Paradoxical inhibition of fibrinogen binding and potentiation of alpha-granule release by specific types of inhibitors of glycoprotein IIb-IIIa.
OBJECTIVE
To determine whether glycoprotein (GP) IIb-IIIa inhibitors can paradoxically augment activation of platelets, activation of GP IIb-IIIa, alpha-granule degranulation, and lysosome release were induced after exposure of platelets to GP IIb-IIIa inhibitors.


METHODS
ADP-induced platelet activation was assessed after exposure of platelets to Abciximab, or to a non-peptide ligand, the free acid of Orbofiban (Orbofiban(a)). Activation of GP IIb-IIIa was detected based on binding of fluorochrome labeled fibrinogen or a labeled monoclonal antibody, PAC-1. alpha-Granule degranulation was detected based on surface expression of P-selectin and lysosome release was detected based on surface expression of CD63.


RESULTS
Despite significant inter-individual variability in inhibition of fibrinogen binding in response to each of the GP IIb-IIIa inhibitors used, a concentration dependent decrease in fibrinogen binding was seen with each agent in samples from each subject. Binding of PAC-1 was inhibited in a parallel manner. Abciximab increased ADP-induced P-selectin expression. Orbofiban(a) did not alter ADP-induced P-selectin expression. Neither agent altered ADP-induced CD63 expression. When platelets were exposed to Abciximab and Orbofiban(a), both Abciximab and Orbofiban(a) were found in the alpha-granules (by confocal microscopy), consistent with potentiation of agonist-induced release of alpha-granular products associated with uptake of proteins.


CONCLUSIONS
Specific types of GP IIb-IIIa inhibitors can paradoxically augment agonist-induced release of alpha-granules despite inhibiting agonist-induced fibrinogen binding.
Schneider
D J
DJ
Department of Medicine, University of Vermont, Burlington, Colchester 05446, USA. djschnei@zoo.uvm.edu
Taatjes
D J
DJ
Sobel
B E
BE
D000077284:Abciximab; D000409:Alanine / Q000378:metabolism / Q000494:pharmacology; D000911:Antibodies, Monoclonal / Q000378:metabolism / Q000494:pharmacology*; D015703:Antigens, CD / Q000032:analysis; D001792:Blood Platelets / Q000187:drug effects* / Q000276:immunology / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D005340:Fibrinogen / Q000378:metabolism*; D005434:Flow Cytometry; D006801:Humans; D007140:Immunoglobulin Fab Fragments / Q000378:metabolism / Q000494:pharmacology*; D066298:In Vitro Techniques; D018613:Microscopy, Confocal; D019007:P-Selectin / Q000032:analysis; D015539:Platelet Activation / Q000187:drug effects; D010975:Platelet Aggregation Inhibitors / Q000378:metabolism / Q000494:pharmacology*; D019039:Platelet Glycoprotein GPIIb-IIIa Complex / Q000037:antagonists & inhibitors*; D010980:Platelet Membrane Glycoproteins / Q000032:analysis; D011485:Protein Binding / Q000187:drug effects; D011759:Pyrrolidines / Q000378:metabolism / Q000494:pharmacology*; D060149:Tetraspanin 30
D000911:Antibodies, Monoclonal; D015703:Antigens, CD; C554586:CD63 protein, human; D007140:Immunoglobulin Fab Fragments; D019007:P-Selectin; D010975:Platelet Aggregation Inhibitors; D019039:Platelet Glycoprotein GPIIb-IIIa Complex; D010980:Platelet Membrane Glycoproteins; D011759:Pyrrolidines; D060149:Tetraspanin 30; D005340:Fibrinogen; C120234:orbofiban; D000409:Alanine; D000077284:Abciximab