after joining the study, a complete baseline evaluation is performed. this includes a physical exam, measurement of height and weight, vital sign check, neurologic exam, 12‑lead ecg, and collection of blood and urine samples for safety laboratory tests such as blood count, chemistry, liver function, coagulation, serum pcsk9, and urinalysis.

the baseline also records any previous hyperammonemic crises, current medication use, and dietary protein allowance.

participants who are assigned to receive the study drug undergo a single intravenous infusion of either ecur-506d or ecur-506a.

the infusion is delivered through a vein over a period determined by the clinical team; the protocol does not specify an exact dose amount or unit.

the infusion is performed in a clinical setting where immediate monitoring of vital signs and observation for any acute reactions are carried out.

following the infusion, the participant remains under observation for a defined period, typically several hours, during which vital signs, neurologic status, and any adverse symptoms are recorded.

additional blood samples may be taken to assess vector pharmacokinetics (pk) in the blood and to check for shedding of the viral vector in plasma, saliva, urine, and feces.

the participant attends regular follow‑up visits for up to fifteen years, as the study runs from may 2026 to june 2041.

at each visit, a physical exam, neurologic exam, and 12‑lead ecg are repeated. vital signs, height, weight, and body surface area are measured.

blood and urine are collected for safety tests, including liver function, serum pcsk9, ammonia, citrulline, glutamine, blood urea nitrogen, and antibody response to the viral vector.

the schedule may include visits every six months or as defined by the study protocol.

if a hyperammonemic crisis occurs, the participant or caregiver records the event, including daily ammonia levels, duration of hospitalization, need for intensive care, and any required treatments such as oral scavenger medication, intravenous scavenger therapy, or hemodialysis.

the severity of each crisis (mild, moderate, severe) is documented according to the protocol definitions.

age‑appropriate developmental assessments are performed using standardized tools such as the bayley scale of infant development iv or the kaufman assessment battery for children.

these evaluations are scheduled at intervals specified by the study to monitor neurodevelopment over time.

the study concludes when the participant reaches the end of the follow‑up period or withdraws consent.

a final comprehensive assessment, including all safety examinations, laboratory tests, and documentation of any serious adverse events, is completed.

the collected data are used to evaluate the long‑term safety of the investigational product.