Comparison of coagulant activity of factor VII and activated factor VII activity assays when used for determination of recombinant activated factor VII levels in plasma

Two assays [coagulant activity of factor VII (FVII:C) and activated factor VII (FVIIa) activity] are currently available for the assessment of factor VII and FVIIa pharmacokinetics. This article presents the results of a comparison of the two assays when applied both in vitro as well as during clinical pharmacokinetic trials of recombinant FVIIa (rFVIIa) administered to healthy individuals and haemophilia patients. The in-vitro data showed that, for the FVII:C assay, plasma samples do not dilute in parallel. For the FVIIa activity assay, dilutions of samples are both parallel and linear with different dilutions of the calibrator. Moreover, intra-assay variation was found to be smaller for the FVIIa activity assay than for the FVII:C assay. When adding different amounts of rFVIIa (0-6 microg/ml) to normal plasma, a mean specific activity of rFVIIa of 48.6 U/mug was observed on applying the FVII:C assay; however, the specific activity decreased with increasing levels of rFVIIa. For the FVIIa activity assay, the mean specific activity was 45.4 IU/mug. Direct comparison of the two activity assays showed that no simple conversion between FVII:C and FVIIa activity measurements are possible. When applying the two assays for pharmacokinetic assessments in two clinical trials, statistically significant different estimates for the area under the curve, half-life, clearance and volume of distribution were obtained. In conclusion, for evaluation of rFVIIa pharmacokinetic properties, activity should be measured with the FVIIa activity assay - which is a more specific and reliable assay of the two available factor VII activity assays, especially when assessing low activity levels.