Inhibition of Bcl-2 Synergistically Enhances the Antileukemic Activity of Midostaurin and Gilteritinib in Preclinical Models of FLT3-Mutated Acute Myeloid Leukemia

Acute myeloid leukemia (AML) is defined by the clonal proliferation of immature myeloid elements and survival rates have remained low in both pediatric and adult patients, with 5-year survival rates approximating 61.5%–65.1% and 25%, respectively (1). FMS-like tyrosine kinase 3 (FLT3) is a receptor tyrosine kinase and important early regulator of hematopoiesis, in which mutations are seen in approximately one third of patients with AML (2). FLT3-internal tandem duplications (ITD) mutations, seen in approximately 25% of patients with AML, result in constitutive activation of downstream pathways that promote cell survival and proliferation, including the MAPK/ERK, PI3K/AKT, and JAK/STAT pathways; understandably, it confers a poor prognosis (2, 3).

Direct inhibition of FLT3 is therefore a promising therapeutic avenue, with some agents already available for immediate use. Midostaurin, a first-generation multikinase inhibitor was approved by the FDA in April 2017 for use in newly diagnosed patients with FLT3-mutated AML, in combination with standard 7+3 cytarabine and daunorubicin induction and cytarabine consolidation, on the basis of results of the RATIFY trial (4). In patients over the age of 60, however (e.g., the largest population of patients with AML), and those with other comorbidities, 7+3 chemotherapy is not well tolerated because of its extensive toxicities and severe side effects (5). Gilteritinib was approved by the FDA in November 2018 for use in adult patients with relapsed or refractory FLT3-mutated AML, following the ADMIRAL trial (NCT02421939; ref. 6). As such, treatment options for FLT3-mutated AML are beginning to exist, but carry with them certain limitations—particularly in relation to monotherapy, with efficacious but short-lived responses. As such, new therapies to combine with FLT3 inhibitors are an unmet clinical need.

Venetoclax (ABT-199) is a selective inhibitor of Bcl-2, and was granted approval by the FDA in November 2018 for first-line use in newly diagnosed AML in adults over 75 or those with comorbidities precluding use of intensive induction chemotherapy, in combination with low-dose cytarabine or the hypomethylating agents azacitidine or decitabine (7). Importantly, patients with FLT3 mutations did not fare worse than their peers while receiving venetoclax, despite the usual poor prognosis associated with this mutation (8, 9). Again however, while encouraging, there is room to improve upon these results; these studies show median complete remission times of 4–6 months, across all arms, complete remission rates of 21%–54% (8, 9).

Our previous work demonstrated that venetoclax reduces the association of Bcl-2 with Bim; however, once freed, a compensatory increase in Bim/Mcl-1–binding occurs, particularly in venetoclax-resistant cell lines, preventing apoptosis (10). Mcl-1 inhibition is capable of halting this association and abrogating venetoclax resistance via a reduction in Bim/Mcl-1, freeing Bim (11–14). It therefore appears that joint inhibition of both Mcl-1 and Bcl-2 is required for effective induction of apoptosis via this mechanism. We therefore hypothesized that combining FLT3 inhibition via gilteritinib or midostaurin with selective Bcl-2 inhibition via venetoclax would result in synergistic antileukemic activity against FLT3-mutated AML. We theorized that FLT3 inhibition, via downstream downregulation of MAPK/ERK, JAK/STAT, and PI3K/AKT pathways, would result in decreased Mcl-1 expression and that the addition of the selective Bcl-2 inhibitor, venetoclax, would prevent free Bim from being sequestered by Bcl-2, thereby freeing it and allowing induction of apoptosis.

Midostaurin (PKC-412), gilteritinib (ASP-2215), venetoclax (ABT-199), and SCH772984 (an ERK-selective inhibitor) were purchased from Selleck Chemicals.

MOLM-13 cells were purchased from AddexBio (2012). MV4-11, THP-1, and U937 cell lines were purchased from the ATCC (2006, 2014, 2002, respectively). Cell lines were cultured using RPMI1640 media plus 10%–20% FBS (CLARK Bioscience), 2 mmol/L l-glutamine, and 100 U/mL penicillin and 100 μg/mL streptomycin and incubated in a humidified, 5%CO₂/95% air environment at 37°C. All lines were tested for the presence of Mycoplasma using the PCR method described by Uphoff and Drexler (15) on a monthly basis. The cell lines were authenticated in 2017 at the Genomics Core at Karmanos Cancer Institute (Detroit, MI) using the PowerPlex 16 System from Promega.

Primary patient samples were obtained from the First Hospital of Jilin University (Changchun, China), following obtainment of informed consent according to the Declaration of Helsinki. The study itself and the obtainment of patient samples were first approved by the Human Ethics Committee of The First Hospital of Jilin University (Changchun, China). All patient samples were screened for FLT3-ITD, NPM1, C-kit, CEBPA, IDH1, IDH2, N-RAS, and DNMT3A mutations by PCR amplification and automated DNA sequencing, cytogenetics, and fusion genes utilizing RT-PCR, as described previously (16, 17). Individual patient characteristics are provided in Supplementary Table S1. Patient samples were purified using Ficoll-Hypaque density centrifugation and cultured in RPMI1640 media plus 20% FBS, ITS Solution (Sigma-Aldrich), and 20% supernatant of the 5637 bladder cancer cell line (to provide sources of GM-CSF, G-CSF, IL1β, macrophage colony–stimulating factor, and stem cell factor; refs. 16, 18, 19).

Cells were treated with either midostaurin, gilteritinib, SCH772984, or venetoclax, alone or in combination, for up to 24 hours. Flow cytometry analysis using the Annexin V-FITC/PI Apoptosis Kit (Beckman Coulter) was performed as described previously (20, 21). Results are displayed as percentage of Annexin V–positive cells. All experiments were repeated in triplicate independently; displayed cell line data are from one experiment representative of findings of all three, and patient sample experiments were performed once in triplicate due to limited sample. The extent and direction of the antileukemic interaction was identified via calculation of the combination index (CI) using CompuSyn Software (Combosyn Inc.), where CI < 1, CI = 1, and CI > 1 are indicative of synergistic, additive, and antagonistic effects, respectively (20, 22).

Cell lysis was performed in the presence of protease and phosphatase inhibitors (Roche Diagnostics). Whole-cell lysates underwent SDS-PAGE, electrophoretically transferred onto polyvinylidene difluoride membranes (Thermo Fisher Scientific), and subsequently immunoblotted utilizing anti-Mcl-1, -Bcl-2, -Bcl-xL, -Bax, -β-actin, -ERK (Proteintech), -p-AKT (T308), -p-AKT (S473; Affinity Biosciences), -Bim, -cf-Caspase 3, -p-S6 (S240/244), -p-STAT5(Y694), -p-Mcl-1 (T163; Cell Signaling Technologies), -Bak, -p-ERK, -AKT (Abcam) as described previously (23, 24). Visualization of immunoreactive proteins was performed via the Odyssey Infrared Imaging System (LI-COR), as described by the manufacturer. Western blots were repeated in triplicate (at minimum), with one representative blot displayed. Densitometry measurements were performed using Odyssey V3.0 (LI-COR) and normalized to β-actin, calculated as fold change compared with the corresponding control no-drug treatment specimen.

AML cell lines were treated for 4 hours with either midostaurin or venetoclax, alone or in combination, then lysed with 1% CHAPS, 5 mmol/L MgCl₂, 150 mmol/L NaCl, 1 mmol/L EDTA, 1 mmol/L EGTA, 20 mmol/L Tris, and 0.05% Tween-20 in the presence of protease inhibitors. Coimmunoprecipitation of Bim was carried out as described previously (25). Two micrograms of anti-Bim antibody (Cell Signaling Technology), 1 mg protein lysate, and Protein A Agarose Beads (Roche Diagnostics). Proteins were then eluted with 50 mmol/L glycine at pH 2.0, and then analyzed via Western blotting as described above.

The pMD-VSV-G and delta 8.2 plasmids were gifts from Dr. Dong at Tulane University (New Orleans, LA). Red fluorescent protein (RFP) and Mcl-1 cDNA constructs were purchased from Thermo Fisher Scientific Biosciences. Bak and Bax shRNA constructs were purchased from Sigma Aldrich. Lentivirus production and transduction were carried out as described previously (26).

tRNA was extracted using TRIzol (Thermo Fisher Scientific) and cDNAs were prepared from 2 μg tRNA using random hexamer primers and a RT-PCR Kit (Thermo Fisher Scientific), and then purified using the QIAquick PCR Purification Kit (Qiagen), as described previously (16, 22). Mcl-1 transcripts were quantitated using the TaqMan probe Hs01050896_m1 (Thermo Fisher Scientific) and a LightCycler 480 Real-Time PCR Machine (Roche Diagnostics), based on the manufacturer's instructions. RT-PCR results were expressed as means from three independent experiments and were normalized to GAPDH transcripts measured by TaqMan probe (Hs02786624_g1). Fold changes were calculated using the comparative C_t method (27).

MV4-11 cells (1 × 10⁶ cells/mouse; 0.2 mL/injection) were injected intravenously (day 0) into immunocompromised triple transgenic NSG-SGM3 female mice at eight weeks of age (NSGS, JAX#0103062; nonobese diabetic scid gamma (NOD.Cg-Prkdc^scid Il2rg^tm1Wjl Tg(CMV-IL3, CSF2, KITLG)1Eav/MloySzJ) Jackson Laboratory). Mice were randomly placed (5 mice/group; day 4) into vehicle control, 40 mg/kg gilteritinib (administered orally), 85 mg/kg ABT-199 (orally), or 40 mg/kg gilteritinib + 85 mg/kg ABT-199 cohorts [orally; 3% ethanol (200 proof), 1% Tween-80 (polyoxyethylene (20) sorbitan monooleate) and sterile water; all USP grade; v/v]. For combined treatment, gilteritinib was administered first, followed by venetoclax within 1 hour. Mice were treated daily for 27 days (treatment was stopped when one mouse in the control group showed leukemic symptoms). Mice were assessed 2×/day minimum for the duration of the study and body weights recorded daily. Mice were humanely euthanized when they presented with: >20% weight loss, decreased mobility limiting food and water access, lymph node metastases, progressive anemia, or lateral recumbency. Percent increase in lifespan (%ILS) was calculated as follows: % ILS = [T-C/C] × 100 where “T” = treated and “C” = control median day of death. In vivo experiments were approved by the Institutional Animal Care and Use Committee at Wayne State University (Detroit, MI).

Differences were compared utilizing two-sample t test. Statistical analyses were performed utilizing GraphPad Prism 5.0. Error bars represent ± SEM; significance level was set at P < 0.05. Overall survival probability was estimated (Kaplan–Meier method) and statistical analysis was performed using the log-rank test.

To determine whether midostaurin synergizes with venetoclax in inducing cell death in AML, we treated the FLT3-ITD–positive cell lines MOLM-13 and MV4-11 as well as 9 primary patient samples [8 harbor FLT3-ITD and 1 harbor FLT3-TKD (tyrosine kinase domain) mutations] with midostaurin and venetoclax, alone or in combination, for 24 hours. Synergistic cell death induction was observed in both cell lines (MOLM-13 CI < 0.20, MV4-11 CI < 0.18; Fig. 1A), and in all nine primary patient samples (CI < 0.001 to < 0.39; Fig. 1B). Increase in Annexin V–positive cells was accompanied by increased cleaved caspase-3, demonstrating induction of apoptosis (Fig. 1C). We also assessed the combination in the FLT3 wild-type (FLT3-wt) cell lines U937 and THP-1 and three FLT3-wt primary patient samples, and found CI values ranging from 0.002 to 2.12 (Fig. 1D). While the CI values indicate a synergistic interaction, the magnitude of cell death induced by the combination was substantially lower than the FLT3-ITD cells, suggesting a minor contribution of a non-FLT3 mechanism of action. Evaluation of the combination using normal peripheral blood mononuclear cells revealed statistically significant but biologically minimal increase of apoptosis in drug-treated compared with vehicle control–treated cells (Fig. 1E).

To begin to examine the mechanism of action in FLT-ITD AML, we assessed protein levels across the MAPK/ERK, PI3K/AKT, and JAK/STAT pathways via Western blot in two FLT3-ITD–positive AML cell lines and a primary patient sample 24 hours after drug treatment. Midostaurin treatment resulted in downregulation of p-S6 and p-STAT5, which was preserved in cells treated with midostaurin in combination with venetoclax in all three samples tested (Fig. 2A). Downregulation of p-AKT (both S473 and T308) was detected after venetoclax treatment and was further decreased in the combination treatment; total AKT levels decreased to a similar extent as p-AKT, while the effect of midostaurin was cell-specific (Fig. 2A). The level of p-ERK decreased following midostaurin treatment in the primary AML patient sample; however, it remained unchanged in MV4-11 cells and increased in MOLM-13 cells. Interestingly, venetoclax treatment also resulted in substantial increase of p-ERK in all three samples, which was abrogated by combination with midostaurin (Fig. 2A). To better understand this phenomenon, we conducted a time-course analysis in both MOLM-13 and MV4-11 cell lines. Increased p-ERK was detected within 12–24 hours of venetoclax exposure. Midostaurin treatment downregulated p-ERK as early as 4 hours, unlike at the 24-hour endpoint (Fig. 2B and C). In the combined treatment, p-ERK levels were nearly undetectable after just 4 hours of treatment and remained below baseline up to 24 hours during treatment. Next, we performed a time-course Annexin V-fluorescein isothiocyanate (FITC)/PI flow cytometry experiment and found that the combination induced apoptosis as early as 4 hours (Fig. 2D), which coincided with the early enhanced downregulation of p-ERK.

To begin to determine whether p-ERK is a mechanism of resistance to venetoclax, we treated AML cells with variable concentrations of venetoclax and found that there was a concentration-dependent increase of p-ERK (Fig. 2E). Furthermore, treatment with the ERK-selective inhibitor SCH772984 completely abolished venetoclax-induced expression of p-ERK and significantly enhanced venetoclax-induced apoptosis (Fig. 2F and G). Taken together, these results suggest that induction of p-ERK is a mechanism of resistance to venetoclax.

To assess the role of p-ERK in the antileukemic activity of midostaurin, we treated AML cells with midostaurin alone or in combination with SCH772984 and found that SCH772984 significantly enhanced cell death induced by midostaurin (Fig. 2H). Western blots confirmed downregulation of p-ERK by SCH772984 alone and reduction of midostaurin-induced upregulation of p-ERK (Fig. 2I). These results suggest that upregulation of p-ERK is also a mechanism of resistance to midostaurin.

Next, we examined the expression of the Bcl-2 family of proteins across two AML cell lines and a primary AML patient sample because their expression can be modulated by the FLT3 downstream signaling pathways. We identified consistent downregulation of Mcl-1 across the samples when treated with midostaurin alone and in combination with venetoclax for 24 hours (Fig. 3A). Bcl-2, Bcl-xL, and Bim protein levels were not notably affected by either agent, alone or in combination. While reduced levels of Bak and Bax were detected, the decrease was not consistent across all three samples. Time-course evaluation showed that Mcl-1 downregulation occurs within 4 hours of exposure to midostaurin in both MOLM-13 and MV4-11 (Fig. 3B); these results relate to the previously noted flow cytometric time-point analysis of enhanced cell death at 4 hours (Fig. 2D). Coimmunoprecipitation revealed that midostaurin treatment reduced Mcl-1 binding to Bim, but increased Bim binding to Bcl-2, in both MOLM-13 and MV4-11 cells. Consistent with our published work (10, 11, 28), venetoclax treatment reduced Bcl-2 binding to Bim, but increased binding of Mcl-1 to Bim (Fig. 3C). In the combined treatment, both Mcl-1 and Bcl-2 binding to Bim were reduced. To further confirm the role Mcl-1 plays in the combined treatment, we overexpressed Mcl-1 in MV4-11 cells. Its overexpression significantly reduced apoptosis induced by venetoclax alone and in combination with midostaurin (Fig. 3D), suggesting that Mcl-1 plays an important role in the mechanism of action of combined midostaurin and venetoclax. To determine whether the combination induces intrinsic apoptosis, Bak and Bax shRNA knockdown were performed. Knockdown of Bak and Bax significantly reduced Annexin V positivity (Fig. 3E). While the reduction of Annexin V–positive cells was only partial, potentially due to the redundancy of Bak and Bax, these results show that the combination induces apoptosis, at least partially through the intrinsic apoptosis pathway.

To begin to determine how midostaurin treatment reduces Mcl-1 protein level, we first performed RT-PCR. Midostaurin treatment significantly reduced Mcl-1 transcript levels 4 hours after drug treatment. Reduced levels were maintained in the combined drug-treated cells (Fig. 4A), indicating that midostaurin transcriptionally downregulates Mcl-1. To determine whether midostaurin treatment affects Mcl-1 protein stability, we treated MOLM-13 and MV4-11 cells with midostaurin and venetoclax, alone or in combination, for 4 hours. Cycloheximide treatment (10 μg/mL) revealed that Mcl-1 levels decreased significantly faster in MV4-11 cells treated with midostaurin alone and in combination with venetoclax (P < 0.01; Fig. 4B and C), while we did not detect a significant change in MOLM-13 cells. Because phosphorylation of Mcl-1 at T163 has been shown to stabilize Mcl-1 by prolonging its half-life (29), we looked at phosphorylation of Mcl-1 postmidostaurin treatment. Consistent with our cycloheximide results, we detected significantly reduced phosphorylation of Mcl-1 at T163 in MV4-11 cells treated with midostaurin alone and in combination with venetoclax, while we did not detect obvious change for MOLM-13 cells (Fig. 4D and E). Taken together, these results show that midostaurin treatment reduces Mcl-1 transcript levels and decreases expression of Mcl-1 protein. While midostaurin can affect Mcl-1 stability, it may be a cell line–specific response. Nonetheless, downregulation of Mcl-1 transcript levels occurred in both cell lines, suggesting that downregulation of Mcl-1 by midostaurin is predominantly due to alteration of transcripts.

As midostaurin is a first-generation FLT3 inhibitor and displays low selectivity, we sought to confirm that the efficacy and mechanism demonstrated were indeed due to FLT3-inhibition and not due to off-target effects. We therefore used the second-generation FLT3 inhibitor gilteritinib. Consistent with midostaurin, gilteritinib synergized with venetoclax to induce Annexin V positivity in MOLM-13 and MV4-11 cells (Fig. 5A) accompanied by cleavage of caspase-3 (Fig. 5B), demonstrating that the two agents synergize in inducing apoptosis in FLT3-ITD AML cells. Synergy was confirmed in 8 primary FLT3-ITD AML patient samples (Fig. 5C). Similar to midostaurin, the combination of venetoclax and gilteritinib also demonstrated synergy against FLT3-wt cell lines and patient samples (Fig. 5D), although the magnitude of cell death was substantially less than in the FLT3-ITD samples. The combination treatment induced apoptosis in normal peripheral blood mononuclear cells (PBMCs), although the magnitude was minimal compared with vehicle control–treated cells (PBMC#1: 4.4% vs. 7.4% and PBMC#2: 2.2% vs. 8.8%; Fig. 5E).

In vivo efficacy potential of gilteritinib in combination with venetoclax was evaluated in an early-stage MV4-11–derived xenograft mouse model. We first conducted a toxicity/preliminary efficacy study with 2 mice/group. The mice were treated with vehicle control, 40 mg/kg gilteritinib, 85 mg/kg venetoclax, or gilteritinib + venetoclax on a daily basis for 13 days (Supplementary Fig. S1A). Moderate body weight loss (3% or less) was observed during treatment, although this was completely reversible (Supplementary Fig. S1B). Median survival for vehicle control and venetoclax-treated mice was 41.5 days and 43.5 days, respectively, while gilteritinib alone was 61.5 days and the combination was 95.5 days, an ILS of 130% was achieved (Supplementary Fig. S1C). On the basis of these encouraging results, we conducted a full trial using 5 mice/group and extending treatment to 27 consecutive days (Fig. 5F). Moderate body weight loss (6% or less) was observed during treatment, although it was completely reversible (Fig. 5G). One mouse from both the venetoclax group and the combination group was excluded because of technical issues. Median survival for vehicle control, venetoclax, and gilteritinib-treated mice was 37 days, 41 days, and 91 days, respectively (Fig. 5H). On day 190, the one remaining gilteritinib-treated mouse and the four combination-treated mice were asymptomatic. At this point, we rechallenged the mice with tumor to rule-out a leaky system. Two weeks after implanting tumors, the tumors were significantly larger, demonstrating the survival of these mice was not due to regain of immunity.

Early induction of cell death in both FLT3-ITD cell lines was observed, with significantly enhanced killing demonstrated within 4 hours of combined exposure (Fig. 6A). Next, we evaluated the downstream effects of gilteritinib upon the MAPK/ERK, PI3K/AKT, and JAK/STAT pathways via Western blot and observed downregulation of p-ERK, p-S6, and p-STAT5, which were maintained when combined with venetoclax (Fig. 6B). Combined gilteritinib and venetoclax treatment reduced p-AKT (T308 in both cell lines, while S473 in MV4-11 only) and total AKT levels. In accordance with midostaurin treatment, gilteritinib treatment resulted in downregulation of Mcl-l protein levels. Similar to midostaurin, treatment with single-agent gilteritinib or venetoclax for 24 hours resulted in increased p-ERK, while the combination resulted in an overall decrease of p-ERK (Fig. 6C). To determine whether upregulation of p-ERK may be a mechanism of resistance to gilteritinib, we treated AML cells with gilteritinib alone and in combination with the ERK-selective inhibitor SCH772984 and found that SCH772984 significantly enhanced apoptosis induced by gilteritinib and prevented gilteritinib or venetoclax upregulation of p-ERK (Fig. 6D and E). To determine whether Mcl-1 plays a role in the mechanism of action, we overexpressed Mcl-1 in MV4-11 cells. Overexpression of Mcl-1 significantly reduced apoptosis induced by venetoclax treatment alone or in combination with gilteritinib (Fig. 6F). Consistent with midostaurin treatment, gilteritinib treatment caused downregulation of Mcl-1 transcript levels (Fig. 6G). In agreement with midostaurin treatment, gilteritinib combined with venetoclax was at least partially dependent on Bak and Bax (Fig. 6H), showing that the combination induces apoptosis, at least partially through the intrinsic apoptosis pathway. In summary, upregulation of p-ERK plays a role in resistance to gilteritinib treatment and the addition of venetoclax prevents induction of ERK phosphorylation. Furthermore, downregulation of Mcl-1 by gilteritinib enhances cell death induced by venetoclax.

In combining the FDA-approved FLT3 inhibitors midostaurin or gilteritinib with an additional FDA-approved anti-AML agent, venetoclax, we identified notable antileukemic synergy in FLT3-ITD AML cell lines and primary patient samples. The synergy observed between midostaurin/gilteritinib and venetoclax appears to be mechanistically based around two key points (Fig. 6I). First, Mcl-1 is downregulated by midostaurin/gilteritinib, although the precise molecular mechanism still needs to be determined—we have previously established that venetoclax frees Bim from Bcl-2, but also that this Bim can then be sequestered by Mcl-1, and therefore that joint Bcl-2 inhibition and Mcl-1 downregulation/inhibition is required for effective induction of apoptosis (10–14). Second, we expected to see suppression of multiple cell survival pathways due to multikinase inhibition with the FLT3 inhibitors, but we were surprised to observe the potent induction of p-ERK by venetoclax monotherapy. This induction was entirely abrogated by the addition of midostaurin or gilteritinib, leading us to theorize that this is another potential mechanism responsible for synergistic induction of apoptosis by the two classes of drugs (Fig. 6I). By using the selective ERK inhibitor SCH772984, we were able to confirm that resistance to venetoclax was indeed, at least, partially due to p-ERK induction.

In agreement with Bruner and colleagues (30), we found that midostaurin/gilteritinib treatment downregulates p-ERK as early as 4 hours; although, by 24 hours p-ERK levels were at least back to baseline, if not substantially increased. This is consistent with previous reports that FLT3 inhibitor treatment can cause persistent ERK activation in the presence of bone marrow stroma (31–33), although, our results were obtained in the absence of bone marrow stroma. We also found that this rebound of p-ERK was a mechanism of resistance to both midostaurin and gilteritinib. On the basis of these findings, we speculate that the timing of drug administration could potentially affect the efficacy. Administration of midostaurin or gilteritinib too far in advance of venetoclax could potentially reduce the activity of the combined treatment due to upregulation of p-ERK, a mechanism of resistance to venetoclax treatment. While administration of midostaurin or gilteritinib for a period of time long enough to downregulate Mcl-1 but prior to upregulation of p-ERK would likely afford venetoclax the opportunity to further reduce the amount of Bim bound to Bcl-2/Mcl-1 and induce apoptosis; on the other hand, this will also prevent the rebound of p-ERK during FLT3 inhibitor treatment. Our in vivo study was designed on the basis of our in vitro findings; significantly prolonged survival was realized in mice treated with the combination of gilteritinib and venetoclax. Additional studies are warranted to further optimize the schedule of drug administration, which should take into consideration the pharmacokinetic and pharmacodynamic profiles of these agents.

Although clearly showing marked synergy in FLT3-ITD AML cell lines and patient samples, we additionally identified synergy between venetoclax and midostaurin/gilteritinib in FLT3-wt cell lines and primary patient samples. We postulate that this is reflective of the existence of non–FLT3-related mechanisms. Specifically, both gilteritinib and midostaurin have known efficacy against FLT3-wt AML in the clinical setting (34, 35), so it is perhaps unsurprising to see evidence of such activity here. This synergy likely arises due to the multikinase inhibitory activity of midostaurin and the AXL-targeting of gilteritinib, which do not depend on the presence of FLT3-ITD/TKD mutations or constitutive activation of FLT3.

In summary, we found that combining midostaurin or gilteritinib with venetoclax potently induces cell death in FLT3-ITD AML cell lines and primary patient samples. Our study demonstrates that p-ERK expression is induced by venetoclax but abolished by the combination with midostaurin or gilteritinib and that downregulation of Mcl-1 by midostaurin or gilteritinib enhances venetoclax-induced cell death. Results from our MV4-11–derived xenograft mouse model show that the combination has potent antileukemic activity against FLT3-ITD AML in vivo, supporting the clinical translation of midostaurin/gilteritinib plus venetoclax. Fittingly, clinical investigations of FLT3-ITD inhibitors in combination with venetoclax were recently initiated in relapsed/refractory AML (NCT03625505 and NCT03735875).

No potential conflicts of interest were disclosed.

The funders had no role in study design, data collection, analysis and interpretation of data, decision to publish, or preparation of the manuscript.

Conception and design: J. Ma, J.W. Taub, Y. Ge

Development of methodology: L. Polin, J.W. Taub, Y. Ge

Acquisition of data (provided animals, acquired and managed patients, provided facilities, etc.): J. Ma, S. Zhao, X. Qiao, L. Polin, S.H. Dzinic, K. White, L. Zhao, H. Lin

Analysis and interpretation of data (e.g., statistical analysis, biostatistics, computational analysis): J. Ma, X. Qiao, H. Edwards, L. Polin, J. Kushner, S.H. Dzinic, K. White, J.W. Taub, Y. Ge

Writing, review, and/or revision of the manuscript: J. Ma, S. Zhao, T. Knight, H. Edwards, L. Polin, J.W. Taub, Y. Ge

Administrative, technical, or material support (i.e., reporting or organizing data, constructing databases): S. Zhao, L. Polin, J. Kushner, S.H. Dzinic, K. White, G. Wang, Y. Wang, Y. Ge

Study supervision: L. Polin, Y. Ge

This study was supported by Jilin University (Changchun, China), the Barbara Ann Karmanos Cancer Institute (Detroit, MI), Wayne State University School of Medicine (Detroit, MI), and by grants from the National Natural Science Foundation of China, NSFC 31671438 and NSFC 31471295, Graduate Innovation Fund of Jilin University (Changchun, China), Hyundai Hope on Wheels, LaFontaine Family/U Can-Cer Vive Foundation (R2-2017-53), Kids Without Cancer (R2-2017-01), Children's Hospital of Michigan Foundation (Detroit, MI), Decerchio/Guisewite Family, Justin's Gift, Elana Fund, Ginopolis/Karmanos Endowment, and the Ring Screw Textron Endowed Chair for Pediatric Cancer Research. The Animal Model and Therapeutics Evaluation Core and Genomics Core are supported, in part, by NIH Center Grant P30 CA022453 to the Karmanos Cancer Institute at Wayne State University (Detroit, MI)

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.