In a recent study involving B-cell acute lymphoblastic leukemia (B-ALL) with KMT2A rearrangement (KMT2A-r), researchers developed chimeric antigen receptor T (CAR T) cells directed at Fms-related tyrosine kinase-3 (FLT3) that demonstrated potential for antitumor efficacy. This potential was shown using both in vitro and in vivo experiments. Results of the study were reported in the journal Cancer Immunology, Immunotherapy.

CAR-T therapy has improved outcomes with B-ALL, but long-term remission has been difficult to achieve in some patients with B-ALL through this approach. For example, patients receiving CD19-targeted CAR-T therapy may experience relapse with a loss of CD19 surface expression. Because of this, some efforts have been aimed at developing CAR-T cells directed at multiple antigens.

However, in some patients, particularly those with KMT2A-r, even CAR-T therapy designed to target multiple B-cell antigens can lose efficacy upon a lineage switch to myeloid malignancy. Some research has shown FLT3-targeted CAR-T treatment to show antitumor potency in a KMT2A-r B-ALL model system, including after a lineage switch to acute myeloid leukemia, the researchers explained in their report. Additionally, they noted, prior research has shown increased expression of FLT3 in KMT2A-r B-ALL.

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The researchers developed CAR T cells directed at FLT3 or CD19. They also performed CD19-gene knockout in KMT2A-r B-ALL model cells and tested the antitumor efficacy of each CAR-T product against B-ALL cells with or without CD19. Cytotoxicity of CAR-T cells was tested both in vitro and through in vivo experiments using mice.

In vitro assays showed the FLT3-directed CAR-T cells had cytotoxicity with cell lines in which FLT3 was present. In vitro experiments also showed that CD19-directed CAR-T cells demonstrated cytotoxicity with only the B-ALL cells containing CD19. Through in vivo analysis of CD19-depleted mice, greater tumor reduction (P <.001) and longer median survival (P <.01) were seen with FLT3-directed CAR-T cells than with CD19-directed CAR-T cells.

CAR-T cells targeting both CD19 and FLT3 appeared cytotoxic to KMT2A-r B-ALL cells both with and without CD19, through in vitro analyses. Results of assays with CD19-depleted cell lines pointed to cytotoxicity with FLT3-targeted CAR-T cells in the context of CD19-negative immune escape in KMT2A-r B-ALL cells.

The researchers concluded that the results showed utility against KMT2A-r B-ALL with FLT3-directed CAR T cells. “Although dual CAR T cells should be further optimized to increase their in vivo cytotoxicity, targeting FLT3-specific CAR T cells would be a promising strategy for KMT2A-r B-ALL cells even with a CD19-negative relapsed model,” the researchers wrote in their report.


Suematsu M, Yagyu S, Yoshida H, et al. Targeting FLT3‑specific chimeric antigen receptor T cells for acute lymphoblastic leukemia with KMT2A rearrangement. Cancer Immunol Immunother. Published online October 10, 2022. doi:10.1007/s00262-022-03303-4