PHARMACOKINETICS OF RECOMBINANT FACTOR-VIIA IN THE RAT - A COMPARISONOF BIOASSAY, IMMUNOASSAY AND ISOTOPE-ASSAY

Recombinant human factor VIIa (rFVIIa) is an activated coagulation fac tor for intravenous use as a haemostatic agent in haemophiliacs who ge nerate antibodies against factor VIII or IX. Plasma kinetic studies ar e important for the understanding of the action of rFVIIa which is exe rted in the vascular compartment of the body, more specifically on the vessel walls at the site of injury. In the present study, rats were d osed 100 or 500 mug/kg I-125-rFVIIa i. v., without any side effects be ing observed, and the plasma profile of rFVIIa was studied by 3 differ ent assays that were shown to correlate well at early times post-dose: trichloroacetic acid (TCA)-precipitable drug-related radioactivity, r FVIIa antigen determination by ELISA technique, and the assay of clot activity which is the only clinically applicable assay. The plasma con centration curve could be resolved into 1-3 exponentials, depending on the FVIIa detection principle that was employed. Initially, there was a short (ca. 10 min), phase of increasing concentrations before the a ttainment of C(max). This was followed by a plasma recovery (C(max) pl asma volume/dose) in the vicinity of one half of the administered dose . The initial volume of distribution (V1) corresponded to the vascular compartment whereas the volume of distribution at steady state (V(ss) ) was somewhat larger. Whole body clearance (CL-B) of rFVIIa was appro x. 1 ml/min per kg, and mean residence time (MRT) and the half-life as sumed to be associated with the loss of biological activity was approx . 1 h and 20-45 min, respectively. From these plasma data, rFVIIa appe ars to be a low clearance compound with limited tissue distribution an d a short half-life. Tissue distribution studies showed that high I-12 5 levels, assumed,to be rFVIIa-related, included mineralised bone and well-perfused organs such as the liver which suggested that this organ was responsible for a major proportion of CL-B. Finally, mass balance studies showed that almost 90% of the administered radioactivity coul d be accounted for following an i. v. dose, predominantly as non drug- related radioactivity, even though a small amount of TCA-precipitable radioactivity was excreted via the biliary route. In conclusion, dose- or sex-dependent plasma kinetics and tissue distribution within a dos e range of 100 to 500 mug/kg of rFVIIa was not observed. In the early and pharmacologically relevant phase after rFVIIa administration there appears to be good agreement between the various plasma assays employ ed in the study, indicating that the clot assay yields useful informat ion in studies of rFVIIa plasma pharmacokinetics.