Regulation of clot retraction by glycoprotein IIb/IIIa antagonists

Binding of fibrinogen to platelet glycoprotein (GP) IIb/IIIa induces clot retraction. Significant differences among GP IIb/IIIa antagonists were previously noted to inhibit thromboelastography in whole blood specimens. The relationship between efficacy of these agents and inhibition of clot retraction is unclear. Here, we use a plasma-free clot retraction assay to evaluate potency of GP IIb/IIIa antagonists to inhibit clot retraction and modulate platelet signaling, and to address whether these effects are realized in the clinically relevant dose range. The potencies for inhibition of clot retraction and aggregation are similar for antagonists with high affinity for resting platelets and slow off-rates, whereas lower affinity and fast off-rate antagonists are disproportionately less effective in blocking clot retraction. A positive correlation is observed between inhibition of clot retraction and inhibition of tyrosine dephosphorylation across a number of GP IIb/IIIa antagonist pharmacophores. For lower affinity and fast off-rate antagonists, the concentrations required for inhibition of clot retraction clearly exceed the clinical dose range. Site occupancy studies combined with clot retraction experiments addressed whether high affinity and slow off-rate compounds can alter clot retraction during the dosing interval. Binding studies using [3H] Roxifiban, a high affinity GP IIb/IIIa antagonist, indicate that occupancy of >95% of GP IIb/IIIa sites is required to inhibit clot retraction. This level of occupancy is not routinely achieved in the clinic and is not tolerated, at least for chronic therapy. These results suggest that inhibition of clot retraction is not necessary for efficacy of GP IIb/IIIa antagonists.