Benefit of CDK 4/6 inhibition in less common breast cancer subsets: A U.S. Food and Drug Administration pooled analysis.

1024Background: Cyclin dependent kinase 4/6 inhibitors (CDKIs) are approved for 1st and 2nd line endocrine-based therapy in hormone-receptor positive, HER2-negative advanced breast cancer. There is limited clinical data on the benefit and value of adding CDKIs to endocrine therapy in less common subtypes, such as progesterone receptor negative (PR-), de novo metastatic, and lobular breast cancer, which may have differing degrees of endocrine sensitivity. Methods: We pooled data from 5 phase 3 randomized registration trials of CDKI with an aromatase inhibitor (AI) in the 1st or fulvestrant in the 2nd-line setting. Exploratory subset analyses focused on patients with de novo metastatic, PR-, and lobular cancer. Progression free survival (PFS) was examined in the ITT populations using Kaplan Meier plots and hazard ratios (HR) with 95% confidence intervals (CI). Results: We estimated median PFS differences between the CDKI and control arms (Table 1). In patients with PR- tumors (n = 490), the estimated median...