Evaluation Of The Er Alpha Agonist, Gtx-758 (250 Mg Daily), In Men With Metastatic (Mcrpc) And Non-Metastatic Castration Resistant Prostate Cancer (Nmcrpc)

201 Background: LHRH agents used for androgen deprivation therapy (ADT) were intended to lower total testosterone (T) levels to those achieved by orchiectomy. Studies have now demonstrated that a significant percentage of men do not reach orchiectomy equivalent levels and that lower serum T levels correlate with improved outcomes. GTx-758 (Capesaris®) is an oral estrogen receptor α agonist that has the potential to increase sex hormone binding globulin (SHBG), lower T levels, and ameliorate estrogen deficiency side effects associated with ADT. Methods: In an ongoing Phase 2 open label study (G200712, NCT01615120), men with high risk nmCRPC and mCRPC were continued on their current form of ADT along with either 125 mg or 250 mg of GTx-758 daily, for at least 90 days. Men at increased risk for venous thromboembolic events (VTE) are being excluded. The primary endpoint of this trial is the proportion of men with a PSA decline ≥50%, while secondary endpoints include serum total and free T, SHBG, bone turnover and hot flashes. Results: As of October 2014, the 250 mg cohort of GTx-758 has completed enrollment of 28 patients, 20 of which (1 nmCRPC and 19 mCRPC) are evaluable, having been on study ≥90 days. The principal mechanism of drug action, induction of SHBG is confirmed in these patients with median SHBG levels increasing to 297% of baseline. 250 mg GTx-758 impacts T levels in an additive fashion to LHRH agents, as while on study, 4/4 subjects with total T levels >20 ng/dL have fallen below that level and 9/10 subjects with serum free T levels >0.7 pg/ml have fallen below that level. Six of the 20 (30%) subjects have exhibited a ≥50 decrease in PSA level while 17/20 (85%) had PSA declines of varying magnitude. The bone turnover biomarker C-telopeptide decreased in 81% of the subjects. 250 mg of GTx-758 has been generally well tolerated with one reported possibly drug related SAE (VTE). Conclusions: While the Phase 2 clinical trial is ongoing, these preliminary findings indicate that 250 mg of GTx-758 is generally well tolerated and has additive activity to LHRH agents to further decrease testosterone, increase SHBG, lower PSA and decrease bone turnover. Clinical trial information: NCT01615120.