Targeting the receptor tyrosine kinase RET in combination with aromatase inhibitors in ER positive breast cancer xenografts

// Elena Andreucci 1, 2, * , Paola Francica 1, 2, 4, * , Antony Fearns 2, 5 , Lesley-Ann Martin 2 , Paola Chiarugi 1, 3 , Clare M. Isacke 2 , Andrea Morandi 1 1 Department of Experimental and Clinical Biomedical Sciences, University of Florence, Florence, Italy 2 The Breast Cancer Now Toby Robins Research Centre, The Institute of Cancer Research, London, United Kingdom 3 Tuscany Tumor Institute (ITT) and Excellence Centre for Research, Transfer and High Education DenoTHE, Florence, Italy 4 Current address: Department of Clinical Research, Radiation Oncology Laboratory, University of Bern, Bern, Switzerland 5 Current address: The Francis Crick Institute, Mill Hill Laboratory, The Ridgeway, London, United Kingdom * Equal first author contribution Correspondence to: Clare M. Isacke, email: clare.isacke@icr.ac.uk Andrea Morandi, email: andrea.morandi@unifi.it Keywords: RET, GDNF, endocrine therapy, aromatase inhibitors, resistance Received: March 15, 2016      Accepted: August 24, 2016      Published: September 02, 2016 ABSTRACT The majority of breast cancers are estrogen receptor positive (ER+). Blockade of estrogen biosynthesis by aromatase inhibitors (AIs) is the first-line endocrine therapy for post-menopausal women with ER+ breast cancers. However, AI resistance remains a major challenge. We have demonstrated previously that increased GDNF/RET signaling in ER+ breast cancers promotes AI resistance. Here we investigated the efficacy of different small molecule RET kinase inhibitors, sunitinib, cabozantinib, NVP-BBT594 and NVP-AST487, and the potential of combining a RET inhibitor with the AI letrozole in ER+ breast cancers. The most effective inhibitor identified, NVP-AST487, suppressed GDNF-stimulated RET downstream signaling and 3D tumor spheroid growth. Ovariectomized mice were inoculated with ER+ aromatase-overexpressing MCF7-AROM1 cells and treated with letrozole, NVP-AST487 or the two drugs in combination. Surprisingly, the three treatment regimens showed similar efficacy in impairing MCF7-AROM1 tumor growth in vivo . However in vitro , NVP-AST487 was superior to letrozole in inhibiting the GDNF-induced motility and tumor spheroid growth of MCF7-AROM1 cells and required in combination with letrozole to inhibit GDNF-induced motility in BT474-AROM3 aromatase expressing cells. These data indicate that inhibiting RET is as effective as the current therapeutic regimen of AI therapy but that a combination treatment may delay cancer cell dissemination and metastasis.