Purpose:

The BCL2 inhibitor, venetoclax, has transformed clinical care in acute myeloid leukemia (AML). However, subsets of patients do not respond or eventually acquire resistance. Venetoclax-based regimens can lead to considerable marrow suppression in some patients. Bromodomain and extraterminal inhibitors (BETi) are potential treatments for AML, as regulators of critical AML oncogenes. We tested the efficacy of novel BET inhibitor INCB054329, and its synergy with venetoclax to reduce AML without induction of hematopoietic toxicity.

Experimental Design:

INCB054329 efficacy was assessed by changes in cell cycle and apoptosis in treated AML cell lines. In vivo efficacy was assessed by tumor reduction in MV-4-11 cell line–derived xenografts. Precision run-on and sequencing (PRO-seq) evaluated effects of INCB054329. Synergy between low-dose BETi and venetoclax was assessed in cell lines and patient samples in vitro and in vivo while efficacy and toxicity was assessed in patient-derived xenograft (PDX) models.

Results:

INCB054329 induced dose-dependent apoptosis and quiescence in AML cell lines. PRO-seq analysis evaluated the effects of INCB054329 on transcription and confirmed reduced transcriptional elongation of key oncogenes, MYC and BCL2, and genes involved in the cell cycle and metabolism. Combinations of BETi and venetoclax led to reduced cell viability in cell lines and patient samples. Low-dose combinations of INCB054329 and venetoclax in cell line and PDX models reduced AML burden, regardless of the sensitivity to monotherapy without development of toxicity.

Conclusions:

Our findings suggest low dose combinations of venetoclax and BETi may be more efficacious for patients with AML than either monotherapy, potentially providing a longer, more tolerable dosing regimen.

Translational Relevance

Overexpression of the antiapoptotic protein, BCL2, is common in AML. Despite the use of the targeted BCL2 inhibitor venetoclax, some venetoclax-treated patients do not respond, with most treatment responsive patients eventually acquiring resistance. Furthermore, while the combination of venetoclax and DNA methyltransferase inhibitors has proven successful, myelosuppression is common and can be severe. In efforts to test drug combinations aimed at increasing venetoclax response without toxicity, we sought to test the efficacy of a novel bromodomain extraterminal domain inhibitor (BETi), INCB054329 in AML and the capacity of BETi to synergize with BCL2 inhibition. We found that within AML blast cells, BETi decreased production of BCL2 and halted the cell cycle which allowed for an effective combination with lower dose venetoclax, resulting in negligible marrow toxicities. This novel combination and alternative treatment regimens may translate to a safer combination in AML.

Acute myeloid leukemia (AML) is characterized by uncontrolled proliferation of myeloblasts and arrest of normal hematopoietic differentiation, resulting in a heterogeneous clonal disease with an aggressive clinical course. Many pathogenic genetic aberrations in AML are associated with dysregulated MYC expression, including: common translocations such as RUNX1-ETO, MLL, and PML-RARalpha, MYC amplifications, (1, 2) aberrations frequently found in NPM1 (3, 4), and FLT3-ITD (5, 6). Notably, the BET family protein, BRD4, regulates the transcription of MYC target genes, and disruption of BRD4 at the MYC locus results in the silencing of MYC gene expression and reduced cell proliferation (6, 7). Because MYC is dysregulated across genomic subtypes of AML, BET family proteins are attractive targets in antileukemic therapy. Early clinical trials with BETi have reported mixed results in both solid and hematologic malignancies, (8, 9) but enthusiasm with the use of BETi in combination regimens continues to grow (10–13).

Overexpression of the antiapoptotic protein, BCL2, is another common characteristic of AML (14, 15). Venetoclax (VEN), a BCL2-specific inhibitor, in combination with DNA methyltransferase inhibitors (DNMTI—decitabine or azacitidine) or low-dose cytarabine (LDAC) was recently FDA approved for the treatment of AML. These combinations exhibit a complete response (CR) or CR with incomplete blood count recovery of up to 70% of older patients deemed unfit to receive induction cytotoxic chemotherapy (16, 17). These responses appear to also translate to overall survival benefit, and venetoclax-based therapy is quickly entering into the standard of care for patients > 65 years of age with a new diagnosis of AML. Despite this success, refractory disease or acquired resistance ultimately occurs in most patients, and coadministration of venetoclax with cytotoxic agents incurs significant hematologic toxicity which remains an issue in the clinic (16, 18–20).

Here, we investigated the BETi, INCB054329, and its synergistic potential with venetoclaxto induce leukemia cell death. We show that BETi leads to apoptosis in AML cell lines and patient samples and use precision nuclear run-on transcription sequencing (PRO-seq) to confirm reduced transcriptional elongation of key oncogenes and known BETi targets such as MYC and BCL2. In addition, BETi reduced cell cycling and altered mitochondrial stress responses in AML cell lines, supporting coadministration of BETi and venetoclax. The combination of venetoclax and BETi was synergistic in both venetoclax-sensitive and venetoclax-resistant AML cells lines. Assessment of AML patient samples treated with venetoclax combined with BETi also showed increased efficacy over monotherapy. The response to INCB054329 and venetoclax in an AML patient-derived xenograft (PDX) with a low-dose, intermittent schedule confirms BETi and venetoclax activity in combination (21) and reveals a potential means to reduce venetoclax-associated marrow suppression.

Patient samples

Experiments were conducted on primary patient samples which were provided by the Vanderbilt-Ingram Cancer Center Hematopoietic Malignancies Repository and in accordance with the tenets of the Declaration of Helsinki and approved by the Vanderbilt University Medical Center Institutional Review Board. Written informed consent was obtained from all patients included within the study. Normal human primary bone marrow samples were purchased from Hemacare.

Cell lines

AML cell lines MV-4-11, Kasumi-1, HL-60, and K562 were purchased from the ATCC. MOLM-13 and OCI-AML-3 cell lines were purchased from Deutsche Sammelung von Mikroorganismen und Zellkulturen (DSMZ). ATCC and DSMZ cell bank cell lines are authenticated by short tandem repeat profiling and cytochrome C oxidase gene analysis. Cultured cells were split every 3 to 4 days and maintained in exponential growth phase. Cell lines were tested for Mycoplasma as per lab standard of practice using the Universal Mycoplasma Detection Kit (ATCC). Cells were used for the experiments presented here within 10–30 passages from thawing. MV-4-11 cell line was grown in IMDM. OCIAML3 was grown in alpha-MEM, and all other cell lines were cultured in RPMI. All cell lines were supplemented with 10%–20% FBS and 100 U/mL penicillin and 100 μg/mL streptomycin. Cells were kept at 37°C in a 5% CO2 incubator.

BrdU/propidium iodide staining

Cell lines were treated with drug for 48 hours and pulsed with BrdU for 2 hours. After washing, cells were fixed in EtOH and stored overnight. The following day cells were spun and resuspended in 2N HCI with 0.5 mg/mL Pepsin at 37°C for 30 minutes. Cells were then neutralized with 0.1 mol/L Sodium tetraborate, washed and resuspended for staining with BrdU antibody (BD Biosciences) for 45 minutes. Cells were again washed and resuspended in buffer with propidium iodide (PI) and RNaseA and subjected to flow cytometric analysis.

Flow cytometry

For flow cytometry, red blood cells were lysed with EL Buffer on ice (Qiagen), with remaining cells washed and resuspended in 1× PBS with 1% BSA and stained for 15 minutes with the following antibodies: human CD45-APC (Clone 2D1; BioLegend), human CD33-PE-Cy7 (Clone P67.6; BioLegend), murine CD45-PE (Clone 30-F11; BioLegend), and DAPI (BioLegend). For cell cycling using Ki67 and DAPI, cells were fixed/permeabilized with Cytofix Kit (Becton Dickinson) and incubated overnight with APC-Ki67 antibody (BioLegend) and stained with DAPI 5 minutes prior to flow cytometric analysis. For cycling analysis, sub-G1 cells were removed. For differentiation analysis of myeloid cell lines, cells were stained with CD11b (BioLegend) and CD14 (BioLegend), along with positive control Ultracomp beads (Invitrogen). Cells were washed and submitted for flow cytometric analysis using a three-laser LSRII (Becton Dickinson).

Assessment of apoptosis

For annexin/PI staining, an annexin V apoptosis kit was used as per manufacturer’s instructions (BD Pharmingen). For patient samples, mononuclear cells (MNCs) were subjected to 24 hours of drug treatment and stained for flow cytometry. Primary AML blast cells were gated as CD45lo-mid/CD33hi/SSC-Alo with stem cells gated as CD45lo-mid/CD19/CD34+/CD38.

Histology and IHC

Tissues were fixed in 4% paraformaldehyde for 48 hours and stored in 70% ethanol before being embedded in paraffin and sectioned at 5 μm. The bone tissue was decalcified prior to being embedded in paraffin. Sections were dewaxed in xylene and rehydrated in successive ethanol baths. Standard Mayer hematoxylin and eosin (H&E) staining was performed. Antigen retrieval using a standard pH 6 sodium citrate buffer (BioGenex) was performed and sections were stained with Monoclonal Mouse Anti-Human CD45 (Dako, M0701, dilution 1:200) using the M.O.M. Kit (Vector).

PRO-seq and analysis

Nuclear run-on reactions and library preparation were performed as described previously (22). Samples were submitted for sequencing on Illumina NextSeq500. After adapter trimming with Trimmomatic and removal of low-quality sequences, reads were reverse complemented. Reads were aligned to hg19 genome with Bowtie2, and reads with mapping quality <10 were removed. NRSA package was used to calculate polymerase density based on DESeq-normalized read counts. Motif analysis of enhancers was done using HOMER software (23, 24).

Quantitative immunoblot

Cells were grown in their respective media with indicated treatment conditions. Total protein lysates were extracted from 1.6 × 105 cells per treatment condition in laemmli sample buffer (Bio-Rad), sonicated, and boiled at 95°C for 10 minutes. The samples were loaded into a 10% SDS-PAGE (1.6 × 105 cells/well). Western blot analysis was performed according to standard protocol with antibodies to MYC and ACTIN. Antibodies were obtained from the following sources: MYC (Vanderbilt Antibody Core) and ACTIN (Sigma-Aldrich). Membranes were imaged and band densitometry was performed using imageJ. The ratio of band intensity of MYC was calculated relative to loading control (Actin).

Extracellular flux analyses (Seahorse)

Experiments were carried out on Agilent Seahorse XF96 bioanalyzer (Agilent). MOLM-13, Kasumi-1, and MV-4-11 cells were treated for 3 days with 30, 100, or 300 nmol/L INCB054329. Cells were spun onto XF96 Cell-Tak (BD Biosciences, catalog No. 354240) coated plates and rested in Seahorse XF RPMI1640 media supplemented with glutamine, sodium pyruvate, and glucose. Extracellular acidification rate (ECAR)/oxygen consumption rate (OCR) values were normalized to 2 × 105 cells per well. Mito Stress assay was performed using kit (Agilent, catalog No. 10301) and were used as per manufacturer’s protocol.

ROS/tetramethylrhodamine/Mitotracker staining for flow cytometry

Cells were pretreated with BETi 48 hours before incubation with CM-H2DCFDA (General Oxidative Stress Indicator; Life Technologies, catalog No. C6827), tetramethylrhodamine (TMRE) reagent (Mitochondrial potential; Life Technologies, catalog No. T-669), Mitotracker, catalog No. M7514 or Mitosox, catalog No. M3600 (Superoxides). Cells were then incubated for 30 minutes at 37°C, covered from light, washed twice, and submitted for flow cytometric analysis using a three-laser LSRII (Becton Dickinson).

Cell viability assay

Compounds were diluted in DMSO (final concentration containing 0.2% DMSO) and dispensed into a 384-well plate using the Echo 555 liquid handler (Labcyte) with venetoclax and BETi concentrations up to 1 and 0.33 μmol/L in patient samples, 0.1 μmol/L venetoclax and 0.31 μmol/L BETi for MV-4-11 and MOLM-13 cell lines, and 10 μmol/L venetoclax and 0.31 μmol/L BETi in cell lines K-562, OCI-AML-3, HL-60, and Kasumi-1. Following the addition of compounds, cells were pipetted into the 384-well plates at a concentration of between 2,000 and 8,000 cells per well in Iscove’s modified Dulbecco’s medium or RPMI media, as noted above, supplemented with 10% FBS and incubated at 37°C, 5% CO2 in a tissue culture incubator. Plates were incubated for 48 hours and cell viability was measured using the CellTiter-Glo reagent (Promega Corp.). Percent viability was defined as relative luminescence units (RLU) of each well divided by the RLU of cells in DMSO control. Ten-point dose-response curves were generated with GraphPad Prism version 6.0 h. The 50% growth inhibition concentration (GI50) values were determined using nonlinear regression of double log-transformed data.

Clonogenic assay

The methylcellulose medium for MV-4-11 cells and AML patient cells were prepared by StemCell Technologies according to manufacturer’s protocol. Cells were thawed and washed by centrifugation and resuspended in 2% IMDM medium with corresponding amounts of BETi, venetoclax, and DMSO. A total of 10,000 cells were plated in MethoCult GF H4434 medium (Falcon) in 35 mm petri dishes in duplicate. Dishes were incubated at 37°C with 5% CO2 and 95% humidity for 10–14 days. Colonies were scored using an inverted microscope.

In vivo murine modeling

All animal experiments were conducted in accordance to guidelines approved by the IACUC at Vanderbilt University Medical Center (Nashville, TN). Female NSGS [NOD-scid IL2Rgnull3Tg (hSCF/hGM-CSF/hIL3)] mice, 6–8 weeks old were irradiated with 100 cGy microwave radiation. Twenty-four hours later, mice were transplanted intravenously with cells of interest. In the cell line–derived xenografts (CDX), 1 × 106 MV-4-11 cells were transplanted via tail vein injections in each irradiated mouse. For patient sample–derived xenografts (PDX), 2 × 106 AML MNC were transplanted via tail vein injections. Mice were randomized post cell injection into cages of five. Prior to treatment, peripheral microchimerism was documented at week 1 in CDX. For AML PDX, peripheral chimerism was established by 2 weeks. Mice showing no peripheral chimerism by 2 weeks in CDX, or 3 weeks in AML PDX, were considered engraftment failures and removed from the study. Upon establishing microchimerism, mice were treated with either venetoclax (Chemietek), INCB054329 (Incyte), or vehicle by gavage. INCB054329 was dissolved in N,N-dimethylacetamide (DMAC) and diluted in 5% methylcellulose. Venetoclax was dissolved in polyethylene glycol and ethanol and diluted with Phosal 50 PG. Peripheral blood was assessed weekly for human chimerism. Spleen/body ratio was calculated as organ weight (gram) per gram of body weight. Murine complete blood count was conducted on blood collected into EDTA tubes (Greiner Bio-One) and analyzed with a Hemavet (Drew Scientific) analysis system.

Statistical analysis

Unless otherwise noted, data were summarized using the mean (± SD). Per group sample sizes are presented in figures and results reported from two separate experiments, unless stated otherwise. To avoid normality assumptions, pairwise group comparisons were made using the nonparametric Mann–Whitney U test. The nonparametric Spearman correlation was used to assess pairwise variable associations. Zero-interaction potency (ZIP) modeling was applied to the dose response matrices to assess drug synergy, represented by the ZIP score, which is the average combinatory effect of both drugs over the entire matrix of tested concentrations (25). Data were analyzed using Graph Pad Prism 6.0 for Windows (GraphPad Software, www.graphpad.com) and R (R Core Team 2017). R: A language and environment for statistical computing. R Foundation for Statistical Computing, https://www.R-project.org/.

Effects of INCB054329 on AML cell proliferation, cell cycle, and apoptosis

To test the efficacy of INCB054329 (BETi) in AML, we treated AML cell lines with a series of drug concentrations for 72 hours. BETi treatment reduced the cell growth kinetics in a dose dependent fashion in all cell lines, although, to a lesser degree in MOLM-13 cells (Fig. 1A). To determine whether the effect of BETi on cell growth was due to cell death or a reduction in cycling, we performed an array of assays investigating effects on the cell cycle and apoptosis. To detect any shifts in cell cycling, we performed BrdU staining in cells treated with 300 nmol/L BETi at 48 hours which revealed an increase in cell populations in G0–G1 and a reduction in populations in S-phase (Fig. 1B; Supplementary Fig. S1A). To clarify whether the shift into G0–G1 represented quiescence, we further analyzed populations using Ki67 and DAPI under the same treatment to differentiate G0 and G1 phases. Ki67 and DAPI staining revealed a dose-dependent accumulation of cells in G0 in MV-4-11 and Kasumi-1 cells (Fig. 1C; Supplementary Fig. S1B). At 72 hours, cells were assessed for apoptosis. MV-4-11 and Kasumi-1 cells showed significant increases in apoptosis (Fig. 1D). At this time point, live cells from BETi-sensitive lines exhibited significant decreases in cell size, further supporting the finding that these cells had halted in a G0 quiescent state and were undergoing apoptosis (Supplementary Fig. S1C). Not surprisingly, BETi exhibited the greatest efficacy in cell lines when cell cycling and apoptosis were both affected.

Figure 1.

INCB054329 inhibits expansion of AML cell lines in vitro. A, AML cell cultures under BET inhibition with INCB054329 in MV-4-11, MOLM13, and Kasumi-1 cell lines (mean ± SD, n = 3). B, Staining with BRDU reveals changes in cell cycle (mean ± SD, n = 2). C, Further cell-cycle analysis with Ki67/DAPI reveals increases in number of cells in G0 with BETi treatment (mean ± SD, n = 3). D, Annexin V staining reveals increases in early and late apoptotic cells with treatment at 72 hours (mean ± SD, n = 3). E, NSGS mice were engrafted with MV-4-11 human leukemia cells and were then treated with either vehicle (n = 4), 10 (n = 4), 30 (n = 5), or 75 (n = 5) mg/kg of BETi. Peripheral blood, spleen (SPL), and bone marrow (BM) were harvested for chimerism analysis. A nonparametric, unpaired, two-tailed t test was used to calculate significance (mean ± SD, n = 2). F, IHC of femurs and spleen (20×) stained with mAb for hCD45 reveal AML cells left within the bone marrow and spleen of experimental mice.

Figure 1.

INCB054329 inhibits expansion of AML cell lines in vitro. A, AML cell cultures under BET inhibition with INCB054329 in MV-4-11, MOLM13, and Kasumi-1 cell lines (mean ± SD, n = 3). B, Staining with BRDU reveals changes in cell cycle (mean ± SD, n = 2). C, Further cell-cycle analysis with Ki67/DAPI reveals increases in number of cells in G0 with BETi treatment (mean ± SD, n = 3). D, Annexin V staining reveals increases in early and late apoptotic cells with treatment at 72 hours (mean ± SD, n = 3). E, NSGS mice were engrafted with MV-4-11 human leukemia cells and were then treated with either vehicle (n = 4), 10 (n = 4), 30 (n = 5), or 75 (n = 5) mg/kg of BETi. Peripheral blood, spleen (SPL), and bone marrow (BM) were harvested for chimerism analysis. A nonparametric, unpaired, two-tailed t test was used to calculate significance (mean ± SD, n = 2). F, IHC of femurs and spleen (20×) stained with mAb for hCD45 reveal AML cells left within the bone marrow and spleen of experimental mice.

Close modal

Given previous reports of differentiation of AML cell lines under BETi, (26) we assessed the effects of INCB054329 on differentiation. We assessed for myelomonocytic differentiation via CD11b and CD14 after 72 hours of BETi, and found no significant increases in either marker (Supplementary Fig. S1D).

Effects of INCB054329 on disseminated AML in vivo

On the basis of the efficacy observed for INCB054329 in vitro, we tested the inhibitor as a monotherapy in a xenograft transplantation model of MV-4-11 in NSGS mice. After establishing disseminated leukemia, mice were treated with 10, 30, or 75 mg/kg of INCB054329 bid for 21 days. Weekly chimerism analyses were conducted, and the percentage of MV-4-11 cells were quantified in murine peripheral blood using anti-human CD45 (hCD45) and anti-human CD33 (hCD33) mAbs. Twenty-eight days posttransplant, vehicle-treated mice had developed significant leukemia burden, and all experimental mice were sacrificed for tissue harvest. INCB054329 treatment of disseminated human AML resulted in a dose-dependent decrease in tumor burden, as determined by hCD45+/CD33+ MV-4-11 cells in the blood, bone marrow, and spleen (Fig. 1E). Congruently, treatment with INCB054329 reduced disease-associated splenomegaly (Supplementary Fig. S1E). Tumor burden was also assessed by immunohistochemistry for hCD45 in bone marrow and spleen (Fig. 1F).

BETi affects the expression of genes that drive the cell cycle and metabolic pathways

Given Kasumi-1 cells responded most significantly to BETi, we aimed to interrogate the direct transcriptional effects of INCB054329 with PRO-seq (22, 27). Using PRO-seq, transcriptionally engaged RNA polymerases are mapped with high resolution which allows for distinguishing differential gene transcription as a result of the drug treatment. As JQ1 and other bromodomain inhibitors have shown down-regulation of MYC and MYC-dependent target genes (28), we sought to confirm that INCB054329 also downregulated MYC and MYC-dependent target genes. Importantly, we saw that RNA polymerase peaks were diminished as early as 15 minutes after treatment, indicating reduced transcription of MYC, CCND1, and BCL2. Furthermore, we noted that transcription of genes regulating cell cycle and proliferation, such as CDK4 and CDK6 were inhibited by 1-hour posttreatment with 300 nmol/L BETi (Fig. 2A). Gene set enrichment analysis (GSEA) of downregulated genes at 15, 30, and 60 minutes included MYC target genes (Fig. 2B). At 15, 30, and 60 minutes posttreatment, pathway analysis of downregulated genes revealed enrichment in genes related to cell cycle, protein processing, translation, and degradation, and metabolic pathways (Fig. 2C).

Figure 2.

PRO-seq studies reveal multiple gene expression changes in Kasumi-1 cells treated with BETi. A, Transcriptional changes over time in MYC, CCND1, BCL2, CDK4, CDK6, and after treatment with BETi (coding strand in red signifies transcription from left to right, and coding strand in blue signifies transcription from right to left). B, Confirmed signatures of MYC upregulated genes and MYC target genes at 15, 30, and 60 minutes. C, GSEA of significantly changed genes over time. P values for each enrichment category are represented by the black bars.

Figure 2.

PRO-seq studies reveal multiple gene expression changes in Kasumi-1 cells treated with BETi. A, Transcriptional changes over time in MYC, CCND1, BCL2, CDK4, CDK6, and after treatment with BETi (coding strand in red signifies transcription from left to right, and coding strand in blue signifies transcription from right to left). B, Confirmed signatures of MYC upregulated genes and MYC target genes at 15, 30, and 60 minutes. C, GSEA of significantly changed genes over time. P values for each enrichment category are represented by the black bars.

Close modal

Because not all cell lines were as sensitive to the BETi as Kasumi-1 cells, we sought to confirm the PRO-seq findings in all three cell lines. We first confirmed that the BETi was effective with the doses used in all cell lines by assessing MYC protein expression via Western blot analysis. MYC protein expression was reduced in a dose-dependent manner in all cell lines, confirming the on-target efficacy (Fig. 3A). Given the metabolic pathway enrichment in the PRO-seq analysis and the increased number of AML cells in G0–G1 when treated with INCB054329, we tested the mitochondrial functions of the cell lines under BETi. We treated cells with 30, 100, or 300 nmol/L of BETi for 48 hours and assessed metabolic function using the Agilent Seahorse assay system. We assessed oxidative phosphorylation (OXPHOS) through measurement of OCR. At the initial time point, a dose-dependent suppression of OXPHOS was noted in both MV-4-11 and Kasumi-1 cells (Fig. 3B). At this time point, MOLM-13 cells, which showed little to no effect on quiescence or apoptosis in presence of INCB054329, maintained OXPHOS under BETi treatment (Fig. 3B).

Figure 3.

BETi induced metabolic and mitochondrial changes in AML cells. A, Lysates from cells treated with 100 or 300 nmol/L of INCB054329 for 24 or 48 hours were probed for MYC and actin by immunoblot. MYC protein densitometry measurements for each lane are indicated below the MYC membrane. B, OCR measurement after injection of oligomycin (O), FCCP (F), and antimycin plus rotenone (A+R) under mitostress testing. C, ECAR measurement in cells subjected to sequential injections of glucose (Glc), oligomycin (O), and 2-deoxyglucose. D and E, Cytoplasmic (DCFDA) and mitochondrial (Mitosox) measurement of ROS by flow cytometric analysis. Findings recorded as mean fluorescence intensity (MFI). F, Mitochondrial membrane potential detected though TMRE staining. G, Mitochondrial mass measured by Mitotracker. For A–F, data are shown as (mean ± SD, n = 2).

Figure 3.

BETi induced metabolic and mitochondrial changes in AML cells. A, Lysates from cells treated with 100 or 300 nmol/L of INCB054329 for 24 or 48 hours were probed for MYC and actin by immunoblot. MYC protein densitometry measurements for each lane are indicated below the MYC membrane. B, OCR measurement after injection of oligomycin (O), FCCP (F), and antimycin plus rotenone (A+R) under mitostress testing. C, ECAR measurement in cells subjected to sequential injections of glucose (Glc), oligomycin (O), and 2-deoxyglucose. D and E, Cytoplasmic (DCFDA) and mitochondrial (Mitosox) measurement of ROS by flow cytometric analysis. Findings recorded as mean fluorescence intensity (MFI). F, Mitochondrial membrane potential detected though TMRE staining. G, Mitochondrial mass measured by Mitotracker. For A–F, data are shown as (mean ± SD, n = 2).

Close modal

While AML cells preferentially rely on OXPHOS, glycolysis is often upregulated in drug-resistant cells when OXPHOS is disrupted (29–31). For this reason, we also evaluated the glycolytic capability of each cell line in presence of BETi via ECAR. Both BETi-sensitive cell lines were incapable of maintaining or upregulating glycolytic activity whereas ECAR was minimally affected in MOLM-13 cells (Fig. 3C), consistent with sustained OXPHOS observed in response to BETi.

Further mitochondrial studies in these cell lines in the presence of BETi elucidated direct responses within mitochondria. Reactive oxygen species (ROS) were measured in both cytoplasm (DCFDA) and mitochondria (Mitosox) of treated cells. After 48 hours of BETi, levels of superoxide remained stable in both MV-4-11 and Kasumi-1 cell lines, while MOLM-13 showed dose-dependent increases in ROS (Fig. 3D and E). This increase in ROS within the MOLM-13 cells is consistent with cell maintenance of OCR in the face of BETi. Measurement of mitochondrial membrane potential, using TMRE, revealed dose-dependent decreases in MV-4-11 and Kasumi-1 cells, while MOLM-13 cells maintained mitochondrial membrane potential at higher doses of INCB054329 (Fig. 3F). Finally, highly BETi-sensitive cell line Kasumi-1 exhibited a dose-dependent loss in mitochondrial mass (Fig. 3G).

BETi cooperates with BCL2 inhibition in reducing expansion of human AML cell lines in vitro and in vivo

Given the reduction in BCL2 transcription, transition into G0–G1, and altered metabolism as a result of INCB0540329 treatment, we aimed to identify synergistic doses between INCB054329 and inhibitors of antiapoptotic BCL2 family proteins. We conducted cell viability studies via CellTiter-Glo and generated dose–response curves for the combination of VEN + INCB054329 in venetoclax-sensitive (GI50 < 1 μmol/L; MV-4-11, MOLM-13, and Kasumi-1) and venetoclax-resistant(GI50 > 1 μmol/L; HL-60, OCI-AML-3, and K562) AML cell lines (Supplementary Table S1). To assess for any synergistic action, we employed ZIP score analysis (25). ZIP analysis considers the entire dose–response matrix dataset to quantify antagonistic and synergistic doses by combining both Loewe and Bliss modeling theory. The ZIP analysis assesses the change in potency of each dose in the dose-response curves between both individual drugs and their combinations. A synergy score is given for each dose combination, and the average of all scores in the matrix is given as an overall ZIP score. ZIP scores less than −10 suggest an antagonistic effect, while scores larger than 10 suggest synergy. Any ZIP scores between −10 and 10 suggest additive dose effects. The ZIP analysis identified synergistic doses of INCB054329 and venetoclax in a majority of the AML cell lines, including two of the three venetoclax-resistant cell lines tested (Fig. 4A).

Figure 4.

Combinatory therapy with venetoclax and BETi reduces AML cell line expansion in vitro and in vivo. A, ZIP synergy analysis of cell lines under treatment with BETi and venetoclax at 48 hours. The most synergistic drug concentrations are found within the white outlined box in the two-dimensional (2-D) map. Average ZIP synergy score for the matrices are stated. On the 2-D and three-dimensional maps, red indicates synergy, white indicates additive effect, and green indicates antagonism. B, Tissues were harvested for chimerism analysis from NSGS treated with either vehicle (n = 6), 25 mg/kg venetoclax (n = 7), 50 mg/kg BETi (n = 9) or combination venetoclax/BETi (n = 7). A nonparametric, unpaired, two-tailed t test was used to calculate significance (mean ± SD, n = 2). C, IHC of femurs and spleen (20×) stained with monoclonal antibody for hCD45 reveal residual AML cells within the bone marrow and spleen of experimental mice.

Figure 4.

Combinatory therapy with venetoclax and BETi reduces AML cell line expansion in vitro and in vivo. A, ZIP synergy analysis of cell lines under treatment with BETi and venetoclax at 48 hours. The most synergistic drug concentrations are found within the white outlined box in the two-dimensional (2-D) map. Average ZIP synergy score for the matrices are stated. On the 2-D and three-dimensional maps, red indicates synergy, white indicates additive effect, and green indicates antagonism. B, Tissues were harvested for chimerism analysis from NSGS treated with either vehicle (n = 6), 25 mg/kg venetoclax (n = 7), 50 mg/kg BETi (n = 9) or combination venetoclax/BETi (n = 7). A nonparametric, unpaired, two-tailed t test was used to calculate significance (mean ± SD, n = 2). C, IHC of femurs and spleen (20×) stained with monoclonal antibody for hCD45 reveal residual AML cells within the bone marrow and spleen of experimental mice.

Close modal

To further validate the combinatory use of BETi and venetoclax, we transplanted the MV-4-11 cell line into immunocompromised mice. Engrafted mice were treated with INCB054329 (50 mg/kg twice daily), and suboptimal dosing of venetoclax (25 mg/kg every day 5 days on/2 days off) or the combination of each at these doses. Previous PDX models revealed venetoclax doses within this range to have little to insignificant responses as a monotherapy (21, 32, 33). Twenty-eight days after transplant, mice were sacrificed, and analysis of flow cytometric measurement of human CD45+/CD33+ in INCB054329 + VEN treated mice revealed significant decreases of AML cells in the bone marrow compared with either inhibitor alone (Fig. 4B). IHC analysis of hCD45 in the bone marrow and spleen of treated mice also revealed a reduction in tumor burden under combinatory therapy compared with single-agent therapy (Fig. 4C).

BETi and BCL2 inhibition ameliorate AML in primary patient cells in vitro and in vivo

To further validate the efficacy of combined BETi and venetoclax, we treated primary patient AML cells with each of the single agents or the combination. Mononuclear bone marrow cells from nine mutationally diverse patients with AML (Supplementary Table S1) were tested and evaluated for cell viability via CellTiter-Glo, and combination therapy was more effective against AML patient samples than either single-agent therapy in most cases (Fig. 5A). To verify induction of apoptosis, we subjected the same patient samples to Annexin V/PI staining 48 hours after exposure to INCB054329 and venetoclax single and combination therapy. Induction of apoptosis, noted through Annexin+/PI cell staining, was enhanced with the combination therapy in AML blast cells at 24 hours (Fig. 5B). Importantly, while primary AML patient samples treated with INCB054329 and venetoclax revealed early apoptosis of CD34+/CD38 stem cells at 24 hours, effects of the combination on normal human primary CD34+/CD38 cells were not significant (Supplementary 3a). We then tested the clonogenic capacity of AML primary cells in vitro. Primary AML patient samples were plated in methylcellulose with the single or combined therapeutic agents and incubated for 21 days to allow colony formation. Treatment with INCB054329 + VEN led to reduced colony formation in three of the four AML samples (Fig. 5C). Furthermore, the addition of INCB054329 to venetoclax in AML001 cells, which were insensitive to venetoclax (Supplementary Table S1), resulted in a ZIP score just below 10 (Fig. 5D).

Figure 5.

Primary AML samples respond to combinatory therapy with venetoclax and BETi in vitro and in vivo. A, AML patient samples show decreased viability 48 hours posttreatment with 10 nmol/L venetoclax in combination with 330 nmol/L BETi. B, At 24 hours posttreatment, significant increases in early apoptosis were determined by flow cytometric analysis of Annexin/PI staining in AML blasts. C, Methylcellulose assays confirmed the reduced clonal capacity of combination treated cells over a 21-day period. D, ZIP score analysis from treatment with BETi and venetoclax in AML001. E, In a PDX from AML001, primary AML cells were engrafted in NSGS mice and were then treated with either vehicle (n = 5), 20 mg/kg venetoclax (n = 4), 75 mg/kg BETi (n = 5) or 20 mg/kg venetoclax + 75 mg/kg BETi (n = 6) where venetoclax was dosed daily and BETi was administered by oral gavage twice daily 7 days on/7 days off over 10 weeks. At 12 weeks bone marrow (BM) was harvested for chimerism analysis. F, IHC reveals decreases in hCD45 cells in bone marrow and splenic compartments of treated PDX mice.

Figure 5.

Primary AML samples respond to combinatory therapy with venetoclax and BETi in vitro and in vivo. A, AML patient samples show decreased viability 48 hours posttreatment with 10 nmol/L venetoclax in combination with 330 nmol/L BETi. B, At 24 hours posttreatment, significant increases in early apoptosis were determined by flow cytometric analysis of Annexin/PI staining in AML blasts. C, Methylcellulose assays confirmed the reduced clonal capacity of combination treated cells over a 21-day period. D, ZIP score analysis from treatment with BETi and venetoclax in AML001. E, In a PDX from AML001, primary AML cells were engrafted in NSGS mice and were then treated with either vehicle (n = 5), 20 mg/kg venetoclax (n = 4), 75 mg/kg BETi (n = 5) or 20 mg/kg venetoclax + 75 mg/kg BETi (n = 6) where venetoclax was dosed daily and BETi was administered by oral gavage twice daily 7 days on/7 days off over 10 weeks. At 12 weeks bone marrow (BM) was harvested for chimerism analysis. F, IHC reveals decreases in hCD45 cells in bone marrow and splenic compartments of treated PDX mice.

Close modal

To determine the capacity of INCB054329 to sensitize a venetoclax-resistant AML, we treated AML001 with the INCB054329 + VEN combination in a PDX transplantation assay. AML001 patient cells were transplanted into NSGS mice, and treatment began at 8 weeks posttransplant when mice exhibited 5%–10% chimerism. While we did not see any toxicity in our mouse studies, intermittent dosing of INCB54329 was established in this model in an attempt to abate cardiac toxicity seen with other studies in rodents with the BETi, iBET-151 (34). In attempts to further minimize overall toxicity, mice were treated with lower doses of venetoclax than in previous studies (21, 32, 33). INCB054329 was dosed at 75 mg/kg (25% of most effective dose tested in vitro) intermittently (7 days on, 7 days off), and venetoclax was administered at 20 mg/kg (daily) for 10 weeks. As vehicle mice developed systemic leukemia at 13 weeks posttransplant, the experiment was terminated to measure degree of AML infiltration in all animals. AML xenografts were limited by each single agent and near resolved in combination-treated mice (Fig. 5E). IHC from the bone marrow and spleen of combination-treated animals had significantly lower hCD45/CD33+ staining (Fig. 5F). With this dosing regimen, nontarget tissues were unaffected, and histologic analyses yielded no deleterious effects on spleen, kidney, liver, or heart tissue of the PDX AML001 mice (Supplementary Fig. S2A) suggesting a potential role for BETi intermittent dosing in combination with venetoclax. To investigate the potential of the BETi/venetoclax drug combination to induce direct hematologic toxicity, we performed a complete blood count analysis for cancer naïve NSGS mice treated with the same regimen as that of PDX AML001. Blood analysis revealed no significant differences in white blood cell, red blood cell, or platelet counts (Supplementary Fig. S2B). An additional PDX generated with blasts from a venetoclax-sensitive patient, AML004, was treated similarly yielding enhancement of response compared to venetoclax alone (Supplementary Fig. S3B and S3C).

While BET inhibitors have been associated with clinical response in AML, the success is short lived as resistance develops and disease progresses (8, 9). The BCL2 inhibitor, venetoclax, has recently emerged as a transformative component of AML therapy. As a monotherapy, responses to venetoclax were disappointing in relapsed and refractory patients with AML (35). However, induction regimens of venetoclax with DNMTis or LDAC have yielded responses in 50%–70% of all previously untreated older patients, exceeding previous responses to other therapies for a population which represents the majority of all AML (16, 21, 35, 36). Nonetheless, relapse is inevitable in most patients, and some subsets of patients have AML that is primarily refractory to venetoclax. Furthermore, while the venetoclax-based regimens are less toxic than traditional cytotoxic chemotherapy, hematologic toxicity is common and can be profound in some patients (19, 35, 37). Mitigation of venetoclax-associated toxicity on hematopoiesis is a focus of further development of this therapy (16, 17). Indeed, cytotoxic agents have been combined with venetoclax successfully in AML, and combining targeted therapies or immunotherapies with venetoclax is appealing (38–40).

Venetoclax combinations with targeted agents present opportunities to mitigate hematologic toxicities of VEN + DNMTi or VEN + LDAC therapy in some patient populations. As venetoclax-induced hematologic toxicity appears to be dose dependent, we performed several studies with venetoclax at doses below standard of care dosing in hopes to leverage VEN + BETi synergy while limiting toxicity. BETis while targeted, do have potential off-target effects, and previous experience with alternative BETis led us to minimize BETi dosing and schedule with this in mind (34). Drug synergy was seen with INCB054329 +VEN in cell lines, and experiments in primary samples with reduced doses yielded impressive activity against AML, both resistant and sensitive to single-agent venetoclax. These findings support previous studies combining BETi and venetoclax in AML (21) and explore alternate dosing strategies to abate venetoclax and BETi toxicity. While our pathologic analyses revealed no evidence of cardiac, renal, hepatic, or hematologic toxicity in the PDX and cancer naïve NSGS mice used here (Supplementary Fig. S2), further study of the regimen is needed.

We investigated the novel BET inhibitor, INCB054329, by assessing its in vitro effect on MYC expression, cell proliferation, cell cycling, induction of apoptosis, and cell metabolism. We found that this BETi effectively reduced MYC expression in all cell line models tested. In most cases INCB054329 also reduced cell proliferation, causing cells to transition into a quiescent G0–G1 state or undergo apoptosis. Cells sensitive to BETi also exhibited significant reductions in OXPHOS. Leukemia-initiating cells (LIC) generally rely on OXPHOS, and it has been shown that VEN + AZA therapy eliminates LICs by inhibiting OXPHOS via disruption of amino acid–driven metabolism, critical for the survival of LICs, leading to quick elimination of leukemia cells in patients (29, 41, 42). On the basis of these findings, we reasoned that decreases in OXPHOS and the quiescent state noted in cells after BETi would lead to increased sensitivity to venetoclax treatment. Importantly, the combination of these two therapies effectively induced apoptosis in AML CD34+/CD38 stem cells but not in CD34+/CD38 stem cells from normal bone marrow.

Interestingly, while MYC was effectively downregulated in MOLM-13 cells with INCB054329, the cells did not exhibit significant changes in cell cycle, apoptosis, or metabolic capabilities. ROS species were significantly increased at higher doses of BETi, indicating an active effort to maintain OXPHOS under the stress of the BETi. The lack of response indicates that MOLM-13 are either not likely driven by MYC dysregulation or that the mechanism of response to BETi is not MYC dependent. Importantly, however, MOLM-13 cells, as well as venetoclax-resistant cell lines, exhibited synergistic responses to the combination of INCB054329 and VEN. INCB054329 and venetoclax combined also exhibited greater efficacy than monotherapy in genetically diverse patient samples ex vivo, and the combination was effective against both venetoclax-insensitive and venetoclax-sensitive PDX models.

Importantly, we present a dosing scheme that has potential means to reduce BETi-associated toxicity and venetoclax-associated marrow suppression, which occurs in most patients at standard doses (10, 11). The use of intermittent biweekly dosing of the BETi INCB054329 with subtherapeutic doses of VEN preserved off-target tissues while maintaining potent activity against a venetoclax-insensitive AML PDX. This is important, considering BETi-associated toxicities alone often can often result in the removal of patients from drug studies (43). Pharmacokinetic clearance studies on BETi INCB054329 suggest that its high peak serum concentration and short half-life (<5 hours), in comparison with other BETi inhibitors, may be advantageous in providing a short period of target inhibition with subsequent vacation periods allowing for recovery of potential toxicities (43). Overall, these findings suggest that the combination of low doses of venetoclax and INCB054329 is more efficacious for patients with AML than either monotherapy and may potentially provide a longer and more tolerable dosing regimen.

K.R. Stengel reports grants from NCI during the conduct of the study. M.C. Stubbs is an employee of Incyte Corporation/Incyte Research Institute. K.R. Stengel is a shareholder of Incyte Corporation stock. P. Liu reports other from Incyte Corp outside the submitted work. J.C. Rathmell reports personal fees and nonfinancial support from Sitryx and Caribou outside the submitted work. S.W. Hiebert reports grants from NCI and Incyte, Inc. during the conduct of the study. M.R. Savona reports grants from Incyte Corporation during the conduct of the study; and grants from Astex; personal fees from Bristol Myers Squibb, Celgene, Geron, Ryvu, Sierra Oncology, and Abbvie; personal fees and other from Karyopharm outside the submitted work; in addition, M.R. Savona has a patent for Boehringer licensed. No disclosures were reported by the other authors.

H. Ramsey: Data curation, formal analysis, investigation, methodology, writing-original draft, writing-review and editing. D. Greenwood: Formal analysis, investigation, writing-review and editing. S. Zhang: Investigation, writing-review and editing. M. Childress: Formal analysis, investigation, project administration, writing-review and editing. M.P. Arrate: Formal analysis, investigation, writing-review and editing. A.E. Gorska: Formal analysis, investigation, writing-review and editing. L. Fuller: Investigation, writing-review and editing. Y. Zhao: Formal analysis, investigation, writing-review and editing. K. Stengel: Formal analysis, investigation, writing-review and editing. M.A. Fischer: Formal analysis, investigation, writing-review and editing. M.C. Stubbs: Resources, formal analysis, writing-review and editing. P.C.C. Liu: Resources, formal analysis, writing-review and editing. K. Boyd: Formal analysis, writing-review and editing. J.C. Rathmell: Formal analysis, investigation, writing-review and editing. S. Hiebert: Formal analysis, investigation, writing-review and editing. M.R. Savona: Conceptualization, resources, supervision, funding acquisition, investigation, methodology, writing-original draft, project administration, writing-review and editing.

The Vanderbilt-Ingram Cancer Center (VICC) Hematopoietic Malignancies Tissue Repository, the Vanderbilt University Medical Center Translational Pathology Shared Resource, and other VICC shared Core Services were critical in completion of this work.

This study was supported by the E.P. Evans Foundation Discovery Research Grant (to M.R. Savona), the Leukemia and Lymphoma Society (to M.R. Savona), the Adventure Allie Discovery Research Grant (to M.R. Savona), the Larry Adelman Fund (to M.R. Savona), the Biff Rittenburg Foundation (to M.R. Savona), and the Vanderbilt-Incyte Research Alliance Grant (to M.R. Savona and S. Hiebert). The Vanderbilt-Ingram Cancer Center is supported by a NIH P30 CA068485–19. The REDCap database tool is supported by grant UL1 TR000445 from NCATS/NIH. The content of this publication does not necessarily reflect the views or policies of the Department of Health and Human Services, nor does mention of trade names, commercial products, or organizations imply endorsement by the U.S. Government.

The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

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Supplementary data