Abstract PO4-27-08: A Phase II Multi-Institutional Study of Concurrent Radiotherapy, Palbociclib, and HormoneTherapy for Treatment of Bone Metastasis in Breast Cancer Patients

Abstract Background: This Phase II study aimed to determine the efficacy and safety of administering palliative radiotherapy (RT) to bone metastases (mets) in patients (pts) receiving concurrent palbociclib and hormone therapy (HT) for hormone receptor positive, Her2/neu negative breast cancer. The primary endpoint was response rate 3 months after RT. Methods: Pts with painful bone mets or asymptomatic bone mets with risk for impending clinical event were treated with RT to 30 Gy in 10 fractions or 20 Gy in 5 fractions with concurrent palbociclib and HT. Change in maximum pain score on the Brief Pain Inventory was used to assess pain response 3 months post RT relative to baseline. Among pts with asymptomatic bone lesions, response was defined as prevention of a clinical event (e.g., bone fracture) without evidence of local tumor growth on surveillance imaging. Patient reported outcomes (PROs) of fatigue, depression, anxiety, and quality of life were collected. Blood-based biomarkers were examined to determine their relationship with response and PROs. Using a non-inferiority study design, a sample size of at least 33 pts was needed to achieve 80% power to detect a non-inferiority proportion of 60% using a one-sided binomial test and assuming a Type I error of 0.05. Results: Among 38 patients enrolled, 79% had painful bone mets. 35 pts completed baseline and 3-month post RT assessments. 61% and 37% of pts received aromatase inhibitors and fulvestrant, respectively, with palbociclib. Median age was 60 years (31-83) and 25% of pts were non-Hispanic Black. The majority (72%) of pts received 5 fraction RT and 69% received RT to one bone region. 29 patients (83%) were responders [95% CI: 66%-93%, p=.003]. Median progression free survival (PFS) was 30.4 months (1.3 - 44.0). Three-year PFS and overall survival were 45.6% and 67.2%, respectively. Grade 3 neutropenia developed in 4 (11%), 4 (11%), and 7 (20%) pts at end of RT, 1 month, and 3 months post RT, respectively. No pts developed Grade 4 neutropenia. One patient developed Grade 3 gastrointestinal toxicity during RT, another developed Grade 3 dyspnea 1 month post XRT, and another developed a bone fracture 6 months after RT. >While patient reported measures of fatigue, depression, and overall quality of life did not significantly change during or after RT relative to baseline, pain scores (i.e., BPI pain severity and interference, BM22 pain characteristics and pain site subscales, EORTC C15 pain) significantly decreased 3 months post XRT relative to baseline. Heightened inflammatory marker levels (e.g., TNFRII, IL1ra, CRP, TNF alpha) were significantly associated with worse symptoms of fatigue (MFI) and depression (HADS), increased pain interference (BPI), and lower overall quality of life (SF-36), controlling for age and time using mixed effect models. Patients with improved pain symptoms and lower pain scores had significantly lower inflammatory marker levels relative to baseline (e.g., interleukin-6). Conclusions: Our study demonstrated high rates of response to concurrent RT with palbociclib and HT with acceptable levels of toxicity. These results suggest that RT to bone mets effectively alleviates pain in patients taking palbociclib and pain response to treatment is associated with decreased inflammatory markers. RT may be given to pts receiving palbociclib and HT for breast cancer. Clinical Trial Identification: NCT03691493 Citation Format: Mylin Torres, Jolinta Lin, Sarah Friend, Canhua Xiao, Yichun Cao, Shannon Kahn, Karen Godette, Sheela Hanasoge, David Yu, Tony Eng, Matthew Cheney, Nancy Wiggers, Andrew Pippas, Keerthi Gogineni, Jane Meisel, David Schuster, Aditya Bardia, Andrew Miller, Jennifer Felger, Jeffrey Switchenko, Amelia Zelnak, Kevin Kalinsky, Manali Bhave. A Phase II Multi-Institutional Study of Concurrent Radiotherapy, Palbociclib, and HormoneTherapy for Treatment of Bone Metastasis in 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 PO4-27-08.