Incidence of Inhibitors in a Previously Untreated Patients with Severe Hemophilia Α Cohort Treated with a Single Third-Generation Recombinant Factor VIII Concentrate

Introduction: One of the most important complications in the treatment of patients with hemophilia A is the formation of neutralizing antibodies (inhibitors) interfering in the coagulant activity of factor VIII (FVIII). The presence of inhibitor causes a direct impact on mortality and morbidity in these patients and considerably increases the cost of treatment. Among the non-genetic risk factors for inhibitor development, the influence of the type of factor concentrate used in replacement therapy (recombinant or plasma-derived) remains controversial. Thus, the evaluation of an additional population in the real world setting may contribute to elucidate this problem. Since August 2013, almost all previously untreated patients (PUPs) with hemophilia A in Brazil have been receiving exclusively the same third-generation recombinant FVIII (rFVIII) (Advate^®, Shire).

Objective: The aim of this study is to evaluate the immunogenicity of rFVIII (Advate^®). In this context, we analyzed the occurrence of inhibitor among severe and moderately severe hemophilia A PUPs during the first 50 exposure days (EDs) to Advate^®.

Methods: This is an open-label, multicenter, prospective/retrospective, uncontrolled, observational study conducted in eight reference hemophilia treatment centers from distinct geographic areas in Brazil. The inclusion criteria were (a) diagnosis of severe or moderately severe hemophilia A (FVIII:C <2 IU/dL), (b) absence of previous exposure to other FVIII concentrates, except a maximum of 5 previous exposures to any blood components (whole blood, fresh-frozen plasma, packed red cells, platelets, or cryoprecipitate), and (c) exclusive treatment with Advate^® until the 50^th ED or until inhibitor development (primary endpoint). Positive inhibitor was defined as at least two consecutive plasma samples with Bethesda-Nijmegen assay results ≥0.60 BU/mL. Patients were considered as having low-titer inhibitors when peak titers were <5 BU/mL, and high-titer inhibitors if inhibitor titer was ≥5 BU/mL on at least one occasion. Any clinical information considered relevant for the risk of inhibitor development was analyzed when available, and included family history of inhibitor, F8 genotype, ethnicity (defined according physical traits and ancestry ethnic background in the last three generations), age at first rFVIII exposure, treatment regimens (prophylaxis or episodic), doses, occurrence of a severe bleeding episode, surgery, and use of FVIII concentrate simultaneously to infection or vaccination.

Results: So far, 122 patients were enrolled, and 100 patients reached the 50^th ED to rFVIII or developed inhibitor. Twenty-two are still on Advate^® and have not achieved 50EDs (7 patients: 20 to 50EDs; 15 patients: <20ED). Overall, the median age at first exposure to Advate^® was 11.9 months (interquartile range (IQR): 7.5-16.7), and most patients were African-descendants (48%), followed by Caucasians (45%). Positive inhibitor was detected in 35 of the 100 patients (35%), and 71% occurred during the first 20EDs. Most inhibitors were detected during prophylactic treatment (29 of 35; 82.9%). Twenty-five (25%) patients had high-titer inhibitor. Although not statistically significant, 19/48 (39.6%) of the African-descendants patients developed inhibitor, in contrast to 15/45 (33%) of the Caucasians. Interestingly, inhibitor was detected in only 1/7 (14.6%) of the patients with the indigenous background (native population). The influence of other risk factors, as severe bleeding episodes, presence of infection, surgery and history of blood transfusion, were not statistically significant.

Conclusions: Overall inhibitor development in this cohort is consistent with results reported in other PUP studies with recombinant products. The majority of inhibitors developed during the first 20EDs. However, no other risk factor as intensive treatment was statistically significant, due to the small number of events observed. Although observational studies have limitations to assess the immunogenicity of FVIII products, our study contributes to this knowledge, since it evaluates a single third-generation rFVIII in a distinct population, with similar access to factor concentrate and same treatment regimen.