R 68 070: Thromboxane A2 Synthetase Inhibition and Thromboxane A2/Prostaglandin Endoperoxide Receptor Blockade Combined in One Molecule - II. Pharmacological Effects In Vivo and Ex Vivo

Deletion of vascular thromboxane A2 receptors and its impact on angiotensin II-induced hypertension and atherosclerotic lesion formation in the aorta of Ldlr-deficient mice

The thromboxane A2 receptor (TP) has been shown to play a role in angiotensin II (Ang II)-mediated hypertension and pathological vascular remodeling. To assess the impact of vascular TP on Ang II-induced hypertension, atherogenesis, and pathological aortic alterations, i.e. aneurysms, we analysed Western-type diet-fed and Ang II-infused TPVSMC KO/Ldlr KO, TPEC KO/Ldlr KO mice and their respective wild-type littermates (TPWT/Ldlr KO). These analyses showed that neither EC- nor VSMC-specific deletion of the TP significantly affected basal or Ang II-induced blood pressure or aortic atherosclerotic lesion area. In contrast, VSMC-specific TP deletion abolished and EC-specific TP deletion surprisingly reduced the ex vivo reactivity of aortic rings to the TP agonist U-46619, whereas VSMC-specific TP knockout also diminished the ex vivo response of aortic rings to Ang II. Furthermore, despite similar systemic blood pressure, there was a trend towards less atherogenesis in the aortic arch and a trend towards fewer pathological aortic alterations in Ang II-treated female TPVSMC KO/Ldlr KO mice. Survival was impaired in male mice after Ang II infusion and tended to be higher in TPVSMC KO/Ldlr KO mice than in TPWT/Ldlr KO littermates. Thus, our data may suggest a deleterious role of the TP expressed in VSMC in the pathogenesis of Ang II-induced aortic atherosclerosis in female mice, and a surprising role of the endothelial TP in TP-mediated aortic contraction. However, future studies are needed to substantiate and further elucidate the role of the vascular TP in the pathogenesis of Ang II-induced hypertension, aortic atherosclerosis and aneurysm formation.

Trapped platelets activated in ischemia initiate ventricular fibrillation

BACKGROUND

We tested the hypothesis that ischemia-induced ventricular fibrillation (VF) is facilitated by platelets, trapped regionally in the ischemic zone and activated to release arrhythmogenic secretome.

METHODS AND RESULTS

In a randomized study in blood-free, buffer-perfused isolated rat hearts, ischemic zone territory (34±1% of left ventricle) was selected so that ischemia evoked VF in only 42% of controls. VF incidence was increased to 91% (P<0.05) by coronary ligation-induced trapping of freshly prepared autologous platelets (infused before and during coronary ligation, with trapping confirmed by 111In-labeled platelet autoradiographic imaging). Trapping of platelet secretome prepared ex vivo, or platelet-sized fluorospheres, did not increase ischemia-induced VF incidence. Secretome alone did, however, evoke VF in 2 sham coronary-ligated hearts. Perfusion did not activate infused platelets in sham coronary-ligated hearts, whereas ligation activated trapped platelets (assessed by thromboxane release). In a separate study, trapping whole-heparinized blood mimicked the ability of trapped platelets to increase VF incidence. This effect was not prevented by >5 days oral pretreatment in vivo with clopidogrel (10 mg/kg per day) or indomethacin (2.4 mg/kg per day).

CONCLUSIONS

Platelets facilitate VF during acute ischemia independently of their ability to participate in occlusive thrombosis. Moreover, the effect is unresponsive to antiplatelet drugs commonly used. Labile secretome constituents appear to be responsible. This opens a novel avenue for antiarrhythmic drug research.

Pharmacological profile and therapeutic potential of BM-573, a combined thromboxane receptor antagonist and synthase inhibitor

BM-573 (N-terbutyl-N'-[2-(4'-methylphenylamino)-5-nitro-benzenesulfonyl]urea), a torsemide derivative, is a novel non-carboxylic dual TXA2 synthase inhibitor and receptor antagonist. The pharmacological profile of the drug is characterized by a higher affinity for the thromboxane receptor than that of SQ-29548, one of the most powerful antagonists described to date, by a complete prevention of human platelet aggregation induced by arachidonic acid at a lower dose than either torsemide or sulotroban, and by a significantly prolonged closure time measured by the platelet function analyser (PFA-100). Moreover, at the concentrations of 1 and 10 microM, BM-573 completely prevented production of TXB2 by human platelets activated by 0.6 mM of arachidonic acid. BM-573 prevents rat fundus contraction induced by U-46619 but not by prostacyclin or other prostaglandins. Despite possessing a chemical structure very similar to that of a diuretic torsemide, BM-573 has no diuretic activity. BM-573 does not prolong bleeding time and, unlike some of the other sulfonylureas, has no effect on blood glucose levels. In vivo, BM-573 appears to have antiplatelet and antithrombotic activities since it reduced thrombus weight and prolonged the time to abdominal aorta occlusion induced by ferric chloride. BM-573 also relaxed rat aorta and guinea pig trachea precontracted with U-46619. In pigs, BM-573 completely antagonized pulmonary hypertensive effects of U-46619 and reduced the early phase of pulmonary hypertension in models of endotoxic shock and pulmonary embolism. Finally, BM-573 protected pigs from myocardial infarction induced by coronary thrombosis. These results suggest that BM-573 should be viewed as a promising therapeutic agent in the treatment of pulmonary hypertension and syndromes associated with platelet activation.

Paradox of simultaneous intestinal ischaemia and hyperaemia in inflammatory bowel disease

This review has focused on evidence regarding intestinal perfusion of inflammatory bowel disease (IBD). Basic investigation has defined an altered microvascular anatomy in the affected IBD bowel, which corresponds with diminished mucosal perfusion in the setting of chronic, long-standing inflammation. Diminished perfusion is linked to impaired wound healing, and may contribute to the continued refractory mucosal damage, which characterizes IBD. Alterations in vascular anatomy and physiology in IBD suggests additional possible mechanisms by which micro-vessels may contribute to the initiation and perpetuation of IBD. This begs the following questions: will angiogenesis within the gut lead to sustained inflammation, does the growing vasculature generate factors that transform the surrounding tissue and does angiogenesis generate vascular anastomosis within the gut, with shunting of blood away from the mucosal surface, impairment of metabolism and potentiation of gut damage? Further studies are required to define the mechanisms that underlie the vascular dysfunction and its role in pathophysiology of IBD.

In vitro effects of risperidone and 9-hydroxy-risperidone on human platelet function, plasma coagulation, and fibrinolysis

BACKGROUND

Thrombotic events have been reported with the use of antipsychotic compounds, although the incidence, predisposing factors, and biological mechanisms associated with these events in psychiatric patients are subject to debate.

OBJECTIVE

The in vitro actions of risperidone and its active metabolite 9-hydroxy-risperidone (9-OH-risperidone) on human platelet function, plasma coagulation, and fibrinolysis were examined to explore whether hematologic effects might be a mechanism for thrombotic events with these compounds.

METHODS

Blood was donated by healthy white male subjects who were free of medications (particularly acetylsalicylic acid and nonsteroidal anti-inflammatory compounds). Platelet shape change and adhesion/aggregation reactions to risperidone and 9-OH-risperidone induced by adenosine diphosphate (ADP), collagen, epinephrine, and 5-hydroxytryptamine (5-HT) were tested in human platelet-rich plasma. Arachidonic acid metabolism was assessed in human platelets and rat aortic rings. Plasma coagulation was tested in human platelet-poor plasma. Fibrinolysis was measured in human whole blood.

RESULTS

The 12 study subjects ranged in age from 20 to 40 years (median age 30 years). At concentrations of 1 x 10(-5) mol/L (approximately 4180 ng/mL), neither risperidone nor 9-OH-risperidone induced platelet shape change or aggregation, amplified reactions to ADP, or modified platelet adhesion/aggregation induced by collagen or ADP, but they did attenuate epinephrine-induced platelet aggregation (-50% in the case of 9-OH-risperidone; P < 0.05) and 5-HT-induced platelet aggregation (drug concentrations yielding 50% inhibition of 5-HT-induced platelet aggregation, 0.5 and 0.2 ng/mL, respectively). Cyclooxygenase, thromboxane A2 synthase, 12-lipoxygenase, prostacyclin synthase, plasma coagulation, and fibrinolysis were unaffected.

CONCLUSIONS

Risperidone and 9-OH-risperidone reduced epinephrine- and 5-HT-induced human platelet aggregation but did not significantly alter other measures of platelet function, plasma coagulation, or fibrinolysis in vitro.

BM-573, a dual thromboxane synthase inhibitor and thromboxane receptor antagonist, prevents pig myocardial infarction induced by coronary thrombosis

The aim of this study was to characterize the effects of BM-573 [N-terbutyl-N'-[2-(4'-methylphenylamino)-5-nitro-benzenesulfonyl] urea], a novel dual thromboxane A2 receptor antagonist and thromboxane synthase inhibitor, on myocardial infarction induced by topical ferric chloride (FeCl3) application to the left anterior descending (LAD) coronary artery in anesthetized pigs. All control animals (n = 6) developed an occlusive thrombus in the LAD coronary artery. The mean infarct size, revealed by triphenyl tetrazolium chloride (TTC), and the area at risk, evidenced by Evans blue, corresponded to 35.3 +/- 2.2 and 36.9 +/- 2.1% of the left ventricular mass, respectively. In the BM-573-treated group (n = 6), a drug infusion (10 mg. kg-1. h-1) started 30 min before FeCl3 application and continued throughout the experimentation. Among the BM-573-treated group, four pigs did not develop coronary artery thrombus and their myocardium appeared healthy. Histopathological examination of FeCl3-injured coronary artery revealed an occlusive and adherent thrombus in control group, while pretreatment with BM-573 prevented thrombus formation. In infarcted zones, lack of desmin staining and muscle structure disorganization were obvious. Depletion of myocardial ATP content was observed in the myocardial necrotic region of the control group, but not in myocardial samples of BM-573-treated pigs that did not develop myocardial infarction. When BM-573 prevented LAD artery occlusion, the area under the curve of plasmatic troponin T was reduced by 77% over 6 h. These data suggest that BM-573 could be useful for the prevention of myocardial infarction.

Relationship between eicosanoids and endothelin-1 in the pathogenesis of erythropoietin-induced hypertension in uremic rats

Recent studies suggest a possible link between recombinant human erythropoietin (rhEPO)-induced hypertension and endothelium-derived vasoconstrictor autocoids. The current study was designed to evaluate the role of eicosanoids such as thromboxane (TX) A and prostacyclin (PGI ) and of endothelin-1 (ET-1) and the relationship between these vasoactive substances in rhEPO-induced hypertension in uremic rats. Renal failure was induced by a two-stage 5/6 nephrectomy followed by a 6-week stabilization period. In protocol A, rats were divided into four groups: vehicle, rhEPO (100 u/kg, subcutaneously, three times per week), a selective ET receptor antagonist (ABT-627, 10 mg/kg/d), and rhEPO + ABT-627 for 5 weeks. In protocol B, uremic animals were divided into two groups: rhEPO and rhEPO + a TX receptor antagonist and synthesis inhibitor, ridogrel (25 mg/kg/d), for 5 weeks. At the end of the study, immunoreactive eicosanoid metabolites (TXB and 6-keto-PGF, stable metabolites of TXA and PGI ), and ET-1 were measured in either the thoracic aorta or in the mesenteric arterial bed. After 5/6 nephrectomy, the animals developed uremia, anemia, and hypertension. rhEPO corrected the anemia but aggravated the hypertension. Both drugs were effective in preventing the progression of hypertension in rhEPO-treated rats although ABT-627 was more potent than ridogrel. rhEPO increased the concentration of ET-1 and TXB in blood vessels and ABT-627 decreased tissue levels of both vasopressors. The concentration of 6-keto-PGF was not significantly changed. Ridogrel significantly decreased tissue TXB concentrations but had no effect on ET-1 levels. These results suggest that endothelium-derived vasoconstrictor autacoids (TXA and ET-1) are involved in the pathogenesis of rhEPO-induced hypertension in uremic rats. TXA probably serves as a mediator of the vascular effect of ET-1.

Pharmacology of the thromboxane receptor antagonist and thromboxane synthase inhibitor BM-531

BM-531 (N-tert-butyl-N'-[(2-cyclohexylamino-5-nitrobenzene)sulfonyl]urea), a torasemide derivative, is a novel noncarboxylic thromboxane receptor antagonist and thromboxane synthase inhibitor. Indeed, its affinity for human washed platelet TXA2 receptors labeled with [3H]SQ-29548 (IC50 = 0.0078 microM) is higher than sulotroban (IC50 = 0.93 microM) and SQ-29548 (IC50 = 0.021 microM). Moreover, BM-531 is characterized by a potent antiaggregatory property. Indeed, on one hand, in human citrated platelet-rich plasma BM-531 prevents platelet aggregation induced by arachidonic acid (600 microM) (ED100 = 0.125 microM), U-46619, a stable TXA2 agonist (1 microM) (ED50 = 0.482 microM) or collagen (1 microgram/mL) (percentage of inhibition: 42.9% at 10 microM) and inhibits the second wave of ADP (2 microM)-induced aggregation. On the other hand, when BM-531 is incubated in whole blood from healthy donors, the closure time measured by the recently developed platelet function analyser (PFA-100) is significantly prolonged. In addition, at the concentrations of 10 and 1 microM, BM-531 totally prevents the production of TXB2 by human platelets activated by arachidonic acid. Finally, at 10 microM, BM-531 significantly prevents rat fundus contractions induced by U-46619 but not by prostacyclin. These results suggest that BM-531, which is devoid of the diuretic property of torasemide, can be regarded as a promising antiplatelet agent.

Efficacy and safety of oral ridogrel in the treatment of ulcerative colitis: two multicentre, randomized, double-blind studies

BACKGROUND

Ridogrel at low doses inhibits thromboxane synthase. Oral ridogrel, from 5 mg once daily to 150 mg twice daily, improves the endoscopic appearance of colonic mucosa and clinical manifestations in mild to moderate ulcerative colitis.

AIM

One US trial and one international trial were conducted to determine the effect of ridogrel on mild to severe active ulcerative colitis.

METHODS

Two 12-week, double-blind, randomized, parallel-group trials were conducted. A US trial compared 0.5 mg, 2.5 mg and 5 mg of ridogrel once daily with placebo. An international trial compared 0.5 mg of ridogrel once daily with 2.5 mg and 5.0 mg of ridogrel once daily and 800 mg of mesalazine (known as mesalamine in the USA) three times daily. The primary efficacy outcome measure was the rate of complete remission.

RESULTS

In the US trial, complete remission was achieved in 20.8% of patients in the 0.5 mg ridogrel group, 17.9% in the 2.5 mg ridogrel group, 20.6% in the 5.0 mg ridogrel group and 13.6% in the placebo group. In the international trial, 14.4% of patients in the 0.5 mg ridogrel group, 19.6% in the 2.5 mg ridogrel group, 19.4% in the 5.0 mg ridogrel group and 16.4% in the mesalazine group experienced complete remission. In the international trial, rates of complete remission at the end-point were greater in the 2.5 mg and 5.0 mg ridogrel groups than in the 0.5 mg ridogrel group, but the differences were not statistically significant. In the US trial, rates of complete remission at the end-point were greater in the 2.5 mg and 5.0 mg ridogrel groups than in the placebo group, but the differences were not statistically significant. Approximately 30% of the patients in each group discontinued treatment before the 12-week end-point owing to a lack of therapeutic response. All doses of ridogrel were well tolerated and comparable with placebo or mesalazine in terms of safety.

CONCLUSIONS

No significant differences in the primary efficacy outcome measure were found between either the 2.5 mg or the 5.0 mg dose of ridogrel and placebo in the US trial and between either the 2.5 mg or the 5.0 mg dose of ridogrel and the 0.5 mg dose of ridogrel, a surrogate dose for placebo, in the international trial. There was no clear indication in either trial of an effective dose of ridogrel in the treatment of ulcerative colitis.

Antiplatelet and anticoagulant effects of "HN-11 500," a selective thromboxane receptor antagonist

The antiplatelet and anticoagulant effect of a thromboxane receptor (TX receptor) antagonist developed by Nycomed (Linz) has been studied in a placebo-controlled double-blind phase I study. Sixteen healthy male volunteers received different single oral doses of "HN-11 500" (C(14)H(15)NO(5)S(2); 1, 10, 100, 200, and 400 mg). Eight volunteers received placebo. The washout period between each dosage applied was at least 12 days. Platelet aggregation induced by the thromboxane mimetic "U 46 619" (C(21)H(34)0(4)) and platelet adhesion to siliconized glass were significantly and dose-dependently inhibited. The effect lasted between 3 and 4 h (10 mg) and 8 h (400 mg), respectively, and correlated well with the pharmacokinetic data. Platelet aggregation seems to be more sensitive to monitor the effects of HN-11 500 on platelet function than platelet adhesion. Plasma levels of 300 ng/ml HN-11 500 probably leads to >90% inhibition of platelet aggregation. The template bleeding time slightly increased but did not exceed the normal range. Furthermore, there was a wide variation of results. There were no significant changes in platelet counts, platelet-induced thrombin generation time (PITT), and blood coagulation parameters. All doses of HN-11 500 were well tolerated. HN-11 500 is a potent TX receptor antagonist (TXRA), which inhibits either platelet aggregation or platelet adhesion, which has not yet been described. In clinical routine, TXRAs have to demonstrate the effectiveness in large clinical trials for different clinical indications and to compete with single or combined administrations of cyclooxygenase (COX) inhibitors, thienovridines, thromboxane synthase inhibitors, and GIIb/IIIa inhibitors.

Studies on antiplatelet agents from natural safrole. II. Synthesis and pharmacological properties of novel functionalized oxime O-benzylether derivatives

In an ongoing research program aiming at the synthesis and pharmacological evaluation of new possible prototype candidates exploring the molecular hybridation and bioisosterism principles for molecular designing, we describe in this paper the design and synthesis of a series of new functionalized oxime O-benzylethers (4a-b) and (14a-b) as antiplatelet agents based on the inhibition of arachidonic acid (AA) cascade enzymes. For the synthesis of these new bioactive derivatives we used safrole (5), a Brazilian abundant natural product, as starting material. The platelet anti-aggregating evaluation of these oxime O-benzylether compounds (4a-b) and (14a-b) in model induced by ADP, collagen and AA, has permitted to evidence an antithrombotic profile to these new derivatives, being the most active the derivative methyl [[3,4-methylenedioxyphenyl]methylene]amino]oxy]-4-methylenephenylacet ic acid (14a).

Placental transfer of the thromboxane synthetase inhibitor ridogrel in the late-pregnant ewe

OBJECTIVES

To assess the occurrence of placental transfer of the thromboxane synthetase inhibitor ridogrel in the pregnant ewe and to determine its effect on prostanoid levels in the ewe and fetal lamb, on uterine contractility and on maternal and fetal hemodynamics.

STUDY DESIGN

Five chronically instrumented pregnant ewes at 122 days of gestation received intravenous infusions of 5 mg/kg/3 h ridogrel and solvent. Maternal and fetal arterial samples were obtained at predetermined intervals to determine concentrations of ridogrel and prostaglandin metabolites TXB2, 6-keto-PGF1alpha, PGF2alpha, and PGE2. Maternal and fetal responses of blood flow and pressures were determined.

RESULTS

Fetal ridogrel levels were 25% of maternal concentrations. Ridogrel showed rapid and marked thromboxane synthetase inhibition and augmentation of levels of prostaglandin metabolites. There was no evidence of change in amniotic pressure, uterine blood flow, maternal and fetal blood pressure and heart rate.

CONCLUSION

Ridogrel is a potent thromboxane synthetase inhibitor which passes the sheep placenta, does not influence maternal and fetal hemodynamics and uterine contractility, and shows similar antiplatelet activity in the ewe and the fetal lamb.

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.

The use of pegorgotein in the treatment of frostbite

Free oxygen radicals have been postulated to be an important mediator of injury in frostbite. A long-acting version of the endogenous scavenger enzyme, superoxide dismutase, has been created by conjugating it with polyethylene glycol (pegorgotein, formerly known as PEG-SOD). This study evaluated the efficacy of pegorgotein on frostbite tissue survival when administered prior to rewarming. In a prospective study, two groups of nine rabbits received a standardized frostbite injury using a modified Weatherley-White model. A control group received no pharmacologic therapy; the treatment group received 10,000 IU/kg pegorgotein intravenously immediately postinjury. Healing was followed until a clear line of demarcation was apparent (10 days). The percentage of viable ear surface remaining at the end of the study was measured and used to compare the effectiveness of treatment. Student's t-test was used to determine statistical significance. The study was designed to have an 80% ability to detect a 35% difference in tissue survival. No significant difference in frostbite injury (p = 0.967) was observed between the control and treatment groups. The treatment group showed a 9.3 +/- 15.5% tissue survival, whereas the control group had 9.6 +/- 14.5% tissue survival. These results indicate no significant treatment effect for pegorgotein on tissue survival in a rabbit frostbite injury model when administered immediately postinjury.

Pharmacokinetic and pharmacodynamic properties of FK070 (KDI-792), a novel thromboxane receptor antagonist/thromboxane synthetase inhibitor, after single and multiple oral administrations to healthy volunteers

FK070, a thromboxane A2 (TXA2) receptor antagonist/TXA2 synthetase inhibitor, was given orally to healthy male volunteers in a single- and multiple-dose study. In the single-dose study (200, 300, 400 mg), the area under the plasma concentration-time curve (AUC) and the maximum plasma concentration (Cmax) increased non-linearly with dose, while the mean elimination half-life (V0) was essentially unchanged (3.9-7.3h). Recovery of the unchanged drug in the urine was 12-25%. Cmax and AUC as determined with 200 mg of drug after a meal decreased by about 60 and 30%, respectively. Ex-vivo platelet aggregation in the plasma by a TXA2 analogue, U46619, was almost completely inhibited within 1 h, after all doses of drug, with a significant dose-dependent inhibition maintained for 8 h or more, which was much longer than was expected from drug plasma concentration. The aggregation by adenosine diphosphate (ADP) was inhibited to a lesser extent. FK070 also inhibited TXA2 synthetase as evidenced by decreased production of TXB2 and reciprocally increased production of 6-keto-prostaglandin F1 alpha in the serum during ex-vivo whole blood coagulation. These effects peaked 1 h after drug and lasted until 4 h with the higher doses. In the multiple-dose study (300 mg, twice a day, after meals for 6.5 days), drug concentrations in the plasma were well fitted to a three-compartment open model with first-order absorption. FK070 afforded extensive inhibition of platelet aggregation by U46619 throughout the administration period, with a significant inhibition lasting as long as 48 h after conclusion of administration. No clearly drug-related changes were found in routine laboratory tests, subjective and objective findings, or vital signs. FK070 was concluded to be well tolerated and to provide long-lasting blockade of TXA2 receptors, and plasma concentration-dependent inhibition of TXA2 synthetase in the platelets.

Ridogrel as an adjunct to thrombolysis in acute myocardial infarction

An open pilot study was performed to assess the safety and preliminary efficacy of ridogrel, a selective thromboxane-A2 synthetase inhibitor and thromboxane-A2/prostaglandin endoperoxide receptor blocker, as adjunct to thrombolysis, with alteplase and heparin. In 50 patients with acute myocardial infarction, 300 mg ridogrel was injected intravenously in addition to alteplase and heparin. Ridogrel was continued orally (300 mg) twice daily for 5 days. Patency rate at initial (90 min) angiography, defined as thrombolysis in myocardial infarction perfusion grades 2 or 3, was 86%. Rescue percutaneous transluminal coronary angioplasty was performed in 10 patients; immediate results were good in nine, while a large dissection occurred in one patient. New ischemia occurred in 10 patients within 24 h, and after the second angiogram in seven cases. Three underwent coronary artery bypass grafting and seven percutaneous transluminal coronary angioplasty without further complication. Patency rate at second angiography (between 6 and 24 h) was 94%. New Q-waves appeared in 56% of the patients; 36% had a non-Q-wave infarction and 8% had no enzyme rise. Enzymatic infarct size, estimated by the cumulative quantity of alpha-hydroxybutyrate dehydrogenase released in 72 h, was substantially smaller than in comparable studies with rt-PA and heparin. One patient died due to a cerebrovascular hemorrhage. No other deaths occurred. Bleeding complications were seen in 18 patients (36%), necessitating blood transfusion in three. Reinfarction did not occur. Eventually 49 patients were discharged in good condition. Safety with regard to bleeding complications of ridogrel in conjunction with alteplase is about the same as that of aspirin. Immediate and late patency rates were high. Rescue percutaneous transluminal coronary angioplasty could be performed with relative safety and early reocclusion could be successfully dealt with by repeat percutaneous transluminal coronary angioplasty. Further studies with this or similar compounds seem warranted.

P450 inhibitors of use in medical treatment: focus on mechanisms of action

A number of cytochrome P450s are targets for compounds that are clinically used or under clinical evaluation for treatment of patients with mycotic infections, such as dermatophytosis, superficial and systemic candidiasis, cryptococcosis and aspergillosis, with skin diseases, such as psoriasis or ichthyosis, and other retinoid-sensitive malignancies, e.g., neuro-ectodermal glioma. Some of the P450 inhibitors are candidates for the treatment of hirsutism or prostate cancer, others are potent inhibitors of the P450 isomerase involved in the synthesis of thromboxane A2, a potent platelet aggregation inducer and vasoconstrictor.

Development of dual-acting agents for thromboxane receptor antagonism and thromboxane synthase inhibition--I. Synthesis, structure-activity relationship, and evaluation of substituted omega-phenyl-omega-(3-pyridyl)alkenoic acids

A series of arylsulfonamido-substituted omega-phenyl-omega-(3-pyridyl)alkenoic acids were synthesized and evaluated in vitro for their ability to act as both a thromboxane A2 receptor antagonist (TRA) and thromboxane synthase inhibitor (TSI). Variations of alkenoic acid chain length, olefin geometry, substituent effect on the benzenesulfonamido group, and conformational flexibility of the substituted arylsulfonamido group were examined. Among the various substituents, iodo-substitution gave the most potent compound. Conformational flexibility between the arylsulfonamido group and the phenyl ring attached to the alkenoic acid side chain significantly enhanced the dual activities. The compound (E)-21c was identified as the most potent TRA/TSI (TRA: Kd = 53 nM; TSI: IC50 = 23 nM) in the series studied. The compounds 9c and 10c have indicated that these series of compounds are orally active and are specific TSIs as exhibited by the so-called 'shunt' effect on prostacyclin synthesis in vitro.

Thrombin-induced endothelium-dependent inhibition and direct activation of platelet-vessel wall interaction. Role of prostacyclin, nitric oxide, and thromboxane A2

BACKGROUND

Platelet-vessel wall interaction plays an important role in acute cardiovascular disorders. Thrombin is a potent platelet activator but also has profound effects on the endothelium. Endothelial cells possess antithrombotic activity by releasing nitric oxide and prostacyclin, both potent vasodilators and platelet inhibitors. We studied the role of thrombin as a regulator of platelet-vessel wall interaction in isolated human arteries suspended in organ chambers for isometric tension recording.

METHODS AND RESULTS

In arteries with endothelium, thrombin (0.01 to 1 U/mL) induced endothelium-dependent relaxations, which were reduced by the nitric oxide synthase inhibitor NG-nitro-L-arginine methyl ester (L-NAME; 10(-4) mol/L) and/or indomethacin (10(-5) mol/L). Human platelets (75,000/microL) evoked only marginal contractions in arteries with endothelium (3 +/- 3% of the contraction to KCl 100 mmol/L; NS), which were markedly enhanced by endothelial removal (22 +/- 4%; P < .05). Thrombin (1 U/mL) did not affect the response to platelets in arteries with (6 +/- 5%; NS) but induced a huge contraction in rings without endothelium (53 +/- 6%; P < .01 versus control without endothelium). The potent contraction to thrombin-activated platelets (1000 to 75,000/microL) in arteries without endothelium was markedly inhibited by the thromboxane A2 synthetase/receptor antagonist ridogrel (10(-5) mol/L; P < .005 versus control) and the single-acting thromboxane receptor blocker SQ-30741 (10(-7) mol/L; P < .01 versus control).

CONCLUSIONS

Thus, thrombin directly stimulates platelets to release thromboxane A2, inducing potent vasoconstriction, which is prevented by the simultaneous thrombin-induced release of prostacyclin and nitric oxide from endothelial cells. In arteries devoid of functional endothelial cells, as occurs in patients with coronary artery disease, a combined inhibition of thromboxane production and action provides a potent therapeutic tool to interfere with the thrombin-induced activation of platelet-vessel wall interaction.

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.

Time course of the effects of a single bolus injection of F(ab')2 fragments of the antiplatelet GPIIb/IIIa antibody 7E3 on arterial eversion graft occlusion, platelet aggregation, and bleeding time in dogs

The time course of the effects of a single intravenous bolus injection of 10 mg/kg aspirin or 0.8 mg/kg F(ab')2 fragments of the monoclonal antiplatelet glycoprotein IIb/IIIa receptor antibody 7E3 [7E3-F(ab')2] on arterial occlusion, platelet aggregation, and bleeding time was studied in 30 dogs with an everted (inside out) carotid arterial segment inserted into the femoral artery. In the absence of an antiplatelet agent, the eversion grafts occluded spontaneously with platelet-rich thrombus within 30 minutes. With aspirin, arterial occlusion persisting for 2 hours occurred in 5 of 10 dogs and cyclic occlusion and reflow in 4 animals; arterial occlusion was observed in all dogs at 24 hours. With 7E3-F(ab')2, arterial patency persisted throughout a 2-hour observation period in all of 10 dogs and for 24 hours in 4 of the 10 dogs. Contralateral eversion grafting 24 hours after aspirin or 7E3-F(ab')2 injection was associated with graft patency for 2 hours in 1 of 5 aspirin dogs and in 3 of 5 7E3-F(ab')2 dogs; patency persisted for 24 hours. In dogs grafted 48 hours after aspirin or 7E3-F(ab')2 injection, patency at 24 hours was seen in 0 of 5 dogs given aspirin and 3 of 5 dogs given 7E3-F(ab')2. The overall frequencies of arterial graft patency at 2, 24, 48, and 72 hours after study drug injection were significantly higher in the 7E3-F(ab')2 groups than in the aspirin groups (P < .0005, n = 10 in each group; P < .05, n = 15; P < .005, n = 15; and P = .05, n = 5, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacological characterization of cinnamophilin, a novel dual inhibitor of thromboxane synthase and thromboxane A2 receptor

1. The pharmacological effects of cinnamophilin, a new lignan, isolated from Cinnamomum philippinense, was determined in vitro in human platelet, rat isolated aorta and guinea-pig isolated trachea and in vivo in mice and guinea-pigs. 2. Cinnamophilin inhibited dose-dependently human platelet-rich plasma (PRP) aggregation induced by arachidonic acid (AA), collagen and U-46619 with IC50 of 5.0 +/- 0.4, 5.6 +/- 0.6 and 3.0 +/- 0.4 microM, respectively. The second wave of ADP- or adrenaline-induced platelet aggregation was inhibited by cinnamophilin, while the first wave was only slightly inhibited by cinnamophilin above 30 microM. 3. Cinnamophilin was found to be a thromboxane A2 (TXA2) receptor blocking agent in human platelet, rat aorta and guinea-pig trachea as revealed by its competitive antagonism of U-46619-induced aggregation of human-PRP, contraction of rat aortic rings and guinea-pig tracheal rings with pA2 values of 7.3 +/- 0.2, 6.3 +/- 0.1 and 5.2 +/- 0.2, respectively. 4. [3H]-inositol monophosphate formation and the rise of intracellular Ca2+ caused by U-46619 in human platelet was suppressed by cinnamophilin (10 microM). 5. Cinnamophilin induced a dose-dependent inhibition of thromboxane B2 (TXB2) formation, while the prostaglandin E2 (PGE2) formation was increased. Cinnamophilin did not affect unstimulated platelet adenosine 3':5'-cyclic monophosphate (cyclic AMP) levels. When the platelets were challenged with AA, a dose-dependent rise in cyclic AMP was observed. Dazoxiben (a pure TX synthase inhibitor) and SQ 29548 (a pure TXA2 receptor antagonist) did not affect cyclic AMP levels in AA-treated platelets. 6. A high concentration of cinnamophilin (100 MicroM), failed to attenuate the contractile response of rat aorta to endothelin-l, angiotensin II, 5-hydroxytryptamine or noradrenaline. Contraction of tracheal rings induced by histamine, carbachol or KCl was also not inhibited by cinnamophilin (100 MicroM).7. Thirty min after intraperitoneal (i.p.) administration of cinnamophilin (100 microg kg-1), tail bleeding time of mice was prolonged more markedly than with indomethacin, dazoxiben or SQ 29548.8. Intravenous administration of AA (50 microg kg-1) to guinea-pig induced bronchoconstriction. Cinnamophilin(0.1 mg kg-1, i.v.) was administered 1 min before AA, the bronchoconstriction response to AA was abolished.9. It is concluded that cinnamophilin is a novel dual TX synthase inhibitor and TXA2 receptor antagonist and that it may be a useful tool for the investigation and treatment of diseases involving TXA2 disorders.

Randomized trial of ridogrel, a combined thromboxane A2 synthase inhibitor and thromboxane A2/prostaglandin endoperoxide receptor antagonist, versus aspirin as adjunct to thrombolysis in patients with acute myocardial infarction. The Ridogrel Versus Aspirin Patency Trial (RAPT)

BACKGROUND

Aspirin, by nonselectively blocking cyclooxygenase both in platelets and in endothelial cells, not only inhibits the thromboxane A2 pathway of platelet activation but at the same time also the generation of vasodilating and platelet-inhibitory prostanoids, such as prostacyclin, by the endothelial cells. Ridogrel, by inhibiting thromboxane A2 synthase and blocking the thromboxane A2/prostaglandin endoperoxide receptors, is a more potent antiplatelet agent than aspirin and might offer an advantage over aspirin as an adjunct to thrombolysis. This study was performed to compare the efficacy and safety of ridogrel with that of aspirin as conjunctive therapy for thrombolysis in patients with acute myocardial infarction.

METHODS AND RESULTS

A total of 907 patients with acute myocardial infarction were randomized between aspirin and ridogrel given in addition to streptokinase (1.5 MU over a period of 1 hour). The primary end point was coronary patency (TIMI flow grades 2 and 3) at predischarge angiography to be performed between 7 and 14 days after admission. A patent infarct-related vessel was found in similar proportions of patients in the two treatment groups: 72.2% in the ridogrel and 75.5% in the aspirin group. The presence of clinical markers of reperfusion at 2 hours and the incidence of major clinical events during hospital stay were also similar in both groups. However, in a post hoc analysis, a lower incidence of new ischemic events (reinfarction, recurrent angina, ischemic stroke) was observed with ridogrel: 13% versus 19% in the aspirin group (a 32% reduction; P < .025). No excess of serious bleeding complications, including hemorrhagic stroke, was found.

CONCLUSIONS

Ridogrel is not superior to aspirin in enhancing the fibrinolytic efficacy of streptokinase but might be more effective in preventing new ischemic events.

The effects of thromboxane inhibitors on the microvascular and tumor response to photodynamic therapy

Vascular stasis and tissue ischemia are known to cause tumor cell death in several experimental models after photodynamic therapy (PDT); however, the mechanisms leading to this damage remain unclear. Because previous studies indicated that thromboxane release is implicated in vessel damage, we further examined the role of thromboxane in PDT. Rats bearing chondrosarcoma were injected with 25 mg/kg Photofrin (intravenously) 24 h before treatment. Light (135 J/cm2, 630 nm) was delivered to the tumor area after injection of one of the following inhibitors: (1) R68070: a thromboxane synthetase inhibitor; (2) SQ-29548: a thromboxane receptor antagonist; and (3) Flunarizine: an inhibitor of platelet shape change. Systemic thromboxane levels were determined. Vessel constriction and leakage were evaluated by intravital microscopy. Tumor response was assessed after treatment. Thromboxane levels were decreased more than 50% with SQ-29548 as compared to controls. Thromboxane levels in animals given R68070 and Flunarizine remained at baseline levels. SQ-29548 and R68070 reduced vessel constriction compared to controls, while Flunarizine totally prevented vessel constriction. R68070 and SQ-29548 inhibited vessel permeability compared to PDT controls; Flunarizine did not. Animals given these inhibitors showed markedly reduced tumor cure. These results indicate that the release of thromboxane is linked to the vascular response in PDT.

Thromboxane A2 receptor antagonism and synthase inhibition in essential hypertension

Short-term effects of ridogrel, a combined thromboxane synthase inhibitor and receptor antagonist, were investigated in 16 patients with uncomplicated essential hypertension. After a 2-week placebo period without antihypertensive medication, patients were admitted to the hospital overnight on two occasions 3 weeks apart. On each occasion, they received two doses of either placebo or ridogrel (300 mg) 12 hours apart according to a double-blind crossover protocol. Renal and systemic thromboxane A2 and prostacyclin biosynthesis were investigated by measuring urinary excretion of thromboxane B2, 6-oxo-prostaglandin F1 alpha, and their respective 2,3-dinor metabolites using gas chromatography/mass spectrometry. Responses of platelets to a thromboxane A2 mimetic and to adenosine diphosphate were studied turbidometrically. Blood pressure was measured automatically at 20-minute intervals. Ridogrel reduced excretion of 2,3-dinor-thromboxane B2 and thromboxane B2 compared with placebo (21 +/- 6 versus 279 +/- 28 and 14 +/- 4 versus 39 +/- 9 ng/g creatinine, respectively; P < .0001 and P < .05). Excretion of 2,3-dinor-6-oxoprostaglandin F1 alpha and 6-oxoprostaglandin F1 alpha was increased by ridogrel compared with placebo (184 +/- 20 versus 146 +/- 11 and 86 +/- 9 versus 58 +/- 6 ng/g creatinine, respectively; P < .05). Ridogrel selectively antagonized platelet aggregation to the thromboxane mimetic (P < .0001). Blood pressure did not differ significantly between ridogrel and placebo treatment periods. Thus, in patients with essential hypertension, acute administration of ridogrel reduces renal and extrarenal thromboxane A2 biosynthesis, increases renal and extrarenal prostacyclin biosynthesis, inhibits thromboxane receptor-activated platelet aggregation, but has no effect on systemic arterial pressure.

Beneficial effects of combined thromboxane synthase inhibition/receptor blockade with CGS 22652 in a canine model of coronary thrombosis

Various antiplatelet agents were examined for their effectiveness as adjuncts to thrombolytic therapy in a canine model of thrombin-induced coronary thrombosis. Aspirin (5 mg/kg i.v. bolus), CGS 15435A (thromboxane synthase inhibitor (TxSI), 0.1 mg/kg i.v. bolus +0.04 mg/kg per h) and BM 13.505 (thromboxane receptor antagonist (TxRA), 0.5 mg/kg i.v. bolus +0.2 mg/kg per h) administered concurrently with streptokinase (750,000 units/h) were examined for their effects on reperfusion and reocclusion, as were a combination therapy with CGS 15435A + BM 13.505 or the dual TxRA/TxSI inhibitor, CGS 22652 (1 mg/kg i.v. bolus +0.4 mg/kg per h). All dogs received heparin (150 U/kg bolus + 50 U/kg per h) throughout the experimental protocol. Survival analysis at reperfusion indicated that thrombolysis was significantly improved in dogs treated with CGS 15435A, BM 13.505, CGS 15435A+BM 13.505 or CGS 22652 over that of vehicle-treated animals. Both dual inhibitor groups and the BM 13.505 group were significantly different from aspirin. Aspirin-treated dogs were not different from vehicle. Otherwise, all treatments differed from the vehicle-treated group at reocclusion. Time and incidence of reocclusion for CGS 22652 was significantly improved over that of BM 13.505. Residual thrombus weight was significantly reduced in the CGS 22652-treated and BM 13.505 + CGS 15435A-treated animals. These findings demonstrate that streptokinase-induced thrombolysis is accompanied by TxA2/prostaglandin H2 synthesis and platelet activation and suggest a role for platelet activation during reocclusion following clot lysis. These studies also show it is possible to combine the beneficial effects of both a TxRA and TxSI into a single chemical entity, CGS 22652, which, when administered as adjunctive therapy to streptokinase, results in an apparent synergistic antithrombotic effect.

Combined thromboxane A2 synthase inhibition and prostaglandin endoperoxide receptor antagonism limits myocardial infarct size after mechanical coronary occlusion and reperfusion at doses enhancing coronary thrombolysis by streptokinase

OBJECTIVES

We sought to examine to what extent a combination of strong thromboxane A2 synthase inhibition and moderate endoperoxide receptor blockade enhances streptokinase-induced coronary thrombolysis and provides anti-ischemic activity independent from its thrombolytic activity.

METHODS

Coronary thrombi, induced by crush injury and stenosis of the coronary artery, were lysed with streptokinase, 10,000 IU/kg body weight over 90 min, in anesthetized dogs receiving solvent (n = 11), ridogrel, 0.31 mg/kg intravenously, for thromboxane A2 synthase inhibition (n = 7) or ridogrel, 5 mg/kg, for additional prostaglandin endoperoxide receptor antagonism in addition to thromboxane A2 synthase inhibition (n = 7) 10 min before the administration of streptokinase.

RESULTS

Thrombolytic efficacy was greatest in animals receiving both dual-acting ridogrel, 5 mg/kg intravenously, and streptokinase as evidenced by the highest incidence of high grade coronary reperfusion (solvent 3 of 11; ridogrel, 0.31 mg/kg, 5 of 7; ridogrel, 5 mg/kg, 7 of 7; p < 0.05 vs. solvent) within the shortest delay (solvent 210 min; ridogrel, 0.31 mg/kg, 85 min; ridogrel, 5 mg/kg, 37 min; p < 0.05 vs. solvent and ridogrel, 0.31 mg/kg) and the lowest incidence of reocclusion (solvent 5 of 7; ridogrel, 0.31 mg/kg, 2 of 7; ridogrel, 5 mg/kg, 1 of 7; p < 0.05 versus solvent). Myocardial infarct size after coronary artery ligation (90 min) and subsequent reperfusion (150 min) in anesthetized dogs was 49.3 +/- 4.3% versus 29 +/- 3.9% (p < 0.05 vs. solvent) of the area of the left ventricle at risk in dogs receiving solvent (n = 9) or ridogrel, 5 mg/kg intravenously (n = 10), respectively, despite similar hemodynamic characteristics, collateral blood flow and area at risk in both groups.

CONCLUSIONS

Combined thromboxane A2 synthase inhibition and endoperoxide receptor antagonism 1) upgrades thrombolysis with streptokinase in canine coronary arteries, 2) limits myocardial infarct size after nonthrombotic coronary occlusion and reperfusion, and 3) may preserve ventricular function compromised by coronary occlusion through dual manipulation of the arachidonic acid cascade in blood and myocardial tissue, respectively.

Thrombolytic profiles of clot-targeted plasminogen activators. Parameters determining potency and initial and maximal rates

BACKGROUND

Targeting of plasminogen activators to the thrombus by means of fibrin-specific monoclonal antibodies may enhance their thrombolytic potency. The kinetics of clot binding of two human fibrin-specific monoclonal antibodies (MA-12B3 and MA-15C5) and of clot lysis with their chemical 1:1 stoichiometric complexes with recombinant single-chain urokinase-type plasminogen activator (rscu-PA) (rscu-PA/MA-12B3 and rscu-PA/MA-15C5) were determined in hamsters and rabbits. Thrombolytic potencies, maximal rates of clot lysis, and the duration of the lag phases before clot lysis of the antibody/rscu-PA conjugates were compared with those of rscu-PA and tissue-type plasminogen activator (rt-PA).

METHODS AND RESULTS

Bolus injection of 7.5 micrograms of 125I-labeled antibody in rabbits with an extracorporeal arteriovenous loop containing a 0.3-mL human plasma clot produced clot-to-blood ratios of 6.6 +/- 1.0 (mean +/- SEM) for MA-12B3 and 1.1 +/- 0.15 for MA-15C5 (p < 0.001 versus MA-12B3) within 6 hours. Progressive digestion of the clot did not alter the binding of MA-12B3 but resulted in as much as a 10-fold increase of the binding of MA-15C5. The conjugates infused intravenously over 90 minutes in hamsters with a human plasma clot in the pulmonary artery produced dose-related in vivo clot lysis. Thrombolytic potencies (maximal slope of the percent lysis versus dose in milligrams of u-PA equivalent per kilogram body weight) were 2,500 +/- 440 for rscu-PA/MA-12B3, 3,600 +/- 640 for rscu-PA/MA-15C5 (p = NS vs. rscu-PA/MA-12B3), 60 +/- 8 for rscu-PA (p < 0.001 versus both conjugates), and 380 +/- 66 for rt-PA (p < 0.001 versus both conjugates). The plasma clearances of the conjugates were fourfold to sixfold slower than those of rscu-PA and rt-PA. Maximal rates of clot lysis, determined by continuous external radioisotope scanning over the thorax, were 0.90 +/- 0.13%, 0.91 +/- 0.17%, 0.84 +/- 0.12%, and 1.1 +/- 0.16% lysis per minute for rscu-PA/MA-12B3, rscu-PA/MA-15C5, rscu-PA, and rt-PA, respectively; these maximal rates were obtained with 0.016, 0.016, 1.0, and 0.25 mg/kg, respectively, and were associated with minimal lag phases of 18 +/- 3.2, 28 +/- 4.9, 34 +/- 3.7, and 25 +/- 3.9 minutes, respectively.

CONCLUSIONS

The thrombolytic potency of the rscu-PA/antifibrin conjugates is determined by their clearance, as well as by rate and extent of initial binding to clots and by changes in binding during clot lysis. Clot targeting of rscu-PA with fibrin-specific antibodies increases its thrombolytic potency but does not alter the maximal rate or the minimal lag phase of clot lysis. These parameters appear to be independent of the nature of the plasminogen activator and of targeting.

Endogenous prostaglandin endoperoxides may alter infarct size in the presence of thromboxane synthase inhibition: studies in a rabbit model of coronary artery occlusion-reperfusion

OBJECTIVES

The aim of this study was to assess whether prostaglandin endoperoxides, which continue to be formed in the setting of thromboxane A2 synthase inhibition, might influence the fate of ischemic myocardium in a model of coronary occlusion and reperfusion.

BACKGROUND

It was recently demonstrated that thromboxane A2 synthase inhibitors reduce ischemic myocardial injury through a redirection of prostaglandin (PG) endoperoxides toward the synthesis of "cardioprotective" prostaglandins, such as PGI2, PGE2 and PGD2. However, part of these prostaglandin endoperoxides may also stimulate a receptor, shared with thromboxane A2, mediating platelet aggregation and vasoconstriction.

METHODS

New Zealand White rabbits were subjected to 30 min of coronary occlusion, followed by 5.5 h of reperfusion. Fifteen minutes before reperfusion, the animals were randomized to receive 1) saline solution (control animals, n = 8); 2) SQ 29548, a potent and selective thromboxane A2/PGH2 receptor antagonist (n = 8); 3) dazoxiben, a selective thromboxane A2 synthase inhibitor (n = 8); 4) R 68070 (Ridogrel), a drug with dual thromboxane A2 synthase-inhibiting and thromboxane A2/PGH2 receptor-blocking properties (n = 8); or 5) aspirin + R 68070 (n = 8).

RESULTS

Dazoxiben and R 68070, but not SQ 29548, significantly reduced thromboxane B2 formation and increased plasma levels of 6-keto-PGF1 alpha, PGE2 and PGF2 alpha. Ex vivo platelet aggregation induced by U46619 (a thromboxane A2 mimetic) was inhibited by SQ 29548 and R 68070 but not by dazoxiben. In control animals, infarct size determined at the end of the experiment by triphenyltetrazolium chloride staining averaged 57.7 +/- 3.2% of the area at risk of infarction. The administration of SQ 29548 did not significantly reduce infarct size compared with that in control animals, whereas dazoxiben and R 68070 significantly reduced infarct size to 36.7 +/- 2.8% and 16.6 +/- 3.6% of area at risk of infarction, respectively (p < 0.001 vs. control values). In rabbits treated with R 68070, infarct size was also significantly smaller than that of dazoxiben-treated rabbits (p < 0.01). This protective effect of R 68070 was completely abolished when the drug was administered with aspirin, infarct size in this group averaging 59.7 +/- 1.6% (p = NS vs. control values). No differences in regional myocardial blood flow, systemic blood pressure, heart rate or extent of area at risk were observed among groups.

CONCLUSIONS

Thus, prostaglandin endoperoxides play an important role in modulating the cardioprotective effects of thromboxane A2 synthase inhibitors. The simultaneous inhibition of thromboxane A2 synthase and blockade of thromboxane A2/PGH2 receptors by R 68070 identify a pharmacologic interaction of potential therapeutic importance.

Combination of inhibition of thrombin and blockade of thromboxane A2 synthetase and receptors enhances thrombolysis and delays reocclusion in canine coronary arteries

BACKGROUND

The efficacy of thrombolytic therapy in treating patients with acute myocardial infarction is limited by failure to achieve reperfusion in some patients, by the prolonged time required to achieve reperfusion, and by reocclusion of some coronary arteries. We designed this study to examine the effect of combined inhibition of thrombin and thromboxane synthesis and blockade of thromboxane A2 receptors in addition to tissue-type plasminogen activator (t-PA) on thrombolysis and reocclusion in an experimental canine model with coronary thrombosis.

METHODS AND RESULTS

Blood flow velocity in the left anterior descending coronary artery (LAD) of 32 anesthetized mongrel dogs was monitored by a pulsed Doppler flow probe. Coronary thrombosis was induced by applying electrical stimulation to the LAD at the site where an external constrictor was used to narrow the artery. Three hours after the formation of occlusive thrombus, animals were randomly assigned to receive one of the following: 1) t-PA (80 micrograms/kg + 8 micrograms.kg-1.min-1 i.v.) and saline; 2) t-PA and hirulog, a hirudin-based synthetic peptide and specific thrombin inhibitor (2 mg/kg + 2 mg.kg-1.hr-1 i.v.); 3) t-PA and ridogrel, a combined thromboxane A2 synthetase inhibitor and receptor antagonist (5 mg/kg + 2.5 mg.kg-1.hr-1 i.v.); or 4) t-PA, hirulog, and ridogrel. Reperfusion developed in 14% (one of seven) of dogs treated with t-PA alone at an average of 86 +/- 4 minutes after treatment, in 78% (seven of nine) of dogs treated with t-PA plus hirulog at 53 +/- 11 minutes, in 13% (one of eight) of dogs treated with t-PA plus ridogrel at 85 +/- 5 minutes, and in 88% (seven of eight) of dogs treated with t-PA, hirulog, and ridogrel at 37 +/- 10 minutes (comparison of the frequency of and the time to reperfusion, both p < 0.01). Among the dogs with reestablished coronary blood flow, reocclusion developed in the one treated with t-PA alone at 36 minutes after reperfusion, in seven of the seven treated with t-PA plus hirulog at 66 +/- 15 minutes, and in two of the seven treated with t-PA, hirulog, and ridogrel at 151 +/- 21 minutes (comparison of the frequency of and time to reocclusion, both p < 0.05). Reocclusion was not detected in the one dog treated with t-PA and ridogrel or in the other five dogs treated with t-PA, hirulog, and ridogrel within 180 minutes after reperfusion. Hirulog prolonged and maintained activated clotting times at a level twice that of baseline values. Hirulog inhibited ex vivo platelet aggregation induced by thrombin, and ridogrel inhibited platelet aggregation induced by U46619, a thromboxane mimetic.

CONCLUSIONS

Inhibition of thrombin in addition to treatment with t-PA enhances thrombolysis. A combination of inhibition of thrombin and thromboxane synthetase and blockade of thromboxane A2 receptor enhances thrombolysis and delays or may prevent reocclusion of the recanalized coronary arteries.

Synthesis of some guanylhydrazones and imidazolinylhydrazones as thromboxane-synthase and platelet aggregation inhibitors

The imidazolinylhydrazones of (3-pyridinyloxy)-acetaldehyde and of 6-[3-(2-formyl-pyridinyl)oxy]hexanoic acid were synthesized as cyclic analogues of the corresponding guanylhydrazones which were found to be selective inhibitors of human thromboxane-synthase. The benzene isosters were also prepared in order to define the importance of the ring nitrogen for the activity. Moreover, the guanyl- and imidazolinyl-hydrazones of two 6-[(3-pyridinyl)oxy]hexanoic acids showing in the 2 position an alkyl chain with an alpha, beta-unsaturated ketonic function were prepared. Imidazolinylhydrazones 7 and 18 are selective inhibitors of thromboxane-synthase, while the two guanylhydrazones 14 and 15 which do not affect prostanoid biosynthesis seemed to be antagonists at the thromboxane receptor.

Gas chromatographic-mass spectrometric determination of 11-dehydrothromboxane B2 in human urine

The extension of a method for the determination of thromboxane B2 (TxB2), 2,3-dinor-TxB2, 6-keto-prostaglandin F1 alpha (6-keto-PGF1 alpha) and 2,3-dinor-6-keto-PGF1 alpha to quantify 11-dehydro-TxB2 in the same urinary sample is described. After phenylboronic acid and C18 column chromatography, 11-dehydro-TxB2, which is present in urine as the lactone and its corresponding hydroxy acid, was quantitatively converted into its lactone form for a thin-layer purification step and pentafluorobenzyl esterification. Quantification of eicosanoids was achieved by analysing their trimethylsilyl ethers with gas chromatography and negative-ion chemical ionization mass spectrometry. The overall recovery from urine for tritiated 11-dehydro-TxB2 was 80%. The detection limit was 10 pg/ml. The method was applied to the determination of these eicosanoids in volunteers and in patients suffering from acute myocardial infarction.

Comparative thrombolytic properties of tissue-type plasminogen activator (t-PA), single-chain urokinase-type plasminogen activator (u-PA) and K1K2Pu (a t-PA/u-PA chimera) in a combined arterial and venous thrombosis model in the dog

The chimeric molecule K1K2Pu, comprising the two kringle domains (K1 and K2) of tissue-type plasminogen activator (t-PA) and the COOH-terminal region with the serine protease domain (Pu) of urokinase-type plasminogen activator (u-PA), was previously shown to have a 5- to 10-fold reduced clearance rate with maintained specific thrombolytic activity, resulting in an increased thrombolytic potency in animal models of venous and arterial thrombosis. To document the thrombolytic potential of K1K2Pu, the thrombolytic potency and fibrin specificity were studied in a combined platelet-rich arterial eversion graft thrombosis and venous whole blood clot model in heparinized dogs (100 U/kg bolus and 50 U/kg per h infusion). Dose-response effects of bolus injections of K1K2Pu (0.032 to 0.25 mg/kg) were compared with those of recombinant t-PA (rt-PA) and of recombinant single chain u-PA (rscu-PA) (0.25 to 1.0 mg/kg each) in groups of five or six dogs, each given heparin with or without the thromboxane synthase inhibitor/prostaglandin endoperoxide receptor antagonist ridogrel. Heparin and ridogrel in the absence of a thrombolytic agent did not produce arterial reflow or venous clot lysis in five dogs. Addition of K1K2Pu, rt-PA or rscu-PA resulted in a dose-dependent induction of arterial reflow and of venous clot lysis in the absence of systemic fibrinolytic activation and fibrinogen breakdown. Consistent arterial reflow required 0.063 mg/kg of K1K2Pu and 0.5 mg/kg of rt-PA or of rscu-PA. The thrombolytic potency for venous clot lysis, expressed as percent lysis per mg compound administered per kg body weight, was (mean +/- SEM) 750 +/- 160 for K1K2Pu, 68 +/- 17 for rscu-PA (p less than 0.001 vs. K1K2Pu) and 110 +/- 29 for rt-PA (p less than 0.001 vs. K1K2Pu). The plasma clearance rates were significantly lower for K1K2Pu than for rscu-PA and rt-PA. In the absence of ridogrel, arterial reflow was significantly slower and was followed by cyclic reocclusion and reflow; however, venous clot lysis was unaffected. Template bleeding times were not significantly altered in the absence but were markedly prolonged in the presence of ridogrel. These results confirm and establish that, when given as a bolus injection, K1K2Pu has an approximately 10-fold higher thrombolytic potency for arterial and venous thrombolysis than does rt-PA or rscu-PA. Thrombolysis with K1K2Pu is obtained in the absence of systemic fibrinolytic activation and fibrinogen breakdown. These properties suggest that K1K2Pu offers potential for thrombolytic therapy by bolus administration in patients with thromboembolic disease.

Ridogrel: a selective inhibitor of the cytochrome P450-dependent thromboxane synthesis

Ridogrel [(E)-5-[[[(3-pyridinyl)[3-(trifluoromethyl)phenyl] methylene]amino]oxy] pentanoic acid] is a potent inhibitor of the P450-dependent human platelet thromboxane A2 (TxA2) synthase. Fifty percent inhibition is already achieved at 5.0 +/- 0.37 nM. This IC50 value is close to half the P450 concentration used, i.e. 10.7 nM. Ridogrel binds to human platelet microsomal P450 as proven by the type II spectral changes induced by the addition of increasing concentrations of ridogrel to solubilized microsomes. The calculated half-maximal spectral change (SC50 value) is 3.78 +/- 1.79 nM. These results indicate that ridogrel binds stoichiometrically and suggest that inhibition of thromboxane synthesis may originate from liganding of its basic nitrogen to the haem-iron of P450 and from the attachment of the hydrophobic carboxylic side chain to or near the substrate binding place. Ridogrel is a selective inhibitor of the TxA2 synthase. At a high concentration (10 microM), ridogrel has a slight, if any, effect on the P450-mediated cholesterol synthesis in human liver and hepatoma cells and androgen synthesis from 17 alpha-hydroxy-20-dihydroprogesterone or pregnenolone in subcellular fractions from rat testes. These results indicate that ridogrel is a poor inhibitor of the P450-dependent 14 alpha-demethylase, 17 alpha-hydroxylase and 17,20-lyase. It has, up to 10 microM, no effect on the adrenal mitochondrial 11 beta-hydroxylase and cholesterol side-chain cleavage enzyme and does not inhibit aromatase activity in human placental microsomes. Ridogrel has no significant effect on the regio- and stereoselective P450-dependent oxidations of testosterone in liver microsomes from unpretreated or from 5-pregnen-3 beta-ol-20-one-16 alpha-carbonitrile-, phenobarbital- or 3-methylcholanthrene-pretreated male and female Sprague-Dawley rats. It does not interfere with the reduction of testosterone into 5 alpha-dihydrotestosterone and 5 alpha androstane 3 beta, 17 beta-diol.

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.

Thrombolytic and pharmacokinetic properties of chimeric tissue-type and urokinase-type plasminogen activators

BACKGROUND

Chimeric molecules comprising the A-chain of tissue-type plasminogen activator (t-PA) and the catalytic domain of urokinase-type plasminogen activator (u-PA) have intact enzymatic characteristics of u-PA, partial fibrin-binding properties of t-PA, and thrombolytic properties in animal models comparable with but not superior to those of single-chain u-PA (scu-PA). Deletion of the finger and growth factor domains (t-PA-delta FE/scu-PA-e) in such chimeras further reduces their affinity for fibrin.

METHODS AND RESULTS

A detailed investigation of the thrombolytic potency and the pharmacokinetics of t-PA and u-PA chimeras was performed in quantitative animal models for thrombolysis. In hamsters with pulmonary embolism, in rabbits with jugular vein thrombosis, and in baboons with femoral vein thrombosis, the thrombolytic potency (percent lysis per milligram of compound administered per kilogram of body weight) of t-PA-delta FE/scu-PA-e was significantly higher than that of recombinant scu-PA (rscu-PA, Saruplase) as shown by a maximal rate of 720 +/- 170% versus 45 +/- 5% lysis per milligram of compound per kilogram of body weight (mean +/- SEM, p less than 0.01) in hamsters, 210 +/- 18% versus 49 +/- 3% lysis per milligram of compound per kilogram of body weight (mean +/- SEM, p less than 0.01) in rabbits, and 310 +/- 73% versus 90 +/- 0.3% lysis per milligram of compound per kilogram of body weight (p less than 0.01) in baboons. However, the specific thrombolytic activity (percent lysis per microgram per milliliter steady-state plasma antigen level) of t-PA-delta FE/scu-PA-e was not significantly different from that of rscu-PA in hamsters (210 +/- 57% versus 160 +/- 27% lysis per microgram per milliliter antigen level) and was lower than that of rscu-PA in rabbits (37 +/- 4% versus 130 +/- 5% lysis per microgram per milliliter antigen level; p less than 0.01). In dogs with a combined femoral vein blood clot and a platelet-rich femoral arterial eversion graft thrombosis, 0.25 mg/kg body wt bolus injections of t-PA-delta FE/scu-PA-e produced significantly more venous clot lysis (90 +/- 5%, n = 10) than 0.25 mg/kg rscu-PA (26 +/- 3%, n = 10) (p less than 0.001) and, at the arterial side, more frequent (10 of 10 dogs versus three of 10 dogs) and more persistent (six of 10 dogs versus none of 10 dogs) recanalization (p = 0.002). After bolus injection in hamsters, rabbits, or baboons, t-PA-delta FE/scu-PA-e had a fourfold to sixfold longer initial half-life than rscu-PA and a slower plasma clearance of sixfold in hamsters, 10-fold in rabbits, and more than 10-fold in baboons.

CONCLUSIONS

These results indicate that t-PA-delta FE/scu-PA-e has a markedly enhanced thrombolytic potency toward venous and arterial thrombi caused by a delayed in vivo clearance with relatively maintained specific thrombolytic activity. These properties suggest that the chimera may be clinically useful for thrombolytic therapy by bolus administration in patients with thromboembolic disease.

Ridogrel prevents the thromboxane-mediated pressor response and oedema induced by hydrogen peroxide in isolated rabbit lungs

Perfusion of isolated rabbit lungs with hydrogen peroxide (H2O2, 3 x 10(-5) M) raised the overflow of thromboxane B2 (TXB2) and the perfusion pressure. H2O2 induced oedema formation and endothelial distress, as evidenced by an increased production of 6-oxo-prostaglandin F1 alpha (6-oxo-PGF1 alpha). Endothelial cell death did not occur since there was no release of lactate dehydrogenase. The thromboxane A2 (TXA2)-synthase inhibitor/receptor antagonist ridogrel (R68070) further enhanced 6-oxo-PGF1 alpha output, while inhibiting TXB2 release. Ridogrel prevented the rise in pulmonary artery pressure and oedema formation. These data indicate that TXA2 is probably involved in the acute pulmonary pressor response and concomitant oedema formation induced by H2O2. In order to assess the functional activity of the pulmonary endothelium, the uptake of 5-hydroxytryptamine (5-HT) was measured before and 15 min after exposure to H2O2. As the H2O2-induced effects were not associated with any change in the uptake of 5-hydroxytryptamine (5-HT), we conclude that the endothelial injury was reversible or that the 5-HT uptake was not sensitive enough to evaluate the integrity of the pulmonary endothelium during oxidant-induced injury.

Differential effects of thromboxane A2 synthase inhibition, singly or combined with thromboxane A2/prostaglandin endoperoxide receptor antagonism, on occlusive thrombosis elicited by endothelial cell injury or by deep vascular damage in canine coronary arteries

In open-chest dogs, cyclic flow reductions (CFRs, 5.1-6.6/hr in controls; n = 24) caused by platelet deposition/dislodgment at sites of endothelial cell injury in critically stenosed left anterior descending coronary arteries (59% flow reduction) were attenuated to the same extent either by single thromboxane A2 (TXA2) synthase inhibition (0.31 mg/kg i.v. ridogrel; CFR, 0.16 +/- 0.16/hr; n = 6; p less than 0.05) or by a comparatively modest degree of TXA2/prostaglandin endoperoxide receptor antagonism on top of TXA2 synthase inhibition (5 mg/kg i.v. ridogrel; CFR, 0.22 +/- 0.1/hr; n = 10; p less than 0.05). By contrast, occlusive thrombosis on deep vascular damage elicited by intraluminal stimulation (150-microA anodal constant current) in nonpreconstricted canine coronary arteries (time to occlusion, 237.1 +/- 13.9 minutes; n = 7; incidence of occlusion within 300 minutes, six of seven experiments) was not affected by platelet cyclooxygenase inhibition (5 mg/kg i.v. acetylsalicylic acid; n = 7), single TXA2 synthase inhibition (1.25 mg/kg i.v. ridogrel; n = 7), or single TXA2/prostaglandin endoperoxide receptor antagonism (10 mg/kg + 10 mg/kg/hr i.v. sulotroban for 300 minutes; n = 5). However, such an occlusive thrombus formation was significantly reduced by combined TXA2 synthase/prostaglandin endoperoxide receptor inhibition (5 mg/kg i.v. ridogrel; time to occlusion greater than 300 minutes, n = 7; incidence of occlusion within 300 minutes, one of seven experiments; p less than 0.05). This study reveals 1) a differential efficacy of TXA2 synthase inhibition, singly or combined with TXA2/prostaglandin endoperoxide receptor antagonism, depending on the extent of the vessel wall lesion triggering thrombosis and the size of the thrombus required to obstruct the vascular lumen and 2) a significant synergism in preventing occlusive thrombosis of extensively damaged coronary arteries between strong TXA2 synthase inhibition and comparatively modest TXA2/prostaglandin endoperoxide receptor antagonism with ridogrel.

Kistrin, a polypeptide platelet GPIIb/IIIa receptor antagonist, enhances and sustains coronary arterial thrombolysis with recombinant tissue-type plasminogen activator in a canine preparation

BACKGROUND

Kistrin is a 68-amino acid polypeptide from the venom of the Malayan pit viper Agkistrodon rhodostoma, which inhibits the platelet GPIIb/IIIa receptor. Its effect on thrombolysis, reocclusion, and bleeding associated with administration of recombinant tissue-type plasminogen activator (rt-PA) was studied in a canine model of coronary artery thrombosis.

METHODS AND RESULTS

Coronary patency was monitored for 2 hours by ultrasonic flow probe and repeated coronary angiography. The rt-PA was given as 0.45-mg/kg bolus injections at 15-minute intervals until recanalization or to a maximum of four boluses. Four groups of four or five dogs were studied: a control group that received intravenous heparin (4,000-unit bolus and 1,000 units each hour) and three groups that received heparin and 0.48, 0.24, or 0.12 mg/kg kistrin, administered as a 10% bolus injection and an infusion during a 60-minute period. In the control group, reflow occurred in four of five dogs within 37 +/- 47 minutes but was followed by cyclic reflow and reocclusion. Kistrin at a dose of 0.48 and 0.24 mg/kg reduced the time to reflow to 6 +/- 5 and 10 +/- 3 minutes, respectively, and abolished reocclusion. With 0.12 mg/kg kistrin, reflow occurred in all four animals, within 27 +/- 23 minutes, and reocclusion occurred in two animals. Kistrin induced a dose-related prolongation of the template bleeding time: with 0.48 mg/kg kistrin, the bleeding time was prolonged from 3.8 +/- 1.3 minutes before infusion to 29 +/- 2 minutes during infusion, but it was shortened to 8.3 +/- 2.6 minutes at 90 minutes after the end of infusion. Kistrin also caused a dose-related inhibition of platelet aggregation with ADP and collagen: with 0.48 mg/kg kistrin, platelet aggregation was abolished during the infusion but had partially recovered toward the end of the observation period. Pathological examination of recanalized coronary arterial segments of dogs given 0.48 or 0.24 mg/kg kistrin revealed widely patent arteries with some platelets layered on the damaged intimal surface.

CONCLUSIONS

Kistrin increases the rate and extent of thrombolysis with a reduced dose of rt-PA, and it prevents reocclusion. At an effective dose, it is associated with a transient prolongation of the bleeding time and inhibition of platelet aggregation. Kistrin may offer promise as adjunctive treatment to thrombolytic agents in patients with acute myocardial infarction.