Although only 4–5% of patients with severe Hemophilia B (HB) develop factor IX (FIX) antibodies that cause inactivation of transfused FIX concentrate (conc), about 1/3 of these are associated with life-threatening anaphylactic reactions; immune tolerance induction (ITI) with high-dose FIX conc is often unsuccessful. We present individualized novel approaches to ITI in 2 boys with severe HB and high-responding inhibitors. ELISA assays utilizing recombinant FIX (rFIX) to capture patient IgG followed by detection with subclass specific monoclonal antibodies were developed to evaluate the characteristics of the factor IX inhibitors before, during and following ITI. Patient 1, a 2 yo boy, presented with a subdural hemorrhage; his inhibitor titer was 14 BU. He was treated with recombinant VIIa (rVIIa), 200 mcg/kg followed by 100 mcg/kg q2 hours plus rFIX conc (BeneFix), 1000 U/kg prior to and post subdural hematoma evacuation; a continuous infusion, 40U/kg/hour rFIX conc was started. FIX:C was >100%, so rVIIa was discontinued and the rFIX infusion was continued to maintain FIX:C levels above 50%. Rituximab (375 mg/m2 q week x 4) was started. On the 6th day, he developed anamnesis; plasma FIX:C dropped to the 20% range in spite of increases in his rFIX conc drip to 68 u/kg/hour. Investigation of right leg edema revealed a large thrombus involving the popliteal, iliac and inferior vena cava with pulmonary embolism. In order to remove the inhibitor antibody and achieve plasma FIX levels that would allow safe anticoagulation with heparin, plasmapheresis with an immunoadsorption Protein A sepharose column (Fresenius) was undertaken. FIX:C levels were unexpectedly lower immediately following each cycle. Investigation of FIX: Ag and anti-FIX IgG, IgG1 and IgG4 by ELISA assays before and after each cycle revealed the presence of FIX: Ag and specific anti-FIX IgG in the column eluates. After the 5th cycle, increasing FIX:C levels allowed weaning of the rFIX conc; the thromboses completely resolved. The patient currently is on standard prophylactic doses of rFIX conc with expected recoveries with no evidence of inhibitor. Patient 2 was a 9 year old boy with a high responding anaphylactoid inhibitor; he had severe and frequent hemarthroses treated with rVIIa with variable success resulting in significant hemophilic arthropathy. He had previously received 2 courses of rituximab with recurrence of inhibitor 3 weeks post-therapy. Therefore, in order to suppress T-cell as well as B-cell immune responses, after desensitization with increasing infusions of rFIX conc, he was treated with cyclophosphamide (10 mg/kg IV on days 2, 3 and PO on days 4 and 5) a standard course of rituximab (375 mg/m2 on days 1, 8, 15, 22), IVIG (100 mg/kg on days 2–5) initially, and high dose rFIX conc, 100U/kg/day. He is now maintained on every-other monthly doses of rituximab and replacement doses of IVIG. As FIX levels rose during ITI, rFIX conc was weaned; eight months after initiation of ITI, he has expected recoveries of FIX: C on standard prophylactic doses of rFIX conc. Investigation of the nature of the patient’s inhibitors revealed that both patients had high titer IgG1 and IgG 4 anti-factor IX antibodies that disappeared after ITI. Unlike the persistence of non-inhibitory IgG4 factor VIII antibodies reported in some patients with hemophilia A, in these two patients, there was no detectable FIX-specific pan-IgG, IgG1 or IgG4 following ITI. We conclude that novel approaches to ITI can be successfully undertaken in severe HB patients with high titer factor IX inhibitors.

Author notes

Disclosure: No relevant conflicts of interest to declare.