Expression of Markers of Platelet Activation and the Interpatient Variation in Response to Abciximab

Abstract

Abstract-Our study concerns the biological effects of abciximab (c7E3 Fab, ReoPro), a powerful new antiplatelet drug that blocks glycoprotein (GP) IIb-IIIa complexes. Samples were examined from 6 patients with coronary artery disease who received a bolus of abciximab followed by a 10-g/min infusion for at least 18 hours before percutaneous transluminal coronary angioplasty. Inhibition of ADP-induced PA was maximal for 4 patients but partial (79% and 53%) for 2 others during the infusion. Flow cytometry performed with monoclonal antibodies (PAC-1, AP-6, and F26) specific for the "activated" GP IIb-IIIa complex revealed large decreases in the expression of activation markers on platelets during therapy, but these decreases were less marked when inhibition of ADP-induced PA was incomplete.Residual aggregation was seen for all patients during the infusion when TRAP 14-mer peptide or thrombin was the stimulus. Unblocked GP IIb-IIIa complexes were detected on thrombin-stimulated platelets from the patients by immunoelectron microscopy performed using the monoclonal antibody AP-2. Unblocked GP IIb-IIIa complexes were also detected by flow cytometry when platelets preincubated for 1 hour in vitro with abciximab under saturating conditions were (1) incubated with TRAP 14-mer or (2) permeabilized with Triton X-100. In confirming interpatient variation in the platelet response to a standard dose of abciximab, our results also show that an uninhibited internal pool of GP IIb-IIIa complexes may mediate a residual response to strong agonists. (Arterioscler Thromb Vasc Biol.

Platelet Glycoprotein IIb/IIIa Inhibitors

“…Although ADP-induced platelet aggregation was extensively inhibited by abciximab in our previous study, 26 platelet responses to thrombin receptor-activating peptide (14mer) during abciximab infusion ranged from 28% to 71% of pretreatment levels. Similar results have previously been reported for patients after a single bolus injection of abciximab.…”

“…Further studies are required to confirm that such events occur in vivo and to determine whether this can lead to clinical drawbacks. In our previous study, 26 activated platelets progressively reappeared in the circulation after the stopping of the abciximab, but this can be due to the fact that the cause of increased platelet stimulation had not been removed (eg, fissured atherosclerotic plaques, stenosis, and resistant fibrin-rich thrombi).…”

“…24,25 We have recently compared the expression of activation-dependent markers on platelets during abciximab therapy given according to the CAPTURE protocol 9 to patients with unstable angina undergoing PTCA. 26 Before the onset of therapy, the percentage of platelets positive for one or more of the monoclonal antibodies directed against GPIIb-IIIa complexes that are "activated and unoccupied" (PAC-1) or "activated and occupied with fibrinogen" (AP6 and F26) were elevated for 5 of 6 patients studied. Abciximab therapy reduced these levels appreciably, presumably through the blockade of the active site of GPIIbIIIa.…”

“…4,9,26 Among the other factors that may influence the recovery rate after anti-GPIIb-IIIa therapy are (1) variations in the plasma levels of free drug, (2) the rate at which surface-bound drug dissociates from circulating platelets, 5 and (3) the extent to which GPIIb-IIIa receptors are exchanged between surface and internal pools. 28 Therefore, it is reasonable to suggest that biological surveillance, with testing of platelet function and flexibility with regard both to the doses used and the duration of therapy, may help to improve the success rate with anti-GPIIb-IIIa inhibitors in acute situations.…”

“…Using the monoclonal antibody AP-2, competitive for GPIIb-IIIa with c7E3, in flow cytometry or immunoelectron microscopy, we have shown that unblocked (with abciximab) GPIIb-IIIa receptors from the internal pool do indeed become exposed after thrombin stimulation of platelets taken from patients during abciximab infusion. 26 A residual aggregation with thrombin receptor-activating peptide during the infusion and after the administration of the abciximab bolus suggests that the concentration of free abciximab in plasma is insufficient to block these newly exposed receptors within the time scale of platelet aggregation, which will occur in seconds. The ability of peptides, peptidomimetics, and orally bioavailable drugs to inhibit aggregation with strong agonists will therefore depend on their affinity for GPIIb-IIIa and their sustained concentration in plasma.…”