Bone-sialoprotein (BSP), Dickkopf-1 (DKK1) and CXCR4 as potential biomarkers of breast cancer metastasis to bone: Analysis within the AZURE (BIG 01/04) study of adjuvant zoledronic acid.

Biomarkers predicting the risk of breast cancer metastatic spread to bone (BCBM) are urgently required. Our previous in vitro studies have identified Wnt-family inhibitor Dickopf-1 (DKK1) as a potential such biomarker. Also, bone sialoprotein (BSP) has shown elevated expression within breast primary tumours that metastasize to bone and BSP levels are also increased within serum samples from BCBM patients. The receptor protein CXCR4 is another potential biomarker of bone metastasis shown to bind stromal-derived-factor-1 (SDF1 / CXCL12) released by bone. In the current study, we tested these proteins as predictors of bone-metastasis within tissue-microarrays from the AZURE trial in which 3360 patients with early breast cancer at moderate/high risk of recurrence, were randomised 1:1 to receive standard adjuvant therapy +/-zoledronic acid (ZOL). In this pilot study, primary tumour cores from AZURE trial patients were stained with antibodies towards DKK1, BSP or CXCR4. Cytoplasmic staining was assessed independently by two trained operators, blinded to outcome data, under supervision of an experienced breast histopathologist. DKK1, BSP and CXCR4 levels were correlated with local and distant recurrences by multivariate analysis, adjusted for systemic therapy plan, ER status and lymph node involvement. Distant events examined included: first distant recurrence (metastasis to any site), first recurrence in bone (even if concurrent with relapse in other sites) and first recurrence in non-skeletal sites. Pilot-scale analysis of 462 patients revealed statistically significant association of BSP levels with first distant recurrence (whole population, p = 0.005; ZOL arm, p = 0.013) and with first event in non-skeletal sites (whole population, p = 0.008; ZOL arm, p = 0.046). • CXCR4 levels displayed statistically significant association with first distant recurrence (whole population p = 0.031) and first recurrence in bone (ZOL arm, p = 0.030). • DKK1 was not significantly associated with distant events in this analysis. • No significant associations were found for other distant events. Our pilot studies suggest that BSP and CXCR4, but not DKK1 may have potential as biomarkers predictive of metastasis to bone and/or nonbone sites. However these are initial data and we are currently extending these analyses within a larger patient group.