Whole Exome Sequencing (Wes) In Hormone-Receptor Positive (Hr Plus ) Metastatic Breast Cancer (Mbc) To Identify Mediators Of Resistance To Cyclin-Dependent Kinase 4/6 Inhibitors (Cdk4/6i).

12016 Background: Combining CDK4/6i with endocrine therapy results in prolongation of disease control in HR+ MBC, though resistance invariably occurs. There is a critical need to understand mechanisms governing response and resistance to these agents. Methods: WES was performed on 51 baseline metastatic tumor biopsies obtained at treatment initiation with CDK4/6i in combination with various anti-estrogens. Tumor samples were classified as sensitive (S, from patients with clinical benefit) or intrinsically resistant (IR, from patients without clinical benefit). WES was also performed in 11 acquired resistance (AR) specimens obtained from responding patients after progression. In 6 patients, WES was performed on matched pre-treatment S and post-progression AR specimens. Putative resistance drivers were introduced into HR+/HER2- breast cancer cells (T47D, MCF7) via lentiviral infection or knockout via CRISPR. Sensitivity of the modified cell lines to anti-estrogens and CDK4/6i was characterized. Results: WES of 62 tumors revealed multiple potential mechanisms of resistance to CDK4/6i (Table), including biallelic RB1 inactivation, AKT1 mutation (mut) and/or amplification (amp), RAS mut, AURKA amp, IGF1R amp, ERBB2 mut, and FGFR2 mut and/or amp. Introduction of candidates into HR+ breast cancer cells conveyed resistance to CDK4/6i in vitro, including loss of Rb and overexpression of AKT1, AURKA, FGFR2, mut-KRAS, and mut-ERBB2. Additional sequencing efforts and characterization of variants is ongoing and will be presented. Conclusions: These results provide new insight into the diverse spectrum of genomic events driving resistance to CDK4/6i and set the stage for additional mechanistic studies. For patients with AKT1, RAS, AURKA, IGF1R, ERBB2, and FGFR2-dependent resistance, clinical trials incorporating novel combinations of targeted therapies could be designed to circumvent or overcome resistance. Phenotype S IR AR IR+AR (%) Tumor samples (n) 21 30 11 41 RB1 biallelic inactivation 0 3 1 4 (10) IGF1R amp 0 2 1 3 (7) RAS mut 0 2 1 3 (7) AKT1 mut or amp 1 3 2 5 (12) FGFR2 mut or amp 0 3 0 3 (7) AURKA amp 0 7 4 11 (27) HER2 mut 1 1 3 4 (10)