A single dose of the potent gonadotropin-releasing hormone antagonist acyline suppresses gonadotropins and testosterone for 2 weeks in healthy young men

Acyline is a novel GnRH antagonist that reliably inhibits gonadotropins and testosterone ( T) levels in men for 48 h after a single dose up to 75 mug/kg. In this study we examined gonadotropin and T levels in 28 healthy young men administered acyline as single doses of 150 or 300 mug/kg or serial injections of 75 mug/ kg. A single 300 mug/kg dose of acyline suppressed gonadotropins and T to castrate levels for 15 d ( baseline, 21.1 +/- 3.1; nadir, 1.95 +/- 0.4 nmol/liter; mean +/- SEM; P < 0.05). Serum acyline levels peaked 90 min after the injection of 300 mu g/kg acyline to a maximum concentration of 112.4 +/- 18 ng/ ml ( n = 7; t(1/2) = 4.9 d). Injections of 75 mu g/kg acyline every 2 d for five doses suppressed gonadotropins for more than 20 d (nadir T, 1.06 +/- 0.17 nmol/liter; P < 0.05 compared with baseline). Adverse events were mild and included erythema and pruritus at the injection site. Acyline, therefore, is one of the most potent peptide GnRH antagonists studied to date with minimal adverse events. A twice monthly injection of acyline could be used as a potent suppressor of the GnRH axis to advance the development of a hormonal male contraceptive or for treatment of hormonally dependent disease.