Fibrinogen receptor antagonists induce conformational changes of the human platelet glycoprotein IIb

BACKGROUND: Controversial results have been reported concerning the ability of fibrinogen receptor antagonists (fibans) to induce conformational changes in the fibrinogen receptor after binding to it as the initial step of fibrinogen binding and platelet activation. METHODS: Platelets in citrated whole blood were stained with several pairs of anti-glycoprotein (anti-GP) IIb-directed monoclonal antibodies conjugated to phycoerythrin (PE) or indirectly labeled with Cy5. Pairs of monoclonal antibodies that induced a high-fluorescence resonance energy transfer (FRET) efficiency served as tools to detect activation-dependent changes of GP IIb after addition of adenosine diphosphate and several fibans. RESULTS: Using the combination of the clones 5B12-PE and P2-biotin/SA-Cy5, a concentration-dependent alteration of the GP IIb conformation was observed after addition of tirofiban, eptifibatide, and lotrafiban. Magnitude and kinetics differed among the investigated substances. CONCLUSION: The newly developed FRET assay allows the direct investigation of conformational changes of GP IIb after addition of platelet agonists or receptor ligands, as shown for three fibans.