Abstract 5048: Platelet FAK is a critical regulator of tumor growth after withdrawal of anti-angiogenic therapy
Authors: Haemmerle Monika , Bottsford-Miller Justin , Pradeep Sunila , Taylor Morgan L. , Choi Hyun-Jin , Stone Rebecca L. , Cho Min Soon , Nick Alpa M. , Lopez-Berestein Gabriel , Afshar-Khargan Vahid , Sood Anil K. 
Published: July 2016

Abstract
Objective: In most clinical trials, anti-angiogenic therapies have offered only modest improvements in progression-free survival without impacting overall survival. Following cessation of anti-angiogenic therapy, concerns have been raised about a possible rebound in tumor growth, but the underlying mechanisms are poorly understood. Therefore, in this study we aimed at comparing tumor growth and the effects on tumor microenvironment after therapy withdrawal compared to continuous treatment with anti-angiogenic agents.
Methods: Mice were intraperitoneally injected with human or mouse ovarian cancer cells and were treated with the anti-angiogenic drugs pazopanib, bevacizumab or B20 for either a short-term with subsequent withdrawal or continuous therapy until necropsy. Immunohistochemical staining was used to evaluate platelet infiltration into the tumor microenvironment, tumor angiogenesis and vascular leakage. To assess the significance of focal adhesion kinase (FAK) in the process of platelet infiltration and tumor rebound after withdrawal of therapy, we either used the FAK inhibitor GSK2256098 or a mouse model with platelet-specific FAK deletion.
Results: Cessation of therapy with pazopanib, bevacizumab and the cross-human and murine anti-VEGF antibody B20 was associated with up to 4-fold increased tumor growth in mouse models of ovarian cancer when compared to continuous treatment. Tumor outgrowth was associated with significant tumor hypoxia, increased tumor angiogenesis and vascular leakage. More importantly, we found 380% increased hypoxia-induced ADP production and 3-fold increased platelet infiltration into tumors where anti-angiogenic therapy was withdrawn. Lowering platelet levels significantly inhibited tumor rebound after withdrawal of anti-angiogenic therapy. Interestingly, FAK in platelets regulated their migration into tumor microenvironment and FAK knock-out specifically in platelets completely prevented the rebound tumor growth. Additionally, combined therapy with the FAK inhibitor GSK2256098 and the anti-angiogenic agents pazopanib and bevacizumab led to up to 5-fold reduced orthotopic tumors and likewise inhibited negative effects of withdrawal of anti-angiogenic therapy.
Conclusions: Collectively, our results characterize a previously unknown role for platelets in the tumor microenvironment and provide a potential therapeutic benefit for FAK inhibitors in preventing rebound in tumor growth following discontinuation of anti-angiogenic agents. Additionally, dual targeting of FAK and VEGF could have important therapeutic implications for ovarian cancer management.
Citation Format: Monika Haemmerle, Justin Bottsford-Miller, Sunila Pradeep, Morgan L. Taylor, Hyun-Jin Choi, Rebecca L. Stone, Min Soon Cho, Alpa M. Nick, Gabriel Lopez-Berestein, Vahid Afshar-Khargan, Anil K. Sood. Platelet FAK is a critical regulator of tumor growth after withdrawal of anti-angiogenic therapy. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 5048.