A Study to Evaluate the Efficacy and Safety of Factor IX Gene Therapy With PF-06838435 in Adult Males With Moderately Severe to Severe Hemophilia B

This study will evaluate the efficacy and safety of PF-06838435 (a gene therapy drug) in adult male participants with moderately severe to severe hemophilia B (participants that have a Factor IX circulating activity of 2% or less). The gene therapy is designed to introduce genetic material into cells to compensate for missing or non-functioning Factor IX. Eligible study participants will have completed a minimum 6 months of routine Factor IX prophylaxis therapy during the lead in study (C0371004). Participants will be dosed once (intravenously) and will be evaluated over the course of 6 years. The main objective of the study will evaluate the annualized bleeding rate \[ABR\] for participants treated with gene therapy versus standard of care (SOC) therapy (FIX prophylaxis replacement regimen).

Inclusion Criteria * Males who completed 6 months of Factor IX prophylaxis therapy during the lead-in study (C0371004) prior to providing consent at the screening visit for this study. * Documented moderately severe to severe hemophilia B (Factor IX activity \< =2%) * Previous experience with FIX therapy (=\>50 documented exposure days to a FIX protein product) * Suspension of prophylaxis therapy for hemophilia B after administration of the study drug * Laboratory values (hemoglobin, platelets and creatinine) within study specified limits * Agree to contraception until components of the drug are eliminated from their body * Capable of giving signed informed consent Exclusion Criteria * Anti-AAVRh74var neutralizing antibodies (nAb) titer above the established threshold (ie, positive for nAb). * History of inhibitor to Factor IX or inhibitor detected during screening. Clinical signs or symptoms of decreased response to Factor IX * Hypersensitivity to Factor IX replacement product or IV immunoglobulin administration * History of chronic infection or other chronic disease * Any conditions associated with increased thromboembolic risk * Concurrent clinically significant major disease or condition unsuitable for participation and/or may interfere with the interpretation of study results * Laboratory values at screening visit that are abnormal or outside acceptable study limits * Current unstable liver or biliary disease * Currently on antiviral therapy for hepatitis B or C * Planned surgical procedure requiring Factor IX surgical prophylactic factor treatment 15 months from screening visit * Use of restricted therapies (e.g., blood products, acetylsalicylic acid \[aspirin\] or ibuprofen, other investigational therapy, and by-passing agents) * Previously dosed in a gene therapy research trial at any time or in an interventional clinical study within 12 weeks of screening visit * Active hepatitis B or C; hepatitis B surface antigen, hepatitis B virus deoxyribonucleic acid positivity, or hepatitis C virus ribonucleic acid positivity * Significant liver disease * Serological evidence of HIV1 or HIV2 infection with either CD4+ cell count \<=200 mm3 and/or a viral load \>20 copies/mL * Study and sponsor staff involved in the conduct of the study and their families * Unable to comply with study procedures * Sensitivity to heparin or heparin induced thrombocytopenia * Sensitivity to any of the study interventions, or components thereof, or drug or other allergy