Binding of Released Bim to Mcl-1 is a Mechanism of Intrinsic Resistance to ABT-199 which can be Overcome by Combination with Daunorubicin or Cytarabine in AML Cells

Targeting Bcl-2 Proteins in Acute Myeloid Leukemia

B cell lymphoma 2 (BCL-2) family proteins play an important role in intrinsic apoptosis. Overexpression of BCL-2 proteins in acute myeloid leukemia can circumvent resistance to apoptosis and chemotherapy. Considering this effect, the exploration of anti-apoptotic BCL-2 inhibitors is considered to have tremendous potential for the discovery of novel pharmacological modulators in cancer. This review outlines the impact of BCL-2 family proteins on intrinsic apoptosis and the development of acute myeloid leukemia (AML). Furthermore, we will also review the new combination therapy with venetoclax that overcomes resistance to venetoclax and discuss biomarkers of treatment response identified in early-phase clinical trials.

Venetoclax-Based Combinations in Acute Myeloid Leukemia: Current Evidence and Future Directions

The past decade has witnessed major advances in our understanding of molecular biology, which led to breakthrough novel therapies, importantly including the B-cell lymphoma-2 (BCL-2) inhibitor venetoclax. Notably, venetoclax-based combinations have improved outcomes, including both remission rates and overall survival, of older patients with acute myeloid leukemia (AML) deemed "unfit" for intensive chemotherapy due to age or comorbidities. This has translated into a rapid and widespread use of venetoclax-based combinations in both academic and community-based settings. Other venetoclax-based combinations are being investigated in AML with the ultimate goal of improving cure rates across many subgroups; frontline and relapsed/refractory, in combination with intensive chemotherapy, in the post-transplant setting, or as maintenance strategy. In this article, we summarize the current available data on venetoclax-based combinations. We also highlight areas of unmet medical need, and we offer practical clinical pearls for management of patients receiving such therapy.

Pairing MCL-1 inhibition with venetoclax improves therapeutic efficiency of BH3-mimetics in AML

OBJECTIVES

Venetoclax combined with hypomethylating agents is a new therapeutic strategy frequently used for treating AML patients who are not eligible for conventional chemotherapy. However, high response rates are heterogeneous due to different mechanisms mediating resistance to venetoclax such as up-regulation of MCL-1 expression. We thus tested the anti-leukemic activity of S63845, a specific MCL-1 inhibitor.

METHODS

Apoptosis induces by S63845 with or without venetoclax was evaluated in primary AML samples and in AML cell lines co-cultured or not with bone marrow (BM) mesenchymal stromal cells. Sensitivity of leukemic cells to S63845 was correlated to the expression level of BCL-2, MCL-1, and BCL-XL determined by Western Blot and mass spectrometry-based proteomics.

RESULTS

We observed that even if MCL-1 expression is weak compared to BCL-2, S63845 induces apoptosis of AML cells and strongly synergizes with venetoclax. Furthermore, AML cells resistant to venetoclax are highly sensitive to S63845. Interestingly, the synergistic effect of S63845 toward venetoclax-mediated apoptosis of AML cells is still observed in a context of interaction with the BM microenvironment that intrinsically mediates resistance to BCL2 inhibition.

CONCLUSION

These results are therefore of great relevance for clinicians as they provide the rational for combining BCL-2 and MCL-1 inhibition in AML.

Combination strategies to overcome resistance to the BCL2 inhibitor venetoclax in hematologic malignancies

Venetoclax has been approved by the United States Food and Drug Administration since 2016 as a monotherapy for treating patients with relapsed/refractory chronic lymphocytic leukemia having 17p deletion. It has led to a breakthrough in the treatment of hematologic malignancies in recent years. However, unfortunately, resistance to venetoclax is inevitable. Multiple studies confirmed that the upregulation of the anti-apoptotic proteins of the B-cell lymphoma 2 (BCL2) family mediated by various mechanisms, such as tumor microenvironment, and the activation of intracellular signaling pathways were the major factors leading to resistance to venetoclax. Therefore, only targeting BCL2 often fails to achieve the expected therapeutic effect. Based on the mechanism of resistance in specific hematologic malignancies, the combination of specific drugs with venetoclax was a clinically optional treatment strategy for overcoming resistance to venetoclax. This study aimed to summarize the possible resistance mechanisms of various hematologic tumors to venetoclax and the corresponding clinical strategies to overcome resistance to venetoclax in hematologic malignancies.

A novel kinase inhibitor, LZT-106, downregulates Mcl-1 and sensitizes colorectal cancer cells to BH3 mimetic ABT-199 by targeting CDK9 and GSK-3β signaling

Colorectal cancer (CRC) is one of the most common malignancies worldwide and is associated with poor prognosis and high mortality. Despite advances in treatment with chemotherapy, CRC remains a major cause of drug resistance-related cancer deaths. One of the main reasons for such resistance is dysregulation of Mcl-1 expression. In this study, we identified LZT-106 as a novel kinase inhibitor that was able to bind to CDK9 with potent inhibitory ability, and indirectly regulate the expression of Mcl-1. However, different regulatory profiles were observed between LZT-106 and the well-studied CDK9 inhibitor flavopiridol with regards to Mcl-1 inhibition. Via Western blotting, real-time PCR and immunoprecipitation, we confirmed that LZT-106 was also able to target GSK-3β signaling and facilitate the degradation of Mcl-1. And LZT-106 was shown to synergize with ABT-199 to induce apoptosis even in the RKO cell line that overexpressed Mcl-1. Finally, LZT-106 significantly inhibited tumor growth in a xenograft mouse model with minimal toxicity. Overall, our findings suggest that LZT-106 is a promising candidate drug for the treatment of patients with CRC.

BH3 Mimetics in AML Therapy: Death and Beyond?

B cell lymphoma 2 (BCL2) homology domain 3 (BH3) mimetics are targeted therapeutic agents that allow response prediction and patient stratification. BH3 mimetics are prototypical activators of the mitochondrial death program in cancer. They emerged as important modulators of cellular mechanisms contributing to poor therapeutic responses, including cancer cell stemness, cancer-specific metabolic routes, paracrine signaling to the tumor microenvironment, and immune modulation. We present an overview of the antagonism between BH3 mimetics and antiapoptotic BCL2 proteins. We focus on acute myeloid leukemia (AML), a cancer with reduced therapeutic options that have recently been improved by BH3 mimetics.

Involvement of CYP2E1-ROS-CD36/DGAT2 axis in the pathogenesis of VPA-induced hepatic steatosis in vivo and in vitro

Valproic acid (VPA) is a widely prescribed antiepileptic drug, which may cause steatosis in the liver. Oxidative stress is associated with the progression of VPA-induced hepatic steatosis. However, the potential mechanisms are not fully understood. In this study, we demonstrated the involvement of CYP2E1-ROS-CD36/DGAT2 axis in the pathogenesis of VPA-induced hepatic steatosis in vitro and in vivo. First, VPA treatment (500 mg/kg in mice, 5 mM in LO₂ cells) induced hepatic steatosis and enhanced reactive oxidative stress (ROS) level, and ROS scavenger, N-acetyl-L-cysteine (NAC, 200 mg/kg in mice, 1 mM in LO₂ cells) reversed the changes. Next, we observed the enhanced expression and enzymatic activity of cytochrome P450 2E1 (CYP2E1) in VPA-treated mice and LO₂ cells. Importantly, VPA-induced ROS accumulation and hepatic steatosis were attenuated when CYP2E1 was inhibited using CYP2E1 inhibitor, diallyl sulfide (DAS, 100 mg/kg in mice, 1 mM in LO₂ cells) or in CYP2E1-knockdown cell line, suggesting that CYP2E1 plays a potential role in ROS production following hepatic steatosis. Furthermore, gene expression analysis showed that the mRNA levels of cluster of differentiation 36 (CD36), a fatty acid translocase protein and distinct diacylglycerol acyltransferase 2 (DGAT2) were significantly upregulated in mice and LO₂ cells after VPA treatment, while the change was alleviated by NAC and DAS. Meanwhile, time course experiments demonstrated that the increase of CYP2E1 level occurred earlier than that of ROS, CD36 and DGAT2, and ROS generation preceded the onset of hepatic steatosis. Taken together, VPA treatment enhances the expression and enzymatic activity of CYP2E1, which promotes ROS production and then causes CD36 and DGAT2 overproduction and hepatic steatosis in mice and LO₂ cells, which provides a novel insight into VPA-induced hepatic steatosis.

Preclinical evaluation of a regimen combining chidamide and ABT-199 in acute myeloid leukemia

Acute myeloid leukemia (AML) is a heterogeneous myeloid neoplasm with poor clinical outcome, despite the great progress in treatment in recent years. The selective Bcl-2 inhibitor venetoclax (ABT-199) in combination therapy has been approved for the treatment of newly diagnosed AML patients who are ineligible for intensive chemotherapy, but resistance can be acquired through the upregulation of alternative antiapoptotic proteins. Here, we reported that a newly emerged histone deacetylase inhibitor, chidamide (CS055), at low-cytotoxicity dose enhanced the anti-AML activity of ABT-199, while sparing normal hematopoietic progenitor cells. Moreover, we also found that chidamide showed a superior resensitization effect than romidepsin in potentiation of ABT-199 lethality. Inhibition of multiple HDACs rather than some single component might be required. The combination therapy was also effective in primary AML blasts and stem/progenitor cells regardless of disease status and genetic aberrance, as well as in a patient-derived xenograft model carrying FLT3-ITD mutation. Mechanistically, CS055 promoted leukemia suppression through DNA double-strand break and altered unbalance of anti- and pro-apoptotic proteins (e.g., Mcl-1 and Bcl-xL downregulation, and Bim upregulation). Taken together, these results show the high therapeutic potential of ABT-199/CS055 combination in AML treatment, representing a potent and alternative salvage therapy for the treatment of relapsed and refractory patients with AML.

Cotargeting of Mitochondrial Complex I and Bcl-2 Shows Antileukemic Activity against Acute Myeloid Leukemia Cells Reliant on Oxidative Phosphorylation

Targeting oxidative phosphorylation (OXPHOS) is a promising strategy to improve treatment outcomes of acute myeloid leukemia (AML) patients. IACS-010759 is a mitochondrial complex I inhibitor that has demonstrated preclinical antileukemic activity and is being tested in Phase I clinical trials. However, complex I deficiency has been reported to inhibit apoptotic cell death through prevention of cytochrome c release. Thus, combining IACS-010759 with a BH3 mimetic may overcome this mechanism of resistance leading to synergistic antileukemic activity against AML. In this study, we show that IACS-010759 and venetoclax synergistically induce apoptosis in OXPHOS-reliant AML cell lines and primary patient samples and cooperatively target leukemia progenitor cells. In a relatively OXPHOS-reliant AML cell line derived xenograft mouse model, IACS-010759 treatment significantly prolonged survival, which was further enhanced by treatment with IACS-010759 in combination with venetoclax. Consistent with our hypothesis, IACS-010759 treatment indeed retained cytochrome c in mitochondria, which was completely abolished by venetoclax, resulting in Bak/Bax- and caspase-dependent apoptosis. Our preclinical data provide a rationale for further development of the combination of IACS-010759 and venetoclax for the treatment of patients with AML.

B-cell lymphoma-2 inhibition and resistance in acute myeloid leukemia

Spurred by better understanding of disease biology, improvements in molecular diagnostics, and the development of targeted therapies, the treatment of acute myeloid leukemia (AML) has undergone significant evolution in recent years. Arguably, the most exciting shift has come from the success of treatment with the B-cell lymphoma-2 inhibitor venetoclax. When given in combination with a hypomethylating agent or low dose cytarabine, venetoclax demonstrates high response rates, some of which are durable. In spite of this, relapses after venetoclax treatment are common, and much interest exists in elucidating the mechanisms of resistance to the drug. Alterations in leukemic stem cell metabolism have been identified as a possible escape route, and clinical trials focusing on targeting metabolism in AML are ongoing. This review article highlights current research regarding venetoclax treatment and resistance in AML with a focus on cellular metabolism.

An expert overview of emerging therapies for acute myeloid leukemia: novel small molecules targeting apoptosis, p53, transcriptional regulation and metabolism

INTRODUCTION

Acute myeloid leukemia (AML) is an aggressive malignancy of clonal myeloid precursor cells. Curative therapy has classically involved the use of intensive induction chemotherapy followed by consolidation with additional chemotherapy or allogeneic hematopoietic stem cell transplant. For many patients, such an approach is prohibitive because of high treatment-related toxicities. Advancements in the molecular understanding of AML have led to the introduction of new targeted therapies that are changing the treatment landscape for AML.

AREAS COVERED

We review emerging small molecule inhibitors that have shown preclinical efficacy for the treatment of AML. The compounds discussed affect apoptosis, p53-mediated interactions, transcriptional regulation, and cellular metabolism. We performed a literature search of PubMed and primarily included relevant sources published from 2000 to the present, though earlier sources are also referenced.

EXPERT OPINION

Most clinical trials for AML currently employ novel targeted therapies that demonstrate promising activity in preclinical models. We anticipate that new small molecule inhibitors will continue to enter the clinical realm and alter the treatment paradigm for AML. In a field where clinical advancement was comparatively slow for many years, it appears that we are now starting to see the rapid growth borne out of the deepening molecular understanding of AML.

Combining triptolide with ABT-199 is effective against acute myeloid leukemia through reciprocal regulation of Bcl-2 family proteins and activation of the intrinsic apoptotic pathway

Bcl-2 inhibitors display an effective activity in acute myeloid leukemia (AML), but its clinical efficacy as a monotherapy was limited in part owing to failure to target other antiapoptotic Bcl-2 family proteins, such as Mcl-1. In this context, the combination strategy may be a promising approach to overcome this barrier. Here, we report the preclinical efficacy of a novel strategy combining ABT-199 with triptolide (TPL), a natural product extracted from a traditional Chinese medicine, in AML. Combination treatment exhibited markedly increased cytotoxicity in leukemic cells irrespective of p53 status while largely sparing normal cells of the hematopoietic lineage. Moreover, co-administration of ABT-199 with TPL dramatically suppressed leukemia progression as well as prolonged animal survival in a xenograft AML model. The potentiated effect of ABT-199 and TPL against AML was associated with activation of the mitochondrum-related intrinsic apoptotic pathway through a mechanism reciprocally modulating Bcl-2 family proteins. In this case, TPL not only downregulated Mcl-1 but also upregulated proapoptotic BH3-only proteins, thereby overcoming the resistance toward ABT-199. Conversely, ABT-199 abrogated Bcl-2-mediated cytoprotection against TPL. Together, these findings suggest that the regimen combining TPL and ABT-199 might be active against AML by inducing robust apoptosis through reciprocal regulation of anti- and proapoptotic Bcl-2 family proteins, therefore providing a strong rationale for the clinical investigation of this combination regimen for the treatment of AML.

Chemotherapy and Venetoclax in Elderly Acute Myeloid Leukemia Trial (CAVEAT): A Phase Ib Dose-Escalation Study of Venetoclax Combined With Modified Intensive Chemotherapy

PURPOSE

The B-cell lymphoma 2 (BCL-2) inhibitor venetoclax has an emerging role in acute myeloid leukemia (AML), with promising response rates in combination with hypomethylating agents or low-dose cytarabine in older patients. The tolerability and efficacy of venetoclax in combination with intensive chemotherapy in AML is unknown.

PATIENTS AND METHODS

Patients with AML who were ≥ 65 years (≥ 60 years if monosomal karyotype) and fit for intensive chemotherapy were allocated to venetoclax dose-escalation cohorts (range, 50-600 mg). Venetoclax was administered orally for 14 days each cycle. During induction, a 7-day prephase/dose ramp-up (days -6 to 0) was followed by an additional 7 days of venetoclax combined with infusional cytarabine 100 mg/m² on days 1-5 and idarubicin 12 mg/m² intravenously on days 2-3 (ie, 5 + 2). Consolidation (4 cycles) included 14 days of venetoclax (days -6 to 7) combined with cytarabine (days 1-2) and idarubicin (day 1). Maintenance venetoclax was permitted (7 cycles). The primary objective was to assess the optimal dose schedule of venetoclax with 5 + 2.

RESULTS

Fifty-one patients with a median age of 72 years (range, 63-80 years) were included. The maximum tolerated dose was not reached with venetoclax 600 mg/day. The main grade ≥ 3 nonhematologic toxicities during induction were febrile neutropenia (55%) and sepsis (35%). In contrast to induction, platelet recovery was notably delayed during consolidation cycles. The overall response rate (complete remission [CR]/CR with incomplete count recovery) was 72%; it was 97% in de novo AML and was 43% in secondary AML. During the venetoclax prephase, marrow blast reductions (≥ 50%) were noted in NPM1-, IDH2-, and SRSF2-mutant AML.

CONCLUSION

Venetoclax combined with 5 + 2 induction chemotherapy was safe and tolerable in fit older patients with AML. Although the optimal postremission therapy remains to be determined, the high remission rate in de novo AML warrants additional investigation (ANZ Clinical Trial Registry No. ACTRN12616000445471).

Targeting nuclear import and export in hematological malignancies

The transport of proteins across the nuclear membrane is a highly regulated process, essential for the cell function. This transport is actively mediated by members of the karyopherin family, termed importins, or exportins, depending on the direction of transport. These proteins play an active part in tumorigenesis, through aberrant localization of their cargoes, which include oncogenes, tumor-suppressor genes and mediators of key signal transduction pathways. Overexpression of importins and exportins is reported in many malignancies, with implications in cell growth and viability, differentiation, drug resistance, and tumor microenvironment. Given their broad significance across tumors and pathways, much effort is being put to develop specific inhibitors as a novel anticancer therapeutics. Already, selinexor, a specific inhibitor of exportin-1 (XPO1), is approved for clinical use. This review will focus on the role of importins and exportins in hematological malignancies. We will discuss current preclinical and clinical data on importins and exportins, and demonstrate how our growing understanding of their functions has identified new therapeutic targets.

Mitochondria: A Galaxy in the Hematopoietic and Leukemic Stem Cell Universe

Mitochondria are the main fascinating energetic source into the cells. Their number, shape, and dynamism are controlled by the cell’s type and current behavior. The perturbation of the mitochondrial inward system via stress response and/or oncogenic insults could activate several trafficking molecular mechanisms with the intention to solve the problem. In this review, we aimed to clarify the crucial pathways in the mitochondrial system, dissecting the different metabolic defects, with a special emphasis on hematological malignancies. We investigated the pivotal role of mitochondria in the maintenance of hematopoietic stem cells (HSCs) and their main alterations that could induce malignant transformation, culminating in the generation of leukemic stem cells (LSCs). In addition, we presented an overview of LSCs mitochondrial dysregulated mechanisms in terms of (1) increasing in oxidative phosphorylation program (OXPHOS), as a crucial process for survival and self-renewal of LSCs,(2) low levels of reactive oxygen species (ROS), and (3) aberrant expression of B-cell lymphoma 2 (Bcl-2) with sustained mitophagy. Furthermore, these peculiarities may represent attractive new “hot spots” for mitochondrial-targeted therapy. Finally, we remark the potential of the LCS metabolic effectors to be exploited as novel therapeutic targets.

Venetoclax in combination with cytarabine with or without idarubicin in children with relapsed or refractory acute myeloid leukaemia: a phase 1, dose-escalation study

BACKGROUND

Outcomes for children with relapsed or refractory acute myeloid leukaemia remain poor. The BCL-2 inhibitor, venetoclax, has shown promising activity in combination with hypomethylating agents and low-dose cytarabine in older adults for whom chemotherapy is not suitable with newly diagnosed acute myeloid leukaemia. We aimed to determine the safety and explore the activity of venetoclax in combination with standard and high-dose chemotherapy in paediatric patients with relapsed or refractory acute myeloid leukaemia.

METHODS

We did a phase 1, dose-escalation study at three research hospitals in the USA. Eligible patients were aged 2-22 years with relapsed or refractory acute myeloid leukaemia or acute leukaemia of ambiguous lineage with adequate organ function and performance status. During dose escalation, participants received venetoclax orally once per day in continuous 28-day cycles at either 240 mg/m2 or 360 mg/m2, in combination with cytarabine received intravenously every 12 h at either 100 mg/m2 for 20 doses or 1000 mg/m2 for eight doses, with or without intravenous idarubicin (12 mg/m2) as a single dose, using a rolling-6 accrual strategy. The primary endpoint was the recommended phase 2 dose of venetoclax plus chemotherapy and the secondary endpoint was the proportion of patients treated at the recommended phase 2 dose who achieved complete remission or complete remission with incomplete haematological recovery. Analyses were done on patients who received combination therapy. The study is registered with ClinicalTrials.gov (NCT03194932) and is now enrolling to address secondary and exploratory objectives.

FINDINGS

Between July 1, 2017, and July 2, 2019, 38 patients were enrolled (aged 3-22 years; median 10 [IQR 7-13]), 36 of whom received combination therapy with dose escalation, with a median follow-up of 7·1 months (IQR 5·1-11·2). The recommended phase 2 dose of venetoclax was found to be 360 mg/m2 (maximum 600 mg) combined with cytarabine (1000 mg/m2 per dose for eight doses), with or without idarubicin (12 mg/m2 as a single dose). Overall responses were observed in 24 (69%) of the 35 patients who were evaluable after cycle 1. Among the 20 patients treated at the recommended phase 2 dose, 14 (70%, 95% CI 46-88) showed complete response with or without complete haematological recovery, and two (10%) showed partial response. The most common grade 3-4 adverse events were febrile neutropenia (22 [66%]), bloodstream infections (six [16%]), and invasive fungal infections (six [16%]). Treatment-related death occurred in one patient due to colitis and sepsis.

INTERPRETATION

The safety and activity of venetoclax plus chemotherapy in paediatric patients with heavily relapsed and refractory acute myeloid leukaemia suggests that this combination should be tested in newly diagnosed paediatric patients with high-risk acute myeloid leukaemia.

FUNDING

US National Institutes of Health, American Lebanese Syrian Associated Charities, AbbVie, and Gateway for Cancer Research.

Inhibition of CDK9 by voruciclib synergistically enhances cell death induced by the Bcl-2 selective inhibitor venetoclax in preclinical models of acute myeloid leukemia

Venetoclax, an FDA-approved Bcl-2 selective inhibitor for the treatment of chronic lymphocytic leukemia and acute myeloid leukemia (AML), is tolerated well in elderly patients with AML and has good overall response rates; however, resistance remains a concern. In this study, we show that targeting CDK9 with voruciclib in combination with venetoclax results in synergistic antileukemic activity against AML cell lines and primary patient samples. CDK9 inhibition enhances venetoclax activity through downregulation of Mcl-1 and c-Myc. However, downregulation of Mcl-1 is transient, which necessitates an intermittent treatment schedule to allow for repeated downregulation of Mcl-1. Accordingly, an every other day schedule of the CDK9 inhibitor is effective in vitro and in vivo in enhancing the efficacy of venetoclax. Our preclinical data provide a rationale for an intermittent drug administration schedule for the clinical evaluation of the combination treatment for AML.

The Time Has Come for Targeted Therapies for AML: Lights and Shadows

Acute myeloid leukemia (AML) is a complex disease characterized by genetic and clinical heterogeneity and high mortality. After 40 years during which the standard of care for patients evolved very little, the therapeutic landscape has recently seen rapid changes, with the approval of eight new drugs by the Food and Drug Administration (FDA) within the last 2 years, providing new opportunities, as well as new challenges, for treating clinicians. These therapies include FLT3 inhibitors midostaurin and gilteritinib, CPX-351 (liposomal cytarabine and daunorubicin), gemtuzumab ozogamicin (GO, anti-CD33 monoclonal antibody conjugated with calicheamicin), IDH1/IDH2 inhibitors ivosidenib and enasidenib, Hedgehog inhibitor glasdegib, and BCL-2 inhibitor venetoclax. In this review, we summarize currently available data on these new drugs and discuss the rapidly evolving therapeutic armamentarium for AML, focusing on targeted therapies.

Saga of Mcl-1: regulation from transcription to degradation

The members of the Bcl-2 family are the central regulators of various cell death modalities. Some of these proteins contribute to apoptosis, while others counteract this type of programmed cell death, thus balancing cell demise and survival. A disruption of this balance leads to the development of various diseases, including cancer. Therefore, understanding the mechanisms that underlie the regulation of proteins of the Bcl-2 family is of great importance for biomedical research. Among the members of the Bcl-2 family, antiapoptotic protein Mcl-1 is characterized by a short half-life, which renders this protein highly sensitive to changes in its synthesis or degradation. Hence, the regulation of Mcl-1 is of particular scientific interest, and the study of Mcl-1 modulators could aid in the understanding of the mechanisms of disease development and the ways of their treatment. Here, we summarize the present knowledge regarding the regulation of Mcl-1, from transcription to degradation, focusing on aspects that have not yet been described in detail.

Venetoclax for AML: changing the treatment paradigm

Venetoclax is a specific B-cell lymphoma-2 (BCL-2) inhibitor that can restore activation of apoptosis in malignancies, the survival of which depends on dysregulation of this pathway. Preclinical data, using various model systems including cell lines and patient samples, suggested targeting BCL-2 could be a successful therapeutic strategy in patients with acute myeloid leukemia (AML). As predicted by this work, the use of venetoclax in the clinical setting has resulted in promising outcomes for patients with this disease. Although venetoclax showed limited activity as a single agent in the relapsed disease setting, recent studies have shown that when combined with a backbone therapy of a hypomethylating agent or low-dose cytarabine, high response rates with encouraging remission durations for older patients with newly diagnosed AML who were not candidates for intensive induction chemotherapy were observed. Furthermore, venetoclax-based therapies allowed for rapid responses and were able to effectively target the leukemia stem cell population. Here we review the preclinical data that supported the development of venetoclax in AML, as well as the results of the promising clinical trials.

Transcriptional Silencing of MCL-1 Through Cyclin-Dependent Kinase Inhibition in Acute Myeloid Leukemia

Acute myeloid leukemia (AML) is the most common adult acute leukemia. Survival remains poor, despite decades of scientific advances. Cytotoxic induction chemotherapy regimens are standard-of-care for most patients. Many investigations have highlighted the genomic heterogeneity of AML, and several new targeted therapeutic options have recently been approved. Additional novel therapies are showing promising clinical results and may rapidly transform the therapeutic landscape of AML. Despite the emerging clinical success of B-cell lymphoma (BCL)-2 targeting in AML and a large body of preclinical data supporting myeloid leukemia cell (MCL)-1 as an attractive therapeutic target for AML, MCL-1 targeting remains relatively unexplored, although novel MCL-1 inhibitors are under clinical investigation. Inhibitors of cyclin-dependent kinases (CDKs) involved in the regulation of transcription, CDK9 in particular, are being investigated in AML as a strategy to target MCL-1 indirectly. In this article, we review the basis for CDK inhibition in oncology with a focus on relevant preclinical mechanism-of-action studies of CDK9 inhibitors in the context of their therapeutic potential specifically in AML.

Side-by-side comparison of BH3-mimetics identifies MCL-1 as a key therapeutic target in AML

Despite advances in the treatment of acute myeloid leukemia (AML), prognosis of AML patients is still dismal and better treatment options are required. B-cell Lymphoma 2 (BCL-2) homology domain 3 (BH3)-mimetics are emerging as a novel class of apoptosis-inducing agents that are currently being tested for the treatment of different hematological malignancies including AML. Particularly, the selective BCL-2 inhibitor ABT-199/Venetoclax is demonstrating clinical responses and has recently been approved in combination for the treatment of AML. Compounds targeting the related protein MCL-1 have recently entered clinical trials, highlighting the urgency to compare the different BH3-mimetics and identify the most promising antiapoptotic target in AML. We performed a side-by-side comparison of different highly selective and potent BH3-mimetics targeting BCL-2 (ABT-199), MCL-1 (S63845) or BCL-x_L (A1331852) in a panel of AML cell lines and primary patient cells. Gene knockdown using siRNAs was utilized to investigate the functional relevance of BCL-2 proteins. Western blotting and immunoprecipitations were used to explore the influence of BH3-mimetics on interactions between pro- and antiapoptotic BCL-2 proteins. A1331852 induced apoptosis only in selected cases, indicating that BCL-x_L is not a very promising therapeutic target in AML. However, S63845 displayed higher potency than ABT-199, with more cell lines and primary cells responding to S63845 than to ABT-199. MCL-1 dependency in AML cells was confirmed by siRNA-mediated knockdown of MCL-1, which was sufficient to induce apoptosis. S63845-induced cell death was accompanied by a displacement of the BH3-only protein BIM as well as BAK, resulting in BAK-dependent apoptosis. In contrast, ABT-199-induced cell death was mediated by BAX rather than BAK, indicating distinct non-redundant molecular functions of BCL-2 and MCL-1 in AML. Our study reveals that MCL-1 may be a more prevalent therapeutic target than BCL-2 in AML and identifies BIM and BAK as important mediators of S63845-induced apoptosis in AML.

Targeting Apoptotic Pathways in Acute Myeloid Leukaemia

Acute Myeloid Leukaemia is a devastating disease that continues to have a poor outcome for the majority of patients. In recent years, however, a number of drugs have received FDA approval, following on from successful clinical trial results. This parallels the characterization of the molecular landscape of Acute Myeloid Leukaemia (AML) over the last decade, which has led to the development of drugs targeting newly identified recurring mutations. In addition, basic biological research into the pathobiology of AML has identified aberrant programmed cell death pathways in AML. Following on from successful outcomes in lymphoid malignancies, drugs targeting the B Cell Lymphoma 2 (BCL-2) family of anti-apoptotic proteins have been explored in AML. In this review, we will outline the preclinical and clinical work to date supporting the role of drugs targeting BCL-2, with Venetoclax being the most advanced to date. We will also highlight rationale combinations using Venetoclax, ongoing clinical trials and biomarkers of response identified from the early phase clinical trials performed.

Venetoclax for the treatment of newly diagnosed acute myeloid leukemia in patients who are ineligible for intensive chemotherapy

Acute myeloid leukemia (AML) is an aggressive hematological malignancy with a globally poor outcome, especially in patients ineligible for intensive chemotherapy. Until recently, therapeutic options for these patients included low-dose cytarabine (LDAC) or the hypomethylating agents (HMA) azacitidine and decitabine, which have historically provided only short-lived and modest benefits. The oral B-cell lymphoma 2 inhibitor, venetoclax, Venetoclax, an oral B-cell lymphoma 2 (BCL2) inhibitor, is now approved by the USA Food and Drug Administration (FDA) in combination with LDAC or HMA in older AML patients ineligible for intensive chemotherapy. Is now approved by the US Food and Drug Administration for this indication. In the pivotal clinical trials evaluating venetoclax either in combination with LDAC or with HMA, the rates of complete remission (CR) plus CR with incomplete hematological recovery were 54% and 67%, respectively and the median overall survival (OS) was 10.4 months and 17.5 months, respectively, comparing favorably with outcomes in clinical trials evaluating single-agent LDAC or HMA. The most common adverse events with venetoclax combinations are gastrointestinal symptoms, which are primarily low grade and easily manageable, and myelosuppression, which may require delays between cycles, granulocyte colony-stimulating factor (G-CSF) administration, or decreased duration of venetoclax administration per cycle. A bone marrow assessment after the first cycle of treatment is critical to determine dosing and timing of subsequent cycles, as most patients will achieve their best response after one cycle. Appropriate prophylactic measures can reduce the risk of venetoclax-induced tumor lysis syndrome. In this review, we present clinical data from the pivotal trials evaluating venetoclax-based combinations in older patients ineligible for intensive chemotherapy, and provide practical recommendations for the prevention and management of adverse events associated with venetoclax.

Cyclin-dependent kinase (CDK) 9 and 4/6 inhibitors in acute myeloid leukemia (AML): a promising therapeutic approach

Introduction: Despite advancements over the last 2 years, outcomes for acute myeloid leukemia (AML) are poor; however, a greater comprehension of disease mechanisms has driven the investigation of new targeted treatments. Cyclin-dependent kinases (CDKs) regulate cell cycle progression, transcription and DNA repair, and are aberrantly expressed in AML. Targeting the CDK pathway is an emerging promising therapeutic strategy in AML.Areas covered: We describe the rationale for targeting CDK9 and CDK4/6, the ongoing preclinical and clinical trials and the potential of these inhibitors in AML. Our analysis included an extensive literature search via the Pubmed database and clinicaltrials.gov (March to August, 2019).Expert opinion: While CDK4/6 inhibitors are early in development for AML, CDK9 inhibition with alvocidib has encouraging clinical activity in newly diagnosed and relapsed/refractory AML. Preclinical data suggests that leukemic MCL-1 dependence may predict response to alvocidib. Moreover, MCL-1 plays a key role in resistance to BCL-2 inhibition with venetoclax. Investigational strategies of concomitant BCL-2 and CDK9 inhibition represent a promising therapeutic platform for AML. Furthermore, preclinical data suggests that CDK4/6 inhibition has selective activity in patients with KMT2A-rearrangements and FLT3 mutations. Incorporation of CDK9 and 4/6 inhibitors into the existing therapeutic armamentarium may improve outcomes in AML.

CPX-351 (vyxeos) in AML

For decades, the standard induction for patients with acute myeloid leukemia (AML) has been the combination of cytarabine with anthracycline (7 + 3 regimen). In August 2017 the US FDA approved CPX-351 (vyxeos), a liposomal formulation of cytarabine and daunorubicin at a fixed 5:1 molar ratio, for the treatment of adults with newly diagnosed AML with myelodysplasia-related changes (AML-MRC) and therapy-related AML (t-AML). This is the first approved treatment specifically for patients with this subgroup of AML. The approval was based on findings from a multicenter, randomized, open-label, phase III study of CPX-351 Versus 7 + 3 in patients 60-75 years old with newly diagnosed AML-MRC or t-AML. In this study CPX-351 had a higher median OS than 7 + 3 (9.56 vs 5.95 months, HR 0.69; 95% CI: 0.52 to 0.90, p = 0.005). In this profile, we review preclinical and clinical data, and discuss limitations and future directions with CPX-351 use in AML.

Inhibition of Bcl-2 Synergistically Enhances the Antileukemic Activity of Midostaurin and Gilteritinib in Preclinical Models of FLT3-Mutated Acute Myeloid Leukemia

PURPOSE

To investigate the efficacy of the combination of the FLT3 inhibitors midostaurin or gilteritinib with the Bcl-2 inhibitor venetoclax in FLT3-internal tandem duplication (ITD) acute myeloid leukemia (AML) and the underlying molecular mechanism.

EXPERIMENTAL DESIGN

Using both FLT3-ITD cell lines and primary patient samples, Annexin V-FITC/propidium iodide staining and flow cytometry analysis were used to quantify cell death induced by midostaurin or gilteritinib, alone or in combination with venetoclax. Western blot analysis was performed to assess changes in protein expression levels of members of the JAK/STAT, MAPK/ERK, and PI3K/AKT pathways, and members of the Bcl-2 family of proteins. The MV4-11-derived xenograft mouse model was used to assess in vivo efficacy of the combination of gilteritinib and venetoclax. Lentiviral overexpression of Mcl-1 was used to confirm its role in cell death induced by midostaurin or gilteritinib with venetoclax. Changes of Mcl-1 transcript levels were assessed by RT-PCR.

RESULTS

The combination of midostaurin or gilteritinib with venetoclax potently and synergistically induces apoptosis in FLT3-ITD AML cell lines and primary patient samples. The FLT3 inhibitors induced downregulation of Mcl-1, enhancing venetoclax activity. Phosphorylated-ERK expression is induced by venetoclax but abolished by the combination of venetoclax with midostaurin or gilteritinib. Simultaneous downregulation of Mcl-1 by midostaurin or gilteritinib and inhibition of Bcl-2 by venetoclax results in "free" Bim, leading to synergistic induction of apoptosis. In vivo results show that gilteritinib in combination with venetoclax has therapeutic potential.

CONCLUSIONS

Inhibition of Bcl-2 via venetoclax synergistically enhances the efficacy of midostaurin and gilteritinib in FLT3-mutated AML.See related commentary by Perl, p. 6567.

Evaluating venetoclax and its potential in treatment-naïve acute myeloid leukemia

Venetoclax (ABT-199), a BH3-mimetic and selective BCL-2 inhibitor, was recently approved by the US Food and Drug Administration (FDA) for the treatment of acute myeloid leukemia (AML) in adult patients aged 75 years or older, or otherwise unable to tolerate intensive induction chemotherapy, in combination with either hypomethylating agents or low-dose cytarabine. In this review article, we discuss venetoclax’s mechanism of action, in relation to both the BCL-2 protein family in general and BH3-mimetic activity in particular. We then outline the pharmacological advances that preceded and facilitated its development, as well as providing an overview of key preclinical and clinical studies which lead to its use first in chronic lymphoid leukemia (CLL), then in small lymphocytic leukemia (SLL), and subsequently in AML. Finally, we seek to offer an overview of the challenges and opportunities encountered as venetoclax moves into more widespread use, including its use and activity against leukemia initiating cells and oxidative phosphorylation.

Combining BH3-mimetics to target both BCL-2 and MCL1 has potent activity in pre-clinical models of acute myeloid leukemia

Improving outcomes in acute myeloid leukemia (AML) remains a major clinical challenge. Overexpression of pro-survival BCL-2 family members rendering transformed cells resistant to cytotoxic drugs is a common theme in cancer. Targeting BCL-2 with the BH3-mimetic venetoclax is active in AML when combined with low-dose chemotherapy or hypomethylating agents. We now report the pre-clinical anti-leukemic efficacy of a novel BCL-2 inhibitor S55746, which demonstrates synergistic pro-apoptotic activity in combination with the MCL1 inhibitor S63845. Activity of the combination was caspase and BAX/BAK dependent, superior to combination with standard cytotoxic AML drugs and active against a broad spectrum of poor risk genotypes, including primary samples from patients with chemoresistant AML. Co-targeting BCL-2 and MCL1 was more effective against leukemic, compared to normal hematopoietic progenitors, suggesting a therapeutic window of activity. Finally, S55746 combined with S63845 prolonged survival in xenograft models of AML and suppressed patient-derived leukemia but not normal hematopoietic cells in bone marrow of engrafted mice. In conclusion, a dual BH3-mimetic approach is feasible, highly synergistic, and active in diverse models of human AML. This approach has strong clinical potential to rapidly suppress leukemia, with reduced toxicity to normal hematopoietic precursors compared to chemotherapy.

The Role of Inhibition of Apoptosis in Acute Leukemias and Myelodysplastic Syndrome

Avoidance of apoptosis is a key mechanism that malignancies, including acute leukemias and MDS, utilize in order to proliferate and resist chemotherapy. Recently, venetoclax, an inhibitor of the anti-apoptotic protein BCL-2, has been approved for the treatment of upfront AML in an unfit, elderly population. This paper reviews the pre-clinical and clinical data for apoptosis inhibitors currently in development for the treatment of AML, ALL, and MDS.

Venetoclax Combined With Low-Dose Cytarabine for Previously Untreated Patients With Acute Myeloid Leukemia: Results From a Phase Ib/II Study

PURPOSE

Effective treatment options are limited for patients with acute myeloid leukemia (AML) who cannot tolerate intensive chemotherapy. An international phase Ib/II study evaluated the safety and preliminary efficacy of venetoclax, a selective B-cell leukemia/lymphoma-2 inhibitor, together with low-dose cytarabine (LDAC) in older adults with AML.

PATIENTS AND METHODS

Adults 60 years or older with previously untreated AML ineligible for intensive chemotherapy were enrolled. Prior treatment of myelodysplastic syndrome, including hypomethylating agents (HMA), was permitted. Eighty-two patients were treated at the recommended phase II dose: venetoclax 600 mg per day orally in 28-day cycles, with LDAC (20 mg/m² per day) administered subcutaneously on days 1 to 10. Key end points were tolerability, safety, response rates, duration of response (DOR), and overall survival (OS).

RESULTS

Median age was 74 years (range, 63 to 90 years), 49% had secondary AML, 29% had prior HMA treatment, and 32% had poor-risk cytogenetic features. Common grade 3 or greater adverse events were febrile neutropenia (42%), thrombocytopenia (38%), and WBC count decreased (34%). Early (30-day) mortality was 6%. Fifty-four percent achieved complete remission (CR)/CR with incomplete blood count recovery (median time to first response, 1.4 months). The median OS was 10.1 months (95% CI, 5.7 to 14.2), and median DOR was 8.1 months (95% CI, 5.3 to 14.9 months). Among patients without prior HMA exposure, CR/CR with incomplete blood count recovery was achieved in 62%, median DOR was 14.8 months (95% CI, 5.5 months to not reached), and median OS was 13.5 months (95% CI, 7.0 to 18.4 months).

CONCLUSION

Venetoclax plus LDAC has a manageable safety profile, producing rapid and durable remissions in older adults with AML ineligible for intensive chemotherapy. High remission rate and low early mortality combined with rapid and durable remission make venetoclax and LDAC an attractive and novel treatment for older adults not suitable for intensive chemotherapy.

A delicate balance - The BCL-2 family and its role in apoptosis, oncogenesis, and cancer therapeutics

Evasion of apoptosis is fundamental to the pathogenesis of cancer. Members of the B-cell Lymphoma 2 (BCL-2) protein family are key pro- and anti-apoptotic regulators, and in healthy cells are held in a fine, delicate balance - perturbations of which may tip a cell irreversibly towards cellular death or, conversely, allow a cell to permanently escape apoptosis and immortalize itself as a malignant clone. The restoration of this balance or, indeed, adjustment in favor of apoptosis via manipulation of the BCL-2 family, is a promising area in the realm of molecular therapeutics, and one in which breathtaking advances are currently being made. The purpose of this review is to outline the role of the BCL-2 family in apoptosis, to contrast its optimal functioning with those disruptions seen in malignancy, and to provide an overview of the medications both presently available and currently under development which selectively target members of this family.

Epigenetic therapy combinations in acute myeloid leukemia: what are the options?

Epigenetics refers to the regulation of gene expression mainly by changes in DNA methylation and modifications of histone proteins without altering the actual DNA sequence. While epigenetic modifications are essential for normal cell differentiation, several driver mutations in leukemic pathogenesis have been identified in genes that affect epigenetic processes, such as DNA methylation and histone acetylation. Several therapeutic options to target epigenetic alterations in acute myeloid leukemia (AML) have been successfully tested in preclinical studies and various drugs have already been approved for use in clinical practice. Among these already approved therapeutics are hypomethylating agents (azacitidine and decitabine) and isocitrate dehydrogenase inhibitors (ivosidenib, enasidenib). Other agents such as bromodomain-containing epigenetic reader proteins and histone methylation (e.g. DOT1L) inhibitors are currently in advanced clinical testing. As several epigenetic therapies have only limited efficacy when used as single agents, combination therapies that target AML pathogenesis at different levels and exploit synergistic mechanisms are also in clinical trials. Combinations of either epigenetic therapies with conventional chemotherapy, different forms of epigenetic therapies, or epigenetic therapies with immunotherapy are showing promising early results. In this review we summarize the underlying pathophysiology and rationale for epigenetically-based combination therapies, review current preclinical and clinical data and discuss the future directions of epigenetic therapy combinations in AML.

Targeted therapy with a selective BCL-2 inhibitor in older patients with acute myeloid leukemia

Background

Older patients with acute myeloid leukemia are particularly difficult to treat, as they have a high risk of comorbidities, poor performance status and less tolerability to chemotherapy, as well as a more aggressive disease biology, responsible for the resistance to treatment. There is a need to explore novel therapeutic agents that are more effective and tolerable. Venetoclax, a BCL-2 inhibitor is a promising agent, as BCL-2 overexpression is present in 84% of acute myeloid leukemia patients at diagnosis and 95% of patients at relapse and has been associated with leukemia cell survival, chemotherapy resistance and poor prognosis.

Objective

To review the available data about venetoclax in acute myeloid leukemia and how it can influence the treatment in older patients.

Methods

Using the Pubmed database, we selected 29 articles published within the last 15 years, considering preclinical and clinical trials and review studies that combined venetoclax with acute myeloid leukemia.

Results

Venetoclax has demonstrated promising results in preclinical and clinical trials, especially in patients with poor prognosis and the IDH mutation, with an excellent side-effect profile. However, resistance seems to develop rapidly with venetoclax monotherapy, because of antiapoptotic escape mechanisms.

Conclusions

While the results with the use of venetoclax seem encouraging, it is not likely that targeting a single pathway will result in long-term disease control. The solution includes the use of combined therapy to block resistance mechanisms and enhance apoptosis, by reducing MCL-1, increasing BIM or inhibiting the complex IV in the mitochondria.

Genetic biomarkers predict response to dual BCL-2 and MCL-1 targeting in acute myeloid leukaemia cells

Acute myeloid leukaemia (AML) cells often up-regulate pro-survival members of the BCL-2 protein family, such as BCL-2 and MCL-1, to avoid apoptosis. Venetoclax (ABT-199) targets BCL-2 and has shown promising efficacy in AML but over-expression of MCL-1 can cause resistance. A co-operative approach, targeting both BCL-2 and MCL-1 may therefore prove beneficial. This study investigated the potential synergistic relationship between Venetoclax and the MCL-1 inhibitor S63845 in AML cells. We treated MV4-11 cells and primary AML samples for 4 hours with Venetoclax, S63845 or the combination. We used a short-term flow cytometric technique to assess synergy using cytochrome C release as a read out of response. The combination of Venetoclax and S63845 produced a synergistic apoptotic response in MV4-11 cells and primary samples, including the leukaemia re-populating leukaemic stem cell (LSC) population, in 92% of the samples. Known molecular biomarkers of response to BCL-2 and MCL-1 targeting agents were corroborated, and augmented, with the short-term functional assay. The assay also predicted potential biomarkers of response to the combination of BCL-2 and MCL-1 targeting agents. Primary samples with an IDH2_140 mutation were more sensitive to Venetoclax as a single agent whereas samples with a FLT3-ITD mutation were more resistant. This resistance could be reversed when combined with S63845. All FLT3-ITD and NPM1 mutated samples were sensitive to the combination of drugs.

We report that co-operatively targeting BCL-2 and MCL-1 may be beneficial in AML and a short-term in vitro assay can identify patients who might best respond to this combination.

Discovery of Mcl-1-specific inhibitor AZD5991 and preclinical activity in multiple myeloma and acute myeloid leukemia

Mcl-1 is a member of the Bcl-2 family of proteins that promotes cell survival by preventing induction of apoptosis in many cancers. High expression of Mcl-1 causes tumorigenesis and resistance to anticancer therapies highlighting the potential of Mcl-1 inhibitors as anticancer drugs. Here, we describe AZD5991, a rationally designed macrocyclic molecule with high selectivity and affinity for Mcl-1 currently in clinical development. Our studies demonstrate that AZD5991 binds directly to Mcl-1 and induces rapid apoptosis in cancer cells, most notably myeloma and acute myeloid leukemia, by activating the Bak-dependent mitochondrial apoptotic pathway. AZD5991 shows potent antitumor activity in vivo with complete tumor regression in several models of multiple myeloma and acute myeloid leukemia after a single tolerated dose as monotherapy or in combination with bortezomib or venetoclax. Based on these promising data, a Phase I clinical trial has been launched for evaluation of AZD5991 in patients with hematological malignancies (NCT03218683).

BH3-Mimetic Drugs: Blazing the Trail for New Cancer Medicines

Defects in apoptotic cell death can promote cancer and impair responses of malignant cells to anti-cancer therapy. Pro-survival BCL-2 proteins prevent apoptosis by keeping the cell death effectors, BAX and BAK, in check. The BH3-only proteins initiate apoptosis by neutralizing the pro-survival BCL-2 proteins. Structural analysis and medicinal chemistry led to the development of small-molecule drugs that mimic the function of the BH3-only proteins to kill cancer cells. The BCL-2 inhibitor venetoclax has been approved for treatment of refractory chronic lymphocytic leukemia and this drug and inhibitors of pro-survival MCL-1 and BCL-XL are being tested in diverse malignancies.

Inhibition of XPO1 enhances cell death induced by ABT-199 in acute myeloid leukaemia via Mcl-1

The antiapoptotic Bcl-2 family proteins play critical roles in resistance to chemotherapy in acute myeloid leukaemia (AML). The Bcl-2-selective inhibitor ABT-199 (Venetoclax) shows promising antileukaemic activity against AML, though Mcl-1 limits its antileukaemic activity. XPO1 is a nuclear exporter overexpressed in AML cells and its inhibition decreases Mcl-1 levels in cancer cells. Thus, we hypothesized that the XPO1-selective inhibitor KPT-330 (Selinexor) can synergize with ABT-199 to induce apoptosis in AML cells through down-regulation of Mcl-1. The combination of KPT-330 and ABT-199 was found to synergistically induce apoptosis in AML cell lines and primary patient samples and cooperatively inhibit colony formation capacity of primary AML cells. KPT-330 treatment decreased Mcl-1 protein after apoptosis initiation. However, binding of Bim to Mcl-1 induced by ABT-199 was abrogated by KPT-330 at the same time as apoptosis initiation. KPT-330 treatment increased binding of Bcl-2 to Bim but was overcome by ABT-199 treatment, demonstrating that KPT-330 and ABT-199 reciprocally overcome apoptosis resistance. Mcl-1 knockdown and overexpression confirmed its critical role in the antileukaemic activity of the combination. In summary, KPT-330 treatment, alone and in combination with ABT-199, modulates Mcl-1, which plays an important role in the antileukaemic activity of the combination.

A CHAF1B-Dependent Molecular Switch in Hematopoiesis and Leukemia Pathogenesis

CHAF1B is the p60 subunit of the chromatin assembly factor (CAF1) complex, which is responsible for assembly of histones H3.1/H4 heterodimers at the replication fork during S phase. Here we report that CHAF1B is required for normal hematopoiesis while its overexpression promotes leukemia. CHAF1B has a pro-leukemia effect by binding chromatin at discrete sites and interfering with occupancy of transcription factors that promote myeloid differentiation, such as CEBPA. Reducing Chaf1b activity by either heterozygous deletion or overexpression of a CAF1 dominant negative allele is sufficient to suppress leukemogenesis in vivo without impairing normal hematopoiesis.

ABT-199-mediated inhibition of Bcl-2 as a potential therapeutic strategy for nasopharyngeal carcinoma

BACKGROUND

Aberrant overexpression of Bcl-2 protein has been detected in 80% of nasopharyngeal carcinoma (NPC), and Bcl-2 family proteins are implicated in both NPC oncogenesis and chemotherapy resistance. Previous studies have shown that while treatment of NPC cells with Bcl-2 family inhibitors alone is rarely effective, concomitant treatment with a cytotoxic reagent such as cisplatin can increase efficacy through a synergistic effect. The aim of the current work was to determine how we might increase the efficacy of Bcl-2 family inhibitors in the absence of cytotoxic reagents, which are associated with negative side effect profiles.

METHODS

We assessed cell proliferation in Bcl-2 high-expressing NPC cells by CCK-8 assay after treatment with the Bcl-2 inhibitor ABT-199 and/or the Mcl-1 inhibitor S63845. Apoptotic induction by ABT-199 was evaluated by Annexin V-FITC and PI double staining. We also evaluated Bcl-2 family protein expression (Bim, Mcl-1, Bcl-xL, Noxa) after treatment with ABT-199 by western blotting. Finally, xenografted Balb/c nude mice were used to test ABT-199 efficacy in vivo, H&E and immunohistochemistry assay were used to analyze tumor samples.

RESULTS

ABT-199 effectively induced NPC cell apoptosis in vitro and in the xenograft model. Following ABT-199 treatment in NPC cells, upregulation of Mcl-1 and Bcl-xL can lead to drug resistance, while concomitant Noxa overexpression partially neutralized the Mcl-1-caused resistance. Given that ABT-199 induces apoptosis in NPC cells through the Bcl-2/Noxa/Mcl-1 axis, treatment avenues further targeting this pathway should be promising. Indeed, the newly developed Mcl-1 inhibitor S63845 in combination with ABT-199 had a synergistic effect on NPC cell apoptosis.

CONCLUSION

Bcl-2 inhibition in NPC cells with ABT-199 triggers apoptosis through the Bcl-2/Noxa/Mcl-1 axis, and dual inhibition of the anti-apoptotic Bcl-2 family proteins Bcl-2 and Mcl-1 provided a strong synergistic effect without the need for adjunctive cytotoxic agent treatment with cisplatin.