Abstract PO1-17-09: Real-world clinical outcomes in US patients with brain metastases secondary to HR+/HER2- metastatic breast cancer treated with abemaciclib
Authors: Gathirua-Mwangi Wambui , Martin Holly , He Dan , Zheng Shen , Sheffield Kristin , John Jincy , Rybowski Sarah , Brastianos Priscilla 
Published: May 2024

Abstract
Background: Abemaciclib has demonstrated pharmacologically relevant intracranial concentrations and antitumor activity in patients with brain metastases secondary to HR+/HER2- metastatic breast cancer (MBC). However, clinical data is limited, and further assessments are warranted. This retrospective study describes real-world outcomes following abemaciclib initiation in patients with brain metastases secondary to HR+/HER2- MBC. Methods: Data were accessed from the US nationwide Flatiron Health electronic health records-derived de-identified database from 01/01/2011 to 12/31/2021. This real-world study included adult patients with a diagnosis of HR+/HER2- MBC with brain metastases prior to abemaciclib initiation. Patient and treatment characteristics were summarized using descriptive statistics. Kaplan-Meier methods were used to assess clinical outcomes from abemaciclib initiation: time to treatment discontinuation (TTD), progression-free survival (rwPFS), overall survival (rwOS). Intracranial rw best responses were also reported. Results: This study included 82 patients (one male) diagnosed with MBC and brain metastases prior to abemaciclib initiation and the median follow-up was 12.8 months. At data cut-off, 17 (20.7%) patients remained on abemaciclib therapy. Twenty-three (28%) patients received abemaciclib as the first line of therapy for MBC. Of the 82 patients in the study, 68 (82.9%) patients used concomitant endocrine therapy. Prior to abemaciclib initiation, 25 (30.5%) patients received chemotherapy and 33 (40.2%) patients received a CDK4 and 6 inhibitor. Most patients (67.1%) initiated abemaciclib at a dose of 150 mg twice daily. The median time from diagnosis of brain metastasis to abemaciclib initiation was 2.1 months (interquartile range:1.0, 8.0); 16 (19.5%) did not undergo surgical resection or receive radiation to the brain during this time. Sixty-six (80.5%) patients received brain radiation of which 56.1% received stereotactic radiosurgery (SRS) and 47.0% received whole-brain radiation (WBRT) either alone or in combination with craniotomy/metastasectomy, intrathecal chemotherapy, or surgery. The median TTD for abemaciclib was 7.1 months (95% confidence interval [95% CI]: 4.6, 11.3); most common reasons for discontinuation were disease progression (39.0%) or adverse events (29.3%). The median rwPFS was 9.2 months (95% CI: 6.0, 11.6) and median rwOS was 20.8 months (95% CI: 13.9, 26.0). Of the patients where response data were available (n = 51), rw intracranial clinical benefit rate (CBR; complete response [CR]/partial response [PR]/stable disease [SD] ≥24 weeks of abemaciclib initiation) was 62.7% (Table 1). Conclusion: In this real-world study, most patients with brain metastases secondary to HR+/HER2- MBC who initiated abemaciclib treatment received SRS or WBRT prior to or concurrent with abemaciclib initiation. While the clinical outcomes, rwPFS and intracranial CBR were encouraging, these findings should be interpreted with caution due to the nature of the real-world study design.
Table 1. Intracranial Real-World Best Responses
Citation Format: Wambui Gathirua-Mwangi, Holly Martin, Dan He, Shen Zheng, Kristin Sheffield, Jincy John, Sarah Rybowski, Priscilla Brastianos. Real-world clinical outcomes in US patients with brain metastases secondary to HR+/HER2- metastatic breast cancer treated with abemaciclib [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO1-17-09.