Summary
The mechanism for the elimination of factor VII (FVII) from the circulation is unknown,
just as it is unclear how activation of FVII to FVIIa and subsequent complex formation
with antithrombin III (AT) or α2-macroglobulin (α2M) affects clearance. The possibility
that the clearance mechanism involves activation and inhibitor complex formation as
obligatory intermediate reactions is examined in this study. Human and murine sera
were spiked with human FVIIa in the absence and presence of heparin and analysed for
complex formation. Complex formation in vivo was studied after intravenous injection of 125I-VIIa in mice; and the pharmacokinetics (PK) of human and murine FVIIa was studied
in normal mice. Furthermore, comparative PK studies were performed with FVII, FVIIa,
active site blocked FVIIa and a preformed FVIIa-AT complex in normal and α2M-deficient
mice. The data demonstrated that FVIIa-AT complexes and to a much lesser extent FVIIa-α2M-complexes
accumulated in vivo after FVIIa administration. FVIIa-AT accounted for about 50% of total FVIIa antigen
left in the circulation after 3 hours. All FVII derivatives studied including FVII,
FVIIa and FVIIa-AT were cleared with similar rates suggesting an elimination kinetics
which is unaffected by FVII activation and subsequent inactivation by plasma inhibitors.
Keywords
Factor VIIa - antithrombin - alpha2-macroglobulin - complex formation - clearance