PB1723 DEVELOPING NOVEL COMBINED CSF1R/FLT3‐TARGETED THERAPY IN ACUTE MYELOID LEUKAEMIA

Background: Despite emerging insight into the molecular pathogenesis of acute myeloid leukaemia (AML), few new therapeutic drugs have been approved and overall survival in elderly patients remains poor. Although the recently approved kinase inhibitor benzoylstaurosporine (midostaurin, Rydapt) show significant improvement in tyrosine kinase FLT3 mutated AML, the 16‐year time frame between the discovery of its efficacy in FLT3 inhibition and FDA and EU approval is unacceptable. Hitherto, few studies have investigated the supportive tumor microenvironment as a therapeutic target in AML treatment. Malignant myeloid cells in AML communicate with stromal support cells, but therapeutic targeting of this interaction has been limited. Tumor associated macrophages has gained attention in cancer research due to their plasticity and influence on malignant progression in bone marrow. Growth factors such as Colony stimulating factor 1 receptor (CSF1R) signalling is partly responsible for this observed phenotype as it is crucial for macrophage differentiation. Inhibition of CSF1R is proposed to be an effective target to block monocytes and tumor associated macrophages that infiltrate the tumor stroma and support tumor growth. Our laboratory is currently developing a novel class of small molecule kinase inhibitors towards FLT3 and CSF1R through collaboration with BCI Pharma to target high‐risk mutations. This will hopefully contribute significantly to the raising era of targeted therapy in AML molecular subtypes. Aims: We propose the potential of CSF1R/FLT3‐targeted therapy as an attractive treatment combination with standard treatment modalities and immunotherapeutic agents Methods: We have established IC‐50 values of various CSF1R, FLT3 and dual CSF1R/FLTS‐inhibitors on multiple leukemic and stromal cell lines by high‐throughput dose‐response WST‐1 assay. Previously, our laboratory have piloted an in vivo toxicity test (MTD) and proof‐of‐principle‐experiment with a selected inhibitor from BCI. To further investigate the interactions of leukemic cells with the stromal cells/compartment of the bone marrowin AML, 3D co‐culture model usingMOLM13 HS‐5 human BM cell linescultured in alginate gels. This model is currently beingused to explore the interactions and protective effects of the BM stromal compartment in AML, and to investigate the effect of novel targeted therapies and stromal‐conditioned media in a 3D co‐culture setting Results: Dose‐response assays showed significant inhibition and reduced viability by several compounds, where the most promising inhibitor was the dual CSF1R/FLT3 inhibitor BCI‐1124. We found no dose‐limiting toxicity at doses up to 10 mg/kg and tumour volume was significantly reduced in the subcutaneous xenograft model of FLT3‐ITD expressing MV4‐1 cells treated with the inhibitor. Our 3D co‐culture alginate gel is compatible with the leukemic and stromal cells, and allows for further investigations with conditioned media and inhibitor treatment Summary/Conclusion: In summary, we employed a novel approach to develop combined CSF1R/FLT3 targeted therapy for AML to assess the protective tumour microenvironment and by direct leukemic cell death. We screened a selection of CSF1R and FLT3‐targeted inhibitors on several cell lines and established a 3D co‐culture model to investigate the leukemic‐stromal interactions. on. Ultimately, our goal is to identify one or several drug candidate(s) to be tested in a clinical trial setting, in combination or in sequences combined with other targeted therapies.