BH3 Mimetics in AML Therapy: Death and Beyond?
Authors: Claudia Cerella, Mario Dicato, Marc Diederich
Journal: Trends in Pharmacological Sciences (2020)

Acute myeloid leukemia (AML) is efficiently targeted by combinatorial regimens including B cell lymphoma 2 (BCL2) protein antagonists.BCL2 homology domain 3 (BH3) mimetics selectively target leukemic stem cells.Strategies that inhibit BCL2 proteins impair metabolic routes that are preferentially exploited in AML.BCL2 protein antagonists prevent BCL2 family member-dependent paracrine signaling to the AML tumor microenvironment.Inhibition of MCL1 expression/activity modulates the antitumor immune response. B cell lymphoma 2 (BCL2) homology domain 3 (BH3) mimetics are targeted therapeutic agents that allow response prediction and patient stratification. BH3 mimetics are prototypical activators of the mitochondrial death program in cancer. They emerged as important modulators of cellular mechanisms contributing to poor therapeutic responses, including cancer cell stemness, cancer-specific metabolic routes, paracrine signaling to the tumor microenvironment, and immune modulation. We present an overview of the antagonism between BH3 mimetics and antiapoptotic BCL2 proteins. We focus on acute myeloid leukemia (AML), a cancer with reduced therapeutic options that have recently been improved by BH3 mimetics. B cell lymphoma 2 (BCL2) homology domain 3 (BH3) mimetics are targeted therapeutic agents that allow response prediction and patient stratification. BH3 mimetics are prototypical activators of the mitochondrial death program in cancer. They emerged as important modulators of cellular mechanisms contributing to poor therapeutic responses, including cancer cell stemness, cancer-specific metabolic routes, paracrine signaling to the tumor microenvironment, and immune modulation. We present an overview of the antagonism between BH3 mimetics and antiapoptotic BCL2 proteins. We focus on acute myeloid leukemia (AML), a cancer with reduced therapeutic options that have recently been improved by BH3 mimetics. BCL2 family proteins are crucial cell-survival modulators that are aberrantly expressed in several forms of cancer and contribute to disease progression and therapeutic resistance. The development of synthetic small-molecule inhibitors, specifically BH3 mimetics, provides a successful strategy to reactivate cell death signaling in cancer cells. Their mechanism of action is conceptually simple. BH3 mimetics imitate short sequences derived from the BH3 domain of the canonical binding partners of prosurvival BCL2 proteins (the proapoptotic BH3-only proteins), thus preventing BH3-only/prosurvival BCL2 protein family interaction, and eventually inhibiting cell death modulation (Box 1).Box 1The BCL2 Protein Family in CancerCancer cells cope with permanent cellular stress and can withstand a higher stress threshold before dying. Their refractoriness to cell death commitment is a crucial hallmark. The mitochondria are the stage for this prosurvival strategy, and BCL2 family proteins are the major players. In this context, the antiapoptotic members of the BCL2 family, including BCL2L1 (also known as BCLXL), BCL2L2, MCL1, and BCL2-related protein A1 (BCL2A1), antagonize the activation of the proapoptotic members BCL2-associated X apoptosis regulator (BAX) and BCL2 antagonist/killer 1 (BAK1) that are essential for triggering the mitochondrial apoptotic pathway (Figure I). BH3-only proteins act as liaisons between pro- and antiapoptotic family members. Arising in stress conditions, they are functionally different, where the activators (including BCL2-like 11/BCL2L11, also known as BIM), truncated BH3 interacting-domain death agonist (tBID), and BCL2-binding component 3 (BBC3, also known as PUMA) directly bind to BAK1 and BAX to promote their activation, and the sensitizers (including BCL2-associated agonist of cell death, BAD; BCL2-interacting killer, BIK; phorbol-12-myristate-13-acetate-induced protein 1, PMAIP1, also known as NOXA; BCL2-modifying factor, BMF; and harakiri or BCL2-interacting protein, HRK), interact with antiapoptotic BCL2 members to displace them and enable the activating BH3-only proteins and BAK1/BAX to trigger mitochondrial outer membrane permeabilization (MOMP). BH3 mimetics mimic the sensitizers (Figure I). They interact with antiapoptotic BCL2 proteins in a competitive manner, eventually reactivating the death signal. The ability of BH3 mimetics to induce MOMP in the absence of other stimuli is proof of concept that most cancer cells are primed to die and depend on antiapoptotic BCL2 proteins for survival. This vulnerability can be exploited for therapeutic purposes. There are preferential patterns of interaction between anti- and proapoptotic BCL2 family members. Cancer cells derived from different patients (intertumoral heterogeneity), or belonging to distinct subpopulations within the same patient (intratumoral heterogeneity), may rely on different BCL2 proteins for survival. Furthermore, the patterns of BCL2 protein dependency can vary, and therapeutic treatments can change these patterns through the clonal expansion of specific malignant subpopulations. Approaches to monitor the BCL2 dependency of cancer cells could be used to predict responders and to personalize therapies, and these strategies could also be applied after therapy to monitor the potential onset of unresponsiveness (Box 2). Cancer cells cope with permanent cellular stress and can withstand a higher stress threshold before dying. Their refractoriness to cell death commitment is a crucial hallmark. The mitochondria are the stage for this prosurvival strategy, and BCL2 family proteins are the major players. In this context, the antiapoptotic members of the BCL2 family, including BCL2L1 (also known as BCLXL), BCL2L2, MCL1, and BCL2-related protein A1 (BCL2A1), antagonize the activation of the proapoptotic members BCL2-associated X apoptosis regulator (BAX) and BCL2 antagonist/killer 1 (BAK1) that are essential for triggering the mitochondrial apoptotic pathway (Figure I). BH3-only proteins act as liaisons between pro- and antiapoptotic family members. Arising in stress conditions, they are functionally different, where the activators (including BCL2-like 11/BCL2L11, also known as BIM), truncated BH3 interacting-domain death agonist (tBID), and BCL2-binding component 3 (BBC3, also known as PUMA) directly bind to BAK1 and BAX to promote their activation, and the sensitizers (including BCL2-associated agonist of cell death, BAD; BCL2-interacting killer, BIK; phorbol-12-myristate-13-acetate-induced protein 1, PMAIP1, also known as NOXA; BCL2-modifying factor, BMF; and harakiri or BCL2-interacting protein, HRK), interact with antiapoptotic BCL2 members to displace them and enable the activating BH3-only proteins and BAK1/BAX to trigger mitochondrial outer membrane permeabilization (MOMP). BH3 mimetics mimic the sensitizers (Figure I). They interact with antiapoptotic BCL2 proteins in a competitive manner, eventually reactivating the death signal. The ability of BH3 mimetics to induce MOMP in the absence of other stimuli is proof of concept that most cancer cells are primed to die and depend on antiapoptotic BCL2 proteins for survival. This vulnerability can be exploited for therapeutic purposes. There are preferential patterns of interaction between anti- and proapoptotic BCL2 family members. Cancer cells derived from different patients (intertumoral heterogeneity), or belonging to distinct subpopulations within the same patient (intratumoral heterogeneity), may rely on different BCL2 proteins for survival. Furthermore, the patterns of BCL2 protein dependency can vary, and therapeutic treatments can change these patterns through the clonal expansion of specific malignant subpopulations. Approaches to monitor the BCL2 dependency of cancer cells could be used to predict responders and to personalize therapies, and these strategies could also be applied after therapy to monitor the potential onset of unresponsiveness (Box 2). Approval of the first selective BCL2 inhibitor, venetoclax (ABT-199, Venclexta®), by the FDA in 2016 provided an important impulse to the personalized therapeutic use of BH3 mimetics. From a clinical perspective, BCL2 protein dependency became a marker of therapeutic response, thus allowing patient stratification (see Glossary; Box 2). Mechanistically, its emergence fostered the discovery of additional natural and synthetic BH3 mimetics that are selective for other BCL2 family members. Despite the generally positive response to these drugs, primary or secondary forms of resistance have been described. Indeed, pre-existing clones could be dependent on alternative or multiple BCL2 members (BCL2 codependency), and therapy-induced selection of such clones cannot be excluded. Identifying and administrating appropriate BH3 mimetics alone or in combination is currently under preclinical and clinical investigation as a treatment strategy to overcome these forms of resistance.Box 2Diagnostic Tools To Monitor BCL2 Protein DependencyBH3 profiling uses peptides derived from the BH3 domains of BH3-only proteins that recapitulate all possible interaction patterns between BH3-only proteins and antiapoptotic BCL2 members (Box 1). In the original protocol, mitochondria are isolated and probed using the entire panel of peptides. The release of cytochrome c, detected by enzyme-linked immunosorbent assay (ELISA), indicates MOMP and identifies effective peptides derived from sensitizing BH3-only proteins. The concomitant use of peptides derived from BH3-only protein activators confirms the presence of functional BAK1 and BAX in the isolated mitochondria [74.Certo M. et al.Mitochondria primed by death signals determine cellular addiction to antiapoptotic BCL-2 family members.Cancer Cell. 2006; 9: 351-365Abstract Full Text Full Text PDF PubMed Scopus (873) Google Scholar,75.Deng J. et al.BH3 profiling identifies three distinct classes of apoptotic blocks to predict response to ABT-737 and conventional chemotherapeutic agents.Cancer Cell. 2007; 12: 171-185Abstract Full Text Full Text PDF PubMed Scopus (369) Google Scholar]. More recently, this technique has been adapted to primary cells to serve as a therapeutic biomarker for BCL2 dependency. Patient cells undergo partial permeabilization to preserve mitochondrial functionality while allowing the intracellular diffusion of BH3 peptides. MOMP is directly monitored through the use of fluorescent probes (JC-1,5,5′,6,6′-tetrachloro-1,1′,3,3′-tetraethylbenzimidazolocarbo-cyanine iodide or tetramethylrhodamine, TMRE) [76.Ryan J. Letai A. BH3 profiling in whole cells by fluorimeter or FACS.Methods. 2013; 61: 156-164Crossref PubMed Scopus (87) Google Scholar]. BH3 profiling contributed to the elucidation of the mechanisms underlying BCL2 protein dependency. In contrast to lymphoid malignancies, AML cells do not rely exclusively on unique antiapoptotic BCL2 members to survive, and instead frequently develop codependencies [75.Deng J. et al.BH3 profiling identifies three distinct classes of apoptotic blocks to predict response to ABT-737 and conventional chemotherapeutic agents.Cancer Cell. 2007; 12: 171-185Abstract Full Text Full Text PDF PubMed Scopus (369) Google Scholar,77.Butterworth M. et al.BH3 profiling and a toolkit of BH3-mimetic drugs predict anti-apoptotic dependence of cancer cells.Br. J. Cancer. 2016; 114: 638-641Crossref PubMed Scopus (7) Google Scholar] or turnover of BCL2 proteins [78.Moujalled D.M. et al.Combining BH3-mimetics to target both BCL-2 and MCL1 has potent activity in pre-clinical models of acute myeloid leukemia.Leukemia. 2019; 33: 905-917Crossref PubMed Scopus (0) Google Scholar]. BH3 profiling also allows monitoring of resistance that arises from the selection and expansion of pre-existing minor clones bearing different patterns of BCL2 dependency [79.Ramsey H.E. et al.A novel MCL1 inhibitor combined with venetoclax rescues venetoclax-resistant acute myelogenous leukemia.Cancer Discov. 2018; 8: 1566-1581Crossref PubMed Scopus (67) Google Scholar]. This outcome suggests that a combination of agents targeting diverse dependencies (not limited to BCL2 family proteins) is an essential strategy to improve CR rates and OS in AML patients.The concept of BCL2 dependency has inspired several approaches aimed at engineering BCL2 addiction to test the efficacy and selectivity of new BCL2 inhibitors. Cells are engineered to coexpress specific pairs of pro- and antiapoptotic BCL2 proteins and become 'mito-primed' [80.Lopez J. et al.Mito-priming as a method to engineer Bcl-2 addiction.Nat. Commun. 2016; 710538Crossref PubMed Google Scholar, 81.Koss B. et al.Defining specificity and on-target activity of BH3-mimetics using engineered B-ALL cell lines.Oncotarget. 2016; 7: 11500-11511Crossref PubMed Scopus (15) Google Scholar, 82.Lopez J. Tait S.W.G. Application of mito-priming to generate BCL-2 addicted cells.Methods Mol. Biol. 2019; 1877: 45-60Crossref PubMed Scopus (0) Google Scholar]. The proapoptotic member is tagged with GFP, allowing the monitoring of its localization to mitochondria and any perturbation of this condition by immunofluorescence. In addition to assessing the selectivity and potency of new inhibitors, mito-priming profiles any preferential interactions between BH3-only proteins and their effectors BAX and BAK1. Importantly, this technique can be combined with clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9-based genome editing to corroborate mechanistic aspects. CRISPR/Cas9 screening can also be used in combination with drug screening approaches. For example, combining this methodology with a BCL2 inhibitor screen on a broad panel of cell models helped to identify the epithelial–mesenchymal transition (EMT) as a novel mechanism influencing BCL2 dependency in solid tumors [83.Soderquist R.S. et al.Systematic mapping of BCL-2 gene dependencies in cancer reveals molecular determinants of BH3 mimetic sensitivity.Nat. Commun. 2018; 9: 3513Crossref PubMed Scopus (28) Google Scholar]. BH3 profiling uses peptides derived from the BH3 domains of BH3-only proteins that recapitulate all possible interaction patterns between BH3-only proteins and antiapoptotic BCL2 members (Box 1). In the original protocol, mitochondria are isolated and probed using the entire panel of peptides. The release of cytochrome c, detected by enzyme-linked immunosorbent assay (ELISA), indicates MOMP and identifies effective peptides derived from sensitizing BH3-only proteins. The concomitant use of peptides derived from BH3-only protein activators confirms the presence of functional BAK1 and BAX in the isolated mitochondria [74.Certo M. et al.Mitochondria primed by death signals determine cellular addiction to antiapoptotic BCL-2 family members.Cancer Cell. 2006; 9: 351-365Abstract Full Text Full Text PDF PubMed Scopus (873) Google Scholar,75.Deng J. et al.BH3 profiling identifies three distinct classes of apoptotic blocks to predict response to ABT-737 and conventional chemotherapeutic agents.Cancer Cell. 2007; 12: 171-185Abstract Full Text Full Text PDF PubMed Scopus (369) Google Scholar]. More recently, this technique has been adapted to primary cells to serve as a therapeutic biomarker for BCL2 dependency. Patient cells undergo partial permeabilization to preserve mitochondrial functionality while allowing the intracellular diffusion of BH3 peptides. MOMP is directly monitored through the use of fluorescent probes (JC-1,5,5′,6,6′-tetrachloro-1,1′,3,3′-tetraethylbenzimidazolocarbo-cyanine iodide or tetramethylrhodamine, TMRE) [76.Ryan J. Letai A. BH3 profiling in whole cells by fluorimeter or FACS.Methods. 2013; 61: 156-164Crossref PubMed Scopus (87) Google Scholar]. BH3 profiling contributed to the elucidation of the mechanisms underlying BCL2 protein dependency. In contrast to lymphoid malignancies, AML cells do not rely exclusively on unique antiapoptotic BCL2 members to survive, and instead frequently develop codependencies [75.Deng J. et al.BH3 profiling identifies three distinct classes of apoptotic blocks to predict response to ABT-737 and conventional chemotherapeutic agents.Cancer Cell. 2007; 12: 171-185Abstract Full Text Full Text PDF PubMed Scopus (369) Google Scholar,77.Butterworth M. et al.BH3 profiling and a toolkit of BH3-mimetic drugs predict anti-apoptotic dependence of cancer cells.Br. J. Cancer. 2016; 114: 638-641Crossref PubMed Scopus (7) Google Scholar] or turnover of BCL2 proteins [78.Moujalled D.M. et al.Combining BH3-mimetics to target both BCL-2 and MCL1 has potent activity in pre-clinical models of acute myeloid leukemia.Leukemia. 2019; 33: 905-917Crossref PubMed Scopus (0) Google Scholar]. BH3 profiling also allows monitoring of resistance that arises from the selection and expansion of pre-existing minor clones bearing different patterns of BCL2 dependency [79.Ramsey H.E. et al.A novel MCL1 inhibitor combined with venetoclax rescues venetoclax-resistant acute myelogenous leukemia.Cancer Discov. 2018; 8: 1566-1581Crossref PubMed Scopus (67) Google Scholar]. This outcome suggests that a combination of agents targeting diverse dependencies (not limited to BCL2 family proteins) is an essential strategy to improve CR rates and OS in AML patients. The concept of BCL2 dependency has inspired several approaches aimed at engineering BCL2 addiction to test the efficacy and selectivity of new BCL2 inhibitors. Cells are engineered to coexpress specific pairs of pro- and antiapoptotic BCL2 proteins and become 'mito-primed' [80.Lopez J. et al.Mito-priming as a method to engineer Bcl-2 addiction.Nat. Commun. 2016; 710538Crossref PubMed Google Scholar, 81.Koss B. et al.Defining specificity and on-target activity of BH3-mimetics using engineered B-ALL cell lines.Oncotarget. 2016; 7: 11500-11511Crossref PubMed Scopus (15) Google Scholar, 82.Lopez J. Tait S.W.G. Application of mito-priming to generate BCL-2 addicted cells.Methods Mol. Biol. 2019; 1877: 45-60Crossref PubMed Scopus (0) Google Scholar]. The proapoptotic member is tagged with GFP, allowing the monitoring of its localization to mitochondria and any perturbation of this condition by immunofluorescence. In addition to assessing the selectivity and potency of new inhibitors, mito-priming profiles any preferential interactions between BH3-only proteins and their effectors BAX and BAK1. Importantly, this technique can be combined with clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9-based genome editing to corroborate mechanistic aspects. CRISPR/Cas9 screening can also be used in combination with drug screening approaches. For example, combining this methodology with a BCL2 inhibitor screen on a broad panel of cell models helped to identify the epithelial–mesenchymal transition (EMT) as a novel mechanism influencing BCL2 dependency in solid tumors [83.Soderquist R.S. et al.Systematic mapping of BCL-2 gene dependencies in cancer reveals molecular determinants of BH3 mimetic sensitivity.Nat. Commun. 2018; 9: 3513Crossref PubMed Scopus (28) Google Scholar]. BH3 mimetics became a suitable tool to explore the regulatory functions of BCL2 family proteins beyond their role in BCL2 resistance; in addition, they helped to extend the network of BCL2 protein binding partners. These research efforts allowed the discovery of modulatory mechanisms that are potentially relevant for innovating therapeutic approaches by targeting processes such as cancer cell stemness, metabolic functions, signaling to the microenvironment, and immune interactions. Elucidation of these mechanisms, whether interconnected or not with the canonical function of BCL2 family proteins in the modulation of mitochondrial outer-membrane permeabilization (MOMP), will provide new rationales for the therapeutic use of BH3 mimetics. In this context, AML, a disease exhibiting dismal survival rates, has become an important model for investigation. AML is a hematological type of cancer characterized by inhibition of myeloid differentiation, with consequent clonal expansion and accumulation of immature myeloid blasts within the bone marrow (BM) and blood [1.Short N.J. et al.Acute myeloid leukaemia.Lancet. 2018; 392: 593-606Abstract Full Text Full Text PDF PubMed Google Scholar]. This disease typically affects adults (80% of cases) and is particularly prevalent among elderly individuals. AML is one of the most aggressive forms of blood cancer, and patients exhibit poor prognosis and limited 5 year overall survival (OS; 27.8%). The ineligibility of a subset of patients to undergo intensive cycles of chemotherapy, a high rate of relapse even in patients with apparent complete remission (CR) after therapy, and limited access to allogeneic hematopoietic stem cell transplantation (HSCT) are the major reasons for these unfavorable outcomes. AML was treated for many years using a standard 'one-size-fits-all' therapeutic regimen. Since 2017, new drugs, including targeted agents, have been approved for the treatment of AML. These new drugs have substantially replaced standard therapies during the induction therapy and/or consolidation therapy phases, ultimately allowing patient stratification before the implementation of novel personalized therapeutic approaches [1.Short N.J. et al.Acute myeloid leukaemia.Lancet. 2018; 392: 593-606Abstract Full Text Full Text PDF PubMed Google Scholar]. Venetoclax is one such newly approved therapeutic agent. The public availability of data regarding patients who were enrolled into clinical trials has progressively shed light on the mechanisms responsible for the high AML relapse rate and allowed researchers and clinicians to highlight new targetable mechanisms, including BCL2 protein inhibition. Venetoclax (ABT-199; GDC-0199) was approved in 2018 in combination with the hypomethylating agent azacytidine or low-dose cytarabine (LDAC) for the treatment of newly diagnosed AML patients who are unfit for intensive chemotherapy (Table 1 and Box 3). It is also under evaluation for the treatment of myelodysplastic syndrome [2.Jilg S. et al.Venetoclax with azacitidine targets refractory MDS but spares healthy hematopoiesis at tailored dose.Exp. Hematol. Oncol. 2019; 8: 9Crossref PubMed Scopus (8) Google Scholar]. Initially developed to overcome the on-target side effects of thrombocytopenia caused by the broader BCL2/BCL2-like 1 (BCL2L1)/BCL2-like 2 (BCL2L2) inhibitor navitoclax (ABT-263; Table 1) [3.Souers A.J. et al.ABT-199, a potent and selective BCL-2 inhibitor, achieves antitumor activity while sparing platelets.Nat. Med. 2013; 19: 202-208Crossref PubMed Scopus (1476) Google Scholar], its safety and pharmacokinetics have been improved through the use of suitable dosing protocols and prophylaxis to avoid its most dangerous side effects [4.Konopleva M. et al.Efficacy and biological correlates of response in a phase II study of venetoclax monotherapy in patients with acute myelogenous leukemia.Cancer Discov. 2016; 6: 1106-1117Crossref PubMed Scopus (337) Google Scholar, 5.DiNardo C.D. et al.Safety and preliminary efficacy of venetoclax with decitabine or azacitidine in elderly patients with previously untreated acute myeloid leukaemia: a non-randomised, open-label, phase 1b study.Lancet Oncol. 2018; 19: 216-228Abstract Full Text Full Text PDF PubMed Scopus (214) Google Scholar, 6.Pollyea D.A. et al.Venetoclax with azacitidine disrupts energy metabolism and targets leukemia stem cells in patients with acute myeloid leukemia.Nat. Med. 2018; 24: 1859-1866Crossref PubMed Scopus (110) Google Scholar]. Recent clinical developments in regard to venetoclax allow this drug to target BCL2-dependent AML subgroups, including those possessing isocitrate dehydrogenase 1/2 (IDH1/2) mutations [7.Chan S.M. et al.Isocitrate dehydrogenase 1 and 2 mutations induce BCL-2 dependence in acute myeloid leukemia.Nat. Med. 2015; 21: 178-184Crossref PubMed Scopus (235) Google Scholar], or patients with a dependency in response to other therapeutic regimens (synthetic lethality [8.Pan R. et al.Synthetic lethality of combined Bcl-2 inhibition and p53 activation in AML: mechanisms and superior antileukemic efficacy.Cancer Cell. 2017; 32: 748-760Abstract Full Text Full Text PDF PubMed Scopus (65) Google Scholar]). Although this drug achieves excellent CR levels and fast responses, a limitation of venetoclax is the durability of its clinical response [9.Leverson J.D. et al.Found in translation: how preclinical research is guiding the clinical development of the BCL2–selective inhibitor venetoclax.Cancer Discov. 2017; 7: 1376-1393Crossref PubMed Scopus (0) Google Scholar,10.Muller-Tidow C. Schlenk R.F. A new option for remission induction in acute myeloid leukaemia.Lancet Oncol. 2018; 19: 156-157Abstract Full Text Full Text PDF PubMed Scopus (0) Google Scholar]. The outgrowth of AML clones expressing other BCL2 family members that replace BCL2 as prosurvival factors is a major reason for this limitation. In particular, MCL1 (apoptosis regulator, BCL2 family member) is the Achilles heel of venetoclax-based therapy, and this has driven research efforts to develop selective MCL1-targeting approaches.Table 1Chemical Structures of BCL2 Antagonists (Direct Inhibitors and Downregulators)aAbbreviations: ALL, acute lymphoblastic leukemia; DNMT1, DNA methyltransferase 1; eIF4A, eukaryotic initiation factor 4A; MAPK, mitogen-activated protein kinase; MDM2, MDM2 proto-oncogene; MM, multiple myeloma; NKA, sodium/potassium-ATPase; R/R, relapsed/refractory.Compound nameStructureTargetsCommentClinical statusRefsBH3 mimetics targeting BCL2Venetoclax(ABT-199,GDC-0199)BCL2Ki <0.01 nMPotent and selective BCL2 inhibitor; BAD-like BH3 mimeticIt is associated with a fast response in elderly AML patients that are ineligible for other therapies. Good tolerabilitySuitable for AML with IDH1/2 mutations or developing BCL2 dependency in response to other therapeutic regimensLimitations: fast but limited durability of response owing to outgrowth of resistant AML clones; tumor lysis syndrome now controlled with dose escalation administration and suitable prophylaxisApproved since 2017 in combination with the hypomethylating agent azacytidine or LDAC for the treatment of patients with AML who are unfit for chemotherapyUnder evaluation for the treatment of MDSIn clinical trials involving combinational regimens[2.Jilg S. et al.Venetoclax with azacitidine targets refractory MDS but spares healthy hematopoiesis at tailored dose.Exp. Hematol. Oncol. 2019; 8: 9Crossref PubMed Scopus (8) Google Scholar,4.Konopleva M. et al.Efficacy and biological correlates of response in a phase II study of venetoclax monotherapy in patients with acute myelogenous leukemia.Cancer Discov. 2016; 6: 1106-1117Crossref PubMed Scopus (337) Google Scholar,5.DiNardo C.D. et al.Safety and preliminary efficacy of venetoclax with decitabine or azacitidine in elderly patients with previously untreated acute myeloid leukaemia: a non-randomised, open-label, phase 1b study.Lancet Oncol. 2018; 19: 216-228Abstract Full Text Full Text PDF PubMed Scopus (214) Google Scholar,10.Muller-Tidow C. Schlenk R.F. A new option for remission induction in acute myeloid leukaemia.Lancet Oncol. 2018; 19: 156-157Abstract Full Text Full Text PDF PubMed Scopus (0) Google Scholar,67.DiNardo C.D. et al.Molecular patterns of response and treatment failure after frontline venetoclax combinations in older patients with AML.Blood. 2020; 135: 791-803Crossref PubMed Google Scholar]Navitoclax(ABT-263)BCL2, BCL2L1, and BCL2L2Ki ≤0.5–3.3 nMOral selective inhibitor, BAD-like BH3 mimetic, orally available derivative of ABT-737Limitations: on-target effects on platelets of thrombocytopenia, currently manageable with suitable prophylaxis; more recently found to impact on healthy blood progenitors depending on BCL2L1In clinical trials involving solid tumors or hematological malignancies other than AML (e.g., ALL)(NCT03592576)(NCT03181126)[96.Kivioja J.L. et al.Dasatinib and navitoclax act synergistically to target NUP98-NSD1+/FLT3-ITD+ acute myeloid leukemia.Leukemia. 2019; 33: 1360-1372Crossref PubMed Scopus (5) Google Scholar,97.Anstee N.S. et al.Impact of elevated anti-apoptotic MCL-1 and BCL-2 on the development and treatment of MLL-AF9 AML in mice.Cell Death Differ. 2019; 26: 1316-1331Crossref PubMed Scopus (10) Google Scholar]ABT-737BCL2, BCL2L1, and BCL2L2Ki <0.1 nMPrototypical BH3 mimeticInduces the disruption of BCL2/BAK1 complexes by mimicking BADBinds with much lower affinity to BCL2A1 and MCL1 (Ki 0.46 μM)Not orally available; intraperitoneal administration in mouse modelsExperimental[97.Anstee N.S. et al.Impact of elevated anti-apoptotic MCL-1 and BCL-2 on the development and treatment of MLL-AF9 AML in mice.Cell Death Differ. 2019; 26: 1316-1331Crossref PubMed Scopus (10) Google Scholar,98.Pan R. et al.Inhibition of Mcl-1 with the pan-Bcl-2 family inhibitor (−)BI97D6 overcomes ABT-737 resistance in acute myeloid leukemia.Blood. 2015; 126: 363-372Crossref PubMed Scopus (0) Google Scholar]BH3 mimetics targeting MCL1A-1210477MCL1Ki 0.45 nMFirst direct MCL1 binder and inhibitor with relevant selective potencyDerived from indole-2-carboxylic acids, disrupts both MCL1–BCL2L11 and MCL1–PMAIP1 complexes. Synergy with venetoclax and navitoclaxLimitations: bioavailability and potency compared to more recent MCL1 inhibitors; lacks in vivo studiesResearch toolPrecluded from further clinical consideration[99.Leverson J.D. et al.Exploiting selective BCL-2 family inhibitors to dissect cell survival dependencies and define improved strategies for cancer therapy.Sci. Transl. Med. 2015; 7279ra240Crossref Scopus (249) Google Scholar]AZD5991MCL1Ki 0.13 nMMacrocyclic molecule;particularly active against hematological malignancies including MM and AMLCytotoxic activity against sensitive cell lines, synergy in combination with venetoclax in AML (and bortezomib in MM); tumor regression in vivo (in murine and rat models), with evidence of good tolerability even at high dosesIn clin