Targeting RIPK1 in AML cells carrying FLT3-ITD

Mutations of fms-like tyrosine kinase 3 (FLT3) are the most frequent mutations in acute myeloid leukemia (AML). Furthermore, the internal tandem duplication (ITD) represents the most common mutation of FLT3 in AML. To explore therapeutic strategies for AML patients carrying FLT3-ITD, we analyzed death receptor (DR) signaling networks in AML cells comprising FLT3-ITD. We have started with murine myeloid progenitor 32D cells that ectopically express human FLT3-ITD (32D- FLT3-ITD) and found that RIPK1 is strongly upregulated in these cells. Subsequently, we have shown that combinatorial treatment of 32D-FLT3-ITD cells with the SMAC mimetic BV6 and CD95L sensitizes these cells toward apoptosis and necroptosis. Moreover, combinatorial treatment with death ligands (DLs), for example, CD95L or tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) and BV6 enhanced cell death in primary AML blasts from patients carrying FLT3-ITD mutation. Finally, pharmacological and genetic targeting of RIPK1 inhibited DL/BV6-mediated cell death in cells with FLT3-ITD mutations. Taken together, our study suggests a promising therapeutic opportunity for AML cancer cells harboring FLT3-ITD mutation via targeting RIPK1 pathways.     

Use of FLT3 Inhibitors in AML

Multiple FLT3 inhibitors that have been recently approved for use in FLT3 mutated acute myeloid leukemia (AML). These drug approvals represent a new standard of care for patients with FLT3 mutations, including the use of midostaurin in the frontline and gilteritinib in the salvage setting. The success of midostaurin used in combination with induction chemotherapy has prompted exploration of newer, more potent and targeted inhibitors in the upfront setting in combination with chemotherapy. At

FLT3 Inhibitors

Targeting of FLT3 signaling via small molecule inhibitors has been a heavily studied therapeutic strategy over the last decade and half. First generation, multi-targeted FLT3 tyrosine kinase inhibitors (TKIs) such as midostaurin, sorafenib and lestaurtinib were limited by poor drug selectivity, potency and unfavorable protein binding characteristics. Early monotherapy trials with these inhibitors showed little activity beyond transient decrease in circulating peripheral blasts, disappointing

Resistance to FLT3 Inhibitors

Despite observed clinical activity, resistance remains a ubiquitous clinical problem for all FLT3 inhibitors. On-target secondary KD mutations in FLT3 are the most common mechanism of acquired resistance in patients responding to type II inhibitors such as quizartinib and sorafenib, which bind only the inactive kinase conformation.^17-19 The most common resistance-causing mutations occur at the FLT3 gatekeeper F691 and activation loop D835 residues, but may also involve other residues in the

FLT3 Inhibitor Combinations and Maintenance Strategies

Based on the activity of first-generation, less targeted FLT3 inhibitors when used in combination with chemotherapy or with hypomethylating agent (HMA) treatment, current ongoing development efforts have focused on assessment of FLT3 TKI combination strategies.^8,9,24 Thus far, reported results of trials combining next-generation FLT3 TKIs with induction and consolidation chemotherapy in the first-line setting have been encouraging.^25-27 Based on these favorable early outcomes, multiple trials

References

• 1.Cancer Genome Atlas Research N, Ley TJ, Miller C, et al. Genomic and epigenomic landscapes of adult de novo acute myeloid leukemia. N Engl J Med. 2013;368(22):2059-2074.
• 2.Papaemmanuil E, Dohner H, Campbell PJ. Genomic Classification in Acute Myeloid Leukemia. N Engl J Med. 2016;375(9):900-901.
• 3.Bolouri H, Farrar JE, Triche T, Jr., et al. The molecular landscape of pediatric acute myeloid leukemia reveals recurrent structural alterations and age-specific mutational interactions. Nat Med.

     

506 p53 Activation of Mesenchymal Stromal Cells (MSC) Partially Abrogates Microenvironment-Mediated Resistance to FLT3 Inhibition in FLT3/ITD AML

We investigated the role of p53 in BM stromal cells in stromal cell-mediated resistance to FLT3 inhibition in FLT3/ITD AML and found that p53 activation in stroma cells blunts stroma cell-mediated resistance to FLT3 inhibition, in part through regulation of SDF-1.

Full Abstract

FLT3 inhibitors have recently been introduced to overcome the dismal prognosis of AML with FLT3/ITD mutations. However, while ciculating blasts are rapidly eliminated, bone marrow (BM) responses are in general less impressive. One potential explanation for the reduced BM response compared to the striking activity against circulating blast cells may be microenvironmental resistance to FLT3 inhibitors, including protection of FLT3/ITD+ blasts through the SDF-1 (CXCL12)/CXCR4 axis and novel

     

Targeting CD123 in AML

An ideal cell surface target for therapy in leukemia would be: tumor-specific (not expressed on normal cells) or at least enriched on tumor cells, necessary for tumor but not for normal cell survival, internalized efficiently (if the surface-targeted agent is conjugated to chemotherapy or a toxin molecule), and recycled rapidly to the cell surface. While a single target that meets all of these criteria has not yet been discovered in AML, CD123 has emerged as an attractive candidate.¹ The first-in-class CD123-targeting agent, tagraxofusp-erzs (SL-401) was approved in 2018 for patients with blastic plasmacytoid dendritic cell neoplasm (BPDCN)² and is currently in trials for several other hematologic malignancies, including AML. Several other CD123-targeted drugs are in development.     

FLT3 inhibitor KRN383 on xenografted human leukemic cells harboring FLT3-activating mutations FLT3 in AML: much more to learn about biology and optimal targeting

Aberrant FLT3 function in leukemia blasts is associated with a poor prognosis. A number of FLT3 modulators are in development. FLT3 mutations may synergistize with other molecular abnormalities in myeloid transformation. Further insights into FLT3 biology are needed to optimally study the therapeutic role of FLT3 inhibitors.     

BH3 mimetic ABT-737 neutralizes resistance to FLT3 inhibitor treatment mediated by FLT3-independent expression of BCL2 in primary AML blasts.

FLT3 defines a promising target for the treatment of acute myeloid leukemia (AML). In contrast to their efficacy in cell lines, FLT3-specific inhibitors as single agents have only modest clinical activity in patients with AML. As demonstrated here, overexpression of anti-apoptotic proteins of the BCL2 family leads to resistance against FLT3 inhibitors in a hematopoietic cell line model with activating FLT3 mutations. The susceptibility to FLT3 inhibition could be restored by treatment with the novel BH3 mimetic ABT-737. Primary AML samples tested in our study showed a high expression of BCL2 protein, but not of BCL-xL or MCL1. BCL2 protein levels were not reduced after dephosphorylation of FLT3 and its downstream target STAT5 in patient samples with FLT3 internal tandem duplications. Interestingly, treatment with ABT-737 caused apoptotic cell death in all primary AML samples at submicromolar level and synergized efficiently with FLT3 inhibition in AML samples with activating FLT3 mutations. In contrast to AML cell lines, BCR-ABL transformed human cells showed resistance to ABT-737, which might be due to the induction of MCL1 by BCR-ABL. Inhibition of BCL2 family members might define a novel highly efficient and specific strategy in the combined or monotreatment of AML.     

A potential therapeutic target for FLT3-ITD AML: PIM1 kinase

Patients with acute myeloid leukemia (AML) and a FLT3 internal tandem duplication (ITD) mutation have a poor prognosis, and FLT3 inhibitors are now under clinical investigation. PIM1, a serine/threonine kinase, is up-regulated in FLT3-ITD AML and may be involved in FLT3-mediated leukemogenesis. We employed a PIM1 inhibitor, AR00459339 (Array Biopharma Inc.), to investigate the effect of PIM1 inhibition in FLT3-mutant AML. Like FLT3 inhibitors, AR00459339 was preferentially cytotoxic to FLT3-ITD cells, as demonstrated in the MV4-11, Molm-14, and TF/ITD cell lines, as well as 12 FLT3-ITD primary samples. Unlike FLT3 inhibitors, AR00459339 did not suppress phosphorylation of FLT3, but did promote the de-phosphorylation of downstream FLT3 targets, STAT5, AKT, and BAD. Combining AR00459339 with a FLT3 inhibitor resulted in additive to mildly synergistic cytotoxic effects. AR00459339 was cytotoxic to FLT3-ITD samples from patients with secondary resistance to FLT3 inhibitors, suggesting a novel benefit to combining these agents. We conclude that PIM1 appears to be closely associated with FLT3 signaling, and that inhibition of PIM1 may hold therapeutic promise, either as monotherapy, or by overcoming resistance to FLT3 inhibitors.     

Targeting MCL-1 sensitizes FLT3-ITD-positive leukemias to cytotoxic therapies

Patients suffering from acute myeloid leukemias (AML) bearing FMS-like tyrosine kinase-3-internal tandem duplications (FLT3-ITD) have poor outcomes following cytarabine- and anthracyclin-based induction therapy. To a major part this is attributed to drug resistance of FLT3-ITD-positive leukemic cells. Against this background, we have devised an antibody array approach to identify proteins, which are differentially expressed by hematopoietic cells in relation to activated FLT3 signaling. Selective upregulation of antiapoptotic myeloid cell leukemia-1 (MCL-1) was found in FLT3-ITD-positive cell lines and primary mononuclear cells from AML patients as compared with FLT3-wild-type controls. Upregulation of MCL-1 was dependent on FLT3 signaling as confirmed by its reversion upon pharmacological inhibition of FLT3 activity by the kinase inhibitor PKC412 as well as siRNA-mediated suppression of FLT3. Heterologously expressed MCL-1 substituted for FLT3 signaling by conferring resistance of hematopoietic cells to antileukemia drugs such as cytarabine and daunorubicin, and to the proapoptotic BH3 mimetic ABT-737. Conversely, suppression of endogenous MCL-1 by siRNA or by flavopiridol treatment sensitized FLT3-ITD-expressing hematopoietic cells to cytotoxic and targeted therapeutics. In conclusion, MCL-1 is an essential effector of FLT3-ITD-mediated drug resistance. Therapeutic targeting of MCL-1 is a promising strategy to overcome drug resistance in FLT3-ITD-positive AML.     

Stability and prognostic influence of FLT3 mutations in paired initial and relapsed AML samples.

In acute myeloid leukemia (AML), activating mutations in the fms-like tyrosine kinase 3 (FLT3) gene predict poor prognosis. We determined FLT3 internal tandem duplications (FLT3/ITD) and D835 point mutations in paired initial and relapse samples from 80 pediatric and adult AML patients. One D835 point mutation was found in an initial pediatric AML sample. Fms-like tyrosine kinase 3/ITDs were present in 21 initial and 22 relapse samples (26.3 and 27.5%, respectively). Interestingly, FLT3/ITD positivity was related to a significantly shorter time to relapse, most pronounced when the ITD-positive status was found at relapse (P<0.001). However, FLT3/ITD status changed between diagnosis and relapse in 14 cases. In four patients, the FLT3/ITD became undetectable at relapse in five patients FLT3/ITDs were only detected at relapse, and in five patients the length or number of FLT3/ITDs changed. Gain of FLT3/ITDs may suggest oligoclonality with selective outgrowth of the FLT3/ITD-positive clone, whereas losses may reflect ITDs in the more mature leukemic cells rather than in the leukemic stem cell, or, alternatively, that other genetic aberrations provided a greater selective advantage. Studying FLT3/ITD kinetics in minimal residual disease setting may provide some answers for the changes we observed. Fms-like tyrosine kinase 3/ITD is a relevant marker for prognosis, and remains an important target for therapeutic inhibition.     

Meta-analysis for the potential application of FLT3-TKD mutations as prognostic indicator in non-promyelocytic AML

A meta-analysis was performed to demonstrate the prognostic significance of FLT3-TKD mutations in acute myeloid leukemia (AML). The overall hazard ratio (HR) of FLT3-TKD/FLT3-wild type (WT) for disease-free survival (DFS) was 1.20. The overall HRs for overall survival (OS) of FLT3-TKD to FLT3-ITD and FLT3-WT were 0.87 and 1.18. For non-promyelocytic AML with intermediate cytogenetics, the overall HR for DFS of FLT3-TKD/FLT3-ITD was 0.71. The HR for OS of FLT3-TKD/FLT3-ITD was 0.75. Adult AML patients with FLT3-TKD mutations exhibit better outcomes than those with FLT3-ITD. The patients with FLT3-TKD mutations with intermediate cytogenetics had similar OS as those with FLT-WT mutations.     

Allogeneic hematopoietic stem cell transplantation at the first remission for younger adults with FLT3‐internal tandem duplication AML: The JALSG AML209‐FLT3‐SCT study

In this phase II multicenter study (JALSG AML209‐FLT3‐SCT), we aimed to prospectively elucidate the role of allogeneic hematopoietic stem cell transplantation (allo‐HSCT) at first complete remission (CR1) for FLT3‐internal tandem duplication (ITD)‐positive AML. Newly diagnosed de novo AML patients with FLT3‐ITD were enrolled at the achievement of CR1 and received allo‐HSCT as soon as possible after the first consolidation therapy. Mutations of 57 genes in AML cells at diagnosis were also analyzed. Among 48 eligible patients with a median age of 38.5 (17‐49) years, 36 (75%) received allo‐HSCT at a median of 108 days after CR1. The median follow‐up was 1726 days. The primary end‐point, 3‐year disease‐free survival (DFS) based on an intent to treat analysis, was 43.8% (95% confidence interval [CI], 30%‐57%), suggesting the efficacy of this treatment because the lower limit of the 95% CI exceeded the threshold response rate of 20%. The 3‐year overall survival, post‐transplant DFS, and non‐relapse mortality rates were 54.2% (95% CI, 39%‐67%), 58.3% (95% CI, 41%‐72%), and 25.0% (95% CI, 12%‐40%), respectively. The median ITD allelic ratio (AR) was 0.344 (0.006‐4.099). Neither FLT3‐ITD AR nor cooccurring genetic alterations was associated with a poor DFS. This prospective study indicated the efficacy and safety of allo‐HSCT for FLT3‐ITD AML patients in CR1. This study was registered at: www.umin.ac.jp/ctr/ as #UMIN000003433.     

Outcome of older adults with cytogenetically normal AML (CN-AML) and FLT3 mutations

AML patients under the age of 60 whose blasts harbor a FLT3 internal tandem duplication (ITD) mutation have a higher relapse rate and inferior survival compared to those without this mutation. To determine if FLT3ITD also carries a negative prognostic impact in older adults receiving therapies commonly used in this age group, we retrospectively analyzed outcomes of patients ≥60 years with CN-AML according to FLT3 mutation status. We identified 91 newly diagnosed CN-AML patients, 55 with wild-type FLT3 and 36 with FLT3ITD. Of the 91 patients, 36 received supportive care and/or experimental therapies while the remaining 55 received induction chemotherapy, followed by allogeneic SCT in 17 of these patients. Based on univariate analysis, advanced age at diagnosis was significantly associated with shorter overall survival (OS) (p < .0001) while intensive therapies were associated with improved OS (p < .0001). In a multivariate analysis that accounted for type of treatment, patient age, gender, and WBC count, FLT3ITD was significantly associated with shorter OS compared to wtFLT3 [p = .001; hazard ratio (HR) = 2.23; 95% CI: 1.35-3.70]. Our data support the negative prognostic impact of FLT3ITD in older adults with CN-AML.     

Targeting CHK1 inhibits cell proliferation in FLT3-ITD positive acute myeloid leukemia

CHK1 Ser/Thr kinase, a well characterized regulator of DNA damage response, is also involved in normal cell cycle progression. In this study, we investigate how CHK1 participates to proliferation of acute myeloid leukemia cells expressing the mutated FLT3-ITD tyrosine kinase receptor. Pharmacological inhibition of CHK1 as well as its shRNA mediated down regulation reduced the proliferation rate of FLT-ITD expressing leukemic cell lines in a cytostatic manner. Flow cytometry analysis revealed no accumulation in a specific phase of the cell cycle upon CHK1 inhibition. Accordingly, lentiviral-mediated CHK1 overexpression increased the proliferation rate of FLT3-ITD expressing cells, as judged by cell viability and [3H] thymidine incorporation experiments. By contrast, expression of a ser280 mutant did not, suggesting that phosphorylation of this residue is an important determinant of CHK1 proliferative function. Clonogenic growth of primary leukemic cells from patients in semi-solid medium was reduced upon CHK1 inhibition, confirming the data obtained with leukemic established cell lines. Surprisingly, 3 out of 4 CHK1 inhibitory compounds tested in this study were also potent inhibitors of the FLT3-ITD tyrosine kinase receptor. Altogether, these data identify CHK1 as a regulator of FLT3-ITD-positive leukemic cells proliferation, and they open interesting perspectives in terms of new therapeutic strategies for these pathologies.     

Hypomethylating agent and venetoclax with FLT3 inhibitor “triplet” therapy in older/unfit patients with FLT3 mutated AML

In older/unfit newly diagnosed patients with FLT3 mutated acute myeloid leukemia (AML), lower intensity chemotherapy (LIC) in combination with either a FLT3 inhibitor or with venetoclax results in poor overall survival (median 8 to 12.5 months). We performed a retrospective analysis of 87 newly diagnosed FLT3 mutated AML patients treated on triplet (LIC + FLT3 inhibitor + Venetoclax, [N = 27]) and doublet (LIC + FLT3 inhibitor, [N = 60]) regimens at our institution. Data were collected from prospective clinical trials in 75% (N = 65) and 25% (N = 22) who received the same treatment regimens outside of a clinical trial. Triplet therapy was associated with significantly higher rates of complete remission (CR) (67% versus 32%, P = 0.002), CR/CRi (93% versus 70%, P = 0.02), FLT3-PCR negativity (96% versus 54%, P < 0.01), and flow-cytometry negativity (83% versus 38%, P < 0.01) than doublets. At the end of the first cycle, the median time to ANC > 0.5 (40 versus 21 days, P = 0.15) and platelet > 50 K (29 versus 25 days, P = 0.6) among responders was numerically longer with triplets, but 60-day mortality was similar (7% v 10%). With a median follow-up of 24 months (median 12 months for triplet arm, and 63 months for doublet arm), patients receiving a triplet regimen had a longer median overall survival (not reached versus 9.5 months, P < 0.01). LIC combined with FLT3 inhibitor and venetoclax (triplet) may be an effective frontline regimen for older/unfit FLT3 mutated AML that should be further validated prospectively.Subject terms: Medical research, Health care     

The combination of CUDC-907 and gilteritinib shows promising in vitro and in vivo antileukemic activity against FLT3-ITD AML

About 25% of patients with acute myeloid leukemia (AML) harbor FMS-like tyrosine kinase 3 (FLT3) internal tandem duplication (ITD) mutations and their prognosis remains poor. Gilteritinib is a FLT3 inhibitor approved by the US FDA for use in adult FLT3-mutated relapsed or refractory AML patients. Monotherapy, while efficacious, shows short-lived responses, highlighting the need for combination therapies. Here we show that gilteritinib and CUDC-907, a dual inhibitor of PI3K and histone deacetylases, synergistically induce apoptosis in FLT3-ITD AML cell lines and primary patient samples and have striking in vivo efficacy. Upregulation of FLT3 and activation of ERK are mechanisms of resistance to gilteritinib, while activation of JAK2/STAT5 is a mechanism of resistance to CUDC-907. Gilteritinib and CUDC-907 reciprocally overcome these mechanisms of resistance. In addition, the combined treatment results in cooperative downregulation of cellular metabolites and persisting antileukemic effects. CUDC-907 plus gilteritinib shows synergistic antileukemic activity against FLT3-ITD AML in vitro and in vivo, demonstrating strong translational therapeutic potential.Subject terms: Acute myeloid leukaemia, Drug development