Acute reduction of TxA2 platelet binding sites after in vivo administration of a TxA2 receptor inhibitor

A review of picotamide in the reduction of cardiovascular events in diabetic patients

Picotamide is an antiplatelet drug with a dual inhibitory action. Thus, picotamide inhibits both thromboxane A2 (TxA2) receptors and TxA2 synthase and, at variance with aspirin, does not interfere with endothelial prostacyclin (PGI2) production. Two large randomized trials have been performed to assess the clinical efficacy of picotamide in patients at risk of atherothrombosis. The ADEP study compared peripheral artery disease (PAD) patients randomized to picotamide or placebo. This study did not show a significant reduction of cardiovascular events by picotamide but a subgroup analysis showed its potential usefulness in patients with diabetes. To investigate this issue further, the DAVID study recently enrolled diabetic patients with PAD randomized to picotamide versus aspirin; the results showed a significant reduction of overall mortality in the picotamide group. Moreover long-term picotamide treatment in diabetes promotes the reduction of microalbuminuria and the inhibition of growth of carotid plaques. These data suggest that picotamide may represent an interesting drug to be further investigated in future trials in the atherothrombotic setting.

Mechanism of the persisting TxA2 receptor antagonism by picotamide

Picotamide is a dual TxA2 receptor antagonist/TxA2 synthetase inhibitor. As the picotamide effect is maximum only after 5-10 min of incubation, aim of the present work was to study whether the effect of picotamide could be observed even after wash out of the drug from the external buffer. Platelet aggregation, serotonin release and changes in intracellular calcium were analyzed in platelets treated with picotamide and then gel-filtered, in comparison with gel-filtered platelets treated with picotamide. To exclude the possibility that the inhibitory effect of picotamide is due to its intracytosolic storage, serotonin release in digitonin-permeabilized platelets was measured. Platelet aggregation and serotonin release in response either to U46619, a synthetic agonist of the thromboxane A2 receptor, or arachidonic acid or collagen, as well as the changes in intracellular calcium concentration after U46619 or arachidonic acid stimulation, were inhibited by picotamide even when it had been washed out by gel-filtration. Both inhibitions were very similar to that observed in experiments in which picotamide was present in the medium. As the serotonin release in digitonin permeabilized platelets presented the same inhibition it may be excluded that picotamide effect is consequent upon the cytosolic storage of the drug. Since our results clearly indicate that the action of picotamide persists even after washing out of the drug from the medium, the idea that this antagonistic effect may be dependent on its binding to platelet plasma membrane rather than on its cytosolic concentration, is strongly substantiated.

Studies on in vitro effect of picotamide on human platelet aggregation in platelet-rich plasma and whole blood

Picotamide is a new antiaggregating agent influencing the platelet prostaglandin pathway through an inhibition of thromboxane A2 (TXA2) synthesis and a competitive antagonism of platelet TXA2 receptors. In the present study, we investigated the in vitro effect of this drug on human platelet aggregation induced by different agents (adenosine 5'-diphosphate [ADP], collagen, Na arachidonate) both in platelet-rich plasma (PRP; Born's method) and whole blood (WB; impedance method). For each aggregating agent, ED50 value (agonist concentration necessary to induce a maximal aggregation of 50%) was determined in control samples and following addition of different picotamide concentrations on the basis of dose-response curves. Picotamide decreased the response to each aggregating agent in both WB and PRP samples. In WB, 25 microM picotamide was able to induce a highly significant enhancement of ED50 values for ADP (from 6.6 +/- 1 microM to 12.7 +/- 1.7 microM, p < 0.01), Na arachidonate (from 740 +/- 240 microM to 1,080 +/- 280 microM, p < 0.01) and collagen (from 2.4 +/- 0.3 micrograms/ml to 3.8 +/- 0.15 micrograms/ml, p < 0.01). In PRP, the same picotamide concentration significantly enhanced ED50 for each aggregating agent (from 2.0 +/- 0.1 microM to 3.1 +/- 0.3 microM for ADP, p < 0.01; from 960 +/- 80 microM to 1,850 +/- 260 microM for Na arachidonate, p < 0.001; from 3.0 +/- 0.3 microgram/ml to 5.0 +/- 0.8 micrograms/ml for collagen, p < 0.01). Present results show that picotamide effect on platelet response is present also in WB. Data might support the use of picotamide as antiaggregating agent in vascular diseases.

Effect of picotamide on the calcium mobilization and phospholipase C activation in human platelets

The effect of picotamide (G137 or N,N'-bis-3-picolyl-4-methoxyisophthalamide), a dual thromboxane A2 (TxA2) synthetase inhibitor/TxA2 endoperoxide receptor antagonist, on the phospholipase C (PLC) activation and calcium mobilization in human platelets stimulated by arachidonic acid (AA) and TxA2 receptor synthetic agonist U46619, has been studied. Preincubation with picotamide (10(-4) M) for 1 and 3 min significantly reduced (p < 0.03 and p < 0.005 respectively) the calcium concentration changes induced in gel-filtered platelets (GFPs) by U46619 125 nM and 250 nM. Picotamide also reduced the calcium concentration changes induced in GFPs by AA 75 and 150 microM and by ADP 5 and 10 microM. In thrombin degranulated platelets picotamide inhibited the effect of U46619 up to 500 nM. The PLC activation, as indicated by inositol-1,3,4 P3 (Ins 1,3,4 P3) formation in response to U46619 250 nM and AA 150 microM was also inhibited by picotamide. These results may suggest a dual effect of picotamide on the receptor/effector systems through which TxA2 mediates platelet activation.

Binding kinetics and antiplatelet activities of picotamide, a thromboxane A2 receptor antagonist

1. Picotamide was shown to inhibit platelet binding of thromboxane A2 (TxA2)-mimetics and to cause a reduction of TxA2 platelet receptors after in vivo administration. The present study aimed to investigate directly [3H]-picotamide binding to human platelets and in particular the relationship between binding kinetics and antiaggregating properties. 2. [3H]-picotamide time-dependently bound to a single class of platelet TxA2 receptors with a KD of 325 nmol l-1 at equilibrium. The binding was displaceable by TxA2 analogues U46619 and ONO11120 (Ki 19 and 28 nmol l-1 respectively) but not by prostacyclin (PGI2), prostaglandin E2 (PGE2) and TxB2. Antiaggregating activity and TxA2 formation inhibition paralleled with binding kinetics. 3. By prolonging the incubation time from 30 to 120 min, picotamide showed a progressively increasing non-displaceable binding, whereas specific displaceable binding decreased in comparison to the values reached at 30 min. Non displaceable binding was specific, temperature-dependent saturable and followed a Michaelis-Menten kinetic (Vmaxapp = 130 fmol per 10(8) platelets h-1, KMapp = 330 nmol l-1). Picotamide progressively underwent a specific stable interaction with its platelet receptor. 4. In conclusion, after an initial reversible binding, a progressive stabilization of picotamide binding takes place resulting in a progressively more stable interaction with platelets.

Evaluation of the clinical utility of platelet aggregation studies in the long-term follow-up of patients with atherosclerotic vascular disease

The present study was designed to evaluate the usefulness of laboratory monitoring of antiplatelet therapy by means of a multiparametric evaluation of in vitro platelet aggregation tests in the attempt to individually optimize a given therapeutic regimen. The presence of a condition of hyperaggregability was shown in approximately 80% of patients with different forms of atherosclerotic vascular disease not undergoing any therapeutic regimen with antiplatelet agents. Conversely, a significant decrease in platelet activity was observed in patients undergoing different therapies based on acetylsalicylic acid (ASA), ticlopidine, or indobufen. The similar antiaggregatory effect of low-dose vs. high-dose ASA therapies was also shown. Dipyridamole alone showed no antiaggregatory effect, which, in turn, was reached only by the addition of ASA. Nevertheless, the association of ASA plus dipyridamole did not show any stronger antiplatelet effect than ASA alone. The evaluation of in vitro platelet activity in a group of patients treated with picotamide failed to show any significant change in comparison with the untreated group, probably due to the short half-life of picotamide in man and/or to its capability of reversibly antagonizing the action of thromboxane at receptor level. The evaluation of a long-term follow-up of 90 patients treated with different antiplatelet agents supports the idea that a multiparametric analysis of in vitro platelet aggregation may provide valuable help in monitoring and optimizing a given therapeutic regimen.