Background: We previously reported that baseline BMB are independently prognostic for overall survival (OS) in men with castration resistant prostate cancer. We correlated BMB with outcomes in mHSPC as part of S1216, a phase III trial of ADT +/- the novel CYP17 inhibitor orteronel. Methods: Blood was obtained at study entry for bone resorption [C-telopeptide(CTx) & Pyridinoline(PYD)] & formation markers [C-terminal collagen propeptide(CICP) & bone alkaline phosphatase(BAP)]. With prior DSMC approval, patients were sampled to mask potential treatment effect. Logistic regression was used to assess if BMB elevation above median was prognostic for a PFS event w/in 2 years across pooled study treatment arms, adjusting for baseline variables (including disease extent, PSA, age, pre-randomization ADT, & presence of bone mets). An additional interaction term between BMB elevation & presence of bone mets was included; if significant, separate models were developed for men +/- bone mets. Results: Of 1,313 men, 656 were included in this analysis. All 4 BMB levels were higher in men with a PFS event w/in 2 years vs. those with no PFS event. The odds ratio (OR) for a PFS event was significantly higher in men w/ elevated baseline BMB (see table). For BAP, a significant interaction between marker elevation and bone mets was seen (p = 0.003); men w/ bone mets and BAP elevation had an OR of 1.83 for a PFS event in 2 years. Conclusions: In men with newly diagnosed HSPC, elevated baseline levels of BMB were significantly associated with PFS, with about a two-fold increased risk of a progression event w/in 2 years. For CICP, CTx, & PYD, this association was independent of the presence of bone metastases. Baseline BMB levels have strong prognostic value in the mHSPC context. Correlative analysis of BMB & OS is planned. Clinical trial information: NCT01809691.