GTx-758 (tentative brand name Capesaris) is a synthetic nonsteroidal estrogen which was under development by GTx, Inc. for the treatment of advanced prostate cancer.

[1] It has been said that suppression of free testosterone levels by ERα activation-induced increase in hepatic sex hormone-binding globulin (SHBG) production is the primary mechanism of action of GTx-758 in the treatment of prostate cancer.

[1] In animal studies, GTx-758 reversibly suppresses testosterone concentrations to castrate levels, reduces prostate size and levels of prostate-specific antigen (PSA), but does not induce typical side effects associated with hyperestrogenism (or hypoestrogenism) including hot flashes, bone loss, thrombophilia, hypercoagulation, or increased body fat.

[3] However, a higher incidence of venous thromboembolism (VTE) was observed with GTx-78 (4.1%) relative to leuprorelin (0.0%), and this led to early termination of the clinical trial.

[5] Similarly to the prior phase II clinical trial, GTx-758 was regarded as well tolerated, and improved bone parameters, but there were still two possibly drug-related severe adverse effects (VTE and myocardial infarction).