Clinico-molecular characteristics associated with outcomes in breast cancer patients treated with CDK4/6 inhibitors: Results from the AURORA Molecular Screening Initiative

1019 Background: CDK4/6 inhibitor (CDK4/6i) plus endocrine therapy (ET) is the recommended first line standard of care for patients with estrogen receptor positive (ER+), HER2-negative (HER2-) advanced breast cancer (ABC). However, not all patients derive the same benefit from this treatment. We aimed to identify factors associated with outcome in patients treated with CDK4/6i in a real-world setting. Methods: AURORA (NCT02102165) is an international program aimed at studying ABC by performing multi-omics profiling on paired primary tumors and metastases. Here we present exploratory results from AURORA patients treated with first line CDK4/6i + ET. DNA from primary and/or metastatic lesions and germline DNA was sequenced for 411 cancer-related genes. Endocrine resistance was defined according to the 5th ABC international consensus guidelines at the start of the first line. Local histology assessment of the primary tumor was used. Progression-free survival (PFS) was calculated from the start of CDK4/6i treatment until disease progression or death (whichever occurred first). Results: We analyzed 339 patients treated with CDK4/6i + ET in the first line. PFS differed significantly among the endocrine resistance subgroups (p<0.001). Both TP53 and acquired ESR1 mutations were associated with shorter PFS (hazard ratio [HR] 1.59 [95% CI 1.43-6.73], p=0.004 and 3.10 [95% CI 1.16-2.18], p=0.004 respectively). In a multivariable analysis, acquired ESR1 mutations were significantly associated with worse PFS independently of endocrine resistance status (HR 2.42 [95% CI 1.01-5.79], p=0.048). Mutations in PIK3CA were not associated with outcome (HR 0.84 [95% CI 0.63-1.13], p=0.25). No PFS difference was observed between lobular and ductal tumors (HR 1.07 [95% CI 0.66-1.75], p=0.61). Conclusions: Endocrine resistance status and TP53 and acquired ESR1 mutations were associated with shorter PFS. Factors associated with poor outcome may be used to select patients to test alternative treatment strategies in clinical trials. Clinical trial information: NCT02102165 . [Table: see text]