The EphA2 receptor tyrosine kinase is a promising new target for inhibition of tumor angiogenesis and other forms of pathological angiogenesis. Studies with EphA2 knockout mice have demonstrated the critical importance of this receptor in adult angiogenesis. EphA2 is expressed in angiogenic blood vessels, but not in quiescent adult blood vessels or embryonic vasculature, suggesting that agents targeting this receptor will not affect the normal vasculature. Indeed, engineered proteins that block ephrin ligand binding to EphA2 inhibit tumor angiogenesis and tumor growth in a variety of mouse pre-clinical models without obvious toxic effects. EphA2 inhibitors may be particularly beneficial for the treatment of tumors where EphA2 signaling promotes not only angiogenesis but also the malignant properties of the cancer cells. The reagents used so far to inhibit EphA2 do not have good pharmacological properties and/or are not selective for EphA2 versus other related receptors. Chemical compounds that inhibit EphA2 would have more desirable pharmaceutical profiles as candidates for drug development compared to the large protein fragments that have been used so far to inhibit EphA2. However, EphA2 has not yet been widely exploited as a target for small molecules. Therefore, it will be a very significant achievement to identify drug-like EphA2 inhibitors that may be developed for cancer therapy. In this application we propose to develop a high throughput screening assay to identify compounds that inhibit EphA2 activity in cells. As readout, we will use a well established and robust morphological effect of EphA2 that involves retraction of the cell periphery and cell rounding (cell contraction). We propose to: (1) Establish a high throughput screening assay for inhibitors of EphA2-dependent cell contraction and (2) Conduct a pilot screen to evaluate the performance of the assay. Accomplishing these aims will set the stage for a large scale screen to be conducted by the MLPCN in the second year. A series of assays for the validation, prioritization, and characterization of screening hits are already available in our laboratory. The cell-based screen is anticipated to generate new inhibitors with diverse mechanisms of action, which may target the EphA2 ligand-binding domain, kinase domain, or downstream signaling effectors. EphA2 inhibitory compounds may serve as tools to obtain valuable insight into the mechanisms used by EphA2 to influence the properties of cancer cells, which are poorly understood. The inhibitors may also serve as lead compounds for drug development to inhibit cancer progression and treat other diseases characterized by pathological angiogenesis. The identification of EphA2 inhibitors having different mechanisms of action will also enable their combined use, which may be particularly effective to inhibit EphA2 function. Finally, the screening assay developed for the identification of EphA2 inhibitors will be easily adaptable to identify compounds that inhibit the activities of other Eph receptors, many of which are also considered as promising drug targets for cancer treatment. PUBLIC HEALTH RELEVANCE: The EphA2 receptor tyrosine kinase is expressed in tumor blood vessels from early stages of tumor development, but not in normal quiescent blood vessels, and plays an important role in tumor angiogenesis. Large protein fragments that block binding of EphA2 to its ligands have been shown to inhibit tumor growth in pre-clinical mouse cancer models, but they have poor pharmacological properties and selectivity for EphA2 versus other related receptors. Chemical compounds that inhibit EphA2 function will have potential research applications to clarify the role of EphA2 in cancer progression, which is currently not well understood, and therapeutic applications as anti-cancer agents.