Measurement of collagen- and serotonin-induced platelet aggregation in whole blood

Modulation of vasoactivity and platelet aggregation by selective 5-HT receptor antagonism in humans

BACKGROUND

Distinct serotonin [5-hydroxytryptamine (5-HT)] receptors are involved in platelet aggregation and vasoconstriction. Compounds that simultaneously and selectively inhibit the pertaining 5-HT receptors may therefore represent a therapeutic strategy for arterial thrombosis, observed frequently after atherosclerotic plaque rupture. The vasoactive and antiplatelet effects of the combined 5-HT1B and 5-HT2A receptor blocker SL65.0472-00 were investigated in humans to elucidate the functional involvement of these receptors.

METHODS

Twenty-four volunteers, divided into 2 groups of 12, received an oral dose of 20 mg of SL65.0472-00 or placebo in a randomized, double-blind, crossover study. Pre dose and at 2, 4, and 6 hours after dosing, intra-arterial infusions of 5-HT1B agonist sumatriptan (n = 12) or 5-HT (n = 12) were administered. Forearm blood flow (FBF) was measured using plethysmography, and platelet aggregation was measured using whole blood aggregometry induced by a combination of a threshold collagen concentration and an excess of 5-HT. Treatments were compared using analysis of variance.

RESULTS

After placebo treatment, infusion of 1 ng·kg·min 5-HT induced vasodilatation (FBF +80% change from baseline), whereas infusion of 30 and 80 ng·kg·min 5-HT resulted in vasoconstriction (FBF -25% and -50%). After SL65.0472-00 treatment, all 5-HT doses induced vasodilatation (FBF +25%-60%). Sumatriptan dose dependently decreased FBF (maximally -35%), but this effect was not altered by SL65.0472-00 treatment. 5-HT-induced platelet aggregation was effectively inhibited by 90% after SL65.0472-00 treatment for at least 6 hours.

CONCLUSIONS

SL65.0472-00 has potent antagonistic effect on 5-HT-induced vasoconstriction and platelet aggregation but not on sumatriptan-induced vasoconstriction. This suggests that in humans, SL65.0472-00 is a 5-HT2A blocker without clear 5-HT1B antagonistic activity.

Introduction to haemostasis from a pharmacodynamic perspective

Biochemical characterization of the haemostatic system has advanced significantly in the past decades. Sub-systems, such as coagulation, fibrinolysis, blood cells and platelets and the vessel wall have been studied by specialists, mostly separately and independently. The time has come to integrate the approaches, and, in particular, to develop tests to document the state of the whole system and to have available adequate pharmacodynamic tests to evaluate treatments. Many examples are available to show that traditional general methods of clotting and lysis do not provide the information that is desired. The present tendency is to use specific methods for specific factors or effects which are very limited in pharmacological information. There is also increasing awareness of the occurrence of rather broad interindividual variability in the haemostatic system. This suggests that individually tailored treatments are required. This is the more relevant since haemostasis is a balance and treatment should be positioned between efficacy and safety. The conclusion is reached that there is a need for integrated or global methods or sets of methods that reflect the complexity and individual status appropriately and allow the practitioner to judge the effects of interventions and their individual aspects.