NKX101, an Allogeneic Off-the-Shelf CAR NK Therapy Targeting NKG2D-Ls, Has Potent In Vitro Cytotoxicity Against Patient-Derived AML Leukemic Stem Cell (LSC) and Non-Leukemic Stem Cell (Non-LSC) Blasts

Introduction

Acute myeloid leukemia (AML) is a malignancy of immature myeloid cells characterized by rapid proliferation of abnormal myeloblasts. Treatment options for patients with AML are limited, especially for relapsed/refractory (r/r) disease. Leukemic cells with stem cell features, known as leukemic stem cells (LSCs), have been implicated as the origin of relapse in minimal residual disease positive (MRD ⁺) AML (Khaldoyanidi et al. 2022, Kamel et al. 2022). Additionally, absence of NKG2D-ligand (NKG2D-L) expression has been reported to be associated with stemness and the LSC population in AML (Paczulla et. al, 2019). NKX101 is an allogeneic, off-the-shelf, healthy donor-derived chimeric antigen receptor (CAR) NK cell therapy candidate engineered to express an NKG2D-L targeting CAR as well membrane bound IL-15. In a clinical trial, NKX101 has shown promise for the treatment of r/r AML, including generating complete responses (CR) with MRD negativity (MRD ^-). In this study, we utilize a flow cytometry-based approach to (i) evaluate NKG2D-L cell surface expression on normal hematopoietic cells vs. primary AML blasts, (ii) assess the expression pattern of NKG2D-Ls on AML blast subsets, and (iii) determine the potency of NKX101 cytotoxic killing of both LSC and non-LSC AML blasts.

Methods

NKX101 cells were generated from peripheral blood leukopaks from healthy donors. Cryopreserved bone marrow mononuclear cells (BMMCs) were obtained from AML patients in accordance with approved IRB protocols (n=20). BMMCs from healthy donors were used as controls (n=10). AML somatic mutation status was determined using an Illumina TruSight® Myeloid targeted sequencing panel. NKX101 cytotoxicity against patient derived AML blasts was assessed using a flow cytometry-based assay that simultaneously measured specific cytotoxicity AML blast/LSC markers. NKG2D-L protein expression on LSC and non-LSC subsets was determined using flow cytometry with fluorescently labeled monoclonal antibodies specific to MICA, MICB, and ULBP1-6.

Results

Utilizing patient-derived primary samples, we demonstrate that NKG2D-Ls are expressed on AML blast cells and expressed at higher levels on AML blast cells compared to healthy normal hematopoietic cells (p = 0.002). Furthermore, we show that NKG2D-Ls are expressed at similar levels on both LSC and non-LSC AML blast populations. Target cell specific flow cytometry-based cytotoxicity assays revealed that NKX101 cells potently kill patient-derived AML blasts in a dose-dependent manner. Moreover, NKX101 cells had similar cytotoxic potency on both LSC and non-LSC AML blasts subsets.

Conclusions

In summary, we show that NKG2D-Ls are expressed on the surface of AML blasts at levels above those observed in normal hematopoietic cells. NKG2D-Ls are expressed on both non-LSC AML blasts and on the LSC population implicated as the origin of relapse in AML. Additionally, the ability of NKX101 to potently kill AML LSC blasts suggests that NKG2D-L targeting may be a viable mechanism for eliminating LSC in the blood and bone marrow of patients with r/r AML. Taken together, these data support NKG2D-Ls as promising therapeutic targets for AML and support further investigation of NKX101 CAR NK cell therapy for the treatment of r/r AML.