Dependence of the antagonism at human platelet 5-HT2 receptors by ketanserin on the reaction pH

AT-1015, a novel serotonin (5-HT)2 receptor antagonist, blocks vascular and platelet 5-HT2A receptors and prevents the laurate-induced peripheral vascular lesion in rats

The serotonin (5-HT2A) antagonistic activities and the protective effect on laurate-induced peripheral vascular lesions of AT-1015, a novel 5-HT2 receptor antagonist, were investigated. In platelet aggregation, AT-1015 selectively inhibited in vitro 5-HT2A receptor-mediated aggregation, and the activity was almost equivalent to that of ketanserin (5-HT2A/2C receptor antagonist) and 100 times more potent than sarpogrelate (5-HT2A receptor antagonist). AT-1015 also inhibited 5-HT2A receptor-mediated aggregation by oral administration in rat, and the dose required for inhibition was equivalent to ketanserin. In a 5-HT-induced vasoconstriction study in rat, AT-1015 slightly reduced maximal contraction and caused a rightward shift of the concentration-response curve (pKB value, 9.5), which was unlike competitive inhibitors such as ketanserin and sarpogrelate (pA2 value, 9.3 and 8.7, respectively). Moreover, the ex vivo inhibitory activity significantly remained after oral administration (1 mg/kg). In the rat peripheral vascular lesion model, AT-1015 (1 mg/kg, p.o.) effectively prevented progression of peripheral lesions, and it was more potent compared with ketanserin, sarpogrelate, and cilostazol. These results suggest that AT-1015 is a potent 5-HT2A receptor antagonist, and its insurmountable antagonism may be relevant to its therapeutic potential in peripheral vascular disease.

Inhibition of ligand binding to thromboxane A2/prostaglandin H2 receptors by diethylpyrocarbonate. Protection by receptor ligands and reversal by hydroxylamine

The potential of histidines to modulate the binding of agonists and antagonists to human platelet thromboxane A2 (TXA2) receptors was investigated. TXA2 receptors were purified from crude platelet membranes via affinity and wheat germ lectin chromatography. Radioligand binding studies were conducted using the TXA2, mimetic [125I]BOP (I-BOP (I-BOP = [1S-(1 alpha,2 beta(5Z),3 alpha(1E, 3R*),4 alpha)]-7-[3-(3-hydroxy-4-(4'-iodophenoxy)-1-butenyl)7-oxabicyclo- [2.2.1]heptan-2-yl]-5-heptenoic acid) and the TXA2 receptor antagonist [125I]SAP (I-SAP = 7-[(1R,2S,3S,5R)-6,6-dimethyl-3-(4-iodobenzene- sulfonylamino)-bicyclo-[3.1.1]hept-2-yl]-(5Z)-heptenoic acid). The histidine modifying reagent diethyl-pyrocarbonate (DEPC) produced a concentration (30-100 microM) dependent inhibition of binding of both [125I]BOP and [125I]SAP. DEPC treatment significantly (P < 0.05, N = 6) decreased the affinity of the receptor for [125I]SAP (Kd = 2.4 +/- 0.4 and 5.4 +/- 0.4 nM, control and DEPC, respectively) without significantly decreasing the Bmax. The effects of DEPC were reversed by hydroxylamine. The inhibition of [125I]BOP and [125I]SAP binding produced by DEPC was reduced significantly by prior incubation of the purified receptors with the TXA2 receptor agonist U-46619 or the TXA2 receptor antagonist SQ 29548. The results strongly support the notion that one or more histidines reside in a domain that can modulate ligand binding to the TXA2 receptor.

Potentiation of ADP-induced aggregation in human platelet-rich plasma by 5-hydroxytryptamine and adrenaline

1. We have used dose-response curves to quantitate the potentiation of adenosine 5'-diphosphate (ADP)-induced aggregation and thromboxane (TXA2) generation by 5-hydroxytryptamine (5-HT) and adrenaline in human citrated platelet-rich plasma. We have also quantitated the inhibition of these responses by aspirin, ketanserin and yohimbine, singly and in pairs. 2. Ketanserin (5 microM) inhibited TXA2 production and the second wave of platelet aggregation induced by a range of concentrations of ADP alone. This indicates that endogenous 5-HT, released from the platelet dense granules, contributes significantly to responses induced by ADP. 3. When 5-HT (10 microM) was added before ADP, a lower concentration of ADP was required to cause 50% aggregation and TXA2 generation. The ratio of ADP concentrations (CR) to cause 50% aggregation in the presence and absence of 5-HT was 2.1 when only added 5-HT was considered, and 5.0 when endogenous 5-HT was also taken into account. 4. Potentiation of ADP-induced aggregation by 5-HT also occurred in the presence of aspirin, resulting in a CR of 2.3. As expected, ketanserin inhibited potentiation by 5-HT in the presence and absence of aspirin. Although aspirin caused substantial inhibition of aggregation induced by ADP and 5-HT (CR 3.4), further inhibition occurred when ketanserin was also present (CR 6.5). 5. A subthreshold concentration of adrenaline (0.25 microM) caused substantial potentiation of ADP-induced aggregation in the absence (CR 4.0) and presence (CR 2.0) of aspirin. As expected, yohimbine (9 microM) inhibited this potentiation.Maximum TXA2 generation induced by ADP increased from 32.5 to 59.4 pg per 106 platelets when adrenaline was present. Aggregation induced by ADP and adrenaline was markedly inhibited by aspirin (CR 5.1) but was further inhibited when yohimbine (9 microM) was also present (CR 10.0).6. Results from this in vitro study show ketanserin and yohimbine have the potential to be used in combination with aspirin as antithrombotic agents in vivo.

Inhibition of serotonin-amplified human platelet aggregation by ketanserin, ritanserin, and the ergoline 5HT2 receptor antagonists-LY53857, sergolexole, and LY237733

Human platelets are known to possess 5HT2 receptors which, when activated, amplify the aggregation response produced by other aggregating agents. Several 5HT2 receptor antagonists, including ketanserin and ritanserin, are known to antagonize serotonin-mediated aggregation of human platelets. In the present study, we document the ability of three ergoline 5HT2 receptor antagonists, LY53857, sergolexole, and LY237733, to antagonize the serotonergic component of the human platelet aggregation response. Potencies of the ergoline esters (LY53857 and sergolexole) and the ergoline amide (LY237733) to inhibit serotonin-amplified platelet aggregation responses were similar to the potencies of ketanserin and ritanserin under the conditions of our study. Furthermore, all five 5HT2 receptor antagonists were capable of fully inhibiting the serotonergic component of the platelet aggregation response. In contrast to these potent ergoline esters and amides, 1-isopropyl dihydrolysergic acid (up to 10(-5)M), a putative metabolite of the ergoline esters, was ineffective under these in vitro conditions. These data are consistent with the high potency of these ergolines as antagonists of 5HT2 receptors and further support the involvement of 5HT2 receptors on human platelets in the amplifying response to serotonin.

Platelet function during long-term treatment with ketanserin of claudicating patients with peripheral atherosclerosis. A multi-center, double-blind, placebo-controlled trial. The PACK Trial Group

In a multi-center, double-blind, placebo-controlled trial in claudicating patients with peripheral atherosclerosis, the effects of 1 year of treatment with ketanserin (20 mg t.i.d. for 1 month, 40 mg t.i.d. thereafter; n = 63 patients) or placebo (n = 84 patients) on platelet function (aggregation in P.R.P. by 5-HT 5 x 10(-6) M, ADP 1 to 5 x 10(-6) M, collagen 2 micrograms/ml; platelet 5-HT content; plasma beta TG- and PF4-levels; serum TXB2) were analyzed. Before treatment, claudicating patients (n = 173) displayed an higher reactivity of platelets to 5-HT and signs of platelet activation/release in vivo (higher plasma beta TG-PF4, lower platelet 5-HT content and decreased platelet aggregation by ADP, collagen) in comparison with healthy controls (n = 50). After 1 year of treatment with ketanserin, but not with placebo, platelet aggregation induced by 5-HT (slope -41.1%) and platelet 5-HT content (-23.7%) were significantly reduced. PF4 and beta TG were significantly higher than their pre-medication values in the two trial groups. The other platelet function tests were not significantly modified by the treatment. Only the small subgroup of patients with initially elevated plasma beta TG levels (greater than 20 ng/ml) also scrutinized for hidden NSAID consumption or technical bias (exclusion of data with serum TXB2 less than or equal to 10000 pg/100 microliters and/or plasma PF4 greater than 10 ng/ml) had significantly lower plasma beta TG levels (-22.7%) than the pre-medication values after treatment with ketanserin, but not with placebo. The present study confirms that ketanserin affects some platelet functions, during long-term administration in claudicating patients with atherosclerosis.

Effects of some calcium antagonists on aggregation by adrenalin and serotonin and on alpha-adrenoceptor radioligand binding in human platelets

The inhibitory effects of verapamil, nifedipine and diltiazem, representatives of different classes of calcium antagonists, were studied on aggregation of human platelets induced by adrenalin and serotonin (5-HT). For references, the alpha-adrenoceptor-antagonists phentolamine (alpha 1 and alpha 2) and rauwolscine (alpha 2), and the 5-HT 2-receptor-antagonist ketanserin were included. Verapamil in the concentration range 10(-6) 10(-4) M inhibited both adrenalin- and serotonin-induced aggregation in a concentration-dependent manner, whereas nifedipine and diltiazem had little or no effect. Phentolamine and rauwolscine were clearly weaker than verapamil as antagonists of serotonin, and ketanserin lacked effect on adrenalin-induced aggregation. Binding studies with [3H]dihydro-alpha-ergocryptine and [3H]rauwolscine on human platelet membranes showed equal numbers of binding sites, suggesting that only alpha 2-adrenoceptors were present. In the same concentration range as inhibition of aggregation was obtained, verapamil inhibited binding of either radioligand. Nifedipine, diltiazem and 5-HT were all poor inhibitors of radioligand binding. The results suggest that verapamil at high concentrations not only has alpha-adrenoceptor antagonistic properties but also exerts 5-HT-receptor blocking effects. This was not found with the other calcium channel blockers examined (nifedipine, diltiazem).

Serotoninergic mechanisms in hypertension. Focus on the effects of ketanserin

Aggregating platelets release serotonin, which induces contraction of most vascular smooth muscle by activation of S2-serotoninergic receptors. Serotonin released in the circulation may contribute to the increase in peripheral resistance of hypertension as the responsiveness of blood vessels from hypertensive animals and humans to the vasoconstrictor action of the monoamine is augmented. The data obtained with the new antihypertensive agent ketanserin may favor that interpretation. Ketanserin is a selective S2-serotoninergic antagonist with additional alpha 1-adrenergic blocking properties. In humans, it has a terminal half-life of 12 to 25 hours and is eliminated predominantly by the liver. The hemodynamic profile of ketanserin is that of a vasodilator drug with actions on both resistance and capacitance vessels. On short-term intravenous administration, it lowers blood pressure in hypertensive patients with minimal reflex changes in cardiovascular function. When given orally long term to hypertensive patients, ketanserin causes a sustained reduction in arterial blood pressure, comparable to that obtained with either beta-adrenergic blockers or diuretics. Several studies have shown a greater efficacy in older (greater than 60 years of age) than in younger patients independent of starting pressure. Side effects mainly consist of dizziness, somnolence, and dry mouth, but they are usually not severe. The mechanism underlying the antihypertensive effect of ketanserin is unclear. It cannot be attributed to either S2-serotoninergic or alpha 1-adrenergic blockade alone, but an interaction between the two effects appears to be required.