Effect of adherence to monthly zoledronic acid treatment on risk and frequency of skeletal complications and follow-up duration in breast cancer patients

Abstract

9593 Background: Bone is the most common site of metastasis in breast cancer patients (pts). Clinical trials and outcome studies have demonstrated benefit of zoledronic acid (ZA) in reducing risk of skeletal complications (SC). This study explored relationships between persistency on ZA and the intended treatment (TX) outcomes. Methods: Female breast cancer pts with first bone metastasis (BM) diagnosed between Jan 03 to Oct 06 were identified from PharMetrics database. Persistency was defined as duration from initiating ZA TX to date of first TX gap of >45 days. Pts were divided into short (≤90 days, 230 pts), medium (91–180, 171 pts) and long (>180, 313 pts) persistency groups. TX selection bias was handled by calculating propensity scores using year of first BM diagnosis, having other metastases or >1 BM claim, use of opioids and oral bisphoshonate before BM. Propensity scores, age and Charlson's Comorbidity Index (CCI) were included in multiple regressions to investigate impact of ZA persistency on follow-up duration, risk and rate of SC. Results: 2394 pts were enrolled, with 714 (29.72%) ZA treated. Three persistency groups had similar age. Short group had higher CCI than long group (mean 8.53 vs 8.34, p<0.05). Frequency of ZA TX decreased over time in all groups. For short, medium and long persistency groups, means of follow-up durations were 9.8 (median, 7.0), 13.0 (9.0), 19.3 (18.1) months, and means of time to first SC after BM were 249 (median: 167), 323 (217), 418 (365) days. Means of SC rates were 0.27, 0.21, and 0.12 per month for patients who experienced at least 1 SC among the three groups. Medium and long persistency groups had 41% and 139% longer follow-up than short group (p<0.05). Long group had lower risk of SC than short group (HR=0.576, p<0.05). In patients who developed at least 1 SC, long group experienced 39% less SCs (p<0.005) than short group. Conclusions: TX with zoledronic acid in breast cancer pts with BM positively impacts the risk, frequency of SC and follow-up duration. Also, longer persistency on ZA TX is associated with significantly longer follow-up and lower risk of SC. [Table: see text]