No abstract

The role of thrombin inhibition in platelet vessel wall interaction and thrombus growth was studied under controlled flow conditions. Natural hirudin and recombinant hirudin (r-hirudin), which are specific thrombin inhibitors, were compared with heparinized blood (1.8 +/- 0.2 U/mL) and Ca(2+)-chelated blood in their potential to inhibit platelet interaction and thrombus growth on two biologic vascular surfaces and one immobilized vessel wall component. The substrates were perfused by flowing blood at shear rates typical of patent and stenosed arteries (212 to 1,690/s) for 5 minutes. Platelet deposition was measured by In- 111-labeled platelets. We found that both natural and r-hirudin have similar effects on platelet-substrate interaction. As compared with heparin, platelet deposition to mildly damaged vessel wall and digested collagen type I was not reduced by hirudin or citrate. However, hirudin and citrate significantly reduced platelet deposition to severely damaged vessel wall (platelets x 10(6)/cm2: 93 +/- 10 in heparinized blood v 50 +/- 7 in blood treated with 100 U/mL r-hirudin). Therefore, thrombus growth on areas of severe wall damage is in part dependent on local thrombin production at the site of vascular damage. We also found that hirudin added to heparinized blood reduced platelet deposition to severely injured wall but not to subendothelium or collagen-coated slides. Hirudin added to citrated blood did not affect platelet deposition. Our study indicates that local thrombin generation at the site of severe injury will induce platelet activation and deposition even in the presence of average therapeutic heparin levels that inhibit blood coagulation.

Local therapy with the specific thrombin inhibitor r-hirudin significantly reduces short-term quantitative platelet deposition and macroscopic mural thrombus formation following balloon angioplasty compared with systemic treatment of conventional doses of heparin and hirudin and requires a significantly smaller amount of the recombinant drug.

SummaryThrombin plays a key role in platelet activation and thrombosis. Specific inhibition of thrombin appears to be one of the best approaches to prevent thrombus formation. We have studied the effects of a synthetic a-aminoboronic acid derivative - [Ac, (D) Phe-Pro-Boro-Arg-Hydrocloric acid] - on platelet deposition on severely damaged arterial wall. Platelet deposition was evaluated under well characterized rheological conditions in an original perfusion chamber and detected by autologous mIn-labeled platelets. The study was performed “in vivo” in a porcine model of arterial thrombosis triggered by severely damaged vessel wall at blood flow conditions mimicking mild stenosis (1690 s−1) and patent (212 s−1) vessels. In addition, ex-vivo platelet aggregation activity was evaluated by whole blood impedance aggregometry using collagen, ADP and thrombin as agonists. The synthetic a-aminoboronic peptide was intravenously administered as a bolus followed by continuous infusion. Ex vivo thrombin-induced whole blood platelet aggregation was totally abolished, while ADP- and Collagen-induced whole blood platelet aggregation was not modified. The effects of the synthetic antithrombin on platelet deposition were evaluated in native blood (non-anticoagulated) conditions and in combination with heparin. Under both experimental conditions, the synthetic peptide significantly inhibited platelet deposition at local flow conditions of both high (1690 s−1) and low (212s−1) shear rates. Our results suggest that specific inhibition of locally generated thrombin might be a good strategy to prevent platelet dependent arterial thrombus formation independently of the local flow shear rate of the area at risk.