Selective Estrogen Receptor Alpha Agonist GTx-758 Decreases Testosterone with Reduced Side Effects of Androgen Deprivation Therapy in Men with Advanced Prostate Cancer

BackgroundA need remains for new therapeutic approaches for men with advanced prostate cancer, particularly earlier in the disease course.ObjectiveTo assess the ability of an oral selective estrogen receptor α agonist (GTx-758) to lower testosterone concentrations compared with leuprolide while minimizing estrogen deficiency–related side effects of androgen-deprivation therapy.Design, setting, and participantsHormone-naive advanced prostate cancer patients were randomized to oral GTx-758 1000 mg/d, 2000 mg/d, or leuprolide depot.InterventionGTx-758 and leuprolide.Outcome measurements and statistical analysisThe primary end point was the proportion of patients achieving total testosterone ≤50 ng/dl by day 60. Secondary end points included serum free testosterone, prostate-specific antigen (PSA), sex hormone-binding globulin, hot flashes, bone turnover markers, and insulin-like growth factor (IGF)-1 levels.Results and limitationsOf 159 randomized patients, leuprolide reduced total testosterone to ≤50 ng/dl in a greater proportion of patients than GTx-758 by day 60 (43.4%, 63.6%, and 88.2% of subjects receiving GTx-758 1000 mg [p < 0.001], GTx-758 2000 mg [p = 0.004], and leuprolide, respectively). GTx-758 reduced free testosterone and PSA earlier and to a greater degree than leuprolide. GTx-758 led to fewer hot flashes, decreases in bone turnover markers, and alterations in IGF-1 compared with leuprolide. A higher incidence of venous thromboembolic events (VTEs) was seen with GTx-758 (4.1%) compared with leuprolide (0.0%).ConclusionsAlthough leuprolide reduced total testosterone to ≤50 ng/dl in a greater proportion of patients compared with GTx-758, GTx-758 was superior in lowering free testosterone and PSA. GTx-758 reduced estrogen deficiency side effects of hot flashes, bone loss, and insulin resistance but with a higher incidence of VTEs.Patient summaryThis paper reports findings that leuprolide lowered total testosterone more than GTx-758 but that GTx-758 lowered free testosterone and prostate-specific antigen more than leuprolide. GTx-758 also reduced estrogen deficiency side effects, albeit at a higher rate of vascular events.Trial registrationClinicaltrials.gov identifier NCT01615120.