Physical examination included examination of the head, ears, eyes, nose, mouth, skin, heart and lung examinations, lymph nodes, gastrointestinal, musculoskeletal, and neurological systems. The examination assessed the participants for any potential changes in general appearance, the respiratory and cardiovascular systems, as well as towards participant reported symptoms. Findings were considered to be clinically significant based on investigator's decision.

Vital signs (temperature, respiratory rate, pulse rate, height, weight, systolic and diastolic blood pressure) were obtained with participant in the seated position, after having sat calmly for at least 5 minutes. Clinical significance of vital signs was determined at the investigator's discretion.

Following parameters were analyzed for laboratory examination: hematology (neutrophils, lymphocytes, monocytes, eosinophils, basophils, red blood cell \[RBC\] count, hemoglobin, hematocrit, platelet count); liver function (albumin, total bilirubin, total protein, direct bilirubin, indirect bilirubin, aspartate aminotransferase, alanine aminotransferase, lactate dehydrogenase, alkaline phosphatase, GGT); Lipid panel (HDL, VLDL, triglycerides, total cholesterol); clinical chemistry (sodium, potassium, chloride, bicarbonate, glucose, phosphate, serum creatinine, BUN); urinalysis (specific gravity, pH, glucose, protein, blood, ketones; coagulation, immunology, etc. Investigators determined which laboratory abnormalities were reported as treatment-emergent adverse events (TEAEs).

An adverse event (AE) was any untoward medical occurrence in a clinical investigation participant administered a product; the event did not need to have a causal relationship with the treatment. A serious adverse event (SAE) was any untoward medical occurrence at any dose that resulted in death; was life threatening; required inpatient hospitalization or prolongation of existing hospitalization; resulted in persistent or significant disability/incapacity; resulted in congenital anomaly/birth defect. AEs included both SAEs and AEs. An AE was regarded as TEAE if the start date was on or after the infusion of SPK-9001 but before participant's last visit on study (or the date of withdrawal/the date of being lost to follow-up). Severe TEAEs were TEAEs that interfered significantly with participants' usual function. Treatment-related TEAEs were determined by the investigator.

Peripheral blood mononuclear cells (PBMC) results by interferon gamma enzyme-linked immunospot assay (ELISPOT) to assess cellular immune responses to AAV capsid and to FIX were presented. The ELISPOT is a type of assay that focuses on quantitatively measuring the frequency of cytokine secretion for a single cell. The positive ELISPOT results suggested a T-cell reaction to capsid protein.

Based on non-clinical studies in non-human primates (NHPs), it was not predicted that vector-derived FIX:C activity levels \>150% of normal would be achieved in this study. However, thrombin antithrombin (TAT) levels as thrombotic potential were to be measured if vector derived FIX:C activity levels \>150% of normal were achieved in any participant during the study. Blood samples for TAT at Day 0 visit (prior to FIX protein product infusion) were used to establish baseline value.

FIX inhibitors were measured using the Bethesda assay from the central and local laboratory. The Bethesda assay measures the amount of factor (FIX) inactivated when the plasma from the patient is incubated with an external source of factor for 2 hours at 37ºC. Inhibitor levels are quantified in Bethesda units (BU). An inhibitor titer of ≥ 0.6 BU/ml is to be taken as clinically significant.

Incremental recovery was determined as the peak factor level recorded within the first 3 hours after infusion and was reported as (IU/ml)/(IU/kg), using the formula:(\[Activity IU/mL peak post infusion\] - \[Activity IU/mL pre-infusion\]) / (IU/kg infused).