A Study to Evaluate the Safety and Efficacy of PTC518 in Participants With Huntington's Disease (HD)

Participants will first be randomized to Part A or Part B or Parts D or E in a 1:1 randomization ratio, depending on their Huntington's disease Integrated Staging System (HD-ISS) staging criteria and then to active treatment (PTC518 5 mg in Parts A and D and 10 mg in Parts B and E) 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 from the 5 and 10 mg dosing groups and provide a recommendation on when Parts C and F (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. Participants who complete this study and agree to participate in a long-term extension (LTE) study, will be enrolled in a separate LTE study PTC518-CNS-004-HD (NCT06254482).