Thromboxane synthetase inhibition as antithrombotic strategy

Effect of Prostanoids on Human Platelet Function: An Overview

Prostanoids are bioactive lipid mediators and take part in many physiological and pathophysiological processes in practically every organ, tissue and cell, including the vascular, renal, gastrointestinal and reproductive systems. In this review, we focus on their influence on platelets, which are key elements in thrombosis and hemostasis. The function of platelets is influenced by mediators in the blood and the vascular wall. Activated platelets aggregate and release bioactive substances, thereby activating further neighbored platelets, which finally can lead to the formation of thrombi. Prostanoids regulate the function of blood platelets by both activating or inhibiting and so are involved in hemostasis. Each prostanoid has a unique activity profile and, thus, a specific profile of action. This article reviews the effects of the following prostanoids: prostaglandin-D₂ (PGD₂), prostaglandin-E₁, -E₂ and E₃ (PGE₁, PGE₂, PGE₃), prostaglandin F_2α (PGF_2α), prostacyclin (PGI₂) and thromboxane-A₂ (TXA₂) on platelet activation and aggregation via their respective receptors.

Understanding the role of prostaglandin E2 in regulating human platelet activity in health and disease

The platelet thrombus is the major pathologic entity in acute coronary syndromes, and antiplatelet agents are a mainstay of therapy. However, individual patient responsiveness to current antiplatelet drugs is variable, and all drugs carry a risk of bleeding. An understanding of the complex role of Prostaglandin E2 (PGE2) in regulating thrombosis offers opportunities for the development of novel individualized antiplatelet treatment. However, deciphering the platelet regulatory function of PGE2 has long been confounded by non-standardized experimental conditions, extrapolation of murine data to humans, and phenotypic differences in PGE2 response. This review synthesizes past and current knowledge about PGE2 effects on platelet biology, presents a rationale for standardization of experimental protocols, and provides insight into a molecular mechanism by which PGE2-activated pathways could be targeted for new personalized antiplatelet therapy to inhibit pathologic thrombosis without affecting hemostasis.

Non-steroid anti-inflammatory drugs, prostaglandins, and cancer

Fatty acids are involved in multiple pathways and play a pivotal role in health. Eicosanoids, derived from arachidonic acid, have received extensive attention in the field of cancer research. Following release from the phospholipid membrane, arachidonic acid can be metabolized into different classes of eicosanoids through cyclooxygenases, lipoxygenases, or p450 epoxygenase pathways. Non-steroid anti-inflammatory drugs (NSAIDs) are widely consumed as analgesics to relieve minor aches and pains, as antipyretics to reduce fever, and as anti-inflammatory medications. Most NSAIDs are nonselective inhibitors of cyclooxygenases, the rate limiting enzymes in the formation of prostaglandins. Long term use of some NSAIDs has been linked with reduced incidence and mortality in many cancers. In this review, we appraise the biological activities of prostanoids and their cognate receptors in the context of cancer biology. The existing literature supports that these lipid mediators are involved to a great extent in the occurrence and progression of cancer.

Prostanoid receptor antagonists: development strategies and therapeutic applications

Identification of the primary products of cyclo-oxygenase (COX)/prostaglandin synthase(s), which occurred between 1958 and 1976, was followed by a classification system for prostanoid receptors (DP, EP(1), EP(2) ...) based mainly on the pharmacological actions of natural and synthetic agonists and a few antagonists. The design of potent selective antagonists was rapid for certain prostanoid receptors (EP(1), TP), slow for others (FP, IP) and has yet to be achieved in certain cases (EP(2)). While some antagonists are structurally related to the natural agonist, most recent compounds are 'non-prostanoid' (often acyl-sulphonamides) and have emerged from high-throughput screening of compound libraries, made possible by the development of (functional) assays involving single recombinant prostanoid receptors. Selective antagonists have been crucial to defining the roles of PGD(2) (acting on DP(1) and DP(2) receptors) and PGE(2) (on EP(1) and EP(4) receptors) in various inflammatory conditions; there are clear opportunities for therapeutic intervention. The vast endeavour on TP (thromboxane) antagonists is considered in relation to their limited pharmaceutical success in the cardiovascular area. Correspondingly, the clinical utility of IP (prostacyclin) antagonists is assessed in relation to the cloud hanging over the long-term safety of selective COX-2 inhibitors. Aspirin apart, COX inhibitors broadly suppress all prostanoid pathways, while high selectivity has been a major goal in receptor antagonist development; more targeted therapy may require an intermediate position with defined antagonist selectivity profiles. This review is intended to provide overviews of each antagonist class (including prostamide antagonists), covering major development strategies and current and potential clinical usage.

Thromboxane A2: physiology/pathophysiology, cellular signal transduction and pharmacology

Thromboxane A(2) (TXA(2)), an unstable arachidonic acid metabolite, elicits diverse physiological/pathophysiological actions, including platelet aggregation and smooth muscle contraction. TXA(2) has been shown to be involved in allergies, modulation of acquired immunity, atherogenesis, neovascularization, and metastasis of cancer cells. The TXA(2) receptor (TP) communicates mainly with G(q) and G(13), resulting in phospholipase C activation and RhoGEF activation, respectively. In addition, TP couples with G(11), G(12), G(13), G(14), G(15), G(16), G(i), G(s) and G(h). TP is widely distributed in the body, and is expressed at high levels in thymus and spleen. The second extracellular loop of TP is an important ligand-binding site, and Asp(193) is a key amino acid. There are two alternatively spliced isoforms of TP, TPalpha and TPbeta, which differ only in their C-terminals. TPalpha and TPbeta communicate with different G proteins, and undergo hetero-dimerization, resulting in changes in intracellular traffic and receptor protein conformations. TP cross-talks with receptor tyrosine kinases, such as EGF receptor, to induce cell proliferation and differentiation. TP is glycosylated in the N-terminal region for recruitment to plasma membranes. Furthermore, TP conformation is changed by coupling to G proteins, showing several states of agonist binding. Finally, several drugs modify TP-mediated events; these include cyclooxygenase inhibitors, TXA(2) synthase inhibitors and TP antagonists. Some flavonoids of natural origin also have TP receptor antagonistic activity. Recent advances in TP research have clarified TXA(2)-mediated events in detail, and further study will supply more beneficial information about TXA(2) pathophysiology.

The calcium ionophore A23187 induces endothelium-dependent contractions in femoral arteries from rats with streptozotocin-induced diabetes

BACKGROUND AND PURPOSE

To study the importance of endothelium-derived contracting factors (EDCFs) in arteries of rats with type I diabetes.

EXPERIMENTAL APPROACH

Rat femoral arteries were collected four or twelve weeks after induction of diabetes with streptozotocin. Rings, with or without endothelium, were suspended in organ chambers for isometric tension measurement. COX protein levels were determined by Western blotting.

KEY RESULTS

Four weeks after the injection of streptozotocin, the endothelium-dependent relaxations (during contractions to phenylephrine) to A23817 were attenuated, but the endothelium-dependent contractions (quiescent preparations) to the ionophore were augmented. Indomethacin and S18886 prevented the endothelium-dependent contractions, while dazoxiben reduced them in rings from streptozotocin-treated rats, suggesting that thromboxane A2, activating TP- receptors, is involved. Twelve weeks after the injection of streptozotocin, the changes in endothelium-dependent relaxations and contractions to A23187 were even more noticeable. The protein expression of COX-1 was increased in femoral arteries of the diabetic rats. Valeryl salicylate and SC560 inhibited the contractions, suggesting that the EDCFs are produced by COX-1. At that time, a combination of S18886 with EP1-blockers was required to abolish the contractions, suggesting that the EDCFs involved act at both TP- and EP-receptors. Rings without endothelium from streptozotocin-treated rats exhibited a reduced maximal contraction to potassium chloride and U46619, combined with hyper-responsiveness to the latter, suggesting that more prolonged diabetes also alters the responsiveness of vascular smooth muscle.

CONCLUSION AND IMPLICATIONS

The production of EDCFs is progressively increased in the course of type I diabetes. Eventually, the disease also damages vascular smooth muscle.

Postmenopausal hormone replacement therapy increases plasmatic thromboxane β2

OBJECTIVE

This study shows the effect of hormone replacement therapy (HRT), using oral estrogen exclusively or in combination with progestin, on platelet activation in healthy menopaused women.

BACKGROUND

Recent evidence from studies of postmenopausal HRT in healthy women demonstrated a short-time increased risk of coronary heart disease. Platelet activation, which generates vasoconstrictory thromboxane A(2) (TxA(2)), has been related to the risk of cardiovascular diseases.

METHODS

By means of a placebo-controlled study twenty-seven postmenopausal patients were continuously orally administered estrogen in combination with progestin or estrogen exclusively for an 8-week period. Platelet activation was evaluated by flow cytometric P-selectin expression and by enzyme immunoassay plasmatic TxA(2) (TxB(2)) concentrations.

RESULTS

P-selectin binding index changed from 6.3+/-3.6 to 7.0+/-3 in the placebo group (n=10); from 5.9+2.2 to 7.9+/-3.3 in the E+P group (n=8) and from 6.4+2.7 to 7.1+/-1.9 in the E group (n=9). Plasma concentrations of TxB(2) before and after intervention, changed from 1.2+1.2 to 1.5+1.4 (pg/well) in the placebo group; significantly (p=0.005) in the E+P group (n=8), from 0.9+0.3 to 6.1+6.5 (pg/well), and from 1.3+1.5 to 0.8+0.4 (pg/well) in the E group (n=8; mean+standard deviation, basal x therapy, p<0.05).

CONCLUSIONS

Healthy menopaused women who were administered estradiol in association with norethisterone continuously had an increase of plasmatic thromboxane, possibly determined by platelet activation, which indicates a higher short-term thrombotic risk. P-selectin expression analyses failed to demonstrate the impact of HRT on platelets.

Sexual dimorphism in prostanoid-potentiated vascular contraction: roles of endothelium and ovarian steroids

The effects of constrictor prostanoid (CP) pathway inhibitors on vascular reactivity to vasopressin (VP) and phenylephrine (PE) were examined in thoracic aortas of male, female, and ovariectomized (OVX) female Sprague-Dawley rats. Maximal contractile response of control (Cont) aortas to VP was markedly higher in females (3,885 +/- 332 mg/mg ring wt) than in males (810 +/- 148 mg). Indomethacin (Indo; 10 microM) attenuated maximal response to VP in females (3,043 +/- 277 mg) but not in males. SQ-29,548 (SQ; 1 microM) attenuated maximal response to VP in females (3,042 +/- 290 mg) to a similar extent as Indo. Dazoxiben (Daz; 10 microM) alone had no effect, but Daz + SQ attenuated maximal contractile response to VP to a similar extent as SQ alone. Removal of the endothelium in female aortas attenuated contractile responses to VP in Cont aortas. OVX attenuated maximal contractile response to VP in Cont aortas (2,093 +/- 329 mg) and abolished the attenuating effects of Indo. Indo, SQ, and Daz exerted identical effects on contractile responses of male, female, and OVX female aortas to PE. These findings establish the following in the rat aorta: 1) CP, probably thromboxane and/or endoperoxide, is responsible for approximately 25-30% of contractile responses of females, but not males, to VP and PE; 2) CP production by the female aorta is primarily endothelial in origin; and 3) ovarian steroids modulate production and/or actions of CP in female aortas.

New antiplatelet drugs

Antiplatelet drugs are used in a wide range of disorders, either as sole agents or as adjuncts to other therapies. Aspirin has been shown to be clinically effective in a number of ischaemic conditions and has been in use for many years. The newer agents, ticlopidine and clopidogrel (which are thought to inhibit ADP-mediated platelet reactions) are also effective and may prove to be superior to aspirin in certain indications. However, ticlopidine in particular has a different spectrum of side-effects, which may eventually limit its widespread use. The Gp IIb/IIIa antagonists have been most extensively investigated in the acute coronary syndromes, and shown to significantly improve outcome. Most of these studies have utilized agents which need to be given parenterally, and subsequently oral compounds are currently being developed. A number of other antiplatelet drugs such as prostacyclin and its analogues, as well as thromboxane inhibitors have been studied over the years, but overall they have failed to demonstrate any real clinical advantage.

Disorders of platelet function

Qualitative platelet disorders are described and reviewed above. The acquired platelet function defects are very common, and sometimes result in hemorrhage, especially in association with trauma or surgery. However, the specific biochemical defect is absent, and no characterized platelet abnormalities have been recognized. On the other hand, the hereditary qualitative platelet defects are rare, but the platelet abnormalities are characteristic. The study of these patients had led to an increased understanding of the normal primary hemostatic mechanism. Recently, the molecular basis analysis of the platelet defects has been developed. This will help us understand the molecular events involved in platelet adhesion and aggregation.

Thrombosis and the pharmacology of antithrombotic agents

OBJECTIVE

To provide an overview of the current state of knowledge regarding the physiology of hemostasis, the pathophysiology of thrombosis, and the pharmacology of antithrombotic agents.

DATA SOURCES

A MEDLINE search was conducted to identify pertinent literature published since 1984. Recently published textbooks devoted to the subjects of hematology, hemostasis, and thrombosis also were reviewed, particularly their bibliographies. The bibliographies of selected review articles also were reviewed.

STUDY SELECTION

As the amount of literature was vast, only the most significant and noteworthy published studies were reviewed. Review articles and book chapters authored by researchers of international reputation also were reviewed.

DATA EXTRACTION

Identified studies from the primary literature and selected reviews were analyzed carefully. Information regarding hemostasis, thrombosis, and antithrombotic drugs was extracted. Particular attention was given to data regarding drugs currently available or soon to be available on the US market.

DATA SYNTHESIS

Knowledge regarding the regulation of blood coagulation has expanded substantially in recent years. Hemostasis involves the dynamic interplay of numerous intravascular constituents, including the vessel wall, circulating procoagulants and anticoagulants, platelets, and fibrinolytic proteins. Thrombosis is the abnormal formation of a clot within the vascular system. When sufficiently large, thrombi can prevent the flow of blood and nutrients to vital tissues. Thrombosis is associated with many common diseases and is among the leading causes of death in developed countries. Many drugs are now available to prevent the formation and propagation of thrombi. These agents work by different pharmacologic mechanisms and are useful in different clinical situations.

CONCLUSIONS

Thrombosis research has increased our understanding of the pharmacology of antithrombotic drugs and promoted the discovery of new agents targeted more specifically toward the critical steps in pathologic clot formation. New agents have the potential for greater efficacy and fewer adverse effects. An increased understanding of hemostasis, thrombosis, and the pharmacology of antithrombotic drugs should enable the clinician to use these agents appropriately.

In vitro studies on the effect of thromboxane receptor blockade on platelet deposition on vascular surfaces

The efficacy of platelet inhibition by a thromboxane receptor antagonist (Bay U3405) and acetylsalicylic acid was investigated in vitro using a new perfusion system. Rabbit 51Cr-labeled platelets were suspended in saline as perfusate. Vascular grafts, PTFE or Dacron (precoated with blood or uncoated) as well as native aorta were perfused in vitro. Three sets of grafts (vessels) were perfused simultaneously with either acetylsalicylic acid (10(-5) M), Bay U3405 (10(-6) M) treated or untreated (control) labeled platelets. Platelet deposition on the grafts (vessels) was measured in a gamma counter. Values were normalised to graft weight or vessel surface area respectively and to the platelet concentration of perfusate. The results of these experiments indicated that thromboxane receptor inhibition was superior to cyclooxygenase blockade in preventing platelet deposition on synthetic grafts and native aorta vessels in vitro. Whether this is valid in vivo has to be confirmed.

Thromboxane synthase inhibitors and receptor antagonists

Platelet activation plays a major role in myocardial infarction and in reocclusion following successful thrombolysis, as corroborated by several clinical studies using aspirin. However, the overall reduction of new vascular complications in patients with symptomatic arterial disease by aspirin was only around 25%. Therefore, there is great interest in finding new means to inhibit platelet activation more efficiently. One line of research has focused on ways to interfere with the action of thromboxane A2 in a more selective way than aspirin does. As such, the development of thromboxane synthase inhibitors, followed by thromboxane receptor antagonists, raised hopes for a better treatment. However, both classes of drugs have some drawbacks, which could be overcome by combining them. This aim has led to the development of compounds that intrinsically possess both activities. Ongoing research indicates that such a dual inhibitor may indeed be more powerful than either aspirin or drugs with the single actions.

Thromboxane (Tx) A2 receptor blockade and TxA2 synthase inhibition alone and in combination: comparison of anti-aggregatory efficacy in human platelets

1. The present study has compared the relative anti-aggregatory effect of various compounds which interfere with thromboxane (Tx) A2-dependent aggregation of human platelets in whole blood in vitro. These included the cyclo-oxygenase inhibitor aspirin, the TxA2 synthase inhibitor dazoxiben, the TxA2 (TP-) receptor blocking drug GR32191 and two compounds, R.68070 ((E)-5-[[[(3-pyridinyl) [3-(trifluoromethyl)phenyl]-methylen] amino]oxy] pentanoic acid) and CV-4151 [E)-7-phenyl-7-(3-pyridyl)-6-heptenoic acid), which possess both TP-receptor blocking and TxA2 synthase inhibitory activities in the same molecule. 2. GR32191, R.68070 and CV-4151 all antagonized aggregation to the TxA2 mimetic U-46619, with pA2 values of approximately 8.2, 5.4 and 4.8 respectively. This effect was specific, platelet aggregation induced by adenosine 5'-diphosphate (ADP) being unaffected by concentrations up to 10, 1000 and 300 microM respectively. In contrast, neither aspirin nor dazoxiben exhibited any measurable TP-receptor blocking activity. 3. The rank order of potency (pIC50) for inhibition of TxA2 formation in serum was R.68070 (7.4) greater than CV-4151 (6.9) greater than dazoxiben (5.7) greater than aspirin (5.3). In addition, all four drugs abolished collagen-induced platelet TxA2 formation. In contrast, GR32191 produced no consistent inhibition of TxA2 formation in either system up to concentrations of 10-30 microM. 4. The specificity of R.68070, CV-4151 and dazoxiben for TxA2 synthase was indicated by their ability to increase serum levels of prostaglandin E2 (PGE2) and PGD2 in parallel with decreases in TxA2 formation. This profile was not observed with aspirin or GR32191. However, high concentrations of R.68070 (100,microM) and CV-4151 (1000 microM) necessary for maximum TP-receptor blocking activity, produced substantially smaller increases in PGE2 and PGD2, consistent with an aspirin-like effect of these compounds upon cyclo-oxygenase. With dazoxiben (1000 microM), PGE2 and PGD2 levels remained elevated. 5. Aspirin inhibited collagen-induced platelet aggregation, the effect correlating with inhibition of TxA2 formation. Dazoxiben, whilst also achieving maximal inhibition of TxA2 formation, produced significantly less inhibition of aggregation than aspirin. In contrast, GR32191 (0.1-1O microM), at concentrations specific for TP-receptor blockade, produced a significantly greater antagonism of collagen-induced platelet aggregation than aspirin. This additional effect of GR32191 was absent in platelets pretreated with aspirin, indicating the probable involvement of an endogenous anti-aggregatory cyclo-oxygenase product in response to collagen stimulation. 6. R.68070 and CV-4151 also inhibited collagen-induced aggregation, with very high concentrations of R.68070 (100 microM) producing an effect equivalent to that of GR32191. 7. In contrast, the combination of GR32191 with either dazoxiben, R.68070 or CV-4151, at concentrations specific for TxA2 synthase, produced a synergistic inhibitory effect upon collagen-induced platelet aggregation which was greater than that achieved with either aspirin or any of the compounds used alone. Pretreatment of platelets with aspirin reversed this synergistic effect, consistent with it being dependent upon the formation and action of anti-aggregatory prostaglandins. 8. In conclusion, the present study has confirmed the superior platelet inhibitory profile of a combination of a TP-receptor blocking drug and a TxA2 synthase inhibitor to that of either activity alone. However, the maximum inhibitory effect of the currently available compounds, R.68070 and CV4151, which possess both activities in the same molecule, appears to be no greater in vitro than that obtained with the potent TP-receptor blocking drug, GR32191. This most probably reflects the inhibition by R.68070 and CV-4151 of platelet cyclo-oxygenase at the concentrations required for effective TP-receptor blockade which results in a reduction in the formation of anti-aggregatory prostanoids.

Correlation between pharmacokinetics and pharmacologic effects of a new imidazole thromboxane synthetase inhibitor

A new imidazole derivative, sodium 4-[alpha-hydroxy-5-(1-imidazolyl)-2-methylbenzyl]-3,5-dimethylbenzoat e dihydrate (1; Y-20811), is a potent selective inhibitor of thromboxane (TX) synthetase and a candidate drug for preventing ischemic circulatory disorders. After oral dosing in beagle dogs, 1 persistently inhibited TX production, although the elimination of the drug was relatively rapid from plasma. The amount of drug distributed in platelets was small. However, 1 was eliminated from platelets very slowly (t1/2 = 41-130 h), the rate of elimination being comparable to the turnover rate of platelets. From the in vitro experiments, an obvious positive correlation was noted between the amount of the drug bound with TX synthetase in platelets and the inhibition of TX production. This correlation suggested that the small amount of 1 detected in platelets in vivo was large enough to inhibit TX production. From these observations, 1 is thought to dissociate extremely slowly from the active sites of TX synthetase. Consequently, the duration of the effect was dependent on the turnover of platelets.

Protective effect of Y-20811, a long-lasting thromboxane synthetase inhibitor, on endotoxin shock in rabbits

The effect of sodium 4-[alpha-hydroxy-5-(imidazolyl)-2-methylbenzyl]-3,5-dimethyl benzoate dihydrate (Y-20811), a selective thromboxane (TX) synthetase inhibitor, on endotoxin shock was investigated in comparison with aspirin. The drugs were orally administered to rabbits at 24 and 1 hour before injection of endotoxin (5 mg/kg, i.v.). Y-20811 (1 mg/kg) promoted the recovery of decreased platelet counts, and inhibited hypotension induced by endotoxin. It also inhibited the increase in plasma TXB2 and 6-keto PGF1 alpha. Aspirin at 30 mg/kg, inhibited hypotension and the increase in both plasma TXB2 and 6-keto PGF1 alpha levels, but it failed to inhibit the decrease in platelet counts. In the control group, all rabbits died within 180 min after endotoxin injection, while Y-20811 completely protected animals against death at a dose of 0.3 mg/kg. Aspirin also protected animals against death at a dose of 30 mg/kg, which was, however, about one hundredth potent of Y-20811. These results indicate that Y-20811 is useful in treating endotoxin shock.

Phase I study of Y-20811, a new long-acting thromboxane synthetase inhibitor by oral administration

The safety, pharmacological action and pharmacokinetics of the thromboxane (TX) synthetase inhibitor Y-20811, a prospective medication for ischemic vascular disorder, were investigated in Phase I clinical trials in which single and repeated doses were administered orally to 32 healthy adult male volunteers. In the single-dose study, subjects were given single doses of 2.5, 5, 10, 25, 50, and 100 mg; in the repeated-dose study, a single dose of 50 mg was administered daily for 5 days. In the repeated-dose study, slight elevation of liver function test parameters in 1 subject was observed, but throughout the trials, no abnormality attributable to the test drug was found in other routine laboratory tests, subjective and objective findings, vital signs (blood pressure, pulse rate, body temperature, respiration rate), ECG, or bleeding time; nor did any finding indicate a problem concerning the safety of Y-20811. In both single- and repeated-dose studies, inhibition of the serum TXB2 production and an increase in the 6-keto-prostaglandin F1 alpha production during whole-blood coagulation, and inhibition of arachidonic acid-induced platelet aggregation were observed from 1 hour after administration of Y-20811. The duration of these actions was long, the former two lasting 168 hours (1 week) and the latter 48-72 hours or more. Slight inhibition of adenosine diphosphate-induced secondary platelet aggregation was also observed. The maximum plasma concentration time (tmax) of Y-20811 was 0.5-1.1 hours. The initial-phase half-life (t1/2 alpha) was 0.8-1.1 hours and the final-phase half-life (t1/2 beta) was 4.7-9.2 hours.(ABSTRACT TRUNCATED AT 250 WORDS)

Redirection of prostaglandin endoperoxide metabolism at the platelet-vascular interface in man

Prostacyclin (PGI2) is an inhibitor of platelet function in vitro. We tested the hypothesis that PGI2 is formed in biologically active concentrations at the platelet-vascular interface in man and can be pharmacologically modulated to enhance its inhibitory properties. This became feasible when we developed a microquantitative technique that permits the measurement of eicosanoids in successive 40-microliters aliquots of whole blood emerging from a bleeding time wound. In 13 healthy volunteers the rate of production of thromboxane B2 (TXB2) gradually increased, reaching a maximum of 421 +/- 90 (mean +/- SEM) fg/microliters per s at 300 +/- 20 s. The hydration product of PGI2, 6-keto-PGF1 alpha, rose earlier and to a lesser degree, reaching a peak (68 +/- 34 fg/microliters per s) at 168 +/- 23 s. The generation of prostaglandins PGE2 and D2 resembled that of PGI2. Whereas the threshold concentration of PGI2 for an effect on platelets in vitro is approximately 30 fg/microliters, only less than 3 fg/microliters circulates under physiological conditions. By contrast, peak concentrations of 6-keto-PGF1 alpha obtained locally after vascular damage averaged 305 fg/microliters. Pharmacological regulation of PG endoperoxide metabolism at the platelet-vascular interface was demonstrated by administration of a TX synthase inhibitor. The rate of production of PGI2, PGE2, and PGD2 increased coincident with inhibition of TXA, as reflected by three indices; the concentration of TXB2 in bleeding time blood and serum, and excretion of the urinary metabolite, 2,3-dinor-TXB2. These studies indicate that PGI2 is formed locally in biologically effective concentrations at the site of vessel injury and provide direct evidence in support of transcellular metabolism of PG endoperoxides in man.

Exchange of cyclooxygenase dependent metabolites between vessel wall and platelets in arterial thrombosis

In this investigation, transport of endoperoxides from the vessel wall to the platelets in an in vivo model for the induction and quantification of platelet thrombosis is described. Thrombi are induced in a branch of the mesenteric artery of the male white Wistar rat through topical superfusion of adenosine diphosphate solutions following focal de-endothelialization by means of a small electrical current in two groups, comparing untreated rats to acetylsalicylic acid (ASA) treated ones. In the latter group the cyclooxygenase activity of the platelets is completely inhibited but not so that of the endothelial cells. In both groups arachidonic acid enhances adenosine diphosphate induced arterial platelet thrombosis most likely through the conveyance of endoperoxides generated in the vessel wall to the platelets as thromboxane synthetase antagonists as well as thromboxane receptor blocking agents completely inhibit this enhancement in both groups. Furthermore it is demonstrated that ASA treated animals show an increased propensity to thrombosis induced by adenosine diphosphate.

Inhibition of leukotriene and thromboxane biosynthesis by a 9-(4-chlorophenyl) analogue of arachidonic acid

The synthesis of 9-(4-chlorophenyl)-7,7-dimethyl-5(Z), 8-nonadienoic acid (7) and its methyl ester 6, and their effects on arachidonic acid metabolism in vitro are described. The IC50 values of 19.6 and 20.6 microM were observed for inhibition of leukotriene synthesis in human granulocytes for 6 and 7, respectively. Additionally, the compounds inhibited thromboxane B2 (TxB2) synthesis, with respective IC50 values of 6.1 and 20 microM, while producing pronounced 3-8-fold increases in PGE2 synthesis in human mononuclear cells. Increased PGE2 synthesis may have reflected shunting of free arachidonic acid substrate at the thromboxane synthetase and endoperoxide E isomerase branchpoint of arachidonic acid metabolism.

Dietary biotin effects on polyunsaturated fatty acids in chick tissue lipids and prostaglandin E2 levels in freeze-clamped hearts

Chicks were fed a purified diet with 0, 100, 200, 300, 400, or 500 micrograms/kg diet of added biotin to determine the effects of biotin deficiency on polyunsaturated fatty acids in tissue lipids. Body weight was reduced by biotin deficiency and liver and heart biotin levels varied with the biotin in the diet. Fatty acids in liver and lung from biotin-deficient chicks at 15 days contained elevated (P less than .03) 18:3 omega 3 and 18:2 omega 6 but prostaglandin precursors 20:3 omega 6 and 20:4 omega 6 were reduced (P less than .03) in liver lipids. Heart tissues from 15-day-old chicks fed the biotin-deficient diet were low (P less than .03) in 20:3 omega 6. Feeding acetylsalicylic acid in diets containing added biotin (0, 100, 400, and 500 micrograms/kg) did not significantly alter fatty acid levels in chick tissue lipids but significantly reduced plasma prostaglandin E2 (PGE2). Biotin deficiency reduced heart PGE2 levels in 22-day-old chicks. An 8-h fast reduced (P less than .04) 20:4 omega 6 in chick heart total fatty acids.

Role of proaggregatory and antiaggregatory prostaglandins in hemostasis. Studies with combined thromboxane synthase inhibition and thromboxane receptor antagonism

Thromboxane synthase inhibition can lead to two opposing effects: accumulation of proaggregatory cyclic endoperoxides and increased formation of antiaggregatory PGI2 and PGD2. The elimination of the effects of the cyclic endoperoxides by an endoperoxide-thromboxane A2 receptor antagonist should enhance the inhibition of hemostasis by thromboxane synthase blockers. We have carried out a series of double-blind, placebo-controlled, crossover studies in healthy volunteers to check if this hypothesis may be operative in vivo in man. In a first study, in 10 healthy male volunteers, the combined administration of the thromboxane receptor antagonist BM 13.177 and the thromboxane synthase inhibitor dazoxiben gave stronger inhibition of platelet aggregation and prolonged the bleeding time more than either drug alone. In a second study, in 10 different healthy male volunteers, complete inhibition of cyclooxygenase with indomethacin reduced the prolongation of the bleeding time by the combination BM 13.177 plus dazoxiben. In a third study, in five volunteers, selective cumulative inhibition of platelet TXA2 synthesis by low-dose aspirin inhibited platelet aggregation and prolonged the bleeding time less than the combination BM 13.177 plus dazoxiben. In vitro, in human platelet-rich plasma stimulated with arachidonic acid, the combination of BM 13.177 and dazoxiben increased intraplatelet cAMP while the single drugs did not affect it. Our results indicate that prostaglandin endoperoxides can partly substitute for the activity of TXA2 in vivo in man and that an increased formation of endogenous antiaggregatory and vasodilatory prostaglandins, as obtained with selective thromboxane synthase inhibitors, may contribute to the impairment of hemostasis.

Dissociation between thromboxane generation and metastatic potential in cells from a murine fibrosarcoma. Studies with a selective thromboxane synthase inhibitor

Since the highly metastatic variant M4 of the mFS6 fibrosarcoma has the peculiar feature of generating larger amounts of immunoreactive thromboxane B2 (TxB2) than the non-metastatic variant (M9), we used the thromboxane synthase inhibitor dazmegrel (UK-38,485) in an effort to influence its metastatic potential. TxB2 formation by tumor cells freshly harvested from the primary tumor could be completely inhibited by drug addition in vitro. TxB2 generation was inhibited with a dose-response curve, 2 microM being the lowest dazmegrel concentration giving 100% inhibition. Chronic treatment of tumor-bearing mice with dazmegrel (150 mg/kg b.w. twice daily by gavage) from the day of tumor-cell implantation until killing of the animals caused a more than 10-fold reduction in serum TxB2 formation; TxB2 generation by tumor cells was also significantly depressed. This treatment, however, did not significantly modify either primary tumor weight or metastasis formation. Our data suggest that selective inhibition of thromboxane generation in either blood or tumor cells does not prevent spontaneous metastasis formation in the murine model studied.

Mechanism of antagonism of thromboxane receptors in vascular smooth muscle

The mechanism and profile of antagonism of thromboxane receptors were studied in isolated perfused cat coronary arteries and in rat aortic rings. In cat coronary arteries, the thromboxane receptor antagonist (TxRA) BM-13,505 at concentrations from 3 to 300 ng/ml, significantly attenuated the vasoconstrictor effects of both a thromboxane A2 analog (CTA2) and an endoperoxide analog (U-46,619) and did not alter the constrictor responses to leukotriene D4, arginine vasopressin, or angiotensin II. In rat aortic rings precontracted by CTA2 or U-46,619, the effective threshold concentration of BM-13,505 for relaxation was 5 ng/ml. Lower concentrations of BM-13,505 exerted no relaxation, and higher concentrations exhibited faster relaxation to the precontracted baseline levels. This relaxation was not observed in aortic rings precontracted by norepinephrine or angiotensin II. The action of TxRA was not influenced by the absence of the endothelium or by pretreatment with a selective guanylate cyclase inhibitor, methylene blue. Also, thromboxane receptor antagonists do not appear to block thromboxane induced constriction by action as free radical scavengers. It can be concluded that replacing thromboxane A2 on the vascular receptor by a TxRA is the main factor responsible for the antagonism of thromboxane induced vasoconstriction in vascular smooth muscle preparations, not, the presence of the endothelium, activation of guanylate cyclase, or scavenging of superoxide free radicals.

Indomethacin, dazoxiben and extravascular lung water after Escherichia coli infusion

The effects of selectively inhibiting synthesis of thromboxane A2 (TXA2) with dazoxiben and of all cyclooxygenase products with indomethacin were studied in goats after infusion of 5 X 10(8) live Escherichia coli bacteria/kg. Pulmonary and systemic pressures, cardiac output, and double indicator dilution extravascular lung water (EVLW) were measured at 15-min intervals. EVLW was determined gravimetrically at 6 hr to confirm the final double indicator dilution values. Plasma levels of TXA2 and prostacyclin (PGI2) were measured as their stable metabolites, TXB2 and 6-keto-PGF1 alpha, respectively. Dazoxiben blocked the increase in plasma TXB2, prevented pulmonary hypertension, and attenuated the increase in EVLW after E. coli. Mean gravimetric EVLW was 8.7 ml/kg in the dazoxiben-treated group compared to 11.3 ml/kg in the untreated control group. Indomethacin blocked the increased plasma TXB2 and 6-keto-PGF1 alpha, attenuated pulmonary hypertension, and prevented almost all increases in EVLW. Mean gravimetric EVLW was 8.2 ml/kg after indomethacin. We conclude that in acute bacteremia, the early pulmonary hypertension is mediated largely by TXA2 (however, a second phase of hypertension results from non-cyclooxygenase products), either production of cyclooxygenase products (perhaps PGI2) inhibits part of the action of pulmonary vasoconstrictors, or indomethacin stimulates the production of other vasoconstrictors (such as lipoxygenase products), and indomethacin prevents the accumulation of EVLW by blocking formation of cyclooxygenase products or by other nonspecific actions.

The in vitro effect of amrinone on thromboxane B2 synthesis in human whole blood

Thromboxane B2 production in vitro in human whole blood was inhibited at pharmacological levels of amrinone. There was no observed effect on prostacyclin production as measured by 6-ketoprostaglandin F1 alpha levels. The biochemical basis for the inhibition of platelet aggregation by amrinone and the inter-relationships between arachidonic acid metabolism, c-AMP and Ca2+ levels are discussed.

Increased thromboxane biosynthesis in a human preparation of platelet activation: biochemical and functional consequences of selective inhibition of thromboxane synthase

Although thromboxane A2 is a potent platelet agonist and vasoconstrictor in vitro, our knowledge of its pathophysiologic importance in human disease is limited. To facilitate the elucidation of its role in vivo, we sought to define a human syndrome in which pharmacologic interventions designed to inhibit the biosynthesis or biologic actions of thromboxane A2 might be appropriately assessed. Patients with severe peripheral vascular disease were selected on the basis of elevated plasma beta-thromboglobulin and circulating platelet aggregates and compared with healthy, age-matched control subjects. In addition to the platelet indexes, their bleeding time was shorter and excretion of 2,3-dinor-thromboxane B2, a noninvasive index of thromboxane formation in vivo, and 2,3-dinor-6-keto-prostaglandin F 1 alpha, the major urinary metabolite of prostacyclin, was markedly increased. A selective inhibitor of thromboxane synthase, imidazo (1,5-2) pyridine-5-hexanoic acid, was administered to these patients under randomized, double-blind, controlled conditions. Platelet aggregation ex vivo, the circulating platelet aggregate ratio, and the bleeding time were all unaltered, despite almost maximal inhibition of platelet thromboxane formation 1 hr after dosing. By contrast, pronounced inhibition of aggregation was observed when platelet cyclooxygenase was inhibited by aspirin. During long-term dosing with the synthetic inhibitor, inhibition of thromboxane biosynthesis was incomplete, which would permit continued thromboxane-dependent platelet aggregation to occur. However, the failure of enzyme blockade to influence platelet function at the time of maximal drug action, despite efficient inhibition of serum thromboxane B2, suggests that accumulation of proaggregatory endoperoxides is also likely to have contributed to the persistence of platelet activation. We have characterized a human preparation in which platelet activation coexists with increased thromboxane biosynthesis. In this setting, platelet activation persists despite long-term administration of a thromboxane synthase inhibitor in a dosing regimen representative of that employed in clinical trials. Prolongation of drug action and combination with antagonists of the shared endoperoxide/thromboxane A2 receptor may be necessary to assess the potential of selective inhibition of thromboxane synthase as a therapeutic strategy in man.

A new combination therapy for selective and prolonged antiplatelet effect: results in the dog

This study characterizes the effect of dazmegrel, a thromboxane synthetase inhibitor, on platelet function in the dog and introduces its potential use in combination with aspirin therapy. Ex vivo testing of dazmegrel alone was performed with three dosages and three administration regimens. Platelet aggregation response, malondialdehyde formation and prostaglandin metabolites generation were evaluated. To maintain complete thromboxane A2 inhibition, dazmegrel had to be given 3 times per day at dosages of not less than 6 mg/kg. The same result was achieved with a single daily administration of combined dazmegrel and aspirin in equal dosages of 3 mg/kg. Dazmegrel, both alone and with aspirin, increased and sustained heightened levels of prostacyclin, unlike the simultaneous inhibition of both prostaglandin metabolites seen with aspirin therapy alone. Because the combination of dazmegrel and aspirin effectively blocks thromboxane A2 formation and also enhances prostacyclin formation, the synergistic action of these agents increases their combined antiplatelet effect to a level not attainable by either agent alone. The significance of this combined therapy warrants further experimental study and may soon merit clinical trial for the prevention of stroke and other major thrombotic complications.

Effects of ketanserin, a selective 5-HT2 serotonergic antagonist, on the secondary recruitment of human platelets in vitro

Ketanserin, a selective 5-HT2 serotonergic receptor antagonist, reduces in vitro the release-associated human platelet aggregation induced by threshold concentrations of collagen and curtails the second wave of aggregation/release induced by critical concentrations of ADP in particular and, to a lesser extent, of 1-epinephrine. Its inhibitory effect on the second waves becomes more pronounced when the reaction is already attenuated by moderate cyclo-oxygenase inhibition with esculetin, by yohimbine or by propranolol. The first wave of aggregation induced by ADP or 1-epinephrine is not affected. Such an inhibition of secondary platelet recruitment by ketanserin in vitro may be due to an inhibition of the 5-HT2 receptor-mediated amplifying effects of platelet-released 5-HT or to a non-specific interference with the platelet membrane, reducing the release of mediators from the platelets. Reduction of the increased plasma BTG levels in patients after ketanserin may result from such release-inhibiting mechanisms.

Thromboxane synthase inhibition: implications for prostaglandin endoperoxide metabolism. I. Characterisation of an acute intravenous challenge model to measure prostaglandin endoperoxide metabolism

It has been proposed that thromboxane synthase inhibition (TXSI) may be a useful form of anti-thrombotic therapy and that this is due, in part, to redirection of PGH2 metabolism in favour of PGI2, a potent vasodilator and anti-platelet agent. While redirection has been observed ex vivo there are conflicting reports of its occurrence in vivo. We now describe the characterisation of an acute intravenous challenge model using thrombin, collagen, arachidonic acid (AA) and PGH2 for the study of PGH2 metabolism. Following challenge, plasma concentrations of TXB2, 6-oxo-PGF1 alpha, alleged metabolites of PGI2 (PGI2m) and PGE2 were measured by radioimmunoassay (RIA). Thrombin and collagen challenge resulted in a dose-related increase in plasma TXB2 while AA and PGH2, in addition, elevated 6-oxo-PGF1 alpha and PGI2m. Injection of PGH2 elevated 6-oxo-PGF1 alpha, PGI2m, TXB2 and PGE2 levels. Experimental conditions were defined such that challenge with thrombin (40 NIH units kg-1), collagen (100 micrograms kg-1), AA (1 mg kg-1) and PGH2 (5 micrograms kg-1) and measurement of eicosanoids 0.5 min following challenge were optimal for detection of redirection of PGH2 metabolism in vivo. The identity of immunoreactive TXB2 and 6-oxo-PGF1 alpha was further supported by experiments in which the extracted immunoreactive eicosanoids co-eluted with authentic [3H]standards when subject to reverse phase high performance liquid chromatography (RPHPLC). Evidence is also presented that the levels of plasma eicosanoids measured in this model reflect in vivo biosynthesis.

Thromboxane synthase inhibition: implications for prostaglandin endoperoxide metabolism. II. Testing the 'redirection hypothesis' in an acute intravenous challenge model

In the preceding paper we described the characterisation of an acute intravenous challenge model for the evaluation of the effects of thromboxane synthase inhibition (TXSI) on eicosanoid metabolism. Herein we describe the biochemical pharmacology of two TXSI and aspirin in this model. Both TXSI caused significant inhibition of plasma TXB2 in vivo without elevation of 6-oxo-PGF1 alpha levels. Similar results were obtained when combined levels of 6-oxo-PGF1 alpha,13,14 dihydro 6-oxo-PGF1 alpha,13,14 dihydro 6,15-dioxo-PGF1 alpha and 6-oxo-PGE1 were measured as an index of PGI2 biosynthesis (PGI2m). Thus no evidence of in vivo redirection of PGH2 to PGI2 was found. Ex vivo experiments performed in serum gave an apparent stimulation of immunoreactive 6-oxo-PGF1 alpha following TXSI but RPHPLC analysis of extracted serum showed that this stimulation was accounted for by increase in a product co-eluting with [3H]PGF2 alpha. The implications of these findings in relation to TXSI and receptor antagonists are discussed.

Single dose pharmacokinetics and effects on platelet function of the thromboxane receptor blocker BM 13.177

The pharmacokinetics and pharmacodynamic effect on platelet activation of a single 800 mg oral dose of BM 13.177 have been investigated in 8 male volunteers. BM 13.177 disappeared from plasma with a terminal elimination half-life of 0.85 h. 52% of the dose was excreted unchanged in urine. Assuming complete absorption, total clearance was calculated to be 741.3 ml/min and renal clearance to range from 310.4 to 396.9 ml/min. The pharmacodynamic studies were performed ex vivo/in vitro in studies were performed ex vivo/in vitro in platelets stimulated either with methyl mercury chloride or with U 46619. Methyl mercury chloride is a platelet activator that requires TXA2 formation from endogenous arachidonic acid, whereas U 46619 is a stable PGH2 analogue and thromboxane mimetic at the platelet TXA2/PGH2 receptor. A close correlation between the plasma concentration-time profile of BM 13.177 and inhibition of platelet shape change or aggregation was demonstrated.

AH23848: a thromboxane receptor-blocking drug that can clarify the pathophysiologic role of thromboxane A2

Despite numerous suggestions in the literature that thromboxane A2 is involved in a variety of occlusive vascular diseases, no definitive evidence is available. Arguments have been presented to support the view that such evidence can only come from clinical studies with a highly specific thromboxane receptor-blocking drug. We have now identified such a drug, AH23848, in our laboratories. Preliminary experiments with AH23848, ([1 alpha (Z), 2 beta,5 alpha]-(+/-)-7-[5-[[(1,1'-biphenyl)-4-yl]methoxy]-2-(4-morpholin yl)-3-oxocyclopentyl]-4-heptenoic acid), show that it is a potent, specific thromboxane receptor-blocking drug that is orally active and has a long duration of action. It should be a valuable tool in elucidating any physiologic or pathologic role of thromboxane A2.

The biochemical pharmacology of thromboxane synthase inhibition in man

Selective inhibitors of thromboxane synthase have two theoretical advantages over inhibitors of the cyclooxygenase enzyme as potential antithrombotic compounds. First, they do not prevent formation of prostacyclin, a platelet-inhibitory, vasodilator compound, coincident with inhibiting thromboxane biosynthesis. Second, the prostaglandin endoperoxide substrate that accumulates in the platelet in the presence of thromboxane synthase inhibition may be donated to endothelial prostacyclin synthase at the site of platelet-vascular interactions (endoperoxide "steal"). Selective inhibition of thromboxane biosynthesis coincident with enhanced prostacyclin formation in vivo has been observed after administration of these compounds to man. Despite these attractive features and the efficacy of these compounds in diverse short-term animal preparations of thrombosis, investigations of their efficacy in human disease have proven disappointing. This may reflect on the importance of thromboxane A2 in the diseases that have been investigated. Alternatively, the lack of drug efficacy may have resulted from either incomplete suppression of thromboxane biosynthesis and/or substitution for the biological effects of thromboxane A2 by prostaglandin endoperoxides during long-term dosing studies. Given that selective inhibition of thromboxane formation can be approached with aspirin, the particular value of these compounds is dependent on enhancing prostacyclin formation. Aspirin inhibits thromboxane-dependent platelet activation, but many platelet agonists are likely to act in concert in vivo and prostacyclin inhibits platelet aggregation induced by both thromboxane-dependent and thromboxane-independent mechanisms. To test the hypothesis that thromboxane synthase inhibitors are efficacious in human disease, compounds of longer duration of action are required. Combination with antagonists of the prostaglandin/thromboxane A2 receptor may be necessary to reveal their full beneficial action.

Cerebral platelet thromboembolism and thromboxane synthetase inhibition

Platelet aggregating sodium arachidonate was slowly infused into the internal carotid artery (1 mg, 100 microliters, 1 microliter/s) of nitrous oxide anesthetized rats. The electroencephalographic activity recorded by a Cerebral Function Monitor from the injected hemisphere was reduced within minutes. The somatosensory evoked responses to contralateral electrical stimulation of the whisker area were eliminated on the same side in most cases when measured five and fifteen minutes after the infusion. The brain was frozen in situ with liquid nitrogen after fifteen minutes. Regional tissue analysis showed ipsilateral derangement of the cerebral energy state and increased lactate levels. Pretreatment with the platelet antiaggregating thromboxane synthetase inhibitor OKY-1581 (Sodium-3-4-(3-pyridylmethyl)phenyl-2-methyl-acrylate), 30 mg/kg i.v., fifteen minutes before the sodium arachidonate infusion prevented cerebral energy failure and elimination of the sensory evoked responses.

The role of thromboxane in primate endotoxin shock

Previous studies have suggested an important role of thromboxane (Tx) in the pathogenesis of endotoxin shock in the rat. The present study evaluated the role of thromboxane in an LD70 primate model of endotoxin shock by administering 6 mg/kg of endotoxin to three groups of animals that were pretreated with either saline (5 ml), OKY 1581 (2 mg/kg, 10 min prior), or imidazole (25 mg/kg/hr starting 30 min prior), groups I, II, and III, respectively. There were significant differences between the groups with respect to changes in MAP, PAP, and CO. OKY 1581 effectively blocked endotoxin-induced increase in plasma Tx. However, as a result of shunting of the endoperoxides into the prostacyclin pathway, there was a greater increase in plasma 6-keto PGF1 alpha, the stable hydrolysis product of prostacyclin. Imidazole augmented the formation of both prostacyclin and Tx. Despite the differences in plasma prostanoids, there was no difference between the groups with respect to changes in platelet or WBC counts, nor in survival: I (4/10); II (4/10); III (2/6).

CONCLUSIONS

(i) endotoxin-induced neutropenia and decrease in the platelet count are not Tx mediated; (ii) Tx is not solely responsible for the decrease in CO during endotoxin shock; (iii) it is possible to prevent endotoxin-induced increase in the PAP by either blocking Tx formation or by increasing endogenous PGI2 production; and (iv) Tx may not be a major contributing factor in the mortality of endotoxin shock in baboons.

Inhibition of vasoconstrictor mechanisms by dazoxiben in the rat mesenteric vasculature

Sympathetic neurotransmission can be modulated by prostaglandins in a number of tissues, but it is not known whether thromboxane A2 also influences neurotransmission. In this study, vasoconstrictor responses to electrical stimulation of the sympathetic nerves and to injection of noradrenaline were examined in the blood perfused mesentery of the rat in situ. The thromboxane synthetase inhibitor dazoxiben, infused into the perfusion circuit at 10-100 mumol/l, significantly inhibited constrictor responses to nerve stimulation and to injected noradrenaline and vasopressin. The cyclo-oxygenase inhibitor indomethacin (28 mumol/kg intravenously) had no effect on responses to nerve stimulation or noradrenaline, but pretreatment with indomethacin abolished the inhibitory effect of dazoxiben on vasoconstrictor responses. The thromboxane-mimetic (U46619, 10 nmol/l) slightly reduced responses to nerve stimulation (but not to noradrenaline), whereas prostacyclin (3-10 nmol/l) and PGE2 (3 nmol/l) markedly reduced responses both to nerve stimulation and to injections of noradrenaline. These prostanoids did not alter perfusion pressure at these concentrations. The data suggest that the inhibitory effect of dazoxiben on sympathetic neurotransmission is unlikely to be due directly to inhibition of thromboxane synthesis. Inhibition might result from diversion of endoperoxide metabolism to endogenous prostanoids that, in turn, inhibit activation of vasoconstrictor mechanisms.

Nifedipine in the treatment of Raynaud's phenomenon. Evidence for inhibition of platelet activation

Platelet activation has been reported to occur in patients with Raynaud's phenomenon; however, the effect of calcium channel blockers and thromboxane synthetase inhibitors has not been previously studied. The effect of two drugs that potentially inhibit platelet activation were studied: nifedipine, a calcium channel blocker, and dazoxiben, a specific thromboxane synthetase inhibitor. Two platelet-specific proteins released during platelet activation, beta-thromboglobulin and platelet factor 4, were measured during a double-blind clinical trial of these two drugs in patients with Raynaud's phenomenon. The plasma beta-thromboglobulin level was significantly elevated in the patient population (53.8 +/- 7.6 ng/ml) during the placebo period compared with that in a normal control population (27.0 +/- 3.1 ng/ml) (p less than 0.01). The plasma platelet factor 4 level was 8.7 +/- 2.2 ng/ml in the patients compared with 6.5 +/- 1.0 ng/ml in the normal subjects (p = NS). These findings indicate the presence of in vivo platelet activation in patients with Raynaud's phenomenon. Nifedipine lowered the levels of beta-thromboglobulin to near the normal range (33.4 +/- 4.6 ng/ml). The inhibition of platelet activation by nifedipine was associated with clinical improvement in Raynaud's phenomenon with fewer and less intense episodes. Beta-thromboglobulin was not lowered by dazoxiben (58.1 +/- 9.0 ng/ml) compared with the placebo. The reduction of beta-thromboglobulin levels by nifedipine indicates that in vivo platelet activation was inhibited by this agent. Since this was associated with a reduced frequency of attacks, it is not clear whether this was a direct effect of the drug on platelet activation, leading to decreased frequency of vasospasm, or an effect on vascular smooth muscle leading to decreased vasospasm and a secondary decrease in platelet activation.

A comparative study of the involvement of the prostaglandin H2/thromboxane A2 pathway in intravascular platelet aggregation in guinea-pigs and rats

The effects of indomethacin, dazoxiben and EPO45 on collagen-induced platelet aggregation in vivo were studied in guinea-pigs and rats to determine the involvement of the prostaglandin endoperoxide/thromboxane A2 pathway in the aggregatory response. Indomethacin and EPO45 (a thromboxane receptor antagonist) partially inhibited platelet aggregation in rats. It was concluded that only one third of the aggregatory response to collagen was mediated by the products of cyclo-oxygenase conversion of arachidonic acid. In rats, dazoxiben was inactive although the conversion of the prostaglandin endoperoxides to thromboxane A2 was inhibited (measured as thromboxane B2). 6-keto PGF1 alpha was detected in plasma after collagen was injected into dazoxiben-treated rats. In this species therefore, the endoperoxides have significant aggregatory activity whilst the apparent increase in the level of prostacyclin was not sufficient to have any anti-aggregatory effect. All three drugs were active in the guinea-pig. About 60% of the aggregatory response to collagen was due to the products of the cyclo-oxygenase pathway, the main mediator being thromboxane A2. In guinea-pigs, dazoxiben also elevated 6-keto PGF1 alpha in the plasma after an injection of collagen. However, this apparent increase in prostacyclin production did not contribute to the anti-aggregatory effect.

Prostaglandins and human platelet aggregation. Implications for the anti-aggregating activity of thromboxane-synthase inhibitors

Selective pharmacological blockade of thromboxane-synthase in human platelets by dazoxiben resulted in the reorientation of cyclic-endoperoxides towards PGE2, PGD2 and PGF2 alpha. At concentrations which can be reached when thromboxane-synthase is inhibited, PGE2 (100-500 nM) exerted a marked, concentration-dependent pro-aggregatory effect. This required the formation of endogenous or the addition of exogenous endoperoxides and was prevented by PGD2 or 13-aza-prostanoic acid, a selective antagonist of PGH2/TxA2 receptors. The anti-aggregating effect of PGD2 was evident at concentrations lower than those obtained in dazoxiben-treated platelets. It is proposed that in the absence of TxA2 generation, a combination of endoperoxides and PGE2 may result in normal aggregation. The latter may be inhibited by PGD2. No interference of PGF2 alpha on platelet function could be shown.