Inhibition of platelet activation by prostacyclin: possible consequences in coagulation and anticoagulation

Preservation of platelet function in cryopreserved platelet concentrates with prostacyclin

Prostacyclin (Epoprostenol) or a stable prostacyclin analogue (ZK 36,374) were added to platelet concentrates prior to cryopreservation. This resulted in significantly better preserved function of the thawed platelet concentrate, assessed by platelet aggregation to various concentrations of ADP, collagen and ristocetin, compared to control cryopreserved platelet concentrates. The use of prostacyclin or one of its stable analogues should be considered to reduce platelet activation and subsequent loss of function during the various manipulative procedures when preparing standard and cryopreserved platelet concentrates.

Oxidant stress but not thromboxane decreases with epoprostenol therapy

Epoprostenol has improved the outcome of patients with primary pulmonary hypertension (PPH); however, its mechanism of action remains poorly understood. Isoprostanes are easily measured markers of oxidant stress and can activate platelets leading to increased thromboxane A2 (TxA2) production. We hypothesized that oxidant stress is associated with increased TxA2 synthesis and that epoprostenol decreases oxidant stress and TxA2 production in patients with PPH. Morning urine samples were obtained from 19 patients with PPH. We measured urinary metabolites of the isoprostane, 8-iso-PGF2alpha (F2-IsoP-M), and of TxA2 (Tx-M) before and after treatment with epoprostenol in patients with PPH. Mean (+/-SE) levels of F2-IsoP-M were elevated at baseline in our patients, 863 +/- 97 pg/mg creatinine. During treatment with epoprostenol, values decreased to 636 +/- 77 pg/mg creatinine (P = 0.011), and there was a strong correlation between the change in F2-IsoP-M and follow-up pulmonary vascular resistance (R2 = 0.69, P < 0.001). Tx-M levels were markedly elevated at baseline and were unchanged with therapy. These results indicate that oxidant stress decreases with epoprostenol therapy and is associated with hemodynamic and clinical improvement. The failure of Tx-M to decrease with therapy suggests that epoprostenol does not exert a beneficial effect through inhibition of TxA2 production in patients with PPH.

Physiology and pharmacology of prostaglandins

Prostaglandins (PGs) are products of polyunsaturated acid metabolism, particularly arachidonic acid (AA) released from membrane phospholipids by the action of phospholipase A2 in response to a variety of physical, chemical, and neurohormonal factors. AA is rapidly metabolized to oxygenated products by two distinct enzymatic pathways: cyclooxygenase and lipoxygenase. The intermediate cyclooxygenase products are converted to primary PGs, while the lipoxygenase products are converted to leukotrienes. The generation of various cyclooxygenase products varies from tissue to tissue. Aspirin and related antiinflammatory drugs reduce tissue biosynthesis of all cyclooxygenase products; their therapeutic effects and side effects parallel the inhibition of cyclooxygenase. Exogenous PGs exhibit a broad spectrum of effects. PGs of the E series and PGI2 are generated by the endothelium and the vessel wall to maintain the microcirculation and to counteract the vasoconstrictive and proaggregatory actions of thromboxane A2 (TXA2). Exogenous PGs of the E and I series are potent vasodilators in various vascular beds, and result in decreased systemic blood pressure and reflex stimulation of heart rate. PGEs and PGI2 increase renal blood flow and provoke diuresis and natriuresis, partly by modulating the renin-angiotensin-aldosterone system. PGFs contract the bronchial and gut muscle, while PGEs and PGI2 have opposite effects. PGEs and PGFs, but not PGI2, cause a strong contraction of the uterine muscle, hence their undesirable uterotonic effects. PGEs relax bronchial muscle, whereas PGFs cause bronchoconstriction; their imbalance may contribute to the high bronchial tone in bronchial asthma. PGs of the E and I series and TXA2 are generated by the gastrointestinal mucosa and released into the lumen upon neural or hormonal stimulation; they probably participate in the maintenance of mucosal integrity and microcirculation. Exogenous PGs of the E and I series inhibit gastric acid secretion and stimulate alkaline secretion while increasing mucosal blood flow. All PGs, including those noninhibitory for acid secretion, are cytoprotective against various ulcerogens and necrotizing agents. The classic PGs constitute only a small fraction of biologically active products of AA metabolism, and recent studies on the lipoxygenase products emphasize their biological activity and involvement in a variety of pathological conditions.

A chemically stable analogue, 9 beta-methyl carbacyclin, with similar effects to epoprostenol (prostacyclin, PGI2) in man

The effects of 9 beta-methyl carbacyclin, a chemically stable analogue of epoprostenol (prostacyclin, PGI2) were studied, in comparison with epoprostenol, both in vitro and in vivo in man. In vitro 9 beta-methyl carbacyclin and epoprostenol inhibited platelet aggregation induced by ADP, collagen, the endoperoxide analogue U46619 and arachidonic acid. The potency of 9 beta-methyl carbacyclin relative to epoprostenol was comparable in ADP and collagen-aggregated platelet rich plasma (PRP), 9 beta-methyl carbacyclin being 0.01 times as active as epoprostenol. The anti-aggregatory potencies of the two compounds were comparable in PRP and whole blood. The phosphodiesterase inhibitor isobutyl methyl xanthine enhanced the anti-aggregatory activity of both compounds in vitro. 9 beta-methyl carbacyclin and epoprostenol elevated platelet cyclic AMP, 9 beta-methyl carbacyclin being 0.04 times as active as epoprostenol. In a placebo controlled trial both drugs produces significant headache and facial flushing when compared with placebo. Nasal stuffiness, abdominal discomfort and nausea were reported on all three treatments. Both drugs caused significant and comparable increase in heart rate and decrease in pre-ejection (PEP) and PEP/left ventricular ejection time (LVET) ratio compared with placebo. Systolic and diastolic blood pressure, LVET and QS2 index were unchanged. Platelet aggregation responses to ADP were significantly inhibited by all three doses of both drugs compared with placebo. Bleeding time was significantly longer during epoprostenol infusion than either placebo or 9 beta-methyl carbacyclin infusion. Neither drug had significant effect, compared with placebo, on kaolin activated clotting time in PPP, PRP or in PRP in the presence of heparin, prothrombin time, partial thromboplastin time, thrombin clotting time, fibrinogen, fibrinogen degradation products or euglobulin clot lysis time. The pharmacodynamic effects and duration of action of 9 beta-methyl carbacyclin and of epoprostenol are similar; 9 beta-methyl carbacyclin is approximately 100 times less potent than epoprostenol in man.

Prostaglandin E1 infusion in unstable angina: Effects on anginal frequency and cardiac function

Intermittent vasospasm and/or platelet aggregation may play an important role in producing transient coronary artery obstruction leading to an unstable ischemic syndrome. To determine whether intravenous infusion of prostaglandin E1 (PGE1) a known coronary vasodilator and inhibitor of platelet aggregation, produces salutary effects in unstable angina, we evaluated its effects in 19 patients with an unstable acute ischemic syndrome. PGE1 produced a significant decrease in the number of episodes of rest angina (p less than 0.001) and eliminated the need for intravenous nitroglycerin and morphine in 10 patients. These salutary clinical effects were associated with a significant (p less than 0.05) reduction in mean arterial pressure, mean pulmonary artery pressure, mean pulmonary capillary wedge pressure, and the double product, without a reduction in the endocardial perfusion gradient (aortic diastolic blood pressure--mean pulmonary capillary wedge pressure). Adverse effects were generally minor and easily controlled. Thus PGE1 infusion may be of value in the treatment of acute unstable ischemic syndromes.