Platelet arachidonic acid

The 6- or 7-phenyl-substituted l-ethyl-pyrazino[2,3-r-][l,2,6]thiadiazine 2,2-dioxides 187 were found to show significant platelet aggregation inhibition comparable to other antithrombotic agents. Strong evidence was found that this activity was due to interference with the platelet arachidonic acid pathway <1999JME1698>. 

Roth 01. (1986). Platelet arachidonic acid metabolism and platelet-activating factor. In Biodiemistry of Platelets (Phillips DR, and Shuman MA Eds.X PP. 69-100, Academic Press Inc. (London) Ltd., Londoa... 

The other sources of the arachidonic acid released by platelets in response to thrombin are phosphatidylcholine and phosphatidylethanolamine (Table 1.1). However, each of these phospholipids exists as three species in platelets, namely as the diacyl-, 1-0-alkyl-, or 1-0-alk-l enyl- forms (Table 1.2). Table 1.2 also shows that l-O-alkyl-2-acyl-PC and l-O-alk-l enyl-2-acyl-PE are enriched in arachidonic acid relative to the other classes of PC or PE, giving rise to the idea that they could serve as important sources of arachidonic acid in stimulated platelets. Purdon and Smith " prelabelled platelets with pH]-arachidonic acid and followed the changes in the different species of PC and PE following thrombin stimulation. It was found that while there was a decrease in radiolabel of both 1,2-diacyl-PC and 1,2-diacyl-PE at all times studied, there was no decrease in the other species of PC or PE, and, indeed, radiolabel in l-O-alkyl-2-acyl-PC and l-0-alk-l -enyl-2-acyl-PE increased at later time points (3-5 min) after thrombin. The thrombin-induced incorporation of arachidonic acid in plasmalogen PE was observed previously by others. Purdon and Smith " concluded that, upon stimulation of human platelets, arachidonic acid is released from both 1,2-diacyl-PC and 1,2-diacyl-PE for metabolism by cyclo-oxygenase and lipoxygenase, while certain other pools of phosphatidylcholine and phosphatidylethanolamine act to collect arachidonic acid. 

Lipoxygenation is the major pathway of dioxygenation of arachidonic acid in blood platelets and leads to the 12-5-hydroperoxy acid 12-HPETE and the corresponding 12-hydroxy acid 12-HETE. Several pathways for the synthesis of 12-HETE have been developed. However, despite the availability of this substance, its biological role remains undetermined. 

Prostacyclin I2 (PGI2) inhibits platelet aggregation and relaxes coronary arteries. Like PGE2 and TXA2, it is formed from arachidonic acid via PGH2. 

Animal cells can modify arachidonic acid and other polyunsaturated fatty acids, in processes often involving cyclization and oxygenation, to produce so-called local hormones that (1) exert their effects at very low concentrations and (2) usually act near their sites of synthesis. These substances include the prostaglandins (PG) (Figure 25.27) as well as thromboxanes (Tx), leukotrienes, and other hydroxyeicosanoic acids. Thromboxanes, discovered in blood platelets (thrombocytes), are cyclic ethers (TxBg is actually a hemiacetal see Figure 25.27) with a hydroxyl group at C-15. 

TXA2 is produced by activated platelets via the sequential conversion of arachidonic acid by phospholipase A2, cyclooxygenase-1 (COX-1), and thromboxane synthase. Similar to ADP, TXA2 acts as a... 

CYP5 synthesizes thromboxane A2, a fatty acid in the arachidonic acid cascade that causes platelet aggregation. Aspirin prevents platelet aggregation because it blocks the cyclooxygenases COX1 and COX2 which catalyze the initial step of the biotransformation of arachidonic acid to thromboxane and prostaglandins. 

CoUagen-induced activation of a platelet phospholipase A2 by increased levels of cytosolic Ca results in hberation of arachidonic acid from platelet phospho-hpids, leading to the formation of thromboxane A2 (Chapter 23), which in turn, in a receptor-mediated fashion, can further activate phospholipase C, promot-ing platelet aggregation. 

Lipoxygenases catalyse the regio-specific and stereoselective oxygenation of unsaturated fatty acids. The mammalian enzymes have been detected in human platelets, lung, kidney, testes and white blood cells. The leukotrienes, derived from the enzymatic action of the enzyme on arachidonic acid, have effects on neutrophil migration and aggregation, release of lysosomal enzymes, capillary permeability, induction of pain and smooth muscle contraction (Salmon, 1986). 

There is some evidence that in cells with low anandamide amidase activity, such as platelets and neutrophils, anandamide is inactivated by an oxidative pathway involving 12(5)-lipoxygenase (Edgemond, 1998). Metabolism of anandamide by enzymes of the arachidonic acid cascade... 

Another vasoactive substance produced by the endothelium is thromboxane A2 (TxA2). Normally, small amounts of TxA2 are released continuously however, increased synthesis appears to be associated with some cardiac diseases. Synthesized from arachidonic acid, a plasma membrane phospholipid, TxA2 is a potent vasoconstrictor. Furthermore, this substance stimulates platelet aggregation, suggesting that it plays a role in thrombotic events such as myocardial infarction (heart attack). Nonsteroidal anti-inflammatory drugs such as aspirin and ibuprofen block formation of TxA2 and reduce formation of blood clots. 

Another metabolite of arachidonic acid is prostacyclin (PGI2). As with TxA2, PGI2 is produced continuously. Synthesized by vascular smooth muscle and endothelial cells, with the endothelium as the predominant source, PGI2 mediates effects that are opposite to those of TxA2. Prostacyclin causes vasodilation and inhibits platelet aggregation and, as a result, makes an important contribution to the antithrombogenic nature of the vascular wall. 

Aspirin is maximally effective as an antithrombotic agent at the comparatively low dose of 81 to 325 mg per day. (The antipyretic dose of aspirin in adults is 325 to 650 mg every 4 h.) Higher doses of aspirin are actually contraindicated in patients prone to thromboembolism. At higher doses, aspirin also reduces synthesis of prostacyclin, another arachidonic acid metabolite. Prostacyclin normally inhibits platelet aggregation. The prophylactic administration of low-dose aspirin has been shown to increase survival following myocardial infarction, decrease incidence of stroke, and assist in maintenance of patency of coronary bypass grafts. 

A recent trend in the pharmaceutical industry has been to harness the intrinsic tissue-protective properties of NO for improving the gastric tolerance of nonsteroidal antiinflammatory drugs (NS AIDs). This trend has led to the synthesis of hybrid, chimeric molecules containing an NSAID or aspirin moiety and a NO-donor functionality [153, 154]. One such hybrid is a NO-releasing derivative of aspirin, NCX-4016. In a doubleblind, randomized, placebo-controlled gastrointestinal safety assessment in healthy subjects, NCX-4016 (400 or 800 mg twice daily for 7 days) acted like aspirin as an inhibitor of arachidonic acid-induced platelet aggregation in vitro [155]. Whether... 

As a typical example of a 6-heteroarylsubstituted dihydropyridazinone exhibiting antithrombotic activity motapizone, NAT 05-239 (63) (CAS 90697-57-7) may serve [96], Compound CCI 17810 (64) (CAS 76283-03-9) bearing a substituted phenyl moiety at C-6 of the pyridazine system has been shown to inhibit potently in vitro human platelet aggregation (induced by collagen, ADP, thrombin or arachidonic acid) with EC50 values in the range of 0.5-10/rg/ml [245], ... 

---