Immune Tolerance Induction for FIX Inhibitors Using Combined B and T Cell Immune Modulation Therapy in Severe Hemophilia B

Introduction

Development of an inhibitor to Factor IX is a potentially life-threatening complication that occurs in 2-4% of severe hemophilia B patients. Attempt to eradicate these neutralizing alloantibodies with immune tolerance induction (ITI) regimens using prolonged repeated exposure to FIX fails to induce tolerance in ~70% of cases, and is limited by severe complications including FIX hypersensitivity and nephrotic syndrome. Cases of FIX ITI with immunosuppressive drugs have been reported infrequently. In light of increased understanding of the role of active T lymphocyte regulation of clotting factor-directed B lymphocyte-mediated humoral immunity, an approach combining immune modulating agents and frequent FIX infusions was reported by Beutel, et al (Haemostaseologie, 2009; 2014). We report a multi-institutional retrospective experience with combination immune modulation therapy (CIT), including B cell suppression with anti-CD20 monoclonal antibody Rituximab and T cell modulation using mycophenylate mofetil (MMF) with FIX ITI in 11 males with severe congenital hemophilia B.

Methods

The originally published regimen of Beutel, et al involves a 50 day course including FIX 100 IU/kg twice daily, rituximab 375 mg/m2 x 4 doses, mycophenolate (MMF) 300 mg/m2/dose daily, dexamethasone 6 mg/m2/dose pulses and IVIG 1 g/kg x 6 doses. Following individual Institutional Review Board approval, data on 11 patients was contributed by the INPH investigators and via outreach internationally and collected retrospectively using a uniform data collection form. Cases were included in the CIT series if the combination of FIX, rituximab and MMF was used. Approaches varied in the concomitant use of IVIG and pulse corticosteroids (dexamethasone or prednisone).

Results

CIT was the first immune tolerance therapy for 9/11 patients. Patients were 14-222 months of age at time of CIT, with a median time from inhibitor diagnosis to CIT 20 (range 0-207) months. Median historical peak inhibitor titer prior to initiation of CIT was 3.2 (range 1-42) BU/ml. Inhibitor titers at the initiation of CIT ranged from undetectable to 7 BU/ml. Eighty-onepercent of patients had a history of FIX hypersensitivity reaction. Prior to initiation of therapy, 27% of the patients underwent FIX desensitization during the initial course.

The patients received FIX BID (9/11 patients) or QD (2/11 patients), 4 doses of rituximab 375 mg/m2, MMF for a minimum of 49 days (varying duration from 49-1247 days). IVIG was infused in 10/11 patients, with most receiving 3-7 doses during each course of CIT. 6/11 patients received between 2-10 courses of pulse corticosteroids during a CIT course.

Each course of CIT achieved disappearance of the titer of FIX inhibitor at a median time of 1 month (range 1-41 months) Hypersensitivity reactions did not limit the courses of CIT, but did recur with inhibitor recurrence in 1 patient. Recurrence occurred in 6/11 patients at a median time of 11 months from time of CIT, in some cases soon after documented B cell recovery with a median inhibitor titer of 1.8 (range 0.7-7) BU/ml. In cases of recurrence, a negative inhibitor titer was achieved again in 1 patient by increasing FIX dose and in 4 patients who received additional courses of CIT. Repeat recurrence was seen in these patients following repeat CIT, although at low titer with a median of 1 BU/ml (range 1-4), allowing ongoing management with FIX in 3 of the 4. Overall, patients have had a median follow-up of 61 months following CIT, 9/11 patients are currently managed with factor IX for prophylaxis and bleeding, while 2/11 use bypassing agents.

The most common complication was hospitalization for central venous catheter-related bacterial infection that occurred from < 1 to > 12 months after the use of rituximab in 3/11 patients. Nephrotic syndrome occurred in 3/11 patients. In each case, nephrotic syndrome responded to corticosteroids or re-initiation of CIT and FIX could be resumed.

Conclusions

FIX ITI with CIT targeting both B and T cells in hemophilia B is an effective means of achieving a negative inhibitor titer with tolerable safety in this series, allowing return to use of FIX for hemostasis and prophylaxis in most patients. Although recurrence is common and longer follow up is needed, when compared to the experience with using FIX alone, outcomes appear to be improved using CIT with FIX ITI as an initial approach.