Safety and Efficacy of Palbociclib and Radiation Therapy in Patients With Metastatic Breast Cancer: Initial Results of a Novel Combination

PURPOSE: Palbociclib is a selective cyclin-dependent kinase 4/6 inhibitor approved for metastatic ER+/HER2- breast cancer. Preclinical evidence suggests a possible synergistic effect of palbociclib when combined with radiation therapy (RT); however, the toxicity of this pairing is unknown. We report preliminary results on the use of this combination. METHODS AND MATERIALS: Records of patients treated with palbociclib at our institution from 2015 to 2018 were retrospectively reviewed. Patients who received RT for symptomatic metastases concurrently or within 14 days of palbociclib were included. Local treatment effect was assessed by clinical examination and subsequent computed tomography/magnetic resonance imaging. Toxicity was graded based on Common Terminology Criteria for Adverse Events version 5.0. RESULTS: A total of 16 women received palliative RT in close temporal proximity to palbociclib administration. Four patients received palbociclib before RT (25.0%), 5 concurrently (31.3%), and 7 after RT (43.8%). The median interval from closest palbociclib use to RT was 5 days (range, 0-14). The following sites were irradiated in decreasing order of frequency: bone (11 axial skeleton [9 vertebra and 2 other]; 4 pelvis; 3 extremity), brain (4: 3 whole brain RT and 1 stereotactic radiosurgery), and mediastinum (1). The median and mean follow-up time is 14.7 and 17.6 months (range, 1.7-38.2). Pain relief was achieved in all patients. No radiographic local failure was noted in the 13 patients with evaluable follow-up imaging. Leukopenia, neutropenia, and thrombocytopenia were seen in 4 (25.0%), 5 (31.3%), and 1 (6.3%) patient before RT. After RT, 5 (31.3%), 1 (6.3%), and 3 (18.8%) patients were leukopenic, neutropenic, and thrombocytopenic, respectively. All but 2 (grade 2) hematologic toxicities were grade 1. No acute or late grade 2+ cutaneous, neurologic, or gastrointestinal toxicities were noted. Toxicity results did not differ based on disease site, palbociclib-RT temporal association, or irradiated site. CONCLUSIONS: The use of RT in patients receiving palbociclib resulted in minimal grade 2 and no grade 3+ toxicities. This preliminary work suggests that symptomatic patients receiving palbociclib may be safely irradiated.