Serum Igf-1 Levels In Men With Advanced Prostate Cancer Treated With The Er Alpha Agonist, Gtx-758

171 Background: The insulin pathway and, in particular, insulin-like growth factor-1 (IGF-1), has been implicated in the development of prostate cancer, and serum IGF-1 levels have been associated with advanced disease. GTx-758, an ERα agonist that is being evaluated for the treatment of advanced prostate cancer, reduces serum total testosterone (T) and free T and increases SHBG. Herein, we compare the effects of GTx-758 and leuprolide on serum IGF-1 levels in men with advanced prostate cancer treated with ADT.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 from all of the men in the study and serum IGF-1 levels (ng/ml) were analyzed by a reference laboratory. All p values describe the comparison of GTx-758 treatment groups to the leuprolide treated men.Through day 90, in men receiving the 1000 mg and 2000 mg doses of GTx-758 , serum IGF-1 levels were decreased by more than 70 ng/ml (greater than 50%), while levels stayed constant or increased slightly in men on leuprolide (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 in the study reached the 90 day treatment date (92 men reached that date).In men with advanced prostate cancer, the ERα agonist, GTx-758, can decrease the active form of testosterone, serum free T, to significantly lower levels than leuprolide. Patients receiving GTx-758 experienced a significant decrease in serum IGF-1. Since changes in insulin metabolism are involved in regulating prostate cancer as well as bone and systemic metabolism, the observed decreases in IGF-1 could be significant. A Phase II clinical trial utilizing lower doses of GTx-758 (G200712) is currently ongoing to determine if similar effects can be observed with a lower rate of VTEs.NCT01326312.