Thrombin-induced platelet activation and its inhibition by anticoagulants with different modes of action

Abstract

Thrombin-induced platelet activation involves cleavage of protease-activated receptors (PARs) 1 and 4, and interaction, via glycoprotein (Gp)Ibalpha, with the platelet GpIb/IX/V complex. This study investigated inhibition of platelet activation by thrombin inhibitors with different modes of action: two reversible direct thrombin inhibitors, melagatran and inogatran; hirudin, a tightly binding direct thrombin inhibitor; and two indirect thrombin inhibitors, heparin and dalteparin. Up-regulation of P-selectin (CD62P) and PAR-1 cleavage was measured in human whole blood, by flow cytometry. The thrombin concentration that induced 50% of maximum (EC50 ) PAR-1 cleavage was 0.028 nmol/l, while that of platelet activation (CD62P) was over two-fold higher (0.64 nmol/l). The EC50 of a PAR-1-independent component, defined as a further activating effect of thrombin on top of the maximum PAR-1-activating peptide (AP) effect, was 3.2 nmol/l. All anticoagulants were concentration-dependent inhibitors of thrombin-induced platelet activation and PAR-1 cleavage, but none inhibited PAR-1-AP or PAR-4-AP induced activation. Melagatran and inogatran were more potent inhibitors of CD62P up-regulation than of PAR-1 cleavage; conversely, hirudin, heparin and dalteparin were more potent inhibitors of PAR-1 cleavage.Thus, reversible direct thrombin inhibitors, such as melagatran, are potent inhibitors of thrombin-induced platelet activation, acting mainly by inhibition of a PAR-1-independent component.

Effect of low and high dose melagatran and other antithrombotic drugs on platelet aggregation

“…It appears to interact directly with the GPlba and PAR-4 thrombin receptors without affecting PAR-1, in addition to directly inhibiting all forms of active thrombin. Similar conclusions of receptor specificity were indicated by studies where platelet activation, as measured by expression of P-selectin on the platelet membrane, was monitored under different conditions plus or minus melagatran [10]. Drugs that inhibit all three active forms of thrombin (a, b, c) may prove to be of significant clinical advantage over those that just target a-thrombin when we better understand how the three different platelet thrombin receptors function in normal hemostasis and abnormal thrombosis.…”

“…Other indirect inhibitors block the thrombin receptor(s) on platelets such as the PAR-1 inhibitor, SCH203099, [3] and the PAR-4 inhibitor, histone 1 [4]. Inhibitors that act directly on the thrombin molecule include melagatran, hirudin, heparin and argatroban [5][6][7][8][9][10][11].…”

“…Recent studies demonstrated that these three active forms of thrombin are differentially inhibited by antithrombotic agents [4,12]. The antithrombotic, melagatran, was also shown to inhibit a-thrombin-induced platelet aggregation [10,13].…”

The ATP-gated P2X1 Receptor Plays a Pivotal Role in Activation of Aspirin-treated Platelets by Thrombin and Epinephrine

“…A biphasic model of thrombin-induced platelet activation has been proposed in which PAR1 is initially activated by a low concentration of thrombin (1), possibly aided by binding to glycoprotein Ib␣. Notably, picomolar concentrations of thrombin have been observed to induce 50% of maximum PAR1 cleavage (24). On the other hand, it is assumed that PAR4 is a low affinity thrombin receptor that is cleaved and activated by higher concentrations of thrombin (6).…”

Anesthetic implications of the new anticoagulant and antiplatelet drugs

“…These drugs have been in use for a while in the prophylaxis of venous thromboembolism and prevention of arterial thrombosis [126][127][128][129][130][131][132][133]. Unfortunately, both drugs are no longer FDAapproved and were withdrawn from the market because of their severe adverse effects, mainly cellular liver damage, with elevation of liver enzymes, and the development of subsequent liver failure [134,135].…”