This study is checking a new treatment called pegtibatinase for people with a condition called classical homocystinuria (HCU). HCU is a genetic disorder where the body has trouble breaking down certain proteins, leading to health problems. Participants will be either given pegtibatinase or a placebo (a fake treatment) to see how effective and safe the new treatment is. The study will last up to 38 weeks, which includes different phases like screening, diet planning, and treatment periods. Participants will have to visit the study center regularly and some visits might be possible from home. The study is meant for people aged between 12 and 65 years who meet certain health criteria. Participants need to follow a stable diet and treatment plan throughout the study.

• The study will last for a total of 38 weeks.
• Regular visits are required, including some from home.
• Participants will be monitored for diet and treatment adherence.

Participants must be careful about their diet and medication intake and should be ready for multiple visits and evaluations throughout the study duration.