Effects on bone resorption markers of continuing pamidronate or switching to zoledronic acid in patients with high risk bone metastases from breast cancer

BACKGROUND: Switching patients who remain at high risk of skeletal related events (SREs) despite pamidronate to the more potent bisphosphonate zoledronate, may be an effective treatment strategy. As part of a previously reported clinic study in this setting, we evaluated whether biomarkers for bone resorption, such as Bone-Specific Alkaline Phosphatase (BSAP), bone sialoprotein (BSP), and N-terminal telopeptide (NTX) correlated with subsequent SRE risk. METHODS: Breast cancer patients who remained at high risk of SREs despite at least 3 months of q.3-4 weekly pamidronate were randomized to either continue on pamidronate or to switch to zoledronate (4 mg) once every 4 weeks for 12-weeks. High risk bone metastases were defined by either: occurrence of a prior SRE, bone pain, radiologic progression of bone metastases and/or serum C-terminal telopeptide (CTx) levels > 400 ng/L despite pamidronate use. Serum samples were collected at baseline and weeks 1, 4, 8 and 12 (CTx and BSAP) and baseline and week 12 (NTx and BSP), and all putative biomarkers were measured by ELISA. Follow up was extended to 2 years post trial entry for risk of subsequent SREs. The Kaplan-Meier method was used to estimate time-to-event outcomes. Generalized estimating equations (GEE) were used to evaluate if laboratory values over time or the change in laboratory values from baseline were associated with having a SRE within the time frame of this study. RESULTS: From March 2012 to May 2014, 76 patients were screened, with 73 eligible for enrolment. All 73 patients were available for biochemical analysis, with 35 patients receiving pamidronate and 38 patients receiving zoledronate. The GEE analysis found that no laboratory value was associated with having a subsequent SRE. Interaction between visit and laboratory values was also investigated, but no interaction effect was statistically significant. Only increased number of lines of prior hormonal treatment was associated with subsequent SRE risk. CONCLUSION: Our analysis failed to find any association between serum BSAP, BSP, CTx or NTx levels and subsequent SRE risk in this cohort of patients. This lack of correlation between serum biomarkers and clinical outcomes could be due to influences of prior bisphosphonate treatment or presence of extra-osseous metastases in a significant proportion of enrolled patients. As such, caution should be used in biomarker interpretation and use to direct decision making regarding SRE risk for high risk patients in this setting.