Impact of FLT3 Receptor (CD135) Detection by Flow Cytometry on Clinical Outcome of Adult Acute Myeloid Leukemia Patients

How acute myeloid leukemia (AML) escapes from FMS-related tyrosine kinase 3 (FLT3) inhibitors? Still an overrated complication?

FMS-related tyrosine kinase 3 (FLT3) mutations, present in about 25%-30% of acute myeloid leukemia (AML) patients, constitute one of the most frequently detected mutations in these patients. The binding of FLT3L to FLT3 activates the phosphatidylinositol 3-kinase (PI3K) and RAS pathways, producing increased cell proliferation and the inhibition of apoptosis. Two types of FLT3 mutations exist: FLT3-ITD and FLT3-TKD (point mutations in D835 and I836 or deletion of codon I836). A class of drugs, tyrosine-kinase inhibitors (TKI), targeting mutated FLT3, is already available with 1^st and 2^nd generation molecules, but only midostaurin and gilteritinib are currently approved. However, the emergence of resistance or the selection of clones not responding to FLT3 inhibitors has become an important clinical dilemma, as the duration of clinical responses is generally limited to a few months. This review analyzes the insights into mechanisms of resistance to TKI and poses a particular view on the clinical relevance of this phenomenon. Has resistance been overlooked? Indeed, FLT3 inhibitors have significantly contributed to reducing the negative impact of FLT3 mutations on the prognosis of AML patients who are no longer considered at high risk by the European LeukemiaNet (ELN) 2022. Finally, several ongoing efforts to overcome resistance to FLT3-inhibitors will be presented: new generation FLT3 inhibitors in monotherapy or combined with standard chemotherapy, hypomethylating drugs, or IDH1/2 inhibitors, Bcl2 inhibitors; novel anti-human FLT3 monoclonal antibodies (e.g., FLT3/CD3 bispecific antibodies); FLT3-CAR T-cells; CDK4/6 kinase inhibitor (e.g., palbociclib).

Flow Cytometric Identification of Hematopoietic and Leukemic Blast Cells for Tailored Clinical Follow-Up of Acute Myeloid Leukemia

Acute myeloid leukemia (AML) is a myeloid malignancy that is characterized by the accumulation of leukemic blast cells, which originate from hematopoietic stem cells that have undergone leukemic transformation and/or are more mature progenitors that have gained stemness features. Currently, no consensus exists for the flow cytometric identification of normal blast cells and their leukemic counterparts by their antigenic expression profile. Differentiating between the benign cells and the malignant cells is crucial for the further deployment of immunophenotype panels for the clinical follow-up of AML patients. This review provides an overview of immunophenotypic markers that allow the identification of leukemic blast cells in the bone marrow with multiparameter flow cytometry. This technique allows the identification of hematopoietic blast cells at the level of maturing cells by their antigen expression profile. While aberrant antigen expression of a single immunophenotypic marker cell cannot be utilized in order to differentiate leukemic blast cells from normal blast cells, combinations of multiple immunophenotypic markers can enable the distinction of normal and leukemic blast cells. The identification of these markers has provided new perspectives for tailored clinical follow-up, including therapy management, diagnostics, and prognostic purposes. The immunophenotypic marker panels, however, should be developed by carefully considering the variable antigen marker expression profile of individual patients.

Clinical implication and prognostic significance of FLT3-ITD and ASXL1 mutations in Egyptian AML patients: A single-center study

BACKGROUND

Both Fms-like tyrosine kinase 3-internal tandem duplication (FLT3-ITD) and Additional Sex Comb-like 1 (ASXL1) mutations are frequent and early genetic alteration events in acute myeloid leukemia (AML) patients. These genetic alterations may be associated with an unfavorable prognosis.

OBJECTIVE

Up to our knowledge, this is the first study performed to evaluate the clinical implication and prognostic significance of FLT3-ITD and ASXL1 mutations and their coexistence on the outcome of Egyptian AML patients.

METHODS

Our study included 83 patients with AML who were subjected to immunophenotyping and detection of FLT3-ITD and ASXL1 gene mutation by polymerase chain reaction (PCR) and real-time PCR, respectively.

RESULTS

FLT3-ITD and ASXL1 mutations were detected in 20.5% and 18.1% of AML patients respectively. Seven patients (8.4%) had co-expression of both genes' mutations. FLT3-ITD mutation was significantly higher in younger age, higher WBCs count and poor cytogenetic risk patients (P= 0.01, < 0.001 and 0.008 respectively). ASXL1 mutation was significantly higher in intermediate cytogenetic risk patients (P= 0.2). The mean period of survival and relapse-free survival (RFS) were significantly reduced in FLT3-ITD and ASXL1 mutations compared with their non-mutant types (P= 0.01 and 0.03 respectively). Both mutations were independent risk factors for overall survival (OS) and (RFS) in univariate and multivariate analysis in AML patients.

CONCLUSION

FLT3-ITD and ASXL1 gene mutations or their coexistence can predict a poor prognosis in AML patients.

Potential targeting of FLT3 acute myeloid leukemia

Aberrant FLT3 receptor signaling is common in acute myeloid leukemia (AML) and has important implications for the biology and clinical management of the disease. Patients with FLT3-mutated AML frequently present with critical illness, are more likely to relapse after treatment, and have worse clinical outcomes than their FLT3 wild type counterparts. The clinical management of FLT3-mutated AML has been transformed by the development of FLT3 inhibitors, which are now in use in the frontline and relapsed/refractory settings. However, many questions regarding the optimal approach to the treatment of these patients remain. In this paper, we will review the rationale for targeting the FLT3 receptor in AML, the impact of FLT3 mutation on patient prognosis, the current standard of care approaches to FLT3-mutated AML management, and the diverse array of FLT3 inhibitors in use and under investigation. We will also explore new opportunities and strategies for targeting the FLT3 receptor. These include targeting the receptor in patients with non-canonical FLT3 mutations or wild type FLT3, pairing FLT3 inhibitors with other novel therapies, using minimal residual disease (MRD) testing to guide the targeting of FLT3, and novel immunotherapeutic approaches.

T-cell-based immunotherapy of acute myeloid leukemia: current concepts and future developments

Acute myeloid leukemia (AML) is a heterogeneous disease linked to a broad spectrum of molecular alterations, and as such, long-term disease control requires multiple therapeutic approaches. Driven largely by an improved understanding and targeting of these molecular aberrations, AML treatment has rapidly evolved over the last 3-5 years. The stellar successes of immunotherapies that harness the power of T cells to treat solid tumors and an improved understanding of the immune systems of patients with hematologic malignancies have led to major efforts to develop immunotherapies for the treatment of patients with AML. Several immunotherapies that harness T cells against AML are in various stages of preclinical and clinical development. These include bispecific and dual antigen receptor-targeting antibodies (targeted to CD33, CD123, CLL-1, and others), chimeric antigen receptor (CAR) T-cell therapies, and T-cell immune checkpoint inhibitors (including those targeting PD-1, PD-L1, CTLA-4, and newer targets such as TIM3 and STING). The current and future directions of these T-cell-based immunotherapies in the treatment landscape of AML are discussed in this review.

CAR T cells for other pediatric non-B-cell hematologic malignancies

As CAR T-cell therapy has advanced in B-cell acute lymphoblastic leukemia, research is now underway to develop similar therapies for other lymphoid and myeloid malignancies for pediatric patients. Barriers, including antigen selection and on-target/off-tumor toxicity, have prevented the rapid development of immune-based therapies for T-lineage and myeloid malignancies. More recently, unique strategies have been developed to overcome these barriers, with several products advancing to clinical trials. For T-lineage diseases, targets have focused on CD5, CD7, and CD38, whereas myeloid disease targets have predominately focused on CD123, CD33, and, more recently, CLL-1. This review provides a comprehensive overview of these targets and approaches to overcoming safety concerns in the development of CAR T-cell therapies for pediatric patients with T-lineage and myeloid malignancies.

FLT3-ITD and CD135 Over-Expression are Frequent Findings of Poor Survival in Adult Patients with Acute Leukemias

BACKGROUND

Fms-like tyrosine kinase 3 (FLT3) expression and mutation have been considered a poor prognostic factor in acute myeloid leukemia (AML). FLT3-ITD mutation is present in 30% of adult patients with AML and 2-5% in childhood acute lymphoblastic leukemia (ALL). The impact of these mutations on the prognosis of ALL patients, has not yet been established. Moreover, a limited number of publications regarding the level of expression of the FLT3 receptor (CD135) in both leukemias exist. This study aimed to analyze the clinical outcomes associated to the presence of FLT3-ITD mutation and the expression of CD135.

METHODS

82 adult patients with newly diagnosed acute leukemia (39 with AML and 43 with ALL) were included. Flow cytometry and RT-PCR were done to analyze the expression of CD135 and the presence of FLT3 ITD mutation, respectively.

RESULTS

FLT3-ITD was present in 14 (36%) of AML and 15 (35%) of ALL patients. Disease free survival (DFS) and overall survival (OS) were lower in ALL patients having a CD135 expression >3000 cells/μL. There was a trend for poor OS in AML patients expressing FLT3 ITD. OS was worse in AML patients with high expression of CD135.

CONCLUSION

A higher (35%) frequency of FLT3-ITD was found in adult ALL patients. The presence of FLT3-ITD was associated with a trend of poor OS in AML cases, and overexpression of CD135 was correlated with poor DFS in ALL cases and poor OS in both acute leukemias.

Midostaurin in combination with chemotherapy is most effective in patients with acute myeloid leukemia presenting with high FLT3-ITD allelic ratio who proceed to allogeneic stem cell transplantation while in first complete remission

OBJECTIVES

Midostaurin, a multikinase and FLT3 inhibitor, is the first non-chemotherapy agent approved and widely adopted for the treatment of FLT3-ITD acute myeloid leukemia (AML). Yet, its role in improving survival of patients referred to allogeneic stem cell transplantation (allo-SCT) in first complete remission (CR1) needs to be defined.

METHODS

This multicenter study retrospectively evaluated the outcome of 119 FLT3-ITD AML patients [59 (49.6%) males and 60 females] intensively treated between 2015 and 2019 at five Israeli centers. In our cohort, allo-SCT in CR1 was widely implemented (47%) and patient stratification was based on the current allelic ratio (AR) cutoff of 0.5.

RESULTS

Ninety-eight patients (82.3%) achieved CR1/CR with incomplete count recovery (CRi). Death during induction was reported in 7 (5.9%) patients. In multivariate analysis, midostaurin use and allo-SCT in CR1 were the most significant factors affecting overall survival (OS). Midostaurin incorporation in chemotherapy regimens significantly improved CR + CRi rates (P = .002), reduced relapse rates (P = .02), and was remarkably advantageous for high-AR patients (2-year OS 82%). In low-AR patients, the midostaurin effect was much less prominent.

CONCLUSIONS

Our results demonstrate benefits of midostaurin incorporation in intensive chemotherapy regimens, particularly for high-AR AML patients to whom it should be offered along with allo-SCT in CR1.

γδ T Cells Number, CD200, and Flt3 Expression Is Associated with Higher Progression Free Survival in Patients with Chronic Myeloid Leukemia

BACKGROUND

Tumor immunoedition involves alterations in cells of immune system, which may play an important role in the immunosurveillance of patients with cancer diseases.

AIM OF THE STUDY

To determine the association between the number of immune cells and the expression of surface markers in leukemic cells of patients with de novo CML who achieved molecular response.

METHODS

A longitudinal study was conducted in 31 patients with de novo CML. Peripheral blood samples were obtained at diagnosis for quantification of immune cells and tumor cells expressing CD200, CD135, GpP, and Bcl-2. Results were compared with a group of 60 healthy donors. Lymphocyte subsets were analyzed during a 48 month follow-up period and molecular response to treatment was assessed simultaneously by QT-PCR. The group of patients with deep molecular response was compared with de novo CML patients; the cut-off value of cell count was determined by ROC analysis. Kaplan-Meier and Cox proportional hazard model were used to determine the significant association between the number of cells and progression-free survival.

RESULTS

Differences in number of CD4, CD4Tregs, NK, γδT, monocytes, and pDC's, tumor-cells expressing CD200⁺, CD135⁺, GpP⁺, and Bcl-2⁺ were observed between patients and healthy donors. The number of γδT lymphocytes, CD200⁺, and CD135⁺ cells were associated with longer progression-free survival (p = 0.0112, p = 0.0012 and p = 0.0201 respectively).

CONCLUSION

A γδT lymphocyte count <63 cel/uL, CD200⁺ <997 cel/uL, and CD135⁺ <23 317 cel/uL at diagnosis is associated with the maintenance of deep molecular response at 48 months in patients with de novo CML.

Tracing Leukemia Stem Cells and Their Influence on Clinical Course of Adult Acute Myeloid Leukemia

BACKGROUND

Acute myeloid leukemia (AML) evolves from neoplastic transformation of stem cell disease termed "leukemia stem cells" (LSCs). An unsatisfactory response to AML therapy is determined by the presence of minimal residual disease (MRD). The predominance of LSCs might anticipate sustained MRD results. The present study aimed to demonstrate the effect of LSCs on MRD at induction days 14 and 28 on overall survival (OS) and disease-free survival (DFS) and to compare LSC expression with MRD status.

PATIENTS AND METHODS

A total of 84 patients with de novo adult AML underwent testing using LSC panels for CD38/CD123/CD34/CD45 and CD90/CD133/CD45/CD33 and different regular MRD panels.

RESULTS

At day 14 after induction, the high expression of CD123 and CD133 had adverse effects on both OS and DFS (P = .004 and P ≤ .001 and P ≤ .001 and P ≤ .001, respectively). Greater expression of CD34⁺/CD38^-/CD123⁺ resulted in unfavorable OS and DFS (P ≤ .001 for both). Both CD34⁺/CD38^-/CD123⁺ and CD34^-/CD38⁺/CD123⁺ expression at day 14 after induction had an adverse effect on DFS only (P < .001 and P = .029, respectively). On multivariate analysis, CD133 expression and MRD status were independent prognostic parameters (hazard ratio [HR], 2.3; 95% confidence interval [CI], 1.2-4.4; P = .015; and HR, 2.9; 95% CI, 1.0-7.9; P = .041). At day 28 after induction, MRD and increased CD123⁺/CD34^-, CD34⁺/CD38^-/CD123⁺, CD133⁺/CD33^- expression were associated with inferior OS (P = .016, P = .0035, P = .0.002, and P = .002, respectively). MRD and high expression of CD34⁺CD123⁺, CD133⁺/CD33^-, CD34⁺/CD38^-/CD123⁺ were associated with inferior DFS (P < .001, P = .002, P < .001, P < .001, respectively). On multivariate analysis, only CD133⁺/CD33^- expression was the independent prognostic factor (HR, 3.1; 95% CI, 1.5-6.7; P = .003).

CONCLUSIONS

Estimation of LSC expression is a sensitive indicator of the response to therapy in adult patients with AML and might be a better prognosticator than the findings from regular MRD panels.