Abstract C174: Establishment of luminal breast cancer patient-derived xenografts with acquired resistance to both hormonotherapy and everolimus.
Authors: El Botty Rania , Cottu Paul , Assayag Franck , de la Grange Pierre , Gentien David , Thuleau Aurélie , Vacher Sophie , Chateau-Joubert Sophie , Bièche Ivan , Marangoni Elisabetta 
Published: August 2014

Abstract
The addition of everolimus (Rad) to endocrine therapies (ET) in luminal advanced breast cancers has shown to increase overall survival in randomized studies. However drug resistance is an emerging problem and most patients eventually experience progression of disease. We aimed to establish and characterize patient-derived xenografts (PDX) of luminal breast cancers with acquired resistance in vivo to both ET and Rad. Two PDX xenografts (HBCx22 and HBCx34), established from early stage ER+ breast cancers, were treated in vivo with different ET (tamoxifen, fulvestrant, ovariectomy and letrozole) during 12 months and tumors escaping to therapies were re-engrafted and maintained under treatment. Four hormono-resistant (HR) xenografts were established, 2 resistant to Tamoxifen (HBCx22TamR and HBCx34TamR) and 2 resistant to ovariectomy (HBCx22OvaR and HBCx34OvaR). Expression of estrogen receptor (ER) was maintained, while expression of several ER-controlled genes (MYB, PS2 and PR) was decreased in HR tumors indicating modifications of ER transcriptional activity. The expression of ER co-regulators was additionally studied: GREB1 was reduced specifically in TamR, while FOXA1 were increased specifically in TamR. Expression analysis of P-AKT, P-mTOR and P-S6 showed similar levels of PI3 kinase pathway activation between parental and HR xenografts. Gene expression profiling, performed with Affymetrix Gene Expression Arrays, showed that genes differentially expressed in TamR and OvaR tumors were only partially overlapping, suggesting treatment-specific mechanisms of hormono-resistance. In addition, TamR and OvaR signatures were tumor-specific. Gene ontology enrichment and pathway analyses of gene expression datasets will be presented at the meeting. HR xenografts were then treated with Rad alone and combined to ET. In HBCx22OvaR and TamR, resistant to all ET, treatment with Rad arrested tumor growth in vivo but did not show any additive effect when combined to ET. Conversely, the HBCx34TamR model was still responding to fulvestrant, and the combination Rad+Fulvestrant resulted in tumor regressions. In the HBCx34TamR xenograft, tumor escape to Rad treatment occurred in one mouse after 8 months of continuous treatment. The tumor was re-engrafted and maintained as tumorgraft resistant to both tamoxifen and Rad (HBCx34Tam-RadR). Western blot and IHC studies showed that mTOR signaling was still inhibited in this tumor, but it was associated to MAP kinase activation and increased IGF-1R expression. In conclusion, we have developed PDX models of luminal breast cancer with cross-resistance to ET and everolimus in vivo. These models provide a valuable preclinical tool to investigate molecular mechanisms of acquired resistance and to test new targeted therapies and new combinations in patients’ tumors.
Citation Information: Mol Cancer Ther 2013;12(11 Suppl):C174.
Citation Format: Rania El Botty, Paul Cottu, Franck Assayag, Pierre de la Grange, David Gentien, Aurélie Thuleau, Sophie Vacher, Sophie Chateau-Joubert, Ivan Bièche, Elisabetta Marangoni. Establishment of luminal breast cancer patient-derived xenografts with acquired resistance to both hormonotherapy and everolimus. [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 C174.