First-line treatment with a cyclin-dependent kinase 4/6 inhibitor combined with an aromatase inhibitor for hormone receptor-positive, human epidermal growth factor receptor-2 negative metastatic breast cancer: Population-based outcomes for patients treated in Alberta, Canada.

e13020 Background: Cyclin-dependent kinase 4/6 inhibitor (CDK4/6i) in combination with an aromatase inhibitor (AI) has emerged as the standard first line treatment in patients with hormone receptor positive, human epidermal growth factor receptor-2 (HER-2) negative metastatic breast cancer (MBC). In this analysis, we describe population-based outcomes for first-line treatment with a CDK4/6i combined with an AI. Methods: All patients who were prescribed CDK4/6i + AI from January 2016 through June 2019 in a large Canadian province were included. Descriptive statistics were used to summarize patient demographics, tumor and treatment characteristics. Survival distributions were estimated using the Kaplan-Meier method. Multivariate analysis (MVA) using a Cox proportional hazards model was constructed to examine associations between potentially prognostic clinical variables and progression free survival (PFS). Results: A total of 316 patients were included. Median age was 61 years (interquartile range, 53-70 years), 82% were postmenopausal women, 39% had de novo MBC, and 48% had non-visceral disease. Palbociclib was prescribed in 94% of patients and the remaining patients received ribociclib. The CDK4/6i was dose-reduced upfront or during treatment in 47%. While 70% of the patients discontinued treatment due to progression, 30% stopped due to toxicity/patient preference/physician recommendation. With a median follow-up of 28.1 months, the median PFS was 37.9 months (95% CI, 26.7-NR). In the MVA, PR-negative tumour (HR, 2.37; 95% CI, 1.45-3.88; P = 0.001) and dose reduction of the CDK4/6i (HR, 1.51; 95% CI, 1.06-2.16; P = 0.022) predicted worse PFS. Median overall survival (OS) was not reached. The 30-month and 36-month OS rates were 74% and 68%, respectively. Of patients who progressed (n = 131), 89% received second-line treatment (chemotherapy in 46%, single agent hormonal therapy in 35%, hormonal therapy plus a targeted agent in 15%, and other in 4%). Median time to progression on second line chemotherapy was 9.0 (5.8-17.6) months and second line hormonal therapy +/- targeted agent was 4.0 (3.4-8.6) months (P = 0.012). Conclusions: The real-world outcomes of first-line use of CDK4/6i and AI are encouraging. PR negative tumors and dose reduction appear to be negative prognostic markers. CDK4/6i + AI as first-line treatment for HR-positive, HER2-negative MBC in Alberta is justified based on favorable PFS and early OS outcomes.