Participants will first be randomized to Part A or Part B in a 1:1 randomization ratio, and then to active treatment (PTC518 5 mg in Part A or 10 mg in Part B) or matching placebo within each part in a 2:1 ratio of active treatment to placebo. A Drug Safety Monitoring Board (DSMB) Charter will undertake an unblinded review of safety data in Part A and Part B and provide a recommendation on when Part C (with a 20 mg active treatment arm) can be initiated. At that time, participants will be randomized to any study Part that is currently open for enrollment, and then to either active treatment or placebo (in a 2:1 ratio) within that Part.
The primary goal of this study is to evaluate the safety and pharmacodynamic effects of PTC518 and placebo in participants with HD.