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(Arteriosclerosis, Thrombosis, and Vascular Biology. 2007;27:E9.)
© 2007 American Heart Association, Inc.

Thrombosis

A Mechanistic Model for Paradoxical Platelet Activation by Ligand-Mimetic _IIbß₃ (GPIIb/IIIa) Antagonists

Nicole Bassler; Christoph Loeffler; Pierre Mangin; Yuping Yuan; Meike Schwarz; Christoph E. Hagemeyer; Steffen U. Eisenhardt; Ingo Ahrens; Christoph Bode; Shaun P. Jackson; Karlheinz Peter

From the Centre for Thrombosis & Myocardial Infarction (N.B., C.E.H., S.U.E., K.P.), Baker Heart Research Institute, Melbourne, Australia; the Department of Cardiology (C.L., M.S., I.A., C.B.), Albert-Ludwigs-University, Freiburg, Germany; and the Australian Centre for Blood Diseases (P.M., Y.Y., S.P.J.), Monash University, Melbourne, Australia.

Correspondence to Karlheinz Peter, MD, Baker Heart Research Institute, PO Box 6492 St Kilda Road Central, Melbourne, Victoria 8008, Australia. E-mail karlheinz.peter{at}baker.edu.au

Objective— Integrins are attractive therapeutic targets. Inhibition of integrin _IIbß₃ effectively blocks platelet aggregation. However, limitations with intravenous _IIbß₃ antagonists and failure of oral _IIbß₃ antagonists prompted doubts on the current concept of ligand-mimetic integrin blockade.

Methods and Results— Evaluating P-selectin expression on platelets by flow cytometry, we report a mechanism of paradoxical platelet activation by ligand-mimetic _IIbß₃ antagonists and define three requirements: (1) Induction of ligand-bound conformation of _IIbß₃, (2) receptor clustering, (3) prestimulation of platelets. Conformational change is inducible by clinically used ligand-mimetic _IIbß₃ antagonists, RGD-peptides, and anti-LIBS antibodies. In a mechanistic experimental model, clustering is achieved by crosslinking integrins via antibodies, and preactivation is induced by low-dose ADP. Finally, we demonstrate that platelet adhesion on collagen represents an in vivo correlate of platelet prestimulation and receptor clustering, in which the presence of ligand-mimetic _IIbß₃ antagonists results in platelet activation as detected by P-selectin, CD63, and CD40L expression as well as by measuring Ca²⁺-signaling. Blockade of the ADP receptor P2Y₁₂ by AR-C69931MX and clopidogrel inhibits _IIbß₃ antagonist-induced platelet activation.

Conclusion— These findings can explain limitations of ligand-mimetic anti-_IIbß₃ therapy. They describe potential benefits of concomitant ADP receptor blockade and support a shift in drug development from ligand-mimetic toward allosteric or activation-specific integrin antagonists.

Limitations with intravenous and failure of oral _IIbß₃ antagonists question the current concept of ligand-mimetic _IIbß₃ blockade. We provide a model explaining paradoxical platelet activation as consequence of _IIbß₃ antagonist-induced conformational change of _IIbß₃. Concomitant blockade of the ADP-receptor P2Y₁₂, activation-specific and allosteric blockade are described/discussed as alternative _IIbß₃-blocking strategies.

Key Words: _IIbß₃ • GPIIb/IIIa • _IIbß₃ • antagonists • integrin • antiplatelet therapy