Summary
A biophysical approach tested the hypothesis that tirofiban, like eptifibatide, perturbs
GpIIb/IIIa structure. Tirofiban bound tightly to platelet GpIIb/IIIa (EC50 ∼ 24 nmol/L) and effectively inhibited platelet aggregation (IC50 ∼ 37 nmol/L) but blocked platelet adhesion to clotted fibrin only at much higher
doses (IC50 ∼ 580 nmol/L). Electrophoretic analyses demonstrated that tirofiban protected GpIIb/IIIa
from SDSinduced subunit dissociation. However, saturating tirofiban concentrations
had little or no effect on GpIIb/IIIa secondary or tertiary structure, as determined
by circular dichroic spectroscopy, dynamic light scattering, and sedimentation velocity
measurements performed with purified receptors in octyl glucoside. Moderate dose-dependent
effects on GpIIb/IIIa quaternary structure were detected by sedimentation equilibrium.
Transmission electron microscopy showed minimal tirofiban-induced receptor activation
or oligomerization. Thus, even at the increased concentrations needed to block platelet:fibrin adhesive interactions, tirofiban exhibited only limited effects on GpIIb/IIIa conformation
and clustering. Our results provide new insights into the mechanisms and potential
prothrombotic complications of integrin antagonists.
Keywords
GpIIb/IIIa - integrin α
IIbβ
3
- integrin antagonist - fibrin - structure