Medical Castration Using the Investigational Oral GnRH Antagonist TAK-385 (Relugolix): Phase 1 Study in Healthy Males.

ContextTAK-385 is a highly selective, oral, nonpeptide GnRH antagonist being investigated as a possible prostate cancer treatment.ObjectiveThe objectives were to evaluate safety, tolerability, pharmacokinetics, and pharmacodynamics of TAK-385 on LH and testosterone.Design, setting, and participantsThis was a three-part, randomized, double-blind, placebo-controlled, phase 1 dose-escalation study in 176 healthy male UK volunteers.InterventionsPart 1, single doses of TAK-385 (0 [placebo], 80, 120, 180, or 360 mg). Part 2, 14-day TAK-385 (0, 20, 40, 80, or 180 mg) daily. Part 3, 28-day TAK-385 (40 [with loading dose], 60, 80, or 160 mg) or placebo daily. Parts 2 and 3 included men aged 40-75 years.Main outcome measuresMain outcome measures included plasma concentrations of TAK-385, LH, and testosterone.ResultsOral TAK-385 was readily absorbed, and steady state was reached in ≤ 14 days. Food reduced TAK-385 systemic exposure by 47-52%. Mean serum testosterone levels declined ≤ 6 hours after TAK-385 administration. Loading doses up to 360 mg on day 1 or 360 mg on day 1 followed by 240 mg on day 2 reduced the time to achieve castrate testosterone levels from ≥ 7 to 4 ng/mL. After discontinuation of TAK-385 on day 28, testosterone levels normalized in most subjects in ≤ 28 days. Common adverse events included bradycardia, headache, and hot flush (all grade ≤ 2).ConclusionsOral TAK-385 (40-180 mg/d) was well tolerated and effectively lowered testosterone in healthy men. Planned phase 2 doses in men with hormone-sensitive prostate cancer are 80 and 120 mg/d.