Pegtibatinase as a Treatment for Patients With Classical Homocystinuria (HCU) (Also Known as the COMPOSE Study)

Inclusion Criteria: * Age * Cohort 7 (currently enrolling): ≥5 to \<12 years of age. * Completed Cohorts 1-6: ≥12 to 65 years of age. * Diagnosis of classical homocystinuria (HCU) * Cohort 7 (currently enrolling): Diagnosis based on clinical, biochemical, and/or molecular genetic testing. * Completed Cohorts 1-6: Genetically confirmed cystathionine beta-synthase (CBS)-deficient HCU. * Plasma total homocysteine (tHcy) * Cohort 7 (currently enrolling): Plasma tHcy ≥50 μM at Screening. * Completed Cohorts 1-6: Plasma tHcy ≥50 μM at Screening and documented historical plasma tHcy ≥80 μM. * Willing and able (or parent/legal guardian willing and able) to provide informed consent/assent and comply with study procedures. * Willing to maintain a generally stable standard-of-care treatment regimen, including dietary management and HCU-related therapies, unless changes are medically necessary. * Participants of childbearing potential must have a negative pregnancy test before study treatment and agree to use protocol-specified contraception, if applicable. Exclusion Criteria: Cohort 7 only: * Diagnosis of Marfan syndrome, methylenetetrahydrofolate reductase (MTHFR) deficiency, or a disorder of cobalamin metabolism. * History of a major thrombotic event within the previous 6 months. * Body weight \<15 kg. All Cohorts: * Previous treatment with pegtibatinase or pegtarviliase) * Participation in a pegtibatinase clinical study. * Receipt of another investigational drug or investigational medical device within 30 days before Screening or planned use during study participation. * Use of injectable polyethylene glycol (PEG)-containing medications (other than pegtibatinase or PEG-containing vaccines) within 3 months before Screening or during study participation. * Known hypersensitivity to pegtibatinase or a history of severe hypersensitivity to a PEG-containing product. * Active HIV, hepatitis B, or hepatitis C infection. * History of organ transplantation or immunosuppressive therapy. * Clinically significant medical conditions that could interfere with study participation or participant safety. * Pregnant or breastfeeding, or planning to become pregnant during study participation. * Major surgery planned during the study period. * Any condition that could prevent the participant from complying with study procedures or completing the study.