Abstract P5-01-13: Efficacy of abemaciclib and endocrine therapy on the overall survival for advanced ER-positive/HER2-negative breast cancer according to the treatment of lines

Abstract Backgrounds: Combination therapy of CDK4/6 inhibitors and endocrine therapy (ET) is standard care for ER-positive/HER2-negative (ER+/HER2-) advanced breast cancer. Progression free survival of each treatment is important, however, overall survival (OS) form the diagnosis of advanced breast cancer is also important for the patients. Instead of the high efficacy, CDK4/6 inhibitors also have high cost and toxicity. Not all patients will want to use CDK4/6 inhibitors from first-line therapy, depending on their circumstances and preferences. Objective:This study was conducted to investigate the efficacy of the combination therapy of abemaciclib and ET according to the treatment line in terms of time to treatment failure (TTF) and OS. Patients and Methods: A total of 114 patients who started combination therapy with abemaciclib and ET between 2018/12 and 2024/1 and were followed for at least three months were included in the study. Clinicopathological characteristics and TTF, OS from the start of abemaciclib treatment, OS from the diagnosis of the advanced breast cancer were analyzed. according to the treatment lines. Results: Median age of the patients was 61 (range 25-90) years. Twenty-nine (25.4%) were pre-menopausal and 85 (74.6%) were postmenopausal women. Abemaciclib was used as the firtst line treatment in 50(43.9%), second line 23 (20.2%) and third line or later 41 (35.9%) patients. ET for combination was aromatase inhibitor (AI) 44 (38.6%), AI +LH-RHa 8 (7.0%), fulvestrant (FUL) 44 (38.6%) and FUL+LH-RHa 17 (14.9%) patients. De novo stage IV 31 (27.2%), relapse while on the first 2 years of adjuvant endocrine therapy (adjET) 13 (11.4%), relapse while on adjET but after the first 2 years, or relapse within 12 months of completing adjET 28 (24.5%), relapse after 12 months of completing adjET 32 (28.0%), and no adjET was 10 (8.7%). Abemaciclib use from the 1st line treatment was associated with the presence of visceral metastasis, liver metastasis. Abemaciclib use from the 3rd line or later was associated with relapse after 12 months of completing adjET. Median TTF of the abemaciclib treatment was 19.6 months as the first line, 12.7 months as the second line, 14.9 month as the third or later line. Median OS from the start of abemaciclib 43.3, 40.7 and 57.8 months in which abemaciclib was used as the first, second and third or later line. Thus, there were no statistical differences in TTF of the abemaciclib treatment and OS from the start of abemaciclib according to the treatment lines of abemaciclib. Median OS from the diagnosis of advanced breast cancer was 43.7, 58.8, 135.4 months in which abemaciclib was used as the first, second and third or later line (p<0.0001). Conclusion:In routine practice, abemaciclib was used in different lines of treatment, depending on physician’s choice and patient preference. The TTF of the abemaciclib treatment or OS from the start of abemaciclib was not different among the treatment lines. Median OS from diagnosis of advanced breast cancer was significantly longer in patients who used abemaciclib as a third or later line treatment compared to those who used abemaciclib as a first or second line. Although there is a selection bias, the results of this study suggest that abemaciclib does not necessarily have to be used as the first line treatment for advanced ER-positive/HER2-negative breast cancer. Citation Format: Eriko Tokunaga, Yumiko Koi, Junji Kawasaki, Wakako Tajiri, Sayuri Akiyoshi, Hideki Ijichi, Chinami Koga, Yoshiaki Nakamura. Efficacy of abemaciclib and endocrine therapy on the overall survival for advanced ER-positive/HER2-negative breast cancer according to the treatment of lines [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2024; 2024 Dec 10-13; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2025;31(12 Suppl):Abstract nr P5-01-13.