130 Background: In patients with metastatic castration-resistant prostate cancer (CRPC), the degree of perturbation from normal bone turnover provides a strong indication of risk for disease progression, skeletal complications and death. Biomarkers of bone cell function and bone collagen degradation provide an integrated index of the underlying disease for patients with bony metastases, but available bone markers are not precise or accurate. In this study, patients with CRPC with bony metastases will consume a single oral calcium-41 dose and the pharmacokinetics of this will be measured over an 18 month period. Participants will also be assessed clinically for time to progression, skeletal related events and death. Methods: Patients with metastatic castration-resistant prostate cancer and bony metastatses, as diagnosed via bone scintigraphy, who were on bisphosphonates were enrolled into the study. 12 consenting research subjects consumed a single 1.2 microgram 41Ca tracer dose and provided 30-250mL urine specimens (single voids) after dose on day 1 (6h after dosing), days 7, 14, 28, 42, 60, and monthly thereafter. Isotope ratios were measured via accelerator mass spectrometry. Results: Urinary 41Ca/Ca was significantly and inversely associated with increased skeletal tumor burdens suggesting that development of an isotopic urine test for bone metastasis extent is feasible, providing a non-invasive and quantitative measure of disease extent. A calculation of the area under the curve of the measurements between day zero and 14 were inversely correlated with disease extent at baseline. Clinical deterioration with worsening bony disease was associated with a significant decrease in the urinary 41Ca/Ca value. We are currently assessing correlations between bone turnover and outcomes such as therapy effectiveness, disease progression, skeletal related events and death. Conclusions: This work is the first direct measurement of long-term calcium metabolism in advanced prostate cancer, providing a basic scientific complement to cellular and collagen-based measures of bone formation and resorption rates as well as a correlation to clinically relevant outcomes.
