Abstract 927: Genomic profiling of metastatic breast cancer identifies the Hippo pathway to be a key determinant of CDK4/6 inhibitor sensitivity
Authors: Razavi Pedram , Li Qing , Li Zhiqiang , Brown David N. , Ping Christina , Shen Ronglai , Sanchez-Vega Francisco , Schultz Nikolaus D. , Baselga Jose , Scaltriti Maurizio , Reis-Filho Jorge S. , Chandarlapaty Sarat 
Published: August 2019

Abstract
CDK4/6 inhibitors (CDK4/6i) have proven benefits in estrogen receptor-positive (ER+) metastatic breast cancer (MBC), however the activity is variable among patients and biomarkers of sensitivity are lacking. We sought to determine whether any genomic alterations might characterize extreme responders and non-responders from over 500 patients with ER+ MBC who received CDK4/6i plus aromatase inhibitors or fulvestrant.
We first analyzed a cohort of 348 patients who has MSK-IMPACT sequencing performed on a pre-treatment tumor sample and asked whether any somatic mutation or copy number alteration was associated with altered progression-free survival (PFS) on subsequent CDK4/6i-based therapy. As expected, common alterations in CCND1, PIK3CA or ESR1 were not associated with altered PFS on CDK4/6i. Also as anticipated, RB1 loss was associated with resistance to CDK4/6i (median PFS: 3.6 months, 95% confidence interval [CI]: 2.2, not reached, p=0.00041). Unexpectedly, alterations in components of the Hippo pathway (FAT1, YAP, LATS1/2, and NF2) were mutually exclusive with each other and with RB1 and were collectively associated with resistance to CDK4/6i in ER+ MBC (Hazard Radio [HR]: 3.6; 95% CI: 1.7, 7.8, p=0.0044). FAT1 was the most frequently mutated gene in the Hippo pathway with alterations resulting in FAT1 loss associated with a markedly shortened PFS (median: 2.4 months, HR: 11.1; 95% CI: 4.65, 26.62; p=2.2x10-11). No alterations were clearly linked with extremely prolonged PFS on CDK4/6i. Additional analyses from a larger cohort of patients with genomic sequencing prior to starting CDK4/6i are now underway and will be presented.
To further understand the mechanisms of Hippo pathway regulation of cell cycle, cell line and patient derived xenograft models were utilized. These studies revealed that different mechanisms of suppression of Hippo signaling such as FAT1 knockdown (CRISPR or shRNA) and NF2 knockdown (shRNA) lead to YAP activation and YAP-dependent CDK4/6i resistance. Moreover, in all cases, resistance was mediated by upregulation of CDK6 as knockdown of CDK6 prevented drug resistance.
Finally, we have sought to understand the implications of these distinct mechanisms of resistance to CDK4/6i. Clinical follow up on subsequent lines of therapy and duration of treatment after CDK4/6i (e.g. chemotherapy or other hormone therapy) are now being analyzed based on class of resistance alteration and will be presented.
In conclusion, RB1 loss and Hippo pathway alterations mediate resistance to CDK4/6i in patients with ER+ MBC, nominating these as potential biomarkers for this class of drugs.
Citation Format: Pedram Razavi, Qing Li, Zhiqiang Li, David N. Brown, Christina Ping, Ronglai Shen, Francisco Sanchez-Vega, Nikolaus D. Schultz, Jose Baselga, Maurizio Scaltriti, Jorge S. Reis-Filho, Sarat Chandarlapaty. Genomic profiling of metastatic breast cancer identifies the Hippo pathway to be a key determinant of CDK4/6 inhibitor sensitivity [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 927.