血栓形成机理及溶血栓药物的研究进展

血栓形成机理及溶血栓药物的研究进展,讲解：张荣春,主要内容,一、血液二、凝血机制三、血栓及其形成机理四、溶血栓药物及其发展五、News六、外文文献,一、血液,血液是流动在心脏和血管内的不透明红色液体，主要成分为血浆、血细胞。属于结缔组织。血液中含有各种营养成分，如无机盐、氧、以及细胞代谢产物、激素、酶和抗体等，有营养组织、调节器官活动和防御有害物质的作用。,二、凝血机制,2.1 凝血（Blood Coagulation）凝血：血液凝固，是指血液由流动的液体状态变成不能流动的凝胶状态的过程，是生理性止血的重要环节。血液凝固的实质就是血浆中的可溶性纤维蛋白原变成不可溶的纤维蛋白的过程。,2.2 凝血途径,三、血栓及其形成机理,3.1 血栓（thrombus ） 活动物机体在心脏或血管内某一部分因血液成分发生析出、凝集和凝固所形成的固体状物质。,3.2 血栓的形成（thrombosis） 在活体的心脏或血管腔内，血液发生凝固或血液中的某些有形成分互相粘集，形成固体质块的过程，称为血栓形成。,3.3 机体的溶纤系统 在哺乳动物血液中存在一个溶纤系统，它担当溶解血栓的作用。在纤溶酶原激活剂（PA）的作用下，纤溶酶原（Pg）转变成纤溶酶（Pm）,后者将血凝块上不溶性纤维蛋白降解为可溶性产物，从而使血栓溶解。,血液中有两种PA，一种来自血管内皮细胞的组织型纤溶酶原激活剂（t- PA），另一种是由肾细胞分泌的单链尿激酶激活剂（scu- PA）。 在正常情况下，血液中只存在少量PA，生成微量Pm，而纤溶酶原激活剂- 1（PAI- 1）和2- 抗纤溶酶原（2- AP）这两种抑制剂占优势，并保持该系统的平衡。 当机体出血时，通过形成纤维蛋白完成止血过程。但是，如果纤维蛋白长期存在于血管内，就会引起血管内腔狭窄、闭塞和末梢功能障碍，发生血栓性疾病。,3.3 影响血栓形成的因素3.3.1 血管壁的改变： 损伤的血管内膜粗糙不平，使血小板易于粘集，同时，内摸损伤使内皮下的胶原纤维暴露，后者可激活血中的因子，并进而激活内源性凝血系统；损伤的内膜所释放的组织凝血因子又可激活外源性凝血系统，引起血液凝固促使血栓形成。,3.3.2 血流速度的改变： 血流速度的改变包括血流缓慢、漩涡形成和血流停止等。由于在正常情况下，血液中的有形成分在血流中轴流动，与血管壁之间隔着一层血浆。当血流缓慢时，此种轴流消失，血小板逐渐析出从而进入边流并同损伤的内膜接触而发生粘集；同时，粘集的血小板及局部形成的一些凝血因子，也因血流缓慢不易被稀释和冲走，这些都有利于血栓形成。因此，血栓常发生在血流缓慢的静脉中。血流中的涡流在血栓形成中也有重要意义。血液中的涡流能促使血小板从血流中分离、沉淀和粘集而逐渐形成血栓。,3.3.3 血小板的改变： 许多实验证明血小板在血栓形成中起重要作用，血小板的数量、释放的物质等都与血栓形成有关，如血小板数量超过100万/mm3 形成血栓的可能性大大增加；幼稚的血小板粘附，聚集的能力强于成熟的血小板。,3.3.4 促凝物质的影响： 促凝物质进入血液使血液出现凝固性增高的病理状态促进血栓的形成。,3.3.5 血液流速的改变 血液在流速低时，粘度增高，血粘度的增高又会引起红细胞变形性降低和聚集性增高，使粘度进一步增高，使血流进一步变慢，这样有利于血小板的粘附和聚集。,3.3.6 其他因素： 在生理或病理情况下，使血液中凝血因子增多或抗凝功能减弱，纤维蛋白溶解系统功能受到抑制，以及血液中脂肪酸过高都会使血栓更易发生。,四、血栓治疗,血栓栓塞性疾病：由血栓造成某处管道闭塞所导致的一切病变。随着人类寿命的延长，血栓栓塞性病变也在明显增加，因而近年来血栓栓塞症及其治疗方法受到了人们的广泛关注。临床疗法共有3种，即外科手术、抗栓疗法和溶栓疗法。前2种疗法均有较大缺陷，目前研究的重点主要在于溶栓疗法和血栓溶酶的研究。,4.1 抗栓疗法,4.1.1 抗栓药物 : 预防血小板聚集（抗血小板聚集剂）； 抑制凝血过程（抗凝剂）； 清除循环中的纤维蛋白原（去纤维蛋白制剂）,使用的药物有肝素（Heparinum）、噻氯匹定（tidopidine）、氯吡格雷（clopidogrel）、利多格雷（ridogrel）、西卡前列素（cicaprost）、蛋白质C（PC）、抗凝血酶（AT），水蛭素（hirudin）及其衍生物（hirulog，hirugen）等,水蛭素：水蛭始载于神农本草经Hzycraft于1884年首先发现医用水蛭的提取物中含有抗凝血的物质1957年 Markwardt 从医用水蛭中成功地分离出这种抗凝物质的纯品，并定名为水蛭素。,水蛭素是凝血酶的直接抑制剂，它与凝血酶的活性中心的可识别位点牢固结合，并且结合区域大大超过活性中心，两者紧密接触，形成高度稳定的非共价复合体，使其不能与血纤维蛋白原结合，从而抑制凝血酶的活性，因此具有抗凝、抗血栓等药理作用。hirulog和hirugen是从水蛭素结构衍化而来的，它们的药理活性及作用与天然水蛭素基本相同。,缺陷：疗效低，副作用明显，需要定期进行血液学检测或需长期治疗，口服一般无效等。目前临床上比较合理的抗血栓治疗方法为抗凝剂与抗血小板药物联合运用，能抑制血小板活化和血纤维蛋白形成，对病人已形成的血栓难以直接溶解，故主要用于预防。,4.2 溶栓疗法,溶栓疗法是使已形成的血栓直接溶解的唯一疗法，即利用溶栓剂直接溶解血栓或通过激活血液中的血纤维蛋白溶酶原转化成血纤维蛋白溶酶来催化血栓的主要基质血纤维蛋白水解。,在过去的10年中，用溶栓剂治疗冠状动脉疾病、肺栓塞、血栓性心脏病的比例有所增加，其中对急性心肌梗死（AMI ）、脑梗死的治疗最为成功。临床统计结果证明，用链激酶或组织血纤维蛋白溶酶原激活剂使AMI的再灌注百分比从13%21%提高到60%90%。临床上已正式批准使用的溶栓剂有：链激酶，尿激酶，重组组织血纤维蛋白溶酶原激活剂，对甲氧苯甲酰纤溶酶原链激酶激活合剂复合物等。,4.3 血栓溶酶研究进展,最初使用的血栓溶酶是链激酶和尿激酶,链激酶（Streptokinase,简称SK）是从溶血性链球菌培养液中分离提取出的一种非酶糖蛋白，单链，分子量47 00050 000。Tillet和Gavner于1943年首次发现SK具有溶栓作用，并于1955年将其用于临床。,SK本身无酶活性，它不直接激活血纤维蛋白溶酶原，而是首先以11的分子比与血纤维蛋白溶酶原前激活物形成复合物，此复合物使血纤维蛋白溶酶原构象发生变化，成为有活性的复合物，再催化血纤维蛋白溶酶原转变为血纤维蛋白溶酶，引起血栓内部的崩解和血栓表面的溶解。 优点：有效、其冠脉开通率为50缺点：抗原性强、易引起变态反应，还可能出现低血压，即副作用过大、禁忌症多、连续用药不可超过7 d。,尿激酶（Urokinase,简称UK）是从人尿或肾组织培养物中提取出来的一种丝氨酸类蛋白水解酶。 1951年Willams发现的存在于哺乳动物尿中激活血纤维蛋白溶酶原的水解酶。1961年，Hansen最先将UK应用于临床。1965年，日本将其制成UK制剂。我国的上海生物化学制药厂于1990年正式获准生产UK制剂。,UK在新鲜的尿中主要有两种类型：双链大分子UK（HMWUK），分子量为54 000D；单链小分子UK（LMWUK），分子量为33 000D。HMWUK系天然存在形式，而LMWUK为前者的自发降解产物或被同时存在的蛋白水解酶降解HMWUK后产生，它直接激活血纤维蛋白酶原而提高对血纤维蛋白的溶解能力，从而达到溶解血栓的目的。优点：无抗原性，无热原性，可连续用药半个月以上。缺点：选择性差，治疗的同时会降解血纤维蛋白原。,广东药学院学报(ACAD J GCP) 2000，16(4)第4期 杜冰等：血栓溶酶研究进展SK和UK这两种药物在体内的半衰期短，只有320 min，要使其达到应有的疗效，必须大剂量长期用药，容易产生全身性出血的倾向。,针对UK半衰期短的特点，1991年日本有人提出利用尿激酶淀粉结合物改善单独使用尿激酶的各种缺点并能充分且持续的发挥尿激酶的活性。,为了克服SK的缺点，1986年，英国Beccham公司成功研制了一种新的溶解血栓药Eminase（茴香酰化的纤溶酶原链激酶活化复合剂）。优点：在45 min内一次性静脉给药（SK、UK则需要灌注数小时），不但给药途径快速方便，而且溶栓作用持久，不需要反复注射。,SK的换代产品还有APSAC(acylated plasminogen streptokinase activator complex)。其商品名是anistreplase，分子量为2.1710-22 kg(131 kD)，它的形成是通过SK纤溶酶原复合物的活性中心丝氨酸上引入一个可逆的酰基基团，暂时封闭活性中心，但不影响与血纤维蛋白的结合。,APSAC在体内无溶栓活性，进入血液后，能与血纤维蛋白结合，并发生脱酰基水解反应，形成有活性的SK纤溶酶原复合物，激活血纤维蛋白溶酶原。,优点：半衰期长达105 min，给药时间短，只需 min，并具有与纤维蛋白结合的能力。缺点：非特异性，具有抗原性，易发生变态反应，还可引起低血压。,AT是血浆中最重要的抗凝血酶，它能和已激活的凝血酶结合并使其灭活。肝素在血液凝固的三个阶段均发挥抗凝血作用。UK主要清除已形成的血栓。,还有报道采用UK和透明质酸酶合剂来治疗血栓栓塞症，透明质酸酶本身的扩散作用可以增加血流量，能大大增加UK疗效。但上述这些药多为静脉注射或点滴使用，所以药剂要求高纯度，导致价格昂贵，而且并未克服UK和SK产生的全身性出血的倾向。,1963年，Heid从马来西亚产红口腹蛇毒中提纯出一种抗凝物质安克洛酶，治疗血栓栓塞性疾病效果很好。此后，各国学者对此进行了深入的基础理论研究，并探讨用于治疗各种血栓栓塞性疾病，我国发现尖吻蝮蛇、蝮蛇、眼镜蛇、竹叶青蛇毒等具有纤溶作用。,1984年获准生产蝮蛇抗栓酶（Ahalysantinfarctasum），它是从蝮蛇毒中分离、纯化所得的凝血酶制剂，具有抗凝、溶栓、去纤、抗血小板粘附聚集、降脂、扩张血管等多种功效，因而广泛的用于各种血管闭塞及循环障碍性疾病，其效果与日本产东菱精纯克栓酶一致。不良反应：发热、头痛、嗜睡、头晕、皮下出血、肢体胀痛等，同时还伴有频发房性早搏、增生性龈炎、阴囊巨大型荨麻疹、过敏性休克、多脏器严重出血、肝细胞性黄疸、急性肾功能衰竭等。,为解决长期用药全身性出血的倾向，日本旭化工业公司与兴和公司创产了人体正常细胞培养生产tPA的方法。tPA即组织血纤维蛋白溶酶原激活剂(tissuetype plasminogen activator，简称tPA或TPA)，是体内各组织中都含有的一种丝氨酸蛋白酶，能选择的将血栓上的血纤维蛋白溶酶原变成血纤维蛋白溶酶使血栓溶解，在没有血栓时几乎无此作用，与尿激酶等不同，血栓溶解时不伴有全身性出血的倾向。,1991年tPA已上市，药品名为Tisokinase，是由人肺二倍体纤维细胞产生的526个氨基酸残基组成的丝氨酸蛋白，分子量为1.1110-22 kg（67 kD）。制剂商品名为Plasvata注射剂（东洋酿造）和Hapase注射剂（兴和新药）。,tPA是一条单链分子，可以经特异血纤维蛋白酶作用在Arg275Ile276处断裂形成双链，其N端重链包括的4个结构功能： Finger区（即F区，可与血纤维蛋白连接） E区（即类表皮生长因子区） Kringle 1区（可简称K1区） Kringle 2区（可简称K2区）,目前公认治疗血栓栓塞症的最佳药物应为单克隆抗体。最早是Bode等在1985年将抗纤维蛋白单克隆抗体（59D8）与UK共价连接，制成单抗导向溶血栓剂，从而开辟了溶血栓药物研究的新领域。,随着现代分子生物学，基因工程技术，蛋白质工程技术的不断发展，许多新的导向溶血栓剂不断的进入临床试验。 美国生物遗传科学公司（ABC公司）最近成功地开发出具有良好血栓溶解作用单克隆抗体称为MH1，能特异性识别血栓成分血纤维蛋白，并显示了高度溶解作用，在给药后1530 min产生作用而使血栓消失。,ABC公司于1993年6月开始作为新抗血栓药用于治疗心肌梗死和肺栓塞等疾病的临床试验，并在1993年底至1994年初向美国食品和药物管理局（FDA）申请了生产许可。我国北大蛋白质工程和植物基因工程国家重点实验室在这方面的研究也取得了一定的进展，但这种单克隆抗体需经静脉注射或滴注，口服几近无效。,以美国为首的GUSTO研究组于1990年至1993年进行了一国际大规模治疗心肌梗死的研究计划。结果表明：早期应用tPA的溶解血栓法是治疗急性心肌梗死的最佳方法。但这种tPA在血中的半衰期较短。,目前又开发出第二代tPA即重组组织血纤维蛋白溶酶原激活剂（MTPA或称rtPA），MTPA系应用蛋白质工程技术，将天然tPA氨基酸结构中的五处变换为其它氨基酸，从而改变了其结构，在血中维持时间为tPA的10倍，日本卫材公司在世界上率先进入临床试验。,1992年，美国重组tPA产值已达1亿8千万美元。自“七五”计划起，我国开始开展tPA的DNA在中国仓鼠卵巢细胞中高效表达的研究，经过多年努力，表达水平为250 U/(106 cellsd)。据最新文献表明，军事医学科学院已有人将其表达水平提高到6000 U/(106 cellsd)。,近年来日本报道了两种激酶即纳豆激酶（Natokinase）和天醅激酶（Tempehkinase），它们分别来自于日本的传统食品纳豆及印度尼西亚的传统食品天醅中，这两种食品都是大豆发酵的产品，历史悠久，因此分离出的酶其安全性不成问题，而且它们是能达到“经口纤溶疗法”目的的高效血栓溶解酶。,纳豆激酶是1987年日本须见洋行等把纳豆放在人工血栓上发现的,它的分子量较小（约为2万），是由含丙氨酸的275个残基所组成的一条单链多肽。不仅能静脉给药，口服用药效果也很好，而且它的体内半衰期很长（8d）。口服后纳豆缴酶除溶解血纤维蛋白外还引起继发性的tPA增加，以及能激活尿激酶原（pro UK）使之转化为尿激酶，这表明其作用是多方面的。,天醅激酶 分子量约为20 00050 000，在pH7.4时，50 下可稳定保持活性10 min，在血液中其活性可维持10h，对血纤维蛋白有强大的分解活性，而且有活化血纤维蛋白溶酶原的活性，对HDValLeuLyspNA，HDPhepipArgpNA，HDValLeuArgpNA强烈分解。,上述两种药其分子量小，易于吸收，故可制成肠溶衣胶囊服用，或制成纯制剂以静脉滴入，可用于一般的血栓栓塞症的治疗，同时还可以作为预防血栓栓塞症的药物或食物。,中科院生物物理研究所和江中制药厂共同研制了新一代全天然抗血栓药江中博洛克，于1995年获卫生部批准正式生产，它是采用生化分离技术从特殊蚯蚓中分离的一种丝氨酸蛋白酶，即蚓激酶（Lumbrokinase）。,蚓激酶具有激活人体内的纤维蛋白溶酶原和溶解体内血栓的作用。在兔中试验，蚓激酶不仅抑制血栓形成，而且有很好的溶解血栓作用；在大鼠实验性脑缺血中应用，蚓激酶可溶解血栓，改善脑缺血状况。又由于博洛克为肠溶衣胶囊，进入胃内后不被破坏，既保证了疗效，又服用方便。但是它的副作用也很明显，容易导致出血等。,目前国内外在血栓栓塞症治疗的方向一方面趋向于价格低，易为人体接收、体内半衰期长以及可以口服的药物或保健性的食品，另一方面趋向于高效，能药到病除的基因工程药物。全世界有血栓栓塞性病人1 500万，所需的溶栓剂的潜在市场约20亿美元。在我国豆豉、腐乳等发酵豆制品的研究中也发现类似的溶栓酶如豆豉纤溶酶等。但我国在这方面的研究很少，故而开发前景广阔。研究开发类似纳豆激酶或天醅激酶的药物或保健品不仅可以充分开发我国食品中的功能性微生物，还可以解决我国目前的血栓溶解药物口服性差、价格昂贵、并发症多的问题，但并不能因此而忽略了后一种基因工程药物的开发。,五 NEWS,国家一类生物溶栓新药普佑克即将上市 2011-11-10 “我国十一五重大新药创制科技重大专项中的首个治疗用生物剖品一类新药普佑克即将登陆中国。” 普佑克通用名为注射用重组人尿激酶原，是尿激酶的前体，其进入血液后并无活性，但在吸附在血栓表面后，经激肽酶作用被激活，转变为尿激酶，进而发挥溶栓作用，具有明显的血栓位置特异性，从而大大降低了目前许多溶栓药较为严重的出血副作用。用于ST段抬高型心肌梗死治疗具有一次开通率高、出血性脑卒中发生率低等显著优势，将成为中国自主研发的首个换代溶栓药品。 (智信),Emerging antithrombotic drugs: A review,Abstract Thromboembolic disorders are one of the disorders for which the researchers are still in search for a safe and efficient drug. Despite the widespread use of antithrombotic drugs for the prevention and treatment of arterial and venous thrombosis, thromboembolic diseases continue to be a major cause of death and disability worldwide. This shows the researchers inefficiency in searching efficacious and safe antithrombotic drugs. The researchers have reached to the basic mechanism of thrombus formation and by interrupting various steps of this mechanism, they can prevent as well as treat thromboembolic disorders. In continuation of Aspirin, now, the researchers are using clopedogrel, Ticlopidine and GpIIb/IIIa inhibitors (Abciximab, Tirofiban and Eptifibatide). Warfarin is an old antithrombotic drug, which is still being used but due to various side effects and drug interactions, they are bound to use newer drugs. Newer antiplatelet drugs include prasugrel, ticagrelor, cangrelor and elinogrel whereas newer thrombin inhibitors are Ximelgatran and Dabigatran. Apixaban and edoxaban are also newer entry in this category as Factor Xa inhibitors. Idrabiotaparinux is an indirect inhibitor of Xa as it accelerates the activity of antithrombin. Moreover, researches and trials for better and safe drugs are going on Keywords:Antiplatelet drugs, Antithrombotic drugs, Thrombin inhibitors,Introduction One of the major causes of morbidities and mortality worldwide is thromboembolic disorder. Thrombosis is the process of formation of solid mass in circulation from constituents of flowing blood, and the mass itself is called as thrombus. Hemostatic plugs are the blood clots formed in healthy individuals at the site of bleeding i.e. they are useful as they stop the escape of blood and plasma, whereas thrombi developing in the unruptured blood vessels may be harmful. Virchow described three primary events which predispose to thrombus formation (Virchows triad) : endothelial injury; alteration in flow of blood and hypercoagulability of blood. 1 Thrombosis can be either arterial or venous. Both arterial and venous are composed of fibrin, platelets and trapped RBC. Arterial thrombi are commonly formed after endothelial injury due to rapidly flowing blood, thus, platelets are abundant and fibrin is relatively sparse in arterial thrombus, whereas venous thrombus is formed due to blood stasis in veins, thus venous thrombus is mainly composed of fibrin and trapped RBC but less platelets. This fact is important to treat the patient effectively. Antithrombotic drugs are mainly of three types: (I) antiplatelet agents; (II) fibrinolytic drugs and (III) anticoagulants. By knowing the nature of thrombus, we can institute effective therapy i.e. arterial thrombus should be treated by antiplatelet agents 2,3 and venous thrombosis should be treated by anticoagulants mainly. 4,5 Despite the widespread use of antithrombotic drugs for the prevention and treatment of arterial and venous thrombosis, thromboembolic diseases continue to be a major cause of death and disability worldwide. That means available drugs are not so much efficacious and sufficient to combat these disorder. 6 Here we are going to review the older drugs in short, their shortcomings i.e. unmet medical needs of currently available antithrombotic therapy and finally the newer drugs, newer targets and opportunities and challenges for them.,The Unmet Need of Current Antithrombotic Therapy Recurrent ischemic events are quite common if patient with previous history of stroke takes aspirin, clopidogrel, ticlopidine or their combinations regularly i.e. at present most widely prescribed antiplatelet agents are not efficient enough to prevent further attacks. This can be either resistance to these drugs or incomplete suppression of platelets by these drugs.The only oral anticoagulant available in the market for more than half a century i.e. warfarin also tells the same story with other limitations too. It has very slow onset of action for which it is combined with rapidly acting parenteral anticoagulants for first few days. Due to some genetic polymorphism, dose adjustment is needed in almost every individual because of variable metabolism of Warfarin. There is one important problem of multiple drug interactions and more than this is its narrow therapeutic index. Because of this inconvenience, patient compliance is poor on long-term basis.Combination therapy (1 antithrombotic agents at a time) has resulted in declining rate of recurrent ischemia but the incidence of bleeding has increased. 6,Natural Anticoagulant Mechanisms In healthy vasculature, circulating platelets are maintained in an inactive state by nitric oxide and prostacyclin released by endothelial cells lining the blood vessels. Endothelial cells also express ADPase, which degrades ADP released from RBC and activated platelets, thereby preventing further platelet aggregation. 7 PGI 2 opposes action of TXA 2 and thus inhibits platelet aggregation and release. Antithrombin III (a plasma protein) blocks the action of factors XII, XI, IX, X and II. Protein C (a plasma protein) inactivates factor V and VIII not blocked by AT-III; it also enhances the action of t-PA. Also Heparan sulfate (a proteoglycan related to the Heparin) is synthesized by endothelial cells and it enhances the activity of AT-III. 8 Both hemostasis and thrombosis depend on three general components-the vascular wall, platelets and the coagulation cascade. Platelets contain two specific types of granules. Alpha granules express adhesion molecule, P selectin, on their membranes and contain factor V, vWF, PDGF, fibrinogen and TGFb, whereas d (delta) granules contain ADP, calcium, 5HT and histamine. 9 Vascular injury leads to transient vasoconstriction and finally, platelets are exposed to ECM via vWF and undergo three reactions: (I) adhesion and shape change; (II) secretion (release reaction) and (III) aggregation. 9,Adhesion of platelets to ECM (extracellular matrix) is facilitated by the interaction of vWF (from endothelium) and Gp IB receptors (on platelets). This adhesion leads to change in shape of platelets and there is release of granules (ADP from dense or delta granules). ADP is a potent mediator of platelet aggregation 9 and it also augments further ADP release. Now, TXA 2 is synthesized and released which is also a mediator of platelets aggregation. On one hand, vessel wall disruption augments platelet activation and aggregation, on the other hand, tissue factor initiates coagulation (extrinsic pathway). Phospholipid complexes on platelets activate intrinsic coagulation pathway. Platelet adhesion involves vWF, whereas aggregation involves GP IIb-IIIa receptors on platelets via fibrinogen. 9 Common coagulation pathway finally produces thrombin, which not only converts fibrinogen to fibrin but also activates platelets. These fibrin strands, along with platelets, now form platelet-fibrin mesh 6 as hemostatic plug or thrombus Antiplatelet agents can be classified on the basis of their site of action into those that inhibit (i) adhesion (ii) activation (iii) aggregation and (iv) platelet-mediated links with inflammation 6 Figure 1.,Inhibitors of Platelet Adhesion This step can be inhibited by interfering interaction between GP Ib (platelet receptor) and collagen or vWF. Many drugs are in queue to market which act through above mechanism but none has reached phase III yet. Strategies for inhibiting the ECM-platelet interaction include humanized monoclonal antibodies and aptamers against the receptors, small molecule peptide inhibitors and proteins derived from the medicinal leech. 6,Inhibitors of Platelet Activation Platelet activation can be inhibited by Inhibitors of TXA 2 pathway Inhibitors of ADP pathway PAR-I inhibitors Phosphodiesterase (PDE) inhibitors,Inhibitors of Platelet Aggregation Tirofiban, eptifibatide and abciximab are Gp IIb/IIIa inhibitors. Abciximab inhibits not only Gp IIb/IIIa but also a IIb/b3 receptors (for vWF) on platelets, thereby, decreasing aggregation through fibrinogen. 16 It is given i.v. and has shown its efficacy in reducing ischemic events in management of ACS and as adjunctive therapy during PCI; 6 however, trials with orally administered Gp IIb/IIIa inhibitors have failed to demonstrate any benefit. Moreover, a pooled data has shown them to significantly increase mortality in ACS cases. 17 Reason behind this is unknown but may be related to partial agonistic activity or proinflammatory effects. 18 These disappointing results have halted development of this class of drugs. 6,Inhibitors of Platelet-dependent Inflammatory Pathway Inflammation is an important determinant of progression of atherosclerosis and post-thrombotic syndrome, which complicates DVT 19 through CD40/CD40 ligand pathway and P-selectin. Platelets also play important role in inflammation. 20 CD40 acts as cell attractant molecule and starts proinflammatory response. Also, it is highly expressed in atheromatous plaques. Thus, inhibitors of CD40/CD40 ligand pathway can be novel molecules for delaying progression of atherosclerosis. 21 P-selectin molecule helps in the formation of platelet leukocyte aggregates. Inhibition of P-selectin will attenuate thrombus stimulus and formation. It has been proved in animal models of DVT 22 but yet to be proved in humans.,Present Anticoagulants and their Future Prospects Oral anticoagulants i.e. warfarin is in use for more than half a century but too much adverse effects push for developments in this field. Drug interactions, very slow onset of action and variable response are major and problematic limitations of warfarin. Now, we are targeting towards inhibition of (i) thrombin and (ii) other coagulation factors especially Xa.Thrombin inhibitors Thrombin is a most potent platelet antagonist; it converts fibrinogen to fibrin and also amplifies its own production. Because of its multiple roles in coagulation, thrombin inhibitors not only block fibrin formation but also attenuate further thrombin formation and platelet activation. 23 The first direct thrombin inhibitor, Ximelgatran has shown its efficacy in atrial fibrillation. The major drawback due to which it has been withdrawn is hepatotoxicity. 24 Latest addition in this series is Dabigatran etexilate. Being a prodrug, it is converted to Dabigatran by esterases after oral administration. T 1/2 is 14-17 h and is excreted out of the kidneys, thus, once or twice daily administration is enough. It is undergoing phase III trial for prevention and treatment of VTE and for stroke prevention in atrial fibrillation. Other trials include comparison between Dabigatran and Warfarin (RE-LY) and two doses of Dabigatran with Warfarin (Re-COVER). Pooled data shows no evidence of hepatotoxicity but Dabigatran level increases when used with P-glycoprotein inhibitors (quinidine, clarithromycin and verapamil). 25,26,Factor Xa inhibitors I