Researchers are looking for a better way to treat people living with classical homocystinuria (HCU), a rare inherited condition. It is caused by changes in the cystathionine beta-synthase (or "CBS") gene and prevents an enzyme from working correctly in the body. This enzyme helps break down homocysteine, which is produced when the body processes methionine from protein-containing foods. This helps keep homocysteine and methionine at healthy levels. When this enzyme is not working, homocysteine and methionine build up in the blood and then spread into different tissues of the body and stop these body tissues from working normally. People living with HCU can experience problems with their vision, bones, blood vessels, thinking, learning, and memory. Available treatments for HCU include a low-methionine diet (typically achieved by limiting protein-containing foods), a methionine-free protein supplement to help meet daily protein needs, and betaine (Cystadane®). These treatments are either not sufficient or are hard to take for many patients. Pegtibatinase was designed by researchers to act like the CBS enzyme, which is missing or not working properly in people living with HCU. Researchers are studying whether adding pegtibatinase to standard treatment can lower homocysteine levels. Researchers want to learn how safe and effective pegtibatinase is, how it moves through and affects the body, and how it may affect the daily lives of people with HCU. The study plans to enroll participants from the US, Australia, Belgium, France, Germany, Ireland, Italy, Poland, Portugal, Qatar, Saudi Arabia, Spain, Turkey, and the United Kingdom. Many study visits may happen at home, depending on where you live and local regulations. Some visits may include a home nurse and telemedicine appointments with the study doctor.
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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