Immunogenicity of BAX 855 in Previously Treated Patients with Congenital Severe Hemophilia Α
Authors: Horling Frank Michael , Allacher Peter , Koppensteiner Herwig , Engl Werner , Scheiflinger Fritz , Abbuehl Brigitt E. , Reipert Birgit M 
Published: October 2019

Abstract
Background and objectives
BAX 855 (Antihemophilic Factor [Recombinant] pegylated, rurioctocog alfa pegol) is an extended half-life (EHL) recombinant human coagulation factor VIII (rFVIII) modified with polyethylene glycol (PEG) (Turecek et al., 2012). It was recently approved in the US and Japan for on-demand treatment of bleeding events and for prophylactic treatment for patients with congenital severe hemophilia A.
The efficacy and safety of BAX 855 were extensively studied during clinical development of this compound (Konkle et al., 2015). The assessment of BAX855 immunogenicity was of particular interest because the development of neutralizing antibodies (FVIII inhibitors) is the most serious complication following replacement therapies with FVIII products. FVIII inhibitors develop in about 20-32% of previously untreated patients (Gouw SC et al., 2013) and with a rate of 1.55- 3.8 per 1000 patients per year in previously treated patients (Kempton CL, 2010) with severe hemophilia A. To fully understand the potential of BAX855 to induce antibody responses, both FVIII inhibitors and total FVIII-binding antibodies were assessed. Furthermore, potential antibody development against PEG-FVIII, PEG and CHO proteins was investigated.
Methods
The clinical protocols (ClinicalTrials.gov identifier: NCT02585960, NCT02210091, NCT01736475, NCT01913405, NCT01945593, NCT01599819, NCT02615691) and the methods used for antibody analytics (Whelan et al 2013; Lubich et al 2016) were previously described. ELISA technologies were used for the analysis of total binding antibodies, the Nijmegen modification of the Bethesda assay was used for the detection of FVIII inhibitors. Correlation analyses were done to assess any potential correlation between the development of antibodies and potential adverse events.
Results
None of the 243 subjects (6 PUPs and 237 PTPs) included in the analysis developed FVIII inhibitors (≥ 0.6 BU/mL) A total of 44 subjects tested positive for binding antibodies against FVIII, PEG-FVIII or PEG at single time points. 28 of these 44 subjects showed pre-existing antibodies against FVIII, PEG-FVIII, or PEG prior to first exposure to BAX 855, which disappeared during the study. 13 subjects who tested negative at screening developed transient antibodies against FVIII, PEG-FVIII, or PEG at one or two consecutive study visits after exposure to BAX 855. Antibodies were transient and not detectable at subsequent visits or at completion of the study. Five subjects showed positive results for binding antibodies at study completion or at the time of the data cutoff. No conclusion can be drawn whether these antibodies are of transient or persistent nature. There was no confirmed causal relationship between the appearance of binding antibodies against FVIII, PEG or PEG-FVIII and adverse events, nor was there an impact on hemostatic efficacy in any of the 44subjects. No subject had pre-existing antibodies or developed de novo antibodies to CHO proteins during the study at any time point.
Conclusion
Our data indicate that BAX855 did not show an increased risk for PTPs to develop FVIII inhibitors. We did not see any FVIII inhibitor development in PUPs, but the small number of overall exposures does not allow general conclusions for PUPs. Importantly, the data suggest that BAX855 did not induce immune responses associated with impaired treatment efficacy or with altered PK parameters.

Disclosures
Horling: Shire: Employment. Allacher:IMC Krems: Research Funding. Koppensteiner:Shire: Employment. Engl:Shire, formerly Baxalta and Baxter: Employment, Equity Ownership. Scheiflinger:Shire: Employment, Research Funding. Abbuehl:Baxalta (now part of Shire): Employment. Reipert:Shire: Employment.