Abstract 4150: Anti-tumor activity of the PI3K/mTOR pathway inhibitors alpelisib (BYL719) and everolimus (RAD001) in xenograft models of acquired resistance to CDK-4/6 targeted therapy
Authors: O'Brien Neil A. , Conklin Dylan , Luo Tong , Ayala Raul , Issakhanian Shawnt , Kalous Ondrej , Euw Erika Von , Hurvitz Sara A. , DiTomaso Emmanuelle , Su Faye , Linnartz Ronald , Scherer Stefan , Hirawat Samit , Slamon Dennis J. 
Published: July 2017

Abstract
The selective cyclin dependent kinases 4 and 6 (CDK-4/6) inhibitor, palbociclib, has recently been approved in combination with endocrine-based therapies for the treatment of advanced ER+/HER2- breast cancer (BC). Despite the improvements in progression-free and overall survivals, a significant number of patients on CDK-4/6 therapy will go on to develop progressive disease. As such, it is critical to identify the mechanisms by which tumor cells evade CDK-4/6 targeted therapy and to develop therapeutic strategies to overcome resistance. In this study, we evaluated the efficacy of a p110α-selective PI3K inhibitor, alpelisib (BYL719) and the mTORC1 specific inhibitor, everolimus (RAD001), in ER+/HER2- xenograft tumors conditioned in vitro and in vivo to acquire resistance to combined palbociclib + endocrine-based therapy. The EFM19 (ER+/HER2-, PIK3CA mt), MDA134 (ER+/HER2-, PIK3CA wt), and MDA361 (ER+/HER2+, PIK3CA mt) BC cell lines were conditioned to acquire palbociclib resistance through long-term culture in the presence of increasing concentrations of drug. Baseline total/phosphoprotein levels of over 280 cancer-associated analytes were measured in the parental and resistant cell lines by Reverse Phase Protein Array (RPPA). In vivo resistant models were developed from xenografts of the MCF7 and HCC1500 ER+ BC cell lines, treated until progression with palbociclib (75 mg/kg) + fulvestrant (5 mg/mse). EFM19-PR cells maintained palbociclib resistance in vivo and were included in the xenograft studies. Resistance to palbociclib was confirmed in vitro by a shift in growth inhibitory IC50 from an average of less 50nM to over 1 µM. Significantly reduced Rb and ER protein levels were detected in each of these palbociclib resistant cell lines by RPPA. Cross-resistance to an alternative CDK-4/6 inhibitor (ribociclib) was found in in vitro studies. In addition, switching treatment of the xenografts progressing on palbociclib + fulvestrant to ribociclib (75mg/kg) + fulvestrant did not impact tumor progression. However, when progressing tumors were switched to BYL719 (30mg/kg) + fulvestrant or RAD001 (10mg/kg) + fulvestrant, sustained tumor regression was observed for over 45 days of treatment in every model tested. The triplet combination of ribociclib + fulvestrant + BYL719 or RAD001 provided marginal additional benefit over the doublet combination of BYL719 or RAD001 + fulvestrant. RPPA analysis of xenograft tissue collected from these studies will help to identify additional molecular alterations involved in resistance to CDK-4/6 targeted therapy. These preclinical data indicate that acquired resistance to CDK-4/6 inhibition occurs through a loss of dependence on Rb-signaling. However, targeting an alternative pathway like PI3K/mTOR with molecules such as BYL719 or RAD001 may be a viable strategy for further clinical investigation.
Citation Format: Neil A. O'Brien, Dylan Conklin, Tong Luo, Raul Ayala, Shawnt Issakhanian, Ondrej Kalous, Erika Von Euw, Sara A. Hurvitz, Emmanuelle DiTomaso, Faye Su, Ronald Linnartz, Stefan Scherer, Samit Hirawat, Dennis J. Slamon. Anti-tumor activity of the PI3K/mTOR pathway inhibitors alpelisib (BYL719) and everolimus (RAD001) in xenograft models of acquired resistance to CDK-4/6 targeted therapy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 4150. doi:10.1158/1538-7445.AM2017-4150