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

Open to males ages 18-65

• 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

• 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