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Background:  Limited data exist on the clinical impact of low-responding inhibitors and the requirement for immune tolerance induction (ITI) treatment to establish tolerance, reduce bleeding, and improve outcome. The aim of this article is to describe the therapeutic management of children with severe hemophilia A and low-responding inhibitors and its effect on bleeding phenotype., Methods:  The REMAIN (Real-life Management of Inhibitors) study is a satellite study of the PedNet registry. It included unselected children with severe hemophilia A (factor VIII [FVIII] < 0.01 IU/mL) born between January 1, 1990 and December 31, 2009 who developed clinically relevant inhibitors and were followed-up for at least 3 years after the first positive inhibitor test., Results:  A total of 260 patients with inhibitors were identified and 68 of them (26%) had low-responding inhibitors (peak < 5 BU/mL). Five patients were lost to follow-up and 63 were included in this study. The median follow-up was 3.7 years (interquartile range: 3.0-7.5). ITI was started in 51/63 (81%) patients. The median time from ITI start to first negative inhibitor titer was similar with low-dose and high-dose ITI regimens (2.5 and 3.1 months, respectively). Ten of the 12 patients who did not receive ITI were treated with regular prophylaxis and reached a negative titer after a median of 6.5 months. Bleeding rate was low in all patients with no difference between treatment regimens., Conclusion:  In children with low-responding inhibitors negative titers were reached with regular FVIII treatment irrespective of the regimen (i.e., prophylaxis or ITI)., Competing Interests: H.M.v.d.B. has received research grants from Bayer, Shire/Takeda, Novo Nordisk, CSL Behring, Pfizer, and Sobi; M.E.M. has acted as paid speaker/consultant for Bayer Healthcare, CSL Behring, Grifols, Roche, Kedrion, Pfizer, Novo Nordisk, Octapharma, Sobi, and Shire/Takeda; C.K. reports grants and personal fees from Bayer Vital GmbH, Biotest AG, CSL Behring, Grifols, Intersero, Pfizer, Sanofi, Sobi, Takeda, Jansen, Roche/Chugai, MSD, and Novo Nordisk; R.D.O. has received personal fees and nonfinancial support from Shire/Takeda, CSL Behring, LFB, Novo Nordisk, Octapharma, Roche, and Spark; H.P. has received honoraria from Bayer, Baxalta, CSL Behring, Novo Nordisk, Roche, Sobi, and Pfizer; T.S.M. has no conflicts to disclose; B.N. has received fees and research grants from Sanofi, Sobi, CSL Behring, and Bayer; E.S. has acted as paid speaker/consultant for Bayer Healthcare, CSL Behring, Grifols, Roche, Kedrion, Pfizer, Novo Nordisk, Octapharma, Sobi, Spark, Uniqure, and Shire/Takeda., (Georg Thieme Verlag KG Stuttgart · New York.)

In children with severe haemophilia A, inhibitors to factor VIII (FVIII) usually develop during the first 50 treatment exposure days and are classified as low or high titre depending on the peak inhibitor titre being greater or less than 5 Bethesda units/mL (BU/mL). Classification of the inhibitor may change with time, as some low-titre inhibitors progress to high titre following re-exposure to FVIII concentrate. The aim of this study was to investigate potential risk factors for such a progression in children with severe haemophilia A and newly diagnosed inhibitors. This study was a follow-up study of the PedNet Registry and included 260 children with severe haemophilia A and inhibitors born between 1990 and 2009 and recruited consecutively from 31 haemophilia centres. Clinical and laboratory data were collected from the date of each child's first positive inhibitor test for at least 3 years. At the time of first positive inhibitor test, 49% (n = 127) had low-titre inhibitors, with 50% of them progressing to high titre and only 25% maintaining low titres. The FVIII gene (F8) mutation type was known in 247 patients (95%), and included 202 (82%) null mutations. The progression to high-titre inhibitors was associated with null F8 mutations (odds ratio [OR]: 2.6; 95% confidence interval [CI]: 1.0–6.5), family history of inhibitors (OR: 7.2; 95% CI: 1.8–28.4) and the use of high-dose immune tolerance induction, defined as ≥100 IU FVIII concentrate/kg/d (OR: 3.9; 95% CI: 1.5–10.0). These results suggest that high-dose immune tolerance induction should be avoided as the initial strategy in patients who develop low-titre FVIII inhibitors., Competing Interests: Conflict of Interest: All authors declare no competing conflicts of interest with respect to the content of the manuscript.

Many studies have reported an increased incidence of inhibitors in previously untreated patients (PUPs) with severe haemophilia A after the introduction of recombinant products. It was the objective of this study to investigate whether the inhibitor incidence has increased between 1990 and 2009 in an unselected cohort of PUPs with severe haemophilia A (FVIII< 1 %). Patients were consecutively recruited from 31 haemophilia treatment centres in 16 countries and followed until 50 exposure days or until inhibitor development. Inhibitor development was studied in five-year birth cohorts comparing cumulative incidences. Furthermore the risk for inhibitor development per five-year birth cohort was studied using multivariable Cox regression, adjusting for potential genetic and treatment-related confounders. A total of 926 PUPs were included with a total cumulative inhibitor incidence of 27.5 %. The inhibitor incidence increased from 19.5 % in 1990-1994 (lowest) to 30.9 % in 2000-2004 (highest; p-value 0.011). Low titre inhibitor incidence increased from 3.1 % in 1990-1994 to 10.5 % in 2005-2009 (p-value 0.009). High titre inhibitor incidences remained stable over time. After 2000, risk of all inhibitor development was increased with adjusted hazard ratios 1.96 (95 % CI 1.06-2.83) in 2000-2004 and 2.34 (1.42-4.92) in 2005-2009. Screening for inhibitors was intensified over this 20-year study period from a median of 1.9 to 2.9 tests/year before 2000 to 2.7 to 4.3 tests/year after 2000. In conclusion, the cumulative inhibitor incidence has significantly increased between 1990 and 2009. The high titre inhibitor incidence has remained stable.

Assessment of quality in terms of safety and efficacy of clotting factor concentrates (CFCs) is very important for all new therapeutic products. In rare diseases this is often complicated due to small number of trial participants. In hemophilia, an extra complication is the large impact previous treatments have on both the risk on inhibitors and the overall response to bleedings. For new CFCs, safety needs to be evaluated against inhibitor risk whereas efficacy is primarily judged against the most common clinical manifestations of the disease, namely, bleeding into joints and muscles. In this article the challenges are described for hemophilia that recruits patients globally. Recommendations of ISTH are discussed; these propose to substitute the single-arm prelicensure study with a two-stage approach, which considers epidemic and endemic incidence rate, and might increase the feasibility of studying multiple new products in populations with rare disease without compromising the assessment of product safety and efficacy. We also suggest that the annual bleeding rate (ABR) is an unreliable predictor of efficacy. The response to treatment highly depends on the current disease status of every patient participating in a clinical trial. The phenotype of patients with hemophilia is highly influenced by previous treatment history. Patients with severe hemophilia of the same age can demonstrate a different response to treatment., (Thieme Medical Publishers 333 Seventh Avenue, New York, NY 10001, USA.)

There is considerable variability in bleeding patterns of severe haemophilia (<1% factor VIII). Knowledge of the contribution of thrombophilic factors in these patterns may improve individually tailored treatment strategies. We reviewed the literature regarding the relation between prothrombotic factors and clinical phenotype of severe haemophilia. Medline and EMBASE were searched for relevant articles. 9369 articles published between 1963 and September 2003 were screened and seven relevant papers were retrieved. Each of these reported on a different combination of thrombophilic factors. Presence of the factor V Leiden mutation appears to decrease the severity of severe haemophilia most consistently. Findings on other thrombophilic factors were inconclusive. There is a clear need for additional research on potential determinants of phenotypes of severe haemophilia before such knowledge can be translated into individual care for severe haemophilia patients with confidence.

Findings from a recent study suggest that earlier start of treatment with factor VIII in patients with severe hemophilia is associated with a higher risk to develop inhibitors. We set out to assess the association between age of first administration of clotting factor VIII and the risk to develop inhibitors in infants with severe hemophilia A. This work was a cohort study, carried out in the national hemophilia treatment center. The study included eighty-one consecutive patients with severe hemophilia A who received their first dose of factor VIII between 1975 and 1998. Patients were followed until their last visit in 2001 or 2002. The average follow-up was 16 years (range 3-26). Persistent inhibitory antibodies developed in 12 of 81 patients (15%). Cumulative incidence at 100 exposure days was 34% (95% confidence interval 7-61%) in patients starting therapy before the age of 6 months, 20% (4-36%) in patients starting therapy between 6 months and 1 year, 13% (0-27%) in those starting therapy between 1 and 1.5 years, and 0% in those who started therapy beyond 1.5 years of age (p for trend 0.03). Our findings confirm that age of first factor VIII administration in children with severe hemophilia A is inversely associated with the risk to develop antibodies against factor VIII. The role of confounding factors such as the type of factor VIII mutation and environmental factors needs to be evaluated.

Since the late 1950s, prophylactic therapy has been used in patients with severe hemophilia in order to convert their phenotype into the phenotype of moderate hemophilia. In the early years, prophylaxis was mostly started after the development of arthropathy. This secondary prophylaxis decreased the number of bleeds but did not halt the development of arthropathy. With the start of primary prophylaxis, however, it was shown that both joint bleeds and arthropathy could be prevented. Although its benefits have been proved, prophylaxis has been used as standard treatment for children in only few countries. There are now favorable results with prophylaxis on bleeding and arthropathy with more than 25 years of follow-up. In general, the strategies used for prophylaxis can be divided into those that are targeted at preinfusion clotting factor levels of > 1% and those that are guided by clinical bleeding patterns of individual patients. In both types of strategies, treatment should be started early, before the start of joint damage. However, clotting factor consumption in the first group of strategies is expected to be higher, whereas bleeding frequencies are expected to be slightly lower. The differences in long-term outcome between both strategies may only become apparent after more than two decades. In addition, some patients may be able to stop taking prophylaxis in adulthood while maintaining a low bleeding frequency. In conclusion, although the optimal prophylactic regimen is still under debate, it is well established that prophylaxis reduces or even prevents joint bleeds and hemophilic arthropathy. Therefore, prophylaxis should be offered to all children with severe hemophilia, at least until adulthood.