Background: Men with advanced prostate cancer are being treated with androgen deprivation therapy (ADT) for longer periods of time. The use of ADT can be limited by estrogen deficiency side effects, including a loss of bone and a higher incidence of fractures. GTx-758 is an ERα agonist that lowers serum free testosterone greater than an LHRH agonist. Herein we compare the effects of GTx-758 and leuprolide on markers of bone turnover, C-terminal telopeptides and bone specific alkaline phosphatase, in men with advanced prostate cancer treated with ADT. Methods: In a Phase II study (G200705), men with advanced prostate cancer (n=159) received 1000 mg or 2000 mg of GTx-758 daily or leuprolide. Serum samples were collected and analyzed by a reference laboratory. C-terminal telopeptides (pg/ml) and bone specific alkaline phosphatase (U/L) were measured as indicators of bone turnover. All p values describe the comparison of the GTx-758 treatment groups to the leuprolide treated men at day 120. Results: Men receiving the 1000 mg and 2000 mg doses of GTx-758 had lower C-terminal telopeptide levels with a mean percentage change of -56.9 ± 12.5 and -54.8 ± 23.1, respectively, as compared with an increase of 46.0 ± 48.9 percentage in the men receiving the leuprolide (p<0.001). Similarly, bone specific alkaline phosphatase levels were lower in men treated with GTx-758, with mean percentage changes of-28.5 ± 11.7 and -19.8 ± 14.7, respectively, compared with an increase of 8.1 ± 24.3 percent in the leuprolide treated group (p<0.001). As a result of an increased risk of venous thromboembolic events (VTEs) at these higher doses of GTx-758, the trial was stopped prior to its completion and not all of the men on the study reached the 120 day treatment dates (71 evaluable). Conclusions: Patients receiving GTx-758 experienced a significant decrease in markers of bone turnover indicating potential improvement and not a loss of bone on ADT. Since changes in bone mineral density are a major side effect that can negatively affect the quality of life in men on ADT, the improvements in bone turnover observed in men treated with GTx-758 could be significant. A Phase II clinical trial utilizing lower doses of GTx-758 (G200712) is currently being performed. Clinical trial information: NCT01326312.