Successful treatment of immune tolerance induction with rituximab in a patient with severe hemophilia B and inhibitor.

Inhibitor development is one of the major problems in hemophilia patients. Whereas the inhibitor incidence in hemophilia A is estimated to be as high as 25-30%, it appears to be less frequent in hemophilia B, occurring in about 1-3% of hemophilia B patients. There are only a few case reports about immune tolerance induction (ITI) for hemophilia B patients. The present report describes ITI with rituximab in a patient with severe hemophilia B and inhibitor. The patient was diagnosed with severe hemophilia B at 9 months. He received prophylactic replacement therapy with plasma-derived factor IX (pd FIX). After 19 exposure days, inhibitor of factor IX was detected in his plasma, and replacement therapy was stopped. However, as he suffered from intracranial hemorrhage at the age of 1 year, he underwent first ITI at the age of 1 year. Unfortunately, this failed to reduce the level of the inhibitor, and this strategy was stopped after 2 years. Second ITI with pd FIX also failed. At the age of 14 years, ITI with rituximab was performed after obtaining informed consent. The patient received rituximab 375 mg/m once a week for four doses and received 40 u/kg of pd FIX every day. At 4 weeks after the start of ITI with rituximab, the level of the inhibitor of factor IX was diminished and was undetectable for 1 year after therapy. In this patient, ITI with rituximab was well tolerated and effective. This method should be considered for patients with hemophilia B and inhibitor.