Summary
In a study designed to demonstrate the safety and pharmacokinetics of a recombinant
factor VIII (Recombinate) manufactured in Andover, MA and Thousand Oaks, CA, two different
methods of factor VIII assay (one-stage clotting and Chromogenic substrate) were compared
in vivo. The study was performed in four centres in the UK: London, Oxford, Cardiff
and Manchester. Two pharmacokinetic studies, at least one week apart, were performed
in 30 patients with severe haemophilia A (VIII:C < 2 IU/dl). A dose of 50 IU/kg was
administered with sampling pre-infusion, and +0.25, 0.5, 1, 3, 6, 9, 12 and 24 h post-infusion.
The aggregate 60 pharmacokinetic study showed a half-life of 12.7 and 13.0 h (p =
0.28) and recovery of 127 and 161 IU/dl (p = 0.0001) using one-stage clotting or chromogenic
substrate respectively. In a supplementary experiment, 20 post-infusion samples were
re-assayed by 1-stage and chromogenic assay using two plasma (20th British plasma standard and an “in-house” pooled normal plasma) and two concentrate
standards, derived from the same type, but different batch of infused concentrate
(Recombinate) and pre-diluted in either individual pre-infusion sample or in pooled
commercial haemophilic plasma. The use of the Recombinate concentrate standard overcame
the significant difference in FVIII levels between 1-stage and chromogenic assay methods
when a plasma standard was used (p <0.0001). It is concluded that where potency dosing
designation is carried out by an assay system different to that used in the clinical
situation, the use of the recombinant concentrate as a standard in post-infusion plasma
samples is likely to give more reliable and reproducible results.
Keywords
Recombinant FVIII (Recombinate) - assay discrepancies
