There is a potential for immunogenicity with therapeutic proteins. The development of factor VIII inhibitors with XYNTHA was evaluated in 144 adult and pediatric PTPs with at least 50 EDs. Laboratory-based assessments for FVIII inhibitor (partial Nijmegen modification of the Bethesda inhibitor assay) were conducted in the clinical studies. The criterion for a positive FVIII result test result was ≥ 0.6 BU/mL. Across all studies, 3 subjects developed factor VIII inhibitors (2.1%).
The clinical studies for XYNTHA examined 124 subjects (94 for bleeding and 30 for surgery) who had previously been treated with factor VIII (PTPs). In the safety and efficacy study, two subjects with inhibitors were observed in 89 subjects (2.2%) who completed ≥ 50 exposure days. In a Bayesian statistical analysis, results from this study were used to update PTP results from a prior supporting study using XYNTHA manufactured at the initial facility (with one de novo and two recurrent inhibitors observed in 110 subjects) and the experience with predecessor product (with one inhibitor observed in 113 subjects). The Bayesian analysis indicated that the population inhibitor rate for XYNTHA, an estimate of the 95% upper limit of the true inhibitor rate, was 4.17%.
None of the PTPs developed anti-CHO (Chinese hamster ovary) or anti-TN8.2 antibodies. One PTP developed anti-FVIII antibodies; but, this subject did not develop an inhibitor.
In the surgery study, one low titer persistent inhibitor and one transient false-positive inhibitor were reported. In this study, one surgical subject developed anti-CHO cell antibodies with no associated allergic reaction. One subject developed anti-FVIII antibodies; but, this subject did not develop an inhibitor.
Across all studies, safety was evaluated in 40 previously treated pediatric patients <16 years of age with at least 50 EDs (25 children, <6 years of age and 15 adolescents, 12 to <16 years of age). Of these, one pediatric subject developed an inhibitor.
The detection of antibody formation is highly dependent on the sensitivity and specificity of the assay. Additionally, the observed incidence of antibody, including neutralizing antibody, positivity in an assay may be influenced by several factors, including assay methodology, sample handling, timing of sample collection, concomitant medications, and underlying disease. For these reasons, comparisons of the incidence of antibodies to XYNTHA with the incidence of antibodies to other products may be misleading.