Pexidartinib plus FLT3-directed CAR-Macrophage for the treatment of FLT3-ITD-mutated acute myeloid leukemia in preclinical model

Acute myeloid leukemia (AML) is the most common type of acute leukemia in adults. Internal tandem duplication of FMS-like tyrosine kinase 3 (FLT3-ITD) mutations occur in about 25%-30% of AML cases and are associated with adverse prognosis. Recent advances indicate that M2-like leukemia-associated macrophages (M2-LAM) are highly infiltrated in the bone marrow of FLT3-ITD+ AML patients; however, the underlying mechanisms and therapeutic implications are still elusive. Herein, we reveal that conditioned medium from FLT3-ITD+ MOLM-13 AML cells polarized M2-LAM and impaired their phagocytic activities. Unexpectedly, co-culture of M2-LAM protected MOLM-13 cells from the treatment of FLT3 inhibitor quizartinib by activating their FLT3 signaling pathway. Pharmaceutically, FLT3/CSF1R dual inhibitor pexidartinb effectively suppressed M2-LAM, reduced leukemic burden, and prolonged the survival of MOLM-13-xenografted mice. To enhance the phagocytic activities of macrophages, FLT3-directed chimeric antigen receptor-engineered macrophages (FLT3L-CAR-Macrophage) were generated using FLT3 ligand (FLT3L) as the recognizing domain of CAR. Transfection of THP-1 monocytic cells-or umbilical cord blood mononuclear cells-derived macrophages with FLT3L-CAR-encoding mRNA enhanced their phagocytic activities to MOLM-13 cells in vitro. Consistently, FLT3L-CAR-Macrophage differentiated from FLT3L-CAR-expressing THP-1 cells effectively phagocytosed MOLM-13 cells in vitro, reduced leukemic burden and prolonged the survival of MOLM-13-xenografted mice. Importantly, treatment of pexidartinib resulted in upregulation and surface localization of FLT3-ITD protein in MOLM-13 cells, sensitized MOLM-13 cells to the treatment of FLT3L-CAR-Macrophage in vitro, and synergized with FLT3L-CAR-Macrophage to further reduce leukemic burden in MOLM-13-xenografted mice. Together, our data indicate that pexidartinib plus FLT3L-CAR-Macrophage could be a promising therapeutic strategy for the treatment of FLT3-ITD+ AML in preclinical model which warrants further investigation.