Abstract 85: Endocrine resistant luminal breast cancer xenografts are powerful models for the analysis of sensitivity to endocrine and everolimus treatments .

Abstract

Abstract Background. Activation of the PI3K pathway is associated with resistance to endocrine treatments (ET) in luminal breast cancer (LBC). Everolimus, an mTOR inhibitor, has been approved in combination with exemestane in endocrine resistant breast tumors. However, no definite signature of ET resistance or predictive factors of sensitivity to everolimus (EVE) treatment have been evidenced so far. In order to further decipher these molecular patterns, we used patient derived LBC xenografts (LBC-PDX) treated with various ET and EVE based combinations. Methods. 3 LBC-PDX (named HBCx-3, HBCx-21 and HBCx-34) were treated with ET during several months to establish hormono-resistant (HR) xenografts. Both parental and HR models were treated by tamoxifen, estrogen deprivation, fulvestrant, EVE alone, and EVE combined with all three ET modalities. Tissue Micro Arrays from control and treated tumor samples were generated for IHC analyses. RT-PCR analyses were conducted on genes related to proliferation, ER, PI3K, IGF-1R and angiogenesis pathways. Results. In primary PDX, ET showed a poor level of sensitivity for HBCx-3 and a high and sustained efficacy of ER targeting and/or ER deprivation for HBCx-21 and HBCx-34 tumors. In vivo resistance to ET was confirmed in all HR variants with tumor growth rates faster than parental xenografts. ER expression was conserved in HR tumors, but expression of the ER-responsive genes PS2, PR and MYB was strongly decreased, indicating impaired ER transcriptional activity. Acquired resistance to tamoxifen and ovariectomy was also associated to a strong decrease of IGF-1R signaling in HBCx-21, and was related to higher P-AKT expression, although P-pS6 was highly expressed in both sensitive and resistant tumors. Furthermore, in vivo experiments showed that EVE alone was highly efficient in all models independently of P-AKT expression/PTEN loss. EVE combined with fulvestrant yielded the most complete and durable tumor growth inhibition. In HBCx-3, proliferation and pS6 levels were markedly decreased only by the EVE-fulvestrant combination. In HBCx-21 and HBCx-34 HR models, EVE-based treatments decreased proliferation, expression of angiogenesis markers, as well as P-pS6 expression with no variation in (P-)AKT nor (P-)mTOR levels. A high IGF-1R protein expression was found only in the HBCx-34 TAM-R tumor, which was strongly decreased in the fulvestrant-treated tumors. Conclusions. LBC-PDX with confirmed HR status have been established and may serve as models to study alternate therapies. Of note, in the HBCx-21 and -34 models, activation of the PI3K pathway is poorly correlated with baseline sensitivity to ET, suggesting other pathways of resistance. EVE alone is highly efficient in all settings, and combination with fulvestrant is promising at the in vivo and molecular levels. Proteomic, genomic and gene expression studies are ongoing. Citation Format: Paul H. Cottu, Thomas Bagarre, Jean-Jacques Fontaine, Franck Assayag, Sophie Richon, Sophie Chateau-Joubert, Aurélie Thuleau, Patricia de Cremoux, Khemaies Slimane, Didier Decaudin, Anne Vincent-Salomon, Ivan Bièche, Elisabetta Marangoni. Endocrine resistant luminal breast cancer xenografts are powerful models for the analysis of sensitivity to endocrine and everolimus treatments . [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 85. doi:10.1158/1538-7445.AM2013-85