title: A mechanistic model for paradoxical platelet activation by ligand-mimetic alphaIIb beta3 (GPIIb/IIIa) antagonists.
abstract: OBJECTIVE
  Integrins are attractive therapeutic targets. Inhibition of integrin alphaIIb beta3 effectively blocks platelet aggregation. However, limitations with intravenous alphaIIb beta3 antagonists and failure of oral alphaIIb beta3 antagonists prompted doubts on the current concept of ligand-mimetic integrin blockade.
  
  
  RESULTS
  Evaluating P-selectin expression on platelets by flow cytometry, we report a mechanism of paradoxical platelet activation by ligand-mimetic alphaIIb beta3 antagonists and define three requirements: (1) Induction of ligand-bound conformation of alphaIIb beta3, (2) receptor clustering, (3) prestimulation of platelets. Conformational change is inducible by clinically used ligand-mimetic alphaIIb beta3 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 alphaIIb beta3 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 alphaIIb beta3 antagonist-induced platelet activation.
  
  
  CONCLUSIONS
  These findings can explain limitations of ligand-mimetic anti-alphaIIb beta3 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.
authors.affiliation: Centre for Thrombosis & Myocardial Infarction, Baker Heart Research Institute, PO Box 6492 St Kilda Road Central, Melbourne, Victoria 8008, Australia.
authors.forename: Nicole
authors.initials: N
authors.lastname: Bassler
authors.forename: Christoph
authors.initials: C
authors.lastname: Loeffler
authors.forename: Pierre
authors.initials: P
authors.lastname: Mangin
authors.forename: Yuping
authors.initials: Y
authors.lastname: Yuan
authors.forename: Meike
authors.initials: M
authors.lastname: Schwarz
authors.forename: Christoph E
authors.initials: CE
authors.lastname: Hagemeyer
authors.forename: Steffen U
authors.initials: SU
authors.lastname: Eisenhardt
authors.forename: Ingo
authors.initials: I
authors.lastname: Ahrens
authors.forename: Christoph
authors.initials: C
authors.lastname: Bode
authors.forename: Shaun P
authors.initials: SP
authors.lastname: Jackson
authors.forename: Karlheinz
authors.initials: K
authors.lastname: Peter
mesh_terms: D002448:Cell Adhesion / Q000187:drug effects; D005455:Fluorescent Antibody Technique; D006801:Humans; D008024:Ligands; D008954:Models, Biological; D015539:Platelet Activation / Q000187:drug effects* / Q000502:physiology; D010975:Platelet Aggregation Inhibitors / Q000494:pharmacology*; D019039:Platelet Glycoprotein GPIIb-IIIa Complex / Q000037:antagonists & inhibitors*; D012680:Sensitivity and Specificity; D015398:Signal Transduction
chemical_list: D008024:Ligands; D010975:Platelet Aggregation Inhibitors; D019039:Platelet Glycoprotein GPIIb-IIIa Complex