Preservation of myocardium in transient ischemia by the thromboxane synthetase inhibitor UK 38.485

Ifetroban sodium: an effective TxA2/PGH2 receptor antagonist

This review presents a comprehensive discussion on the chemistry, pharmacokinetics, and pharmacodynamics of ifetroban sodium, a new thomboxane A2/prostaglandin H2 receptor antagonist. Thromboxane A2 is an arachidonic acid product, formed by the enzyme cyclooxygenase. In contrast to other cyclooxygenase products, thromboxane A2 has been shown to be involved in vascular contraction and has been implicated in platelet activation. In general, results of clinical studies and animal experiments indicate that hypertension is associated with hyperaggregability of platelets and increased thomboxane A2 levels in blood, urine, and tissues. The precursors to thromboxane A2, prostaglandin G2, and prostaglandin H2, also bind and activate the same receptors. Thus, a receptor antagonist was thought to be an improved strategy for reversing the actions of thromboxane A2/prostaglandin H2, rather than a thromboxane synthesis inhibitor. This review describes new methods for the synthesis and analysis of ifetroban, its tissue distribution, and its actions in a variety of animal models and disease states. We describe studies on the mechanisms of how ifetroban relaxes experimentally contracted isolated vascular tissue, and on the effects of ifetroban on myocardial ischemia, hypertension, stroke, thrombosis, and its effects on platelets. These experiments were conducted on several animal models, including dog, ferret, and rat, as well as on humans. Clinical studies are also described. These investigations show that ifetroban sodium is effective at reversing the effects of thromboxane A2- and prostaglandin H2-mediated processes.

Thromboxane A2 receptor blockade improves contractile function following cardiopulmonary bypass in dogs and pigs

Thromboxane A2/prostaglandin endoperoxide (TP) receptor antagonists have been reported to decrease the extent of myocardial damage after coronary ligation. The purpose of this study was to determine if the TP antagonist SQ 30,741 can protect myocardial tissue during cardiac arrest and cardiopulmonary bypass (CPB) in dogs and pigs. In the first part of the study, anesthetized dogs were subjected to normothermic CPB (37.5 degrees C) at a flow rate of 2 liters/m2/min. Dogs were treated with either 5 mg/kg + 5 mg/kg/hr SQ 30,741 or vehicle starting before CPB. The aorta was cross-clamped for 25 min and then released to allow reperfusion. In another study, pigs had hypothermic (28 degrees C) CPB but with arrest for 1 hr. Myocardial recovery was assessed by segment shortening as determined by sonomicrometry. Canine hearts treated with SQ 30,741 had a significantly improved reperfusion contractile function such that at 60 min postreperfusion, segmental shortening returned to 96% of pre-bypass levels vs 70% in vehicle-treated controls (P less than 0.05). In pigs, 70% of vehicle-treated pigs could not be weaned off CPB and died. All six pigs treated with SQ 30,741 survived. SQ 30,741 prevented platelet loss in dogs, but did not in pigs. Thus, SQ 30,741 significantly improved reperfusion function in hearts subjected to CPB.

The combined antiischemic effects of the thromboxane receptor antagonist SQ 30,741 and tissue-type plasminogen activator

The thromboxane-receptor antagonist, SQ 30,741, may be used as adjuvant therapy for thrombolysis and has also been shown to have antiischemic activity that is independent of its thrombolytic activity. Since tissue-type plasminogen activator (t-PA) and SQ 30,741 may be administered simultaneously, we determined whether the antiischemic effects of SQ 30,741 can be potentiated by t-PA. This was accomplished by combining doses of t-PA and SQ 30,741, which alone were not cardioprotective. Anesthetized dogs were subjected to left circumflex coronary artery occlusion for 90 minutes and reperfusion for 5 hours. The dogs were treated during reperfusion with a dose of t-PA that caused approximately a 30% reduction in plasma fibrinogen alone or in combination with 1.5 mg/kg + 0.4 mg/kg/hr SQ 30,741, which started 10 minutes after initiation of ischemia. At these doses, neither t-PA nor SQ 30,741 alone significantly reduced infarct size (57% +/- 6%, 50% +/- 10%, 57% +/- 6% of the left ventricular area at risk for vehicle controls, t-PA, and SQ 30,741 respectively); however, combination treatment resulted in a significant reduction in infarct size (37% +/- 5% of the left ventricular area at risk). Higher doses of t-PA and SQ 30,741 alone significantly reduced infarct size. The protective effects of t-PA and SQ 30,741 occurred without altering peripheral hemodynamic status. No differences in collateral or reperfusion blood flow were observed between groups. Thus although SQ 30,741 may act to improve the efficacy of thrombolysis, t-PA may in turn enhance the antiischemic activity of SQ 30,741 or at least reduce the threshold dose.

Role of thromboxane A2 in the control of myocardial O2 supply/consumption balance and severity of ischemia during pacing-induced ischemia

The role of thromboxane A2 (TXA2) in the control of O2 supply/consumption variables during pacing-induced ischemia was examined using the TXA2 receptor antagonist SQ 29,548. Anesthetized, open-chest dogs were subjected to left anterior descending coronary artery (LAD) stenosis that produced significant epicardial S-T segment elevation (12 mV) only when superimposed on atrial pacing. Regional myocardial blood flow was determined using radioactive microspheres, and O2 consumption was determined by measuring O2 saturation of venous blood draining the ischemic region. The dogs were treated with saline or 0.2 mg/kg + 0.2 mg/kg/hr SQ 29,548, and the effect on ischemia was determined during 5-minute pacing-induced ischemic episodes at 10, 40, and 70 minutes postdrug or saline treatment. SQ 29,548 significantly reduced S-T elevation at 40 and 70 minutes postdrug compared with saline values and at all times measured compared with its paired predrug pace+stenosis values. SQ 29,548 reduced S-T elevation approximately 45% compared with its paired predrug values at 70 minutes. SQ 29,548 resulted in a significantly higher subendocardial-to-subepicardial flow ratio (0.70 +/- 0.10, p less than 0.05) compared with saline-treated animals (0.42 +/- 0.06), with an overall increase of flow to the ischemic region of approximately 40%. This increased flow was matched by a proportional increase in O2 consumption without a change in O2 extraction. The O2 supply/consumption balance was also unchanged by SQ 29,548 implying that despite the increase in blood flow, the ischemic region was still flow-limited.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of the thromboxane A2 receptor antagonist SQ 29,548 on the severity of pacing-induced ischemia

The effect of the thromboxane A2 (TXA2)/prostaglandin endoperoxide receptor antagonist, SQ 29,548 on pacing-induced ischemia was determined in anesthetized open-chest dogs. The dogs were subjected to a left anterior descending coronary artery (LAD) stenosis sufficient to result in ischemia as measured by epicardial ST-segment elevation only when the hearts were paced 70-80 beats/min above baseline. After a recovery (nonpacing) period, either 0.2 mg/kg + 0.2 mg/kg/hr SQ 29,548 or saline was infused i.v. The animals were subjected to 5 min periods of pacing before and 10, 40 or 70 min after the initiation of drug or saline treatment. Before drug treatment, pacing + LAD stenosis resulted in significant ST-elevation in both groups (11.5 +/- 1.1 mV and 12.4 +/- 0.6 mV for saline and SQ 29,548 groups, respectively). During drug treatment, SQ 29,548 significantly reduced ST-elevation during pacing + stenosis at 40 and 70 min. At 70 min, ST-elevation was 4.9 +/- 1.7 mV for the SQ 29,548 group vs 10.8 +/- 1.2 mV for the saline group (p less than 0.05). In separate experiments 0.2 mg/kg + 0.2 mg/kg/hr SQ 29,548 antagonized the activity of a TXA2 mimetic, U-46,619, for femoral vasoconstriction, causing 100-to-200 fold rightward shifts in dose response relationships. Thus, a dose of SQ 29,548 capable of strong TXA2-receptor blockade reduced the severity of ischemia, and this effect was independent of changes in arterial blood pressure and heart rate.

Lack of involvement of thromboxane A2 in postischemic recovery of stunned canine myocardium

Modification of the thromboxane: prostacyclin ratio alters the severity of reperfusion arrhythmias and postischemic damage in long-term, irreversibly injured myocardium. In this study, the effects of the thromboxane synthetase inhibitor dazmegrel and the thromboxane receptor antagonist BM 13.505 on myocardial postischemic functional recovery and preservation of tissue adenine nucleotides was examined after a 15-minute episode of ischemia followed by 3 hours of reperfusion (myocardial stunning). Dazmegrel (3 or 8 mg/kg) or BM 13.505 (10 mg/kg) was given 15 minutes before coronary occlusion and compared with a control group in barbital-anesthetized dogs. Regional segment shortening (percent segment shortening, sonomicrometry), regional myocardial blood flow (microspheres), and coronary venous eicosanoid and high-energy phosphate levels (biopsies after 3 hours of reperfusion) were measured. Areas at risk, regional myocardial blood flow, and regional segment shortening during coronary occlusion were similar in all groups. Dazmegrel (3 mg/kg) attenuated the decrease in endocardial and midmyocardial adenosine 5'-triphosphate, and both doses significantly improved regional segment shortening during reperfusion. Coronary venous thromboxane levels were significantly decreased throughout the experiment in both dazmegrel-treated groups, and thromboxane levels were significantly elevated in the control group 3 hours after reperfusion. Prostacyclin, measured in the form of its main metabolite, 6-keto-prostaglandin F1 alpha, did not change significantly in the control group throughout the experiment, but it was markedly increased in dazmegrel groups throughout reperfusion, particularly in the dazmegrel group receiving 3 mg/kg. BM 13.505 exerted no beneficial effects on postischemic function or metabolism. In conclusion, after a reversible ischemic insult, postischemic recovery of function and metabolic status was not enhanced by preocclusion treatment with a thromboxane receptor blocker, and thus, the beneficial effects of thromboxane synthesis inhibition on postischemic abnormalities was not due to a reduction in thromboxane but was the result of endoperoxide shunting and a subsequent increase in prostacyclin. Therefore, thromboxane does not appear to be an important mediator of reversible ischemia-reperfusion damage.