No abstract

Prostacyclin is a potent vasodilator and platelet inhibitor produced by vascular endothelium. Endogenous production of prostacyclin under physiologic conditions is extremely low, far below the capacity of vascular tissue to generate this substance in response to stimulation in vitro. This may reflect a low frequency or intensity of stimulation of prostacyclin production. We postulated that if prostacyclin does act as an endogenous platelet-inhibitory agent, it should be produced in greater amounts in a clinical setting in which platelet-vascular interactions are likely to be increased. To test this hypothesis, we examined prostacyclin biosynthesis in patients with severe atherosclerosis and evidence of platelet activation in vivo. Excretion of 2,3-dinor-6-keto-prostaglandin F1 alpha, a major urinary prostacyclin metabolite, was significantly higher in 9 patients with severe atherosclerosis and evidence of platelet activation (251 to 1859 pg per milligram of creatinine) than in 54 healthy volunteers (45 to 219 pg per milligram of creatinine; P less than 0.001). This difference represented an alteration in biosynthesis rather than in metabolism, since the fractional conversion of infused prostacyclin to the dinor metabolite was identical in both groups. Prostacyclin production may be low in healthy persons because there is almost no stimulus for its production but enhanced in patients with severe atherosclerosis as a consequence of platelet interactions with endothelium or other vascular insults. These observations are compatible with a role for prostacyclin as a local regulator of platelet-vascular interactions.

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 ,8-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-ketoprostaglandin F,a, the major urinary metabolite of prostacyclin, was markely 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 synthase 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.Circulation 73, No. 6, 1300No. 6, -1309No. 6, , 1986 IN VIEW OF the potential importance of the proaggregatory platelet eicosanoid thromboxane A2 in the mediation of vascular occlusive events in vivo, there has been considerable interest in the effects of selective From the

The effects of nafazatrom on leukocyte function in vitro and in vivo were related to its ability to salvage ischemic myocardium in an occlusion-reperfusion model of myocardial injury in the anesthetized dog. Nafazatrom (0.4-75 microM) produced dose-related inhibition in vitro of neutrophil aggregation, superoxide anion generation, arachidonic acid metabolism, and, to a lesser extent, the release of beta-glucuronidase. In contrast, nafazatrom (0.4-37.5 microM) did not substantially influence platelet aggregation or the platelet metabolism of arachidonic acid. In vivo nafazatrom (10 mg/kg, po) reduced infarct size from 58 +/- 3% of the risk area (mean +/- SEM, n = 9) in control dogs to 23 +/- 2% of the risk area (n = 9, P less than 0.01). Nafazatrom also reduced the incidence of accompanying arrhythmias. Nafazatrom-induced myocardial salvage was not associated with any hemodynamic changes; moreover, it was independent of platelets, since thrombocytopenia did not prevent nafazatrom from exerting a protective effect. Measurements of the neutrophil-specific myeloperoxidase enzyme in ischemic myocardium indicate that the smaller infarct size in dogs treated with nafazatrom is accompanied by diminished leukocyte infiltration. Thus, the ability of nafazatrom to inhibit neutrophil function in vitro and cell infiltration in vivo may underly its myocardial-protective effects.