Safety of concomitant therapy with radium‐223 and abiraterone or enzalutamide in a real‐world population

BACKGROUND: Real-world utilization and outcomes of combination therapy for men with metastatic castrate-resistant prostate cancer (mCRPC) are largely unknown. We evaluated the overall survival (OS) and skeletal-related events (SREs) among men who received radium-223 with or without concomitant abiraterone or enzalutamide in the Veterans Affairs (VA) Health System. METHODS: We reviewed charts of all mCRPC patients who received radium-223 in the VA from January 2013 to September 2017. We used Cox models to test the association between concomitant therapy versus radium-223 alone on OS and SRE. Sensitivity analyses were performed for concomitant use of denosumab/bisphosphonates. RESULTS: Three hundred and eighteen patients treated with radium-223 were identified; 116/318 (37%) received concomitant abiraterone/enzalutamide. Two hundred and seventy-seven (87%) patients died during follow-up. Patients who received concomitant therapy were younger at radium-223 initiation (median age 68 vs. 70, p = .027) and had a longer follow-up (median 29.5 vs. 17.9 months, p = .030). There was no OS benefit for those on concomitant therapy (hazard ratio [HR]: 0.87, 95% confidence interval [CI]: 0.67-1.12, p = .28). There was a trend for an increased SRE risk for patients on concomitant therapy (HR: 1.87, 95% CI: 0.96-3.61, p = .066), but this was not significant. When analyses were limited to men using bone heath agents, similar results were seen for OS (HR: 0.86, 95% CI 0.64-1.15, p = .30) and SRE (HR: 2.36, 95% CI: 0.94-5.94, p = .068). CONCLUSIONS: Despite the common use of concomitant therapy in this real-world study, there was no difference in OS among mCRPC patients. A nonsignificant increased SRE risk was observed. Further work needs to evaluate the optimal sequence, timing, and safety of combination therapies.