Abstract P5-02-49: Efficacy of subsequent-abemaciclib treatment after disease progression on palbociclib combined with endocrine therapy in patients with ER-positive HER2-negative metastatic breast cancer
Authors: Seki Hirohito , Nakai Taketo , Shimizu Ken 
Published: March 2023

Abstract
Purpose: Currently, CDK4/6i combined with endocrine therapy (ET) has become the standard of care as first- or second-line treatment for estrogen receptor-positive (ER+) human epidermal growth factor receptor 2-negative (HER2-) metastatic breast cancer (MBC). However, at present, there are no guidelines for the selection of appropriate treatments after disease progression on prior CDK4/6i treatment combined with ET. Therefore, this retrospective study aimed to verify the efficacy and evaluate predictive factors of clinical outcomes in the patients with ER+/HER2- MBC during subsequent-abemaciclib treatment after disease progression on prior-palbociclib combined with ET.
Methods: In total, 81 patients with ER+/HER2- MBC were treated with palbociclib and ET at our medical center between December 2017 and November 2020. Among them, 25 patients who received subsequent-abemaciclib after disease progression on prior-palbociclib were included. All patients provided informed consent for the indicated treatment. Clinicopathological variables were compared using Fisher’s exact test. The Mann–Whitney U test was used to compare categorical variables. PFS and time to chemotherapy (TTC) were estimated using Kaplan–Meier analysis with 95%CIs.
Results: The median age was 69 years, and four women were premenopausal. Stage IV disease occurred in 28.0% (7/25) and visceral metastases were observed in 68.0% (17/25) of the patients. The treatment line of prior-palbociclib was the first-line in 3 (12.0%), second-line in 11 (44.0%), and third- and late-line in 11 patients (44.0%). The median PFS of prior-palbociclib plus ET was 6.3 months (95%CI=5.814–6.786). Subsequent-abemaciclib combined with fulvestrant after disease progression on prior-palbociclib was administered in 64.0% (16/25) of the patients. Median numbers of previous ET and chemotherapy of subsequent-abemaciclib were 2 and 0, respectively. Subsequent-abemaciclib after disease progression on prior-palbociclib resulted in an ORR and clinical benefit rate (CBR) of 16.0% (4/25) and 44.0% (11/25), respectively (Table 1). Kaplan–Meier curve analysis showed that the median PFS was 5.3 months (95%CI=3.082–7.518). Univariate analysis revealed that the best overall response (BOR) ≥PR and progression-free interval (PFI) ≥6 months in prior-palbociclib contributed to better clinical outcomes. Moreover, in multivariate analysis, BOR to prior-palbociclib was the only independent predictive factor for PFS (HR=0.190; 95%CI=0.050–0.722; p=0.015) (Table 2). With regard to grade ≥3 TRAEs in the subsequent-abemaciclib, neutropenia and diarrhea were observed in 16.0% (4/25); appetite loss and fatigue in 12.0% (3/25); leukopenia and anemia in 8.0% (2/25); and thrombocytopenia and liver dysfunction in 4.0% (1/25) of patients. Twelve patients (48.0%) required dose reduction due to TRAEs grade ≥3 in subsequent-abemaciclib. Of them, 10 patients required one dose-level reduction and 2 needed two dose-level reductions. Three patients (12.0%) required dose discontinuation: two had uncontrollable appetite loss and nausea and one had pneumonia. Of the 25 patients, 12 were not administered any prior chemotherapy and the median TTC in those treated with subsequent-abemaciclib was 33.9 months (95%CI=11.334–56.136). Next-line treatment after disease progression on subsequent-abemaciclib in the patients who were not administered prior chemotherapy was performed in 2 (8.0%) who were treated with ET and in 12 (48.0%) who were treated with chemotherapy (1 taxane-based, 7 eribulin, and 4 oral 5-fluorouracil). The median PFS in patients treated with chemotherapy after disease progression on subsequent-abemaciclib treatment was 6.2 months (95%CI=3.484–8.916).
Table 1. Best overall response rate in patients with ER+/HER- MBC who were treated with subsequent-abemaciclib after disease progression on prior-palbociclib
Table 2. Univariate and multivariate analyses of the progression-free survival in patients with ER+/HER2- MBC treated with subsequent-abemaciclib after progression on prior-palbociclib
Citation Format: Hirohito Seki, Taketo Nakai, Ken Shimizu. Efficacy of subsequent-abemaciclib treatment after disease progression on palbociclib combined with endocrine therapy in patients with ER-positive HER2-negative metastatic breast cancer [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P5-02-49.