Symptomatic skeletal-related events in patients receiving longer term bone-modifying agents for bone metastases from breast and castration resistant prostate cancers.

Background: The effect of longer-term use of bone-modifying agent (BMA) on symptomatic skeletal event (SSE) rates in patients with bone metastases remains unclear. This retrospective study of a cohort of patients in a randomized controlled trial evaluated SSEs in patients receiving BMAs at a single cancer center.

Methods: Data from patients with metastatic breast and castration-resistant prostate cancer (CRPC) were interrogated to evaluate the effects of longer-term use of BMAs on incidence, type, and risk factors for SSEs.

Results: Of 162 patients, 109 (67%) had breast cancer (BC) and 53 (33%) CRPC. Median age at diagnosis of bone metastases was 61.9 years (range 27.5-97.2) for BC patients and 72.1 (range 37.0-92.2) for CRPC patients. Median duration of BMA use was 2.3 years (range 0.1-9.9 years) for BC and 3.8 years (range 1.5-9.4) for CRPC patients. The initial BMAs in BC patients were pamidronate (46.8%), denosumab (31.2%), and zoledronate (22%). All CRPC patients received denosumab. During follow-up, 59% of BC and 75% of CRPC patients had at least one SSE. The number of patients experiencing ≥ 1 SSE per year was higher in the first year after bone metastasis diagnosis (63/162; 38.9%) compared with that in the second (26/149; 17.5%) and third years (30/123; 24.4%). Neither age, visceral disease, multiple bone metastases, nor biological markers for BC had a significant impact on time to first SSE.

Conclusions: The risk for SSEs was greatest in the first year after diagnosis of bone metastasis. Studies evaluating de-escalation and even stopping of BMAs with longer-term use may therefore be warranted.