Acquired resistance to venetoclax (ABT-199) in t(14;18) positive lymphoma cells

The Effects of ABT-199 and Dihydroartemisinin Combination on Cell Growth and Apoptosis in Human U937 and KG-1 Cancer Cells

INTRODUCTION

Change in the balance of Bcl-2 family proteins is one of the main reasons for resistance of tumor cells to ABT-199. In this study, the effect of dihydroartemisinin on cell growth, apoptosis and sensitivity of the AML cells to ABT-199 was investigated.

METHODS

Cell proliferation and survival were assessed by trypan blue staining and MTT assay, respectively. Cell apoptosis was measured by Hoechst 33342 staining and caspase-3 activity assay. The expression levels of Bcl-2, Mcl-1 and Bax mRNA were tested by qRT-PCR.

RESULTS

Our data showed that combination therapy significantly reduced the IC50 value and synergistically decreased the AML cell survival and growth compared with dihydroartemisinin or ABT-199 alone. Treatment with each of ABT-199 or dihydroartemisinin alone clearly enhanced the Bax mRNA expression and inhibited the expression of Mcl-1 and Bcl-2 mRNA. Inhibition of Mcl-1 mRNA by dihydroartemisinin was associated with enhancement of apoptosis induced by ABT-199 in AML cells.

CONCLUSION

In conclusion, dihydroartemisinin not only triggers the intrinsic pathway of apoptosis, but also can increase the sensitivity of the AML cells to ABT-199 via suppression of Mcl-1 expression.

Mechanisms of resistance to hypomethylating agents and BCL-2 inhibitors

Myeloid malignancies such as myelodysplastic syndrome (MDS) & acute myeloid leukemia (AML) are clonal diseases that emerge and progress due to the expansion of disease-initiating aberrant hematopoietic stem cells, that are not eliminated by conventional cytotoxic therapies. Hypomethylating agents(HMA), azacytidine and decitabine are the first line agents for treatment of MDS and a combination with BCL-2 inhibitor, venetoclax, is approved for AML induction in patients above 75 years and is also actively being investigated for use in high risk MDS. Resistance to these drugs has become a significant clinical challenge in treatment of myeloid malignancies. In this review, we discuss molecular mechanisms underlying the development of resistance to HMA and venetoclax. Insights into these mechanisms can help identify potential biomarkers for resistance prediction, aid in the development of combination therapies and strategies to prevent resistance and advance the field of cancer therapeutics.

Genetic events associated with venetoclax resistance in CLL identified by whole-exome sequencing of patient samples

Although BCL2 mutations are reported as later occurring events leading to venetoclax resistance, many other mechanisms of progression have been reported though remain poorly understood. Here, we analyze longitudinal tumor samples from 11 patients with disease progression while receiving venetoclax to characterize the clonal evolution of resistance. All patients tested showed increased in vitro resistance to venetoclax at the posttreatment time point. We found the previously described acquired BCL2-G101V mutation in only 4 of 11 patients, with 2 patients showing a very low variant allele fraction (0.03%-4.68%). Whole-exome sequencing revealed acquired loss(8p) in 4 of 11 patients, of which 2 patients also had gain (1q21.2-21.3) in the same cells affecting the MCL1 gene. In vitro experiments showed that CLL cells from the 4 patients with loss(8p) were more resistant to venetoclax than cells from those without it, with the cells from 2 patients also carrying gain (1q21.2-21.3) showing increased sensitivity to MCL1 inhibition. Progression samples with gain (1q21.2-21.3) were more susceptible to the combination of MCL1 inhibitor and venetoclax. Differential gene expression analysis comparing bulk RNA sequencing data from pretreatment and progression time points of all patients showed upregulation of proliferation, B-cell receptor (BCR), and NF-κB gene sets including MAPK genes. Cells from progression time points demonstrated upregulation of surface immunoglobulin M and higher pERK levels compared with those from the preprogression time point, suggesting an upregulation of BCR signaling that activates the MAPK pathway. Overall, our data suggest several mechanisms of acquired resistance to venetoclax in CLL that could pave the way for rationally designed combination treatments for patients with venetoclax-resistant CLL.

The discrete roles of individual FOXO transcription factor family members in B-cell malignancies

Forkhead box (FOX) class O (FOXO) proteins are a dynamic family of transcription factors composed of four family members: FOXO1, FOXO3, FOXO4 and FOXO6. As context-dependent transcriptional activators and repressors, the FOXO family regulates diverse cellular processes including cell cycle arrest, apoptosis, metabolism, longevity and cell fate determination. A central pathway responsible for negative regulation of FOXO activity is the phosphatidylinositol-3-kinase (PI3K)-AKT signalling pathway, enabling cell survival and proliferation. FOXO family members can be further regulated by distinct kinases, both positively (e.g., JNK, AMPK) and negatively (e.g., ERK-MAPK, CDK2), with additional post-translational modifications further impacting on FOXO activity. Evidence has suggested that FOXOs behave as 'bona fide' tumour suppressors, through transcriptional programmes regulating several cellular behaviours including cell cycle arrest and apoptosis. However, an alternative paradigm has emerged which indicates that FOXOs operate as mediators of cellular homeostasis and/or resistance in both 'normal' and pathophysiological scenarios. Distinct FOXO family members fulfil discrete roles during normal B cell maturation and function, and it is now clear that FOXOs are aberrantly expressed and mutated in discrete B-cell malignancies. While active FOXO function is generally associated with disease suppression in chronic lymphocytic leukemia for example, FOXO expression is associated with disease progression in diffuse large B cell lymphoma, an observation also seen in other cancers. The opposing functions of the FOXO family drives the debate about the circumstances in which FOXOs favour or hinder disease progression, and whether targeting FOXO-mediated processes would be effective in the treatment of B-cell malignancies. Here, we discuss the disparate roles of FOXO family members in B lineage cells, the regulatory events that influence FOXO function focusing mainly on post-translational modifications, and consider the potential for future development of therapies that target FOXO activity.

Synergistic Action of MCL-1 Inhibitor with BCL-2/BCL-XL or MAPK Pathway Inhibitors Enhances Acute Myeloid Leukemia Cell Apoptosis and Differentiation

Acute myeloid leukemia (AML) is a hematological malignancy characterized by excessive proliferation of abnormal myeloid precursors accompanied by a differentiation block and inhibition of apoptosis. Increased expression of an anti-apoptotic MCL-1 protein was shown to be critical for the sustained survival and expansion of AML cells. Therefore, herein, we examined the pro-apoptotic and pro-differentiating effects of S63845, a specific inhibitor of MCL-1, in a single-agent treatment and in combination with BCL-2/BCL-XL inhibitor, ABT-737, in two AML cell lines: HL-60 and ML-1. Additionally, we determined whether inhibition of the MAPK pathway had an impact on the sensitivity of AML cells to S63845. To assess AML cells' apoptosis and differentiation, in vitro studies were performed using PrestoBlue assay, Coulter electrical impedance method, flow cytometry, light microscopy and Western blot techniques. S63845 caused a concentration-dependent decrease in the viability of HL-60 and ML-1 cells and increased the percentage of apoptotic cells. Combined treatment with S63845 and ABT-737 or MAPK pathway inhibitor enhanced apoptosis but also induced differentiation of tested cells, as well as altering the expression of the MCL-1 protein. Taken together, our data provide the rationale for further studies regarding the use of MCL-1 inhibitor in combination with other pro-survival protein inhibitors.

The path to venetoclax resistance is paved with mutations, metabolism, and more

Venetoclax is a B cell lymphoma 2 (BCL-2)-selective antagonist used to treat chronic lymphocytic leukemia (CLL) and acute myelogenous leukemia (AML). Although this has been a promising therapeutic option for these patients, many of these patients develop resistance and relapsed disease. Here, we summarize the emerging mechanisms of resistance to venetoclax treatment, discuss the promising combination strategies, and highlight the combinations that are currently in clinical trials. Efforts to understand mechanisms of resistance are critical to advance the development of new targeted therapeutic strategies and further our understanding of the biological functions of BCL-2 in tumor cells.

Deregulation and epigenetic modification of BCL2-family genes cause resistance to venetoclax in hematologic malignancies

The BCL2 inhibitor venetoclax has been approved to treat different hematological malignancies. Because there is no common genetic alteration causing resistance to venetoclax in chronic lymphocytic leukemia (CLL) and B-cell lymphoma, we asked if epigenetic events might be involved in venetoclax resistance. Therefore, we employed whole-exome sequencing, methylated DNA immunoprecipitation sequencing, and genome-wide clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 screening to investigate venetoclax resistance in aggressive lymphoma and high-risk CLL patients. We identified a regulatory CpG island within the PUMA promoter that is methylated upon venetoclax treatment, mediating PUMA downregulation on transcript and protein level. PUMA expression and sensitivity toward venetoclax can be restored by inhibition of methyltransferases. We can demonstrate that loss of PUMA results in metabolic reprogramming with higher oxidative phosphorylation and adenosine triphosphate production, resembling the metabolic phenotype that is seen upon venetoclax resistance. Although PUMA loss is specific for acquired venetoclax resistance but not for acquired MCL1 resistance and is not seen in CLL patients after chemotherapy-resistance, BAX is essential for sensitivity toward both venetoclax and MCL1 inhibition. As we found loss of BAX in Richter's syndrome patients after venetoclax failure, we defined BAX-mediated apoptosis to be critical for drug resistance but not for disease progression of CLL into aggressive diffuse large B-cell lymphoma in vivo. A compound screen revealed TRAIL-mediated apoptosis as a target to overcome BAX deficiency. Furthermore, antibody or CAR T cells eliminated venetoclax resistant lymphoma cells, paving a clinically applicable way to overcome venetoclax resistance.

SOHO State of the Art Updates and Next Questions | Mechanisms of Resistance to BCL2 Inhibitor Therapy in Chronic Lymphocytic Leukemia and Potential Future Therapeutic Directions

Chronic lymphocytic leukaemia (CLL) constitutively overexpresses B-cell lymphoma 2 (BCL2) with consequent dysregulation of intrinsic apoptosis leading to abnormal cellular survival. Therapeutic use of BCL2 inhibitors (BCL2i, eg, venetoclax) in CLL, as both continuous monotherapy or in fixed duration combination, has translated scientific rationale into clinical benefit with significant rates of complete responses, including those without detectable minimal residual disease. Unlike with chemotherapy, response rates to venetoclax do not appear to be influenced by pre-existing chromosomal abnormalities or somatic mutations present, although the duration of response observed remains shorter for those with traditional higher risk genetic aberrations. This review seeks to describe both the disease factors that influence primary venetoclax sensitivity/resistance and those resistance mechanisms that may be acquired secondary to BCL2i therapy in CLL. Baseline venetoclax-sensitivity or -resistance is influenced by the expression of BCL2 relative to other BCL2 family member proteins, microenvironmental factors including nodal T-cell stimulation, and tumoral heterogeneity. With selection pressure applied by continuous venetoclax exposure, secondary resistance mechanisms develop in oligoclonal fashion. Those mechanisms described include acquisition of BCL2 variants, dynamic aberrations of alternative BCL2 family proteins, and mutations affecting both BAX and other BH3 proteins. In view of the resistance described, this review also proposes future applications of BCL2i therapy in CLL and potential means by which BCL2i-resistance may be abrogated.

NA1-115-7, from Zygogynum pancheri, is a new selective MCL-1 inhibitor inducing the apoptosis of hematological cancer cells but non-toxic to normal blood cells or cardiomyocytes

The overexpression of antiapoptotic members (BCL-2, BCL-xL, MCL-1, etc.) of the BCL-2 family contributes to tumor development and resistance to chemotherapy or radiotherapy. Synthetic inhibitors targeting these proteins have been developed, and some hematological malignancies are now widely treated with a BCL-2 inhibitor (venetoclax). However, acquired resistance to venetoclax or chemotherapy drugs due to an upregulation of MCL-1 has been observed, rendering MCL-1 an attractive new target for treatment. Six MCL-1 inhibitors (S64315, AZD-5991, AMG-176, AMG-397, ABBV-467 and PRT1419) have been evaluated in clinical trials since 2016, but some were affected by safety issues and none are currently used clinically. There is, therefore, still a need for alternative molecules. We previously described two drimane derivatives as the first covalent BH3 mimetics targeting MCL-1. Here, we described the characterization and biological efficacy of one of these compounds (NA1-115-7), isolated from Zygogynum pancheri, a plant belonging to the Winteraceae family. NA1-115-7 specifically induced the apoptosis of MCL-1-dependent tumor cells, with two hours of treatment sufficient to trigger cell death. The treatment of lymphoma cells with NA1-115-7 stabilized MCL-1, disrupted its interactions with BAK, and rapidly induced apoptosis through a BAK- and BAX-mediated process. Importantly, a similar treatment with NA1-115-7 was not toxic to erythrocytes, peripheral blood mononuclear cells, platelets, or cardiomyocytes. These results highlight the potential of natural products for use as specific BH3 mimetics non-toxic to normal cells, and they suggest that NA1-115-7 may be a promising tool for use in cancer treatment.

PIK-75 overcomes venetoclax resistance via blocking PI3K-AKT signaling and MCL-1 expression in mantle cell lymphoma

Therapeutic resistance is the major challenge in clinic for patients with mantle cell lymphoma (MCL), an aggressive subtype of B-cell lymphoma. In addition to the FDA-approved Bruton's tyrosine kinase (BTK) inhibitors, multiple clinical trials have demonstrated clinical benefits in targeting BCL-2 by venetoclax and reported to greatly improve clinical outcome for refractory/relapsed patients with MCL alone or in combination with BTK inhibitors. However, resistance to venetoclax is no exception and marks as a new clinic challenge. To decode the underlying mechanisms driving venetoclax resistance, we established two MCL cell lines, Mino-Re and Rec1-Re, with acquired resistance to venetoclax from sensitive Mino and Rec-1. Using reverse phase protein assay (RPPA), an agnostic proteomic approach, we identified targetable signaling pathways that are associated with acquired venetoclax resistance in Mino-Re and Rec1-Re cells. A panel of pro-survival signals was identified to correlate well with venetoclax-resistance, including increased expression of MCL-1, BCL-xL and AKT phosphorylation, and decreased expression of BIM, BAX and PTEN. Based on a high throughput drug screening of over 320 FDA-approved/investigational drugs in the paired venetoclax-sensitive and -resistant cell lines Mino-Re and Rec1-Re, we identified the top candidates that are capable to overcome acquired venetoclax resistance in these cells. The best candidate is PIK-75, a dual inhibitor targeting both PI3K and CDK9. Its action to overcome venetoclax resistance was further confirmed in additional cell lines with primary venetoclax resistance (n=4) and primary patient samples (n=21). Mechanistically, PIK75 treatment potently diminished the elevated MCL-1 expression and AKT activation in cells with acquired or primary venetoclax resistance and resulted in potent anti-MCL activity to overcome these resistances. In addition, PIK75 is also potent in overcoming tumor microenvironment (TME)-associated venetoclax resistance. Furthermore, PIK-75 treatment is efficacious in overcoming primary and acquired venetoclax resistance in xenograft models and inhibited tumor cell dissemination to spleen in mice. Altogether, our data demonstrated that PIK-75 is highly potent in overcoming primary, acquired, or stromal cells-induced venetoclax resistances in MCL cells and revealed a new tumor vulnerability that can be exploited clinically in difficult to treat MCL cases, especially those with venetoclax resistance.

All-Trans-Retinoic Acid Combined With Valproic Acid Can Promote Differentiation in Myeloid Leukemia Cells by an Autophagy Dependent Mechanism

Acute myeloid leukemia (AML) is an aggressive blood cancer with an overall survival of 30%. One form of AML, acute promyelocytic leukemia (APL) has become more than 90% curable with differentiation therapy, consisting of all-trans-retinoic acid (ATRA) and arsenic trioxide (ATO). Application of differentiation therapy to other AML subtypes would be a major treatment advance. Recent studies have indicated that autophagy plays a key role in the differentiation of ATRA-responsive APL cells. In this study, we have investigated whether differentiation could be enhanced in ATRA resistant cells by promoting autophagy induction with valproic acid (VPA). ATRA sensitive (NB4) and resistant leukemia cells (NB4R and THP-1) were co-treated with ATRA and valproic acid, followed by assessment of autophagy and differentiation. The combination of VPA and ATRA induced autophagic flux and promoted differentiation in ATRA-sensitive and -resistant cell lines. shRNA knockdown of ATG7 and TFEB autophagy regulators impaired both autophagy and differentiation, demonstrating the importance of autophagy in the combination treatment. These data suggest that ATRA combined with valproic acid can promote differentiation in myeloid leukemia cells by mechanism involving autophagy.

The manipulation of apoptosis for cancer therapy using BH3-mimetic drugs

Apoptosis is a form of programmed cell death that is regulated by the balance between prosurvival and proapoptotic BCL-2 protein family members. Evasion of apoptosis is a hallmark of cancer that arises when this balance is tipped in favour of survival. One form of anticancer therapeutic, termed 'BH3-mimetic drugs', has been developed to directly activate the apoptosis machinery in malignant cells. These drugs bind to and inhibit specific prosurvival BCL-2 family proteins, thereby mimicking their interaction with the BH3 domains of proapoptotic BCL-2 family proteins. The BCL-2-specific inhibitor venetoclax is approved by the US Food and Drug Administration and many regulatory authorities worldwide for the treatment of chronic lymphocytic leukaemia and acute myeloid leukaemia. BH3-mimetic drugs targeting other BCL-2 prosurvival proteins have been tested in preclinical models of cancer, and drugs targeting MCL-1 or BCL-X_L have advanced into phase I clinical trials for certain cancers. As with all therapeutics, efficacy and tolerability need to be carefully balanced to achieve a therapeutic window whereby there is significant anticancer activity with an acceptable safety profile. In this Review, we outline the current state of BH3-mimetic drugs targeting various prosurvival BCL-2 family proteins and discuss emerging data regarding primary and acquired resistance to these agents and approaches that may overcome this. We highlight issues that need to be addressed to further advance the clinical application of BH3-mimetic drugs, both alone and in combination with additional anticancer agents (for example, standard chemotherapeutic drugs or inhibitors of oncogenic kinases), for improved responses in patients with cancer.

Cell Survival and Cell Death at the Intersection of Autophagy and Apoptosis: Implications for Current and Future Cancer Therapeutics

Autophagy and apoptosis are functionally distinct mechanisms for cytoplasmic and cellular turnover. While these two pathways are distinct, they can also regulate each other, and central components of the apoptosis or autophagy pathway regulate both processes directly. Furthermore, several upstream stress-inducing signaling pathways can influence both autophagy and apoptosis. The crosstalk between autophagy and apoptosis has an integral role in pathological processes, including those related to cancer, homeostasis, and aging. Apoptosis is a form of programmed cell death, tightly regulated by various cellular and biochemical mechanisms, some of which have been the focus of drug discovery efforts targeting cancer therapeutics. Autophagy is a cellular degradation pathway whereby cells recycle macromolecules and organelles to generate energy when subjected to stress. Autophagy can act as either a prodeath or a prosurvival process and is both tissue and microenvironment specific. In this review we describe five groups of proteins that are integral to the apoptosis pathway and discuss their role in regulating autophagy. We highlight several apoptosis-inducing small molecules and biologics that have been developed and advanced into the clinic and discuss their effects on autophagy. For the most part, these apoptosis-inducing compounds appear to elevate autophagy activity. Under certain circumstances autophagy demonstrates cytoprotective functions and is overactivated in response to chemo- or radiotherapy which can lead to drug resistance, representing a clinical obstacle for successful cancer treatment. Thus, targeting the autophagy pathway in combination with apoptosis-inducing compounds may be a promising strategy for cancer therapy.

Programmed cell death, redox imbalance, and cancer therapeutics

Cancer cells are disordered by nature and thus featured by higher internal redox level than healthy cells. Redox imbalance could trigger programmed cell death if exceeded a certain threshold, rendering therapeutic strategies relying on redox control a possible cancer management solution. Yet, various programmed cell death events have been consecutively discovered, complicating our understandings on their associations with redox imbalance and clinical implications especially therapeutic design. Thus, it is imperative to understand differences and similarities among programmed cell death events regarding their associations with redox imbalance for improved control over these events in malignant cells as well as appropriate design on therapeutic approaches relying on redox control. This review addresses these issues and concludes by bringing affront cold atmospheric plasma as an emerging redox controller with translational potential in clinics.

Venetoclax and pegcrisantaspase for complex karyotype acute myeloid leukemia

Complex karyotype acute myeloid leukemia (CK-AML) has a dismal outcome with current treatments, underscoring the need for new therapies. Here, we report synergistic anti-leukemic activity of the BCL-2 inhibitor venetoclax (Ven) and the asparaginase formulation Pegylated Crisantaspase (PegC) in CK-AML in vitro and in vivo. Ven-PegC combination inhibited growth of multiple AML cell lines and patient-derived primary CK-AML cells in vitro. In vivo, Ven-PegC showed potent reduction of leukemia burden and improved survival, compared with each agent alone, in a primary patient-derived CK-AML xenograft. Superiority of Ven-PegC, compared to single drugs, and, importantly, the clinically utilized Ven-azacitidine combination, was also demonstrated in vivo in CK-AML. We hypothesized that PegC-mediated plasma glutamine depletion inhibits 4EBP1 phosphorylation, decreases the expression of proteins such as MCL-1, whose translation is cap dependent, synergizing with the BCL-2 inhibitor Ven. Ven-PegC treatment decreased cellular MCL-1 protein levels in vitro by enhancing eIF4E-4EBP1 interaction on the cap-binding complex via glutamine depletion. In vivo, Ven-PegC treatment completely depleted plasma glutamine and asparagine and inhibited mRNA translation and cellular protein synthesis. Since this novel mechanistically-rationalized regimen combines two drugs already in use in acute leukemia treatment, we plan a clinical trial of the Ven-PegC combination in relapsed/refractory CK-AML.

Preconditioning improves bleaching tolerance in the reef-building coral Pocillopora acuta through modulations in the programmed cell death pathways

Reef-building corals rely on intracellular algal symbionts to meet energetic demands. Increasing extreme weather driven by climate change often leads to disruption of this symbiosis and to coral death. Corals can better withstand stress after previous exposure to sublethal conditions, but the mechanisms for this resilience remain unclear. Here, we show that a three-day thermal preconditioning increases tolerance of acute heat stress through modulations in cell death pathways in the stony coral Pocillopora acuta. In preconditioned corals, the ratio of pro-survival (pa-Bcl-2 and pa-BI-1) to pro-death (pa-BAK and pa-BAX) gene expression increased and the corals underwent significantly less bleaching. When treated with Bcl-2 inhibitor, corals lost the improved thermal tolerance, suggesting an important role of programmed cell death in coral bleaching and acclimatization. During heat stress, the activity of acid phosphatase increased but caspase-3 did not, suggesting the involvement of autophagy/symbiophagy rather than apoptosis in this process. A similar shift in gene expression also occurs in thermally stressed corals that have been exposed to naturally higher temperatures during summer thermal maxima in Kāne'ohe Bay, Hawai'i, suggesting that corals can increase their resilience to realistic warming events during high-risk periods through alterations in cell signalling. These data suggest that programmed cell death pathways underly coral acclimatization and resilience and may be important for coral reef conservation and management.

Copanlisib synergizes with conventional and targeted agents including venetoclax in B- and T-cell lymphoma models

Copanlisib is a pan-class I phosphoinositide 3-kinase (PI3K) inhibitor with preferred activity toward PI3Kα and PI3Kδ. Despite the clear overall clinical benefit, the number of patients achieving complete remissions with the single agent is relatively low, a problem shared by the vast majority of targeted agents. Here, we searched for novel copanlisib-based combinations. Copanlisib was tested as a single agent, in combination with an additional 17 drugs in 26 cell lines derived from mantle cell lymphoma (MCL), marginal zone lymphoma (MZL), and T-cell lymphomas. In vivo experiments, transcriptome analyses, and immunoblotting experiments were also performed. Copanlisib as a single agent showed in vitro dose-dependent antitumor activity in the vast majority of the models. Combination screening identified several compounds that synergized with copanlisib. The strongest combination was with the B-cell lymphoma 2 (BCL2) inhibitor venetoclax. The benefit of the combination over single agents was also validated in an MZL xenograft model and in MCL primary cells, and was due to increased induction of apoptosis, an effect likely sustained by the reduction of the antiapoptotic proteins myeloid cell leukemia 1 (MCL1) and BCL-XL, observed in MCL and MZL cell lines, respectively. These data supported the rationale for the design of the Swiss Group for Clinical Cancer Research (SAKK) 66/18 phase 1 study currently exploring the combination of copanlisib and venetoclax in relapsed/refractory lymphomas.

Targeting epigenetic mechanisms to overcome venetoclax resistance

The BH-3 mimetic venetoclax overcomes apoptosis and therapy resistance caused by high expression of BCL2 or loss of BH3-only protein function. Although a promising therapy for hematologic malignancies, increased expression of anti-apoptotic MCL-1 or BCL-XL, as well as other resistance mechanisms prevent a durable response to venetoclax. Recent studies demonstrate that agents targeting epigenetic mechanisms such as DNA methyltransferase inhibitors, histone deacetylase (HDAC) inhibitors, histone methyltransferase EZH2 inhibitors, or bromodomain reader protein inhibitors may disable oncogenic gene expression signatures responsible for venetoclax resistance. Combination therapies including venetoclax and epigenetic therapies are effective in preclinical models and the subject of many current clinical trials. Here we review epigenetic strategies to overcome venetoclax resistance mechanisms in hematologic malignancies.

Targeting Bcl-2 Family Proteins: What, Where, When?

Proteins of the Bcl-2 family are known as regulators of apoptosis, one of the most studied forms of programmed cell death. The Bcl-2 protein family is represented by both pro- and antiapoptotic members. Antiapoptotic proteins are often exploited by tumor cells to avoid their death, thus playing an important role in carcinogenesis and in acquisition of resistance to various therapeutic agents. Therefore, antiapoptotic proteins represent attractive targets for cancer therapy. A detailed investigation of interactions between Bcl-2 family proteins resulted in the development of highly selective inhibitors of individual antiapoptotic members. These agents are currently being actively studied at the preclinical and clinical stages and represent a promising therapeutic strategy, which is highlighted by approval of venetoclax, a selective inhibitor of Bcl-2, for medical use. Meanwhile, inhibition of antiapoptotic Bcl-2 family proteins has significant therapeutic potential that is yet to be revealed. In the coming era of precision medicine, a detailed study of the mechanisms responsible for the sensitivity or resistance of tumor cells to various therapeutic agents, as well as the search for the most effective combinations, is of great importance. Here, we discuss mechanisms of how the Bcl-2 family proteins function, principles of their inhibition by small molecules, success of this approach in cancer therapy, and, eventually, biochemical features that can be exploited to improve the use of Bcl-2 family inhibitors as anticancer drugs.

Targeting BCL-2 in Cancer: Advances, Challenges, and Perspectives

Simple Summary

Apoptosis, a programmed form of cell death, represents the main mechanism by which cells die. Such phenomenon is highly regulated by the BCL-2 family of proteins, which includes both pro-apoptotic and pro-survival proteins. The decision whether cells live or die is tightly controlled by a balance between these two classes of proteins. Notably, the pro-survival Bcl-2 proteins are frequently overexpressed in cancer cells dysregulating this balance in favor of survival and also rendering cells more resistant to therapeutic interventions. In this review, we outlined the most important steps in the development of targeting the BCL-2 survival proteins, which laid the ground for the discovery and the development of the selective BCL-2 inhibitor venetoclax as a therapeutic drug in hematological malignancies. The limitations and future directions are also discussed.

Abstract

The major form of cell death in normal as well as malignant cells is apoptosis, which is a programmed process highly regulated by the BCL-2 family of proteins. This includes the antiapoptotic proteins (BCL-2, BCL-XL, MCL-1, BCLW, and BFL-1) and the proapoptotic proteins, which can be divided into two groups: the effectors (BAX, BAK, and BOK) and the BH3-only proteins (BIM, BAD, NOXA, PUMA, BID, BIK, HRK). Notably, the BCL-2 antiapoptotic proteins are often overexpressed in malignant cells. While this offers survival advantages to malignant cells and strengthens their drug resistance capacity, it also offers opportunities for novel targeted therapies that selectively kill such cells. This review provides a comprehensive overview of the extensive preclinical and clinical studies targeting BCL-2 proteins with various BCL-2 proteins inhibitors with emphasis on venetoclax as a single agent, as well as in combination with other therapeutic agents. This review also discusses recent advances, challenges focusing on drug resistance, and future perspectives for effective targeting the Bcl-2 family of proteins in cancer.

BH3 Mimetics for the Treatment of B-Cell Malignancies-Insights and Lessons from the Clinic

Simple Summary

B-cell malignancies, including chronic lymphocytic leukemia (CLL), non-Hodgkin lymphoma (NHL), and plasma cell dyscrasias, are significant contributors to cancer morbidity and mortality worldwide. The pathogenesis of many B-cell malignancies involves perturbations in the intrinsic pathway of apoptosis that allow cells to evade cell death. BH3 mimetics represent a class of anti-cancer agents that can restore the ability of cancer cells to undergo apoptosis. Venetoclax, a recently approved BH3 mimetic, has transformed the therapeutic landscape for CLL. Other BH3 mimetics are currently under development. This review summarizes the available data on existing BH3 mimetics and highlights both the rapidly expanding role of BH3 mimetics in the treatment of B-cell malignancies and the clinical challenges of their use.

Abstract

The discovery of the link between defective apoptotic regulation and cancer cell survival engendered the idea of targeting aberrant components of the apoptotic machinery for cancer therapy. The intrinsic pathway of apoptosis is tightly controlled by interactions amongst members of three distinct subgroups of the B-cell lymphoma 2 (BCL2) family of proteins. The pro-survival BCL2 proteins prevent apoptosis by keeping the pro-apoptotic effector proteins BCL2-associated X protein (BAX) and BCL2 homologous antagonist/killer (BAK) in check, while the BH3-only proteins initiate apoptosis by either neutralizing the pro-survival BCL2 proteins or directly activating the pro-apoptotic effector proteins. This tripartite regulatory mechanism is commonly perturbed in B-cell malignancies facilitating cell death evasion. Over the past two decades, structure-based drug discovery has resulted in the development of a series of small molecules that mimic the function of BH3-only proteins called the BH3 mimetics. The most clinically advanced of these is venetoclax, which is a highly selective inhibitor of BCL2 that has transformed the treatment landscape for chronic lymphocytic leukemia (CLL). Other BH3 mimetics, which selectively target myeloid cell leukemia 1 (MCL1) and B-cell lymphoma extra large (BCLxL), are currently under investigation for use in diverse malignancies. Here, we review the current role of BH3 mimetics in the treatment of CLL and other B-cell malignancies and address open questions in this rapidly evolving field.

Targeting BCL-2 in B-cell malignancies and overcoming therapeutic resistance

Defects in apoptosis can promote tumorigenesis and impair responses of malignant B cells to chemotherapeutics. Members of the B-cell leukemia/lymphoma-2 (BCL-2) family of proteins are key regulators of the intrinsic, mitochondrial apoptotic pathway. Overexpression of antiapoptotic BCL-2 family proteins is associated with treatment resistance and poor prognosis. Thus, inhibition of BCL-2 family proteins is a rational therapeutic option for malignancies that are dependent on antiapoptotic BCL-2 family proteins. Venetoclax (ABT-199, GDC-0199) is a highly selective BCL-2 inhibitor that represents the first approved agent of this class and is currently widely used in the treatment of chronic lymphocytic leukemia (CLL) as well as acute myeloid leukemia (AML). Despite impressive clinical activity, venetoclax monotherapy for a prolonged duration can lead to drug resistance or loss of dependence on the targeted protein. In this review, we provide an overview of the mechanism of action of BCL-2 inhibition and the role of this approach in the current treatment paradigm of B-cell malignancies. We summarize the drivers of de novo and acquired resistance to venetoclax that are closely associated with complex clonal shifts, interplay of expression and interactions of BCL-2 family members, transcriptional regulators, and metabolic modulators. We also examine how tumors initially resistant to venetoclax become responsive to it following prior therapies. Here, we summarize preclinical data providing a rationale for efficacious combination strategies of venetoclax to overcome therapeutic resistance by a targeted approach directed against alternative antiapoptotic BCL-2 family proteins (MCL-1, BCL-xL), compensatory prosurvival pathways, epigenetic modifiers, and dysregulated cellular metabolism/energetics for durable clinical remissions.

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.

Decreased BIM expression in BCL2-negative follicular lymphoma: a potential mechanism for resistance to apoptosis

Follicular lymphoma (FL) is characterized by t(14; 18)(q32; q21), leading to overexpression of the antiapoptotic molecule BCL2; however, a subset of FLs lack BCL2 rearrangement and BCL2 expression as analyzed by immunohistochemistry (IHC). In this study, we evaluated expression of antiapoptotic (MCL1 and BCL-XL) and proapoptotic proteins (BIM) by IHC in both BCL2(-) and BCL2(+) FLs. FLs diagnosed between 2009 and 2019 were reviewed to identify BCL2(-) cases by IHC (assessed by clone 124). Immunohistochemical analyses for BCL2 (EP36), MCL1, BIM, BCL-XL, and Ki-67 were performed on tissue microarrays or whole slides. BCL2 (EP36) was interpreted as positive (≥10%) or negative (<10%). Ki-67 was interpreted on tumor cells in 10% increments. The remaining immunohistochemical analysis results were scored on tumor cells in 10% increments, and intensity was interpreted as weak, moderate, or strong to derive an H-score. Twenty-four BCL2(-) FLs were initially identified, but on further testing with BCL2(EP36) immunohistochemical staining, 5 of 24 were reclassified as BCL2(+), leaving 19 BCL2(-) FLs. Thirty-three BCL2(+) FLs were selected with sufficient tissue for additional immunohistochemical analyses. There was no significant difference in expression of antiapoptotic BCL-XL or MCL1 between BCL2(-) and BCL2(+) FLs (p = 0.75 and 0.28, respectively). However, proapoptotic BIM expression was significantly lower in BCL2(-) FLs than in BCL2(+) FLs (p = 0.002). In our study, 21% of putative BCL2(-) FLs were BCL2(+) when tested with alternative clones, supporting the practice of having more than one BCL2 clone in immunohistochemical laboratories. Decreased BIM expression in BCL2(-) FLs could have an overall antiapoptotic effect and represent an alternate mechanism for cell survival in BCL2(-) FLs.

DLBCL Cells with Acquired Resistance to Venetoclax Are Not Sensitized to BIRD-2 But Can Be Resensitized to Venetoclax through Bcl-XL Inhibition

Anti-apoptotic Bcl-2-family members are frequently dysregulated in both blood and solid cancers, contributing to their survival despite ongoing oncogenic stress. Yet, such cancer cells often are highly dependent on Bcl-2 for their survival, a feature that is exploited by so-called BH3-mimetic drugs. Venetoclax (ABT-199) is a selective BH3-mimetic Bcl-2 antagonist that is currently used in the clinic for treatment of chronic lymphocytic leukemia patients. Unfortunately, venetoclax resistance has already emerged in patients, limiting the therapeutic success. Here, we examined strategies to overcome venetoclax resistance. Therefore, we used two diffuse large B-cell lymphoma (DLBCL) cell lines, Riva WT and venetoclax-resistant Riva (VR). The latter was obtained by prolonged culturing in the presence of venetoclax. We report that Riva VR cells did not become more sensitive to BIRD-2, a peptide targeting the Bcl-2 BH4 domain, and established cross-resistance towards BDA-366, a putative BH4-domain antagonist of Bcl-2. However, we found that Bcl-XL, another Bcl-2-family protein, is upregulated in Riva VR, while Mcl-1 expression levels are not different in comparison with Riva WT, hinting towards an increased dependence of Riva VR cells to Bcl-XL. Indeed, Riva VR cells could be resensitized to venetoclax by A-1155463, a selective BH3 mimetic Bcl-XL inhibitor. This is underpinned by siRNA experiments, demonstrating that lowering Bcl-XL-expression levels also augmented the sensitivity of Riva VR cells to venetoclax. Overall, this work demonstrates that Bcl-XL upregulation contributes to acquired resistance of DLBCL cancer cells towards venetoclax and that antagonizing Bcl-XL can resensitize such cells towards venetoclax.

Double agents of cell death: novel emerging functions of apoptotic regulators

Apoptosis is a highly regulated form of cell death that is required for many homeostatic and pathological processes. Recently, alternative cell death pathways have emerged whose regulation is dependent on proteins with canonical functions in apoptosis. Dysregulation of apoptotic signaling frequently underlies the pathogenesis of many cancers, reinforcing the need to develop therapies that initiate alternative cell death processes. This review outlines the convergence points between apoptosis and other death pathways with the purpose of identifying novel strategies for the treatment of apoptosis-refractory cancers. Apoptosis proteins can play key roles in the initiation, regulation, and execution of nonapoptotic death processes that include necroptosis, autophagy, pyroptosis, mPTP-mediated necrosis, and ferroptosis. Notably, recent evidence illustrates that dying cells can exhibit biochemical and molecular characteristics of more than one different type of regulated cell death. Thus, this review highlights the amazing complexity and interconnectivity of cell death processes and also raises the idea that a top-to-bottom approach to describing cell death mechanisms may be inadequate for fully understanding the means by which cells die.

A novel CDK9 inhibitor increases the efficacy of venetoclax (ABT-199) in multiple models of hematologic malignancies

MCL-1 is one of the most frequently amplified genes in cancer, facilitating tumor initiation and maintenance and enabling resistance to anti-tumorigenic agents including the BCL-2 selective inhibitor venetoclax. The expression of MCL-1 is maintained via P-TEFb-mediated transcription, where the kinase CDK9 is a critical component. Consequently, we developed a series of potent small-molecule inhibitors of CDK9, exemplified by the orally active A-1592668, with CDK selectivity profiles that are distinct from related molecules that have been extensively studied clinically. Short-term treatment with A-1592668 rapidly downregulates RNA pol-II (Ser 2) phosphorylation resulting in the loss of MCL-1 protein and apoptosis in MCL-1-dependent hematologic tumor cell lines. This cell death could be attenuated by either inhibiting caspases or overexpressing BCL-2 protein. Synergistic cell killing was also observed between A-1592668 or the related analog A-1467729, and venetoclax in a number of hematologic cell lines and primary NHL patient samples. Importantly, the CDK9 inhibitor plus venetoclax combination was well tolerated in vivo and demonstrated efficacy superior to either agent alone in mouse models of lymphoma and AML. These data indicate that CDK9 inhibitors could be highly efficacious in tumors that depend on MCL-1 for survival or when used in combination with venetoclax in malignancies dependent on MCL-1 and BCL-2.

Improved in vivo targeting of BCL-2 phenotypic conversion through hollow gold nanoshell delivery

Although new cancer therapeutics are discovered at a rapid pace, lack of effective means of delivery and cancer chemoresistance thwart many of the promising therapeutics. We demonstrate a method that confronts both of these issues with the light-activated delivery of a Bcl-2 functional converting peptide, NuBCP-9, using hollow gold nanoshells. This approach has shown not only to increase the efficacy of the peptide 30-fold in vitro but also has shown to reduce paclitaxel resistant H460 lung xenograft tumor growth by 56.4%.

Restoring Apoptosis with BH3 Mimetics in Mature B-Cell Malignancies

Apoptosis is a highly conserved mechanism enabling the removal of unwanted cells. Mitochondrial apoptosis is governed by the B-cell lymphoma (BCL-2) family, including anti-apoptotic and pro-apoptotic proteins. Apoptosis evasion by dysregulation of anti-apoptotic BCL-2 members (BCL-2, MCL-1, BCL-XL) is a common hallmark in cancers. To divert this dysregulation into vulnerability, researchers have developed BH3 mimetics, which are small molecules that restore effective apoptosis in neoplastic cells by interfering with anti-apoptotic proteins. Among them, venetoclax is a potent and selective BCL-2 inhibitor, which has demonstrated the strongest clinical activity in mature B-cell malignancies, including chronic lymphoid leukemia, mantle-cell lymphoma, and multiple myeloma. Nevertheless, mechanisms of primary and acquired resistance have been recently described and several features such as cytogenetic abnormalities, BCL-2 family expression, and ex vivo drug testing have to be considered for predicting sensitivity to BH3 mimetics and helping in the identification of patients able to respond. The medical need to overcome resistance to BH3 mimetics supports the evaluation of innovative combination strategies. Novel agents including MCL-1 targeting BH3 mimetics are currently evaluated and may represent new therapeutic options in the field. The present review summarizes the current knowledge regarding venetoclax and other BH3 mimetics for the treatment of mature B-cell malignancies.

Mechanisms of intrinsic and acquired resistance to venetoclax in B-cell lymphoproliferative disease

Venetoclax is an oral selective BCL2 inhibitor which is highly efficacious in a variety of B-cell lymphoproliferative diseases (B-LPDs) due to their collective dependency on BCL2 over-expression as a central feature of their pathogenesis. However, despite its general efficacy across the spectrum of B-LPDs, certain subtypes are characterized by significantly higher response rates (RRs) to venetoclax (e.g. chronic lymphocytic leukemia) than others (e.g. diffuse large B-cell lymphoma). This variation in RR is the result of an underlying spectrum of primary (intrinsic) resistance to venetoclax mediated by numerous intracellular and microenvironmental mechanisms. Moreover, despite an initial response, most patients will experience disease progression on venetoclax therapy thus manifesting secondary (acquired) resistance. This review describes the molecular mechanisms in B-LPDs that drive both of these types of clinical resistance, the understanding of which is central to optimizing outcomes using this therapy.

Elevated VMP1 expression in acute myeloid leukemia amplifies autophagy and is protective against venetoclax-induced apoptosis

Vacuole membrane protein (VMP1) is a putative autophagy protein, which together with Beclin-1 acts as a molecular switch in activating autophagy. In the present study the role of VMP1 was analysed in CD34⁺ cells of cord blood (CB) and primary acute myeloid leukemia (AML) cells and cell lines. An increased expression of VMP1 was observed in a subset of AML patients. Functional studies in normal CB CD34⁺ cells indicated that inhibiting VMP1 expression reduced autophagic-flux, coinciding with reduced expansion of hematopoietic stem and progenitor cells (HSPC), delayed differentiation, increased apoptosis and impaired in vivo engraftment. Comparable results were observed in leukemic cell lines and primary AML CD34⁺ cells. Ultrastructural analysis indicated that leukemic cells overexpressing VMP1 displayed a reduced number of mitochondrial structures, while the number of lysosomal degradation structures was increased. The overexpression of VMP1 did not affect cell proliferation and differentiation, but increased autophagic-flux and improved mitochondrial quality, which coincided with an increased threshold for venetoclax-induced loss of mitochondrial outer membrane permeabilization (MOMP) and apoptosis. In conclusion, our data indicate that in leukemic cells high VMP1 is involved with mitochondrial quality control.

The TP53 Apoptotic Network Is a Primary Mediator of Resistance to BCL2 Inhibition in AML Cells

To study mechanisms underlying resistance to the BCL2 inhibitor venetoclax in acute myeloid leukemia (AML), we used a genome-wide CRISPR/Cas9 screen to identify gene knockouts resulting in drug resistance. We validated TP53, BAX, and PMAIP1 as genes whose inactivation results in venetoclax resistance in AML cell lines. Resistance to venetoclax resulted from an inability to execute apoptosis driven by BAX loss, decreased expression of BCL2, and/or reliance on alternative BCL2 family members such as BCL2L1. The resistance was accompanied by changes in mitochondrial homeostasis and cellular metabolism. Evaluation of TP53 knockout cells for sensitivities to a panel of small-molecule inhibitors revealed a gain of sensitivity to TRK inhibitors. We relate these observations to patient drug responses and gene expression in the Beat AML dataset. Our results implicate TP53, the apoptotic network, and mitochondrial functionality as drivers of venetoclax response in AML and suggest strategies to overcome resistance. SIGNIFICANCE: AML is challenging to treat due to its heterogeneity, and single-agent therapies have universally failed, prompting a need for innovative drug combinations. We used a genetic approach to identify genes whose inactivation contributes to drug resistance as a means of forming preferred drug combinations to improve AML treatment.See related commentary by Savona and Rathmell, p. 831.This article is highlighted in the In This Issue feature, p. 813.

Bcl-2 Is a Therapeutic Target for Hypodiploid B-Lineage Acute Lymphoblastic Leukemia

Acute lymphoblastic leukemia (ALL) is the most common cancer in children. The highest rates of treatment failure occur in specific genetic subsets of ALL, including hypodiploid B-cell ALL (B-ALL), for which effective alternative therapies to current intensive chemotherapy treatments have yet to be developed. Here, we integrated biochemical and genomic profiling with functional drug assays to select effective agents with therapeutic potential against hypodiploid B-ALL. ABT-199, a selective Bcl-2 inhibitor, was effective in reducing leukemic burden in vitro and in vivo in patient-derived xenograft models of hypodiploid B-ALL. Daily oral treatment with ABT-199 significantly increased survival in xenografted mice. The unexpected efficacy of ABT-199 observed in hypodiploid leukemias lacking BIM expression (the major reported mediator of ABT-199-induced apoptosis) led us to investigate the mechanism of action of ABT-199 in the absence of BIM. Treatment with ABT-199 elicited responses in a dose-dependent manner, from cell-cycle arrest at low nanomolar concentrations to cell death at concentrations above 100 nmol/L. Collectively, these results demonstrate the efficacy of Bcl-2 inhibition and potential therapeutic strategy in hypodiploid B-ALL. SIGNIFICANCE: These results demonstrate the efficacy of ABT-199 in vivo and provide encouraging preclinical data of Bcl-2 as a potential target for the treatment of hypodiploid B-ALL.

Targeting the Bcl-2 Family in B Cell Lymphoma

Although lymphoma is a very heterogeneous group of biologically complex malignancies, tumor cells across all B cell lymphoma subtypes share a set of underlying traits that promote the development and sustain malignant B cells. One of these traits, the ability to evade apoptosis, is essential for lymphoma development. Alterations in the Bcl-2 family of proteins, the key regulators of apoptosis, is a hallmark of B cell lymphoma. Significant efforts have been made over the last 30 years to advance knowledge of the biology, molecular mechanisms, and therapeutic potential of targeting Bcl-2 family members. In this review, we will highlight the complexities of the Bcl-2 family, including our recent discovery of overexpression of the anti-apoptotic Bcl-2 family member Bcl-w in lymphomas, and describe recent advances in the field that include the development of inhibitors of anti-apoptotic Bcl-2 family members for the treatment of B cell lymphomas and their performance in clinical trials.

BCL2-Family Dysregulation in B-Cell Malignancies: From Gene Expression Regulation to a Targeted Therapy Biomarker

BCL2-family proteins have a central role in the mitochondrial apoptosis machinery and their expression is known to be deregulated in many cancer types. Effort in the development of small molecules that selectively target anti-apoptotic members of this family i.e., Bcl-2, Bcl-xL, Mcl-1 recently opened novel therapeutic opportunities. Among these apoptosis-inducing agents, BH3-mimetics (i.e., venetoclax) led to promising preclinical and clinical activity in B cell malignancies. However, several mechanisms of intrinsic or acquired resistance have been described ex vivo therefore predictive markers of response as well as mechanism-based combinations have to be designed. In the present study, we analyzed the expression of the BCL2-family genes across 10 mature B cell malignancies through computational normalization of 21 publicly available Affimetrix datasets gathering 1,219 patient samples. To better understand the deregulation of anti- and pro-apoptotic members of the BCL2-family in hematological disorders, we first compared gene expression profiles of malignant B cells to their relative normal control (naïve B cell to plasma cells, n = 37). We further assessed BCL2-family expression according to tissue localization i.e., peripheral blood, bone marrow, and lymph node, molecular subgroups or disease status i.e., indolent to aggressive. Across all cancer types, we showed that anti-apoptotic genes are upregulated while pro-apoptotic genes are downregulated when compared to normal counterpart cells. Of interest, our analysis highlighted that, independently of the nature of malignant B cells, the pro-apoptotic BH3-only BCL2L11 and PMAIP1 are deeply repressed in tumor niches, suggesting a central role of the microenvironment in their regulation. In addition, we showed selective modulations across molecular subgroups and showed that the BCL2-family expression profile was related to tumor aggressiveness. Finally, by integrating recent data on venetoclax-monotherapy clinical activity with the expression of BCL2-family members involved in the venetoclax response, we determined that the ratio (BCL2+BCL2L11+BAX)/BCL2L1 was the strongest predictor of venetoclax response for mature B cell malignancies in vivo.

Hematologic Tumor Cell Resistance to the BCL-2 Inhibitor Venetoclax: A Product of Its Microenvironment?

BCL-2 family proteins regulate the intrinsic pathway of programmed cell death (apoptosis) and play a key role in the development and health of multicellular organisms. The dynamics of these proteins' expression and interactions determine the survival of all cells in an organism, whether the healthy cells of a fully competent immune system or the diseased cells of an individual with cancer. Anti-apoptotic proteins like BCL-2, BCL-XL, and MCL-1 are well-known for maintaining tumor cell survival and are therefore attractive drug targets. The BCL-2-selective inhibitor venetoclax has been approved for use in chronic lymphocytic leukemia and is now being studied in a number of other hematologic malignancies. As clinical data mature, hypotheses have begun to emerge regarding potential mechanisms of venetoclax resistance. Here, we review accumulating evidence that lymphoid microenvironments play a key role in determining hematologic tumor cell sensitivity to venetoclax.

The rise of apoptosis: targeting apoptosis in hematologic malignancies

Dysregulation of the B-cell leukemia/lymphoma-2 (BCL-2) family of proteins of the intrinsic apoptotic pathway is fundamental to the pathophysiology of many hematologic malignancies. The BCL-2 family consists of regulatory proteins that either induce apoptosis (proapoptotic) or inhibit it (prosurvival). BCL-2, myeloid cell leukemia-1, and B-cell lymphoma–extra large are prosurvival proteins that are prime targets for anticancer therapy, and molecules targeting each are in various stages of preclinical and clinical development. The US Food and Drug Administration (FDA)-approved BCL-2 inhibitor venetoclax was first proven to be highly effective in chronic lymphocytic leukemia and some B-cell non-Hodgkin lymphoma subtypes. Subsequently, venetoclax was found to be active clinically against a diverse array of hematologic malignancies including multiple myeloma, acute myeloid leukemia, myelodysplastic syndrome, acute lymphoblastic leukemia, and others. Here, we give a brief introduction to BCL-2 family biology and the mechanism of action of BCL-2 Homology 3 (BH3) mimetics, and provide an overview of the clinical data for therapeutically targeting prosurvival proteins in hematologic malignancies, with a focus on BCL-2 inhibition. To prioritize novel agent combinations and predict responders, we discuss the utility of functional assays such as BH3 profiling. Finally, we provide a perspective on how therapies targeting BCL-2 family proteins may be optimally implemented into future therapeutic regimens for hematologic malignancies.

A Novel MCL1 Inhibitor Combined with Venetoclax Rescues Venetoclax-Resistant Acute Myelogenous Leukemia

Suppression of apoptosis by expression of antiapoptotic BCL2 family members is a hallmark of acute myeloblastic leukemia (AML). Induced myeloid leukemia cell differentiation protein (MCL1), an antiapoptotic BCL2 family member, is commonly upregulated in AML cells and is often a primary mode of resistance to treatment with the BCL2 inhibitor venetoclax. Here, we describe VU661013, a novel, potent, selective MCL1 inhibitor that destabilizes BIM/MCL1 association, leads to apoptosis in AML, and is active in venetoclax-resistant cells and patient-derived xenografts. In addition, VU661013 was safely combined with venetoclax for synergy in murine models of AML. Importantly, BH3 profiling of patient samples and drug-sensitivity testing ex vivo accurately predicted cellular responses to selective inhibitors of MCL1 or BCL2 and showed benefit of the combination. Taken together, these data suggest a strategy of rationally using BCL2 and MCL1 inhibitors in sequence or in combination in AML clinical trials. SIGNIFICANCE: Targeting antiapoptotic proteins in AML is a key therapeutic strategy, and MCL1 is a critical antiapoptotic oncoprotein. Armed with novel MCL1 inhibitors and the potent BCL2 inhibitor venetoclax, it may be possible to selectively induce apoptosis by combining or thoughtfully sequencing these inhibitors based on a rational evaluation of AML.See related commentary by Leber et al., p. 1511.This article is highlighted in the In This Issue feature, p. 1494.

Autophagy in cancer: a complex relationship

Macroautophagy is the process by which cells package and degrade cytosolic components, and recycle the breakdown products for future use. Since its initial description by Christian de Duve in the 1960s, significant progress has been made in understanding the mechanisms that underlie this vital cellular process and its specificity. Furthermore, macroautophagy is linked to pathologic conditions such as cancer and is being studied as a therapeutic target. In this review, we will explore the connections between autophagy and cancer, which are tumor- and context-dependent and include the tumor microenvironment. We will highlight the importance of tumor compartment-specific autophagy in both cancer aggressiveness and treatment.

Rational combination strategies to enhance venetoclax activity and overcome resistance in hematologic malignancies

Venetoclax (ABT-199) is a Bcl-2-specific BH3-mimetic that has shown significant promise in certain subtypes of CLL as well as in several other hematologic malignancies. As in the case of essentially all targeted agents, intrinsic or acquired resistance to this agent generally occurs, prompting the search for new strategies capable of circumventing this problem. A logical approach to this challenge involves rational combination strategies designed to disable preexisting or induced compensatory survival pathways. Many of these strategies involve downregulation of Mcl-1, a pro-survival Bcl-2 family member that is not targeted by venetoclax, and which often confers resistance to this agent. Given encouraging clinical results involving venetoclax in both lymphoid and myeloid malignancies, it is likely that such combination approaches will be incorporated into the therapeutic armamentarium for multiple hematologic malignancies in the near future.

Potent anti-leukemic activity of a specific cyclin-dependent kinase 9 inhibitor in mouse models of chronic lymphocytic leukemia

Onset of progression even during therapy with novel drugs remains an issue in chronic lymphocytic leukemia (CLL). Thus, there is ongoing demand for novel agents. Approaches targeting cyclin-dependent kinases (CDK) have reached the clinical trial stage. CDK9 mediating RNA transcriptional elongation is the evolving pivotal CLL CDK inhibitor target. However, more CDK9 selective compounds are desirable. Here, we describe the CDK9 inhibitor LDC526 displaying a low nanomolar biochemical activity against CDK9 and an at least 50-fold selectivity against other CDKs. After demonstrating in vitro MEC-1 cell line and primary human CLL cell cytotoxicity we evaluated the LDC526 in vivo effect on human CLL cells transplanted into NOD/scid/γc^null (NSG) mice. LDC526 administration (75 mg/kg) for 5 days resulted in a 77% reduction of human CLL cells in NSG spleens compared to carrier control treatment. Next, we longitudinally studied the LDC526 impact on circulating CLL cells in the TCL1 transgenic mouse model. LDC526 (50 mg/kg) administration for two days led to a 16-fold reduction of blood CLL cell numbers. Remarkably, residual CLL cells exhibited significantly increased intracellular BCL-2 levels. However, the LDC526 cytotoxic effect was not restricted to CLL cells as also declining numbers of normal B and T lymphocytes were observed in LDC526 treated TCL1 mice. Taken together, our in vivo data provide a strong rational for continued LDC526 development in CLL therapy and argue for the combination with BCL-2 inhibitors.

Induction of apoptosis and suppression of tumor growth by Nur77-derived Bcl-2 converting peptide in chemoresistant lung cancer cells

Resistance to chemotherapy is a major cause of treatment failure and poor overall survival in patients with lung cancer. Identification of molecular targets present in resistant cancer cells is essential for addressing therapeutic resistance and prolonging lung cancer patient survival. Members of the B-cell lymphoma 2 (Bcl-2) family of proteins are associated with chemotherapeutic resistance. In this study, we found that pro-survival protein Bcl-2 is upregulated in paclitaxel resistant cells, potentially contributing to chemotherapy resistance. To exploit the increase in Bcl-2 expression for targeting therapy resistance, we investigated the effects of a peptide derived from the nuclear receptor Nur77 that converts Bcl-2 from an anti-apoptotic protein to a pro-apoptotic protein. The Nur77 derived peptide preferentially induced apoptosis in paclitaxel-resistant cancer cells with high expression of Bcl-2. This peptide also induced apoptosis of multidrug resistant H69AR lung cancer cells that express Bcl-2 and inhibited their growth in 3D spheroids. The Nur77 peptide strongly suppressed the growth of paclitaxel-resistant lung cancer cells in a zebrafish xenograft tumor model. Taken together, our data supports a new strategy for treating lung cancers that acquire resistance to chemotherapy through overexpression of Bcl-2.

Venetoclax and low-dose cytarabine induced complete remission in a patient with high-risk acute myeloid leukemia: a case report

Conventional combination therapies have not resulted in considerable progress in the treatment of acute myeloid leukemia (AML). Elderly patients with AML and poor risk factors have grave prognosis. Midostaurin has been recently approved for the treatment of FLT-3-mutated AML. Venetoclax, a BCL-2 inhibitor, has been approved for the treatment of relapsed and/or refractory chronic lymphoid leukemia. Clinical trials on applying venetoclax in combination with cytarabine and other agents to treat various hematological malignancies are currently underway. Here, we present a case of a male patient with poor performance status and who developed AML following allogeneic hematopoietic stem cell transplant for high-risk myelodysplasia. The patient with high risk AML achieved complete response to the combined treatment regimen of low-dose cytarabine and venetoclax. Furthermore, we reviewed current clinical trials on the use of venetoclax for hematological malignancies.

Venetoclax: A new wave in hematooncology

Inhibitors of antiapoptotic proteins of the BCL2 family can successfully restart the deregulated process of apoptosis in malignant cells. Whereas nonselective agents have been limited by their affinity to different BCL2 members, thus inducing excessive toxicity, the highly selective BCL2 inhibitor venetoclax (ABT-199, Venclexta™) has an acceptable safety profile. To date, it has been approved in monotherapy for the treatment of relapsed or refractory chronic lymphocytic leukemia (CLL) with 17p deletion. Extension of indications can be expected in monotherapy and in combination regimens. Sensitivity to venetoclax is not common in lymphomas, but promising outcomes have been achieved in the mantle cell lymphoma group. Venetoclax is also active in multiple myeloma patients, especially in those with translocation t(11;14), even if high-risk features such as del17p are also present. Surprisingly, positive results are being obtained in elderly acute myeloid leukemia patients, in whom inhibition of BCL2 is able to substantially increase the efficacy of low-dose cytarabine or hypomethylating agents. Here, we provide a summary of available results from clinical trials and describe a specific mechanism of action that stands behind the efficacy of venetoclax in hematological malignancies.

Predicting effective pro-apoptotic anti-leukaemic drug combinations using co-operative dynamic BH3 profiling

The BH3-only apoptosis agonists BAD and NOXA target BCL-2 and MCL-1 respectively and co-operate to induce apoptosis. On this basis, therapeutic drugs targeting BCL-2 and MCL-1 might have enhanced activity if used in combination. We identified anti-leukaemic drugs sensitising to BCL-2 antagonism and drugs sensitising to MCL-1 antagonism using the technique of dynamic BH3 profiling, whereby cells were primed with drugs to discover whether this would elicit mitochondrial outer membrane permeabilisation in response to BCL-2-targeting BAD-BH3 peptide or MCL-1-targeting MS1-BH3 peptide. We found that a broad range of anti-leukaemic agents–notably MCL-1 inhibitors, DNA damaging agents and FLT3 inhibitors–sensitise leukaemia cells to BAD-BH3. We further analysed the BCL-2 inhibitors ABT-199 and JQ1, the MCL-1 inhibitors pladienolide B and torin1, the FLT3 inhibitor AC220 and the DNA double-strand break inducer etoposide to correlate priming responses with co-operative induction of apoptosis. ABT-199 in combination with pladienolide B, torin1, etoposide or AC220 strongly induced apoptosis within 4 hours, but the MCL-1 inhibitors did not co-operate with etoposide or AC220. In keeping with the long half-life of BCL-2, the BET domain inhibitor JQ1 was found to downregulate BCL-2 and to prime cells to respond to MS1-BH3 at 48, but not at 4 hours: prolonged priming with JQ1 was then shown to induce rapid cytochrome C release when pladienolide B, torin1, etoposide or AC220 were added. In conclusion, dynamic BH3 profiling is a useful mechanism-based tool for understanding and predicting co-operative lethality between drugs sensitising to BCL-2 antagonism and drugs sensitising to MCL-1 antagonism. A plethora of agents sensitised cells to BAD-BH3-mediated mitochondrial outer membrane permeabilisation in the dynamic BH3 profiling assay and this was associated with effective co-operation with the BCL-2 inhibitory compounds ABT-199 or JQ1.

Combined venetoclax and alvocidib in acute myeloid leukemia

More effective treatment options for elderly acute myeloid leukemia (AML) patients are needed as only 25-50% of patients respond to standard-of-care therapies, response duration is typically short, and disease progression is inevitable even with some novel therapies and ongoing clinical trials. Anti-apoptotic BCL-2 family inhibitors, such as venetoclax, are promising therapies for AML. Nonetheless, resistance is emerging. We demonstrate that venetoclax combined with cyclin-dependent kinase (CDK) inhibitor alvocidib is potently synergistic in venetoclax-sensitive and -resistant AML models in vitro, ex vivo and in vivo. Alvocidib decreased MCL-1, and/or increased pro-apoptotic proteins such as BIM or NOXA, often synergistically with venetoclax. Over-expression of BCL-XL diminished synergy, while knock-down of BIM almost entirely abrogated synergy, demonstrating that the synergistic interaction between alvocidib and venetoclax is primarily dependent on intrinsic apoptosis. CDK9 inhibition predominantly mediated venetoclax sensitization, while CDK4/6 inhibition with palbociclib did not potentiate venetoclax activity. Combined, venetoclax and alvocidib modulate the balance of BCL-2 family proteins through complementary, yet variable mechanisms favoring apoptosis, highlighting this combination as a promising therapy for AML or high-risk MDS with the capacity to overcome intrinsic apoptosis mechanisms of resistance. These results support clinical testing of combined venetoclax and alvocidib for the treatment of AML and advanced MDS.

Therapeutics targeting Bcl-2 in hematological malignancies

Members of the B-cell lymphoma 2 (BCL-2) gene family are attractive targets for cancer therapy as they play a key role in promoting cell survival, a long-since established hallmark of cancer. Clinical utility for selective inhibition of specific anti-apoptotic Bcl-2 family proteins has recently been realized with the Food and Drug Administration (FDA) approval of venetoclax (formerly ABT-199/GDC-0199) in relapsed chronic lymphocytic leukemia (CLL) with 17p deletion. Despite the impressive monotherapy activity in CLL, such responses have rarely been observed in other B-cell malignancies, and preclinical data suggest that combination therapies will be needed in other indications. Additional selective antagonists of Bcl-2 family members, including Bcl-X_L and Mcl-1, are in various stages of preclinical and clinical development and hold the promise of extending clinical utility beyond CLL and overcoming resistance to venetoclax. In addition to direct targeting of Bcl-2 family proteins with BH3 mimetics, combination therapies that aim at down-regulating expression of anti-apoptotic BCL-2 family members or restoring expression of pro-apoptotic BH3 family proteins may provide a means to deepen responses to venetoclax and extend the utility to additional indications. Here, we review recent progress in direct and selective targeting of Bcl-2 family proteins for cancer therapy and the search for rationale combinations.