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Introduction.In patients with hemophilia treated with factor VIII products, the development of inhibitory antibodies poses the largest safety risk. Especially during the first 50 exposure days (EDs), up to 37% of patients with severe hemophilia A have been reported to develop an inhibitor. To study neo-immunogenicity of products and new treatment strategies, patients have been distinguished into previously untreated (PUPs) and previously treated patients (PTPs); the latter defined as patients treated for more than 150 EDs. The number of 150 EDs was established in the eighties during a time when most patients received on-demand treatment and testing for inhibitors was not frequently performed. More recent studies on inhibitor incidence in PUPs with severe hemophilia A report that 50% of inhibitors develop within 14-15 EDs, however the cut-off number of EDs for a PUP to become a PTP is not well defined. The aim of this study was to define the number of EDs for PUPs to become PTPs based on long-term follow-up of patients with severe hemophilia A Methods.All patients with severe hemophilia A born after January 1, 2000, treated for at least 1 ED and followed prospectively until inhibitor development or the number of EDs at last follow-up, were included. The number of EDs at inhibitor development is the last exposure day before the first positive titer was reported. An inhibitor was defined as positive when at least two positive inhibitor titers were measured. Positivity was defined according to the cut-off level in each individual center's laboratory. Results.Of 1,038 PUPs with severe hemophilia A, 930 (89.6%) were followed until 75 EDs, 429 until 500 EDs and 212 until 1000 EDs. In total, 300 inhibitors developed, of which 298 (99.3%) within the first 75 EDs. Thereafter only two inhibitors developed, both low titer: after 249 and 264 EDs. Conclusion.Almost all inhibitors develop during the first 75 EDs. Patients with severe hemophilia A can be defined as PTP after 75 instead of 150 exposure days. A change of definition of PTP will increase the number of severe hemophilia A patients eligible for new therapies. Disclosures Santagostino: Bioverativ: Membership on an entity's Board of Directors or advisory committees; Bayer: Membership on an entity's Board of Directors or advisory committees; Shire: Membership on an entity's Board of Directors or advisory committees; Pfizer: Membership on an entity's Board of Directors or advisory committees; Kedrion: Membership on an entity's Board of Directors or advisory committees; Sobi: Membership on an entity's Board of Directors or advisory committees; Novo Nordisk: Membership on an entity's Board of Directors or advisory committees; Roche: Membership on an entity's Board of Directors or advisory committees; CSL Behring: Membership on an entity's Board of Directors or advisory committees; Grifols: Membership on an entity's Board of Directors or advisory committees; Octapharma: Membership on an entity's Board of Directors or advisory committees. Male:SOBI: Speakers Bureau; Shire: Speakers Bureau; Roche: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Octapharma: Speakers Bureau; Novo Nordisk: Speakers Bureau; Biotest: Speakers Bureau; Bayer: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; CSL Behring: Membership on an entity's Board of Directors or advisory committees, Research Funding, Speakers Bureau. Oldenburg:Novo Nordisk: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Octapharma: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Shire: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Biogen Idec: Honoraria, Membership on an entity's Board of Directors or advisory committees; Pfizer: Honoraria, Membership on an entity's Board of Directors or advisory committees; Chugai: Honoraria, Membership on an entity's Board of Directors or advisory committees; Grifols: Honoraria, Membership on an entity's Board of Directors or advisory committees; Biotest: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; CSL Behring: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Roche: Honoraria, Membership on an entity's Board of Directors or advisory committees; Bayer: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Swedish Orphan Biovitrum: Honoraria, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau. Liesner:Baxalta: Consultancy, Research Funding; Novo Nordisk: Research Funding, Speakers Bureau; Sobi: Speakers Bureau; Bayer: Consultancy, Research Funding; Roche: Research Funding; Octapharma: Consultancy, Other: Clinical study investigator for NuProtect Study (Octapharma sponsored), Research Funding, Speakers Bureau. Carcao:Shire: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Roche: Honoraria, Membership on an entity's Board of Directors or advisory committees; Bayer: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Octapharma: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Grifols: Honoraria, Membership on an entity's Board of Directors or advisory committees; Novo Nordisk: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; CSL-Behring: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; LFB: Honoraria, Membership on an entity's Board of Directors or advisory committees; Bioverativ/Sanofi: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Biotest: Honoraria, Membership on an entity's Board of Directors or advisory committees; Pfizer: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding. Nolan:CSL Behring: Research Funding; Sobi: Research Funding; Bayer: Research Funding. Álvarez-Roman:Shire: Consultancy; NovoNordisk: Consultancy; SOBI: Consultancy. Koenigs:Gilead: Research Funding; CSL Behring: Consultancy, Research Funding; Pfizer: Research Funding, Speakers Bureau; Bayer: Consultancy, Research Funding, Speakers Bureau; Intersero: Research Funding; Bioverativ: Consultancy; Roche/Chugai: Consultancy; EU (IMI, FP7): Research Funding; Sobi: Consultancy, Research Funding, Speakers Bureau; Shire: Consultancy, Research Funding; Novo Nordisk: Consultancy, Speakers Bureau; Biotest: Research Funding, Speakers Bureau; Jansen: Research Funding.

The development of anti-FVIII antibodies (i.e., inhibitors) is the major side effect of severe hemophilia A treatment. Inhibitors mainly develop in children during the first 50 exposure days and are classified in low-and high-titer (i.e., peak titer < or > 5 UB/ml). High-titer inhibitors have the major clinical impact. At diagnosis however, the real nature of the antibody is not clear in all patients, since some low-titer inhibitors may progress to high-titer. The determinants of the evolution from low- to high-titer inhibitors are still unclear and the aim of the present study was to investigate potential risk factors associated with the progression from low- to high-titer inhibitors. This study is a follow-up study of the PedNet Registry and includes 260 children with severe hemophilia A and clinically relevant inhibitors, born between 1990 and 2009 and consecutively recruited from 31 hemophilia centers in 16 countries. Clinical and laboratory data were collected from the date of first positive inhibitor test and covered a minimum of 3-years follow-up. Factors potentially associated with progression from low- to high-titer inhibitor development were analyzed using univariate and multivariate logistic regression. F8 mutation type was known in 247 patients (95%), including 202 (82%) null mutations (i.e., large deletions, nonsense mutations and inversions). Positive family history of inhibitors was present in 37 of 99 (37%) with positive family history of hemophilia. At diagnosis 49% (n=127) had low-titer inhibitors, however, upon FVIII re-exposure, 50% of low-titer inhibitors progressed to high-titer and only 25% of patients (n=69) had persistent low-titer inhibitors. Within the first 3 years of follow-up, immune tolerance induction (ITI) was equally implemented in around 80% of low-and high-titer patients but it was started later in children with high-titers (median time to ITI start 4.5 vs 0.3 months; p100 IU/kg/day) with an OR of 3.9 (95%CI 1.5-10.0), independent from the effects of F8 mutation type (adjusted OR 3.6, 95%CI 1.4-9.8) and family history of inhibitors (adjusted OR 6.7, 95%CI 1.1-42.6). No difference was found by comparing the use of daily versus non-daily ITI. In conclusion, in a cohort of 260 children with severe hemophilia A and inhibitors, 49% presented with low-titers at diagnosis and 46% of them progressed to high-titers during follow-up. Progression to high-titer inhibitors was associated with the use of high-dose ITI. These results suggest that intensive ITI should be avoided as initial strategy in low-titer inhibitor patients. Disclosures Mancuso: Pfizer: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Sobi/Biogen Idec: Consultancy, Speakers Bureau; Novo Nordisk: Consultancy, Speakers Bureau; CSL Behring: Consultancy, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Baxalta: Consultancy, Speakers Bureau; Bayer Healthcare: Consultancy, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Kedrion: Consultancy. Fischer:Wyeth/Pfizer: Research Funding; Biogen: Consultancy; NovoNordisk: Consultancy, Research Funding, Speakers Bureau; Pfizer: Consultancy, Speakers Bureau; Biotest Octapharma: Speakers Bureau; CSL Behring: Speakers Bureau; Baxter: Consultancy, Research Funding, Speakers Bureau; Freeline: Consultancy; Bayer: Consultancy, Research Funding, Speakers Bureau. Santagostino:Octapharma: Consultancy; Novo Nordisk: Consultancy; Kedrion: Consultancy; Sobi: Consultancy; Biogen Idec: Consultancy; Roche: Consultancy; Grifols: Consultancy; Pfizer: Consultancy; Baxalta: Consultancy; CSL Behring: Consultancy; Bayer: Consultancy. Escuriola:Baxalta, now part of Shire: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: Investigator Clinical Studies, Research Funding; Bayer: Consultancy, Honoraria, Research Funding; Biotest: Consultancy, Honoraria, Research Funding; CSL Behring: Consultancy, Honoraria, Research Funding; Grifols: Consultancy, Honoraria, Research Funding; Octapharma: Consultancy, Honoraria, Research Funding; NovoNordisk: Consultancy, Honoraria, Research Funding. Liesner:BPL: Consultancy, Honoraria, Research Funding; Bayer: Consultancy, Honoraria, Speakers Bureau; Cangene: Research Funding; CSL Behring: Consultancy, Honoraria, Research Funding; Baxalta Innovations GmbH, now a part of Shire: Consultancy, Honoraria, Research Funding; Pfizer: Consultancy, Honoraria, Research Funding; SOBI: Consultancy, Honoraria, Research Funding, Speakers Bureau; Octapharma: Consultancy, Honoraria, Research Funding, Speakers Bureau; Biogen: Consultancy, Honoraria, Research Funding; Grifols: Consultancy, Honoraria. Nolan:Sobi: Research Funding; Biogen: Research Funding.

A 40 year-old Caucasian patient with severe familial hemophilia A (FVIII These preliminary results show that this IT treatment, performed despite a high theoretical risk of failure, resulted in this patient in a dramatic clinical improvement, even though biological criteria of success have not yet been achieved. The respective roles of the type of FVIII concentrate, and of immunosuppression remain to be assessed, as well as the cost/benefit analysis on a longer follow-up period.