• Occurrence of antibody formation [ Time Frame: From date of first dose of CB2679d until date of first occurrence of clinical event, assessed up to treatment Day 28. ]
  Rate of occurrence of antibody formation resulting in a decreased endogenous level of coagulation Factor IX (FIX)
• Occurrence of clinical thrombotic event [ Time Frame: From date of first dose of CB2679d until date of first occurrence of clinical event, assessed up to treatment Day 28 ]
  Rate of occurrence of clinical thrombotic event not attributable to another cause
• Occurrence of an antibody response [ Time Frame: From date of first dose of CB2679d until date of first occurrence of clinical event, assessed up to treatment Day 28 ]
  Rate of occurrence of an antibody response to CB2679d and cross-reactive with wild-type recombinant coagulation FIX
• Thrombogenicity assessment [ Time Frame: From date of first dose of CB2679d until date of first occurrence of clinical event, assessed up to treatment Day 28 ]
  Patients with clinically significant level of thrombogenicity markers (D-dimer, Prothrombin fragment 1+2 (F1+2), and thrombin-antithrombin complex (TAT))
• FIX activity levels [ Time Frame: From date of IV pre-dose to 35 minutes post dose. From date SC pre-dose Day 1 of dose up to treatment Day 28, and was-out period ]
  Patients with a change in FIX activity levels from pre-dose. The frequencies of these events will be summarized as proportions and counts.

Single-center, open-label Phase 2b study will evaluate the PK, PD, efficacy and safety parameters of SC prophylaxis treatment regimens with CB2679d in adult subjects with hemophilia B. The study will enroll and dose subcutaneously, a total of 6 adult male subjects with severe congenital hemophilia B.

During the Treatment Period, the subject will receive an IV dose of 50 IU/kg followed 35 ± 5 minutes later by a SC dose of 100 IU/kg. Daily SC doses of 100 IU/kg will be administered until Day 28 (28 total SC doses). On Day 1, PK, PD, and safety assessments will be done at pre-IV dose and repeated 35 (± 5) minutes later prior to the SC dose. Subsequent PK, PD and safety assessments will be performed pre-dose on days 2, 3, 7, 14, 21 and 28.

During the Washout Period, PK, PD, and safety assessments will be done on Days 29, 30, 31, 32 and 33. Daily FIX activity levels will be measured, unless FIX activity level is known to be < 5%, as measured by local laboratory.

An End of Study visit will occur 30 days (± 2 days) after the last dose of study drug.

• Biological: Coagulation Factor IX variant
  Single intravenous injection of CB2679d/Dalcinonacog alfa
• Biological: Coagulation Factor IX variant
  Daily subcutaneous injections of CB2679d/Dalcinonacog alfa for 28 days

• Experimental: Intravenous Loading Dose
  Coagulation Factor IX variant, 50 IU/kg by intravenous route
  Intervention: Biological: Coagulation Factor IX variant
• Experimental: Subcutaneous Dosing
  Coagulation Factor IX variant, 100 IU/kg by subcutaneous route
  Intervention: Biological: Coagulation Factor IX variant

Inclusion Criteria:

• Confirmed diagnosis of severe (<2%) congenital hemophilia B.
• Male, age 18 or older.
• Agreement to use highly effective birth control throughout the study.
• Affirmation of informed consent with signature confirmation before any trial-related activities.
• Stated willingness to comply with all study procedures and availability for the duration of the study.

Exclusion Criteria:

• History or a family history of FIX inhibitors.
• Positive antibody to FIX detected by central laboratory at screening.
• Previous participation in and subsequent treatment in a clinical trial within the previous 30 days or 3-half-lives, whichever is longer, or absence of clinical effect.
• Have a coagulation disorder other than congenital hemophilia B.
• Factor IX gene mutation 128G>A.
• Significant contraindication to participation.