When overexpressed or constitutively active by mutation, the tyrosine kinase receptor KIT is involved in some proliferative diseases such as gastro-intestinal stromal tumors (GIST), mastocytosis and some leukemia. Therefore, KIT signaling represents a major target in oncology. Development of a new therapeutic class called tyrosine kinase inhibitors is in full expansion. A major example of TKI is imatinib which targets KIT and is efficient in the majority of GIST cases. However, TKI treatment is often unpaired by primary or acquired resistance due to secondary mutations. That is why we aim to develop new compounds to target KIT or associated signaling pathways by three strategies.We have recently shown that oncogenic KIT mutants are intracellularely localized whereas WT KIT is expressed at the cell surface. Kinase activity inhibition leads to membrane mutants’ relocalization. Based on this finding, we developed and validated a screening assay measuring mutants’ relocalization by cytometry. Chemicals library screening allows us to select new KIT signaling inhibitors active on KIT mutant cell lines.We used phage display to generate scFv and VHH antibodies which are specific to KIT intracellular domain. When expressed in cytoplasm (they are called intrabodies), their binding on KIT inhibits kinase activity directly or signaling partners’ recruitment. Selected intrabodies are specific to various KIT isoforms and their functional characterization is ongoing. KIT inhibitory intrabodies will be used to screen chemical libraries by ELISA for drugs that block intrabodies binding on KIT intracellular domain. We will then select molecules that potentially inhibit KIT oncogenicity.We developed scFv-Fc antibodies by phage display that recognize KIT extracellular domain. Two selected antibodies inhibit SCF induced signaling. In WT KIT expressing leukemic cell lines, we showed that antibody treatment reduces cell viability. Moreover, they also diminish cell proliferation of 2 imatinib sensitive and resistant mast cell leukemia cell lines (HMC1.1 and HMC1.2, respectively). They represent potential therapeutic tools for treatment of KIT involved diseases and for bypass TKI resistance of some mutants.
