Ponatinib in patients with refractory acute myeloid leukaemia: findings from a phase 1 study

Identification of a Selective FLT3 Inhibitor with Low Activity against VEGFR, FGFR, PDGFR, c-KIT, and RET Anti-Targets

FLT3 is mainly expressed in immune and various cancer cells and is a drug target for acute myeloid leukemia (AML). Recently, FLT3 has also been identified as a potential target for treating chronic pain. Most FLT3 inhibitors (FLT3i) identified to date, including approved drugs such as gilteritinib, midostaurin, ponatinib, quizartinib, and FLT3i in clinical trials, such as quizartinib and crenolanib, also inhibit closely-related kinases that are important for immune (c-KIT), cardiovascular (KDR/VEGFR2, FGFR, PDGFR) or kidney (RET) functions. While the aforementioned FLT3i may increase survival rates in AML, they are neither ideal for AML maintenance therapy nor for non-oncology applications, such as for the treatment of chronic pain, due to their promiscuous inhibition of many kinase anti-targets. Here, we report the identification of new FLT3i compounds that have low activities against kinases that have traditionally been difficult to differentiate from FLT3 inhibition, such as KDR/VEGFR, FGFR, PGFR, c-KIT, and RET. These selective compounds could be valuable chemical probes for studying FLT3 biology in the context of chronic pain and/or may represent good starting points to develop well-tolerated FLT3 therapeutics for non-oncology indications or for maintenance therapy for AML.

FLT3 targeting in the modern era: from clonal selection to combination therapies

Fms-like tyrosine kinase 3 (FLT3) is the most frequently mutated gene in acute myeloid leukemia (AML). Modern targeting of FLT3 with inhibitors has improved clinical outcomes and FLT3 inhibitors have been incorporated into the treatment of AML in all phases of the disease, including the upfront, relapsed/refractory and maintenance settings. This review will discuss the current understanding of FLT3 biology, the clinical use of FLT3 inhibitors, resistance mechanisms and emerging combination treatment strategies.

Ponatinib vs. Imatinib as Frontline Treatment for Philadelphia Chromosome-Positive Acute Lymphoblastic Leukemia: A Matching Adjusted Indirect Comparison

INTRODUCTION

Efficacy of ponatinib-based treatment for patients with Philadelphia chromosome-positive acute lymphoblastic leukemia (Ph + ALL) has not been compared to imatinib-based treatments in head-to-head clinical trials. We evaluated its efficacy versus imatinib-based regimens using a matching adjusted indirect comparison.

METHODS

Two ponatinib studies were used: the phase 2 MDACC study of ponatinib + hyper-CVAD (cyclophosphamide, vincristine, doxorubicin, and dexamethasone) in adult patients and the phase 2 GIMEMA LAL1811 study of ponatinib + steroids in patients > 60 years/unfit for intensive chemotherapy and stem cell transplant. Studies on imatinib as first-line treatment in adults with Ph + ALL were identified using a systematic literature search. Population adjustment was based on the prognostic factors and effect modifiers identified by clinical experts. Hazard ratios (HRs) were calculated for overall survival (OS) and odds ratios (ORs) for complete molecular response (CMR).

RESULTS

The systematic literature search identified two studies (GRAAPH-2005 and NCT00038610) reporting the efficacy of first-line imatinib + hyper-CVAD and one study reporting the efficacy of first-line imatinib monotherapy induction + imatinib-based consolidation (CSI57ADE10). Ponatinib + hyper-CVAD prolonged OS and gave a higher CMR rate than imatinib + hyper-CVAD. The adjusted HR [95% confidence interval (CI)] for OS was 0.35 (0.17-0.74) for MDACC vs. GRAAPH-2005 and 0.35 (0.18-0.70) for MDACC vs. NCT00038610; the adjusted OR (95% CI) for CMR was 12.11 (3.77-38.87) for MDACC vs. GRAAPH-2005 and 5.65 (2.02-15.76) for MDACC vs. NCT00038610. Ponatinib + steroids prolonged OS and gave a higher CMR rate than imatinib monotherapy induction + imatinib-containing consolidation. The adjusted HR (95% CI) for OS was 0.24 (0.09-0.64) and the adjusted OR (95% CI) for CMR was 6.20 (1.60-24.00) for GIMEMA LAL1811 vs. CSI57ADE10.

CONCLUSION

In adults with newly diagnosed Ph + ALL, first-line treatment with ponatinib was associated with better outcomes than first-line treatment with imatinib.

“FLipping” the Story: FLT3-Mutated Acute Myeloid Leukemia and the Evolving Role of FLT3 Inhibitors

Simple Summary

Patients with acute myeloid leukemia (AML) may have a number of different mutations. Those with mutations in the FLT3 gene have a higher risk of relapse and death than those lacking these mutations. FLT3 is a key receptor on the surface of AML cells, which drives cell survival and growth. Although activation of this receptor is normally tightly controlled, in AML, FLT3 mutations allow it to activate itself, independent of external control. Over the past 5 years, a number of new drugs have been developed to specifically target these mutations. In this article, we discuss these drugs and their uses, as well as the mechanisms by which AML cells may gain resistance to them and how that resistance can be overcome.

Abstract

The treatment of many types of cancers, including acute myeloid leukemia (AML), has been revolutionized by the development of therapeutics targeted at crucial molecular drivers of oncogenesis. In contrast to broad, relatively indiscriminate conventional chemotherapy, these targeted agents precisely disrupt key pathways within cancer cells. FMS-like tyrosine kinase 3 (FLT3)—encoding a critical regulator of hematopoiesis—is the most frequently mutated gene in patients with AML, and these mutations herald reduced survival and increased relapse in these patients. Approximately 30% of newly diagnosed AML carries an FLT3 mutation; of these, approximately three-quarters are internal tandem duplication (ITD) mutations, and the remainder are tyrosine kinase domain (TKD) mutations. In contrast to its usual, tightly controlled expression, FLT3-ITD mutants allow constitutive, “run-away” activation of a large number of key downstream pathways which promote cellular proliferation and survival. Targeted inhibition of FLT3 is, therefore, a promising therapeutic avenue. In April 2017, midostaurin became both the first FLT3 inhibitor and the first targeted therapy of any kind in AML to be approved by the US FDA. The use of FLT3 inhibitors has continued to grow as clinical trials continue to demonstrate the efficacy of this class of agents, with an expanding number available for use as both experimental standard-of-care usage. This review examines the biology of FLT3 and its downstream pathways, the mechanism of FLT3 inhibition, the development of the FLT3 inhibitors as a class and uses of the agents currently available clinically, and the mechanisms by which resistance to FLT3 inhibition may both develop and be overcome.

An evaluation of ponatinib as a therapy in adult patients with resistant/intolerant chronic-phase chronic myeloid leukemia

INTRODUCTION

Chronic myeloid leukemia is now a highly treatable leukemia due to the availability of multiple tyrosine kinase inhibitors (TKIs) inhibiting the BCR-ABL1 oncogene. Some patients with CML can display resistance or intolerance to multiple TKIs, oftentimes due to the presence of mutations in BCR-ABL1, such as T315I, which limits effective treatment options. Ponatinib is a third-generation, rationally-designed TKI with clinically meaningful activity in this difficult-to-treat population. Ponatinib is associated with an increased risk of arterial occlusive events (AOEs) which has required a re-examination of its dosing in order to limit the risk of these events.

AREAS COVERED

This review will provide an overview of the mechanism of action of ponatinib and the safety and efficacy data from clinical trials in chronic myeloid leukemia.

EXPERT OPINION

Ponatinib is a potent pan-BCR-ABL1 TKI with substantial activity in patients with more resistant or advanced CML. Its efficacy needs to be balanced with the increased risk of vascular events, which seems to be at least partially diminished by the implementation of mitigation strategies aimed at modifying cardiovascular risk factors and adaptive dosing of the drug.

Small Molecule Compounds of Natural Origin Target Cellular Receptors to Inhibit Cancer Development and Progression

Receptors are macromolecules that transmit information regulating cell proliferation, differentiation, migration and apoptosis, play key roles in oncogenic processes and correlate with the prognoses of cancer patients. Thus, targeting receptors to constrain cancer development and progression has gained widespread interest. Small molecule compounds of natural origin have been widely used as drugs or adjuvant chemotherapeutic agents in cancer therapies due to their activities of selectively killing cancer cells, alleviating drug resistance and mitigating side effects. Meanwhile, many natural compounds, including those targeting receptors, are still under laboratory investigation for their anti-cancer activities and mechanisms. In this review, we classify the receptors by their structures and functions, illustrate the natural compounds targeting these receptors and discuss the mechanisms of their anti-cancer activities. We aim to provide primary knowledge of mechanistic regulation and clinical applications of cancer therapies through targeting deregulated receptors.

The Use of Inhibitors of Tyrosine Kinase in Paediatric Haemato-Oncology-When and Why?

The fundamental pathophysiology of malignancies is dysregulation of the signalling pathways. Protein tyrosine kinases (PTKs) are among the enzymes which, if mutated, play a critical role in carcinogenesis. The best-studied rearrangement, which enhances PTK activity and causes atypical proliferation, is BCR-ABL1. Abnormal expression of PTKs has proven to play a significant role in the development of various malignancies, such as chronic myelogenous leukaemia, brain tumours, neuroblastoma, and gastrointestinal stromal tumours. The use of tyrosine kinase inhibitors (TKIs) is an outstanding example of successful target therapy. TKIs have been effectively applied in the adult oncology setting, but there is a need to establish TKIs’ importance in paediatric patients. Many years of research have allowed a significant improvement in the outcome of childhood cancers. However, there are still groups of patients who have a poor prognosis, where the intensification of chemotherapy could even cause death. TKIs are designed to target specific PTKs, which lead to the limitation of severe adverse effects and increase overall survival. These advances will hopefully allow new therapeutic approaches in paediatric haemato-oncology to emerge. In this review, we present an analysis of the current data on tyrosine kinase inhibitors in childhood cancers.

INCB84344-201: Ponatinib and steroids in frontline therapy of unfit patients with Ph+ acute lymphoblastic leukemia

In patients with newly diagnosed Ph⁺ ALL, ponatinib and prednisone therapy resulted in long molecular remissions and few resistance mutations.

The observed high rates of discontinuation and dose modification suggest that a lower dose may be more appropriate in older/unfit patients.

Tyrosine kinase inhibitors have improved survival for patients with Philadelphia chromosome–positive (Ph⁺) acute lymphoblastic leukemia (ALL). However, prognosis for old or unfit patients remains poor. In the INCB84344-201 (formerly GIMEMA LAL 1811) prospective, multicenter, phase 2 trial, we tested the efficacy and safety of ponatinib plus prednisone in newly diagnosed patients with Ph⁺ ALL ≥60 years, or unfit for intensive chemotherapy and stem cell transplantation. Forty-four patients received oral ponatinib 45 mg/d for 48 weeks (core phase), with prednisone tapered to 60 mg/m²/d from days-14-29. Prophylactic intrathecal chemotherapy was administered monthly. Median age was 66.5 years (range, 26-85). The primary endpoint (complete hematologic response [CHR] at 24 weeks) was reached in 38/44 patients (86.4%); complete molecular response (CMR) in 18/44 patients (40.9%) at 24 weeks. 61.4% of patients completed the core phase. As of 24 April 2020, median event-free survival was 14.31 months (95% CI 9.30-22.31). Median overall survival and duration of CHR were not reached; median duration of CMR was 11.6 months. Most common treatment-emergent adverse events (TEAEs) were rash (36.4%), asthenia (22.7%), alanine transaminase increase (15.9%), erythema (15.9%), and γ-glutamyltransferase increase (15.9%). Cardiac and vascular TEAEs occurred in 29.5% (grade ≥3, 18.2%) and 27.3% (grade ≥3, 15.9%), respectively. Dose reductions, interruptions, and discontinuations due to TEAEs occurred in 43.2%, 43.2%, and 27.3% of patients, respectively; 5 patients had fatal TEAEs. Ponatinib and prednisone showed efficacy in unfit patients with Ph⁺ ALL; however, a lower ponatinib dose may be more appropriate in this population. This trial was registered at www.clinicaltrials.gov as #NCT01641107.

A phase 1/2 study of the oral FLT3 inhibitor pexidartinib in relapsed/refractory FLT3-ITD-mutant acute myeloid leukemia

FMS-like tyrosine kinase 3 (FLT3) tyrosine kinase inhibitors (TKIs) have activity in acute myeloid leukemia (AML) patients with FLT3 internal tandem duplication (ITD) mutations, but efficacy is limited by resistance-conferring kinase domain mutations. This phase 1/2 study evaluated the safety, tolerability, and efficacy of the oral FLT3 inhibitor PLX3397 (pexidartinib), which has activity against the FLT3 TKI-resistant F691L gatekeeper mutation in relapsed/refractory FLT3-ITD-mutant AML. Ninety patients were treated: 34 in dose escalation (part 1) and 56 in dose expansion (part 2). Doses of 800 to 5000 mg per day in divided doses were tested. No maximally tolerated dose was reached. Plasma inhibitory assay demonstrated that patients dosed with ≥3000 mg had sufficient levels of active drug in their trough plasma samples to achieve 95% inhibition of FLT3 phosphorylation in an FLT3-ITD AML cell line. Based on a plateau in drug exposure, the 3000-mg dose was chosen as the recommended phase 2 dose. The most frequently reported treatment-emergent adverse events were diarrhea (50%), fatigue (47%), and nausea (46%). Based on modified response criteria, the overall response rate to pexidartinib among all patients was 21%. Twenty-three percent of patients treated at ≥2000 mg responded. The overall composite complete response rate for the study was 11%. Six patients were successfully bridged to transplantation. Median overall survival (OS) of patients treated in dose expansion was 112 days (90% confidence interval [CI], 77-150 days), and median OS of responders with complete remission with or without recovery of blood counts was 265 days (90% CI, 170-422 days). This trial was registered at www.clinicaltrials.gov as #NCT01349049.

The safety profile of FLT3 inhibitors in the treatment of newly diagnosed or relapsed/refractory acute myeloid leukemia

INTRODUCTION

FLT3 inhibitors are important drugs in the therapy of FLT3 positive acute myeloid leukemia (AML). Midostaurin was registered in combination with chemotherapy to treat newly diagnosed AML. Gilteritinib and quizartinib demonstrate effectiveness in a randomized trial in relapsed/refractory AML. Several promising FLT3 inhibitors are being evaluated in clinical research.

AREAS COVERED

This review will report the safety of FLT3 inhibitors that are registered for acute myeloid leukemia induction and rescue therapy.

EXPERT OPINION

In the near future, it is possible that all the FLT3 positive non M3-AML patients will receive a FLT3 inhibitor. Therapy adherence and strategies to mitigate adverse events must be pursued. The treatment with FLT3 inhibitors may be optimized in terms of toxicities with a rational evaluation of antifungal prophylaxis and concomitant therapy, cardiology monitoring, and keeping in mind rare adverse events. Future studies on unfit patients, special populations, and maintenance settings are warranted, together with post-market studies and real-life experiences. Whenever new FLT3 inhibitors will come to the clinic, we could face a scenario in which profound knowledge of effectiveness, toxicities, and off-target effects will be relevant to choose the best drug for each patient.

FLT3 Mutations in Acute Myeloid Leukemia: Key Concepts and Emerging Controversies

The FLT3 receptor is overexpressed on the majority of acute myeloid leukemia (AML) blasts. Mutations in FLT3 are the most common genetic alteration in AML, identified in approximately one third of newly diagnosed patients. FLT3 internal tandem duplication mutations (FLT3-ITD) are associated with increased relapse and inferior overall survival. Multiple small molecule inhibitors of FLT3 signaling have been identified, two of which (midostaurin and gilteritinib) are currently approved in the United States, and many more of which are in clinical trials. Despite significant advances, resistance to FLT3 inhibitors through secondary FLT3 mutations, upregulation of parallel pathways, and extracellular signaling remains an ongoing challenge. Novel therapeutic strategies to overcome resistance, including combining FLT3 inhibitors with other antileukemic agents, development of new FLT3 inhibitors, and FLT3-directed immunotherapy are in active clinical development. Multiple questions regarding FLT3-mutated AML remain. In this review, we highlight several of the current most intriguing controversies in the field including the role of FLT3 inhibitors in maintenance therapy, the role of hematopoietic cell transplantation in FLT3-mutated AML, use of FLT3 inhibitors in FLT3 wild-type disease, significance of non-canonical FLT3 mutations, and finally, emerging concerns regarding clonal evolution.

Management of toxicities associated with targeted therapies for acute myeloid leukemia: when to push through and when to stop

The recent advent of myriad targeted therapies for acute myeloid leukemia (AML) has led to new hope for our patients but has also introduced new challenges in managing the disease. For clinicians, the ability to treat AML in the outpatient setting with novel agents of equal or greater efficacy than 7+3 has been transformative. Despite the enthusiasm, however, the reality is that many patients are still frail and remain at risk for treatment-related complications. Translating the results of clinical trials into improved outcomes for these individuals requires an understanding of how best to manage the adverse effects of these agents. Which patients benefit most and what to watch for? When to stop therapy? Using illustrative case presentations, this review details the unique toxicities associated with each of the approved mutation-specific and nonspecific targeted drugs for AML. The goal of this review is to help clinicians determine the risk:benefit ratio in decision making for individual patients with AML.

Current and emerging molecular and epigenetic disease entities in acute myeloid leukemia and a critical assessment of their therapeutic modalities

Acute myeloid leukemia (AML) is the most frequently diagnosed acute leukemia, and its incidence increases with age. Although the etiology of AML remains unknown, exposure to genotoxic agents or some prior hematologic disorders could lead to the development of this condition. The pathogenesis of AML involves the development of malignant transformation of hematopoietic stem cells that undergo successive genomic alterations, ultimately giving rise to a full-blown disease. From the disease biology perspective, AML is considered to be extremely complex with significant genetic, epigenetic, and phenotypic variations. Molecular and cytogenetic alterations in AML include mutations in those subsets of genes that are involved in normal cell proliferation, maturation and survival, thus posing significant challenge to targeting these pathways without attendant toxicity. In addition, multiple malignant cells co-exist in the majority of AML patients. Individual subclones are characterized by unique genetic and epigenetic abnormalities, which contribute to the differences in their response to treatment. As a result, despite a dramatic progress in our understanding of the pathobiology of AML, not much has changed in therapeutic approaches to treat AML in the past four decades. Dose and regimen modifications with improved supportive care have contributed to improved outcomes by reducing toxicity-related side effects. Several drug candidates are currently being developed, including targeted small-molecule inhibitors, cytotoxic chemotherapies, monoclonal antibodies and epigenetic drugs. This review summarizes the current state of affairs in the pathobiological and therapeutic aspects of AML.

Identification and Development of 1,4-Diaryl-1,2,3-triazolo-Based Ureas as Novel FLT3 Inhibitors

A class of 1,4-diaryl-1,2,3-triazolo-based ureas were synthesized and developed as novel FLT3 inhibitors. The representative compound 28 strongly inhibited FLT3-ITD kinase (IC₅₀ = 32.8 nM) and isogenic BaF3-FLT3-ITD cell (GI₅₀ = 0.6 nM). It exhibited potent inhibition against FLT3-ITD positive MV4-11 (GI₅₀ = 3.0 nM) and MOLM-13 (GI₅₀ = 5.9 nM) cell lines and high selectivity over FLT3-WT cell lines. It also displayed good pharmacokinetics properties and demonstrated promising oral in vivo efficacy in a MV4-11 cell xenografted mouse model. It might be a potent lead compound for further development to treat FLT3-ITD driven acute myloid leukemia.

Association of fludarabin, cytarabine, and fractioned gemtuzumab followed by hematopoietic stem cell transplantation for first-line refractory acute myeloid leukemia in children: A single-center experience

CONTEXT

Acute myeloid leukemia (AML) is a rare disease in children, with only 50% to 60% event-free survival. Among patients with AML, 10% do not respond to first-line chemotherapy. There is no recommendation concerning second-line treatments. Gemtuzumab ozogamicin (GO) is a monoclonal antibody targeting CD33, linked to calicheamicin. We report the efficacy and tolerance of a salvage regimen of fludarabin, cytarabine, and GO (FLA-GO) in patients refractory to first-line treatment.

METHODS

Eight patients (median age 14.5 years), who had more than 2% minimal residual disease (MRD) by flow cytometry (MRD flow), received gemtuzumab 3 mg/m² on days 1, 4, 7, associated with cytarabine 2000 mg/m² and fludarabin 30 mg/m² on days 1 to 5.

RESULTS

Six patients achieved complete remission (CR) (blast count morphology ≤5 × 10^-2 , CR-MRD flow <1 × 10^-3 for four patients). Five patients received a second course. We observed 11 episodes of febrile neutropenia, including 6 septicemias without complication. There was no fungal infection or toxic death. Two patients received granulocyte colony stimulating factor. One patient had partial platelet recovery; one, prolonged pancytopenia. All patients received hematopoietic stem cell transplantation (HSCT). We observed five mild-to-severe sinusoidal obstruction syndromes during HSCT procedures, particularly in patients who did not receive defibrotide prophylaxis. At the date of last contact (median follow-up: 58 months; range: 22-78), six patients were in continuous CR with negative MRD. Two patients died of post-HSCT relapse.

CONCLUSION

FLA-GO is a good salvage regimen for pediatric refractory AML, with significant but acceptable toxicity. HSCT is mandatory to achieve sustained CR in these patients.

PROTACs: great opportunities for academia and industry

Although many kinds of therapies are applied in the clinic, drug-resistance is a major and unavoidable problem. Another disturbing statistic is the limited number of drug targets, which are presently only 20-25% of all protein targets that are currently being studied. Moreover, the focus of current explorations of targets are their enzymatic functions, which ignores the functions from their scaffold moiety. As a promising and appealing technology, PROteolysis TArgeting Chimeras (PROTACs) have attracted great attention both from academia and industry for finding available approaches to solve the above problems. PROTACs regulate protein function by degrading target proteins instead of inhibiting them, providing more sensitivity to drug-resistant targets and a greater chance to affect the nonenzymatic functions. PROTACs have been proven to show better selectivity compared to classic inhibitors. PROTACs can be described as a chemical knockdown approach with rapidity and reversibility, which presents new and different biology compared to other gene editing tools by avoiding misinterpretations that arise from potential genetic compensation and/or spontaneous mutations. PRTOACs have been widely explored throughout the world and have outperformed not only in cancer diseases, but also in immune disorders, viral infections and neurodegenerative diseases. Although PROTACs present a very promising and powerful approach for crossing the hurdles of present drug discovery and tool development in biology, more efforts are needed to gain to get deeper insight into the efficacy and safety of PROTACs in the clinic. More target binders and more E3 ligases applicable for developing PROTACs are waiting for exploration.

The growing landscape of FLT3 inhibition in AML

Midostaurin and gilteritinib are FLT3 inhibitors that have been recently approved for use in FLT3-mutant acute myeloid leukemia (AML). These approved drugs represent a new standard of care for patients with FLT3 mutations in both the first-line and salvage settings. The success of midostaurin used in combination with induction chemotherapy has prompted exploration of newer, more potent and targeted inhibitors (including gilteritinib) in the first-line setting in combination with chemotherapy. At the same time, the success of gilteritinib and other newer FLT3 inhibitors as monotherapy in the salvage setting has been tempered by the development of resistance because of diverse mechanisms. Investigational strategies that incorporate FLT3 inhibitors in combination with hypomethylating agents and as maintenance therapy after allogeneic stem cell transplantation have shown promise. Other novel combination strategies are also undergoing clinical investigation. In this article, we review the current landscape of approved and investigational FLT3 inhibitors in AML, including the current standard of care and investigational strategies.

Update on management and progress of novel therapeutics for R/R AML: an Iberian expert panel consensus

A significant proportion of adult patients with acute myeloid leukemia (AML) fail to achieve complete remission or will relapse later on after achieving it. Prognosis for relapsed or refractory (R/R) AML patients remains discouraging, with the main curative option still relying on hematopoietic stem cell transplant (HSCT) for those who are eligible. Beyond morphological bone marrow and peripheral blood assessment, evaluation of patient performance status and comorbidities, as well as genetic/molecular characterization, is crucial to make an accurate diagnosis and prognosis, which will be useful to select the most appropriate treatment. Emerging strategies are mainly focusing on the development of immune- and molecular-based approaches. Novel targeted therapies are generally well tolerated, potentially allowing them to be administered alone or in combination with classical chemotherapy agents. Enrolment in clinical trials should be considered first option for R/R AML patients, either as a bridge to HSCT or to benefit from novel therapies that eventually may prolong survival and improve quality of life. An Iberian expert panel has reviewed the recent advances in the management of R/R AML with the aim to develop updated evidence and expert opinion-based recommendations.

Midostaurin In Acute Myeloid Leukemia: An Evidence-Based Review And Patient Selection

Fms-related-tyrosine kinase 3 (FLT3) mutations occur in approximately a third of acute myeloid leukemia (AML) patients and confer an adverse prognosis. Numerous studies have evaluated FLT3 targeting as single agent and in combination approaches in frontline and relapsed AML. At this time, midostaurin, a multikinase inhibitor, is the only FLT3-inhibitor that is US FDA approved to be used in combination with induction therapy in the frontline FLT3-mutated AML setting based on improved overall survival noted in the RATIFY Phase III trial. The utility of midostaurin in maintenance post stem cell transplantation has shown promising results and further studies are still ongoing. In this review, we discuss the studies that led to the inception of midostaurin as a targeted kinase inhibitor, its evaluation in AML, the early clinical trials and the large Phase III clinical trial that led to its eventual US FDA-approval in FLT3-mutated AML. Our review also discusses data on midostaurin adverse effects, mechanisms of resistance and limitations of its utility. We further discuss emerging second-generation FLT3 inhibitors, with a focus on quizartinib and gilteritinib and future directions to enhance FLT3-inhibitor efficacy and overcome mechanisms of resistance.

Isoliquiritigenin, an Orally Available Natural FLT3 Inhibitor from Licorice, Exhibits Selective Anti-Acute Myeloid Leukemia Efficacy In Vitro and In Vivo

Licorice is a medicinal herb widely used to treat inflammation-related diseases in China. Isoliquiritigenin (ISL) is an important constituent of licorice and possesses multiple bioactivities. In this study, we examined the selective anti-AML (acute myeloid leukemia) property of ISL via targeting FMS-like tyrosine kinase-3 (FLT3), a certified valid target for treating AML. In vitro, ISL potently inhibited FLT3 kinase, with an IC₅₀ value of 115.1 ± 4.2 nM, and selectively inhibited the proliferation of FLT3-internal tandem duplication (FLT3-ITD) or FLT3-ITD/F691L mutant AML cells. Moreover, it showed very weak activity toward other tested cell lines or kinases. Western blot immunoassay revealed that ISL significantly inhibited the activation of FLT3/Erk1/2/signal transducer and activator of transcription 5 (STAT5) signal in AML cells. Meanwhile, a molecular docking study indicated that ISL could stably form aromatic interactions and hydrogen bonds within the kinase domain of FLT3. In vivo, oral administration of ISL significantly inhibited the MV4-11 flank tumor growth and prolonged survival in the bone marrow transplant model via decreasing the expression of Ki67 and inducing apoptosis. Taken together, the present study identified a novel function of ISL as a selective FLT3 inhibitor. ISL could also be a potential natural bioactive compound for treating AML with FLT3-ITD or FLT3-ITD/F691L mutations. Thus, ISL and licorice might possess potential therapeutic effects for treating AML, providing a new strategy for anti-AML.

Targeting FLT3 mutations in AML: review of current knowledge and evidence

Genomic investigations of acute myeloid leukemia (AML) have demonstrated that several genes are recurrently mutated, leading to new genomic classifications, predictive biomarkers, and new therapeutic targets. Mutations of the FMS-like tyrosine kinase 3 (FLT3) gene occur in approximately 30% of all AML cases, with the internal tandem duplication (ITD) representing the most common type of FLT3 mutation (FLT3-ITD; approximately 25% of all AML cases). FLT3-ITD is a common driver mutation that presents with a high leukemic burden and confers a poor prognosis in patients with AML. The prognostic value of a FLT3 mutation in the tyrosine kinase domain (FLT3-TKD), which has a lower incidence in AML (approximately 7-10% of all cases), is uncertain. Accumulating evidence demonstrates that FLT3 mutational status evolves throughout the disease continuum. This so-called clonal evolution, together with the identification of FLT3-ITD as a negative prognostic marker, serves to highlight the importance of FLT3-ITD testing at diagnosis and again at relapse. Earlier identification of FLT3 mutations will help provide a better understanding of the patient's disease and enable targeted treatment that may help patients achieve longer and more durable remissions. First-generation FLT3 inhibitors developed for clinical use are broad-spectrum, multikinase inhibitors; however, next-generation FLT3 inhibitors are more specific, more potent, and have fewer toxicities associated with off-target effects. Primary and secondary acquired resistance to FLT3 inhibitors remains a challenge and provides a rationale for combining FLT3 inhibitors with other therapies, both conventional and investigational. This review focuses on the pathological and prognostic role of FLT3 mutations in AML, clinical classification of the disease, recent progress with next-generation FLT3 inhibitors, and mechanisms of resistance to FLT3 inhibitors.

New drugs for acute myeloid leukemia inspired by genomics and when to use them

We are several years into the "postdiscovery" era in acute myeloid leukemia (AML) thanks to extensive work involving the sequencing of genomes and exomes of countless patients, which has led to routine comprehensive targeted sequencing in clinical care. The ability to unlock the molecular underpinnings of each patient's disease was supposed to usher in a new treatment era in which each patient was assigned, based on her mutational profile, a personalized cocktail of targeted therapies that would snuff the disease into submission with minimal toxicity. Whether we have fully realized the promise of personalized therapy in AML is unclear. Here, I review those new drugs that have been inspired by genomics, discuss others that might be possible and their potential roles, and consider whether the ability to target genomic mutations in a personalized manner constitutes the future of AML therapeutics or is representative of an era that has already passed.

FLT3 inhibitors in acute myeloid leukemia: Current and future

FMS-like tyrosine kinase 3 (FLT3) is a receptor tyrosine kinase that is responsible for the proliferation and survival of hematopoietic stem cells in acute myeloid leukemia. Although patients with FLT3 mutations have similar rates of remission following induction chemotherapy, relapse rates are significantly higher and patients with FLT3 mutations have significantly worse outcomes for overall survival and disease-free survival. Early FLT3 inhibitors, such as sorafenib, were non-selective and inhibited several tyrosine kinase receptors resulting in significant toxicity. The treatment of FLT3-positive acute myeloid leukemia has advanced recently with the development of a several FLT3-targeting agents that are either approved or in development. Midostaurin represents the first FDA-approved treatment targeted against FLT3, and there are several promising agents currently undergoing clinical trials. Although certain mutations confer resistance to earlier generation FLT3-targeted tyrosine kinase inhibitors, newer agents show activity in the presence of these mutations.

Crotonoside exhibits selective post-inhibition effect in AML cells via inhibition of FLT3 and HDAC3/6

Targeted therapies for the treatment of acute myeloid leukemia (AML), specifically the FLT3 inhibitors, have shown promising results. Nevertheless, it is very unlikely that inhibitors which target a single pathway will provide long-term disease control. Here, we report the characterization of crotonoside, a natural product extracted from Chinese medicinal herb, Croton, for the treatment of AML via inhibition of FLT3 and HDAC3/6. In vitro, crotonoside exhibited selective inhibition in AML cells. In vivo, crotonoside treatment at 70 and 35 mg/kg/d produced significant AML tumor inhibition rates of 93.5% and 73.6%, respectively. Studies on the anti-AML mechanism of crotonoside demonstrated a significant inhibition of FLT3 signaling, cell cycle arrest in G0/G1 phase, and apoptosis. In contrast to classic FLT3 inhibitor; sunitinib, crotonoside was able to selectively suppress the expression of HDAC3 and HDAC6 without altering the expression of other HDAC isoforms. Inhibitors of HDAC3 and HDAC6; RGFP966 and HPOB, respectively, also exhibited selective inhibition in AML cells. Furthermore, we established novel signaling pathways including HDAC3/NF-κB-p65 and HDAC6/c-Myc besides FLT3/c-Myc which are aberrantly regulated in the progression of AML. In addition, crotonoside alone or the combination of sunitinib/RFP966/HPOB exhibited a significant post-inhibition effect in AML cells by the inhibition of FLT3 and HDAC3/6. Inhibitors targeting the FLT3 and HDAC3/6 might provide a more effective treatment strategy for AML. Taken together, the present study suggests that crotonoside could be a promising candidate for the treatment of AML, and deserves further investigations.

Molecularly targeted drug combinations demonstrate selective effectiveness for myeloid- and lymphoid-derived hematologic malignancies

Translating the genetic and epigenetic heterogeneity underlying human cancers into therapeutic strategies is an ongoing challenge. Large-scale sequencing efforts have uncovered a spectrum of mutations in many hematologic malignancies, including acute myeloid leukemia (AML), suggesting that combinations of agents will be required to treat these diseases effectively. Combinatorial approaches will also be critical for combating the emergence of genetically heterogeneous subclones, rescue signals in the microenvironment, and tumor-intrinsic feedback pathways that all contribute to disease relapse. To identify novel and effective drug combinations, we performed ex vivo sensitivity profiling of 122 primary patient samples from a variety of hematologic malignancies against a panel of 48 drug combinations. The combinations were designed as drug pairs that target nonoverlapping biological pathways and comprise drugs from different classes, preferably with Food and Drug Administration approval. A combination ratio (CR) was derived for each drug pair, and CRs were evaluated with respect to diagnostic categories as well as against genetic, cytogenetic, and cellular phenotypes of specimens from the two largest disease categories: AML and chronic lymphocytic leukemia (CLL). Nearly all tested combinations involving a BCL2 inhibitor showed additional benefit in patients with myeloid malignancies, whereas select combinations involving PI3K, CSF1R, or bromodomain inhibitors showed preferential benefit in lymphoid malignancies. Expanded analyses of patients with AML and CLL revealed specific patterns of ex vivo drug combination efficacy that were associated with select genetic, cytogenetic, and phenotypic disease subsets, warranting further evaluation. These findings highlight the heuristic value of an integrated functional genomic approach to the identification of novel treatment strategies for hematologic malignancies.

FLT3 activating mutations display differential sensitivity to multiple tyrosine kinase inhibitors

Fms-like tyrosine kinase-3 (FLT3) is a receptor tyrosine kinase that normally functions in hematopoietic cell survival, proliferation and differentiation. Constitutively activating mutations of FLT3 map predominately to the juxtamembrane domain (internal tandem duplications; ITD) or the activation loop (AL) of the kinase domain and are detected in about 1/3 of de novo acute myeloid leukemia (AML) patients. Small molecule tyrosine kinase inhibitors (TKI) effectively target FLT3/ITD mutations, but some activating mutations, particularly those on the AL, are relatively resistant to many FLT3 TKI. We reproduced many of the AL or other non-ITD activating mutations and tested 13 FLT3 TKI for their activity against these and wild-type FLT3. All 13 TKI tested inhibited BaF3/ITD cell proliferation in a concentration-dependent manner as reported, but most TKI exhibited a wide range of differential activity against AL and other point mutants. Western blotting results examining inhibition of FLT3 autophosphorylation and signaling pathways indicate that many AL mutations reduce TKI binding. Most FLT3 TKI effectively target wild-type FLT3 signaling. As a demonstration of this differential activity, treatment of BaF3 D835Y cells transplanted in BALB/c mice with sorafenib showed no effect in vivo against this mutant whereas lestaurtinib proved effective at reducing disease burden. Thus, while FLT3 TKI have been selected based on their ability to inhibit FLT3/ITD, the selection of appropriate TKI for AML patients with FLT3 AL and other activating point mutations requires personalized consideration.

Molecular and Cellular Mechanisms of Myelodysplastic Syndrome: Implications on Targeted Therapy

Myelodysplastic syndrome (MDS) is a group of heterogeneous clonal hematopoietic stem cell disorders characterized by cytopenia, ineffective hematopoiesis, and progression to secondary acute myeloid leukemia in high-risk cases. Conventional prognostication relies on clinicopathological parameters supplemented by cytogenetic information. However, recent studies have shown that genetic aberrations also have critical impacts on treatment outcome. Moreover, these genetic alterations may themselves be a target for treatment. The mutation landscape in MDS is shaped by gene aberrations involved in DNA methylation (TET2, DNMT3A, IDH1/2), histone modification (ASXL1, EZH2), the RNA splicing machinery (SF3B1, SRSF2, ZRSR2, U2AF1/2), transcription (RUNX1, TP53, BCOR, PHF6, NCOR, CEBPA, GATA2), tyrosine kinase receptor signaling (JAK2, MPL, FLT3, GNAS, KIT), RAS pathways (KRAS, NRAS, CBL, NF1, PTPN11), DNA repair (ATM, BRCC3, DLRE1C, FANCL), and cohesion complexes (STAG2, CTCF, SMC1A, RAD21). A detailed understanding of the pathogenetic mechanisms leading to transformation is critical for designing single-agent or combinatorial approaches in target therapy of MDS.

Molecularly targeted therapy in acute myeloid leukemia

Acute myeloid leukemia (AML) is molecularly heterogeneous. Formerly categorized cytogenetically and molecularly, AML may be classified by genomic and epigenomic analyses. These genetic lesions provide therapeutic targets. Genes targeted currently include mutated FLT3, NPM1 and KIT with drugs entering Phase III trials. Complete remission can be achieved in relapsed/refractory AML, albeit mostly transient. Mutated epigenetic modifiers, including DNMT3A, IDH1/2 and TET2, can be targeted by small molecule inhibitors, hypomethylating agents and histone deacetylase inhibitors. Other agents include cellular signaling pathway inhibitors and monoclonal antibodies against myeloid-associated antigens. Combinatorial strategies appear logical, mostly involving smaller molecular inhibitors partnering with hypomethylating agents. Currently limited to relapsed/refractory AML, targeted therapies are increasingly tested in frontline treatment with or without standard chemotherapy.

Clonal evolution of AML on novel FMS-like tyrosine kinase-3 (FLT3) inhibitor therapy with evolving actionable targets

For acute myeloid leukemia (AML), identification of activating mutations in the FMS-like tyrosine kinase-3 (FLT3) has led to the development of several FLT3-inhibitors. Here we present clinical and next generation sequencing data at the time of progression of a patient on a novel FLT3-inhibitor clinical trial (ASP2215) to show that employing therapeutic interventions with these novel targeted therapies can lead to consequences secondary to selective pressure and clonal evolution of cancer. We describe novel findings alongside data on treatment directed towards actionable aberrations acquired during the process. (

CLINICAL TRIAL

NCT02014558; registered at: 〈https://clinicaltrials.gov/ct2/show/NCT02014558〉).

Emerging Agents for the Treatment of Advanced, Imatinib-Resistant Gastrointestinal Stromal Tumors: Current Status and Future Directions

Imatinib is strongly positioned as the recommended first-line agent for most patients with advanced gastrointestinal stromal tumor (GIST) due to its good efficacy and tolerability. Imatinib-resistant advanced GIST continues to pose a therapeutic challenge, likely due to the frequent presence of multiple mutations that confer drug resistance. Sunitinib and regorafenib are approved as second- and third-line agents, respectively, for patients whose GIST does not respond to imatinib or who do not tolerate imatinib, and their use is supported by large randomized trials. ATP-mimetic tyrosine kinase inhibitors provide clinical benefit even in heavily pretreated GIST suggesting that oncogenic dependency on KIT frequently persists. Several potentially useful tyrosine kinase inhibitors with distinct inhibitory profiles against both KIT ATP-binding domain and activation loop mutations have not yet been fully evaluated. Agents that have been found promising in preclinical models and early clinical trials include small molecule KIT and PDGFRA mutation-specific inhibitors, heat shock protein inhibitors, histone deacetylase inhibitors, allosteric KIT inhibitors, KIT and PDGFRA signaling pathway inhibitors, and immunological approaches including antibody-drug conjugates. Concomitant or sequential administration of tyrosine kinase inhibitors with KIT signaling pathway inhibitors require further evaluation, as well as rotation of tyrosine kinase inhibitors as a means to suppress drug-resistant cell clones.

Chronic myeloid leukemia: advances in understanding disease biology and mechanisms of resistance to tyrosine kinase inhibitors

The successful implementation of tyrosine kinase inhibitors (TKIs) for the treatment of chronic myeloid leukemia (CML) remains a flagship for molecularly targeted therapy in cancer. This focused review highlights critical elements of the underlying biology of CML and provides a summary of the molecular mechanisms that lead to TKI resistance: BCR-ABL1 mutation-based resistance and therapy escape through alternative pathway activation despite inhibition of BCR-ABL1 tyrosine kinase activity. We direct attention to the most current manifestations of these issues, including emergence of pan-TKI-resistant BCR-ABL1 compound mutants, new strategies for identification and therapeutic targeting of alternative pathways, and the exciting, controversial topic of cessation of TKI therapy leading to durable treatment-free remissions for a subset of patients. Further gains in our understanding of the biology of Philadelphia chromosome-positive (Ph-positive) leukemia and mechanisms of resistance to BCR-ABL1 TKIs will benefit patients and also provide a blueprint for similar discovery in other cancers.

Computer aided drug discovery of highly ligand efficient, low molecular weight imidazopyridine analogs as FLT3 inhibitors

The FLT3 kinase represents an attractive target to effectively treat AML. Unfortunately, no FLT3 targeted therapeutic is currently approved. In line with our continued interests in treating kinase related disease for anti-FLT3 mutant activity, we utilized pioneering synthetic methodology in combination with computer aided drug discovery and identified low molecular weight, highly ligand efficient, FLT3 kinase inhibitors. Compounds were analyzed for biochemical inhibition, their ability to selectively inhibit cell proliferation, for FLT3 mutant activity, and preliminary aqueous solubility. Validated hits were discovered that can serve as starting platforms for lead candidates.

Will FLT3 inhibitors fulfill their promise in acute meyloid leukemia?

PURPOSE OF REVIEW

'FMS'-like tyrosine kinase 3 (FLT3) mutations in acute myeloid leukemia (AML) have been brought from discovery in the early 1990s to clinical targeting in the past 10 years. Despite several promising leads in preclinical models, no agent has yet been approved for clinical use. Here we will review the development of novel therapies for AML with FLT3 mutations.

RECENT FINDINGS

Initial clinical development focused on broad kinase inhibitors which were found to have limited clinical activity due to insufficient kinase inhibitory activity and high toxicity. Subsequent development has brought forth narrow-spectrum inhibitors with potent in-vivo activity and reasonable clinical tolerance, but many patients still progress with prolonged use.

SUMMARY

The optimal role for targeting FLT3 may depend on multimodality therapy and will likely require hematopoietic transplant. The incorporation of ABL kinase inhibitors into acute lymphoblastic leukemia management should serve as a model for incorporation of FLT3-targeted agents into clinical care. Strategies incorporating FLT3-targeted agents into AML therapy are ongoing, but challenges in trial design, clinical heterogeneity and need for long-term follow-up make these investigations complicated in design and implementation.

BET protein antagonist JQ1 is synergistically lethal with FLT3 tyrosine kinase inhibitor (TKI) and overcomes resistance to FLT3-TKI in AML cells expressing FLT-ITD

Recently, treatment with bromodomain and extraterminal protein antagonist (BA) such as JQ1 has been shown to inhibit growth and induce apoptosis of human acute myelogenous leukemia (AML) cells, including those expressing FLT3-ITD. Here, we demonstrate that cotreatment with JQ1 and the FLT3 tyrosine kinase inhibitor (TKI) ponatinib or AC220 synergistically induce apoptosis of cultured and primary CD34(+) human AML blast progenitor cells (BPC) expressing FLT3-ITD. Concomitantly, as compared with each agent alone, cotreatment with JQ1 and the FLT3-TKI caused greater attenuation of c-MYC, BCL2, and CDK4/6. Simultaneously, cotreatment with JQ1 and the FLT3-TKI increased the levels of p21, BIM, and cleaved PARP, as well as mediated marked attenuation of p-STAT5, p-AKT, and p-ERK1/2 levels in AML BPCs. Conversely, cotreatment with JQ1 and FLT3-TKI was significantly less active against CD34(+) normal bone marrow progenitor cells. Knockdown of BRD4 by short hairpin RNA also sensitized AML cells to FLT3-TKI. JQ1 treatment induced apoptosis of mouse Ba/F3 cells ectopically expressing FLT3-ITD with or without FLT3-TKI-resistant mutations F691L and D835V. Compared with the parental human AML FLT3-ITD-expressing MOLM13, MOLM13-TKIR cells resistant to AC220 were markedly more sensitive to JQ1-induced apoptosis. Furthermore, cotreatment with JQ1 and the pan-histone deacetylase inhibitor (HDI) panobinostat synergistically induced apoptosis of FLT3-TKI-resistant MOLM13-TKIR and MV4-11-TKIR cells. Collectively, these findings support the rationale for determining the in vivo activity of combined therapy with BA and FLT3-TKI against human AML cells expressing FLT3-ITD or with BA and HDI against AML cells resistant to FLT3-TKI.

Investigational FMS-like tyrosine kinase 3 inhibitors in treatment of acute myeloid leukemia

INTRODUCTION

Outcomes for the majority of patients with acute myeloid leukemia (AML) remain poor. Over the past decade, significant progress has been made in the understanding of the cytogenetic and molecular determinants of AML pathogenesis. One such advance is the identification of recurring mutations in the FMS-like tyrosine kinase 3 gene (FLT3). Currently, this marker, which appears in approximately one-third of all AML patients, not only signifies a poorer prognosis but also identifies an important target for therapy. FLT3 inhibitors have now undergone clinical evaluation in Phase I, II and III clinical trials, as both single agents and in combination with chemotherapeutics. Unfortunately, to date, none of the FLT3 inhibitors have gained FDA approval for the treatment of patients with AML. Yet, several promising FLT3 inhibitors are being evaluated in all phases of drug development.

AREAS COVERED

This review aims to highlight the agents furthest along in their development. It also focuses on those FLT3 inhibitors that are being evaluated in combination with other anti-leukemia agents.

EXPERT OPINION

The authors believe that the field of research for FLT3 inhibitors remains promising, despite the historically poor prognosis of this subgroup of patients with AML. The most promising areas of research will likely be the elucidation of the mechanisms of resistance to FLT3 inhibitors, and development of potent FLT3 inhibitors alone or in combination with hypomethylating agents, cytotoxic chemotherapy or with other targeted agents.

Recent advances and novel agents for FLT3 mutated acute myeloid leukemia

Acute myeloid leukemia (AML) is a devastating hematologic malignancy that affects both older adults as well as children. Treatments available for AML largely depend on cytotoxic agents and often the only curative option is an allogeneic bone marrow transplant, an option limited to young persons and associated with high morbidity and mortality. There is an urgent need for the identification of new myeloid targets and an understanding of the key genetic mutations involved in disease progression and prognosis. One such mutation is the internal tandem duplication (ITD) in the FMS-like tyrosine kinase receptor-3 (FLT3) gene which confers an inferior outcome that is attributed to a higher relapse rate. In this review, we evaluate the FLT3-ITD mutation and discuss the recent data regarding emerging approaches using FLT3 inhibitors for the treatment of AML.

Crenolanib is a selective type I pan-FLT3 inhibitor

Tyrosine kinase inhibitors (TKIs) represent transformative therapies for several malignancies. Two critical features necessary for maximizing TKI tolerability and response duration are kinase selectivity and invulnerability to resistance-conferring kinase domain (KD) mutations in the intended target. No prior TKI has demonstrated both of these properties. Aiming to maximize selectivity, medicinal chemists have largely sought to create TKIs that bind to an inactive (type II) kinase conformation. Here we demonstrate that the investigational type I TKI crenolanib is a potent inhibitor of Fms tyrosine kinase-3 (FLT3) internal tandem duplication, a validated therapeutic target in human acute myeloid leukemia (AML), as well as all secondary KD mutants previously shown to confer resistance to the first highly active FLT3 TKI quizartinib. Moreover, crenolanib is highly selective for FLT3 relative to the closely related protein tyrosine kinase KIT, demonstrating that simultaneous FLT3/KIT inhibition, a prominent feature of other clinically active FLT3 TKIs, is not required for AML cell cytotoxicity in vitro and may contribute to undesirable toxicity in patients. A saturation mutagenesis screen of FLT3-internal tandem duplication failed to recover any resistant colonies in the presence of a crenolanib concentration well below what has been safely achieved in humans, suggesting that crenolanib has the potential to suppress KD mutation-mediated clinical resistance. Crenolanib represents the first TKI to exhibit both kinase selectivity and invulnerability to resistance-conferring KD mutations, which is unexpected of a type I inhibitor. Crenolanib has significant promise for achieving deep and durable responses in FLT3-mutant AML, and may have a profound impact upon future medicinal chemistry efforts in oncology.

New resistance mechanisms for small molecule kinase inhibitors of Abl kinase

Mutations in the kinase domain of Bcr-Abl are the most common cause of resistance to therapy with Imatinib in patients with chronic myelogenous leukaemia (CML). Second generation Bcr-Abl inhibitors, such as Nilotinib and Dasatinib, are able to overcome most Imatinib- resistant mutants, with the exception of the T315I substitution. Structural studies of Abl wild-type and T315I mutant have provided better understanding of how this mutation leads to resistance and have been used to support the drug design process for the development of inhibitors able to target the T315I substitution.

Combining the ABL1 kinase inhibitor ponatinib and the histone deacetylase inhibitor vorinostat: a potential treatment for BCR-ABL-positive leukemia

Resistance to imatinib (Gleevec®) in cancer cells is frequently because of acquired point mutations in the kinase domain of BCR-ABL. Ponatinib, also known as AP24534, is an oral multi-targeted tyrosine kinase inhibitor (TKI), and it has been investigated in a pivotal phase 2 clinical trial. The histone deacetylase inhibitor vorinostat (suberoylanilide hydroxamic acid) has been evaluated for its significant clinical activity in hematological malignancies. Thus, treatments combining ABL TKIs with additional drugs may be a promising strategy in the treatment of leukemia. In the current study, we analyzed the efficacy of ponatinib and vorinostat treatment by using BCR-ABL-positive cell lines. Treatment with ponatinib for 72 h inhibited cell growth and induced apoptosis in K562 cells in a dose-dependent manner. We found that ponatinib potently inhibited the growth of Ba/F3 cells ectopically expressing BCR-ABL T315I mutation. Upon BCR-ABL phosphorylation, Crk-L was decreased, and poly (ADP-ribose) polymerase (PARP) was activated in a dose-dependent manner. Combined treatment of Ba/F3 T315I mutant cells with vorinostat and ponatinib resulted in significantly increased cytotoxicity. Additionally, the intracellular signaling of ponatinib and vorinostat was examined. Caspase 3 and PARP activation increased after combination treatment with ponatinib and vorinostat. Moreover, an increase in the phosphorylation levels of γH2A.X was observed. Previously established ponatinib-resistant Ba/F3 cells were also resistant to imatinib, nilotinib, and dasatinib. We investigated the difference in the efficacy of ponatinib and vorinostat by using ponatinib-resistant Ba/F3 cells. Combined treatment of ponatinib-resistant cells with ponatinib and vorinostat caused a significant increase in cytotoxicity. Thus, combined administration of ponatinib and vorinostat may be a powerful strategy against BCR-ABL mutant cells and could enhance the cytotoxic effects of ponatinib in those BCR-ABL mutant cells.

FLT3 mutations in acute myeloid leukemia: what is the best approach in 2013?

Patients with acute myeloid leukemia who harbor an FMS-like tyrosine kinase 3 (FLT3) mutation present several dilemmas for the clinician. The results of an FLT3 mutation test, which can be influenced by several variables, need to be interpreted according to the clinical setting and there is a need for internationally standardized FLT3 mutation assays. Because of the lack of prospective studies, the role of allogeneic transplantation as consolidation therapy is still somewhat controversial, but the preponderance of evidence suggests that transplantation in first remission, if possible, is probably the best option. Clinically useful FLT3 inhibitors are hopefully on the near horizon and are being studied in the context of current treatment paradigms.