CAR T-cells Targeting FLT3 Have Potent Activity Against FLT3ITD AML and Act Synergistically with the FLT3-inhibitor Crenolanib

FMS-like tyrosine kinase 3 (FLT3) is a transmembrane protein expressed on normal hematopoietic stem and progenitor cells (HSC) and retained on malignant blasts in acute myeloid leukemia (AML). We engineered CD8 and CD4 T-cells expressing a FLT3-specific chimeric antigen receptor (CAR) and demonstrate they confer potent reactivity against AML cell lines and primary AML blasts that express either wild-type FLT3 or FLT3 with internal tandem duplication (FLT3-ITD). We also show that treatment with the FLT3-inhibitor crenolanib leads to increased surface expression of FLT3 specifically on FLT3-ITD AML cells and consecutively, enhanced recognition by FLT3-CAR T-cells in vitro and in vivo. As anticipated, we found that FLT3-CAR T-cells recognize normal HSCs in vitro and in vivo, and disrupt normal hematopoiesis in colony-formation assays, suggesting that adoptive therapy with FLT3-CAR T-cells will require subsequent CAR T-cell depletion and allogeneic HSC transplantation to reconstitute the hematopoietic system. Collectively, our data establish FLT3 as a novel CAR target in AML with particular relevance in high-risk FLT3-ITD AML. Further, our data provide the first proof-of-concept that CAR T-cell immunotherapy and small molecule inhibition can be used synergistically, as exemplified by our data showing superior antileukemia efficacy of FLT3-CAR T-cells in combination with crenolanib.