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

Prior treatment

• Receipt of an experimental agent within 90 days or 5 half-lives prior to Screening or anytime over the duration of this study, ribonucleic acid (RNA)- or deoxyribonucleic acid (DNA)-targeted HD-specific investigational agents such as antisense oligonucleotides, cell transplantation, or any other experimental brain surgery
• Participation in an investigational study or investigational paradigm (such as exercise/physical activity, cognitive therapy, brain stimulation, etc) within 90 days prior to Screening or anytime over the duration of this study

Other illness

• Any history of gene therapy exposure for the treatment of HD
• Any medical history of brain or spinal disease that would interfere with the lumbar puncture process safety assessments
• Any medical history or condition that would interfere with the ability to complete the protocol-specified assessments (for example, implanted shunt, conditions precluding magnetic resonance imaging \[MRI\] scans)

Other

• Pregnancy, planning on becoming pregnant during the course of the study or within 6 months of end of treatment, or currently breastfeeding
  • Note: Other inclusion and exclusion criteria may apply.

• Genetically confirmed HD diagnosis with a cytosine-adenine-guanine (CAG) repeat length from 40 to 50, inclusive
  • Eligibility for HD-ISS Stage 2 Group (Parts A, B, and C):
• A Unified Huntington's Disease Rating Scale (UHDRS)-Independence Scale (IS) score of 100
• A UHDRS Total Functional Capacity (TFC) score of 13
• A score between 0.18 and 4.93 inclusive on the normed version of the HD prognostic index (PINHD)
  • Eligibility for HD-ISS Mild Stage 3 Group (Parts D, E, and F):
• A UHDRS Total Functional Capacity (TFC) score of 11 or 12, or a UHDRS TFC score of 13 with an UHDRS IS score of \<100