CVN424 is a GPR6 inverse agonist that provides selective pharmacological control of the indirect striatopallidal pathway. We assessed the safety and efficacy of CVN424 as an adjunctive treatment to levodopa for reducing OFF-time in individuals with Parkinson's disease (PD) experiencing motor-fluctuations. This was a randomised, double-blind, placebo-controlled study conducted at 21 sites across the United States to evaluate two doses of CVN424 (NCT04191577). Patients with PD (Hoehn and Yahr stages 2-4) who were on a stable dose of levodopa and experiencing ≥2h of daily OFF-time were randomised (1:1:1) to receive either once-daily CVN424 (50mg or 150mg) or placebo for a 28-day treatment period. The primary endpoints were safety and tolerability. The key secondary endpoint was the change from baseline to Day 27 in OFF-time. The study was conducted from December 23, 2019, to October 14, 2021. Out of 198 participants screened, 141 eligible participants were randomised to one of the three treatment groups (n=47 per group), and 127 participants completed the 28-day treatment period. The most common treatment emergent adverse events (TEAEs) were headache (2% with CVN424 50mg, 9% with CVN424 150mg, and 2% with placebo) and nausea (4% with CVN424 50mg, 6% with CVN424 150mg and 2% with placebo). No serious treatment-related adverse events were reported. On Day 27, the mean±standard deviation (SD) change from baseline in daily OFF-time was-1.3±3.0h in the CVN424 50mg group,-1.6±2.5h in the CVN424 150mg group, and-0.5±2.9h in the placebo group. The placebo-adjusted LS mean±standard error (SE) treatment difference was significant for the CVN424 150mg dose (1.3±0.56h, [95 CI%-2.41 to-0.19], nominal p=0.02). Treatment with CVN424 was safe and well-tolerated. Despite the short study duration and small sample size, the 150mg CVN424 dose provided a clinically meaningful reduction in daily OFF-time. This study supports the development of CVN424 for the treatment of PD. Cerevance.