Platelet Cyclic Nucleotide Phosphodiesterases

Robert W. Colman, M.D. Sol Sherry Thrombosis Research Center Temple University School of Medicine 

Currently, the two antiplatelet agents with proven efficacy are aspirin, which inhibits cyclooxygenase -dependent synthesis of thromboxane Aj (TXj ), and ticlopidine, wdrich blocks the ability of ADP to inhibit stimulated adenyl cyclase. Bodi of these drags have proven prophylactic uses in reducing the risk of thrombo -occlusive and thromboembolic complications for all major arterial beds in individuals with a previous history of such episodes. Controlled trials show that both aspirin and ticlopidine are indicated in the secondary prevention of stroke, myocardial induction and peripheral vascular occlusion. However, there are limitations to their efficacy. No net changes in vascular events are seen with primary prevention. Moreover, antiplatelet drugs do not alter thrombocytopenia or impairment 

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Dipyridamole (Persantine) is a vasodilator that, in combination with warfarin, inhibits embolization from prosthetic heart valves and, in combination with aspirin, reduces thrombosis in patients with thrombotic diseases. Dipyridamole by itself has little or no benefit in fact, in trials where a regimen of dipyridamole plus aspirin was compared with aspirin alone, dipyridamole provided no additional beneficial effect. Dipyridamole interferes with platelet function by increasing the cellular concentration of adenosine 3, 5 -monophosphate (cyclic AMP). This effect is mediated by inhibition of cyclic nucleotide phosphodiesterase and by blockade of uptake of adenosine, which acts at A2 receptors for adenosine to stimulate platelet adenylyl cyclase. The only current recommended use of dipyridamole is for primary prophylaxis of thromboemboli in patients with prosthetic heart valves the drug is given in combination with warfarin. 

GU-7, a 3-arylcoumarin derivative, has been isolated fi om Glycyrrhizae radix, which is a crude herbal medicine. GU-7 caused inhibition of platelet aggregation, phosphorylation of 40K and 20K dalton proteins, inositol 1,4,5-trisphosphate production, intraplatelet calcium increase and phosphodiesterase activity in vitro. The data indicate that GU-7 inhibits platelet aggregation by increasing intraplatelet cAMP concentration. Antiplatelet action may also explain the mechanism by which traditional medicines are effective in diabetic neuropathy [236]. Osthole causes hypotension in vivo, and inhibits platelet aggregation and smooth muscle contraction in vitro. It may interfere with calcium influx and cyclic nucleotide phosphodiesterases [12]. Cloricromene, a synthetic coumarin derivative, also possesses antithrombotic-antiplatelet activity [237]. Some of these properties of cloricromene have been attributed to the inhibition of arachidonate release from membrane phospholipids [12]. 

Cyclic nucleotide phosphodiesterase (PDE)—key enzyme in promotion of platelet aggregation. 

A frequently cited mechanism of action for these agents is phosphodiesterase (PDE) inhibition and the associated antiplatelet effects that accompany increases in intracellular cyclic adenosine monophosphate (cAMP). In fact, the effects of these drugs go far beyond their direct effect on PDE inhibition or platelet function. This chapter discusses (/) cyclic nucleotides, PDE, and PDE inhibitors (if) the mechanisms of action of dipyridamole and cilostazol (Hi) drug issues and (iv) current clinical applications for dipyridamole and cilostazol, including recent clinical trials that may have changed our perception of the possible utility of these agents for percutaneous intervention. 

Cyclic nucleotide degradative activity occurs in mammalian whole blood, but it is not known whether phosphodiesterase exists in plasma in vivo. The enzyme can be released from damaged platelets and granulocytes in vitro. However, degradation of cyclic AMP in whole blood is slowed by reduction in temperature, immediate centrifugation and addition of methylxanthines, and plasma can be pipetted directly into HCIO4. It is usually necessary to process several milliliters of plasma owing to the low cyclic nucleotide levels and losses with purification. 

It is widely known that cyclic nucleotides are involved in fundamental processes, which include those related to platelet aggregation. In this way, the inhibition of the enzyme phosphodiesterase could be one such mechanism through its effect on cAMP (Cyclic Adenosin Monophosphate) and cGMP (Cyclic Guanosine Monophosphate) [117], Another mechanism might involve inhibition of cycloxygenase, with a consequent depression of thromboxane A2 biosynthesis [115] as well as some influence on the inhibition of the intracellular mobilization of Ca2+ flux and its influx across the plasma membrane, as happens with genistein [118]. 

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