Biomarkers of bone metabolism in [223Ra] RaCl2 therapy - association with extent of disease and prediction of overall survival.

Background: The alpha-emitting radionuclide therapy [²²³Ra]RaCl₂ (Radium-223) improves overall survival (OS) and time to symptomatic skeletal event (SSE) in patients with metastatic castration-resistant prostate cancer (mCRPC). Evidence suggests that the effect of Radium-223 is partly exerted through an impact on the surrounding bone matrix. We hypothesized that bone metabolism markers (BMM) could provide predictive information regarding response to Radium-223. Accordingly, the aim of this study was to investigate changes in BMM during Radium-223 therapy and evaluate association with clinical outcome.

Methods: Prospective study of BMM in patients with mCRPC receiving Radium-223. Blood samples were collected before each administration of Radium-223 and the following BMM were quantified; bone-specific alkaline phosphatase (BALP), osteocalcin, procollagen type I N-propeptide (PINP), C-terminal telopeptide of type I collagen (CTX), C-terminal cross-linking telopeptide of type I collagen generated by matrix metalloproteinases (CTX-MMP), tartrate-resistant acid phosphatase isoform 5b (TRACP5b), receptor-activated nuclear factor κB ligand (RANKL), osteoprotegerin (OPG), and sclerostin. Clinical outcomes were scintigraphic progression during/after therapy, change in bone scan index (BSI), occurrence of SSE, and OS.

Results: A total of 55 mCRPC patients were included. There was a significant linear association between skeletal extent of disease and CTX-MMP, PINP, BALP, and osteocalcin. No significant association between dynamics in BSI and BMM were detected. Median OS for the cohort was 14 months (95% CI: 10.7-16.8). Baseline levels of Log2-CTX-MMP (HR = 2.15 (95%CI: 1.1-4.1)) and Log2-BALP (HR = 1.59 (95%CI: 1.1-2.1)) were associated with OS. Patients with increasing CTX-MMP during therapy had significantly shorter OS (Median OS = 4 mo. (95%CI: 2.3-5.7)) than patients with stable or decreasing CTX-MMP (Median OS = 12 mo. (95%CI: 10.1-13.9), P < 0.001).

Conclusion: BMM are significantly associated with scintigraphic extent of skeletal disease and OS in patients with mCRPC. Particularly, the bone resorption marker CTX-MMP is a promising surrogate marker for prediction of outcome in patients receiving Radium-223 therapy and could potentially improve selection of patients for therapy and assessment of response.

Trial registration: Clinicaltrials.gov, NCT03247010. Registered 10th of August 2017, https://clinicaltrials.gov/study/NCT03247010?term=NCT03247010&rank=1 .