The primary objective of the study is to assess safety of FIXFc at doses ranging from 1 to 100 IU/kg.

This study was previously posted by Syntonix Pharmaceuticals, Inc. In January, 2007, sponsorship of the trial was transferred to Biogen Idec. In February, 2017, sponsorship of the trial was transferred to Bioverativ.

Key Inclusion Criteria:

1. Are previously treated (PTPs) with severe (<2 IU/dL endogenous FIX level) hemophilia B and at least 150 prior documented exposure days to other FIX products.
2. Have no prior history of or currently detectable inhibitor defined as > 0.6 Bethesda units by the local lab. A family history of inhibitors will not exclude the patient.
3. No prior history of an allergic reaction or anaphylaxis associated with any FIX or IVIG administration.
4. No concurrent autoimmune disease.
5. At least 7 days since their last dose of FIX (wash-out period).
6. Certain laboratory testing criteria and other protocol-defined criteria may apply.
7. HIV negative or if HIV positive with a CD4 count ≥ 200 cells/mm3. HIV patients are allowed to receive protease inhibitors per the discretion of the Investigator.

Key Exclusion Criteria:

1. Presence of a major bleeding episode on Day 1 of study.
2. Any coagulation disorder in addition to hemophilia B.
3. A patient currently on a dose and regimen of FIX that would preclude participation in the study due to possible increased risk of bleeding because of the requirement to withhold treatment during the study period.
4. A positive d-dimer at screening.
5. Documented history of liver cirrhosis.
6. Positive for HBsAg and/or positive for hepatitis C antibody, and/or HIV positive with an ALT or AST greater than 5 times upper limit of normal.
7. Certain prior illnesses and other protocol-defined criteria.

NOTE: Other protocol defined Inclusion/Exclusion criteria may apply