In vitro Models of Thrombosis

The statistical significance is evaluated according to the procedure described for flow behavior of erythrocytes described above. 

One of the principal methods for measuring viscosity is based on the rate of flow of a liquid through an orifice according to Harkness (1971). In this test, a defined volume of plasma is transferred into a capillary viscometer and the efflux time required for the plasma to flow from the upper to the lower mark is measured. Using this procedure, the effect of test compounds on the viscosity of blood plasma can be determined. The test can be carried out either ex vivo or in vitro. 

Beagle dogs weighing 12-20 kg, or rabbits weighing 2.0-3.0 kg or Wistar rats weighing 150-300 of either sex are used as test animals. Likewise, the test procedure can be performed in man. The test subjects receive the test compound by oral, subcutaneous or intravenous administration 15, 60, 90 or 180 min before the withdrawal of blood. 

Following addition of the test compound, plasma (obtained as described below) is incubated at 37 °C for 5 or 30 min. 

Freshly collected venous blood is anticoagulated with 1 mg/ml blood K+-EDTA or heparin sodium (5 IU/ml blood) and centrifuged at 3000 rpm for 5 min. The supernatant (plasma) is removed and a sample of 0.9 ml plasma is transferred into a capillary viscometer 

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Abulencia JP, Tien N, McCarty OJT et al. (2001) Comparative antiplatelet efficacy of a novel, nonpeptide GPIIb/IIIa antagonist (XV454) and Abciximab (c7E3) in flow models of thrombosis. Arterioscler Thromb Vase Biol 21 149-156 McCarty OJ, Abulencia JP, Mousa SA, Konstantopoulos K (2004) Evaluation of platelet antagonists in in vitro flow models of thrombosis. Methods Mol Med 93 21-34... 

De ite differmces in their mechanisms of action and in vitro activities, pentasaccharide, DX-9065a and TAP have been shown to be effective antithrombotic agents in experimental models of venous thrombosis, coronary artery occlusion, arterial thrombolysis and acute reocclusion, restenosis after angioplasty, dialysis, and DIG. Pentasaccharide has also demonstrated measurable antithrombotic effects in human trials. Both TAP and DX-9065a produce measurable in vitro anticoagulant effects. In contrast, pentasaccharide does not produce an anticoagulant effect by the typical clot based assays. Thus, with fector Xa inhibitors there is not necessarily a correlation between current lab assays and antithrombotic efficacy as there is with heparin. 

A number of reports on to vivo studies of the antithrombogenic properties of aspirin have appeared recently. In this regard, it is important to realize that the validity of animal models of thrombosis remains open to question. Platelets from aspirin treated rabbits were found to be morphologically normal, but did not aggregate in the presence of collagen to vitro.23 Thrombus formation induced in rats by typhosa endotoxin was inhibited by aspirin previously administered by stomach tube. 4 Platelets from these animals exhibited a reduced tendency to aggregate vitro and the prevention of thrombus formation could be... 

Pyrlmidopyrimidlnes - Dipyridamole and two analogs, RA233 and RA433, have been studied extensively as platelet aggregation Inhibitors rn vitro and in various animal models of thrombosis.53,54 The mechanism of action of these compounds on platelets appears to involve inhibitory effects on glucose metabolism55 55 cydic AMP phosphodiesterase.27... 

In contrast to unfractionated heparin, the factor Xa inhibitor tick anticoagulant peptide (TAP) effectively inhibited coronary arterial thrombosis in a canine electrolytic injury model (57). TAP was also effective in inhibition of the procoagulant properties of whole blood clots in vitro however, it was stated that TAP might be not optimal due to its slow binding kinetics (54). Meanwhile, several low molecular weight direct factor Xa inhibitors are in clinical development (Table I), some of them specifically for the treatment and secondary prevention of ACS. DX-9065a, ZK-807834 and otamixaban have been intensively characterized in vitro and in vivo and are in clinical investigations for the treatment of acute arterial thrombosis. 

PEUs were used since they have excellent hydrolytic stabihty. In the 1980s two types of trileaflet PUR valves made of Biomer (Ethicon) and Lycra Spandex (DuPont) were developed. The Lycra valves showed low regurgitation in vitro, and lasted for one year or more in large animal models, but calcification and thrombosis became evident in growing sheep (Wisman et al., 1982). Calcification was also seen with Biomer valves in juvenile sheep (Hilbert et al., 1987). 

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