Abarelix acetate is a synthetic third generation gonadotropin-releasing hormone receptor (GnRHR) antagonist. It increases histamine release from rat peritoneal mast cells in vitro and from a human skin model ex vivo. In vivo, abarelix decreases plasma luteinizing hormone (LH) levels six hours post-treatment in castrated rats, with levels returning to baseline within 24 hours.3 Abarelix (2 mg kg) also transiently decreases plasma testosterone levels in intact rats, with levels returning to baseline within seven days post-treatment. Formulations containing abarelix have previously been used in the treatment of advanced prostate cancer.