CAR-T Cells Immunotherapies for the Treatment of Acute Myeloid Leukemia-Recent Advances

In order to increase the effectiveness of cancer therapies and extend the long-term survival of patients, more and more often, in addition to standard treatment, oncological patients receive also targeted therapy, i.e., CAR-T cells. These cells express a chimeric receptor (CAR) that specifically binds an antigen present on tumor cells, resulting in tumor cell lysis. The use of CAR-T cells in the therapy of relapsed and refractory B-type acute lymphoblastic leukemia (ALL) resulted in complete remission in many patients, which prompted researchers to conduct tests on the use of CAR-T cells in the treatment of other hematological malignancies, including acute myeloid leukemia (AML). AML is associated with a poorer prognosis compared to ALL due to a higher risk of relapse caused by the development of resistance to standard treatment. The 5-year relative survival rate in AML patients was estimated at 31.7%. The objective of the following review is to present the mechanism of action of CAR-T cells, and discuss the latest findings on the results of anti-CD33, -CD123, -FLT3 and -CLL-1 CAR-T cell therapy, the emerging challenges as well as the prospects for the future.

Bimodal expression of potential drug target CLL-1 (CLEC12A) on CD34+ blasts of AML patients

Objectives

This study aims to retrospectively assess C‐lectin‐like molecule 1 (CLL‐1) bimodal expression on CD34+ blasts in acute myeloid leukemia (AML) patients (total N = 306) and explore potential CLL‐1 bimodal associations with leukemia and patient‐specific characteristics.

Methods

Flow cytometry assays were performed to assess the deeper immunophenotyping of CLL‐1 bimodality. Cytogenetic analysis was performed to characterize the gene mutation on CLL‐1‐negative subpopulation of CLL‐1 bimodal AML samples.

Results

The frequency of a bimodal pattern of CLL‐1 expression of CD34⁺ blasts ranged from 8% to 65% in the different cohorts. Bimodal CLL‐1 expression was most prevalent in patients with MDS‐related AML (P = .011), ELN adverse risk (P = .002), NPM1 wild type (WT, P = .049), FLT3 WT (P = .035), and relatively low percentages of leukemia‐associated immunophenotypes (P = .006). Additional immunophenotyping analysis revealed the CLL‐1⁻ subpopulation may consist of pre‐B cells, immature myeloblasts, and hematopoietic stem cells. Furthermore, (pre)‐leukemic mutations were detected in both CLL‐1⁺ and CLL‐1⁻ subfractions of bimodal samples (N = 3).

Conclusions

C‐lectin‐like molecule 1 bimodality occurs in about 25% of AML patients and the CLL‐1⁻ cell population still contains malignant cells, hence it may potentially limit the effectiveness of CLL‐1‐targeted therapies and warrant further investigation.

Expression of putative leukemia stem cell targets in genetically-defined acute myeloid leukemia subtypes

Although most acute myeloid leukemia (AML) patients achieve complete remissions, the majority still eventually relapse and die of their disease. Rare primitive leukemia cells, so-called leukemia stem cells (LSCs), represent one potential type of resistant cell subpopulation responsible for this dissociation between response and cure. Several LSC targets have been described, but there is limited evidence about their relative utility or that targeting any can prevent relapse. LSCs not only appear to be biologically heterogeneous, but the classic immunocompromised mouse transplantation model also has serious shortcomings as an LSC assay. Out data suggest that the most immature cell phenotype that can be identified within a patient's leukemia may be clinically relevant and represent the de facto LSC. Moreover, although phenotypically heterogeneous, these putative LSCs show consistent phenotypes within individual genetically defined groups. Using this LSC definition, we studied several previously described putative LSC targets, CD25, CD26, CD47, CD96, CD123, and CLL-1, and all were expressed across heterogeneous LSC phenotypes. In addition, with the exception of CD47, there was at most low expression of these targets on normal hematopoietic stem cells (HSCs). CD123 and CLL-1 demonstrated the greatest expression differences between putative LSCs and normal HSCs. Importantly, CD123 monoclonal antibodies were cytotoxic in vitro to putative LSCs from all AML subtypes, while showing limited to no toxicity against normal HSCs and hematopoietic progenitors. Since minimal residual disease appears to be a more homogeneous population of cells responsible for relapse, targeting CD123 in this setting may be most effective.

Targeting CLL-1 for acute myeloid leukemia therapy

Despite major scientific discoveries and novel therapies over the past four decades, the treatment outcomes of acute myeloid leukemia (AML), especially in the adult patient population remain dismal. In the past few years, an increasing number of targets such as CD33, CD123, CLL-1, CD47, CD70, and TIM3, have been developed for immunotherapy of AML. Among them, CLL-1 has attracted the researchers’ attention due to its high expression in AML while being absent in normal hematopoietic stem cell. Accumulating evidence have demonstrated CLL-1 is an ideal target for AML. In this paper, we will review the expression of CLL-1 on normal cells and AML, the value of CLL-1 in diagnosis and follow-up, and targeting CLL-1 therapy-based antibody and chimeric antigen receptor T cell therapy as well as providing an overview of CLL-1 as a target for AML.

High GPR56 surface expression correlates with a leukemic stem cell gene signature in CD34-positive AML

Acute myeloid leukemia (AML) is driven by a minor fraction of leukemic stem cells (LSCs) whose persistence is considered being the primary cause of disease relapse. A detailed characterization of the surface immunophenotype of LSCs to discriminate them from bulk leukemic blasts may enable successful targeting of this population thereby improving patient outcomes in AML. To identify surface markers, which may reflect LSC activity at diagnosis, we performed a detailed analysis of 16 putative LSC markers in CD34/38 leukemic subcompartments of 150 diagnostic AML samples using multicolor flow cytometry. The most promising markers were then selected to determine a possible correlation of their expression with a recently published LSC gene signature. We found GPR56 and CLL-1 to be the most prominently differently expressed surface markers in AML subcompartments. While GPR56 was highest expressed within the LSC-enriched CD34⁺ 38^- subcompartment as compared to CD34⁺ 38⁺ and CD34^- leukemic bulk cells, CLL-1 expression was lowest in CD34⁺ 38^- leukemic cells and increased in CD34⁺ 38⁺ and CD34^- blasts. Furthermore, high GPR56 surface expression in CD34⁺ 38^- leukemic cells correlated with a recently published LSC gene expression signature and was associated with decreased overall survival in patients receiving intensive chemotherapy. In contrast, CLL-1 expression correlated inversely with the LSC gene signature and was not informative on outcome. Our data strongly support GPR56 as a promising clinically relevant marker for identifying leukemic cells with LSC activity at diagnosis in CD34-positive AML.

Preclinical pharmacokinetics and pharmacodynamics of DCLL9718A: An antibody-drug conjugate for the treatment of acute myeloid leukemia

Few treatment options are available for acute myeloid leukemia (AML) patients. DCLL9718A is an antibody-drug conjugate that targets C-type lectin-like molecule-1 (CLL-1). This receptor is prevalent on monocytes, neutrophils, and AML blast cells, and unlike CD33, is not expressed on hematopoietic stem cells, thus providing possible hematopoietic recovery. DCLL9718A comprises an anti-CLL-1 IgG1 antibody (MCLL0517A) linked to a pyrrolobenzodiazepine (PBD) dimer payload, via a cleavable disulfide-labile linker. Here, we characterize the in vitro and in vivo stability, the pharmacokinetics (PK) and pharmacodynamics (PD) of DCLL9718A and MCLL0517A in rodents and cynomolgus monkeys. Three key PK analytes were measured in these studies: total antibody, antibody-conjugated PBD dimer and unconjugated PBD dimer. In vitro, DCLL9718A, was stable with most (> 80%) of the PBD dimer payload remaining conjugated to the antibody over 96 hours. This was recapitulated in vivo with antibody-conjugated PBD dimer clearance estimates similar to DCLL9718A total antibody clearance. Both DCLL9718A and MCLL0517A showed linear PK in the non-binding rodent species, and non-linear PK in cynomolgus monkeys, a binding species. The PK data indicated minimal impact of conjugation on the disposition of DCLL9718A total antibody. Finally, in cynomolgus monkey, MCLL0517A showed target engagement at all doses tested (0.5 and 20 mg/kg) as measured by receptor occupancy, and DCLL9718A (at doses of 0.05, 0.1 and 0.2 mg/kg) showed strong PD activity as evidenced by notable reduction in monocytes and neutrophils.

Prognostic and therapeutic role of CLEC12A in acute myeloid leukemia

CLEC12A has recently been identified as an antigen, expressed on leukemic stem cells and leukemic blasts. Given the fact that this expression profile seems stable throughout diagnosis, treatment and relapse on leukemic blasts and leukemic stem cells, CLEC12A can be considered a highly potent and reliable marker for the detection of measurable residual disease and therefore applicable for risk stratification and prognostication in AML. Low CLEC12A expression on leukemic blasts seems to be independently associated with lower likelihood of achieving complete remission after 1 cycle of induction chemotherapy, shorter event free survival, as well as overall survival, indicating potential prognostic properties of CLEC12A expression itself. Lack of expression on the normal hematopoietic stem and progenitor cells, in contrast to CD123 and CD33, might result in less toxicity regarding cytopenias, making CLEC12A an interesting target for innovating immunotherapies, including monoclonal and bispecific antibodies, antibody-drug conjugates and CAR-T cells therapy.

Development of A Chimeric Antigen Receptor Targeting C-Type Lectin-Like Molecule-1 for Human Acute Myeloid Leukemia

The treatment of patients with acute myeloid leukemia (AML) with targeted immunotherapy is challenged by the heterogeneity of the disease and a lack of tumor-exclusive antigens. Conventional immunotherapy targets for AML such as CD33 and CD123 have been proposed as targets for chimeric antigen receptor (CAR)-engineered T-cells (CAR-T-cells), a therapy that has been highly successful in the treatment of B-cell leukemia and lymphoma. However, CD33 and CD123 are present on hematopoietic stem cells, and targeting with CAR-T-cells has the potential to elicit long-term myelosuppression. C-type lectin-like molecule-1 (CLL1 or CLEC12A) is a myeloid lineage antigen that is expressed by malignant cells in more than 90% of AML patients. CLL1 is not expressed by healthy Hematopoietic Stem Cells (HSCs), and is therefore a promising target for CAR-T-cell therapy. Here, we describe the development and optimization of an anti-CLL1 CAR-T-cell with potent activity on both AML cell lines and primary patient-derived AML blasts in vitro while sparing healthy HSCs. Furthermore, in a disseminated mouse xenograft model using the CLL1-positive HL60 cell line, these CAR-T-cells completely eradicated tumor, thus supporting CLL1 as a promising target for CAR-T-cells to treat AML while limiting myelosuppressive toxicity.

Treatment of Acute Myeloid Leukemia with T Cells Expressing Chimeric Antigen Receptors Directed to C-type Lectin-like Molecule 1

The successful immunotherapy of acute myeloid leukemia (AML) has been hampered because most potential antigenic targets are shared with normal hematopoietic stem cells (HSCs), increasing the risk of sustained and severe hematopoietic toxicity following treatment. C-type lectin-like molecule 1 (CLL-1) is a membrane glycoprotein expressed by >80% of AML but is absent on normal HSCs. Here we describe the development and evaluation of CLL-1-specific chimeric antigen receptor T cells (CLL-1.CAR-Ts) and we demonstrate their specific activity against CLL-1⁺ AML cell lines as well as primary AML patient samples in vitro. CLL-1.CAR-Ts selectively reduced leukemic colony formation in primary AML patient peripheral blood mononuclear cells compared to control T cells. In a human xenograft mouse model, CLL-1.CAR-Ts mediated anti-leukemic activity against disseminated AML and significantly extended survival. By contrast, the colony formation of normal progenitor cells remained intact following CLL-1.CAR-T treatment. Although CLL-1.CAR-Ts are cytotoxic to mature normal myeloid cells, the selective sparing of normal hematopoietic progenitor cells should allow full myeloid recovery once CLL-1.CAR-T activity terminates. To enable elective ablation of the CAR-T, we therefore introduced the inducible caspase-9 suicide gene system and we show that exposure to the activating drug rapidly induced a controlled decrease of unwanted CLL-1.CAR-T activity against mature normal myeloid cells.