Functional characteristics of the novel, human-derived recombinant FVIII protein product, human-cl rhFVIII.

INTRODUCTION: Hemophilia A is routinely treated by administration of exogenous coagulation factor VIII (FVIII). As safety and efficacy of FVIII products have improved over the years, development of FVIII-neutralizing antibodies (FVIII inhibitors) has emerged as the most serious complication. The new human cell line-derived recombinant human FVIII (human-cl rhFVIII) is the first recombinant FVIII product produced in a human cell line without additive animal proteins, with a goal of minimizing the risk of inhibitor development. MATERIALS AND METHODS: Biochemical analyzes of purity, molecular and functional attributes of the novel human-cl rhFVIII were undertaken for product characterization. RESULTS AND CONCLUSIONS: Human-cl rhFVIII was shown to be highly pure, with host-cell protein and DNA traces comparable to, or lower than, currently marketed recombinant FVIII (rFVIII) products. Human-cl rhFVIII was shown to have high specific FVIII activity and characteristics similar to full-length rFVIII products. Furthermore, no significant discrepancy between one-stage and chromogenic assay results were observed for human-cl rhFVIII, indicating potency ratios of these assays comparable to the full-length rFVIII products. In functional tests, human-cl rhFVIII exhibited physiological thrombin generation and a normal rate of inactivation by activated protein C. Importantly, human-cl rhFVIII displayed higher binding capacity with von Willebrand factor than comparator products, thus minimizing circulating unbound FVIII and further reducing the potential risk of inhibitor development.

Assaying the circulating factor VIII activity in hemophilia A patients treated with recombinant factor VIII products.

Large discrepancies between factor VIII assay methods have been reported from pharmacokinetic studies of recombinant factor VIII concentrates. In the assay of postinfusion patient plasma samples, traditional activated partial thromboplastin time (aPTT)-based one-stage clotting methods usually give results that are 20 to 50% lower than those obtained by chromogenic substrate assays. Investigations into the cause of these discrepancies have shown that the choice of phospholipid in the one-stage assay is crucial. The use of platelets or liposomes resembling platelet factor 3 instead of traditional aPTT reagents results in an increase in the apparent one-stage activity and a fairly good correlation with the chromogenic results. These and other functional test results, antigen measurements as well as clinical data, support the view that the chromogenic assay most accurately reflects the therapeutic effect. In addition to the differences among assay methods, there is also a discrepancy between the World Health Organization (WHO) standards for concentrates and plasma. The use of product-specific standards, prepared by diluting the factor VIII concentrate into hemophilic plasma, when assaying postinfusion plasma samples seems to be a feasible approach to overcome the problems encountered in pharmacokinetic studies.

Preclinical pharmacology of albumin-free B-domain deleted recombinant factor VIII.

A second-generation recombinant factor VIII molecule was developed with an albumin-free formulation. In this modified form of factor VIII, the N- and C-terminal sections of the B-domain are retained and fused at serine 743 and glutamine 1638, resulting in a B-domain deleted factor VIII protein known as ReFacto (Genetics Institute, Andover, MA). Preclinical studies of ReFacto have focused on efficacy of the product for the hemophilia A patient population. The efficacy and pharmacokinetic profiles of ReFacto were similar to plasma-derived factor VIII in correcting the hemostatic defect of hemophilia A dogs. Both ReFacto and plasma-derived human factor VIII (Octonativ-M7, Pharmacia, Stockholm, Sweden) were found to associate with von Willebrand factor (vWF) after infusion into hemophilia A dogs as demonstrated by size exclusion chromatography. Infusion of either ReFacto or Octonativ-M7 quickly corrected factor VIII coagulant activity (FVIIIc), whole blood clotting time (WBCT), and activated partial thromboplastin time (aPTT). No obvious differences were seen between ReFacto and Octonativ-M7. Both ReFacto and Octonativ-M7 treatment reduced secondary bleeding time to less than 6 minutes. The clearance was faster and the volume of distribution at steady state was larger for plasma-derived factor VIII compared with ReFacto. The half-life was similar between Octonativ-M7 and ReFacto. These data predict that ReFacto will be effective in correcting human factor VIII deficiency states.