Effect of a pyrimidopyrimidine derivative on thrombus formation in the rabbit

Bicyclic 6 + 6 systems: the chemistry of pyrimido[4,5-d]pyrimidines and pyrimido[5,4-d]pyrimidines

The present study provides an overview of the chemistry and biological significance of pyrimido[4,5-d]pyrimidine and pyrimido[5,4-d]pyrimidine analogs as types of bicyclic [6 + 6] systems. The main sections include: (1) synthesis methods; (2) the reactivities of the substituents linked to the ring carbon and nitrogen atoms; and (3) biological applications. A discussion demonstrating the proposed mechanisms of unexpected synthetic routes is intended. The aim of this study is to discuss the synthetic significance of the titled compounds and to establish the biological characteristics of this class of compounds as studied to date, where the compounds have been applied on a large scale in the medical and pharmaceutical fields. This survey will help researchers in the fields of synthetic organic and medicinal chemistry to undertake and improve new approaches for the construction of new standard biological components.

Clinical pharmacokinetics of antiplatelet agents used in the secondary prevention of stroke

Stroke is one of the leading causes of death and debilitation. Several million stroke survivors are alive throughout the world today. Prevention of recurrent stroke is of major importance to stroke survivors. Several pharmacological agents are currently available for use in secondary stroke prevention.Clopidogrel, the combination of immediate-release aspirin and extended-release dipyridamole and aspirin alone are the most widely recommended agents for use in the secondary prevention of strokes. Clopidogrel has shown superiority over aspirin in the combined endpoints of stroke, death and myocardial infarction. The immediate-release aspirin/extended-release dipyridamole combination has shown superiority to aspirin alone in the secondary prevention of stroke. Dipyridamole has been studied as an antiplatelet agent for several decades. Early trials to prove its efficacy compared with aspirin were not favourable, and patients often experienced many adverse effects. Researchers began developing an extended-release formulation in an effort to maintain therapeutic blood concentrations with less frequent daily administration and better adverse effect profile. Pharmacokinetic analysis of this new product showed it to have a more consistent and reproducible absorption compared with immediate-release dipyridamole. The rate of absorption of extended-release dipyridamole is considerably slower than that of immediate-release dipyridamole, while similar plasma concentrations are maintained to optimise antiplatelet efficacy. This allows extended-release dipyridamole to be administered twice daily rather than four times daily.A large-scale randomised trial was conducted with extended-release dipyridamole 200mg in combination with immediate-release aspirin 25mg given twice daily. The combination product showed a greater efficacy at preventing a recurring stroke then either agent administered alone. Indirect comparisons with clopidogrel show that the combination of immediate-release aspirin/extended-release dipyridamole may be more effective than clopidogrel at preventing a recurring stroke.

Aggrenox: a fixed-dose combination of aspirin and dipyridamole

OBJECTIVE

To describe the pharmacology, pharmacokinetics, efficacy, and safety of a fixed-dose combination of aspirin and extended-release (ER) dipyridamole indicated for the secondary prevention of stroke.

DATA SOURCES

Published articles and abstracts were identified from a MEDLINE search (1966-December 1999) using the search terms dipyridamole, aspirin, antiplatelet, antiaggregation, and stroke prevention. Pertinent articles written in English were considered for review. Additional articles were identified from the references of retrieved literature.

STUDY SELECTION AND DATA EXTRACTION

Studies including a combination of aspirin/dipyridamole in human subjects were evaluated. Emphasis was placed on randomized, controlled trials.

DATA SYNTHESIS

Aspirin is a platelet inhibitor that works by inhibiting platelet cyclooxygenase, which reduces the production of thromboxane A2. Dipyridamole is a platelet inhibitor that is thought to work in part by inhibiting platelet cyclic-3',5'-adenosine monophosphate and cyclic-3',5'-guanosine monophosphate phosphodiesterase. The active metabolite of aspirin, salicylic acid, is highly bound to plasma protein and has a plasma half-life of two to three hours. Dipyridamole is also highly bound to plasma proteins, and the ER formulation has a plasma half-life of 13 hours. The first European Stroke Prevention Study (ESPS-1) found the combination of aspirin/dipyridamole to be superior to placebo in the prevention of stroke and transient ischemic attack (TIA). The ESPS-1, however, did not include an aspirin-only treatment arm. Therefore, it was unclear whether the combination of aspirin/dipyridamole was superior to aspirin alone. As a result, a second trial was conducted that included treatment arms of aspirin alone, ER dipyridamole alone, combination therapy, and placebo. The combination of aspirin 25 mg plus ER dipyridamole 200 mg twice daily was shown in the ESPS-2 to be significantly better than either agent given individually in preventing stroke and TIAs (p < 0.001).

CONCLUSIONS

The American College of Chest Physicians (ACCP) recommends aspirin 50-325 mg/d to be the initial antiplatelet of choice for the prevention of atherothrombotic cerebral ischemic events. However, with the favorable results of the ESPS-2, it may be appropriate to substitute aspirin/ER dipyridamole for aspirin alone as the drug of choice. This combination appears to have a favorable adverse effect profile. The relative effectiveness of aspirin/ER dipyridamole compared with clopidogrel and ticlopidine has yet to be determined. If alternative antiplatelet therapy is needed, the ACCP recommends clopidogrel rather than ticlopidine because of its lower incidence of adverse effects. The ACCP further states that the combination of aspirin plus dipyridamole may be more effective than clopidogrel; these agents have a similarly favorable adverse effect profile.

Efficacy of dipyridamole as prophylaxis for stroke and the added value of dipyridamole in combination with aspirin

Although dipyridamole was identified as a potential antithrombotic agent many years ago,(1) its effectiveness as a single agent in secondary prevention of arterial thrombosis in man has never been proven in properly conducted clinical trials with sufficient power. In many clinical trials it has been tested in combination with aspirin, and until recently meta-analyses have failed to show any significant benefit of the combination of aspirin and dipyridamole beyond that shown with aspirin alone.(2,3).

Dipyridamole directly inhibits vascular smooth muscle cell proliferation in vitro and in vivo: implications in the treatment of restenosis after angioplasty

OBJECTIVES

The effect of dipyridamole on smooth muscle cell proliferation and prevention of intimal thickening after arterial injury was investigated.

BACKGROUND

In addition to antiplatelet activity, dipyridamole also inhibits cell proliferation. We examined whether the antiproliferative action of dipyridamole on smooth muscle cells, as demonstrated here, has a direct effect on intimal thickening after vascular injury.

METHODS

Cell proliferation was determined by measuring deoxyribonucleic acid (DNA) synthesis and by cell counting. The in vivo effect of locally delivered dipyridamole was determined in a rabbit model with carotid or femoral artery injury.

RESULTS

Dipyridamole produced a dose-dependent inhibition of smooth muscle cell proliferation, producing 50% inhibition at 7 micrograms/ml. Structural analogues SH-869 and mopamidol were 10 to 100 times less effective than dipyridamole, suggesting that cell growth inhibition may be unrelated to the antiplatelet activity of dipyridamole. Inhibition of cell proliferation by dipyridamole was attenuated by increasing the serum concentration in the culture medium. Bypassing serum by local delivery of dipyridamole at the periadventitial site produced 63% inhibition (p < 0.05) of cell replication in balloon-injured arteries. Locally delivered dipyridamole also inhibited intimal thickening (20%, p < 0.05) after balloon injury.

CONCLUSIONS

Dipyridamole inhibited smooth muscle cell proliferation in vitro. This activity was attenuated by serum proteins. Locally delivered dipyridamole inhibited cell replication in arteries and intimal thickening after balloon injury. These results suggest that although systemic treatment with dipyridamole may not be efficacious because of inadequate serum levels, its antiproliferative action on smooth muscle cells may reduce restenosis when the drug is delivered locally after coronary angioplasty.

Actions of dipyridamole on endogenous and exogenous noradrenaline in the dog mesenteric vein

1. In the isolated mesenteric vein of the dog, dipyridamole inhibited both the excitatory junction potential (e.j.p.) and the slow depolarization evoked by perivascular nerve stimulation, to 60-70% of control, with no change in the postjunctional membrane potential. These inhibitory actions of dipyridamole were not modified by 8-phenyltheophylline or phentolamine, suggesting that the inhibition did not involve either the actions of endogenous adenosine or the prejunctional alpha-autoregulation mechanism. 2. Dipyridamole did not produce any detectable effects on either the facilitation process of the e.j.ps or the postjunctional membrane depolarization produced by exogenously applied noradrenaline (NA). 3. Dipyridamole reduced the outflow of both the NA and the 3,4-dihydroxyphenylglycol (DOPEG) evoked by perivascular nerve stimulation to below 10% of control, the effect being much greater than that of exogenously applied adenosine (to about 90% of the control). 4. Exogenously-added NA was degraded by incubation with a segment of the vein. Dipyridamole itself produced degradation of NA and accelerated the NA-induced degradation. By contrast, pyrogallol, but not pargyline or imipramine, prevented the NA-induced degradation. 5. It is suggested that dipyridamole degrades NA directly, and also indirectly through activation of catechol-O-methyl transferase, with no alteration of the activity of monoamine oxidase or of the uptake mechanisms of NA into nerve terminals.

Dipyridamole inhibits O2- release and expression of tissue factor activity by peripheral blood monocytes stimulated with lipopolysaccharide

Monocytes can be induced to synthesize and express tissue factor procoagulant activity. They can also be stimulated to release a broad spectrum of inflammatory agents including superoxide anion (O2-) that are thought to contribute to the pathogenesis of inflammatory diseases. Dipyridamole, an inhibitor of platelet aggregation blocks the lipopolysaccharide (LPS)-induced increase in monocyte-associated tissue factor activity and phorbol myristate acetate (PMA) stimulated O2- release from monocytes and polymorphonuclear leukocytes (PMN). Dipyridamole inhibition of O2- release can be reversed by increased glucose in the culture media, whereas dipyridamole inhibition of tissue factor can not be reversed by increased glucose in the culture media. These results reveal that dipyridamole influences monocytes by at least two distinct mechanisms. Further, it may serve as an anti-thrombotic agent by virtue of its effect on both platelet aggregation and monocyte tissue factor activity.

Dipyridamole specifically decreases platelet-derived growth factor release from platelets

The authors have previously reported that dipyridamole decreased platelet-derived growth factor levels in human serum by lowering the release of this factor during blood clotting. In the present study, we have shown that this effect is specific to dipyridamole, and does not occur with other antiplatelet drugs such as aspirin, trapidil or ticlopidine. In addition, dipyridamole has been shown to decrease the PDGF level selectively, but not the levels of other factors from alpha granules in platelets (beta-thromboglobulin and platelet factor 4). These data indicate that dipyridamole may be an effective drug for the prevention of PDGF-related disorders.

Extracellular adenine compounds, red blood cells and haemostasis: facts and hypotheses

Previously, the role of adenine nucleotides was thought to be confined to the intracellular space of the cell. Research of the last decades has revealed that nucleotides also occur in the extracellular milieu. This survey deals with extracellular adenine compounds in the blood, focussing on their role as chemical mediators in the haemostatic effect of red cells. Erythrocytes may act as pro-aggregatory cells by at least two chemical mechanisms. Firstly, they can enhance platelet aggregation by releasing adenosine diphosphate (ADP), a well known platelet stimulatory substance. ADP is set free when red cells are stressed mechanically, for instance by shear forces generated in the blood stream; ample experimental evidence supporting this view is summarized. Secondly, erythrocytes efficiently take up extracellular adenosine via their nucleoside transporters, thereby removing a potent inhibitor of platelet function. Extracellular adenosine occurs in the blood stream, either directly released from various tissues or as the end product of extracellular adenine nucleotide metabolism, e.g. after degradation of red cell-born ADP or ATP. Finally, a novel mechanism of action of the antithrombotic drug dipyridamole, which has very recently been put forward, is demonstrated. Dipyridamole inhibits platelet function indirectly by blocking the uptake of extracellular adenosine via the nucleoside transporter of red cells; increased adenosine levels in turn are responsible for the antiaggregatory effect of dipyridamole.

Adenosine uptake site heterogeneity in the mammalian CNS? Uptake inhibitors as probes and potential neuropharmaceuticals

Inhibitors of adenosine uptake or transport have been used clinically for some time in certain cardiovascular diseases. More recently, some of them have also been investigated for possible clinical use in combination with antimetabolites based on the observed heterogeneity of nucleoside transport in mammalian tumor cells. Such a heterogeneity of adenosine uptake and uptake sites has now also been suggested in the mammalian CNS. The aim of this article is, therefore, to review the present status of our knowledge of adenosine uptake in the mammalian CNS, compare it with our far more advanced knowledge of nucleoside transport in other mammalian cells and suggest direction of future research. The possible implications for the development of adenosine uptake inhibitors as adenosinergic neuropharmaceuticals will be discussed based on our knowledge of the physiological function of adenosine in the CNS.

Synergistic inhibitory effects of dipyridamole and vincristine on the growth of human leukaemia and lymphoma cell lines

The effects of combinations of dipyridamole, an effective blocker of the salvage pathway of DNA synthesis, and 8 types of anti-cancer drugs on the growth of human T, B and myeloid leukaemia/lymphoma cell lines in vitro were examined. In combinations, dipyridamole and vincristine (VCR), and dipyridamole and vindesine had synergistic inhibitory effects. Dipyridamole reduced the efflux of VCR from cells and enhanced their VCR accumulation in a dose-dependent manner at concentrations of up to 10 microM in the lymphoid cell lines, MOLT-3 and BL-TH, and of up to at least 20 microM in the myeloid cell line, ML-1. Dipyridamole also enhanced the accumulation of VCR in PHA-stimulated and un-stimulated lymphocytes of normal donors, but efflux of VCR was more rapid from normal lymphocytes than from cultured cell lines. It is proposed that combination therapy with dipyridamole plus VCR should be effective in the treatment of leukaemia and lymphoma.

Dipyridamole and vascular prostacyclin production

The action of dipyridamole on the vascular production of prostacyclin (PGI2) has been investigated. Dipyridamole (1-100 microM) did not induce a significant stimulation of PGI2 release in any of the following experimental models: rings of rabbit aorta, cultured endothelial cells from bovine aorta or human umbilical vein, cultured explants of bovine aortic smooth muscle. The activity of known stimuli of PGI2 release (ADP, suloctidil, serotonin) and the capacity of dipyridamole to inhibit adenosine uptake into endothelial cells were carefully checked. Pretreatment of the rabbit aorta with dipyridamole (10-100 microM) prolonged the transient stimulation of PGI2 release induced by mechanical deendothelialization: this effect was probably due to a partial protection of the cyclooxygenase against oxidative self-inactivation. Our largely negative results are consistent with the current theory that the antiplatelet action of dipyridamole is mediated by adenosine and not by PGI2.

Dipyridamole-induced epistaxis

The case history of a patient treated with dipyridamole who developed severe epistaxis, accompanied by alteration of the platelet aggregation tests, is presented. The bleeding did not respond to local therapy and stopped only after discontinuation of the drug and normalization of the platelet aggregation. Hemorrhage, especially in elderly patients, should be considered as a possible complication of dipyridamole administration.

Persantine-Aspirin Reinfarction Study. Part II. Secondary coronary prevention with persantine and aspirin

In the Persantine-Aspirin Reinfarction Study, Part II (PARIS II), 3,128 persons who had recovered from myocardial infarction, suffered from 4 weeks to 4 months previously, were randomized into two groups: dipyridamole (Persantine) plus aspirin (n = 1,563) and placebo (n = 1,565). The average length of follow-up was 23.4 months. Prespecified primary end points were coronary incidence (definite nonfatal myocardial infarction plus death due to recent or acute cardiac event), coronary mortality (death due to recent or acute cardiac event) and total mortality, each at 1 year of patient follow-up and at the end of the study. Coronary incidence in the Persantine plus aspirin group was significantly lower than in the placebo group, both at 1 year (30% reduction) and at the end of the study (24% reduction). The statistically significant differences in coronary incidence, at 1 year and at the end of the study, in favor of the combination treatment remained after adjustment for multiple baseline variables and adjustment for multiple testing (three end points for two time periods). Although there were reductions for other end points, these differences were not statistically significant. Coronary mortality was 20% lower in the Persantine plus aspirin group compared with the placebo group at 1 year, and 6% lower overall. Total mortality in the treated group compared with the placebo group was 11% lower at 1 year and 3% lower overall. The reduced rates of coronary incidence largely reflected lower rates of definite nonfatal myocardial infarction in the Persantine plus aspirin group. Several subgroups were defined a priori and at the end of the study. The beneficial effect of Persantine plus aspirin compared with placebo for coronary incidence tended to be greater for the following groups of patients: those who had a non-Q wave infarct; those who were not taking digitalis; those who were receiving beta-receptor blocking drugs at baseline; those who were in New York Heart Association functional class I; those who had had only one myocardial infarction; or those who were enrolled in the study early, that is within 85 days of the qualifying myocardial infarction.

Effect of dipyridamole on experimental disseminated intravascular coagulation in rats

Experimental disseminated intravascular coagulation (DIC) can be induced by 4-h sustained infusion of endotoxin in a dose of 100 mg/kg in rats. The experimental model of DIC in rats was used to study the preventive effect of dipyridamole against DIC. Before the infusion of endotoxin, 0.5, 5.0 or 50.0 mg/kg of dipyridamole was injected intraperitoneally. The preventive effect against DIC was noted in all the parameters, such as fibrinogen and fibrin degradation products, fibrinogen level, prothrombin time, partial thromboplastin time, platelet count, and the number of renal glomeruli with fibrin thrombi, in rats treated with 5.0 or 50.0 mg/kg of dipyridamole. From these results, it was shown that dipyridamole inhibited the aggravation of endotoxin-induced experimental DIC in rats.

Mechanism of action of dipyridamole

Dipyridamole appears to act in vivo by synergistically modifying several biochemical pathways, including: a) inhibition of platelet cAMP-phosphodiesterase; b) potentiation of adenosine inhibition of platelet function by blocking reuptake by vascular and blood cells, and subsequent degradation of adenosine; and possibly, c) potentiation of PGI2 antiaggregatory activity and enhancement of PGI2 biosynthesis. These independent processes inhibit platelet function by increasing platelet cAMP through both a reduction in enzymatic cAMP-degradation, and stimulation of cAMP formation via activation of adenylcyclase by adenosine and possibly PGI2. Only the inhibition of cAMP phosphodiesterase appears to be involved in the dipyridamole inhibition of isolated platelets in vitro, since adenosine and PGI2 originate in vivo from tissues other than platelets and any blood concentrations existing in vivo will disappear before platelet-rich plasma has been prepared for in vitro platelet studies. The antithrombotic effects of dipyridamole in a baboon model of arterial thromboembolism are unaffected by simultaneous administration of dazoxiben, a specific thromboxane synthetase inhibitor, but are optimally potentiated by the simultaneous addition of aspirin in doses of 20 mg/kg/day. Since this dose of aspirin has no detectable antithrombotic effects when used alone, but blocks vascular PGI2 synthesis, the antithrombotic effects of dipyridamole, at least in this model, appear to be independent of prostacyclin.

Myocardial biosynthesis of prostaglandins with special consideration of prostacyclin and the influence of dipyridamole and propranolol

Experiments with isolated perfused hearts of guinea pigs and rats showed that cardiac action is linked to formation of prostaglandinlike substances (PLS) and prostacyclin (PGI2). Perfusion of the hearts with arachidonic acid or pretreatment with a linoleic-acid-supplemented diet significantly increased the content of PLS and PGI2 and exerted an economic effect on the heart performance. Dipyridamole induced a marked increase in the coronary flow and PGI2 formation of the hearts but decreased the enhanced myocardial PGI2 biosynthesis after perfusion with arachidonic acid. Propranolol also caused a rise in PGI2 efflux but did not show any influence on PGI2 formation after arachidonic acid. Dipyridamole and propranolol prevent decreased PGI2 formation after acetylsalicylic acid, supporting the view that a combination of these drugs exerts a preventive effect in patients with angina pectoris and heart infarction.

The effect of intravenous dipyridamole on the cerebral and systemic circulations of the dog

In 7 dogs anesthetized with halothane and nitrous oxide, dipyridamole was administered in a loading dose of 1 mg/kg supplemented with 0.5 mg/kg every 30 minutes. Cardiovascular parameters and organ blood flows (using the radioactive microsphere technique) were measured before and at 30 minute intervals after each administration of dipyridamole, for a total of 105 minutes. The administration of dipyridamole was associated with a 20% reduction in systemic arterial pressure, a 31% reduction in peripheral vascular resistance, and a 13% increase in cardiac index. Cerebrovascular resistance decreased 21%, but regional cerebral blood flow and metabolism were unchanged. Blood flow to the heart increased 355% in the right ventricle and 213% in the left ventricle. Blood flow to the jejunum decreased 52% while blood flow to the kidney and liver decreased slightly. The circulatory effects of dipyridamole are probably related to its interference with the inactivation of endogenous adenosine. The differential effects of dipyridamole on organ flow are similar to those seen following the IV infusion of adenosine.

Lack of effect of ischemia and dipyridamole on prostacyclin production in arteriosclerosis obliterans

Six patients with advanced arteriosclerosis obliterans in the lower extremities were subjected to an exercise test on a tread mill with and without dipyridamole treatment. Prostacyclin (PGI2) release was measured by the concentration of its stable metabolite, 6-keto-prostaglandin F1 alpha in plasma. All the patients suffered from ischemic pain during both tests, but no changes were seen in plasma 6-keto-PGF1 alpha. Dipyridamole did not affect the physical performance. Our results suggest that atherosclerotic vessels do not increase PGI2 production in response to ischemia and that a single dose of dipyridamole does not change PGI2 production.

Effect of various doses of acetylsalicylic acid in combination with dipyridamole on the balance between prostacyclin and thromboxane in human serum

1 Thirty-six healthy human subjects were randomly divided into six groups which were treated with a single dose of 75 mg (1.3 mg/kg) of dipyridamole alone, or 75 mg of dipyridamole in combination with 30 mg (0.5 mg/kg), 50 mg (0.8 mg/kg), 160 mg (2.6 mg/kg) and 330 mg (5.7 mg/kg) of acetylsalicylic acid (ASA), or with placebo. 2 The concentrations of prostacyclin (PGI2) and thromboxane A2 (TxA2) metabolites, 6-keto-prostaglandin F1 alpha (6-keto-PGF1 alpha) and TxB2 respectively, were measured in serum with specific radioimmunoassays before and 1 and 3 h afer the ingestion of the test dose. 3 The basal concentrations of 6-keto-PGF1 alpha and TxB2 correlated significantly (r = 0.588, P less than 0.001). 4 Dipyridamole alone did not change PGI2 or TxA2 production. 5 Dipyridamole-ASA combinations with ASA doses between 0.5 and 0.8 mg/kg inhibited TxB2 production by 48 to 74% and with the ASA doses between 2.6 and 5.7 mg/kg by about 90%. None of these combinations changed PGI2 production. 6 The ratio of 6-keto-PGF1 alpha to TxB2 increased 3.5 to 6 times with ASA doses of 0.5 to 0.8 mg/kg and 21 to 29 times with doses between 2.6 to 5.7 mg/kg. 7 These results suggest that the anti-thrombotic effect of dipyridamole in vivo is not mediated through direct changes in PGI2 and/or TxA2 production.

Role of blood platelets in coronary artery disease

Over the past decade, research in blood platelet physiology has led to the suggestion that platelets play an important part in the pathogenesis and complications of coronary artery disease. Occlusive intravascular platelet aggregates have been shown to cause ischemic myocardial damage in the experimental animal and to be present in some patients who die suddenly. The interplay between endothelial damage and platelet aggregation has been implicated in the etiology of atherosclerosis. Products released from platelets during aggregation may cause arterial spasm. Patients with overt ischemic heart disease and with the risk factors associated with coronary artery disease have been found to have abnormally reactive platelets. Clinical studies of drugs that inhibit platelet aggregation have been reported to show a beneficial effect in preventing cardiac deaths or myocardial infarction; other studies have been negative or shown only a trend toward benefit. This report reviews the theoretical and experimental basis for the platelet hypothesis and the current data on the use of antiplatelet drugs in patients with coronary disease.

New 5-hydroxy-2-indolecarbohydrazides as platelet aggregation inhibitors in ethylene glycol

The effect of ethylene glycol on blood platelet aggregation was examined using a previously described method. This method also was used to investigate several derivatives of 2-indolecarbohydrazide in vitro. All compounds inhibited platelet aggregation induced by collagen, epinephrine, or adenosine diphosphate at concentrations below 5 x 10(-4) M.

Dipyridamole and other phosphodiesterase inhibitors act as antithrombotic agents by potentiating endogenous prostacyclin

The antithrombotic effect of dipyridamole is through phosphodiesterase inhibition and depends on stimulation of platelet cyclic A.M.P. by circulating prostacyclin in the bloodstream. Low doses of aspirin selectively inhibit platelet cyclooxygenase and potentiate the antithrombotic effects of dipyridamole and theophylline. High doses of aspirin also prevent prostacyclin formation, thereby abolishing the effects of dipyridamole. Thus, the antithrombotic effectiveness of the combination of aspirin and dipyridamole depends critically on the doses used.

Biocompatibility of coated and uncoated charcoal during haemoperfusion in healthy dogs

The biocompatibility of two commercially available charcoal columns, one containing coated and the other uncoated but immobilized charcoal, was compared during four haemoperfusions with each in eight healthy greyhounds. Reductions in arterial levels of platelets (49% and 42% respectively) and leucocytes (both 21%) were similar. Microaggregates, detected by the Swank screen filtration pressure technique, were found in blood leaving the columns during three of the four perfusions with each column. Another twelve perfusions with the uncoated column were carried out with the addition of one or other of the following three agents which inhibit platelet aggregation: sulfinpyrazone, dipyridamole, or citrate-phosphate-dextrose. With none of these were platelet losses less as compared with the four perfusions in which heparin only was used. However, rises in screen filtration pressure were less pronounced. In other perfusions, where its dosage was varied, heparin was shown to reverse, and in large doses delay, the appearance of micro-aggregates. Thrombus in the column itself may be a source of microaggregates, but platelet aggregation in the absence of thrombus deposition may be responsible. The relation of these findings to micro-aggregate formation, which has constituted a clinical problem during charcoal haemoperfusion in humans with fulminant hepatic failure, is considered.

The effect of dipyridamole on platelet function: correlation with blood levels in man

1 The effect on platelet functions of dipyridamole (a pyrimido-pyrimidine compound) was compared with a control group of patients taking warfarin. 2 Adhesion, aggregation and platelet factor 4 availability showed a significant decrease in the dypyridamole group. 3 Aggregation and platelet factor 4 showed a significant correlation with blood dipyridamole level. 4 Adhesion, aggregation and platelet factor 4 were reduced below the lower limit of normal at blood dipyridamole levels above 3.5 micronmol/1.

Preventing venous thromboembolism in elderly patients with hip fractures: studies of low-dose heparin, dipyridamole, aspirin, and flurbiprofen

Low-dose heparin, dipyridamole (alone and in combination with aspirin), and flurbiprofen were evaluated as potential prophylactic agents against deep venous thrombosis in elderly patients with hip fractures. None of the agents that modify platelet behaviour could reduct the frequency of isotopically diagnosed venous thrombosis. Low-dose heparin reduced the overall frequency of venous thrombosis and its extent as judged by the frequency of bilaterally abnormal scans, but this reduction did not achieve statistical significance.

[Experiments on the medicamental treatment of the noise-induced cochlear damage. Part I. The effect of dipyridamol and allopurinol on the RMP of the cochlea (guinea pig) after noise (author's transl)]

Dipyridamol has in addition to a coronardilatating effect an ati-thrombotic one too. Moreover it facilitates the release of O2 from haemoglobin by increase of the concentration of 2,3-diphosphoglycerate in the erythrocytes. Allopurinol causes a potential resynthesis of ATP in the cells by inhibition of the metabolism of uric acid. Both substances therefore seemed to be suitable for a medicamental therapy of the noise-induced cochlear damage. The expected favourable effect of Dipyridamol and Allopurinol on the organ of Corti after sound exposure (RMP-measurement before and after exposure to pure tone 120 dB SPL) however had not been confirmed by experiment on animals (guinea pig).

Effect of anticoagulants and inhibitors of platelet aggregation on thrombotic occlusion of endarterectomized cat carotid arteries

Cat carotid arteries measuring 1.8 to 2 mm in diameter were endarterectomized under the operating microscope over a 1-cm segment and the arteriotomy was closed with a 9-0 monofilament nylon suture. Vessels exhibiting significant narrowing of the lumen due to faulty closure of the arteriotomy were excluded from the study. The vessels were divided into six groups according to the method of treatment of the animals: control, aspirin, Coumadin, Coumadin plus aspirin, heparin for less than four hours, and heparin for four to eight hours. All vessels in the untreated group subjected to simple arteriotomy and closure remained patent. Only heparin demonstrated an apparent beneficial effect after endarterectomy with 100% of the vessels treated more than four hours and 30% of those treated less than four hours remaining patent. This is contrasted to a 0% patency in other endarterectomized vessels.

Failure of heparin therapy to affect the clinical course of severe pre-eclampsia

In view of the association between pre-eclampsia and disseminated intravascular coagulation, three patients presenting with severe pre-eclampsia before the 28th week of pregnancy were treated with heparin. In all three patients, there was deterioration of hypertension and proteinuria that necessitated the withdrawal of treatment after five to six days. During treatment, serum and urinary fibrinolytic degradation products (FDPs) continued to rise or remained unaltered, plasminogen levels showed a steady fall, and the platelet count remained at a reduced level. These data suggest that heparin was an ineffective form of treatment and did not prevent the intravascular fibrin deposition associated with severe pre-eclampsia.