Phenotype-based drug screening reveals association between venetoclax response and differentiation stage in acute myeloid leukemia

MCL-1 inhibitors, fast-lane development of a new class of anti-cancer agents

Cell death escape is one of the most prominent features of tumor cells and closely linked to the dysregulation of members of the Bcl-2 family of proteins. Among those, the anti-apoptotic family member myeloid cell leukemia-1 (MCL-1) acts as a master regulator of apoptosis in various human malignancies. Irrespective of its unfavorable structure profile, independent research efforts recently led to the generation of highly potent MCL-1 inhibitors that are currently evaluated in clinical trials. This offers new perspectives to target a so far undruggable cancer cell dependency. However, a detailed understanding about the tumor and tissue type specific implications of MCL-1 are a prerequisite for the optimal (i.e., precision medicine guided) use of this novel drug class. In this review, we summarize the major functions of MCL-1 with a special focus on cancer, provide insights into its different roles in solid vs. hematological tumors and give an update about the (pre)clinical development program of state-of-the-art MCL-1 targeting compounds. We aim to raise the awareness about the heterogeneous role of MCL-1 as drug target between, but also within tumor entities and to highlight the importance of rationale treatment decisions on a case by case basis.

Ex Vivo Analysis of Primary Tumor Specimens for Evaluation of Cancer Therapeutics

The use of ex vivo drug sensitivity testing to predict drug activity in individual patients has been actively explored for almost 50 years without delivering a generally useful predictive capability. However, extended failure should not be an indicator of futility. This is especially true in cancer research where ultimate success is often preceded by less successful attempts. For example, both immune- and genetic-based targeted therapies for cancer underwent numerous failed attempts before biological understanding, improved targets, and optimized drug development matured to facilitate an arsenal of transformational drugs. Similarly, the concept of directly assessing drug sensitivity of primary tumor biopsies-and the use of this information to help direct therapeutic approaches-has a long history with a definitive learning curve. In this review, we will survey the history of ex vivo testing as well as the current state of the art for this field. We will present an update on methodologies and approaches, describe the use of these technologies to test cutting-edge drug classes, and describe an increasingly nuanced understanding of tumor types and models for which this strategy is most likely to succeed. We will consider the relative strengths and weaknesses of predicting drug activity across the broad biological context of cancer patients and tumor types. This will include an analysis of the potential for ex vivo drug sensitivity testing to accurately predict drug activity within each of the biological hallmarks of cancer pathogenesis.

Targeting Bcl-2 Proteins in Acute Myeloid Leukemia

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

Differentiation of acute myeloid leukemia (AML) cells with ATRA reduces 18F-FDG uptake and increases sensitivity towards ABT-737-induced apoptosis

Acute myeloid leukemia (AML) is a malignant disease of the bone marrow, comprising various subtypes. We have investigated seven different AML cell lines that showed different sensitivities toward the inducer of apoptosis ABT-737, with IC₅₀ concentrations ranging from 9.9 nM to 1.8 µM. Besides, the AML cell lines revealed distinct differences in ¹⁸F-FDG uptake ranging from 4.1 to 11.0%. Moreover, the Pearson coefficient (0.363) suggests a moderate correlation between ¹⁸F-FDG uptake and the IC₅₀ values of ABT-737. Differentiation of the AML cell lines NB-4 and AML-193 with all-trans-retinoic-acid (ATRA) induced a significant increase in sensitivity towards ABT-737 along with a reduced uptake of ¹⁸F-FDG. Therefore, ¹⁸F-FDG uptake could be predictive on sensitivity to treatment with ABT-737. Furthermore, because differentiation treatment of AML cells using ATRA reduced ¹⁸F-FDG uptake and increased sensitivity towards ABT-737, a combined treatment regimen with ATRA and ABT-737 might be a promising therapeutic option in the future.

Fatty acid metabolism underlies venetoclax resistance in acute myeloid leukemia stem cells

Venetoclax with azacitidine (ven/aza) has emerged as a promising regimen for acute myeloid leukemia (AML), with a high percentage of clinical remissions in newly diagnosed patients. However, approximately 30% of newly diagnosed and the majority of relapsed patients do not achieve remission with ven/aza. We previously reported that ven/aza efficacy is based on eradication of AML stem cells through a mechanism involving inhibition of amino acid metabolism, a process which is required in primitive AML cells to drive oxidative phosphorylation. Herein we demonstrate that resistance to ven/aza occurs via up-regulation of fatty acid oxidation (FAO), which occurs due to RAS pathway mutations, or as a compensatory adaptation in relapsed disease. Utilization of FAO obviates the need for amino acid metabolism, thereby rendering ven/aza ineffective. Pharmacological inhibition of FAO restores sensitivity to ven/aza in drug resistant AML cells. We propose inhibition of FAO as a therapeutic strategy to address ven/aza resistance.

Ex vivo cultures and drug testing of primary acute myeloid leukemia samples: Current techniques and implications for experimental design and outcome

New treatment options of acute myeloid leukemia (AML) are rapidly emerging. Pre-clinical models such as ex vivo cultures are extensively used towards the development of novel drugs and to study synergistic drug combinations, as well as to discover biomarkers for both drug response and anti-cancer drug resistance. Although these approaches empower efficient investigation of multiple drugs in a multitude of primary AML samples, their translational value and reproducibility are hampered by the lack of standardized methodologies and by culture system-specific behavior of AML cells and chemotherapeutic drugs. Moreover, distinct research questions require specific methods which rely on specific technical knowledge and skills. To address these aspects, we herein review commonly used culture techniques in light of diverse research questions. In addition, culture-dependent effects on drug resistance towards commonly used drugs in the treatment of AML are summarized including several pitfalls that may arise because of culture technique artifacts. The primary aim of the current review is to provide practical guidelines for ex vivo primary AML culture experimental design.

Single-center pediatric experience with venetoclax and azacitidine as treatment for myelodysplastic syndrome and acute myeloid leukemia

BACKGROUND

The BCL-2 inhibitor venetoclax (ven) has revolutionized the treatment of acute myeloid leukemia (AML) in elderly adults, leading to its recent FDA approval for this population in combination regimens. Although extensive data exist for adult myeloid malignancies, there are limited preclinical data on the efficacy and/or dosing of venetoclax for pediatric myelodysplastic syndrome (MDS) or AML and thus little information to guide use of this regimen in pediatric patients. Our objective was to describe our single-center experience with venetoclax in combination with the hypomethylating agent 5-azacitidine (aza) in pediatric patients with MDS or AML.

PROCEDURE

We conducted a retrospective chart review of patients treated at Children's Hospital Colorado prior to March 2020 with at least one cycle of ven/aza. Patients were included if between the ages of 1 and 25 years with a diagnosis of high-grade MDS or AML. AML patients had relapsed or primary refractory disease or were deemed poor candidates for standard chemotherapy.

RESULTS

Eight patients received ven/aza, two for high-grade MDS and six for AML. Ven/aza was well tolerated by all patients. The most common adverse events seen with this regimen were gastrointestinal and hematologic. Morphologic responses were seen in six patients including both patients with MDS. All four AML responders became minimal residual disease negative. Three responders have thus far proceeded to allogeneic hematopoietic stem cell transplant following ven/aza.

CONCLUSIONS

Our clinical experience suggests that ven/aza is a safe and promising regimen that should be further explored with late-phase clinical trials.

Absence of BCL-2 Expression Identifies a Subgroup of AML with Distinct Phenotypic, Molecular, and Clinical Characteristics

Acute myeloid leukemia (AML) is a hematologic malignancy characterized by the rapid and uncontrolled clonal growth of myeloid lineage cells in the bone marrow. The advent of oral, selective inhibitors of the B-cell leukemia/lymphoma-2 (BCL-2) apoptosis pathway, such as venetoclax, will likely induce a paradigm shift in the treatment of AML. However, the high cost of this treatment and the risk of additive toxicity when used in combination with standard chemotherapy represent limitations to its use and underscore the need to identify which patients are most—and least—likely to benefit from incorporation of venetoclax into the treatment regimen. Bone marrow specimens from 93 newly diagnosed AML patients were collected in this study and evaluated for BCL-2 protein expression by immunohistochemistry. Using this low-cost, easily, and readily applicable analysis method, we found that 1 in 5 AML patients can be considered as BCL-2⁻. In addition to a lower bone marrow blast percentage, this group exhibited a favorable molecular profile characterized by lower WT1 expression and underrepresentation of FLT3 mutations. As compared to their BCL-2⁺ counterparts, the absence of BCL-2 expression was associated with a favorable response to standard chemotherapy and overall survival, thus potentially precluding the necessity for venetoclax add-on.

Integrated analysis of patient samples identifies biomarkers for venetoclax efficacy and combination strategies in acute myeloid leukemia

Deregulation of the BCL2 gene family plays an important role in the pathogenesis of acute myeloid leukemia (AML). The BCL2 inhibitor, venetoclax, has received FDA approval for the treatment of AML. However, upfront and acquired drug resistance ensues due, in part, to the clinical and genetic heterogeneity of AML, highlighting the importance of identifying biomarkers to stratify patients onto the most effective therapies. By integrating clinical characteristics, exome and RNA sequencing, and inhibitor data from primary AML patient samples, we determined that myelomonocytic leukemia, upregulation of BCL2A1 and CLEC7A, as well as mutations of PTPN11 and KRAS conferred resistance to venetoclax and multiple venetoclax combinations. Venetoclax in combination with an MCL1 inhibitor AZD5991 induced synthetic lethality and circumvented venetoclax resistance.

How we treat older patients with acute myeloid leukaemia

After decades when intensive chemotherapy remained the only effective anti-acute myeloid leukaemia (AML) treatment, a torrent of novel, less toxic agents are about to revolutionise AML therapy. Prolonged remissions with good quality of life become achievable for many patients previously considered only for palliative care because they could not tolerate intensive therapy. As treatment options multiply, the importance of genetic profile is recognised, even for advanced-age patients for whom cure is unlikely. With lack of randomised comparative trials for most treatment regimens, one can only extrapolate data from existing studies to make evidence-based decisions. We herein present seven common clinical scenarios illustrating the complexity of treating older AML patients and describe our approach to their management. In each case, up-to-date data on relevant agents to be offered to a particular patient are discussed. The current review is limited to the drugs, available and approved in the Western world and many promising agents, still under investigation, are not discussed.

Monocytic Subclones Confer Resistance to Venetoclax-Based Therapy in Patients with Acute Myeloid Leukemia

Venetoclax-based therapy can induce responses in approximately 70% of older previously untreated patients with acute myeloid leukemia (AML). However, up-front resistance as well as relapse following initial response demonstrates the need for a deeper understanding of resistance mechanisms. In the present study, we report that responses to venetoclax +azacitidine in patients with AML correlate closely with developmental stage, where phenotypically primitive AML is sensitive, but monocytic AML is more resistant. Mechanistically, resistant monocytic AML has a distinct transcriptomic profile, loses expression of venetoclax target BCL2, and relies on MCL1 to mediate oxidative phosphorylation and survival. This differential sensitivity drives a selective process in patients which favors the outgrowth of monocytic subpopulations at relapse. Based on these findings, we conclude that resistance to venetoclax + azacitidine can arise due to biological properties intrinsic to monocytic differentiation. We propose that optimal AML therapies should be designed so as to independently target AML subclones that may arise at differing stages of pathogenesis. SIGNIFICANCE: Identifying characteristics of patients who respond poorly to venetoclax-based therapy and devising alternative therapeutic strategies for such patients are important topics in AML. We show that venetoclax resistance can arise due to intrinsic molecular/metabolic properties of monocytic AML cells and that such properties can potentially be targeted with alternative strategies.