A Study to Investigate Efficacy and Safety of Pegtibatinase Compared With Placebo in Participants ≥12 to ≤65 Years of Age With Classical Homocystinuria (HCU) Due to Cystathionine Beta Synthase Deficiency Receiving Standard of Care Treatment

Inclusion Criteria: * Must be ≥12 to ≤65 years of age, at the time of signing the informed consent * Must have a diagnosis of classical HCU based on clinical, biochemical, and/or molecular genetic testing * Plasma tHcy ≥80 µM at Screening visit, with allowance for up to 18 participants who may be enrolled with a Screening plasma tHcy ≥50 to \<80 µM * Participants who can become pregnant must have a negative pregnancy test before starting the study and must use a highly effective form of birth control (less than 1% risk of pregnancy per year) during the study and for at least 4 weeks after the last dose. * Willing to maintain a generally stable diet for the duration of the study (unless changes are required based on medical/safety reasons) * Willing to maintain generally stable intake and doses of betaine, pyridoxine, and medical food for the duration of the study (unless changes are required based on medical/safety reasons) Exclusion Criteria: * Diagnosis of Marfan syndrome, methylenetetrahydrofolate reductase (MTHFR) deficiency, or disorder of cobalamin metabolism * Concurrent disease or condition (eg, history or presence of clinically significant cardiovascular, pulmonary, hepatic, renal, hematologic, gastrointestinal, endocrine, immunologic, dermatologic, neurological, oncologic, or psychiatric disease) that would interfere with study participation or safety (excluding complications of HCU). * History of major thrombotic event (eg, cerebrovascular accident, myocardial infarction, pulmonary embolism) in the previous 6 months. * Body weight ≥160 kg. * Use or planned use of any injectable drugs containing PEG (excluding PEG-containing vaccines) * Any previous exposure to pegtibatinase and/or previous participation in a clinical study that included administration of pegtibatinase or pegtarviliase * Prior severe immune reaction to a PEG-containing product