Incidence of skeletal-related events in men with castration-resistant prostate cancer

Background: Skeletal-related events (SREs) are common in men with bone metastases and have negative consequences for patients with castration-resistant prostate cancer (CRPC), including pain, reduced quality of life, and increased risk of death. Published data on background rates of SREs in men with CPRC in real-world practice are sparse. Methods: We included men aged ≥ 65 years in the SEER-Medicare database with a prostate cancer diagnosis in 2000-2011 if they had no prior malignancy, had surgical or medical castration, and met protocol-defined criteria for castration resistance. Castration resistance was inferred from subsequent treatment with any of these systemic therapies: abiraterone, cabazitaxel, docetaxel, enzalutamide, mitoxantrone, or sipuleucel-T. The first occurrence of an SRE was identified in Medicare claims using diagnosis or procedure codes for fracture, bone surgery, radiation therapy, or spinal cord compression. We estimated incidence rates (IRs) of SREs in all eligible person-time and stratified by person-time before and after any use of the following bone-targeted agents (BTAs): alendronate, denosumab, ibandronate, pamidronate, risedronate, or zoledronic acid. Results: Of 2,234 men with CRPC (84% white, mean age 76.6 years), 896 (40%) had an SRE during follow-up, with 74% occurring within a year after cohort entry. Overall, the IR of SREs was 3.78 (95% CI, 3.53-4.03) per 100 person-months. The IR of SREs before any BTA use was 4.16 (95% CI, 3.71-4.65) per 100 person-months, and after any use was 3.60 (95% CI, 3.32-3.91) per 100 person-months. Conclusions: In this large cohort of elderly men with CRPC in a real-world setting in the U.S., SREs were common, with most occurring within a year after cohort entry. Although a direct causal interpretation of the difference in rates before and after BTA use is not possible (since confounding by indication and other factors cannot be excluded), further analysis may address at least some potential confounders.