Cancers of the breast, prostate, lung, and ovary over-express the EphA2 transmembrane tyrosine kinase receptor, whose ligands are GPI-anchored proteins called ephrins. Activation and down-modulation of the EphA2 receptor has been shown to inhibit migration, anchorage-independent growth, and invasive ability in human tumor cell lines in vitro and to reduce the in vivo growth rates of human tumor xenografts (Coffman, KT et al, Cancer Research 63: 7907-12, 2003). Reduction of EphA2 levels by treatment with anti-EphA2 siRNA has also been reported to inhibit in vivo tumor formation (Duxbury, MS et al, Oncogene 23: 1448-56, 2003), suggesting that the anti-tumor effects of EphA2 agonists and siRNA are mediated by decreasing the levels of EphA2. We have generated monoclonal antibodies that are selective agonists of the murine (i.e., Ab20) or human (i.e., 1G9-H7) EphA2 receptor. Activation of EphA2 phosphorylation and degradation by Ab20 in murine CT26 colorectal and LLC1 Lewis lung carcinoma cell lines was comparable to the activity, potency, and time-dependency of the dimeric ephrinA1 ligand (i.e., ephrinA1-Fc). Ab20 inhibited the invasion of CT26 tumor cells through reconstituted basement membrane in vitro, but had no effects on cell proliferation. Treatment of established, subcutaneously growing CT26 tumors with Ab20 reduced EphA2 protein levels by ∼85% within 24 hours and tumors excised from mice after two weeks of treatment (i.p., 125ug/mouse 3x weekly) had 10-fold lower EphA2 protein levels than controls. Yet, no difference in tumor growth rates was observed in the Ab20- treated vs. mIgG3 control-treated mice. Similar results were obtained with a monoclonal agonistic antibody (i.e., 1G9-H7) directed against human EphA2 in orthotopically-implanted MDA231 human breast tumors. These studies indicate that elevated levels of EphA2 protein are not essential for the primary growth of murine CT26 colorectal or human MDA231 mammary carcinomas.
[Proc Amer Assoc Cancer Res, Volume 46, 2005]