Neutralization of KIT Oncogenic Signaling in Leukemia with Antibodies Targeting KIT Membrane Proximal Domain 5.

KIT is a cell surface tyrosine kinase receptor whose ligand stem cell factor (SCF) triggers homodimerization and activation of downstream effector pathways involved in cell survival, proliferation, homing, or differentiation. KIT-activating mutations are major oncogenic drivers in subsets of acute myeloid leukemia (AML), in mast cell leukemia, and in gastrointestinal stromal tumors (GIST). The overexpression of SCF and/or wild-type (WT) KIT is also observed in a number of cancers, including 50% of AML and small cell lung cancer. The use of tyrosine kinase inhibitors (TKI) in these pathologies is, however, hampered by initial or acquired resistance following treatment. Using antibody phage display, we obtained two antibodies (2D1 and 3G1) specific for the most membrane proximal extracellular immunoglobulin domain (D5) of KIT, which is implicated in KIT homodimerization. Produced as single chain variable antibody fragments fused to the Fc fragment of a human IgG1, bivalent 2D1-Fc and 3G1-Fc inhibited KIT-dependent growth of leukemic cell lines expressing WT KIT (UT7/Epo) or constitutively active KIT mutants, including the TKI imatinib-resistant KIT D816V mutant (HMC1.2 cell line). In all models, either expressing WT KIT or mutated KIT, 2D1 and 3G1-Fc induced KIT internalization and sustained surface downregulation. However, interestingly, KIT degradation was only observed in leukemic cell lines with oncogenic KIT, a property likely to limit the toxicity of these antibodies in patients. These fully human antibody formats may represent therapeutic tools to target KIT signaling in leukemia or GIST, and to bypass TKI resistance of certain KIT mutants.