DESCRIPTION: (Applicant's Abstract) Overwhelming laboratory and clinical evidence has demonstrated that tumor angiogenesis is likely a crucial step for the development and progression of solid tumors. Tie2 (a.k.a. Tek) is a recently cloned novel endothelium-specific receptor tyrosine kinase required for embryonic vascular development. Studies in the applicant's laboratory demonstrated a significant up-regulation of Tie2 expression in breast cancer tissues. Blocking Tie2 activation by local administration of a recombinant, soluble Tie2 receptor (ExTek) significantly inhibited mammary tumor angiogenesis and tumor growth in a window chamber model. Using a gene therapy strategy to deliver ExTek in vivo inhibited the growth of both well-established primary tumors and tumor metastases. Interestingly, a mammary tumor that did not respond to VEGF blockade in earlier studies, showed a significant growth inhibition when ExTek was applied. The ability of ExTek to inhibit angiogenesis despite the presence of VEGF as well as inhibition of VEGF-independent tumor angiogenesis suggests that disrupting the angiogenic program at stages distal to endothelial activation will provide effective and perhaps more globally useful inhibitors of pathologic neovascularization. Based on these findings, the applicant hypothesizes that targeting Tie2 action will provide a novel and more effective anti-angiogenic cancer therapy. The long-range goals of this application are to study the mechanism of Tie2 activation and develop rational approaches to inhibit tumor angiogenesis based on Tie2 pathways. Towards these goals, the specific aims of this application are to: 1) map the ligand-binding domain of Tie2 receptor; 2) study the function of ligand-binding domain generated in Aims 1; 3) use soluble Tie2 receptor (ExTek) as a target to isolate small molecules that inhibit Tie2 activation and 4) develop specific antibodies to Tie2 receptor. Accomplishing these specific aims will improve the understanding of the mechanism of Tie2 activation, will help to understand the cellular function of Tie2 activation, will generate specific diagnostic agents for cancer, and will result in the development of better inhibitors of Tie2 activation. This multi-faceted approach to the design and development of endothelium-specific inhibitors will allow the selection of the most effective and specific inhibitors of tumor angiogenesis.