of chronic myeloid leukemia with tyrosine kinase inhibitors: the evolving role of Bcr–Abl mutations and mutational analysis

Over the last decade, the treatment of chronic myeloid leukemia has progressed tremendously. The first-generation tyrosine kinase inhibitor imatinib is now flanked by two second-generation molecules, dasatinib and nilotinib – and others are in advanced clinical development. One of the reasons for such intensive research on novel compounds is the problem of resistance, that is thought to be caused, in a proportion of cases, by point mutations in Bcr–Abl. In this article, the authors review how the biological and clinical relevance of Bcr–Abl mutations has evolved in parallel with the availability of more and more therapeutic options. The authors also discuss the practical relevance of Bcr–Abl mutation analysis and how this tool should best be integrated in the optimal clinical management of chronic myeloid leukemia patients.

Managing inadequate responses to frontline treatment of chronic myeloid leukemia: a case-based review

The tyrosine kinase inhibitors (TKIs) imatinib, nilotinib, and dasatinib are the standard of care for treating patients with newly diagnosed chronic-phase chronic myeloid leukemia (CML). Compared with interferon-based treatment, the previous standard of care, imatinib is associated with significantly higher cytogenetic response rates and prolonged overall survival. Nilotinib and dasatinib, both newer and more potent TKIs, significantly improve cytogenetic and molecular response rates compared with imatinib. Despite significant advances in CML treatment enabled by the TKIs, a fraction of patients who receive frontline treatment with a TKI demonstrate inadequate response. The reasons for this vary, but in many cases, inadequate response can be attributed to non-adherence to the treatment regimen, intolerance to the drug, intrinsic or acquired resistance to the drug, or a combination of reasons. More often than not, strategies to improve response necessitate a change in treatment plan, either a dose adjustment or a switch to an alternate drug, particularly in the case of drug intolerance or drug resistance. Improved physician-patient communication and patient education are effective strategies to address issues relating to adherence and intolerance. Because inadequate response to TKI treatment correlates with poor long-term outcomes, it is imperative that patients who experience intolerance or who fail to achieve appropriate responses are carefully evaluated so that appropriate treatment modifications can be made to maximize the likelihood of positive long-term outcome.

Genetic mechanisms of chronic myeloid leukemia blastic transformation

The BCR-ABL1 oncogenic tyrosine kinase can transform pluripotent hematopoietic stem cells and initiate chronic myeloid leukemia in chronic phase (CML-CP), a myeloproliferative disorder characterized by excessive accumulation of mature myeloid cells. Patients in CML-CP usually respond to treatment with ABL1 tyrosine kinase inhibitors (TKIs) such as imatinib, though some patients who respond initially may become resistant later. CML-CP leukemia stem cells (LSCs) are intrinsically insensitive to TKIs and thus survive in the long term. These LSCs or their progeny may at some stage acquire additional genetic changes that cause the leukemia to transform further, from CML-CP to a more advanced phase, which has been subclassified as either accelerated phase (CML-AP) or blastic phase (CML-BP). CML-BP is characterized by a major clonal expansion of immature progenitors, which have either myeloid or lymphoid features. CML-BP responds poorly to treatment and is usually fatal. This review discusses the role of genomic instability leading to blastic transformation of CML and proposes some novel therapeutic approaches.

How I treat CML blast crisis

Blast crisis (BC) remains the major challenge in the management of chronic myeloid leukemia (CML). It is now generally accepted that BC is the consequence of continued BCR-ABL activity leading to genetic instability, DNA damage, and impaired DNA repair. Most patients with BC carry multiple mutations, and up to 80% show additional chromosomal aberrations in a nonrandom pattern. Treatment with tyrosine kinase inhibitors has improved survival in BC modestly, but most long-term survivors are those who have been transplanted. Patients in BC should be treated with a tyrosine kinase inhibitor according to mutation profile, with or without chemotherapy, with the goal of achieving a second chronic phase and proceeding to allogeneic stem cell transplantation as quickly as possible. Although long-term remissions are rare, allogeneic stem cell transplantation provides the best chance of a cure in BC. Investigational agents are not likely to provide an alternative in the near future. In view of these limited options, prevention of BC by a rigorous and early elimination of BCR-ABL is recommended. Early response indicators should be used to select patients for alternative therapies and early transplantation. Every attempt should be made to reduce or eliminate BCR-ABL consistent with good patient care as far as possible.

Molecular resistance: an early indicator for treatment change?

Vigilant monitoring of a patient's response to current treatment is imperative to the management of chronic myeloid leukemia (CML). Early identification of treatment failure may increase the probability that alternative therapy will be effective. This review discusses the use of molecular monitoring in the timely detection of failure of imatinib treatment. Changes in the levels of BCR-ABL transcripts are predictive of response or relapse. Patients achieving a major molecular response (MMR) within 12 months of treatment may experience longer cytogenetic remission. Accumulating evidence also suggests that lower transcript levels observed ≤ 6 months after the start of treatment are associated with improved patient outcomes. For patients with primary or secondary imatinib resistance (or intolerance), dasatinib or nilotinib may be prescribed. These agents have demonstrated activity in patients harboring imatinib-resistant BCR-ABL mutations, except for the T315I substitution.

BCR-ABL1 kinase domain mutations: methodology and clinical evaluation

The introduction of tyrosine kinase inhibitors (TKIs), starting with imatinib and followed by second and third generation TKIs, has significantly changed the clinical management of patients with chronic myeloid leukemia (CML). Despite their unprecedented clinical success, a proportion of patients fail to achieve complete cytogenetic remission by 12 months of treatment (primary resistance) while others experience progressive resistance after an initial response (secondary resistance). BCR-ABL1 kinase domain (KD) mutations have been detected in a proportion of patients at the time of treatment failure, and therefore their identification and monitoring plays an important role in therapeutic decisions particularly when switching TKIs. When monitoring KD mutations in a clinical laboratory, the choice of method should take into account turnaround time, cost, sensitivity, specificity, and ability to accurately quantify the size of the mutant clone. In this article, we describe in a "manual" style the methods most widely used in our laboratory to monitor KD mutations in patients with CML including direct sequencing, D-HPLC, and pyrosequencing. Advantages, disadvantages, interpretation of results, and their clinical applications are reviewed for each method.

Suppression of autophagy by BCR/ABL

Imatinib and second generation BCR/ABL tyrosine kinase inhibitors (TKIs) serve now as standard therapies for patients with chronic myelogenous leukemia (CML); however, CML stem cells are intrinsically insensitive to the cell death-inducing effects of TKIs, allowing the persistence of a "reservoir" of BCR/ABL-expressing CML-initiating cells potentially responsible for disease relapse and progression. Although it is still controversial whether the "insensitivity" of CML stem cells to treatment with TKI is due to BCR/ABL-dependent or independent mechanisms, treatment with IM appears to suppress BCR/ABL-dependent signaling in CML stem cells with no adverse effects on their survival. Recent evidence indicates that BCR/ABL suppresses and treatment of CML cells with IM/TKIs induces autophagy, a genetically-regulated process of adaptation to metabolic stress which could allow tumor cells to become metabolically inert enabling their survival under conditions that may mimic growth factor/nutrients deprivation. Based on this hypothesis, TKI-induced autophagy may "antagonize" TKI-induced cell death and inhibition of autophagy may eliminate this survival mechanism by restoring "sensitivity" of CML stem cells to treatment with IM/TKI. Consistent with this, phenotypically and functionally defined CML-enriched stem cells insensitive to treatment with TKI are efficiently eliminated by the combination of TKI and chloroquine, an inhibitor of late stage autophagy. Thus, inhibition of autophagy may improve the potent and specific effects of TKIs by rendering CML stem cells sensitive to these targeted therapies.

Evolution of therapies for chronic myelogenous leukemia

The clinical outcome for patients with chronic myelogenous leukemia (CML) has changed dramatically in the past 15 years. This has been due to the development of tyrosine kinase inhibitors (TKIs), compounds that inhibit the activity of the oncogenic BCR-ABL1 protein. Imatinib was the first TKI developed for CML, and it led to high rates of complete cytogenetic responses and improved survival for patients with this disease. However, approximately 35% of patients in chronic phase treated with imatinib will develop resistance or intolerance to this drug. The recognition of the problem of imatinib failure led to the design of second-generation TKI (dasatinib, nilotinib, and bosutinib). These drugs are highly active in the scenario of imatinib resistance or intolerance. More recently, both nilotinib and dasatinib were approved for frontline use in patients with chronic phase CML. Ponatinib represents the last generation of TKI, and this drug has been developed with the aim of targeting a specific BCR-ABL1 mutation (T315I), which arises in the setting of prolonged TKI therapy and leads to resistance to all commercially available TKI. Parallel to the development of specific drugs for treating CML, major advances were made in the field of disease monitoring and standardization of response criteria. In this review, we summarize how therapy with TKI for CML has evolved during the last decade.

Galectin-3 (Gal-3) induced by leukemia microenvironment promotes drug resistance and bone marrow lodgment in chronic myelogenous leukemia

Bone marrow (BM) microenvironment (BMME) constitutes the sanctuary for leukemic cells. In this study, we investigated the molecular mechanisms for BMME-mediated drug resistance and BM lodgment in chronic myelogenous leukemia (CML). Gene-expression profile as well as signal pathway and protein analyses revealed that galectin-3 (Gal-3), a member of the β-gal-binding galectin family of proteins, was specifically induced by coculture with HS-5 cells, a BM stroma cell-derived cell line, in all five CML cell lines examined. It was also found that primary CML cells expressed high levels of Gal-3 in BM. Enforced expression of Gal-3 activated Akt and Erk, induced accumulation of Mcl-1, and promoted in vitro cell proliferation, multidrug resistance to tyrosine kinase inhibitors for Bcr-Abl and genotoxic agents as a result of impaired apoptosis induction, and chemotactic cell migration to HS-5-derived soluble factors in CML cell lines independently of Bcr-Abl tyrosine kinase. The conditioned medium from Gal-3-overexpressing CML cells promoted in vitro cell proliferation of CML cells and HS-5 cells more than did the conditioned medium from parental cells. Moreover, the in vivo study in a mice transplantation model showed that Gal-3 overexpression promoted the long-term BM lodgment of CML cells. These results demonstrate that leukemia microenvironment-specific Gal-3 expression supports molecular signaling pathways for disease maintenance in BM and resistance to therapy in CML. They also suggest that Gal-3 may be a candidate therapeutic target to help overcome BMME-mediated therapeutic resistance.

Managing resistance in chronic myeloid leukemia

Chronic myeloid leukemia (CML) is a myeloproliferative disorder that affects 5000 new patients per year in the United States. Prior to 10 years ago, durable remission was rare and patients often underwent bone marrow transplantation with substantial morbidity and mortality. Fortunately, CML has been the epicenter of exciting advances in cancer therapy with the discovery of the Bcr-Abl gene fusion and the subsequent development of imatinib mesylate, a small molecule tyrosine kinase inhibitor, to target the kinase activity of the bcr-abl protein product. Despite unprecedented durability for complete hematologic, cytogenetic, and molecular responses seen with front-line imatinib therapy, many patients require alternative therapy because of drug intolerance, suboptimal response, primary resistance, secondary resistance, or progression to advanced phase disease. Further, up to 5% of patients present with advanced disease that does not sustain a durable response to tyrosine kinase inhibitors. Thus, up to one third of CML patients require alternate therapy. Chronic myeloid leukemia has become an exemplary model system for understanding molecular targeting and overcoming mechanisms of drug resistance. This review will discuss potential mechanisms of resistance and ongoing research into novel targets and agents for CML resistant to standard of care.

Chronic myeloid leukemia 2011: successes, challenges, and strategies--proceedings of the 5th annual BCR-ABL1 positive and BCR-ABL1 negative myeloproliferative neoplasms workshop

This report is based on the presentations and discussions at the 5th annual BCR-ABL1 positive and BCR-ABL1 negative myeloproliferative neoplasms (MPN) workshop, which took place immediately following the 52nd American Society of Hematology (ASH) meeting in Orlando, Florida on December 7th-8th, 2011. Relevant data which was presented at the ASH meeting as well as all other recent publications were presented and discussed at the workshop. This report covers front-line therapies of BCR-ABL1-positive leukemias, in addition to addressing some topical biological, pre-clinical and clinical issues, such as new insights into genomic instability and resistance to tyrosine kinase inhibitors (TKIs), risk stratification and optimizing molecular monitoring. A report pertaining to the new therapies and other pertinent preclinical and clinical issues in the BCR-ABL1 negative MPNs is published separately.

Evolving treatment strategies for patients newly diagnosed with chronic myeloid leukemia: the role of second-generation BCR-ABL inhibitors as first-line therapy

In patients with chronic myeloid leukemia (CML), disease in the initial chronic phase (CP) and subsequent progression are driven by the oncogenic activity of the BCR-ABL fusion kinase. Imatinib, a tyrosine kinase inhibitor of BCR-ABL, has been the mainstay of first-line therapy for CML for 10 years. Although patients with CML-CP respond well to imatinib, those who have delayed reductions in leukemic burden during imatinib therapy, such as not achieving a complete cytogenetic response (CCyR) by 12 months, have an increased risk of disease progression. It has been recognized, with 8 years of observation, that patients who achieve an early major molecular response (MMR) on imatinib have a very low probability of disease progression. Recent randomized phase 3 trials have shown that first-line treatment with dasatinib or nilotinib-more potent BCR-ABL inhibitors-results in significantly higher rates and more rapid achievement of CCyR and MMR in comparison with standard-dose imatinib. These trials suggest that CML treatment can be improved with more potent BCR-ABL inhibition during initial therapy, but further follow-up is needed to confirm that the improved response rates with dasatinib and nilotinib are maintained long term.

Application of cost-effectiveness analysis to demonstrate the potential value of companion diagnostics in chronic myeloid leukemia

AIM

A cost-effectiveness analysis was performed to assess the potential value of companion diagnostics in supporting treatment decisions for dasatinib and nilotinib in chronic myeloid leukemia.

MATERIALS & METHODS

A decision model was developed, and model inputs were taken from the literature and publicly available sources. The perspective of the healthcare sector in the Netherlands was used. Sensitivity and scenario analyses were performed to assess uncertainty in the results.

RESULTS

Companion diagnostics could improve health and reduce costs, despite the estimates being uncertain owing to limited evidence for comparative effectiveness between dasatinib and nilotinib. The results were sensitive to the cost of treatment, utility of progression and progression-free survival.

CONCLUSION

This case demonstrates the use of cost-effectiveness analysis at an early stage of health technology assessment to generate economic evidence for the use of companion diagnostics in treatment decisions and to support decision-making for their development.

Measuring response to BCR-ABL inhibitors in chronic myeloid leukemia

In patients with chronic myeloid leukemia (CML), the hallmark Philadelphia chromosome is the marker of disease that can be detected by conventional metaphase cytogenetics, fluorescence in situ hybridization, or polymerase chain reaction. The current "gold standard" of treatment response is cytogenetic response. Cytogenetic response to imatinib is strongly associated with disease progression and survival. Various strategies aimed at improving cytogenetic response have been explored, such as escalation of imatinib and switching to the newer breakpoint cluster region/v-abl Abelson murine leukemia viral oncogene (BCR-ABL) inhibitors dasatinib and nilotinib. Data from recent randomized trials of dasatinib and nilotinib as first-line therapy of newly diagnosed chronic-phase CML suggest that these agents are more effective than imatinib in achieving 6-month and 12-month complete cytogenetic responses. However, it is still too early to know whether or not this early response will translate into a long-term survival advantage. In addition, more sensitive assays to detect residual disease also may be associated with improved long-term outcomes. The deepest measure of response-a complete molecular response-may help identify patients who can stop taking imatinib for the short term, although the long-term consequences of this strategy remain unknown.

Targeting RAD51 phosphotyrosine-315 to prevent unfaithful recombination repair in BCR-ABL1 leukemia

Chronic myeloid leukemia chronic phase (CML-CP) CD34(+) cells contain numerous DNA double-strand breaks whose unfaithful repair may contribute to chromosomal instability and disease progression to blast phase (CML-BP). These phenomena are often associated with the appearance of imatinib-resistant BCR-ABL1 kinase mutants (eg, T315I) and overexpression of BCR-ABL1. Here we show that BCR-ABL1 (nonmutated and T315I mutant) promoted RAD51 recombinase-mediated unfaithful homeologous recombination repair (HomeoRR) in a dosage-dependent manner. BCR-ABL1 SH3 domain interacts with RAD51 proline-rich regions, resulting in direct phosphorylation of RAD51 on Y315 (pY315). RAD51(pY315) facilitates dissociation from the complex with BCR-ABL1 kinase, migrates to the nucleus, and enhances formation of the nuclear foci indicative of recombination sites. HomeoRR and RAD51 nuclear foci were strongly reduced by RAD51(Y315F) phosphorylation-less mutant. In addition, peptide aptamer mimicking RAD51(pY315) fragment, but not that with Y315F phosphorylation-less substitution, diminished RAD51 foci formation and inhibited HomeoRR in leukemia cells. In conclusion, we postulate that BCR-ABL1 kinase-mediated RAD51(pY315) promotes unfaithful HomeoRR in leukemia cells, which may contribute to accumulation of secondary chromosomal aberrations responsible for CML relapse and progression.

Human cancers express mutator phenotypes: origin, consequences and targeting

Recent data on DNA sequencing of human tumours have established that cancer cells contain thousands of mutations. These data support the concept that cancer cells express a mutator phenotype. This Perspective considers the evidence supporting the mutator phenotype hypothesis, the origin and consequences of a mutator phenotype, the implications for personalized medicine and the feasibility of ablating tumours by error catastrophe.

BCR-ABL kinase domain mutation analysis in chronic myeloid leukemia patients treated with tyrosine kinase inhibitors: recommendations from an expert panel on behalf of European LeukemiaNet

Mutations in the Bcr-Abl kinase domain may cause, or contribute to, resistance to tyrosine kinase inhibitors (TKIs) in chronic myeloid leukemia patients. Recommendations aimed to rationalize the use of BCR-ABL mutation testing in chronic myeloid leukemia have been compiled by a panel of experts appointed by the European LeukemiaNet (ELN) and European Treatment and Outcome Study and are here reported. Based on a critical review of the literature and, whenever necessary, on panelists' experience, key issues were identified and discussed concerning: (1) when to perform mutation analysis, (2) how to perform it, and (3) how to translate results into clinical practice. In chronic phase patients receiving imatinib first-line, mutation analysis is recommended only in case of failure or suboptimal response according to the ELN criteria. In imatinib-resistant patients receiving an alternative TKI, mutation analysis is recommended in case of hematologic or cytogenetic failure as provisionally defined by the ELN. The recommended methodology is direct sequencing, although it may be preceded by screening with other techniques, such as denaturing-high performance liquid chromatography. In all the cases outlined within this abstract, a positive result is an indication for therapeutic change. Some specific mutations weigh on TKI selection.

Practical advice for determining the role of BCR-ABL mutations in guiding tyrosine kinase inhibitor therapy in patients with chronic myeloid leukemia

Data demonstrating the superiority of nilotinib over imatinib in the frontline treatment of chronic myeloid leukemia (CML) and ongoing studies with dasatinib and bosutinib are rapidly changing the treatment landscape for CML. In this review, the authors discuss currently available therapies for CML, focusing on mechanisms of resistance to imatinib and treatment strategies to overcome resistance. Relevant articles were identified through searches of PubMed and abstracts from international hematology/oncology congresses. Additional information sources were identified from the bibliographies of these references and from the authors' own libraries and expertise. In vitro 50% inhibitory concentration (IC(50) ) data alone are not sufficient to guide the choice of a tyrosine kinase inhibitor (TKI) in the presence of a mutant breakpoint cluster region-v-abl Abelson murine leukemia viral oncogene homolog (BCR-ABL) clone, because there is a lack of data regarding how well such IC(50) values correlate with clinical response. A small subset of BCR-ABL mutant clones have been associated with impaired responses to second-generation TKIs (tyrosine to histidine mutation at codon 253 [Y253H], glutamic acid to lysine or valine mutation at codon 255 [E255K/V], and phenylalanine to cysteine or valine mutation at codon 359 [F359C/V] for nilotinib; valine to leucine mutation at codon 299 [V299L] and F317L for dasatinib); neither nilotinib nor dasatinib is active against the threonine to isoleucine mutation at codon 315 (T315I). For each second-generation TKI, the detection of 1 of a small subset of mutations at the time of resistance may be helpful in the selection of second-line therapy [corrected]. For the majority of patients, comorbidities and drug safety profiles should be the basis for choosing a second-line agent. Clinical trial data from an evaluation of the response of specific mutant BCR-ABL clones to TKIs is needed to establish the role of mutation testing in the management of CML.

Techniques for risk stratification of newly diagnosed patients with chronic myeloid leukemia

Chronic myeloid leukemia (CML) is a myeloproliferative neoplasm caused by BCR-ABL, a constitutively active tyrosine kinase generated as a result of the t(9;22)(q34;q11). The natural history of CML is progression from a relatively benign chronic phase to an acute leukemia termed blast crisis. Imatinib, an inhibitor of BCR-ABL tyrosine kinase activity, has a dramatic effect on the natural history of the disease. Despite the favorable outcomes with imatinib, a subset of patients have primary refractory disease, or experience relapse after an initial response. Recently identified molecular predictors of drug response might help predict outcome with tyrosine kinase inhibitor therapy more accurately than clinical prognostication scores, but have not yet been introduced into clinical routine. These techniques include analysis of drug transport proteins, in vitro drug assays, measurement of imatinib plasma levels, BCR-ABL activity monitoring, and gene expression profiling. In this article we review the current status of these technologies, which may ultimately allow us to tailor therapy to a specific patient.

Suboptimal responses in chronic myeloid leukemia: milestones and mechanisms

Patients with chronic myeloid leukemia who fail to achieve timely treatment responses have a worse prognosis. Although many patients respond well to first-line treatment with imatinib, a significant proportion relapse or experience an inadequate response. Since effective alternative Bcr-Abl inhibitors are available (i.e., dasatinib or nilotinib), several regional groups have proposed milestones for imatinib failure or suboptimal response based on the achievement of specified levels of response within a defined treatment duration. A suboptimal response indicates that, although patients may continue to receive a benefit from continuing imatinib treatment at the assigned dose, long-term outcome may be better with an alternative strategy. The underlying mechanisms behind suboptimal responses are multifactorial and may differ from those causing relapse.

Results of allogeneic hematopoietic stem cell transplantation for chronic myelogenous leukemia patients who failed tyrosine kinase inhibitors after developing BCR-ABL1 kinase domain mutations

Hematopoietic stem cell transplantation (HSCT) is effective therapy for patients with chronic myelogenous leukemia (CML) but is now mostly indicated for patients who develop resistance to tyrosine kinase inhibitors (TKIs), which can be associated with point mutations in BCR-ABL1. We reviewed the outcomes of imatinib-resistant CML patients (chronic phase, n = 34; accelerated phase [AP], n = 9; and blast phase [BP], n = 4) who underwent HSCT and had BCR-ABL1 sequencing. Mutations were found in 19 patients (40%); 15 of 19 had advanced CML (AP + BP + second chronic phase). Patients with mutations were more likely to transform to AP/BP at time of imatinib failure (69% vs 35%, P = .03). Forty-two patients (89%) responded to HSCT: 32 (68%) had at least a major molecular response. The 2-year event-free survival was 36% and 58% (P = .05) for the mutant and nonmutant groups, respectively; and the 2-year overall survival was 44% and 76% (P = .02), respectively. HSCT is an important salvage option for TKI-resistant patients with or without BCR-ABL1 mutations. Patients with mutations were more likely to develop advanced disease and had worse outcomes after HSCT. HSCT should be considered early for patients deemed to have a low probability of responding to second-generation TKI.

NCCN Task Force report: tyrosine kinase inhibitor therapy selection in the management of patients with chronic myelogenous leukemia

The advent of imatinib has dramatically improved outcomes in patients with chronic myelogenous leukemia (CML). It has become the standard of care for all patients with newly diagnosed chronic-phase CML based on its successful induction of durable responses in most patients. However, its use is complicated by the development of resistance in some patients. Dose escalation might overcome this resistance if detected early. The second-generation tyrosine kinase inhibitors (TKIs) dasatinib and nilotinib provide effective therapeutic options for managing patients resistant or intolerant to imatinib. Recent studies have shown that dasatinib and nilotinib provide quicker and potentially better responses than standard-dose imatinib when used as a first-line treatment. The goal of therapy for patients with CML is the achievement of a complete cytogenetic response, and eventually a major molecular response, to prevent disease progression to accelerated or blast phase. Selecting the appropriate TKI depends on many factors, including disease phase, primary or secondary resistance to TKI, the agent's side effect profile and its relative effectiveness against BCR-ABL mutations, and the patient's tolerance to therapy. In October 2010, NCCN organized a task force consisting of a panel of experts from NCCN Member Institutions with expertise in the management of patients with CML to discuss these issues. This report provides recommendations regarding the selection of TKI therapy for the management of patients with CML based on the evaluation of available published clinical data and expert opinion among the task force members.

Chronic myeloid leukemia cells refractory/resistant to tyrosine kinase inhibitors are genetically unstable and may cause relapse and malignant progression to the terminal disease state

BCR-ABL1 kinase-induced chronic myeloid leukemia in chronic phase (CML-CP) usually responds to treatment with ABL tyrosine kinase inhibitors (TKIs) such as imatinib, dasatinib, and nilotinib. In most patients TKIs reduce the leukemia cell load substantially, but some leukemia cells, for example leukemia stem cells (LSCs), are intrinsically refractory to TKIs. In addition, some patients who respond initially may later become resistant to TKIs due to accumulation of point mutations in BCR-ABL1 kinase. LSCs or their progeny, leukemia progenitor cells (LPCs), at some stage may acquire additional genetic changes that cause the leukemia to transform further to a more advanced blast phase (CML-BP), which responds poorly to treatment and is usually fatal. We postulate that LSCs and/or LPCs refractory or resistant to TKIs may be 'ticking time-bombs' accumulating additional genetic aberrations and eventually 'exploding' to generate additional TKI-resistant clones and CML-BP clones with complex karyotypes.

Inhibition of autophagy: a new strategy to enhance sensitivity of chronic myeloid leukemia stem cells to tyrosine kinase inhibitors

Imatinib mesylate (IM) has become standard therapy for patients with chronic myeloid leukemia (CML), but CML stem cells are intrinsically resistant to IM and to second/third-generation tyrosine kinase inhibitors (TKIs), allowing the persistence of a 'reservoir' of BCR-ABL-expressing CML-initiating cells potentially responsible for disease progression. Although it is still controversial whether the 'insensitivity' of CML stem cells to treatment with TKIs is due to BCR-ABL-dependent or independent mechanisms, recent evidence indicates that treatment with IM suppresses BCR-ABL-dependent signaling in CML stem cells with no adverse effects on their survival. Treatment of CML cells with IM/TKIs induces autophagy, a genetically regulated process of adaptation to metabolic stress which may allow tumor cells to become metabolically inert, enabling their survival under conditions that may mimic growth factor/nutrient deprivation. Based on this hypothesis, TKI-induced autophagy may 'antagonize' TKI-induced cell death and inhibition of autophagy may eliminate this survival mechanism by restoring 'sensitivity' of CML stem cells to treatment with IM/TKIs. Consistent with this, recent evidence indicates that phenotypically and functionally defined CML-enriched stem cells that are insensitive to treatment with TKIs are efficiently eliminated by the combination of TKI and chloroquine, an inhibitor of late stage autophagy. Thus, inhibition of autophagy may 'sensitize' CML stem cells to treatment with TKIs, thus preserving the high specificity of TKI-based therapies.

BCR-ABL isoforms associated with intrinsic or acquired resistance to imatinib: more heterogeneous than just ABL kinase domain point mutations?

Imatinib, a small molecule inhibitor of ABL, PDGFR and C-KIT, has revolutionized treatment of chronic myeloid leukaemia (CML). However, resistance to treatment is of increasing importance and often is due to point mutations in the Abl kinase domain (Abl KD). Here, we analysed clinical outcome and mutation status in two independent Nordic populations (n = 77) of imatinib-resistant CML patients. We detected BCR-ABL transcripts containing point mutations of residues in the P-loop, A-loop and other kinase domain residues in 32 patients (42%). In contrast to previous data, mutations in BCR-ABL were as frequently found in patients with primary resistance (56%) as with secondary resistance (53%). No T315I mutations were found in the study cohort. BCR-ABL splice variants were identified in a significant number of our cases (19%): BCR-ABL transcripts of variable length; a variant fusion transcript joining BCR exon 14 sequences to ABL exon 4; partial, in-frame-deletion of exon 4 due to induction of a cryptic splice site by the L248V and finally, alternative splicing of ABL exon 7 sequences. Though the majority of splice variants observed in this study do not encode functional proteins, alternative splicing appears to represent a common phenomenon in the biology of CML. We conclude that Abl KD point mutations represent a major mechanism of imatinib resistance. Other sequence irregularities were also detected, but their significance in conferring resistance is unclear. Diagnostic strategies looking for imatinib-resistant clones should be designed to detect a broader profile of BCR-ABL variants than just point mutations.

Philadelphia-positive acute lymphoblastic leukemia patients already harbor BCR-ABL kinase domain mutations at low levels at the time of diagnosis

BACKGROUND

In patients with Philadelphia-positive acute lymphoblastic leukemia, resistance to treatment with tyrosine kinase inhibitors is frequent and most often associated with the development of point mutations in the BCR-ABL kinase domain. We aimed to assess: (i) in how many patients BCR-ABL kinase domain mutations are already detectable at relatively low levels at the time of diagnosis, and (ii) whether mutation detection correlates with subsequent response to therapy.

DESIGN AND METHODS

We retrospectively analyzed samples collected at diagnosis from 15 patients with Philadelphia-positive acute lymphoblastic leukemia who subsequently received tyrosine kinase inhibitor therapy (dasatinib) by cloning the BCR-ABL kinase domain in a bacterial vector and sequencing 200 independent clones per sample.

RESULTS

Mutations at relatively low levels (2-4 clones out of 200) could be detected in all patients--eight who relapsed and seven who achieved persistent remission. Each patient had evidence of two to eight different mutations, the majority of which have never been reported in association with resistance to tyrosine kinase inhibitors. In two patients out of six who relapsed because of a mutation, the mutation (a T315I) was already detectable in a few clones at the time of diagnosis. On the other hand, a patient who was found to harbor an F317L mutation is in persistent remission on dasatinib.

CONCLUSIONS

Our results suggest that the BCR-ABL kinase domain is prone to randomly accumulate point mutations in Philadelphia-positive acute lymphoblastic leukemia, although the presence of these mutations in a relatively small leukemic subclone does not always preclude a primary response to tyrosine kinase inhibitors.

Proceedings of the Third Global Workshop on Chronic Myeloid Leukemia

Tyrosine kinase inhibitor (TKI) therapy has resulted in unprecedented responses and survival rates in Philadelphia chromosome-positive chronic myelogenous leukemia (CML) that are durable for years. However, a third of patients either fail to respond or respond suboptimally to imatinib therapy, while some others are intolerant to imatinib. Increased understanding of the molecular basis of imatinib resistance has led to rational development of second-generation TKIs as effective second-line treatment options for imatinib-resistant patients. However, persistence of minimal residual disease (MRD) and development of resistance against TKI therapy are proving to be significant challenges. Treatment options are evolving for patients with CML and the promising results with several novel agents showing activity in CML, including in patients with the T315I mutation, are encouraging advancements in the field. Recently, a panel of global experts in CML deliberated on the imminent approval of second-generation TKIs in the frontline setting, ways to improve on frontline therapy, integration of new agents in current treatment algorithms, and design of future clinical trials; the proceedings of the discussion are summarized in this article.

Philadelphia chromosome-positive acute lymphoblastic leukemia: current treatment and future perspectives

The Philadelphia chromosome (Ph) is the most common cytogenetic abnormality associated with adult acute lymphoblastic leukemia (ALL). Before the advent of tyrosine kinase inhibitors (TKIs), Ph-positive ALL carried a dismal prognosis and was characterized by a poor response to most chemotherapy combinations, short remission durations, and poor survival rates. Outcomes for patients with Ph-positive ALL improved substantially with the introduction of TKIs, and the TKI imatinib induced complete remissions in >95% of patients with newly diagnosed Ph-positive ALL when it was combined with chemotherapy. However, imatinib resistance remains a problem in a substantial proportion of patients with Ph-positive ALL, and multiple molecular mechanisms that contribute to imatinib resistance have been identified. Second-generation TKIs (eg, dasatinib and nilotinib) have demonstrated promising efficacy in the treatment of imatinib-resistant, Ph-positive ALL. Future strategies for Ph-positive ALL include novel, molecularly targeted treatment modalities and further evaluations of TKIs in combination with established antileukemic agents. For this article, the authors reviewed past, current, and future treatment approaches for adult and elderly patients with Ph-positive ALL with a focus on TKIs and combined chemotherapeutic regimens.

A new nonpeptidic inhibitor of 14-3-3 induces apoptotic cell death in chronic myeloid leukemia sensitive or resistant to imatinib

Resistance of chronic myeloid leukemia (CML) to tyrosine kinase inhibitor imatinib mesylate (IM) is most often due to point mutations in the Bcr-Abl fusion gene. T315I mutation (resulting in substitution of Ile for a Thr residue at the "gatekeeper" position 315) raises particular concern, because it also provides resistance to second-generation kinase inhibitors already approved for clinical use (nilotinib and dasatinib). Much effort is therefore focused on alternative molecular-based strategies. Previous studies proved that binding to 14-3-3 scaffolding proteins leads to cytoplasmic compartmentalization and suppression of proapoptotic and antiproliferative signals associated with Bcr-Abl protein kinase, hence contributing to leukemic clone expansion. Here we investigated the effect of 14-3-3 inhibition disruption on hematopoietic cells expressing the IM-sensitive wild type Bcr-Abl and the IM-resistant T315I mutation. Using a virtual screening protocol and docking simulations, we identified a nonpeptidic inhibitor of 14-3-3, named BV02, that exhibits a remarkable cytotoxicity against both cell types. c-Abl release from 14-3-3σ, promoting its relocation to nuclear compartment (where it triggers transcription of p73-dependent proapoptotic genes) and to mitochondrial membranes (where it induces the loss of mitochondrial transmembrane potential) combined with c-Abl enhanced association with caspase 9 (a critical step of sequential caspase activation further contributing to c-Abl pro-apoptotic function) has a prominent role in the effect of BV02 on Bcr-Abl-expressing cells. In conclusion, BV02 may be considered as a treatment option for CML and, in particular, for more advanced phases of the disease that developed IM resistance as a consequence of Bcr-Abl point mutations.

BCR-ABL tyrosine kinase inhibitors in the treatment of Philadelphia chromosome positive chronic myeloid leukemia: a review

Chronic Myeloid Leukemia (CML) is a clonal disease characterized by the presence of the Philadelphia (Ph+) chromosome and its oncogenic product, BCR-ABL, a constitutively active tyrosine kinase, that is present in >90% of the patients. Epidemiologic data indicates that almost 5000 new cases are reported every year and 10% of these patients eventually succumb to the disease. The treatment of CML was revolutionized by the introduction of imatinib mesylate (IM, Gleevec), a BCR-ABL tyrosine kinase inhibitor (TKI). The clinical use of specific BCR-ABL inhibitors has resulted in a significantly improved prognosis, response rate, overall survival, and patient outcome in CML patients compared to previous therapeutic regimens. However, the complete eradication of CML in patients receiving imatinib was limited by the emergence of resistance mostly due to mutations in the ABL kinase domain and to a lesser extent by molecular residual disease after treatment. The second-generation BCR-ABL TKIs nilotinib (Tasigna) and dasatinib (Sprycel), showed significant activity in clinical trials in patients intolerant or resistant to imatinib therapy, except in those patients with the T315I BCR-ABL mutation. Identifying key components involved in the CML pathogenesis may lead to the exploration of new approaches that might eventually overcome resistance mediated to the BCR-ABL TKIs. Here, we present an overview about the current treatment of Ph+ CML patients with the TKIs and the obstacles to successful treatment with these drugs.

Mutations in ABL kinase domain are associated with inferior progression-free survival

Imatinib mesylate is currently the drug of choice for all phases of chronic myeloid leukemia (CML). Despite the initial high rates of hematological and cytogenetic responses, many patients develop resistance. We prospectively studied 40 patients with CML with primary cytogenetic resistance to imatinib mesylate. A semi-nested reverse transcriptase-PCR approach was used for amplification of the ABL kinase domain, which was then subjected to direct sequencing for the detection of point mutations. Expression of BCR-ABL transcripts was quantified using the real-time Taqman assay, and BCR-ABL gene amplification was detected using fluorescence in situ hybridization. Twelve different point mutations were detected in 18/40 (45%) patients. Five patients had mutations in the P-loop region and 13 had mutations in other regions of the BCR-ABL kinase domain. Progression-free survival at 2 years was inferior for patients with mutations compared to those without mutations (72% vs. 95%, p < 0.0045). The BCR-ABL fusion gene was over-expressed in five patients (5/18); the mean BCR-ABL/ABL ratio was 75.38 vs. 28.72 for imatinib responders, p < 0.001. Amplification of the BCR-ABL fusion gene was detected in 4/40 (10%) patients. Point mutation was the major mechanism of primary cytogenetic resistance to imatinib mesylate in the present study. Patients with mutations had inferior progression-free survival compared to those without mutations. Regular monitoring for the presence of point mutations in the ABL kinase region may identify such patients early, with an opportunity to intervene.

Chronic myeloid leukemia: mechanisms of blastic transformation

The BCR-ABL1 oncoprotein transforms pluripotent HSCs and initiates chronic myeloid leukemia (CML). Patients with early phase (also known as chronic phase [CP]) disease usually respond to treatment with ABL tyrosine kinase inhibitors (TKIs), although some patients who respond initially later become resistant. In most patients, TKIs reduce the leukemia cell load substantially, but the cells from which the leukemia cells are derived during CP (so-called leukemia stem cells [LSCs]) are intrinsically insensitive to TKIs and survive long term. LSCs or their progeny can acquire additional genetic and/or epigenetic changes that cause the leukemia to transform from CP to a more advanced phase, which has been subclassified as either accelerated phase or blastic phase disease. The latter responds poorly to treatment and is usually fatal. Here, we discuss what is known about the molecular mechanisms leading to blastic transformation of CML and propose some novel therapeutic approaches.

Treatment options for patients with chronic myeloid leukemia who are resistant to or unable to tolerate imatinib

BACKGROUND

Imatinib has been found to substantially improve outcomes in patients with chronic myeloid leukemia (CML) compared with previously available therapies. However, its use is complicated by development of resistance or drug intolerance, prompting dose escalation or a trial of dasatinib or nilotinib, the second-generation tyrosine kinase inhibitors (TKIs).

OBJECTIVES

This article reviews the mechanisms of TKI resistance; discusses the tolerability and efficacy of high-dose imatinib, dasatinib, and nilotinib; and provides background for the rational use of second-line treatment options.

METHODS

MEDLINE (1966-December 2009) and EMBASE (1993-December 2009) were searched for pertinent English-language publications using search terms that included, but were not limited to, chronic myeloid leukemia, imatinib, dasatinib, nilotinib, and clinical trial. Abstracts from American Society of Hematology annual meetings (2005-2009) were also reviewed. There were no prespecified inclusion or exclusion criteria.

RESULTS

Major and complete cytogenetic responses (MCyR and CCyR, respectively) to second-line treatment with high-dose (600-800 mg/d PO) imatinib were restricted to CML patients who had achieved a CyR to standard-dose imatinib: >90% of patients without a previous CyR failed to respond. The expected durability of the response to this approach remains unclear. Grade 3/4 thrombocytopenia, neutropenia, and anemia occurred in 14%, 39%, and 8%, respectively, of patients receiving high-dose imatinib. In patients who failed first-line treatment with imatinib, dasatinib (70 mg BID PO) was associated with higher rates of CCyR at 2 years compared with imatinib (44% vs 18%, respectively; P = 0.003), as well as higher estimated rates of progression-free survival at 2 years (86% vs 65%; P = 0.001). Dasatinib use was complicated by grade 3/4 thrombocytopenia and neutropenia in 57% and 63% of patients, respectively, and pleural effusion in 5%. Nilotinib treatment was effective in patients who were resistant to or unable to tolerate imatinib, with 46% and 58% achieving a CCyR and MCyR, respectively, at 2 years. Nilotinib use was complicated by grade 3/4 thrombocytopenia and neutropenia in 28% and 40% of patients, respectively, and QTc-interval prolongation in 1% to 10% of patients. Neither agent was clinically effective in patients with the common T315I mutation.

CONCLUSION

Dasatinib and nilotinib were effective and generally well tolerated as second-line treatments for CML patients with a suboptimal response to standard doses of imatinib or imatinib intolerance.

Optimizing therapy for patients with chronic myelogenous leukemia in chronic phase

Identification of BCR-ABL as the defining leukemogenic event in chronic myelogenous leukemia (CML) revolutionized the treatment of the disease. Imatinib, a potent BCR-ABL inhibitor, is the standard of care for the first-line treatment of patients with chronic-phase CML because of its high long-term response rates and favorable tolerability profile compared with previous standard therapies. However, resistance to imatinib develops in 2% to 4% of patients annually. For patients with acquired cytogenetic resistance to standard-dose imatinib (400 mg daily), imatinib dose escalation (600-800 mg daily) is an excellent first option for managing patients and achieving cytogenetic responses. However, for patients with primary resistance to imatinib, hematologic disease recurrence, or emergent BCR-ABL kinase domain mutations, imatinib dose escalation may not be sufficient to control the disease. For these patients, the potent second-generation tyrosine kinase inhibitors dasatinib and nilotinib are available. Both agents provide effective therapeutic options for patients with imatinib resistance or intolerance. For the current overview, the authors reviewed the data supporting the use of both dasatinib and nilotinib in imatinib-resistant or imatinib-intolerant patients, and they have highlighted the future of CML therapy. Overall, the article is intended to offer physicians a comprehensive review of the current literature and to provide data supporting various treatment options for patients with CML throughout the course of imatinib therapy and beyond.

Pyrrolo[1,2-b][1,2,5]benzothiadiazepines (PBTDs) exert their anti-proliferative activity by interfering with Akt-mTOR signaling and bax:bcl-2 ratio modulation in cells from chronic myeloid leukemic patients

In our study we found that pyrrolo[1,2-b][1,2,5]benzothiadiazepines (PBTDs) mediated apoptosis in primary leukemia cells from 27 chronic myelogenous leukemia (CML) patients at onset through the activation of the caspase-9 and -3, and cleavage of poly (ADP-ribose) polymerase (PARP). The bax:bcl-2 ratio was increased as a consequence of down-regulation of bcl-2 and up-regulation of bax proteins in response to treatment with PBTDs. In addition, PBTDs were able to induce cell death in primary leukemia cells derived from 23 CML-chemoresistant patients. Furthermore, the effects of PBTDs on the Akt-mTOR (mammalian target of rapamycin) pathway were determined by Western blot. PBTDs possessed inhibitory activity against mTOR and also impeded hyper-phosphorylation of Akt as a feedback of inhibition of mTOR by rapamycin. The results presented in this study demonstrate that we have identified the PBTDs as restoring the apoptotic pathways both in primary leukemia cells derived from CML patients at onset and in primary leukemia cells derived from CML-chemoresistant patients, thus showing their ability to undergo apoptosis. These compounds constitute a promising therapeutic approach for patients with leukemia. They provide the basis for new strategies for an additional anticancer drug in leukemia therapies, especially when conventional ones fail.

Molecular monitoring of patients with chronic myeloid leukemia: clinical examples from a non-trial setting

The molecular monitoring of chronic myeloid leukemia allows the clinician a minimally invasive method to judge response to tyrosine kinase therapy and to predict outcome and relapse. Because there are several treatment options for patients with suboptimal response, the ability to proactively predict and respond to relapse makes the "personalization" of treatment a realizable goal. There are practical issues with molecular monitoring, however, including availability of assays, standardization of tests, and the learning curve as doctors and patients learn to follow BCR-ABL levels with interest and reason. This review will examine the use of molecular monitoring in the non-trial setting, concentrating on pitfalls that can occur in the real-world delivery of complex medical care.

Cause and management of therapy resistance

A minority of patients treated with imatinib are either refractory to imatinib or eventually relapse. Relapse frequently depends on re-emergence of BCR-ABL kinase activity but may also indicate BCR-ABL-independent disease progression. Over 90 point mutations coding for single amino acid substitutions in the BCR-ABL kinase domain have been isolated from CML patients resistant to imatinib treatment. These mutations affect amino acids involved in imatinib binding or in regulatory regions of the BCR-ABL kinase domain, resulting in decreased sensitivity to imatinib while retaining aberrant kinase activity. The early detection of BCR-ABL mutants during therapy may aid in risk stratification as well as molecular-based treatment decisions. Therapeutic strategies of imatinib resistant disease include novel tyrosine kinase inhibitors with activity against imatinib-resistant mutations and/or with inhibition of alternative pathways, dose escalation to optimise imatinib levels, treatment interruption to stop selection of resistant cells and allogeneic stem cell transplantation in eligible patients.

Chronic myeloid leukemia 2010: where are we now and where can we go?

Chronic myeloid leukemia is a model of how the molecular understanding of a disease can provide the platform for therapy and diagnostics. Clinicians are now empowered with first- and second-generation tyrosine kinases, as well as molecular tools to monitor disease and characterize resistance. However, there are still unanswered questions regarding optimization of therapy, the utility of molecular monitoring, and the search (or need) of "cure" that bears thought. In this review, we will discuss these issues, as they provide a roadmap for what may lie ahead in the therapy of other hematologic malignancies, particular the other myeloproliferative syndromes, where specific genetic lesions, and targeted therapy, are now being realized.