OBJECTIVES: This study aims to assess the effectiveness of combination therapy of cyclin-dependent kinase 4 and 6 (CDK4/6) inhibitors as a first line treatment in postmenopausal patients with hormone receptor positive (HR+), human epidermal growth factor receptor 2 negative (HER2-) advanced breast cancer (ABC), focusing on the age group, underrepresented in clinical trials. (under 45- or over 78-year old).

METHODS: We analyzed nationwide claims data from the Health insurance review & assessments service (HIRA) from Nov 2016 to Feb 2021. In this retrospective cohort study HR+, HER2- ABC who used CDK4/6 inhibitors in combination with aromatase inhibitors (AI) were selected. Based on the age range in both pivotal studies of East Asians (45 to 78 years old), patients were grouped by age: age of 45 less (Young), 45-78, and 79 over (Old). We observed time to next treatment (TTNT), time to chemotherapy (TTC), and time to in-hospital death. Kaplan-Meier method and log-rank test were used to compare groups.

RESULTS: Of 2,830 total patients, 358 young patients (12.65%) and 148 old patients (5.23%) were included. The TTNT was not significantly different by age group. Whereas, results on the TTC and the time to in-hospital death differed by age group. The TTC was significantly shorter in the young age group and the time to in-hospital death was significantly shorter in the old age group (P < 0.05).

CONCLUSIONS: There were approximately 18% of younger or older patients than pivotal trials in real-world clinical practice. Moreover, these understudied age groups showed different real-world effectiveness such as time to chemotherapy and time to in-hospital death. This study might provide fragmentary information which could fill the evidence gap between trial population and real-world patients.