Background: Questions remain around the optimal use of bone-targeted agents (BTA) in patients with bone metastases (BM) from breast cancer (BC). In Canada pamidronate (PAM) is the most commonly used BTA in BC.We explored whether a switch to a more potent BTA, like zoledronic acid (ZA), in patients who remain at high risk of skeletal related events (SREs) despite PAM use is associated with significant palliative benefit. Methods: BC patients with high risk BM (prior SRE, bone progression, bone pain or levels of bone turnover marker serum C-telopeptide (sCTX) >400ng/L) despite >3 months of PAM use were eligible. Patients were randomized in a double–blind manner to either switch to ZA or continue on PAM every 4 weeks for 12 weeks. Primary outcome was the proportion of patients achieving a fall in sCTX at 12 weeks. Secondary outcomes were pain control (BPI and FACT-BP) and toxicity. Results 73 patients completed the study. Median age 61 years (range 37 – 87), prior duration of PAM use 10 months (range 3 – 118). sCTX levels for all patients at baseline, 372+/- 471, week 12, 209 +/-290. Proportion of patients achieving a fall in sCTX from week 0 to week 12, 26/31 (84%) in ZA arm, 17/30 (57%) in PA arm, p=0.0262. Two patients were unable to complete the study due to deterioration in renal function (both receiving PAM), four due to progressive disease (two receiving ZA, two PAM), two patients chose to discontinue the study before completion (both receiving ZA). Four patients (5%) had SRE’s during the study, two receiving ZA, two receiving PAM. Quality of life and pain analysis shows no difference between week 0 and week 12 scores in either arm. Toxicity was predominantly grade 1 and 2, numerically there were more adverse effects in the ZA arm than PAM. Conclusion Switching patients with high risk BM from PAM to ZA leads to a reduction in sCTX levels but may be associated with more toxicity. Quality of life and pain scores were similar between the two treatments. While the literature suggests that a reduction in sCTX may correlate with reduced rate of SREs, given the lack of symptom improvement a switching strategy cannot be recommended. NCT01907880 Clinical trial information: NCT01907880.