FK633: A Potent and Selective Platelet GPIIb/IIIa Antagonist

Binding of thrombin-activated platelets to a fibrin scaffold through α(IIb)β₃ evokes phosphatidylserine exposure on their cell surface

Recently, by employing intra-vital confocal microscopy, we demonstrated that platelets expose phosphatidylserine (PS) and fibrin accumulate only in the center of the thrombus but not in its periphery. To address the question how exposure of platelet anionic phospholipids is regulated within the thrombus, an in-vitro experiment using diluted platelet-rich plasma was employed, in which the fibrin network was formed in the presence of platelets, and PS exposure on the platelet surface was analyzed using Confocal Laser Scanning Microscopy. Almost all platelets exposed PS after treatment with tissue factor, thrombin or ionomycin. Argatroban abrogated fibrin network formation in all samples, however, platelet PS exposure was inhibited only in tissue factor- and thrombin-treated samples but not in ionomycin-treated samples. FK633, an α(IIb)β₃ antagonist, and cytochalasin B impaired platelet binding to the fibrin scaffold and significantly reduced PS exposure evoked by thrombin. Gly-Pro-Arg-Pro amide abrogated not only fibrin network formation, but also PS exposure on platelets without suppressing platelet binding to fibrin/fibrinogen. These results suggest that outside-in signals in platelets generated by their binding to the rigid fibrin network are essential for PS exposure after thrombin treatment.

Novel synthetic approach to N-aryl-4-(3-pyridyl)thiazol-2-amine and analogues using HMCM-41 as catalyst, and their biological evaluation as human platelet aggregation inhibitors

A novel synthetic approach to N-aryl-4-(3-pyridyl)thiazol-2-amine and analogues using HMCM-41, a mesoporous aluminosilicate catalyst and their in vitro ADP-induced platelet aggregation inhibitory activity on human blood platelets is described. Among the test compounds N-(2'-flourophenyl)-4-(3-pyridyl)thiazol-2-amine (9e) was found to be the most potent, IC(50)=4.84x10(-7)M.

Association of the antagonism of von Willebrand factor but not fibrinogen by platelet alphaIIbbeta3 antagonists with prolongation of bleeding time

BACKGROUND

The alphaIIbbeta3 antagonists inhibit platelet aggregation and are used as antithrombotic agents for cardiothrombotic disease. The present study investigates the correlation of inhibition of fibrinogen and von Willebrand factor (VWF) binding by alphaIIbbeta3 antagonists with the inhibition of platelet aggregation and prolongation of bleeding time (BT).

METHODS

Inhibition of fibrinogen and VWF binding were assessed in a purified alphaIIbbeta3-binding assay. As an in vitro cell-based assay, platelet aggregation and VWF-mediated adhesion studies were performed using human platelets. In vivo effects on BT were measured using a template device in dogs at the same time as an ex vivo aggregation study was performed.

RESULTS

In vitro studies demonstrated that the antiaggregatory effects of alphaIIbbeta3 antagonists correlate with their inhibition of fibrinogen binding, but not VWF. Interestingly, the effects of alphaIIbbeta3 antagonists on BT could be differentiated from the inhibition of platelet aggregation. Furthermore, this differentiation was strongly correlated with the different inhibitory potencies between fibrinogen and VWF binding, as well as that between VWF-mediated adhesion and aggregation.

CONCLUSIONS

Our study provides novel evidence showing that the inhibitory effect of alphaIIbbeta3 antagonists on VWF, but not fibrinogen binding, correlates with their ability to prolong BT.

Platelet Preservation With a Glycoprotein IIb/IIIa Inhibitor in a Porcine Cardiopulmonary Bypass Model

BACKGROUND

We tested whether administration of FK633, a short-acting glycoprotein IIb/IIIa inhibitor, before median sternotomy and cardiopulmonary bypass was able to interrupt the platelet activation loop and thereby preserve platelet number and function.

METHODS

This study investigated 16 pigs that underwent median sternotomy and 120 minutes of normothermic cardiopulmonary bypass (100 mL/kg) adding pericardial blood to the perfusate. FK633 was administered with heparin to one group (group F, n = 8), whereas only heparin was administered to the control group (group C, n = 8). Blood samples were obtained at several times, and complete blood count, platelet aggregation to adenosine diphosphate, thrombin-antithrombin complex, and bradykinin were evaluated. P-selectin expression and fibrinogen binding on platelet surfaces were measured by flow cytometry. Template bleeding times were measured before and after cardiopulmonary bypass. Chest tube drainage and hematocrit were determined at 2 and 6 hours after cardiopulmonary bypass.

RESULTS

In group F, platelet counts were preserved from 90 minutes of cardiopulmonary bypass. Platelet aggregation was inhibited at the beginning of cardiopulmonary bypass and showed no change at wound closure, and bleeding times were shortened at 2 hours after cardiopulmonary bypass. There were significant reductions in hematocrit of drainage. Flow cytometry showed no changes in P-selectin expression and fibrinogen binding in group F, whereas P-selectin expression and fibrinogen binding were elevated in group C.

CONCLUSIONS

Platelet inhibition with FK633 before invasive surgical procedure preserved platelet counts during and after cardiopulmonary bypass, and produced normal or near-normal bleeding times in the immediate postoperative period.