Effect of non-heparin thrombin antagonists on thrombin generation, platelet function, and clot structure in whole blood

Platelet contractile force (PCF), which is absent in blood obtained during cardiopulmonary bypass, significantly recovers after protamine sulfate administration. In vitro studies reveal this effect to be primarily caused by heparin. Because many of heparin's effects are mediated by suppression of thrombin generation and activity, this study assessed the influence of thrombin inhibition on PCF. The effects of natural and synthetic antithrombins were measured. Clots were formed by the addition of batroxobin (0.21 μg/mL) to whole blood (platelet count 200,000/μL). Force development was measured from the moment of batroxobin addition. After 1200 s of clotting, purified antithrombin III (22 μM) reduced PCF by 74%. Thrombomodulin (0.014 μM) reduced PCF by 60%. At 0.040 μM, PCF was reduced by 82% (6.5-1.2 Kdynes). Hirudin decreased PCF in a dose-dependent fashion, with complete suppression at concentrations ≥ 0.30 μM. At concentrations between 0.04 and 0.29 μM, Lepirudin (Refludan, a recombinant therapeutic hirudin) produced dose-dependent delay and suppression of PCF. Above 0.29 μM Lepirudin, PCF was totally suppressed. At 1.60 μM, bivalirudin (a synthetic, 20 amino acid peptide) delayed and reduced PCF by 50%. At 6.40 μM, PCF was completely suppressed. Although 20 μM of P-PACK II (d-Phenylalanyl-L-Phenylalanyl-arginine-chloro- methyl ketone 2 HCl) had little effect, 40 μM delayed onset of force development from 300 to 600 s and reduced PCF at 1200 s from 5.2 to 3.3 Kdynes. At 120 μM, force development was totally suppressed. Four micromol Thromstop® (BNas-Gly-(pAM)Phe-Pip) delayed force development by greater than 800 s and PCF at 1200 s was reduced by 70%. At 0.20 μM, Argatroban (a synthetic polypeptide direct thrombin antagonist) delayed onset of PCF from 300 to 540 s and decreased PCF by 40%. At a concentration of 0.40 μM and above, Argatroban totally suppressed PCF. These results indicate that some of the antiplatelet effects of heparin are the result of thrombin inhibition and that low-level thrombin generation is essential for clot retraction. The sensitivity of PCF to the presence of thrombin may permit monitoring of antithrombin agents via this assay.