Action of a pyrimido-pyrimidine compound on platelet behaviour in vitro

Platelets, thrombosis and drugs

The development of thrombosis involves 4 main factors: the vessel wall, the formed elements of the blood, blood coagulation, and blood flow. In venous thrombosis, however, the major part in both the initiation and growth of thrombi is played by the platelets. In selecting drugs which inhibit platelet function it is helful to know which of the platelet reactions that contribute to thrombus formation can be inhibited by various agents. Platelets adhere to the damaged vessel wall, collagen being probably the most important constituent involved. They are then stimulated to release the contents of their storage granules. Release-inducing agents promote the discharge of adenosine diphosphate (ADP) which causes platelets in the vicinity to swell to a more spherical shape, extend pseudopods and adhere to each other. Platelet aggregation is reversible, and a number of drugs have been shown to be capable of inhibiting platelet function at various stages, both in vitro and in vivo. Adrenaline, noradrenaline, oestrogens and nicotine enhance aggregation. Drugs which inhibit platelet function include the non-steroidal anti-inflammatory drugs, the pyrimido-pyrimidines (e.g. dipyridamole), hydroxychloroquine, clofibrate, and dextran. In this review the effects of drugs which inhibit platelet function are outlined and the extent to which they can be used to influence the course of thromboembolic disease in man is discussed. It is suggested that combination of anti-platelet drugs with anticoagulants could prove clinically useful.

Effect in vitro on platelet function of two compounds developed from the pyrimido-pyrimidines

VK 774 and VK 744, two new compounds developed from the pyrimido-pyrimidines, have been found to be powerful inhibitors of platelet function tested in vitro. They inhibit adenosine diphosphate (ADP)-induced platelet aggregation, and the release of platelet factor 3 by kaolin, and VK 774 also reduces platelet adhesiveness and inhibits platelet aggregation (;snowstorm' effect) in the Chandler tube system. Although measured percentage whole blood clot retraction was uninfluenced by these drugs the clot produced with VK 774 was friable and soft. VK 774 appears to be the most powerful of these compounds reported so far, being active in some test systems at 10(-6)M, and, if the results of toxicity testing are satisfactory, it should be an important agent for therapeutic trial.

Effect of RA233 on platelet function in vitro

RA233, a new pyrimido-pyrimidine compound, is a powerful inhibitor of platelet function tested in vitro; it inhibits calcium and adenosine diphosphate (A.D.P.)-induced platelet aggregation, inhibits the retention of platelets by glass beads, decreases the release of platelet factor 3 by kaolin, and inhibits clot retraction. In some in-vitro systems RA233 is significantly more potent that its analogue RA433 in inhibiting platelet function.