Second-line treatment with dasatinib in patients resistant to imatinib can select novel inhibitor-specific BCR-ABL mutants in Ph+ ALL

Current Management of Chronic Myeloid Leukemia Myeloid Blast Phase

Despite the major advancements in the management of chronic phase (CP) chronic myeloid leukemia (CML), blast crisis (BC) remains a major therapeutic challenge. BC can be myeloid, lymphoid, or mixed lineage with myeloid BC being the most common type. BC in CML is mediated by aberrant tyrosine kinase activity of the BCR::ABL fusion protein. The introduction of BCR::ABL tyrosine kinase inhibitor (TKI) has been a gamechanger in the treatment of CML and there has been a significant reduction in the incidence of BC. The main treatment goal in BC is to achieve a second CP and consolidate that with an allogeneic stem cell transplantation (SCT) in eligible patients. The outcomes in BC remain dismal even in the current era. In this review, we provide an overview of the biology and current therapeutic approach in myeloid BC.

Investigation on the interaction behavior of afatinib, dasatinib, and imatinib docked to the BCR-ABL protein

Chronic myeloid leukemia (CML) is a pathological condition associated with the uncontrolled proliferation of white blood cells and respective loss of function. Imatinib was the first drug that could effectively treat this condition, but its use is hindered by the development of mutations of the BCR-ABL protein, which are the cause of resistance. Therefore, dasatinib and afatinib present similarities that can be explored to discover new molecules capable of overcoming the effects of imatinib. Afatinib exhibited electronic and docking behavior, indicating that a replacement with some minor modifications could design a new potential inhibitor. The amide group in each candidate is clearly of pharmacophoric importance, and it needs to concentrate a negative region. Sulfur group presents a good pharmacophoric profile, which was shown by dasatinib results, adding to the influence of the Met318 residue in the target protein active site configuration. This behavior suggests that the sulfur atom and other fragments that have an affinity for the methionine sidechain may provide a significant positive effect when present in TKI molecules such as afatinib or dasatinib.

Discovery and Protein Modeling Studies of Novel Compound Mutations Causing Resistance to Multiple Tyrosine Kinase Inhibitors in Chronic Myeloid Leukemia

OBJECTIVE

BCR-ABL fusion oncogene is the hallmark of chronic myeloid leukemia (CML), causing genomic instability which leads to accumulation of mutations in BCR-ABL as well as other genes. BCR-ABL mutations are the cause of tyrosine kinase inhibitors (TKIs) resistance in CML. Recently, compound BCR-ABL mutations have been reported to resist all FDA approved TKIs. Therefore, finding novel compound BCR-ABL mutations can help and clinically manage CML. Therefore, our objective was to find out novel drug-resistant compound BCR-ABL mutations in CML and carry out their protein modelling studies.

METHODOLOGY

Peripheral blood samples were collected from ten imatinib resistant CML patients receiving nilotinib treatment. BCR-ABL transcript mutations were investigated by employing capillary sequencing. Patient follow-up was carried out using European LeukemiaNet guidelines. Protein modeling  studies were carried out for new compound mutations using PyMol to see the effects of mutations at structural level.

RESULTS

A novel compound mutation (K245N mutation along with G250W mutation) and previously known T351I utation was detected in two of the nilotinib resistance CML patients respectively while in the rest of 8 nilotinib responders, no resistant mutations were detected. Protein modelling studies indicated changes in BCR-ABL mutant protein which may have negatively impacted its binding with nilotinib leading to drug resistance.

CONCLUSION

We report a novel nilotinib resistant BCR-ABL compound mutation (K245N along with G250W mutation) which impacts structural modification in BCR-ABL mutant protein leading to drug resistance. As compound mutations pose a new threat by causing resistance to all FDA approved tyrosine kinase inhibitors in BCR-ABL+ leukemias, our study opens a new direction for in vitro characterization of novel BCR-ABL compound mutations and their resistant to second  generation and third generation TKIs.

Mutations in the BCR-ABL1 Kinase Domain and Elsewhere in Chronic Myeloid Leukemia

Chronic myeloid leukemia (CML) has been the first human malignancy to be associated, more than 50 years ago, with a consistent chromosomal abnormality--the t(9;22)(q34;q11) chromosomal translocation. The resulting BCR-ABL1 fusion gene, encoding a tyrosine kinase with deregulated activity, has a central role in the pathogenesis of CML. Ancestral or additional genetic events necessary for CML to develop have long been hypothesized but never really demonstrated. CML can successfully be treated with tyrosine kinase inhibitors (TKIs). Mutations in the BCR-ABL1 kinase domain might arise, however, that confer resistance to 1 or more of the currently available TKIs. Hence, the critical role of BCR-ABL1 mutation screening for optimal therapeutic management, with the current gold standard technique, conventional sequencing, likely to be replaced soon by ultra-deep sequencing. Mutations in genes other than BCR-ABL1 include ASXL1, TET2, RUNX1, DNMT3A, EZH2, and TP53 in chronic phase patients and RUNX1, ASXL1, IKZF1, WT1, TET2, NPM1, IDH1, IDH2, NRAS, KRAS, CBL, TP53, CDKN2A, RB1, and GATA-2 mutations in advanced phase patients. The latter also display additional cytogenetic abnormalities, including submicroscopic regions of gain or loss that only single nucleotide polymorphism arrays or array comparative genomic hybridization can detect. Whether whole genome/exome sequencing studies will uncover novel mutations relevant for pathogenesis, progression, and risk-adapted therapy is still unclear.

Best Practices in Chronic Myeloid Leukemia Monitoring and Management

: Optimal use of current therapeutic opportunities for chronic myeloid leukemia patients requires integration of clinical and laboratory monitoring. Assessment of molecular response (MR) by real-time quantitative polymerase chain reaction is the most sensitive way to monitor tyrosine kinase inhibitor (TKI) treatment efficacy. Besides major molecular response, which has emerged as a safe haven for survival since the initial studies of first-line imatinib treatment, two additional MR milestones have recently been defined: early molecular response and deep molecular response. The achievement of such MR milestones within defined time points during therapy is thought to draw the ideal trajectory toward optimal long-term outcome and, possibly, successful treatment discontinuation. Sensitive and reproducible MR measurement and proper interpretation of MR results are therefore critical to correctly inform therapeutic decisions. In patients who do not achieve an optimal response to TKI therapy, BCR-ABL1 mutation screening should also be performed, because it may deliver useful information for TKI choice. This review aims to help clinicians apply and translate the latest response definitions and clinical recommendations into practice. We provide a critical update on how these recommendations have incorporated MR levels in the clinical decision algorithms and how detection of BCR-ABL1 mutations should be interpreted. We also include a practical guide for pathologists and molecular biologists to best perform molecular testing and for hematologists and oncologists to best integrate it into routine practice.

IMPLICATIONS FOR PRACTICE

Ever-more-potent therapeutic strategies have been developed for chronic myeloid leukemia (CML) in parallel with the evolution of therapeutic goals and the refinement of response definitions and monitoring schemes and procedures. Terminology and methodology continue to evolve rapidly, making it difficult for busy hematology/oncology professionals to keep abreast of the newest developments. Optimal CML patient management results from the timely and rational use of molecular testing, the critical assessment of the power and pitfalls of current technology, and the appropriate interpretation and contextualization of results.

The development of dasatinib as a treatment for chronic myeloid leukemia (CML): from initial studies to application in newly diagnosed patients

PURPOSE

Dasatinib is a dual Abl/Src tyrosine kinase inhibitor (TKI) designed as a prototypic short-acting BCR-ABL-targeted TKI that inhibits BCR-ABL with greater potency compared with imatinib, nilotinib, bosutinib, and ponatinib and has been shown to have potential immunomodulatory effects. Dasatinib is approved for the treatment of all phases of chronic myeloid leukemia (CML) and Philadelphia chromosome-positive acute lymphoblastic leukemia resistant or intolerant to prior imatinib treatment and first-line treatment for CML in chronic phase. In this article, the development of dasatinib as a treatment for patients with CML is reviewed.

METHODS

This is a review of the relevant literature regarding dasatinib development in CML (2003-2013).

RESULTS

Dasatinib demonstrates efficacy against most BCR-ABL mutations arising during imatinib therapy and is effective in treating patients with imatinib resistance due to other mechanisms. Randomized trial data show that first-line dasatinib provides superior responses compared with imatinib and enables patients to achieve early, deep responses correlated with improved longer-term outcomes. Dasatinib has a generally acceptable safety profile, with most adverse events (AEs) proving manageable and reversible. Cytopenias are commonly observed with dasatinib, and some nonhematologic AEs including pleural effusion have been consistently reported.

CONCLUSION

Dasatinib is an effective treatment option for patients with CML.

Resistance to tyrosine kinase inhibition therapy for chronic myelogenous leukemia: a clinical perspective and emerging treatment options

The development of tyrosine kinase inhibitors (TKIs) has led to extended lifespans for many patients with chronic myelogenous leukemia (CML). However, 20% to 30% of patients fail to respond, respond suboptimally, or experience disease relapse after treatment with imatinib. A key factor is drug resistance. The molecular mechanisms implicated in this resistance include those that involve upregulation or mutation of BCR-ABL kinase and those that are BCR-ABL independent. The clinical consequences of these molecular mechanisms of resistance for disease pathogenesis remain open for debate. This review summarizes the molecular mechanisms and clinical consequences of TKI resistance and addresses the current and future treatment approaches for patients with TKI-resistant CML.

Dasatinib in chronic myeloid leukemia: a limited Indian experience

AIM

To report our experience of the use of dasatinib in various phases of chronic myeloid leukemia (CML).

METHODS

Ten patients in various phases of CML, not responding to imatinib and started on dasatinib, were included and analyzed. The baseline characteristics of the patients and their salient features including the duration and response to initial therapy as well as to dasatinib, were noted.

RESULTS

Before starting dasatinib three patients were in chronic phase of CML while seven others were in the progressive phase (accelerated and blast phase) of CML. Half the patients developed transient grade 3 and 4 toxicities to dasatinib. Overall, the tolerability of the drug in all 10 patients was acceptable and none discontinued treatment. Three patients died due to progressive disease while the remaining seven are continuing the drug with the disease still under cytogenetic or hematological remission. Of the 10 patients, seven achieved complete hematological response and two of the accelerated phase/blast crisis patients achieved complete cytogenetic response. Overall, dasatinib was able to control disease for a median of 20.6 months.

CONCLUSION

Despite small sample size and insufficient information on mutational analysis, dasatinib is effective in CML in India. Cost limits the use of second-generation tyrosine kinase inhibitors in India. Our observation is not suitable for survival analysis but the difference made by dasatinib in progressive disease and its tolerability needs to be acknowledged.

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.

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.

Dasatinib: a review of its use in the treatment of chronic myeloid leukaemia and Philadelphia chromosome-positive acute lymphoblastic leukaemia

Dasatinib (Sprycel®) is an orally administered small molecule inhibitor of multiple tyrosine kinases, including BCR-ABL and SRC family kinases, which is indicated for the treatment of adults with newly diagnosed chronic-phase chronic myeloid leukaemia (CML), CML (chronic-, accelerated- or blast-phase) with resistance or intolerance to prior therapy, including imatinib, or Philadelphia chromosome-positive (Ph+) acute lymphoblastic leukaemia (ALL) with resistance or intolerance to prior therapy. Dasatinib is ≈325-fold more active than imatinib in inhibiting wild-type ABL kinase in vitro and is active against a wide variety of imatinib-resistant BCR-ABL mutants, except for T315I. This article reviews the efficacy and tolerability of dasatinib in the treatment of patients with newly diagnosed chronic-phase CML or imatinib-resistant or -intolerant CML or Ph+ ALL, as well as summarizing its pharmacological properties. In clinical trials, oral dasatinib was effective in achieving major or complete cytogenetic responses in both newly diagnosed and imatinib-resistant or -intolerant chronic-phase CML. Dasatinib was likewise effective in achieving major or overall haematological responses in imatinib-resistant or -intolerant, accelerated- or blast-phase CML, or Ph+ ALL. Responses were rapidly achieved within 1-3 months and were durable over 1-5 years of follow-up. The majority of adverse events with dasatinib were of mild to moderate severity. Fluid retention (including pleural effusion) was the most common adverse event. Haematological abnormalities were common and cytopenias were the most common grade 3/4 adverse events. Dasatinib 100 mg administered once daily was as effective as dasatinib 70 mg administered twice daily, and was better tolerated, being associated with lower incidences of pleural effusion and grade 3/4 thrombocytopenia, in particular. Dasatinib was more effective than high-dose imatinib in the treatment of patients with imatinib-resistant chronic-phase CML and was more effective than standard dosages of imatinib, as well as being associated with less frequent fluid retention, in patients with newly diagnosed chronic-phase CML. Dasatinib was generally equally effective in patients with or without BCR-ABL mutations at baseline. Therefore, oral dasatinib is a highly effective once-daily therapy for the first-line treatment of newly diagnosed patients with chronic-phase CML, as well as for the treatment of patients with imatinib-resistant or -intolerant chronic- and advanced-phase CML or Ph+ ALL.

Destabilization of Bcr-Abl/Jak2 Network by a Jak2/Abl Kinase Inhibitor ON044580 Overcomes Drug Resistance in Blast Crisis Chronic Myelogenous Leukemia (CML)

Bcr-Abl is the predominant therapeutic target in chronic myeloid leukemia (CML), and tyrosine kinase inhibitors (TKIs) that inhibit Bcr-Abl have been successful in treating CML. With progression of CML disease especially in blast crisis stage, cells from CML patients become resistant to imatinib mesylate (IM) and other TKIs, resulting in relapse. Because Bcr-Abl is known to drive multiple signaling pathways, the study of the regulation of stability of Bcr-Abl in IM-resistant CML cells is a critical issue as a possible therapeutic strategy. Here, we report that a new dual-kinase chemical inhibitor, ON044580, induced apoptosis of Bcr-Abl+ IM-sensitive, IM-resistant cells, including the gatekeeper Bcr-Abl mutant, T315I, and also cells from blast crisis patients. In addition, IM-resistant K562-R cells, cells from blast crisis CML patients, and all IM-resistant cell lines tested had reduced ability to form colonies in soft agar in the presence of 0.5 µM ON044580. In in vitro kinase assays, ON044580 inhibited the recombinant Jak2 and Abl kinase activities when the respective Jak2 and Abl peptides were used as substrates. Incubation of the Bcr-Abl+ cells with ON044580 rapidly reduced the levels of the Bcr-Abl protein and also reduced the expression of HSP90 and its client protein levels. Lysates of Bcr-Abl+ cell lines were found to contain a large signaling network complex composed of Bcr-Abl, Jak2, HSP90, and its client proteins as detected by a gel filtration column chromatography, which was rapidly disrupted by ON044580. Therefore, targeting Jak2 and Bcr-Abl kinases is an effective way to destabilize Bcr-Abl and its network complex, which leads to the onset of apoptosis in IM-sensitive and IM-resistant Bcr-Abl+ cells. This inhibitory strategy has potential to manage all types of drug-resistant CML cells, especially at the terminal blast crisis stage of CML, where TKIs are not clinically useful.

Choosing the best second-line tyrosine kinase inhibitor in imatinib-resistant chronic myeloid leukemia patients harboring Bcr-Abl kinase domain mutations: how reliable is the IC₅₀?

Development of drug resistance to imatinib mesylate in chronic myeloid leukemia (CML) patients is often accompanied by selection of point mutations in the kinase domain (KD) of the Bcr-Abl oncoprotein, where imatinib binds. Several second-generation tyrosine kinase inhibitors (TKIs) have been designed rationally so as to enhance potency and retain the ability to bind mutated forms of Bcr-Abl. Since the preclinical phase of their development, most of these inhibitors have been tested in in vitro studies to assess their half maximal inhibitory concentration (IC₅₀) for unmutated and mutated Bcr-Abl-that is, the drug concentration required to inhibit the cell proliferation or the phosphorylation processes driven by either the unmutated or the mutated forms of the kinase. A number of such studies have been published, and now that two inhibitors-dasatinib and nilotinib-are available for the treatment of imatinib-resistant cases, it is tempting for clinicians to reason on the IC₅₀ values to guess, case by case, which one will work best in patients harboring specific Bcr-Abl KD mutations. Here, we discuss the pros and cons of using this approach in TKI selection.

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.

Seeking the causes and solutions to imatinib-resistance in chronic myeloid leukemia

Although only 5000 new cases of chronic myeloid leukemia (CML) were seen in the United States in 2009, this neoplasm continues to make scientific headlines year-after-year. Advances in understanding the molecular pathogenesis coupled with exciting developments in both drug design and development, targeting the initiating tyrosine kinase, have kept CML in the scientific limelight for more than a decade. Indeed, imatinib, a small-molecule inhibitor of the leukemia-initiating Bcr-Abl tyrosine kinase, has quickly become the therapeutic standard for newly diagnosed chronic phase-CML (CP-CML) patients. Yet, nearly one-third of patients will still have an inferior response to imatinib, either failing to respond to primary therapy or demonstrating progression after an initial response. Significant efforts geared toward understanding the molecular mechanisms of imatinib resistance have yielded valuable insights into the cellular biology of drug trafficking, enzyme structure and function, and the rational design of novel small molecule enzyme inhibitors. Indeed, new classes of kinase inhibitors have recently been investigated in imatinib-resistant CML. Understanding the pathogenesis of tyrosine kinase inhibitor resistance and the molecular rationale for the development of second and now third generation therapies for patients with CML will be keys to further disease control over the next 10 years.

Philadelphia-positive patients who already harbor imatinib-resistant Bcr-Abl kinase domain mutations have a higher likelihood of developing additional mutations associated with resistance to second- or third-line tyrosine kinase inhibitors

Dasatinib and nilotinib are tyrosine kinase inhibitors (TKIs) developed to overcome imatinib resistance in Philadelphia-positive leukemias. To assess how Bcr-Abl kinase domain mutation status evolves during sequential therapy with these TKIs and which mutations may further develop and impair their efficacy, we monitored the mutation status of 95 imatinib-resistant patients before and during treatment with dasatinib and/or nilotinib as second or third TKI. We found that 83% of cases of relapse after an initial response are associated with emergence of newly acquired mutations. However, the spectra of mutants conferring resistance to dasatinib or nilotinib are small and nonoverlapping, except for T315I. Patients already harboring mutations had higher likelihood of relapse associated with development of further mutations compared with patients who did not harbor mutations (23 of 51 vs 8 of 44, respectively, for patients who relapsed on second TKI; 13 of 20 vs 1 of 6, respectively, for patients who relapsed on third TKI).

Impact of baseline BCR-ABL mutations on response to nilotinib in patients with chronic myeloid leukemia in chronic phase

PURPOSE

Nilotinib is a second-generation tyrosine kinase inhibitor indicated for the treatment of patients with chronic myeloid leukemia (CML) in chronic phase (CP; CML-CP) and accelerated phase (AP; CML-AP) who are resistant to or intolerant of prior imatinib therapy. In this subanalysis of a phase II study of nilotinib in patients with imatinib-resistant or imatinib-intolerant CML-CP, the occurrence and impact of baseline and newly detectable BCR-ABL mutations were assessed.

PATIENTS AND METHODS

Baseline mutation data were assessed in 281 (88%) of 321 patients with CML-CP in the phase II nilotinib registration trial.

RESULTS

Among imatinib-resistant patients, the frequency of mutations at baseline was 55%. After 12 months of therapy, major cytogenetic response (MCyR) was achieved in 60%, complete cytogenetic response (CCyR) in 40%, and major molecular response (MMR) in 29% of patients without baseline mutations versus 49% (P = .145), 32% (P = .285), and 22% (P = .366), respectively, of patients with mutations. Responses in patients who harbored mutations with high in vitro sensitivity to nilotinib (50% inhibitory concentration [IC(50)] <or= 150 nM) or mutations with unknown nilotinib sensitivity were equivalent to those responses for patients without mutations (not significant). Patients with mutations that were less sensitive to nilotinib in vitro (IC(50) > 150 nM; Y253H, E255V/K, F359V/C) had less favorable responses, as 13%, 43%, and 9% of patients with each of these mutations, respectively, achieved MCyR; none achieved CCyR.

CONCLUSION

For most patients with imatinib resistance and with mutations, nilotinib offers a substantial probability of response. However, mutational status at baseline may influence response. Less sensitive mutations that occurred at three residues defined in this study, as well as the T315I mutation, may be associated with less favorable responses to nilotinib.

Targeting autophagy potentiates tyrosine kinase inhibitor-induced cell death in Philadelphia chromosome-positive cells, including primary CML stem cells

Imatinib mesylate (IM), a potent inhibitor of the BCR/ABL tyrosine kinase, has become standard first-line therapy for patients with chronic myeloid leukemia (CML), but the frequency of resistance increases in advancing stages of disease. Elimination of BCR/ABL-dependent intracellular signals triggers apoptosis, but it is unclear whether this activates additional cell survival and/or death pathways. We have shown here that IM induces autophagy in CML blast crisis cell lines, CML primary cells, and p210BCR/ABL-expressing myeloid precursor cells. IM-induced autophagy did not involve c-Abl or Bcl-2 activity but was associated with ER stress and was suppressed by depletion of intracellular Ca2+, suggesting it is mechanistically nonoverlapping with IM-induced apoptosis. We further demonstrated that suppression of autophagy using either pharmacological inhibitors or RNA interference of essential autophagy genes enhanced cell death induced by IM in cell lines and primary CML cells. Critically, the combination of a tyrosine kinase inhibitor (TKI), i.e., IM, nilotinib, or dasatinib, with inhibitors of autophagy resulted in near complete elimination of phenotypically and functionally defined CML stem cells. Together, these findings suggest that autophagy inhibitors may enhance the therapeutic effects of TKIs in the treatment of CML.

Mechanisms of resistance to tyrosine kinase inhibitors in chronic myeloid leukemia and recent therapeutic strategies to overcome resistance

Given its relative rarity, it may at first seem surprising that chronic myeloid leukemia (CML) has garnered so much attention over the last decade. Yet, the advances in molecular pathogenesis that have been derived from studying this leukemia have clearly benefited all of oncology. Moreover, the strides in drug design and development that have also ensued around CML have given rise to what others have called a molecular revolution in cancer therapy. While a majority of patients with chronic phase CML (CP-CML) have an excellent durable response to imatinib (Gleevec, Novartis, Basel, Switzerland), a clear minority will unfortunately have signs of primary or secondary resistance to therapy. Significant efforts geared toward understanding the molecular mechanisms of imatinib resistance have yielded valuable insights into the biology of drug trafficking into and out of cells, epigenetic control of cellular processes, alterations in enzymatic structures, and the rational structural-based design of small molecule enzyme inhibitors. This review will describe the efforts at understanding the pathogenesis of imatinib resistance and the molecular rationale for the development of second- and now third-generation therapies for patients with CML.

Laboratory practice guidelines for detecting and reporting BCR-ABL drug resistance mutations in chronic myelogenous leukemia and acute lymphoblastic leukemia: a report of the Association for Molecular Pathology

The BCR-ABL tyrosine kinase produced by the t(9;22)(q34;q11) translocation, also known as the Philadelphia chromosome, is the initiating event in chronic myeloid leukemia (CML) and Ph+ acute lymphoblastic leukemia (ALL). Targeting of BCR-ABL with tyrosine kinase inhibitors (TKIs) has resulted in rapid clinical responses in the vast majority of patients with CML and Philadelphia chromosome+ ALL. However, long-term use of TKIs occasionally results in emergence of therapy resistance, in part through the selection of clones with mutations in the BCR-ABL kinase domain. We present here an overview of the current practice in monitoring for such mutations, including the methods used, the clinical and laboratory criteria for triggering mutational analysis, and the guidelines for reporting BCR-ABL mutations. We also present a proposal for a public database for correlating mutational status with in vitro and in vivo responses to different TKIs to aid in the interpretation of mutation studies.

Comparison of mutated ABL1 and JAK2 as oncogenes and drug targets in myeloproliferative disorders

Constitutively activated mutants of the non-receptor tyrosine kinases (TK) ABL1 (Abelson murine leukemia viral (v-abl) homolog (1) protein) and JAK2 (JAnus Kinase 2 or Just Another Kinase 2) play a central role in the pathogenesis of clinically and morphologically distinct chronic myeloproliferative disorders but are also found in some cases of de novo acute leukemia and lymphoma. Ligand-independent activation occurs as a consequence of point mutations or insertions/deletions within functionally relevant regulatory domains (JAK2) or the creation of TK fusion proteins by balanced reciprocal translocations, insertions or episomal amplification (ABL1 and JAK2). Specific abnormalities are correlated with clinical phenotype, although some are broad and encompass several World Health Organization-defined entities. TKs are excellent drug targets as exemplified by the activity of imatinib in BCR-ABL1-positive disease, particularly chronic myeloid leukemia. Resistance to imatinib is seen in a minority of cases and is often associated with the appearance of secondary point mutations within the TK domain of BCR-ABL1. These mutations are highly variable in their sensitivity to increased doses of imatinib or alternative TK inhibitors such as nilotinib or dasatinib. Selective and non-selective inhibitors of JAK2 are currently being developed, and encouraging data from pre-clinical experiments and initial phase-I studies regarding efficacy and potential toxicity of these compounds have already been reported.

EXEL-7647 inhibits mutant forms of ErbB2 associated with lapatinib resistance and neoplastic transformation

PURPOSE

Mutations associated with resistance to kinase inhibition are an important mechanism of intrinsic or acquired loss of clinical efficacy for kinase-targeted therapeutics. We report the prospective discovery of ErbB2 mutations that confer resistance to the small-molecule inhibitor lapatinib.

EXPERIMENTAL DESIGN

We did in vitro screening using a randomly mutagenized ErbB2 expression library in Ba/F3 cells, which were dependent on ErbB2 activity for survival and growth.

RESULTS

Lapatinib resistance screens identified mutations at 16 different ErbB2 amino acid residues, with 12 mutated amino acids mapping to the kinase domain. Mutations conferring the greatest lapatinib resistance cluster in the NH2-terminal kinase lobe and hinge region. Structural computer modeling studies suggest that lapatinib resistance is caused by multiple mechanisms; including direct steric interference and restriction of conformational flexibility (the inactive state required for lapatinib binding is energetically unfavorable). ErbB2 T798I imparts the strongest lapatinib resistance effect and is analogous to the epidermal growth factor receptor T790M, ABL T315I, and cKIT T670I gatekeeper mutations that are associated with clinical drug resistance. ErbB2 mutants associated with lapatinib resistance transformed NIH-3T3 cells, including L755S and T733I mutations known to occur in human breast and gastric carcinomas, supporting a direct mechanism for lapatinib resistance in ErbB2-driven human cancers. The epidermal growth factor receptor/ErbB2/vascular endothelial growth factor receptor inhibitor EXEL-7647 was found to inhibit almost all lapatinib resistance-associated mutations. Furthermore, no ErbB2 mutations were found to be associated with EXEL-7647 resistance and lapatinib sensitivity.

CONCLUSIONS

Taken together, these data suggest potential target-based mechanisms of resistance to lapatinib and suggest that EXEL-7647 may be able to circumvent these effects.

The Bcr-Abl tyrosine kinase inhibitor imatinib and promising new agents against Philadelphia chromosome-positive leukemias

Chronic myeloid leukemia (CML) was the first human malignant disease to be linked to a single, acquired genetic abnormality. Identification of the Bcr-Abl kinase fusion protein and its pivotal role in the pathogenesis of CML provided new opportunities to develop molecular-targeted therapies. Imatinib mesylate (IM, Gleevec, Novartis Pharmaceuticals, Basel, Switzerland), which specifically inhibits the autophosphorylation of the Abl TK, has improved the treatment of CML. However, resistance is often reported in patients with advanced-stage disease. Several novel TK inhibitors have been developed that override IM resistance mechanisms caused by point mutations within the Abl kinase domain. Inhibitors of Abl TK are divided into two main groups, namely, ATP-competitive and ATP noncompetitive inhibitors. The ATP-competitive inhibitors fall into two subclasses, the Src/Abl inhibitors, and the 2-phenylaminopyrimidine-based compounds. Dasatinib (formerly BMS-354825), AP23464, SKI-606, and PD166326 are classified as Src/Abl inhibitors, while nilotinib (AMN107) and INNO-406 (NS-187) belong to the latter subclass of inhibitors. Of these agents, dasatinib and nilotinib underwent clinical trials earlier than the others and favorable results are now accumulating. Clinical studies of the other compounds, including SKI-606 and INNO-406, have been performed in rapid succession. Because of their strong affinities for the ATP-binding site compared to IM, most ATP-competitive inhibitors may be effective in IM-resistant patients. However, an ATP-competitive inhibitor that can inhibit the phosphorylation of T315I Bcr-Abl has not yet been developed. Instead, ATP noncompetitive inhibitors, such as ON012380, Aurora kinase inhibitor MK0457 (VX-680), and p38 MAP kinase inhibitor BIRB-796, have been developed to address this problem. This review provides an update on the underlying pathophysiologies of disease progression and IM resistance, and discusses the development of new targeted TK inhibitors for managing CML and the importance of future strategies targeting CML stem cells.

Philadelphia chromosome positive acute lymphocytic leukemia: a new era of challenges

Significant advances in the treatment of Philadelephia chromosome (Ph)- or BCR-ABL-positive acute lymphocytic leukemia (ALL) have been made since the discovery of the selective ABL tyrosine kinase inhibitors (TKIs). Whereas the outcome with standard chemotherapy was previously dismal, incorporation of imatinib mesylate into frontline therapy has improved relapse-free and overall survival. The benefit of imatinib extends particularly to instances where allogeneic stem cell transplantation in first complete remission is prohibited by older age, comorbidities, or lack of a suitable donor. However, the emergence of resistance to imatinib presents new therapeutic challenges. The development of novel TKIs with enhanced inhibitory potency against ABL and other kinases may further improve on the results observed with imatinib. Optimal use of these novel agents in the treatment schema of Ph(+) ALL will be paramount in ensuring continued success in the eradication of this disease. Herein, the new approaches to the management of Ph(+) ALL are reviewed.