We present 25-year experience with inhibitors in previously untreated patients (PUPs) with severe hemophilia A in Slovakia, where safe factor VIII (FVIII) concentrates have been used since 1990. A prospective study focused on inhibitor incidence in PUPs was established in 1997. Out of a total 61 PUPs born between January 1997 and October 2015, 59 were eligible for evaluation; 50 and 9 were treated with > 20 exposure days (ED) of plasma-derived FVIII (pdFVIII) and recombinant FVIII (rFVIII) products, respectively. In the entire group 13/59 (22%) PUPs developed inhibitors; i.e. 7/50 (14%) and 6/9 (67%) treated with pdFVIII and rFVIII, respectively. Univariate analysis of inhibitor risk factors in patient groups with and without inhibitors showed the rFVIII and serious/ recurrent infections within the first 50 EDs to be associated with inhibitor development (OR of 12.3 [95% CI 2.48-60.83; p ¼ 0.002] and 5.0; [95% CI 1.16-21.9; p ¼ 0.03), respectively]). Also, in multivariate Cox regression analysis, peak treatment ! 5 EDs reached statistical significance. The hazard ratio (HR) was 7.15 (95% CI 1.65-31.36) p ¼ 0.0086 for rFVIII and 4.38 (95% CI 1.02-18.67) p ¼ 0.046 for intensive treatment. Between 1993 and 2015, 21 immune tolerance inductions (ITIs) in 19 inhibitor patients were performed in the two largest hemophilia centers in Slovakia. In all but one ITI courses pdFVIII containing von Willebrand factor (FVIII/VWF) was used with preferred use of high-dose ITI (HD ITI) in high responders (HRs). Complete or partial success was achieved in 17/19 (89.5%) patients. Evaluating only the patients who already completed ITI, the success rate was even higher (15/16; 94%), including 7/7 low responders and

Development of factor IX (FIX) inhibitor is a rare but challenging complication in hemophilia B. In addition to inefficacy of specific replacement therapy, FIX inhibitors increase morbidity due to serious allergic reactions/anaphylaxis upon treatment with FIX. Limited experience with immune tolerance induction (ITI) shows a high risk of nephrotic syndrome development and poor ITI outcomes. Recently, immunomodulation therapy has been used in ITI regimens in hemophilia B; however, relevant guidelines for ITI in hemophilia B are still lacking. We describe a 7-year-old hemophilia B patient with "null" mutation Arg29 stop who underwent surgery and massive transfusion therapy in the neonatal period and developed an FIX inhibitor after consecutive 20 exposures to FIX concentrate. At the age of 6 years, a high-dose ITI was commenced combined with immunomodulation therapy including rituximab, dexamethasone, and intravenous immunoglobulin. Allergic reactions that occurred in the third week of ITI were resolved by premedication with antihistamines and continued immunomodulation protocol without any need for ITI interruption. Inhibitor was negative from week 10; however, doses of FIX continued unchanged until pharmacokinetic criteria for success were met at month 9 of ITI. One year after the start of ITI, the patient started regular prophylaxis with FIX 41 IU/kg three times a week. No further allergic reactions or any signs of nephrotic syndrome have occurred.

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