Freeline is developing adeno-associated virus (AAV) vector based gene therapies for a number of diseases and is actively advancing a programme in Haemophilia B (HB). This study aims to collect prospective data to characterise bleeding events and Factor IX (FIX) concentrate consumption in HB patients that can be used as baseline for participants who elect to participate in a subsequent Freeline gene therapy study. The study will also screen participants for antibodies to a novel AAV vector to assess their suitability for inclusion in a Freeline gene therapy treatment study.

This is a prospective study to collect baseline disease characteristics in patients with HB that are required to establish eligibility for treatment with a novel AAV gene therapy in a subsequent Freeline gene therapy treatment study.

Participants providing consent will attend an enrolment visit to complete eligibility evaluations, collect health and demographic information and receive instruction for completing the study diary. Participants will then complete a diary recording bleeding episodes and FIX usage until they enrol into a gene therapy treatment study. A blood sample will be drawn at a convenient timepoint during the study to assess the participant's AAV neutralising antibody (NAb) status.

No treatment intervention will occur as part of this study.

Inclusion Criteria

1. Male participants, ≥ 16 years of age.

2. Able to give full informed consent or obtain full informed consent/assent (according to local regulations) and/or obtain full informed consent from the participant's legally acceptable representative (as appropriate), and able to understand and comply with all requirements of the study, including diary completion.

3. Interested in participation in future gene therapy clinical studies.

4. Subjects with Haemophilia B with known severe or moderately severe FIX deficiency (≤2% of normal circulating FIX activity) for which the subject is either on

   1. Continuous routine FIX prophylaxis, OR
   2. On demand FIX treatment

5. If receiving prophylaxis, participant has been on stable and adequate prophylaxis for at least 2 months prior to enrolment.

Exclusion Criteria:

1. Documented evidence of liver fibrosis and/ or liver dysfunction
2. Prior treatment with a gene transfer medicinal product.
3. Known presence or history of neutralising anti-human FIX antibodies (inhibitors)
4. Previously established serological evidence of HIV-1
5. Documented active hepatitis B or C, and HBsAg or HCV RNA viral load positivity, respectively, or currently on antiviral therapy for hepatitis B or C
6. Participants at high risk of thromboembolic events (history of arterial or venous thromboembolism
7. Known coagulation disorder other than Haemophilia B
8. Known history of an allergic reaction or anaphylaxis to Factor IX products or known uncontrolled allergic conditions
9. Known history of allergy to corticosteroids or to tacrolimus or any other macrolide
10. Known medical condition that would require chronic administration of corticosteroids (excluding topical formulations)
11. History of alcohol or drug dependence.
12. Planned surgical procedure within the next 12 months requiring prophylactic FIX treatment.
13. Known active severe infection (including documented COVID-19 infection), or any other significant concurrent, uncontrolled medical condition evaluated by the investigator to interfere with adherence to the protocol procedures or with tolerance to gene therapy in a future treatment study including, but not limited to, renal, hepatic, cardiovascular, opthalmological, hematological, immunological, gastrointestinal, endocrine, pulmonary, neurological, cerebral or psychiatric disease, malignancy or any other psychological disorder.

Haemophila B patients