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

Elena Andreucci,Antony Fearns,Lesley-Ann Martin,Paola Francica,Clare M Isacke,Andrea Morandi,Paola Chiarugi

doi:10.18632/oncotarget.11826

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.

Highlights

Endocrine therapies have shown to be effective in patients that express estrogen receptor-α (ERα, called hereafter ER)
We have reported that activation of the REarranged during Transfection (RET) receptor tyrosine kinase by its ligand GDNF decreases response of ER+ breast cancer cells to endocrine therapy, including aromatase inhibitors (AIs), and that the www.impactjournals.com/oncotarget transcriptional signature of RET downstream signaling has both prognostic and predictive value in breast cancer [4, 11,12,13]
We demonstrate that NVP-AST487 and NVP-BBT594 have comparable potencies as inhibitors of GDNF-induced RET signaling in ER+ breast cancer cells in vitro

Summary

Introduction

Endocrine therapies have shown to be effective in patients that express estrogen receptor-α (ERα, called hereafter ER). Along with others, have demonstrated previously that a major cause of AI resistance is growth factor receptor activation that, via the PI3K/AKT/ mTOR or MAPK pathways, drives ligand-independent ER activation [2,3,4,5,6]. These findings have been exploited clinically by combining AIs with mTOR [7, 8] or PI3K/ AKT (NCT01437566) [9, 10] inhibitiors. The combination of the AI letrozole with the RET inhibitor NVP-BBT594 is more effective at suppressing GDNF-induced proliferation of RET+ ER+ breast cancer cells than either monotherapy [12, 14]

Methods

Results

Conclusion