The platelet ATP and ADP receptors

Adenine nucleotides, ADP and ATP, are coreleased from dense granules during platelet activation, as well as from endothelial cells and damaged red blood cells following vascular injury. Through autocrine and paracrine mechanisms, these extracellular signaling molecules interact with the platelet P2 receptors to amplify ongoing platelet activation. Two receptors for ADP, the G(q)-protein-coupled P2Y1 and G(i)-protein-coupled P2Y12 and one receptor for ATP, the P2X1 ion channel, have been identified on platelets. Due to distinct pharmacological properties and differential regulation, the P2Y and P2X receptors essentially operate on different scales of time and distance and trigger selective intracellular signaling cascades. Recent advances in the understanding of the P2Y receptor physiology have reinforced the concept of these receptors as useful targets for antithrombotic therapy. The function of P2X1 in platelet activation only recently started to be unraveled. This review focuses on recent findings on the physiology of these platelet ADP and ATP receptors, their distinct downstream intracellular signaling pathways as well as on the available agonists, antagonists and inhibitors that allow their pharmacological discrimination.     

Salicylidene salicylhydrazide, a selective inhibitor of beta 1-containing GABAA receptors

1. A high-throughput assay utilizing the voltage/ion probe reader (VIPR) technology identified salicylidene salicylhydrazide (SCS) as being a potent selective inhibitor of alpha2beta1gamma1 GABA(A) receptors with a maximum inhibition of 56+/-5% and an IC(50) of 32 (23, 45) nm. 2. Evaluation of this compound using patch-clamp electrophysiological techniques demonstrated that the compound behaved in a manner selective for receptors containing the beta1 subunit (e.g. maximum inhibition of 68.1+/-2.7% and IC(50) value of 5.3 (4.4, 6.5) nm on alpha2beta1gamma1 receptors). The presence of a beta1 subunit was paramount for the inhibition with changes between alpha1 and alpha2, gamma1 and gamma2, and the presence of a subunit having little effect. 3. On all subtypes, SCS produced incomplete inhibition with the greatest level of inhibition at alpha1beta1gamma1 receptors (74.3+/-1.4%). SCS displayed no use or voltage dependence, suggesting that it does not bind within the channel region. Concentration - response curves to GABA in the presence of SCS revealed a reduction in the maximum response with no change in the EC(50) or Hill coefficient. In addition, SCS inhibited pentobarbitone-induced currents. 4. Threonine 255, located within transmembrane domain (TM) 1, and isoleucine 308, located extracellularly just prior to TM3, were required for inhibition by SCS. 5. SCS did not compete with the known allosteric modulators, picrotoxin, pregnenolone sulphate, dehydroepiandrosterone 3-sulphate, bicuculline, loreclezole or mefenamic acid. Neither was the inhibition by SCS influenced by the benzodiazepine site antagonist flumazenil. 6. In conclusion, SCS is unique in selectively inhibiting GABA(A) receptors containing the beta1 subunit via an allosteric mechanism. The importance of threonine 255 and isoleucine 308 within the beta1 subunit and the lack of interaction with a range of GABA(A) receptor modulators suggests that SCS is interacting at a previously unidentified site.     

The effects of siguazodan, a selective phosphodiesterase inhibitor, on human platelet function

1. The effects of siguazodan (SK&F 94836) a selective phosphodiesterase (PDE) inhibitor with inotropic and vasodilator activity, were studied on human platelets. 2. Siguazodan selectively inhibited the major cyclic AMP-hydrolysing PDE in human platelet supernatants. The inhibited enzyme has been variously termed cyclic GMP-inhibited PDE or PDE-III. 3. In platelet-rich plasma (PRP), siguazodan inhibited U46619-induced aggregation more potently than that induced by ADP and collagen. Treatment of the PRP with aspirin had no effect on the potency of siguazodan. 4. In washed platelets, siguazodan increased cyclic AMP levels and reduced cytoplasmic free calcium [( Ca2+]i). ADP decreased the ability of siguazodan to raise cyclic AMP and this may explain its lower potency in inhibiting responses to ADP. 5. Siguazodan has anti-platelet actions over the same concentration range that it is an inotrope and vasodilator.     

Rofecoxib: a selective COX-2 inhibitor

Drugs & Therapy Perspectives -     

Ezetimibe: a selective cholesterol absorption inhibitor

Ezetimibe is the first agent of a novel class of selective cholesterol absorption inhibitors recently approved by the Food and Drug Administration for treatment in the United States. Ezetimibe inhibits the absorption of biliary and dietary cholesterol from the small intestine without affecting the absorption of fat-soluble vitamins, triglycerides, or bile acids. Ezetimibe localizes at the brush border of the small intestine and decreases cholesterol uptake into the enterocytes. Preclinical studies demonstrated lipid-lowering properties of ezetimibe as monotherapy and showed a synergistic effect in combination with 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors (statins). The efficacy and safety of ezetimibe 10 mg/day have been established in phase III clinical trials. In these trials, ezetimibe was investigated as monotherapy, as an add-on to ongoing statin therapy, and as combination therapy with statins in patients with primary hypercholesterolemia. In addition, ezetimibe has been evaluated in patients with homozygous and heterozygous familial hypercholesterolemia and in those with sitosterolemia. When given as monotherapy or in combination with statins or fenofibrate, ezetimibe reduces low-density lipoprotein cholesterol (LDL) by 15-20% while increasing high-density lipoprotein cholesterol by 2.5-5%. Unlike other intestinally acting lipid-lowering agents, ezetimibe does not adversely affect triglyceride levels and, due to its minimal systemic absorption, drug interactions are few. Ezetimibe's side-effect profile resembles that of placebo when given as monotherapy or in combination with statins. In clinical practice, ezetimibe has a role as monotherapy for patients who require modest LDL reductions or cannot tolerate other lipid-lowering agents. In combination therapy with a statin, ezetimibe is used in patients who cannot tolerate high statin doses or in those who need additional LDL reductions despite maximum statin doses.     

Roflumilast: a selective phosphodiesterase 4 inhibitor

Roflumilast is a selective phosphodiesterase (PDE) 4 inhibitor with a range of anti-inflammatory properties and potential for treatment of inflammatory disease. The therapeutic effects of roflumilast are thought to be mediated via increased levels of cellular 3',5'-cyclic adenosine monophosphate (cAMP) and include inhibition of microvascular leakage, inhibition of trafficking, release of cytokines and chemokines from inflammatory cells, and bronchodilation. The anti-inflammatory and bronchodilator properties of roflumilast have resulted in clinical studies to investigate the effects of roflumilast in inflammatory airway diseases such as asthma and chronic obstructive pulmonary disease. In asthma, roflumilast taken as a once-daily oral dose of 500 ug has been shown to improve clinical symptoms and airway function, reduce exercise-induced asthma and decrease bronchial airway hyperresponsiveness. In chronic obstructive pulmonary disease, roflumilast taken as a once-daily oral dose of 500 ug has been shown to reduce the frequency of exacerbations with small effects on improving lung function. Side effects of roflumilast appear to be mild and short lasting. It is likely that this new class of selective PDE4 inhibitor may provide a therapeutic option for patients with inflammatory airway disease.     

Effects of camonagrel,a selective inhibitor of platelet thromboxane synthase,on the platelet-subendothelium interaction

The aim of this study was to compare the effects of a new thromboxane synthase inhibitor, camonagrel, on platelet aggregation and platelet-subendothelium interaction under flow conditions, in comparison with a standard thromboxane synthase inhibitor (dazoxiben) and a cyclooxygenase inhibitor (acetylsalicylic acid). With respect to platelet aggregation in whole blood, the 50% inhibitory concentrations (IC(50)) of camonagrel were between 318 and 797 micromol/l after induction with collagen and adenosine 5'-diphosphate, respectively. For inhibition of thromboxane B(2) synthesis, the IC(50) values were 868 +/- 68 micromol/l; prostaglandin E(2) was inhibited only by acetylsalicylic acid (IC(50) for camonagrel >2,000 micromol/l), and the leukocyte 6-keto-PGF(1alpha) level was increased by camonagrel. The greatest reduction in percentage subendothelial surface occupied by platelets (mainly in the thrombi) after blood perfusion was seen after incubation with camonagrel in the range of concentrations that inhibited collagen-induced platelet aggregation. In conclusion, camonagrel reduced platelet-subendothelium interaction under flow conditions, showing this effect in a range of concentrations lower than in inhibition of platelet aggregation.     

ADP receptors: inhibitory strategies for antiplatelet therapy

The interaction of adenosine-5'-diphosphate (ADP) with its platelet receptors (P2Y(1) and P2Y(12)) plays a very important role in thrombogenesis. The thienopyridine ticlopidine was the first specific antagonist of the platelet P2Y(12) ADP receptor to be tested in randomized clinical trials for the prevention of arterial thrombotic events. Although ticlopidine reduces the incidence of vascular events in patients at risk, it also unfortunately has some significant drawbacks: a relatively high incidence of toxic effects, which may be fatal in some cases; delayed onset of action; and a high interindividual variability in response. A second thienopyridine, clopidogrel, has superseded ticlopidine, because it is also an efficacious antithrombotic drug and is less toxic than ticlopidine. However, clopidogrel is not completely free from faults: severe toxic effects, albeit occurring much less frequently than with ticlopidine, may still complicate its administration to patients; the onset of pharmacologic action can be accelerated by the use of large loading doses, but may still not be optimal; the high interpatient variability in response remains an important issue. These concerns justify the continued search for agents that can further improve the clinical outcome of patients with atherosclerosis through greater efficacy and/or safety. A new thienopyridyl compound, prasugrel, which is characterized by higher potency and faster onset of action compared with clopidogrel, is currently under clinical evaluation. Two direct and reversible P2Y(12) antagonists, cangrelor and AZD6140, feature very rapid onset and reversal of platelet inhibition, which make them attractive alternatives to thienopyridines, especially when rapid inhibition of platelet aggregation or its quick reversal are required. Along with new the P2Y(12) antagonists, inhibitors of the other platelet receptor for ADP, the antagonists P2Y(1), are under development and may prove to be effective antithrombotic agents.     

Fondaparinux:一个新的选择性Ⅹa因子抑制剂

fondaparinux是一个化学合成的全新戊聚糖化合物 ,能够有效地预防大型的下肢骨科手术后常见的静脉血栓栓塞。本文就其药效学、药动学、临床应用及其现状等作一综述。     

选择性JAK抑制剂——巴瑞替尼

选择性JAK抑制剂巴瑞替尼是一种新上市的靶向合成改善病情的抗风湿药物(disease-modifying anti-rheumatic drugs,DMARDs),具有抑制JAK-1和JAK-2的作用,用于治疗类风湿关节炎。本文将对其作用机制、药动学、临床评价、安全性、药物相互作用等进行综述。     

Platelet ADP receptors and their antagonists

ADP plays a crucial role in haemostasis and thrombosis and its receptors are potential target for antithrombotic drugs. The knowledge of the ADP-receptors has been amplified by recent discoveries. This review highlights the ADP-receptors models and their antagonists described in the recent literature, mainly the thienopyridine derivatives.     

Clinical pharmacology of flurbiprofen: a novel inhibitor of platelet aggregation

Summary

Studies are renewed of the inhibitory effect of flurbiprofen, given in doses ranging from 50?mg to 200?mg per day for 1 week, on platelet aggregation measured by biological tests (adenosine diphosphate and collagen methods). Flurbiprofen at doses of 50?mg and 100?mg daily had a peak time of action of between 1 and 2 hours, the effect usually disappearing after 24 hours, and 100?mg flurbiprofen caused a similar decrease in platelet aggregation to 1 g aspirin daily. In a clinical study of 72 patients with chronic glomerulonephritis treated with doses of flurbiprofen up to 200?mg daily there was a significant correlation between the parameters of aggregation measured and treatment, and between proteinuria and adenosine diphosphate aggregation when the flurbiprofen dose did not exceed 100?mg daily.     

Etodolac: An overview of a selective COX-2 inhibitor

Etodolac is a non-steroidal anti-inflammatory drug (NSAID) which has been shown to be effective in the treatment of rheumatoid arthritis and osteoarthritis and a selective COX-2 inhibitor in a wide range of clinically relevant assays in direct comparisons with other NSAIDs. Studies have shown etodolac to have no overall suppression of gastric or duodenal prostaglandins and endoscopic analysis with etodolac showed placebo level scores in comparison with ibuprofen, which showed inducement of gastro-intestinal (GI) side effects. This high degree of gastric tolerability was further demonstrated by microbleeding studies. The favourable GI tolerability profile of etodolac has been shown in long-term and large-scale trials and by routine clinical observation. In summary, etodolac is a well established selective COX-2 inhibitor that has been shown not to suppress gastric or duodenal prostaglandins, to have minimal hepatic or renal effects and to have favourable GI tolerability in comparison with ibuprofen.     

Clinical pharmacology of lumiracoxib: a selective cyclo-oxygenase-2 inhibitor

Lumiracoxib (Prexige) is a selective cyclo-oxygenase (COX)-2 inhibitor developed for the treatment of osteoarthritis, rheumatoid arthritis and acute pain. Lumiracoxib possesses a carboxylic acid group that makes it weakly acidic (acid dissociation constant [pKa] 4.7), distinguishing it from other selective COX-2 inhibitors. Lumiracoxib has good oral bioavailability (74%). It is rapidly absorbed, reaching maximum plasma concentrations 2 hours after dosing, and is highly plasma protein bound. Lumiracoxib has a short elimination half-life from plasma (mean 4 hours) and demonstrates dose-proportional plasma pharmacokinetics with no accumulation during multiple dosing. In patients with rheumatoid arthritis, peak lumiracoxib synovial fluid concentrations occur 3-4 hours later than in plasma and exceed plasma concentrations from 5 hours after dosing to the end of the 24-hour dosing interval. These data suggest that lumiracoxib may be associated with reduced systemic exposure, while still reaching sites where COX-2 inhibition is required for pain relief. Lumiracoxib is metabolised extensively prior to excretion, with only a small amount excreted unchanged in urine or faeces. Lumiracoxib and its metabolites are excreted via renal and faecal routes in approximately equal amounts. The major metabolic pathways identified involve oxidation of the 5-methyl group of lumiracoxib and/or hydroxylation of its dihaloaromatic ring. Major metabolites of lumiracoxib in plasma are the 5-carboxy, 4'-hydroxy and 4'-hydroxy-5-carboxy derivatives, of which only the 4'-hydroxy derivative is active and COX-2 selective. In vitro, the major oxidative pathways are catalysed primarily by cytochrome P450 (CYP) 2C9 with very minor contribution from CYP1A2 and CYP2C19. However, in patients genotyped as poor CYP2C9 metabolisers, exposure to lumiracoxib (area under the plasma concentration-time curve) is not significantly increased compared with control subjects, indicating no requirement for adjustment of lumiracoxib dose in these subjects. Lumiracoxib is selective for COX-2 compared with COX-1 in the human whole blood assay with a ratio of 515 : 1 in healthy subjects and in patients with osteoarthritis or rheumatoid arthritis. COX-2 selectivity was confirmed by a lack of inhibition of arachidonic acid and collagen-induced platelet aggregation. COX-2 selectivity of lumiracoxib is associated with a reduced incidence of gastroduodenal erosions compared with naproxen and a lack of effect on both small and large bowel permeability. Lumiracoxib does not exhibit any clinically meaningful interactions with a range of commonly used medications including aspirin (acetylsalicylic acid), fluconazole, an ethinylestradiol- and levonorgestrel-containing oral contraceptive, omeprazole, the antacid Maalox, methotrexate and warfarin (although, as in common practice, routine monitoring of coagulation is recommended when lumiracoxib is co-administered with warfarin). As such, dose adjustments are not required when co-administering these agents with lumiracoxib. In addition, moderate hepatic impairment and mild to moderate renal impairment do not appear to influence lumiracoxib exposure.     

5'-N-ethylcarboxamidoadenosine: a potent inhibitor of human platelet aggregation.

1 5'-N-ethylcarboxamidoadenosine (NECA) is an adenosine analogue which is 22,900 times more potent than adenosine as a vasodilator. Adenosine and some of its analogues are also inhibitors of human platelet aggregation. NECA was tested for its effects on human platelets. 2 NECA (1 microM) inhibited human platelet aggregation induced by adenosine 5'-diphosphate (ADP), adrenaline, 5-hydroxytryptamine (5-HT) and thrombin more powerfully than adenosine. NECA was 5 to 10 times more potent than adenosine at inhibiting ADP- and adrenaline-induced aggregation. 3 NECA, like adenosine, caused dose-dependent increases in levels of platelet adenosine 3',5'-cyclic monophosphate (cyclic AMP), which were competitively inhibited by theophylline, an adenosine antagonist. 4 These effects of NECA, like those of adenosine, were completely stereospecific as the L-enantiomer of NECA was inactive. 5 NECA did not interfere with the inhibition by ADP of prostaglandin E1 (PGE1)-stimulated adenylate cyclase. 6 NECA is the most potent analogue of adenosine tested so far on human platelets, and is the first example of a 5' modification to retain affinity for the platelet adenosine receptor.     

不同ADP受体拮抗剂对冠心病血小板功能疗效差异的网状Meta分析

目的用贝叶斯网状模型比较ADP(Adenosine diphosphate)受体拮抗药clopidogrel、prasugrel、dcagrelor、clopidogrel+others、prasugrele及其他药物(Atorvastatin、lansoprazol)治疗冠心病患者时对血小板功能的作用差异。方法运用贝叶斯网状模型拟合直接和间接证据分析ADP受体拮抗药对PRU、IPA、PRI影响的差异,并评估各指标的累积排序概率。结果共纳入10篇文献。结果显示,给药后(6±2)h,clopidogrel、prasugrel、ticagrelor的PRU低于placebo;prasugrel、ticagrelor的PRU值低于clopidogrel。给药2、4、(9±3)h后,prasugrel和Ticagrelor的IPA高于placebo。给药2 h后,prasugrel和ticagrelor的PRI低于placebo与clopidogrel;给药24 h后,ticagrelor的PRI较placebo和clopidogrel降低。结论 ADP受体拮抗药clopidogrel、prasugrel、ticagrelor均有抑制血小板功能的作用,且prasugrel、ticagrelor的抑制效果优于clopidogrel,prasugrel优于ticagrelor。     

Meloxicam: a selective COX-2 inhibitor non-steroidal anti-inflammatory drug

Meloxicam is a new non-steroidal anti-inflammatory drug (NSAID) that selectively inhibits the inducible isoform of the cyclo-oxygenase (COX)-2 enzyme. This enzyme has a major role in mediating the inflammatory response, while synthesis of prostaglandins required for normal physiological functioning of the stomach and kidneys is under the control of the constitutive isoform, COX-1. Other NSAIDs in clinical use show varying degrees of selectivity towards COX-1. Only meloxicam and (albeit to a lesser extent) nimesulide could be described as selective for COX-2. In comparative trials of patients with osteo- and rheumatoid arthritis, meloxicam has been found to be at least as effective as other NSAIDs, but with a greatly reduced incidence of gastrointestinal side-effects. There is no evidence that meloxicam causes any deterioration in renal function in patients with moderate degrees of renal failure, and no evidence of drug accumulation with continued use. Meloxicam's half-life of 20 h makes it ideal for once daily administration, and it is 99% converted to inactive metabolites prior to excretion. No clinically significant drug interactions have been detected, making it suitable for use in patients with co-existing pathology. Meloxicam's safety and tolerability make it a significant advance in the treatment of rheumatic disease.