Summary
Complexation of factor VIIa (FVIIa) and tissue factor (TF) initiates the extrinsic
pathway of blood coagulation. Inappropriate triggering of this pathway has been linked
to thrombotic disorders. We have previously shown that a mutant form of soluble tissue
factor (sTF, residues 1-219) having Lys165 and Lys166 replaced with alanine residues
(hTFAA) gave an antithrombotic effect in a rabbit model of arterial thrombosis. Here
we have improved the potency of hTFAA by incorporating amino acid substitutions that
increase the affinity of sTF for FVIIa. Phage display has been used to select consensus
sequences at two FVIIa-contact regions on sTF, segments 44–50 and 130–140, that individually
showed modestly increased (∼2-fold) FVIIa-affinity. These consensus sequences have
been combined with other previously selected mutations to generate a series of variants
(hTFAA-2, hTFAA-3, and hTFAA-4) having 4 to 10-fold increased FVIIa-binding affinity.
Surface plasmon resonance measurements indicate that the increased affinity results
mostly from an increase in the association rate constant. All of these variants displayed
increased inhibitor potency relative to hTFAA in an assay of Factor X activation catalyzed
by the complex of FVIIa with membrane TF. In addition, these affinity improved hTFAA
variants are more potent anticoagulants and have increased antithrombotic activity
relative to hTFAA in an ex vivo flow chamber model of thrombosis. The potency trend parallels increases in FVIIa-binding
affinity; however, the absolute fold increases in potencies are greater than increases
in binding affinity, consistent with kinetic studies of the FVIIa-binding event. Through
incorporation of 10 amino acid substitutions (hTFAA-3) we have increased the antithrombotic
activity of hTFAA by 20-fold.
Keywords
Tissue factor - coagulation factor VIIa - anticoagulant - phage display