Contribution of platelets to thrombus formation

Characterization of a unique dihydropyrimidinone, ethyl 4-(4'-heptanoyloxyphenyl)-6-methyl-3,4-dihydropyrimidin-2-one-5-carboxylate, as an effective antithrombotic agent in a rat experimental model

OBJECTIVES

To evaluate the potential of a novel dihydropyrimidinone, ethyl 4-(4'-heptanoyloxyphenyl)-6-methyl-3,4-dihydropyrimidin-2-one-5-carboxylate (H-DHPM), as a calcium channel blocker, endowed with the ability to inhibit platelet aggregation effectively.

METHODS

In-vitro and in-vivo studies were conducted for the determination of antiplatelet activity using adenosine diphosphate (ADP), collagen or thrombin as inducers. Calcium channel blocking activity and nitric oxide synthase (NOS) activity were monitored. Lipopolysaccharide (LPS)-mediated prothrombotic conditions were developed in rats to study the efficacy of H-DHPM to suitably modulate the inflammatory mediators such as inducible NOS (iNOS) and tissue factor. The cGMP level and endothelial NOS (eNOS) expression were checked in aortic homogenate of LPS-challenged rats pretreated with H-DHPM. The effect of H-DHPM on FeCl(3) -induced thrombus formation in rats was examined.

KEY FINDINGS

The concentrations of H-DHPM required to give 50% inhibition (IC50) of in-vitro platelet aggregation induced by ADP, collagen or thrombin were 98.2±2.1, 74.5±2.3 and 180.7±3.4µm, respectively. H-DHPM at a dose of 52.0±0.02mg/kg (133µmol/kg) was found to optimally inhibit ADP-induced platelet aggregation in-vivo. The level of nitric oxide was found to be up to 9±0.08-fold in H-DHPM-treated platelets in-vitro and 8.2±0.05-fold in H-DHPM-pretreated rat platelets in-vivo compared with control. OH-DHPM, the parent compound was found to be ineffective both in-vitro and in-vivo. H-DHPM-pretreated rats were able to resist significantly the prothrombotic changes caused by LPS by blunting the expression of iNOS, tissue factor and diminishing the increased level of cGMP to normal. H-DHPM enhanced the eNOS expression in aorta of rats treated with LPS. H-DHPM displayed synergy with antiplatelet activity of aspirin even at lower doses. H-DHPM was found to inhibit the LPS-induced platelet aggregation in younger as well as older rats. H-DHPM exhibited the ability to markedly decrease FeCl(3) -induced thrombus formation in rats.

CONCLUSIONS

H-DHPM has the attributes of a promising potent antiplatelet candidate molecule that should attract further study. H-DHPM displayed antiplatelet activity both in vivo and in vitro, which was due partially by lowering the intraplatelet calcium concentration.

Halysin, an antiplatelet Arg-Gly-Asp-containing snake venom peptide, as fibrinogen receptor antagonist

By means of Sephadex G-75 and CM-Sephadex C-50 column chromatography and reverse-phase HPLC, a low molecular weight (Mr = 7500), cysteine-rich peptide, halysin, was purified from Agkistrodon halys (mamushi) snake venom. Halysin is a potent platelet aggregation inhibitor that concentration-dependently inhibited human platelet aggregation stimulated by ADP, thrombin and collagen (IC50 = 0.16 to 0.36 microM) without affecting platelet secretion. It was active in inhibiting platelet aggregation of platelet-rich plasma and whole blood. Halysin had no effect on thromboxane B2 formation of platelets or intracellular Ca2+ mobilization of Quin 2-AM loaded platelets stimulated by thrombin. It inhibited the fibrinogen-induced aggregation of elastase-treated platelets. Halysin concentration-dependently inhibited the 125I-fibrinogen binding to ADP-stimulated platelets in a competitive manner (IC50 = 0.16 microM). 125I-Halysin bound to resting platelets (Kd = 1.6 x 10(-7) M) and to ADP-stimulated platelets (Kd = 3.4 x 10(-8) M) in a saturable manner. EDTA, the Arg-Gly-Asp (RGD)-containing snake venom peptides trigamin and rhodostomin, Arg-Gly-Asp-Ser (RGDS), and Gly-Gln-Gln-His-His-Leu-Gly-Gly-Ala-Lys-Gln-Ala-Gly-Asp-Val blocked both 125I-fibrinogen binding and 125I-halysin binding to ADP-stimulated platelets. The monoclonal antibody, 7E3, raised against glycoprotein IIb-IIIa complex blocked both 125I-fibrinogen and 125I-halysin binding, whereas 10E5 had no significant effect on halysin binding to ADP-stimulated platelets, indicating that 7E3 and halysin bind to an epitope which is different from that of 10E5. RGDS concentration-dependently inhibited 125I-halysin binding in a competitive manner. We determined the primary structure of halysin which is a single peptide chain of 71 amino acid residues. An RGD sequence appeared in the carboxy-terminal domain of halysin. Halysin showed about an 85% identical sequence with trigamin which is a specific antagonist of fibrinogen receptor associated with glycoprotein IIb-IIIa complex. In conclusion, halysin inhibited platelet aggregation by interfering with fibrinogen binding to the fibrinogen receptor of the activated platelets. The RGD sequence of halysin plays an important role in the expression of its biological activity.

Possible role of Streptococcus pyogenes in mucocutaneous lymph node syndrome. XII. Variable responses of platelets in MCLS seem to be explainable by streptococcal pyrogenic exotoxin

As a model system for mucocutaneous lymph node syndrome (MCLS), we have advocated and used mice which had been rendered tolerant to Streptococcus pyogenes-associated antigens by neonatal infection with group A beta-hemolytic streptococci, because these mice have shown a variety of peculiar bioimmunological characteristics bearing a striking resemblance to those of MCLS patients. The results of our current investigations reaffirmed the reliability of the animal model by indicating that mice subjected to neonatal infection with S. pyogenes, or inoculation with streptococcal pyrogenic exotoxin (SPE) in Freund's adjuvant, were perfect counterparts of patients with MCLS on account of their platelet activation and hyperaggregability in response to provocative treatment, which are familiar findings in this disease.

Drugs used for disorders of coagulation

Bleeding and thrombosis are altered states of hemostasis. Inhibition of normal hemostatic mechanisms results in spontaneous hemorrhage, whereas excessive hemostasis results in thrombus formation. The drugs used to prevent and treat thrombosis and those used to arrest bleeding are the subjects of this article.

Urinary excretion of thromboxane B2 in patients with venous thromboembolic disease

Platelet activation occurs in the initial phase of venous thrombus formation. To determine if thromboxanes (Tx) are released during this process and if Tx measurements are useful in the diagnosis, urinary immunoreactive TxB2 was measured by a rapid, inexpensive assay in 100 consecutive patients with suspected thromboembolic disease. Urinary iTxB2 was not increased in patients who took aspirin, nor in patients studied several weeks after onset of symptoms. Of the remaining patients, iTxB2 was increased in 11 of 15 with confirmed deep vein thrombosis and in seven of ten with confirmed pulmonary emboli. Of the 54 patients in whom acute thrombosis was excluded, iTxB2 was increased in only four (7 percent). A second study evaluated 25 additional patients with nondiagnostic lung scans who required pulmonary angiography; iTxB2 was increased in seven of ten with positive angiograms and in 0 of 15 with negative angiograms. The three patients with negative iTxB2 and positive angiograms were receiving heparin when studied. These data suggest that, in the absence of aspirin, platelet Tx is released during thrombus formation. In combination with other noninvasive tests, urinary iTxB2 is a useful adjunct to diagnosing acute thromboembolic disease.

beta-thromboglobulin and platelet factor 4 levels in retinal vein occlusion

Fifty-six patients with retinal vein occlusion--35 with central and 21 with branch vein occlusion--were investigated for comparison with an age and sex matched control group. Mean levels of beta-thromboglobulin and platelet factor 4 were significantly higher (p less than 0.001) in both the group with central and the group with branch retinal vein occlusion than in the control group. A significant increase of beta-thromboglobulin (p less than 0.001) was also found in the retinal vein occlusion group in those patients who were not hyperlipidaemic or diabetic (n = 39). Weak correlations were found between levels of lipoprotein cholesterol and plasma beta-thromboglobulin. Increased platelet aggregation may contribute to the aetiology of retinal vein occlusion.

Endotoxin, prostaglandins and renal fibrin deposition in obstructive jaundice

The delayed clearance of endotoxins in obstructive jaundice may cause renal impairment by inducing renal vasoconstriction and glomerular fibrin deposition as a consequence of intravascular coagulation. As endotoxins activate arachidonic acid metabolism we have examined the effects of selective inhibitors on mortality, plasma TXB2 and 6-oxo-PGF1 alpha production and renal fibrin deposition in rats with obstructive jaundice following endotoxin administration. Jaundiced rats had a high mortality following endotoxin--58 per cent at 4 h and 83 per cent at 24 h. Pretreatment with indomethacin 3 mg/kg i.p., dazoxiben 3 mg i.p. or prostacyclin 300 ng/kg i.v. produced significant improvements in survival. Endotoxaemia was associated with significant elevations of plasma TXB2 and early inhibition of plasma 6-oxo-PGF1 alpha generation. Renal fibrin deposition, assessed using indirect immunofluorescence and a 125I-labelled fibrinogen uptake ratio, occurred in jaundiced kidneys following endotoxin and could be prevented using indomethacin, dazoxiben and prostacylin. These results suggest that endotoxin-induced TXA2 production can cause renal fibrin deposition in obstructive jaundice, thus contributing in the pathogenesis of the renal impairment.

Enhanced in vivo platelet release reaction and malondialdehyde formation in patients with hyperlipidemia

Plasma level of beta-thromboglobulin (beta TG), a useful marker of in vivo platelet "release reaction,"was determined by radioimmunoassay in 69 patients, with three types of primary hyperlipidemia (IIa, IIb, IV) and compared with the findings in age- and sex-matched healthy controls and 57 patients with established atherosclerosis and peripheral vascular disease. Malondialdehyde (MDA) formation, used for assessment of prostaglandin synthesis, was determined in 51 and plasma platelet factor 4 (PF4), measured by radioimmunoassay, in 48 of the patients with hyperlipidemia. Results were correlated to five serum lipids and lipoprotein levels in the patients with hyperlipidemia. beta TG was significantly increased in the patients with hyperlipidemia and peripheral vascular disease, compared to those in the controls (p < 0.001); it was significantly higher in the patients with hyperlipidemia than in those with peripheral vascular disease. PF4 and MDA formation were also increased in the patients with hyperlipidemia, and significantly higher levels of MDA were obtained in patients with type IIb and type IV hyperlipidemia than in those with type IIa hyperlipidemia (p < 0.02). beta TG and MDA correlated weakly with total serum cholesterol triglycerides and very low density lipoprotein-triglyceride. There was also a significant correlation between beta TG and PF4, and MDA production. These results indicate that in vivo platelet "release reaction" and MDA formation are increased in hyperlipidemic patients. The release reaction is more enhanced in those with hyperlipidemia than in the patients with peripheral vascular disease. They suggest that the abnormal platelet function is related to the elevated levels of serum lipids and lipoproteins in the hyperlipidemic patients and not only to the atherosclerotic changes associated with hyperlipidemia.

Temporary ischaemia induced by degradable starch microspheres. Possible thrombogenic effects in vivo and in vitro

Possible thrombogenic effects of degradable starch microspheres were investigated. Controlled temporary small intestinal ischaemia ws induced by injection into the superior mesenteric artery in cats. Arterial flow consistently recovered after ischaemia. No consumption of blood platelets, fibrinogen, or Factor VIII was observed. Aggregation of human platelets was not influenced by microsphere exposure, and platelet retention in starch microsphere columns was minimal. No thrombosis was detected in feline small intestinal vessels in vivo nor did starch surfaces induce adhesion or aggregation of human platelets in vitro. Thus, no evidence of thrombotic hazards was found by inducing temporary intestinal ischaemia by starch microspheres.

Circulating platelet aggregates: a chronic platelet activation in patients with transient ischaemic attacks

Platelet hyperaggregability was tested in 62 TIA patients by means of the formalin fixing principle of Wu and Hoak. Compared to age-matched controls, the platelet aggregates ratio (PAR) was reduced from 0.99 +/- 0.04 SD to 0.807 +/- 0.14 SD. This means a highly significant increase of circulating platelet aggregates (CPA) in untreated TIA patients, regardless of the interval sinche the last attack (from 2 days to 1 year). PAR was especially reduced in patients with abnormal angiography (expressing pronounced cerebral or diffuse ATS) and it was possibly affected by previous antiplatelet treatment with ASA.

Platelet-release reaction in myocardial infarction

A study was made of the platelet-release reaction in heparinised platelet-rich plasma taken from 26 patients, of whom 22 had sustained a definite and four a possible myocardial infarction, and from 54 age-matched controls. No significant differences in reaction were observed between the two groups. Signficant differences were seen, however, between eight patients who died within a year after infarction and the controls (P less than 0.01) and the remaining 18 patients who survived (P = 0.02). These differences were abolished when sodium citrate was included in the experimental procedure. Poor prognosis was thus related to an increased platelet-release reaction after infarction.

beta-Thromboglobulin, platelet production time and pletelet function in vascular disease

Plasma beta-thromboglobulin (beta TG) levels were measured in 103 healthy controls and 112 patients suffering from either peripheral vascular disease (PVD), or cerebrovascular disease (CVD) or deep vein thrombosis (DVT). Plasma beta TG was significantly elevated in 46 PVD patients and 24 recent DVT patients compared to controls, but did not differ significantly in 18 chronic DVT and 24 old CVD patients. In addition, heparin neutralizing activity (HNA) and platelet aggregation induced by adenosine diphosphate, 1-epinephrine and thrombin were compared in 33 out of the 46 PVD patients to 33 controls. The mean HNA was significantly shorter in the PVD patients than in controls. The rate and extent of platelet aggregation were increased in PVD patients compared to controls, but the difference was not statistically significant. Platelet production time (PPT) was measured in 20 controls, 35 PVD patients, nine chronic DVT and 12 chronic CVD patients; significantly shorter PPT was only observed in 14 patients with advanced PVD compared to controls, suggesting increased platelet consumption in these patients. All four assays (plasma beta TG, HNA, platelet aggregation and PPT) were performed in 25 patients; no correlation between the four tests was found in these patients suggesting that the tests were measuring various aspects of platelet function. These results suggest that in vivo platelet consumption as well as platelet aggregation and 'release reaction' are presumably enhanced in PVD and recent DVT patients and that plasma beta TG and PPT assays may be better and more specific indicators of in vivo platelet activation than in vitro platelet aggregation test.