Abstract C1: Functional normalization of the breast cancer vasculature through activation of Tie2 using a vascular-endothelial protein tyrosine phosphatase inhibitor.

Background: The solid tumor microvasculature is characterized by structural immaturity and functional abnormality, and mediates several deleterious aspects of tumor behavior. The vascular endothelial protein tyrosine phosphatase (VE-PTP) attenuates the activity of the endothelial cell (EC) Tie-2 receptor tyrosine kinase, a key mediator of vessel maturation. Here we determine the role of VE-PTP in the vasculature of primary and metastatic mammary carcinomas. Methods: AKB-9778 is a first-in-class pharmacologic VE-PTP inhibitor. We systematically examined its effects in vitro and in vivo. First we studied the effects of AKB-9778 on Tie-2 signaling in two endothelial cell lines in vitro, and also measured its impact on Tie-2 activation in normal and tumor ECs in mice. We next used embryonic zebrafish assays to determine the effects of AKB-9778 on embryonic angiogenesis. We also studied the impact of AKB-9778 therapy on the tumor vasculature, tumor growth and metastatic progression using orthotopic models of murine mammary carcinoma as well as spontaneous and experimental metastasis models (4T1, E0771, P0008, MMTV-PyMT). Finally, we used endothelial nitric oxide synthase (eNOS) deficient mice to establish the role of eNOS in mediating the effects of VE-PTP inhibition. Results: AKB-9778 induced ligand-independent Tie-2 activation in ECs in vitro and in vivo, and impaired embryonic zebrafish angiogenesis. In mouse models of breast cancer, AKB-9778 (i) delayed the early phase of mammary tumor growth by maintaining vascular maturity; (ii) slowed the growth of spontaneous micrometastases by preventing extravasation of tumor cells into distant organ parenchyma (prolonging survival above adjuvant chemotherapy alone); and (iii) matured established primary tumor blood vessels (increased pericyte coverage, reduced permeability) in turn enhancing tumor perfusion, reducing hypoxia, and improving radiation response. Experiments using eNOS knockout mice showed that the effects of AKB-9778 on tumor vessels were mediated in part by eNOS activation. Conclusions: Phosphatase inhibition is a relatively unexplored field in cancer, and this is to our knowledge the first demonstration of the effects of VE-PTP inhibition in any disease. Our results demonstrate that pharmacological VE-PTP inhibition can normalize the structure and function of tumor vessels through Tie-2 activation, which delays tumor growth, slows metastatic progression, and enhances response to concomitant cytotoxic treatments. Furthermore we provide genetic evidence for the causal role of eNOS in mediating the beneficial effects of VEPTP inhibition in established tumors. By using models of adjuvant therapy (combined with chemotherapy) and radiation treatment, our results indicate possible avenues for further studying the clinical translatability of VE-PTP inhibition in cancer. Citation Information: Mol Cancer Ther 2013;12(11 Suppl):C1. Citation Format: Shom Goel, Nisha Gupta, Brian P. Walcott, Matija Snuderl, Cristina T. Kesler, Benjamin J. Vakoc, Randall T. Peterson, Timothy P. Padera, Dan G. Duda, Dai Fukumura, Rakesh K. Jain. Functional normalization of the breast cancer vasculature through activation of Tie2 using a vascular-endothelial protein tyrosine phosphatase inhibitor. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2013 Oct 19-23; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2013;12(11 Suppl):Abstract nr C1.