Response to Anti CD20 Monoclonal Antibody Rituximab® and Epitope Mapping of Inhibitory Antibodies in Patients with Acquired Haemophilia.

Introduction: Acquired haemophilia (AcH) is associated with the development of polyclonal autoantibodies against FVIII, which affect directly the A2 or C2 domain of the FVIII molecule. Immunomodulatory therapy regimes to normalize FVIII levels and to eliminate the inhibitor are essential options in the treatment of patients (pts) with AcH. The aims of the present study were to investigate the response to Rituximab^® and to localize the inhibitor epitopes on the FVIII domains.

Patients and Methods: In 5 pts with AcH (2 females,3 males; age: 64–81 yrs) the inhibitor titers ranged from 9 to 156 BU and the FVIII activities from <1 % to 6 %. Rituximab^® was administered once weekly for 4 weeks (dosage: 375 mg/m²) in combination with a prednisone therapy (1–1.5 mg/kg).

Random peptide phage display libraries were used to identify peptide sequences specific for inhibitors. The short peptide sequences allow the mapping of conformational epitopes on the surface of the FVIII molecule.

Results: In all pts (median follow up of 24 months, range: 2–49 months) we documented complete remission (CR). The median time to achieve CR was 13 weeks (range: 2–32 weeks). The time to CR was much longer (23/32 weeks) in two pts, who showed high inhibitor titers (50/188 BU) compared to pts (2/4/5 weeks) with low inhibitor titers (9/22/29 BU). One pt with a high inhibitor titer (50 BU) relapsed 19 months after CR, an other pt died in septic shock 8 weeks after the initiation of therapy with Rituximab^®. No other relevant side effects were observed. In the pt with a relapse we identified target epitopes in the A1 and A2 domain of FVIII, and in the other pts (n=3) the peptide sequences mapped to epitopes on the C2 domain of FVIII.

Conclusion: The therapy with Rituximab^®shows high response rates and seems to be effective in the elimination of high and low inhibitors against FVIII in pts with AcH. Limited by the small number of pts, further studies are needed to confirm whether the different target epitopes found in patients with (A1/A2) or without a relapse (C2) might have a relevance in the specific immunomodulatory therapy.