Recombinant full-length factor VIII (FVIII) and extended half-life FVIII products in prophylaxis--new insight provided by pharmacokinetic modelling.

The pharmacokinetics (PK) of extended half-life factor VIII (FVIII) products might allow longer dosing intervals in prophylaxis, potentially affecting its efficacy. We used published population PK models of a recombinant full-length FVIII (rAHF-PFM) and a recombinant B-domain-deleted FVIII Fc fusion product (rFVIIIFc) to assess the time spent weekly with FVIII levels below 3 IU dL(-1) or above 10 IU dL(-1) . These FVIII levels were chosen based on the observation that trough levels of 1 IU dL(-1) may not be sufficient in all patients. This approach was applied to a simulated population of 1000 severe haemophilia A subjects with dosing regimens included in the prescribing information or evaluated in clinical trials. FVIII levels remained ≥3 IU dL(-1) in 57% of patients treated with rAHF-PFM 30 IU kg(-1) every 48 h compared with 41.1%, 18.3%, 0.9% and 0% of patients treated with rFVIIIFc 30 IU kg(-1) every 72 h, 50 IU kg(-1) every 96 h or 120 h and 65 IU kg(-1) every 168 h respectively. Patients on rAHF-PFM 30 IU kg(-1) every 48 h spent more time weekly with FVIII levels above 10 IU dL(-1) than those on rFVIIIFc 50 IU kg(-1) every 96 h or 120 h, and 65 IU kg(-1) every 168 h. In conclusion, PK modelling indicates that choice and dosing intervals of standard and extended half-life FVIII products require careful evaluation of individual PK to allow more time at protective levels, especially in patients with active lifestyles.

The EPIC study: a lesson to learn.

INTRODUCTION: Inhibitory antibodies to factor VIII occur in about 30% of previously untreated patients (PUPs) and are the most serious complication of haemophilia A. It is unclear why some patients develop inhibitors and others do not. AIMS: The Early Prophylaxis Immunologic Challenge (EPIC) study was designed to test the hypothesis that inhibitor incidence in PUPs with severe or moderately severe haemophilia A could be reduced when a once-weekly FVIII prophylaxis starts with 25 IU kg(-1) rAHF-PFM before 1 year of age and immunological danger signals are minimized. METHODS: These signals were minimized by avoiding: surgery; the first FVIII infusion during severe bleeding or an infection; central venous access devices and administering vaccinations intramuscularly 3-4 days before or after FVIII. RESULTS: Eight of the 19 treated subjects (42.1%) developed confirmed inhibitors. Eleven of the 19 treated subjects were PUPs without any prior exposure to FVIII. Three of them (27.3%) developed a confirmed inhibitor together with FVIII-binding antibodies. The study was stopped because the likelihood to reach the primary objective was minimal, a decision endorsed by the data safety monitoring board. CONCLUSION: Because of early termination, the EPIC study hypothesis could not be corroborated. Nonetheless, our data analyses indicate that the current definition of an inhibitor only based on plasma inhibitor activity ≥0.6 BU mL(-1) may not always reflect the presence of FVIII-neutralizing antibodies. The findings of this study teach us that low-level inhibitor activity results need in addition a confirmatory test and/or the assessment of the therapeutic response.

Opportunities and limitations of mouse models humanized for HLA class II antigens.

MHC class II molecules are essential for shaping the CD4+ T-cell repertoire in the thymus and for selecting antigenic peptides that are presented to CD4+ T cells in the periphery. A range of different mouse models humanized for HLA class II antigens have been developed to study the regulation of MHC-class II restricted immune responses. These mouse models have been used to identify immunodominant peptides that trigger diseases and to characterize the interactions of T-cell receptors with disease-associated peptides and MHC class II molecules. Peptides presented to CD4+ T cells in these mouse models were shown to be similar to peptides presented to CD4+ T cells in patients who carry the same MHC class II haplotype. Opportunities and limitations associated with these mouse models will be discussed and the potential application of these models for understanding the regulation of antibody responses against factor VIII in hemophilia A will be indicated.