Abstract PO1-05-01: Cyclin-dependent Kinase 4/6 Inhibitors Combined with Radiotherapy in the Management of Brain Metastases in HR-positive/HER2-negative Breast Cancer Patients

Purpose: Cyclin-dependent kinase 4/6 inhibitors (CDK4/6i) plus endocrine therapy demonstrated overall survival benefits in advanced breast cancer patients (ABC), hormone receptor-positive, HER2-negative (HR+/HER2-) and remains the mainstay in the first and second-line treatment. Brain metastases are rare at the beginning of treatment in this group of patients and are found more often during subsequent lines of treatment and other subtypes of breast cancer. However, there is a paucity of data on the safety and efficacy of multimodality CDK4/6i and radiotherapy treatment in the management of brain metastases. Thus, we performed a retrospective analysis of ABC patients (pts) treated at our institution with radiation therapy to the brain and CDK4/6i. Pts who received radiotherapy before CDK4/6i initiation or concurrently with CDK4/6i (group 1) were compared to the patients who progressed in the brain during CDK4/6i treatment and then received radiotherapy (group 2). The primary endpoint was progression-free survival (PFS) in group 1 vs. group 2; the secondary endpoints were local control (LC) within the brain in group 1 vs. group 2 and severe toxicity. Materials/Methods: Among 379 pts who received CDK4/6i, 26 pts (6.9%) received radiotherapy to the brain. 17 pts received radiotherapy before or concurrent with CDK4/6i and were included in group 1. Nine pts received radiotherapy after CDK4/6i discontinuation due to disease progression and comprised group 2. Results: The median age was 51.5 years (IQR range 41-62). Six pts had de novo metastatic disease. The majority of patients were treated in the first-line setting (15 pts, 58%). 17 pts received ribociclib, 7 palbociclib, and 2 abemaciclib. 15 pts received letrozole as an endocrine compound and 11 pts fulvestrant. 19 pts (73%) received previous chemotherapy. Six pts had an ECOG performance status of 0, 15 pts ECOG 1, and 5 pts ECOG 2. The most common local treatment was whole-brain radiotherapy with a total dose of 20 Gy delivered in 5 fractions (fr) (11 pts: 4 pts VMAT, 2 pts IMRT, 3 pts 3D, and 2 pts 2D). Eight pts received stereotactic radiation therapy (6 pts with Linac: 2 pts 24 Gy/2 fr, 2 pts 24 Gy/3 fr, 2 pts 25 Gy/5 fr; 1 pt GammKnife 20 Gy/1 fr, 1 pt Cyberknife 15 Gy/3 fr). Six- and 12-month PFS in group 1 was 88.2% (95% CI74.1-100) and 58.8% (95% CI: 37.7- 91.8), respectively, whereas in group 2, six- and 12-month PFS was 55.6% (95%CI: 31- 99.7) and 44.4% (95% CI: 21- 92.2), respectively. Six- and 12-month LC in group 1 was 92.3% (95%CI: 78.9-100) and 83.9% (65.7-100), respectively. LC in group 2 was poor: 3-month LC was only 66.7 (30-100). At a median follow-up of 8.8 months, no unanticipated toxicity was reported. Conclusion: Multimodality treatment with CDK4/6i and radiotherapy to the brain is safe and effective. Patients who developed metastases during CDK4/6i treatment tend to have a worse prognosis in comparison to those who received prior radiotherapy. CDK4/6i following radiotherapy seems to be an effective and valuable treatment option in this particular metastatic breast cancer population. Citation Format: Marcin Kubeczko, Dorota Gabryś, Anna Polakiewicz-Gilowska, Aleksandra Krzywon, Donata Gräupner, Barbara Łanoszka, Marta Mianowska-Malec, Aleksandra Leśniak, Katarzyna Świderska, Konstanty Chomik, Barbara Grandys, Michał Jarząb. Cyclin-dependent Kinase 4/6 Inhibitors Combined with Radiotherapy in the Management of Brain Metastases in HR-positive/HER2-negative Breast Cancer Patients [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 PO1-05-01.