99.3% of Inhibitors in Severe Hemophilia a Develop before Exposure Day 75. Time to Change Definition of Previously Treated Patients; Data from 1038 Patients with Severe Hemophilia a of the Pednet Registry
Authors: Van den Berg Marijke , Fischer Kathelijn , Santagostino Elena , Chambost Herve , Kurnik Karin , Kenet Gili , Male Christoph , Rafowicz Anne , Platokouki Helen , Ranta Susanna , Van Geet Christel , Oldenburg Johannes , Liesner Ri , Claeyssens Segolene , Gretenkort-Andersson Nadine , Mäkipernaa Anne , Stamm Mikkelsen Torben , Carcao Manuel , Nolan Beatrice , Chalmers Liz , Cid A.R. , Álvarez-Roman Teresa , Altisent Carmen , Rivard Georges , Molinari Claudio , Buehrlen Martina , Kobelt Rainer , Escuriola Carmen , Carvalho Manuela , Koenigs Christoph , Williams Mike , Ljung Rolf 
Published: October 2019

Abstract
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.