The role of Notch signaling and its ligand JAGGED1 (JAG1) in tumor biology have been firmly established, making them appealing therapeutic targets for cancer treatment. Here we report the development and characterization of human/rat-specific JAG1-neutralizing monoclonal antibodies. Epitope mapping identified their binding to the Notch receptor interaction site within the JAG1 Delta/Serrate/Lag2 domain, where E228D substitution prevented effective binding to the murine Jag1 orthologue. These antibodies were able to specifically inhibit JAG1-Notch binding in vitro, downregulate Notch signaling in cancer cells and to block the heterotypic JAG1-mediated Notch signaling between endothelial and vascular smooth muscle cells. Functionally, in vitro treatment impaired 3D growth of breast cancer cell spheroids, in association with a reduction in cancer stem cell number. In vivo testing showed variable effects on human xenograft growth when only tumor-expressed JAG1 was targeted (mouse models) but a more robust effect when stromal expressed Jag1 was also targeted (rat MDA-MB-231 xenograft model). Importantly, treatment of established triple receptor negative breast cancer brain metastasis in rats showed a significant reduction in neoplastic growth. MRI imaging demonstrated that this was associated with a substantial improvement in blood-brain-barrier function and tumor perfusion. Lastly, JAG1-targeting antibody treatment did not cause any detectable toxicity, further supporting its clinical potential for cancer therapy
