SIPPET: methodology, analysis and generalizability.

The development of anti-FVIII neutralizing alloantibodies (inhibitors), occurring in about one-third of previously untreated patients (PUPs) with severe haemophilia A, depends on various genetic and environmental risk factors. Several previous studies have reported on the immunogenicity of FVIII concentrates, and due to differences in study design, study period, inhibitor testing frequency and follow-up duration the results were inconclusive. The first randomized trial on this unresolved question (SIPPET) included 251 previously untreated or minimally treated patients with severe haemophilia A treated with either a single plasma-derived FVIII (pdFVIII) containing VWF or a recombinant FVIII (rFVIII). The results showed an 87% higher rate of inhibitor development for rFVIII than pdFVIII during the first 50 exposure days of treatment. These results generated interest by patient organizations, physicians and regulatory agencies. This manuscript summarizes answers to the main questions that arose after the full publication of SIPPET.