Summary
Active site-inactivated factor VIIa has potential as an antithrombotic agent. The
effects of D-Phe-L-Phe-L-Arg-chloromethyl ketone-treated factor VIla (FFR-FVIIa) were
evaluated in a cell-based system mimicking in vivo initiation of coagulation. FFR-FVIIa inhibited platelet activation (as measured by
expression of P-selectin) and subsequent large-scale thrombin generation in a dose-dependent
manner with IC50 values of 1.4 ± 0.8 nM (n = 8) and 0.9 ± 0.7 nM (n = 7), respectively.
Kd for factor VIIa binding to monocytes ki for FFR-FVIIa competing with factor VIIa were similar (11.4 ± 0.8 pM and 10.6 ± 1.1
pM, respectively), showing that FFR-FVIIa binds to tissue factor in the tenase complex
with the same affinity as factor VIIa. Using platelets from volunteers before and
after ingestion of aspirin (1.3 g), there were no significant differences in the IC50
values of FFR-FVIIa [after aspirin ingestion, the IC50 values were 1.7 ± 0.9 nM (n
= 8) for P-selectin expression, p = 0.37, and 1.4 ± 1.3 nM (n = 7) for thrombin generation,
p = 0.38]. This shows that aspirin treatment of platelets does not influence the inhibition
of tissue factor-initiated coagulation by FFR-FVIIa, probably because thrombin activation
of platelets is not entirely dependent upon expression of thromboxane A2.
