ABT-199 selectively inhibits BCL2 but not BCL2L1 and efficiently induces apoptosis of chronic lymphocytic leukaemic cells but not platelets

BCL-2 inhibition in haematological malignancies: Clinical application and complications

B-cell lymphoma-2 (BCL-2) family proteins are fundamental regulators of the intrinsic apoptotic pathway which modulate cellular fate. In many haematological malignancies, overexpression of anti-apoptotic factors (BCL-2, BCL-XL and MCL-1) circumvent apoptosis. To address this cancer hallmark, a concerted effort has been made to induce apoptosis by inhibiting BCL-2 family proteins. A series of highly selective BCL-2 homology 3 (BH3) domain mimetics are in clinical use and in ongoing clinical trials for acute myeloid leukaemia (AML), chronic myeloid leukaemia (CML), chronic lymphocytic leukaemia (CLL), and multiple myeloma (MM). These inhibitors serve as promising candidates, both as single agents or in combination therapy to improve patient outcomes. In other diseases such as follicular lymphoma, efficacy has been notably limited. There are also clinical problems with BCL-2 family inhibition, including drug resistance, disease relapse, tumour lysis syndrome, and clinically relevant cytopenias. Here, we provide a balanced view on both the clinical benefits of BCL-2 inhibition as well as the associated challenges.

Allosteric cross-talk between the hydrophobic cleft and the BH4 domain of Bcl-2 in control of inositol 1,4,5-trisphosphate receptor activity

Aim:

Inositol 1,4,5-trisphosphate receptor (IP₃R) is a ubiquitous calcium (Ca²⁺) channel involved in the regulation of cellular fate and motility. Its modulation by anti-apoptotic protein B-cell lymphoma 2 (Bcl-2) plays an important role in cancer progression. Disrupting this interaction could overcome apoptosis avoidance, one of the hallmarks of cancer, and is, thus, of great interest. Earlier reports have shown the involvement of both the Bcl-2 homology 4 (BH4) and the transmembrane domains (TMDs) of Bcl-2 in regulating IP₃R activity, while the Bcl-2 hydrophobic cleft was associated primarily with its anti-apoptotic and IP₃R-independent action at the mitochondria (Oncotarget. 2016;7:55704–20. doi: 10.18632/oncotarget.11005). The aim of this study was to investigate how targeting the BH3 hydrophobic cleft of Bcl-2 affects IP₃R:Bcl-2 interaction.

Methods:

Organelle membrane-derived (OMD) patch-clamp and circular dichroism (CD) thermal melting experiments were used to elucidate the effects of the ABT-199 (venetoclax) on the IP₃R:Bcl-2 interaction. Molecular dynamics (MD) simulations of free and ABT-199 bound Bcl-2 were used to propose a molecular model of such interaction.

Results:

It was shown that occlusion of Bcl-2’s hydrophobic cleft by the drug ABT-199 finely modulates IP₃R gating in the low open probability (P_o) regime, characteristic of the basal IP₃R activity in non-excited cells. Complementary MD simulations allowed to propose a model of this modulation, involving an allosteric interaction with the BH4 domain on the opposite side of Bcl-2.

Conclusions:

Bcl-2 is an important regulator of IP₃R activity and, thus of Ca²⁺ release from internal stores and associated processes, including cellular proliferation and death. The presence of multiple regulatory domains in both proteins suggests a complex interaction. Thus, it was found that the occlusion of the hydrophobic cleft of Bcl-2 by ABT-199 disrupts IP₃R activity, leading to Bcl-2 rebinding with smaller affinity and lesser inhibitory effect. MDs simulations of free and ABT-199 bound Bcl-2 propose a molecular model of such disruption, involving an allosteric interaction with the BH4 domain on the opposite side of Bcl-2.

NF-κB: A Druggable Target in Acute Myeloid Leukemia

Simple Summary

AML is a highly heterogeneous hematological disease and is the second most common form of leukemia. Around 40% of AML patients display elevated nuclear NF-κB activity, providing a compelling rationale for targeting the NF-κB pathway in AML. Here we summarize the main drivers of the NF-κB pathway in AML pathogenesis as well as the conventional and novel therapeutic strategies targeting NF-κB to improve the survival of AML patients.

Abstract

Acute Myeloid Leukemia (AML) is an aggressive hematological malignancy that relies on highly heterogeneous cytogenetic alterations. Although in the last few years new agents have been developed for AML treatment, the overall survival prospects for AML patients are still gloomy and new therapeutic options are still urgently needed. Constitutive NF-κB activation has been reported in around 40% of AML patients, where it sustains AML cell survival and chemoresistance. Given the central role of NF-κB in AML, targeting the NF-κB pathway represents an attractive strategy to treat AML. This review focuses on current knowledge of NF-κB’s roles in AML pathogenesis and summarizes the main therapeutic approaches used to treat NF-κB-driven AML.

Lipase-triggered drug release from BCL2 inhibitor ABT-199-loaded nanoparticles to elevate anti-leukemic activity through enhanced drug targeting on the mitochondrial membrane

Selective BCL2 inhibitor ABT-199 has been approved to treat hematological malignancies including acute myeloid leukemia (AML). However, acquired drug resistance and severe side effects occur after extended treatment limiting the clinical usage of ABT-199. Here, we successfully encapsulated pure ABT-199 in amphiphilic mPEG-b-PTMC₁₆₉ block copolymer, forming mPEG-b-PTMC₁₆₉@ABT-199 nanoparticles (abbreviated as PEG-ABT-199), which presented better aqueous dispersion and higher efficiency of loading and encapsulation than pure ABT-199. We then compared the anti-leukemic ability of pure ABT-199 and PEG-ABT-199 in vitro and in vivo. PEG-ABT-199 had a lower IC50 value compared with pure ABT-199 in MV4-11 and MOLM-13 cell lines. In addition, PEG-ABT-199 significantly induced apoptosis and decreased colony number than pure ABT-199. Most importantly, PEG-ABT-199 markedly reduced leukemic burden, inhibited the infiltration of leukemic blasts in the spleen, and extended the overall survival (OS) in MLL-AF9-transduced murine AML compared with free ABT-199. Meanwhile, the blank PEG169 NP was non-toxic to normal hematopoiesis in vitro and in vivo, suggesting that PEG169 NP is a safe carrier. Mechanistically, PEG-ABT-199 enhanced mitochondria-targeted delivery of ABT-199 to trigger the collapse of mitochondrial membrane potential (MMP), the release of cytochrome c (cyt-c), and mitochondria-based apoptosis. In conclusion, our results suggest that PEG-ABT-199 has more vital anti-leukemic ability than pure ABT-199. PEG-ABT-199 has potential application in clinical trials to alleviate side effects and improve anti-leukemia ability. STATEMENT OF SIGNIFICANCE: ATB-199, an orally selective inhibitor for BCL2 protein, presents marked activity in relapsed or refractory AML, T-ALL, and CLL patients. However, ABT-199 resistance severely limits the further clinical usage because of off-target effects, non-specific toxicities, and low delivery of drugs. To reduce the side-effects and improve the solubility and bioavailability, ABT-199 was encapsulated into the amphiphilic mPEG-b-PTMC block copolymer by co-assembly method to obtain mPEG-b-PTMC@ABT-199 nanoparticles (PEG-ABT-199). PEG-ABT-199 has several advantages compared with pure ABT-199. 1.PEG-ABT-199 presents better aqueous dispersion and higher efficiencies of loading and encapsulation than pure ABT-199. 2. PEG-ABT-199 substantially enhances the anti-leukemic ability in vitro and in vivo compared with pure ABT-199. 3. PEG-ABT-199 has little effects on normal cells. 4. PEG-ABT-199 can reduce treatment cost.

Transcriptome profiling based on larvae at different time points after hatching provides a core set of gene resource for understanding the immune response mechanisms of the egg-protecting behavior against Vibrio anguillarum infection in Amphioctopus fangsiao

Mollusks have recently received increasing attention because of their unique immune systems. Mollusks such as Amphioctopus fangsiao are economically important cephalopods, and the effects of their egg-protecting behavior on the larval immune response are unclear. Meanwhile, little research has been done on the resistance response of cephalopod larvae infected with pathogenic bacteria such as Vibrio anguillarum. In this study, V. anguillarum was used to infect the primary hatching A. fangsiao larvae under different egg-protecting behaviors for 24 h, and a total of 7156 differentially expressed genes (DEGs) were identified at four time points after hatching based on transcriptome analysis. GO and KEGG enrichment analyses showed that multiple immune-related GO terms and KEGG signaling pathways were enriched. Protein-protein interaction networks (PPI networks) were used to search functional relationships between immune-related DEGs. Finally, 20 hub genes related to multiple gene functions or involved in multiple signaling pathways were identified, and their accuracy was verified using quantitative RT-PCR. PPI networks were first used to study the effects A. fangsiao larvae after infection with V. anguillarum under different egg-protecting behaviors. The results provide significant genetic resources for exploring invertebrate larval immune processes. The data lays a foundation for further study the immune response mechanisms for invertebrates after infection.

New Treatment Options for Newly-Diagnosed and Relapsed Chronic Lymphocytic Leukemia

OPINION STATEMENT

The better understanding of the biology of chronic lymphocytic leukemia (CLL) gained over the past decade has led to the development and introduction of several targeted drugs, with an demonstrable improvement in the prognosis for this currently incurable condition. Currently, Bruton's tyrosine kinase (BTK) inhibitors, phosphoinositide 3-kinase (PI3K) inhibitors, venetoclax, and CD20 monoclonal antibodies are the key elements in the treatment of both previously untreated and relapsed/refractory CLL patients. Ibrutinib was the first BTK inhibitor approved for clinical use, and showed excellent efficacy and an acceptable safety profile. Following this, the better-tolerated second-generation irreversible BTK inhibitors acalabrutinib and zanubrutinib have been introduced for the treatment of lymphoid malignancies, and acalabrutinib was approved for CLL. When used as single drugs, BTK inhibitors are given continuously until unacceptable toxicity or disease progression; however, when combined with venetoclax and/or CD20 antibodies, they induce deeper response and can be given for a limited time. Recently, promising new reversible BTK inhibitors pirtobrutinib and nemtabrutinib were discovered, and these seem to be more active and better tolerated than their irreversible predecessors. However, they are in an early phase of development and are not currently approved for CLL. The phosphatidylinositol 3-kinase (PI3K) inhibitors idelalisib and duvelisib are highly effective in patients with relapsed CLL, including high-risk disease. The major limitations for their use are adverse events, mostly of autoimmune origin (hepatitis, enteritis/colitis, and pneumonitis). Otherwise, cellular therapies like allogeneic hematopoietic stem cell transplantation and chimeric antigen receptor (CAR) T cells and bispecific monoclonal antibodies offer promise for patients who have failed BTK inhibitors and venetoclax treatment. In the coming years, it is likely that novel targeted therapies will replace immunochemotherapy regimens in most patients.

The anti-apoptotic Bcl-2 protein regulates hair follicle stem cell function

Maintaining the architecture, size and composition of an intact stem cell (SC) compartment is crucial for tissue homeostasis and regeneration throughout life. In mammalian skin, elevated expression of the anti‐apoptotic Bcl‐2 protein has been reported in hair follicle (HF) bulge SCs (BSCs), but its impact on SC function is unknown. Here, we show that systemic exposure of mice to the Bcl‐2 antagonist ABT‐199/venetoclax leads to the selective loss of suprabasal BSCs (sbBSCs), thereby disrupting cyclic HF regeneration. RNAseq analysis shows that the pro‐apoptotic BH3‐only proteins BIM and Bmf are upregulated in sbBSCs, explaining their addiction to Bcl‐2 and the marked susceptibility to Bcl‐2 antagonism. In line with these observations, conditional knockout of Bcl‐2 in mouse epidermis elevates apoptosis in BSCs. In contrast, ectopic Bcl‐2 expression blocks apoptosis during HF regression, resulting in the accumulation of quiescent SCs and delaying HF growth in mice. Strikingly, Bcl‐2‐induced changes in size and composition of the HF bulge accelerate tumour formation. Our study identifies a niche‐instructive mechanism of Bcl‐2‐regulated apoptosis response that is required for SC homeostasis and tissue regeneration, and may suppress carcinogenesis.

BCL2 inhibitors and MCL1 inhibitors for hematological malignancies

BCL2 and MCL1 are commonly expressed pro-survival (anti-apoptotic) proteins in hematological cancers and play important roles in their biology either through dysregulation or by virtue of intrinsic importance to the cell-of-origin of the malignancy. A new class of small molecule anti-cancer drugs, BH3-mimetics, now enable specific targeting of these proteins in patients. BH3-mimetics act by inhibiting the pro-survival BCL2 proteins to enable the activation of BAX and BAK, apoptosis effectors which permeabilize the outer mitochondrial membrane, triggering apoptosis directly in many cells and sensitizing others to cell death when combined with other anti-neoplastic drugs. Venetoclax, a specific inhibitor of BCL2, is the first approved in class, demonstrating striking single agent activity in chronic lymphocytic leukemia (CLL) and in other lymphoid neoplasms, as well as activity against acute myeloid leukemia (AML), especially when used in combination. Key insights from the venetoclax experience include that responses occur rapidly, with major activity as monotherapy proving to be the best indicator for success in combination regimens. This emphasizes the importance of adequate single agent studies for drugs in this class. Furthermore, secondary resistance is common with long-term exposure and often mediated by genetic or adaptive changes in the apoptotic pathway, suggesting that BH3-mimetics are better suited to limited-duration, rather than continuous, therapy. The success of venetoclax has inspired development of BH3-mimetics targeting MCL1. Despite promising preclinical activity against MYC-driven lymphomas, myeloma and AML, their success may particularly depend on their tolerability profile given physiological roles for MCL1 in several non-hematological tissues.

Potential Biomarkers for Treatment Response to the BCL-2 Inhibitor Venetoclax: State of the Art and Future Directions

Intrinsic apoptotic pathway dysregulation plays an essential role in all cancers, particularly hematologic malignancies. This role has led to the development of multiple therapeutic agents targeting this pathway. Venetoclax is a selective BCL-2 inhibitor that has been approved for the treatment of chronic lymphoid leukemia and acute myeloid leukemia. Given the reported resistance to venetoclax, understanding the mechanisms of resistance and the potential biomarkers of response is crucial to ensure optimal drug usage and improved patient outcomes. Mechanisms of resistance to venetoclax include alterations involving the BH3-binding groove, BCL2 gene mutations affecting venetoclax binding, and activation of alternative anti-apoptotic pathways. Moreover, various potential genetic biomarkers of venetoclax resistance have been proposed, including chromosome 17p deletion, trisomy 12, and TP53 loss or mutation. This manuscript provides an overview of biomarkers that could predict treatment response to venetoclax.

Combination Therapies in Chronic Myeloid Leukemia for Potential Treatment-Free Remission: Focus on Leukemia Stem Cells and Immune Modulation

Although tyrosine Kinase Inhibitors (TKI) has revolutionized the treatment of chronic myeloid leukemia (CML), patients are not cured with the current therapy modalities. Also, the more recent goal of CML treatment is to induce successful treatment-free remission (TFR) among patients achieving durable deep molecular response (DMR). Together, it is necessary to develop novel, curative treatment strategies. With advancements in understanding the biology of CML, such as dormant Leukemic Stem Cells (LSCs) and impaired immune modulation, a number of agents are now under investigation. This review updates such agents that target LSCs, and together with TKIs, have the potential to eradicate CML. Moreover, we describe the developing immunotherapy for controlling CML.

Resistance to venetoclax and hypomethylating agents in acute myeloid leukemia

Despite the success of the combination of venetoclax with the hypomethylating agents (HMA) decitabine or azacitidine in inducing remission in older, previously untreated patients with acute myeloid leukemia (AML), resistance - primary or secondary - still constitutes a significant roadblock in the quest to prolong the duration of response. Here we review the proposed and proven mechanisms of resistance to venetoclax monotherapy, HMA monotherapy, and the doublet of venetoclax and HMA for the treatment of AML. We approach the mechanisms of resistance to HMAs and venetoclax in the light of the agents' mechanisms of action. We briefly describe potential therapeutic strategies to circumvent resistance to this promising combination, including alternative scheduling or the addition of other agents to the HMA and venetoclax backbone. Understanding the mechanisms of action and evolving resistance in AML remains a priority in order to maximize the benefit from novel drugs and combinations, identify new therapeutic targets, define potential prognostic markers, and avoid treatment failure.

Therapeutic development and current uses of BCL-2 inhibition

B-cell lymphoma 2 (BCL2) is a key protein regulator of apoptosis. It is variably highly expressed in many hematological malignancies, providing protection from cell death induced by oncogenic and external stresses. Venetoclax is the first selective BCL2 inhibitor, and the first of a new class of anticancer drug (BH3-mimetics) to be approved for routine clinical practice, currently in chronic lymphocytic leukemia (CLL) and acute myeloid leukemia (AML). To help understand the potential and limitations of this therapy, this brief review will touch on the history of development of venetoclax, dissect its mechanism of action, and summarize critical evidence for its approved use in the management of patients with CLL and AML. It will also consider recent data on mechanisms of resistance and explore concepts pertinent to its future development based on key lessons learned to date.

B-ALL Complexity: Is Targeted Therapy Still A Valuable Approach for Pediatric Patients?

Simple Summary

B-ALL is the more frequent childhood malignancy. Even though significant improvements in patients’ survival, some pediatric B-ALL have still poor prognosis and novel strategies are needed. Recently, new genetic abnormalities and altered signaling pathways have been described, defining novel B-ALL subtypes.Innovative targeted therapeutic drugs may potentially show a great impact on the treatment of B-ALL subtypes, offering an important chance to block multiple signaling pathways and potentially improving the clinical management of B-ALL younger patients, especially for the new identified subtypes that lack efficient chemotherapeutic protocols. In this review, we shed light on the up-to-date knowledge of the novel childhood B-ALL subtypes and the altered signaling pathways that could become new druggable targets.

Abstract

B-cell acute lymphoblastic leukemia (B-ALL) is a hematologic malignancy that arises from the clonal expansion of transformed B-cell precursors and predominately affects childhood. Even though significant progresses have been made in the treatment of B-ALL, pediatric patients’ outcome has to be furtherly increased and alternative targeted treatment strategies are required for younger patients. Over the last decade, novel approaches have been used to understand the genomic landscape and the complexity of the molecular biology of pediatric B-ALL, mainly next generation sequencing, offering important insights into new B-ALL subtypes, altered pathways, and therapeutic targets that may lead to improved risk stratification and treatments. Here, we will highlight the up-to-date knowledge of the novel B-ALL subtypes in childhood, with particular emphasis on altered signaling pathways. In addition, we will discuss the targeted therapies that showed promising results for the treatment of the different B-ALL subtypes.

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.

Venetoclax: A Review in Relapsed/Refractory Chronic Lymphocytic Leukemia

Venetoclax (Venclyxto^®; Venclexta^®) is a first-in-class, oral, selective B cell lymphoma-2 (BCL-2) inhibitor. The drug is approved in numerous countries, including those of the EU and in the USA, for the treatment of adults with relapsed or refractory (RR) chronic lymphocytic leukemia (CLL); the specific indication(s) for venetoclax may vary between individual countries. Venetoclax monotherapy or combination therapy with rituximab was an effective treatment, provided durable responses, and had a manageable safety profile in pivotal clinical trials in adults with RR CLL, including in patients with adverse prognostic factors. In combination with 6 cycles of rituximab, venetoclax (fixed 24 months' treatment) was more effective than bendamustine plus rituximab (6 cycles) in prolonging progression-free survival (PFS) and inducing undetectable minimal residual disease (uMRD) in peripheral blood (PB) and bone marrow (BM), with these benefits sustained during 36 months' follow-up. Hence, with its novel mechanism of action and convenient oral once-daily regimen, venetoclax monotherapy or fixed 24-month combination therapy with rituximab represents an important option for treating RR CLL, including in patients with del(17p) or TP53 mutation and those failing a B cell receptor (BCR) inhibitor and/or chemotherapy.

Inhibition of BCL2 Family Members Increases the Efficacy of Copper Chelation in BRAFV600E-Driven Melanoma

The principal unmet need in BRAF^V600E-positive melanoma is lack of an adequate therapeutic strategy capable of overcoming resistance to clinically approved targeted therapies against oncogenic BRAF and/or the downstream MEK1/2 kinases. We previously discovered that copper (Cu) is required for MEK1 and MEK2 activity through a direct Cu-MEK1/2 interaction. Repurposing the clinical Cu chelator tetrathiomolybdate (TTM) is supported by efficacy in BRAF^V600E-driven melanoma models, due in part to inhibition of MEK1/2 kinase activity. However, the antineoplastic activity of Cu chelators is cytostatic. Here, we performed high-throughput small-molecule screens to identify bioactive compounds that synergize with TTM in BRAF^V600E-driven melanoma cells. Genetic perturbation or pharmacologic inhibition of specific members of the BCL2 family of antiapoptotic proteins (BCL-W, BCL-XL, and MCL1) selectively reduced cell viability when combined with a Cu chelator and induced CASPASE-dependent cell death. Further, in BRAF^V600E-positive melanoma cells evolved to be resistant to BRAF and/or MEK1/2 inhibitors, combined treatment with TTM and the clinically evaluated BCL2 inhibitor, ABT-263, restored tumor growth suppression and induced apoptosis. These findings further support Cu chelation as a therapeutic strategy to target oncogene-dependent tumor cell growth and survival by enhancing Cu chelator efficacy with chemical inducers of apoptosis, especially in the context of refractory or relapsed BRAF^V600E-driven melanoma. SIGNIFICANCE: This study unveils a novel collateral drug sensitivity elicited by combining copper chelators and BH3 mimetics for treatment of BRAF^V600E mutation-positive melanoma.

Clinical application of obinutuzumab for treating chronic lymphocytic leukemia

Alkylators and nucleoside analogs were the main drugs for treatingchronic lymphoblastic leukemia (CLL), which have been replaced by monoclonal antibodies, such as rituximab in the past 10 years for refractory or relapsed CLL. The first-line immunochemotherapy regimen, rituximab combined with nucleoside analogs, significantly increased CLL patients’ first-reaction rate and improved progression-free survival. Despite the long-lasting remissions by the use of chemoimmunotherapy, most CLL patients will relapse eventually. The obinutuzumab (GA101), an updated CD20 antibody, that is thought to achieve a more durable response with unique molecular and functional characteristics. Obinutuzumab is a humanized, monoclonal type II CD20 antibody modified by glycoengineering. The glycoengineered Fc portion enhances the binding affinity to the FcγRIII receptor on immune effector cells, resulting in increased antibody-dependent cellular cytotoxicity and phagocytosis. In addition, the type II antibody binding characteristics of obinutuzumab to CD20 lead to an efficient induction of direct non-apoptotic cell death. This review summarizes the results of clinical studies using obinutuzumab and looks forward to its further application in treating CLL clinically.

Venetoclax: evidence to date and clinical potential

The emergence of targeted therapy for patients with hematological diseases has permanently altered the therapeutic landscape. Immunochemotherapy regimes are now more and more being replaced by targeted therapies due to superior efficacy and better safety profiles. However, evolution and selection of subclones with continuous treatment leads to disease relapse and resistance toward these novel drugs. Venetoclax, the highly selective BCL-2 inhibitor (ABT-199), has an acceptable safety profile. To date, it has been approved for the treatment of first-line and relapsed/refractory chronic lymphocytic leukemia (CLL) and acute myeloid leukemia (AML). However, extension of indications can be expected in monotherapy and in combination regimens with promising outcomes in other hematological diseases. In this article, we describe the mechanism of action that stands behind the efficacy of venetoclax and provide a summary of available results from clinical trials.

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.

DRP-1 functions independently of mitochondrial structural perturbations to facilitate BH3 mimetic-mediated apoptosis

Maintenance of mitochondrial integrity is critical for normal cellular homoeostasis. Most cells respond to stress stimuli and undergo apoptosis by perturbing mitochondrial structure and function to release proteins, such as cytochrome c, which are essential for the execution of the intrinsic apoptotic cascade. Cancer cells evade these events by overexpressing the anti-apoptotic BCL-2 family of proteins on mitochondrial membranes. Inhibitors of the anti-apoptotic BCL-2 family proteins, also known as BH3 mimetics, antagonise the pro-survival functions of these proteins and result in rapid apoptosis. Although the precise mechanism by which BH3 mimetics induce apoptosis has been well characterised, not much is known in terms of the structural changes that occur in mitochondria during apoptosis. Using a panel of highly selective BH3 mimetics and a wide range of cell lines, we demonstrate that BH3 mimetics induce extensive mitochondrial fission, accompanied by swelling of the mitochondrial matrix and rupture of the outer mitochondrial membrane. These changes occur in a BAX/ BAK-dependent manner. Although a major mitochondrial fission GTPase, DRP-1, has been implicated in mitochondrial apoptosis, our data demonstrate that DRP-1 might function independently/downstream of BH3 mimetic-mediated mitochondrial fission to facilitate the release of cytochrome c and apoptosis. Moreover, downregulation of DRP-1 prevented cytochrome c release and apoptosis even when OPA1, a protein mediating mitochondrial fusion, was silenced. Although BH3 mimetic-mediated displacement of BAK and other BH3-only proteins from BCL-XL and MCL-1 was unaffected by DRP-1 downregulation, it prevented BAK activation significantly, thus placing DRP-1 as one of the most critical players, along with BAX and BAK, that governs BH3 mimetic-mediated cytochrome c release and apoptosis.

A literature review of the patent publications on venetoclax - a selective Bcl-2 inhibitor: discovering the therapeutic potential of a novel chemotherapeutic agent

INTRODUCTION

Studies presented in patents show that a novel chemotherapeutic agent, venetoclax, might be useful in additional therapeutic indications. Venetoclax is approved in America for the treatment of patients with chronic lymphocytic leukemia (CLL) or small lymphocytic lymphoma (SLL). Venetoclax selectively inhibits the B-cell lymphoma-2 (Bcl-2) protein, an anti-apoptotic protein that can be overexpressed in most B-cell lymphoid malignancies.

AREAS COVERED

This is a review of all the patents granted until November 2018, with venetoclax in the examples or claim section of the patent document. The patents include the synthesis, polymorphism, formulations, in vitro and in vivo efficacy as well as the therapeutic application of venetoclax.

EXPERT OPINION

The approved indications for treatment with venetoclax are limited but expanding rapidly. Studies suggest that venetoclax might be useful in several other therapeutic indications, mostly other hematological malignancies. Numerous studies use venetoclax in combinations with other therapeutic agents. Such combinational treatment shows promising results in additional indications as well as drug-resistant cancers. Venetoclax is an interesting new therapeutic involved in a variety of clinical research. Patent applications in recent years even include venetoclax in somewhat exotic fields such as type 1 diabetes, asthma, and Zika virus treatment.

Venetoclax-based therapies for acute myeloid leukemia

The prognosis of adult acute myeloid leukemia (AML) remains poor, with the long-term survival rate less than 50%. However, the current paradigms of treatment are changing through a better understanding of the disease genetics and pathophysiology. Since 2017, eight new drugs have been approved by the U.S. Food and Drug Administration for the treatment of AML, including the FLT3 inhibitors midostaurin and gilteritinib, the IDH inhibitors ivosidenib and enasidenib, the anti-CD33 monoclonal antibody gemtuzumab ozogamicin, liposomal daunorubicin and cytarabine, the hedgehog pathway inhibitor glasdegib and the BCL-2 inhibitor venetoclax. Preclinical data demonstrated the anti-leukemic efficacy of venetoclax in AML and its synergy when combined with hypomethylating agents or chemotherapy agents. Clinical trials have demonstrated the clinical benefit of venetoclax-based therapies in newly diagnosed AML, leading to the recent FDA approval of venetoclax in combination with hypomethylating agents or low-dose cytarabine for older adults with newly diagnosed AML. Herein, we focus on the role of single-agent BCL-2 inhibition in AML and review the clinical studies of venetoclax-based combination regimens and the evolving mechanisms of resistance.

Update on the role of venetoclax and rituximab in the treatment of relapsed or refractory CLL

For the treatment of mature B cell malignancies including chronic lymphocytic leukemia (CLL), the last 5 years has brought major advances in the application of targeted therapies. Whilst monoclonal anti-CD20 agents such as rituximab have a central role in combination with traditional cytotoxic therapy, their combination with novel agents that target the B cell receptor signaling pathway and other intracellular mechanisms of B cell proliferation is a new approach to treatment. Venetoclax is a highly specific novel agent inhibiting the bcl-2 anti-apoptotic pathway and has potent activity in CLL. Its combination with rituximab results in deeper and more durable responses and this regimen is a valuable option in the treatment of relapsed or refractory CLL including adverse prognostic variants such as cases that are fludarabine refractory or harbor the 17p chromosomal deletion. This review centers on the use of venetoclax and rituximab in relapsed or refractory CLL.

Targeting intermediary metabolism enhances the efficacy of BH3 mimetic therapy in hematologic malignancies

BH3 mimetics are novel targeted drugs with remarkable specificity, potency and enormous potential to improve cancer therapy. However, acquired resistance is an emerging problem. We report the rapid development of resistance in chronic lymphocytic leukemia cells isolated from patients exposed to increasing doses of navitoclax (ABT-263), a BH3 mimetic. To mimic such rapid development of chemoresistance, we developed simple resistance models to three different BH3 mimetics, targeting BCL-2 (ABT-199), BCL-XL (A-1331852) or MCL-1 (A-1210477), in relevant hematologic cancer cell lines. In these models, resistance could not be attributed to either consistent changes in expression levels of the anti-apoptotic proteins or interactions among different pro- and anti-apoptotic BCL-2 family members. Using genetic silencing, pharmacological inhibition and metabolic supplementation, we found that targeting glutamine uptake and its downstream signaling pathways, namely glutaminolysis, reductive carboxylation, lipogenesis, cholesterogenesis and mammalian target of rapamycin signaling resulted in marked sensitization of the chemoresistant cells to BH3 mimetic-mediated apoptosis. Furthermore, our findings highlight the possibility of repurposing widely used drugs, such as statins, to target intermediary metabolism and improve the efficacy of BH3 mimetic therapy.

Next Generation Sequencing in AML-On the Way to Becoming a New Standard for Treatment Initiation and/or Modulation?

Acute myeloid leukemia (AML) is a clonal disease caused by genetic abberations occurring predominantly in the elderly. Next generation sequencing (NGS) analysis has led to a deeper genetic understanding of the pathogenesis and the role of recently discovered genetic precursor lesions (clonal hematopoiesis of indeterminate/oncogenic potential (CHIP/CHOP)) in the evolution of AML. These advances are reflected by the inclusion of certain mutations in the updated World Health Organization (WHO) 2016 classification and current treatment guidelines by the European Leukemia Net (ELN) and National Comprehensive Cancer Network (NCCN) and results of mutational testing are already influencing the choice and timing of (targeted) treatment. Genetic profiling and stratification of patients into molecularly defined subgroups are expected to gain ever more weight in daily clinical practice. Our aim is to provide a concise summary of current evidence regarding the relevance of NGS for the diagnosis, risk stratification, treatment planning and response assessment in AML, including minimal residual disease (MRD) guided approaches. We also summarize recently approved drugs targeting genetically defined patient populations with risk adapted- and individualized treatment strategies.

Stapled Peptides Inhibitors: A New Window for Target Drug Discovery

Protein-protein interaction (PPI) is a hot topic in clinical research as protein networking has a major impact in human disease. Such PPIs are potential drugs targets, leading to the need to inhibit/block specific PPIs. While small molecule inhibitors have had some success and reached clinical trials, they have generally failed to address the flat and large nature of PPI surfaces. As a result, larger biologics were developed for PPI surfaces and they have successfully targeted PPIs located outside the cell. However, biologics have low bioavailability and cannot reach intracellular targets. A novel class -hydrocarbon-stapled α-helical peptides that are synthetic mini-proteins locked into their bioactive structure through site-specific introduction of a chemical linker- has shown promise. Stapled peptides show an ability to inhibit intracellular PPIs that previously have been intractable with traditional small molecule or biologics, suggesting that they offer a novel therapeutic modality. In this review, we highlight what stapling adds to natural-mimicking peptides, describe the revolution of synthetic chemistry techniques and how current drug discovery approaches have been adapted to stabilize active peptide conformations, including ring-closing metathesis (RCM), lactamisation, cycloadditions and reversible reactions. We provide an overview on the available stapled peptide high-resolution structures in the protein data bank, with four selected structures discussed in details due to remarkable interactions of their staple with the target surface. We believe that stapled peptides are promising drug candidates and open the doors for peptide therapeutics to reach currently "undruggable" space.

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.

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.

Reactive Oxygen Species and Oncoprotein Signaling-A Dangerous Liaison

SIGNIFICANCE

There is evidence to implicate reactive oxygen species (ROS) in tumorigenesis and its progression. This has been associated with the interplay between ROS and oncoproteins, resulting in enhanced cellular proliferation and survival. Recent Advances: To date, studies have investigated specific contributions of the crosstalk between ROS and signaling networks in cancer initiation and progression. These investigations have challenged the established dogma of ROS as agents of cell death by demonstrating a secondary function that fuels cell proliferation and survival. Studies have thus identified (onco)proteins (Bcl-2, STAT3/5, RAS, Rac1, and Myc) in manipulating ROS level as well as exploiting an altered redox environment to create a milieu conducive for cancer formation and progression.

CRITICAL ISSUES

Despite these advances, drug resistance and its association with an altered redox metabolism continue to pose a challenge at the mechanistic and clinical levels. Therefore, identifying specific signatures, altered protein expressions, and modifications as well as protein-protein interplay/function could not only enhance our understanding of the redox networks during cancer initiation and progression but will also provide novel targets for designing specific therapeutic strategies.

FUTURE DIRECTIONS

Not only a heightened realization is required to unravel various gene/protein networks associated with cancer formation and progression, particularly from the redox standpoint, but there is also a need for developing more sensitive tools for assessing cancer redox metabolism in clinical settings. This review attempts to summarize our current knowledge of the crosstalk between oncoproteins and ROS in promoting cancer cell survival and proliferation and treatment strategies employed against these oncoproteins. Antioxid. Redox Signal.

The Ewing Family of Tumors Relies on BCL-2 and BCL-XL to Escape PARP Inhibitor Toxicity

PURPOSE

It was recently demonstrated that the EWSR1-FLI1 t(11;22)(q24;12) translocation contributes to the hypersensitivity of Ewing sarcoma to PARP inhibitors, prompting clinical evaluation of olaparib in a cohort of heavily pretreated Ewing sarcoma tumors. Unfortunately, olaparib activity was disappointing, suggesting an underappreciated resistance mechanism to PARP inhibition in patients with Ewing sarcoma. We sought to elucidate the resistance factors to PARP inhibitor therapy in Ewing sarcoma and identify a rational drug combination capable of rescuing PARP inhibitor activity.

EXPERIMENTAL DESIGN

We employed a pair of cell lines derived from the same patient with Ewing sarcoma prior to and following chemotherapy, a panel of Ewing sarcoma cell lines, and several patient-derived xenograft (PDX) and cell line xenograft models.

RESULTS

We found olaparib sensitivity was diminished following chemotherapy. The matched cell line pair revealed increased expression of the antiapoptotic protein BCL-2 in the chemotherapy-resistant cells, conferring apoptotic resistance to olaparib. Resistance to olaparib was maintained in this chemotherapy-resistant model in vivo, whereas the addition of the BCL-2/X_L inhibitor navitoclax led to tumor growth inhibition. In 2 PDXs, olaparib and navitoclax were minimally effective as monotherapy, yet induced dramatic tumor growth inhibition when dosed in combination. We found that EWS-FLI1 increases BCL-2 expression; however, inhibition of BCL-2 alone by venetoclax is insufficient to sensitize Ewing sarcoma cells to olaparib, revealing a dual necessity for BCL-2 and BCL-X_L in Ewing sarcoma survival.

CONCLUSIONS

These data reveal BCL-2 and BCL-X_L act together to drive olaparib resistance in Ewing sarcoma and reveal a novel, rational combination therapy that may be put forward for clinical trial testing.

Re-inforcing the cell death army in the fight against breast cancer

Metastatic breast cancer is responsible for most breast cancer-related deaths. Disseminated cancer cells have developed an intrinsic ability to resist anchorage-dependent apoptosis (anoikis). Anoikis is caused by the absence of cellular adhesion, a process that underpins lumen formation and maintenance during mammary gland development and homeostasis. In healthy cells, anoikis is mostly governed by B-cell lymphoma-2 (BCL2) protein family members. Metastatic cancer cells, however, have often developed autocrine BCL2-dependent resistance mechanisms to counteract anoikis. In this Review, we discuss how a pro-apoptotic subgroup of the BCL2 protein family, known as the BH3-only proteins, controls apoptosis and anoikis during mammary gland homeostasis and to what extent their inhibition confers tumor suppressive functions in metastatic breast cancer. Specifically, the role of the two pro-apoptotic BH3-only proteins BCL2-modifying factor (BMF) and BCL2-interacting mediator of cell death (BIM) will be discussed here. We assess current developments in treatment that focus on mimicking the function of the BH3-only proteins to induce apoptosis, and consider their applicability to restore normal apoptotic responses in anchorage-independent disseminating tumor cells.

Selective BH3-mimetics targeting BCL-2, BCL-XL or MCL-1 induce severe mitochondrial perturbations

Induction of apoptosis by selective BH3-mimetics is currently investigated as a novel strategy for cancer treatment. Here, we report that selective BH3-mimetics induce apoptosis in a variety of hematological malignancies. Apoptosis is accompanied by severe mitochondrial toxicities upstream of caspase activation. Specifically, the selective BH3-mimetics ABT-199, A-1331852 and S63845, which target BCL-2, BCL-XL and MCL-1, respectively, induce comparable ultrastructural changes including mitochondrial swelling, a decrease of mitochondrial matrix density and severe loss of cristae structure. These shared effects on mitochondrial morphology indicate a similar function of these anti-apoptotic BCL-2 proteins in maintaining mitochondrial integrity and function.

Resistance to Ibrutinib in B Cell Malignancies: One Size Does Not Fit All

Ibrutinib resistance, as a result of coordinated rewiring of signaling networks and enforced tumor microenvironment (TME)-lymphoma interactions, drives unrestrained proliferation and disease progression. To combat resistance mechanisms, we must identify the compensatory resistance pathways and the central modulators of reprogramming events. Targeting the transcriptome and kinome reprogramming of lymphoma cells represents a rational approach to mitigate ibrutinib resistance in B cell malignancies. However, with the apparent heterogeneity and plasticity of tumors shown in therapy response, a one size fits all approach may be unattainable. To this end, a reliable and real-time drug screening platform to tailor effective individualized therapies in patients with B cell malignancies is warranted. Here, we describe the complexity of ibrutinib resistance in B cell lymphomas and the current approaches, including a drug screening assay, which has the potential to further explore the mechanisms of ibrutinib resistance and to design effective individualized combination therapies to overcome resistance and disable aggressive lymphomas (see Outstanding Questions).

Advances in targeted therapy for acute myeloid leukaemia

In the past few years, research in the underlying pathogenic mechanisms of acute myeloid leukaemia (AML) has led to remarkable advances in our understanding of the disease. Cytogenetic and molecular aberrations are the most important factors in determining response to chemotherapy as well as long-term outcome, but beyond prognostication are potential therapeutic targets. Our increased understanding of the pathogenesis of AML, facilitated by next-generation sequencing, has spurred the development of new compounds in the treatment of AML, particularly the creation of small molecules that target the disease on a molecular level. Various new agents, such as tyrosine kinase inhibitors, immune checkpoint inhibitors, monoclonal or bispecific T-cell engager antibodies, metabolic and pro-apoptotic agents are currently investigated within clinical trials. The highest response rates are often achieved when new molecularly targeted therapies are combined with standard chemotherapy. Presented here is an overview of novel therapies currently being evaluated in AML.

Reactivation of Smac-mediated apoptosis in chronic lymphocytic leukemia cells: mechanistic studies of Smac mimetic

Dysfunctional apoptotic machinery is a hallmark feature of chronic lymphocytic leukemia (CLL). Accordingly, targeting apoptosis regulators has been proven a rational approach for CLL treatment. We show that CLL lymphocytes express high levels of XIAP, cIAP1, and cIAP2 compared to normal lymphocytes. Smac mimetic, Smac066, designed to bind to BIR3-domain of IAPs, induce apoptosis in primary CLL cells (n=71; p<0.0001), irrespective of prognostic markers. Apoptosis was mediated by diminished levels of IAPs (XIAP-p=0.02; cIAP-p<0.0001) and increased activation of caspases-8,-9,-3. The caspase-cleavage was in direct association with the levels of apoptosis (r2=0.8 for caspases-8,-9,-3). Correlative analysis revealed a direct relationship between reduction in IAPs and degree of apoptosis (r2=0.6 (XIAP); 0.5 (cIAP2)). There was a strong association between apoptosis, IAP-degradation, and concurrent caspase-activation. Pan-caspase inhibitor Z-Vad-fmk reversed the degradation of Mcl-1, but not IAPs suggesting that smac066 is selective to IAPs, however, Mcl-1 degradation is through caspase-mediated cleavage. Immunoprecipitation experiments revealed physical interaction between caspase-3 and XIAP that was disrupted by smac066. Importantly, XIAP and cIAP2 were markedly induced in bone-marrow and lymph-node microenvironments, providing a basis for IAP antagonists as anti-tumor agents in CLL. Smac066 synergized with ABT-737, revealing a mechanistic rationale to jointly target BH3 and BIR3 domains.

Venetoclax: Targeting BCL2 in Hematological Cancers

Over the last years, targeted anti-cancer therapy with small-molecule inhibitors and antibodies moved to the forefront as a strategy to treat hematological cancers. These novel agents showed outstanding effects in treatment of patients, often irrespective of their underlying genetic features. However, evolution and selection of subclones with continuous treatment leads to disease relapse and resistance toward these novel drugs. Venetoclax (ABT-199) is a novel, orally bioavailable small-molecule inhibitor for selective targeting of B-cell lymphoma 2 (BCL2). Venetoclax is in clinical development and shows high efficacy and safety in particular in the treatment of chronic lymphocytic leukemia (CLL), but preliminarily also in acute myeloid leukemia (AML) and acute lymphoblastic leukemia (ALL). The most important and impressive outcomes of venetoclax treatment include a rapid induction of apoptosis and drastic reduction of the tumor bulk within a few hours after administration. Venetoclax was approved by the FDA and EMA in 2016 for patients with previously treated CLL with del(17p13) and patients failing B cell receptor signaling inhibitors (EMA only), on the basis of a single-arm phase II trial demonstrating a tremendous response rate of 79% with complete remission in 20% of cases and an estimated 1-year progression-free survival of 72%. This review focuses on the mode of action, the preclinical models, and outcomes from various clinical trials with venetoclax in different hematologic cancers as well as future development.

Inhibition of CHK1 enhances cell death induced by the Bcl-2-selective inhibitor ABT-199 in acute myeloid leukemia cells

Resistance to standard chemotherapy agents remains a major obstacle for improving treatment outcomes for acute myeloid leukemia (AML). The Bcl-2-selective inhibitor ABT-199 has demonstrated encouraging preclinical results, drug resistance remains a concern. Mcl-1 has been demonstrated to contribute to ABT-199 resistance, thus combining with therapies that target Mcl-1 could overcome such resistance. In this study, we utilized a CHK1 inhibitor, LY2603618, to decrease Mcl-1 and enhance ABT-199 efficacy. We found that LY2603618 treatment resulted in abolishment of the G2/M cell cycle checkpoint and increased DNA damage, which was partially dependent on CDK activity. LY2603618 treatment resulted in decrease of Mcl-1, which coincided with the initiation of apoptosis. Overexpression of Mcl-1 in AML cells significantly attenuated apoptosis induced by LY2603618, confirming the critical role of Mcl-1 in apoptosis induced by the agent. Simultaneous treatment with LY2603618 and ABT-199 resulted in synergistic induction of apoptosis in both AML cell lines and primary patient samples. Our findings provide new insights into overcoming a mechanism of intrinsic ABT-199 resistance in AML cells and support the clinical development of combined ABT-199 and CHK1 inhibition.

Therapeutic opportunities based on caspase modulation

Caspases are a family of proteolytic enzymes that play a critical role in the regulation of programmed cell death via apoptosis. Activation of caspases is frequently impaired in human cancers, contributing to cancer formation, progression and therapy resistance. A better understanding of the molecular mechanisms regulating caspase activation in cancer cells is therefore highly important. Thus, targeted modulation of caspase activation and apoptosis represents a promising approach for the development of new therapeutic options to elucidate cancer cell death.

Combined targeting of BCL-2 and BCR-ABL tyrosine kinase eradicates chronic myeloid leukemia stem cells

BCR-ABL tyrosine kinase inhibitors (TKIs) are effective against chronic myeloid leukemia (CML), but they rarely eliminate CML stem cells. Disease relapse is common upon therapy cessation, even in patients with complete molecular responses. Furthermore, once CML progresses to blast crisis (BC), treatment outcomes are dismal. We hypothesized that concomitant targeting of BCL-2 and BCR-ABL tyrosine kinase could overcome these limitations. We demonstrate increased BCL-2 expression at the protein level in bone marrow cells, particularly in Lin(-)Sca-1(+)cKit(+) cells of inducible CML in mice, as determined by CyTOF mass cytometry. Further, selective inhibition of BCL-2, aided by TKI-mediated MCL-1 and BCL-XL inhibition, markedly decreased leukemic Lin(-)Sca-1(+)cKit(+) cell numbers and long-term stem cell frequency and prolonged survival in a murine CML model. Additionally, this combination effectively eradicated CD34(+)CD38(-), CD34(+)CD38(+), and quiescent stem/progenitor CD34(+) cells from BC CML patient samples. Our results suggest that BCL-2 is a key survival factor for CML stem/progenitor cells and that combined inhibition of BCL-2 and BCR-ABL tyrosine kinase has the potential to significantly improve depth of response and cure rates of chronic-phase and BC CML.

Reciprocal sensitivity of diffuse large B-cell lymphoma cells to Bcl-2 inhibitors BIRD-2 versus venetoclax

Bcl-2 is often upregulated in cancers to neutralize the BH3-only protein Bim at the mitochondria. BH3 mimetics (e.g. ABT-199 (venetoclax)) kill cancer cells by targeting Bcl-2's hydrophobic cleft and disrupting Bcl-2/Bim complexes. Some cancers with elevated Bcl-2 display poor responses towards BH3 mimetics, suggesting an additional function for anti-apoptotic Bcl-2 in these cancers. Indeed, Bcl-2 via its BH4 domain prevents cytotoxic Ca²⁺ release from the endoplasmic reticulum (ER) by directly inhibiting the inositol 1,4,5-trisphosphate receptor (IP₃R). The cell-permeable Bcl-2/IP₃R disruptor-2 (BIRD-2) peptide can kill these Bcl-2-dependent cancers by targeting Bcl-2's BH4 domain, unleashing pro-apoptotic Ca²⁺-release events. We compared eight "primed to death" diffuse large B-cell lymphoma cell lines (DLBCL) for their apoptotic sensitivity towards BIRD-2 and venetoclax. By determining their IC₅₀ using cytometric cell-death analysis, we discovered a reciprocal sensitivity towards venetoclax versus BIRD-2. Using immunoblotting, we quantified the expression levels of IP₃R2 and Bim in DLBCL cell lysates, revealing that BIRD-2 sensitivity correlated with IP₃R2 levels but not with Bim levels. Moreover, the requirement of intracellular Ca²⁺ for BIRD-2- versus venetoclax-induced cell death was different. Indeed, BAPTA-AM suppressed BIRD-2-induced cell death, but promoted venetoclax-induced cell death in DLBCL cells. Finally, compared to single-agent treatments, combining BIRD-2 with venetoclax synergistically enhanced cell-death induction, correlating with a Ca²⁺-dependent upregulation of Bim after BIRD-2 treatment. Our findings suggest that some cancer cells require Bcl-2 proteins at the mitochondria, preventing Bax activation via its hydrophobic cleft, while others require Bcl-2 proteins at the ER, preventing cytotoxic Ca²⁺-signaling events via its BH4 domain.

Found in Translation: How Preclinical Research Is Guiding the Clinical Development of the BCL2-Selective Inhibitor Venetoclax

Since the discovery of apoptosis as a form of programmed cell death, targeting the apoptosis pathway to induce cancer cell death has been a high-priority goal for cancer therapy. After decades of effort, drug-discovery scientists have succeeded in generating small-molecule inhibitors of antiapoptotic BCL2 family proteins. Innovative medicinal chemistry and structure-based drug design, coupled with a strong fundamental understanding of BCL2 biology, were essential to the development of BH3 mimetics such as the BCL2-selective inhibitor venetoclax. We review a number of preclinical studies that have deepened our understanding of BCL2 biology and facilitated the clinical development of venetoclax.Significance: Basic research into the pathways governing programmed cell death have paved the way for the discovery of apoptosis-inducing agents such as venetoclax, a BCL2-selective inhibitor that was recently approved by the FDA and the European Medicines Agency. Preclinical studies aimed at identifying BCL2-dependent tumor types have translated well into the clinic thus far and will likely continue to inform the clinical development of venetoclax and other BCL2 family inhibitors. Cancer Discov; 7(12); 1376-93. ©2017 AACR.

Pharmacological modulation of cell death in organ transplantation

New options to pharmacologically modulate fundamental mechanisms of regulated cell death are rapidly evolving and found first clinical applications in cancer therapy. Here, we present an overview on how the recent advances in the understanding of the biology and pharmacology of cell death might influence research and clinical practice in solid organ transplantation. Of particular interest are the novel opportunities related to organ preservation and immunomodulation, which might contribute to promote organ repair and to develop more selective ways to modulate allogeneic immune responses to prevent rejection and induce immunological tolerance.

BCL-2: Long and winding path from discovery to therapeutic target

In 1988, the BCL-2 protein was found to promote cancer by limiting cell death rather than enhancing proliferation. This discovery set the wheels in motion for an almost 30 year journey involving many international research teams that has recently culminated in the approval for a drug, ABT-199/venetoclax/Venclexta that targets this protein in the treatment of cancer. This review will describe the long and winding path from the discovery of this protein and understanding the fundamental process of apoptosis that BCL-2 and its numerous homologues control, through to its exploitation as a drug target that is set to have significant benefit for cancer patients.