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. 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.     

Ever-advancing chronic myeloid leukemia treatment

Treatment of chronic myeloid leukemia (CML) has been drastically changed by the emergence of the ABL tyrosine kinase inhibitor (TKI), imatinib mesylate. However, resistance and intolerance have frequently been reported, particularly in patients with advanced-stage disease. Point mutations within the ABL kinase domain that interfere with imatinib binding are the most critical cause of imatinib resistance. To overcome this resistance, four second-generation ATP competitive ABL TKIs, dasatinib, nilotinib, bosutinib and bafetinib, have been developed. Dasatinib and nilotinib also demonstrated higher efficacy than imatinib in previously untreated CML patients in chronic phase. Despite promising clinical results, the frequently observed mutant T315I is not effectively targeted by any of the second-generation ABL TKIs. Thus, a third-generation ABL TKI, ponatinib, was developed to inhibit all mutated BCR-ABL and showed clinical efficacy in CML cells harbouring T315I. CML treatment is rapidly progressing and further evolution is surely expected. Moreover, it was recently reported that some CML patients who achieved sustained complete molecular response could stop TKI. CML may become the first human cancer to be conquered solely with oral medicines.     

All tyrosine kinase inhibitor-resistant chronic myelogenous cells are highly sensitive to Ponatinib

The advent of tyrosine kinase inhibitor (TKI) therapy has considerably improved the survival of patients suffering chronic myelogenous leukemia (CML). Indeed, inhibition of BCR-ABL by imatinib, dasatinib or nilotinib triggers durable responses in most patients suffering from this disease. Moreover, resistance to imatinib due to kinase domain mutations can be generally circumvented using dasatinib or nilotinib, but the multi-resistant T315I mutation that is insensitive to these TKIs, remains to date a major clinical problem. In this line, ponatinib (AP24534) has emerged as a promising therapeutic option in patients with all kinds of BCR-ABL mutations, especially the T315I one. However and surprisingly, the effect of ponatinib has not been extensively studied on imatinib-resistant CML cell lines. Therefore, in the present study, we used several CML cell lines with different mechanisms of resistance to TKI to evaluate the effect of ponatinib on cell viability, apoptosis and signaling. Our results show that ponatinib is highly effective on both sensitive and resistant CML cell lines, whatever the mode of resistance and also on BaF3 murine B cells carrying native BCR-ABL or T315I mutation. We conclude that ponatinib could be effectively used for all types of TKI-resistant patients.     

Emerging strategies for the treatment of mutant Bcr-Abl T315I myeloid leukemia

The lessons learned from the remarkably successful use of the first-generation tyrosine kinase inhibitor (TKI) imatinib in patients with chronic myeloid leukemia resulted in a major paradigm shift in the treatment of many human cancers, and now further lessons are being learned from our enhanced understanding of the molecular mechanisms of resistance to imatinib and second-generation TKIs, particularly dasatinib and nilotinib. Although diverse mechanisms seem to be involved, the principal cause appears to be the emergence of point mutations in the Abl kinase domain that affect drug affinity and some of which impair the efficacy with which the drugs bind. Currently, > 50 different mutations have been identified, and the extent to which they confer resistance varies considerably. One of the more common mutations results from the substitution of isoleucine for threonine at Abl amino acid position 351, known as the T315I mutation. It appears that the precise position of the substitution within the kinase domain dictates the degree of resistance to TKIs, and patients with the T315I mutation develop almost complete resistance to imatinib, dasatinib, and nilotinib. Herein, we discuss the emerging strategies for circumventing resistance associated with the Bcr-Abl T315I mutation.     

AP24534 (Ponatinib) : le prochain traitement de la leucémie aiguë lymphoblastique à chromosome Philadelphie ?

Distinct clinicopathologic acute lymphoblastic leukemia (ALL) entities have been identified, resulting in the adoption of risk-oriented treatment approaches. In Philadelphia chromosome-positive (Ph(+)) ALL, the optimal treatment requires the addition of BCR-ABL tyrosine kinase inhibitors, as imatinib. However, the outcome remains poor in absence of allogeneic stem cell transplantation, and novel agents are desperately required. Resistance attributable to kinase domain mutations can lead to relapse despite the development of second-generation compounds, including dasatinib and nilotinib. Despite these therapeutic options, the cross-resistant BCR-ABL (T315I) mutation remains a major clinical challenge. The first evaluations of AP24534 present this drug as a potent multi-targeted kinase inhibitor active against T315I and all other BCR-ABL mutants. AP24534 could be the next treatment of choice in hematological malignancies with Philadelphia-positive chromosome, particularly Ph(+) ALL known for its frequent occurrence of T315I mutation.     

Comparison of imatinib, dasatinib, nilotinib and INNO-406 in imatinib-resistant cell lines

We compared the growth-inhibitory effects and inhibition profile of the SRC family kinases (SFKs) of imatinib, dasatinib, nilotinib and INNO-406. Dasatinib exhibited the strongest potency against BCR-ABL with little selectivity over SFKs. Nilotinib exhibited a weaker affinity than the other inhibitors, but was highly specific for ABL and may be useful for the treatment of P-glycoprotein overexpressing leukemic cells. INNO-406 had an intermediate affinity for BCR-ABL between that of dasatinib and nilotinib, and inhibited only SFKs LCK and LYN among SFKs. Both nilotinib and INNO-406 were potent inhibitors of the dasatinib-resistant T315A, F317L and F317V BCR-ABL mutations.     

Extending the duration of response in chronic myelogenous leukemia: targeted therapy with sequential tyrosine kinase inhibitors

Tyrosine kinase inhibitors (TKIs) are the mainstay for treatment of chronic myelogenous leukemia (CML). Imatinib was the first TKI approved for use in CML, but resistance to this therapy has emerged as a significant issue, and second-line options are often necessary. Increased-dose imatinib may elicit responses in some patients, but clinical evidence suggests only a minority experience sustained benefit. The second-generation TKIs, dasatinib and nilotinib, have demonstrated efficacy in patients resistant or intolerant to imatinib. Changes in therapy, with the aim of inducing durable response, should occur promptly after imatinib failure is identified as all agents are more effective in chronic phase disease than in later stages. Selection of second-line agents should be driven by efficacy and safety: dasatinib may be more effective in patients with P-loop or F359C mutations; nilotinib may be more effective in those with F317L mutations.     

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.     

Dasatinib combined with interferon-alfa induces a complete cytogenetic response and major molecular response in a patient with chronic myelogenous leukemia harboring the T315I BCR-ABL1 mutation

Mutations of BCR-ABL1 are observed in 50% of patients with imatinib-resistant chronic myeloid leukemia (CML). The T315I mutation is resistant to imatinib and second-generation tyrosine kinase inhibitors (TKIs). We report the case of a 57-year-old man diagnosed with CML in 2003 in whom imatinib therapy failed after which he acquired the T315I mutation. He was treated sequentially with an anti-T315I-specific agent, KW-2449, that led to eradication of the mutation without any further improvement. Subsequent introduction of combination therapy that included dasatinib and pegylated interferon led to the achievement of a sustained complete cytogenetic and major molecular response (MMR). This case illustrates the benefit of combination therapy that includes a TKI and a second agent with a different mechanism of action, either sequentially (TKI followed by KW-2449) or concomitantly (TKI + interferon), in eradicating resistant disease with the T315I clone.     

Strategies for overcoming imatinib resistance in chronic myeloid leukemia

Imatinib was the first treatment for chronic myeloid leukemia (CML) that specifically targeted the causative BCR-ABL oncoprotein, and represented a major therapeutic advance in this disease; however, some patients develop resistance or intolerance. Resistance can be classified as BCR-ABL-dependent (e.g., mutation in the BCR-ABL gene) or BCR-ABL-independent (alternative pathways of disease progression, e.g., SRC-family tyrosine kinases). The investigation of therapeutic options post-imatinib failure resulted in the development and regulatory approval of dasatinib, a BCR-ABL and SRC-family kinase inhibitor. Dasatinib is active across all phases of CML and Philadelphia chromosome-positive acute lymphoblastic leukemia, and demonstrates activity in almost all imatinib-resistant mutations. Other therapeutic options are also under investigation, with nilotinib being the most clinically advanced. Nilotinib is an analog of imatinib with similar multiple kinase targets, but without inhibition of SRC, and reduced in vitro activity against BCR-ABL P-loop mutations compared with dasatinib. Similar to dasatinib, nilotinib has no activity against T315I mutations. The availability of dasatinib and development of other tyrosine kinase inhibitors provide positive prospects for patients with imatinib-resistant or -intolerant CML. Here, we discuss several of these new strategies for treating patients after imatinib failure.     

Combined effects of novel heat shock protein 90 inhibitor NVP-AUY922 and nilotinib in a random mutagenesis screen

To overcome imatinib resistance, more potent ABL tyrosine kinase inhibitors (TKIs), such as nilotinib and dasatinib have been developed, with demonstrable preclinical activity against most imatinib-resistant BCR-ABL kinase domain mutations, with the exception of T315I. However, imatinib-resistant patients already harboring mutations have a higher likelihood of developing further mutations under the selective pressure of potent ABL TKIs. NVP-AUY922 (Novartis) is a novel 4,5-diaryloxazole adenosine triphosphate-binding site heat shock protein 90 (HSP90) inhibitor, which has been shown to inhibit the chaperone function of HSP90 and deplete the levels of HSP90 client protein including BCR-ABL. In this study, we investigated the combined effects of AUY922 and nilotinib on random mutagenesis for BCR-ABL mutation (Blood, 109; 5011, 2007). Compared with single agents, combination with AUY922 and nilotinib was more effective at reducing the outgrowth of resistant cell clones. No outgrowth was observed in the presence of 2?μM of nilotinib and 20?nM of AUY922. The observed data from the isobologram indicated the synergistic effect of simultaneous exposure to AUY922 and nilotinib even in BaF3 cells expressing BCR-ABL mutants including T315I. In vivo studies also demonstrated that the combination of AUY922 and nilotinib prolonged the survival of mice transplanted with mixture of BaF3 cells expressing wild-type BCR-ABL and mutant forms. Taken together, this study shows that the combination of AUY922 and nilotinib exhibits a desirable therapeutic index that can reduce the in vivo growth of mutant forms of BCR-ABL-expressing cells.     

Tailoring tyrosine kinase inhibitor therapy to tackle specific BCR-ABL1 mutant clones

Several tyrosine kinase inhibitors (TKIs) are currently under development for the treatment of patients with chronic myelogenous leukemia (CML) resistant or intolerant of imatinib therapy, including nilotinib, dasatinib, and bosutinib. The current paradigm of TKI therapy involves a sequential use of these compounds, with imatinib invariably used as frontline therapy followed by either dasatinib or nilotinib on an empiric basis. A more sensible approach to this sequence is the selection of the TKI best suited to overcome the resistance conferred by BCR-ABL1 mutations detected at each time-point. As more TKIs are becoming available, the management of patients with CML will require degree of “finesse” to better match each patient with the best TKI available. This match is best made based on available in vitro data regarding the activity of each agent against each specific mutation. The case herein reported supports such strategy.     

Chronic Myeloid Leukemia in the Tyrosine Kinase Inhibitor Era: What Is the “Best” Therapy?

The introduction of imatinib mesylate, a Bcr-Abl1 tyrosine kinase inhibitor (TKI), has revolutionized the treatment of chronic myeloid leukemia (CML). By directly targeting the Bcr-Abl kinase, imatinib leads to durable cytogenetic remissions and in turn improved survival. However, many patients with CML develop resistance, fail to respond, or become intolerant to imatinib due to side effects. This has spurred interest in developing second-generation TKIs to overcome the mechanisms of resistance that lead to treatment failure, specifically Bcr-Abl1 kinase domain mutations. Two second-generation TKIs, nilotinib and dasatinib, are approved for the treatment of CML after imatinib failure or intolerance. Unfortunately, many patients fail subsequent treatment with these agents, as they can develop highly resistant mutations such as T315I. Various other strategies are now in use to optimize the treatment of CML, including dose optimization of imatinib, combination therapy, upfront use of second-generation TKIs, and use of maintenance therapy with interferon-α and vaccines. This review highlights progress made in the treatment of CML in the past year.     

Abl tyrosine kinase inhibitors for overriding Bcr-Abl/T315I: from the second to third generation

Treatment of chronic myeloid leukemia (CML) has changed drastically with the emergence of the Abl tyrosine kinase inhibitor (TKI), imatinib mesylate. However, primary and secondary resistance have frequently been reported, particularly in patients with advanced-stage disease. Point mutations within the Abl kinase domain that interfere with imatinib binding are the most critical cause of imatinib resistance. In order to override this resistance, several second generation ATP-competitive Abl TKIs including dasatinib, nilotinib, bosutinib and INNO-406 have been developed. Despite promising clinical results from these novel Abl TKIs for most mutations, the frequently observed mutant T315I is not effectively targeted by any of these agents. Thus, identification of novel agents and the development of new strategies for the effective treatment of CML patients with the T315I mutation are important and challenging tasks. In this review, the current status of novel agents for CML treatment is overviewed as follows: pathogenesis and features of CML; imatinib and second-generation Abl TKIs; why Abl TKIs are not effective against T315I; and novel agents that may override the T315I mutation.     

Outcome of treatment of chronic myeloid leukemia with second-generation tyrosine kinase inhibitors after imatinib failure

Although imatinib revolutionized the management of chronic myeloid leukemia (CML), recent data indicate a transformation in the treatment approach likely in the near future. For patients whose CML does not respond to standard-dose imatinib therapy, increasing the imatinib dose is a second-line option. However, high-dose imatinib is not an appropriate approach for patients experiencing drug toxicity, and there remain questions concerning the durability of responses achieved with this strategy. Alternative second-line options include the newer tyrosine kinase inhibitors (TKIs) such as dasatinib and nilotinib. A substantial amount of long-term data for these agents is available. Although both are potent and specific BCR-ABL TKIs, dasatinib and nilotinib exhibit unique pharmacological profiles and response patterns relative to different patient characteristics, such as disease stage and BCR-ABL mutational status. The superiority of second-generation TKIs over imatinib in newly diagnosed disease has been recognized as well. They induce high and rapid rates of cytogenetic and molecular response, with less progression to advanced forms of disease in comparison with imatinib. Several investigational agents specific for those patients with the T315I mutation remain under evaluation. The future of CML therapy may include early use of these potent agents to help more patients achieve molecular remission and potentially be a path to a CML cure.     

Abl tyrosine kinase inhibitors for overriding Bcr–Abl/T315I: from the second to third generation

Treatment of chronic myeloid leukemia (CML) has changed drastically with the emergence of the Abl tyrosine kinase inhibitor (TKI), imatinib mesylate. However, primary and secondary resistance have frequently been reported, particularly in patients with advanced-stage disease. Point mutations within the Abl kinase domain that interfere with imatinib binding are the most critical cause of imatinib resistance. In order to override this resistance, several second generation ATP-competitive Abl TKIs including dasatinib, nilotinib, bosutinib and INNO-406 have been developed. Despite promising clinical results from these novel Abl TKIs for most mutations, the frequently observed mutant T315I is not effectively targeted by any of these agents. Thus, identification of novel agents and the development of new strategies for the effective treatment of CML patients with the T315I mutation are important and challenging tasks. In this review, the current status of novel agents for CML treatment is overviewed as follows: pathogenesis and features of CML; imatinib and second-generation Abl TKIs; why Abl TKIs are not effective against T315I; and novel agents that may override the T315I mutation.     

T315I mutation in Ph-positive acute lymphoblastic leukemia is associated with a highly aggressive disease phenotype: three case reports

T315I mutation in BCR-ABL causes resistance to therapy with tyrosine kinase inhibitors (TKIs) in Philadelphia chromosome-positive acute lymphoblastic leukemia (Ph(+) ALL) cases. We report three cases of relapse accompanied by T315I mutation during rapid disease progression. Case 1 was a 64-year-old male. During chemotherapy, qPCR detected a decrease of BCR-ABL to 190 copies once, but this suddenly increased to 22,000 copies. The patient received dasatinib, but the disease relapsed hematologically when the T315I mutation was detected. Retrospective analysis revealed that the T315I mutated clone already existed at the molecular relapse occurrence. Case 2 was a 25-year-old male. The patient underwent bone marrow transplantation (BMT) at the first molecular complete remission (CR), but 102 days after BMT, the ALL relapsed at the molecular level. Although he received imatinib, ALL immediately fully relapsed; the T315I mutation was detected. Case 3 was a 40-year-old female. Molecular CR was achieved by induction therapy, but ALL relapsed at the molecular level (9,200 copies). The patient received dasatinib, but relapsed hematologically, and the T315I mutation was observed. She underwent umbilical cord blood transplantation, but relapsed. In these three cases, survival from the time of the T315I mutation detection was 4, 2, and 6 months, respectively. The T315I mutation in Ph(+) ALL was associated with a highly aggressive disease phenotype. In order to make appropriate therapeutic decisions, it is important to analyze the mutations immediately at the time of molecular relapse.     

Tyrosine Kinase Inhibitors: The First Decade

The treatment of chronic myeloid leukemia (CML) drastically changed with the introduction of imatinib mesylate, a Bcr-Abl1 tyrosine kinase inhibitor (TKI), in 1998. By directly targeting this leukemogenic protein kinase, imatinib affords patients with CML sustained chromosomal remissions, which translate into prolonged survival. However, there has been concern over the emergence of resistance to imatinib, and some patients fail to respond or are intolerant of imatinib therapy because of untoward toxicity. This has spurred interest in developing novel TKIs to overcome the mechanisms of resistance that lead to treatment failure—most importantly, Bcr-Abl1 kinase domain mutations. Two of these second-generation TKIs, nilotinib and dasatinib, are approved worldwide for the treatment of CML after imatinib failure or intolerance. Although these agents are active, they fail in many patients because of the development of highly resistant mutations such as the T315I, against which several novel agents are currently being tested in clinical trials. This review provides an account of the progress made in the field of TKI therapy for CML over the past decade.     

How Much and How Long: Tyrosine Kinase Inhibitor Therapy in Chronic Myeloid Leukemia

As the first clinically successful tyrosine kinase inhibitor (TKI), imatinib pioneered a new approach to treating patients with cancer. Dramatic results from chronic myeloid leukemia (CML) clinical trials spurred the development of TKIs for other malignancies such as acute myeloid leukemia as well as kidney and lung cancer. In CML, imatinib resistance led to the rapid development of dasatinib and nilotinib, more potent second-generation ABL kinase inhibitors that can often overcome imatinib resistance. While the clinical efficacy of TKIs in CML is well established, a number of important questions remain about the optimal dose and duration of therapy. Even the best initial dose for imatinib is still under investigation. Although laboratory and clinical studies had led to the prevailing view that continual inhibition of the BCR-ABL kinase was required for optimal efficacy, recent data on dasatinib have upended this notion and have led to a change in the recommended dosing schedule. The availability of dasatinib and nilotinib also begs the question of whether they might be superior to imatinib as first-line agents. Finally, the question of whether it may be possible to stop TKI therapy at least in some patients with CML has attracted considerable attention. More than 10 years after the introduction of imatinib, optimization of TKI therapy for CML continues.