Role of new antiplatelet agents as adjunctive therapies in thrombolysis

Role of New Antiplatelet Agents as Adjunctive Therapies in Thrombolysis James T. Willerson, MD, Paolo Golino, MD, Janice McNatt, John Eidt, MD, Sheng-Kun Yao, MD, and L. Maximilian Buja, MD

Coronary thrombolysis is the treatment of choice for patients with acute Q-wave myocardial jnfarcts who have no contrainditions to such therapy. However, the time required for thrombolysis and the possibility of reocclusion of the infarct-related artery remain problematic. Herein are described experimental animal studies and clinical evaluations in which attempts have been made to develop adjunctive therapies that, when coupled with available thrombolytic interventions, might shorten the time to thrombolysis and delay or prevent reocclusion. From the studiis conducted to date, it is clear that a combined thromboxane synthesis inhibitor and receptor antagonist with a serotonin receptor antagonist and heparin shorten the time to thrombolysis and delay or prevent coronary artery reocelusion in experimental canine models with copper coil-induced coronary artery thrombi. A monoclonal antibody to the platelet glycoprotein Ilb/llla receptor coupled with tissue plasminogen activator (t-PA) and heparin also shortens the time to thrombolysis and delays or prevents reocclusion in experimental canine models. Thrombin inhibitors, including heparin and synthetic inhibitors, given with t-PA and aspirin, appear to shorten the time to thrombolysis and delay or prevent coronary artery reocclusion in experimental canine models. Aspirin coupled with intravenous streptokinase reduces mortality in patients with presumed acute myocardial infarction, and a combination of heparin and t-PA results in infarct-artery patency more frequently than t-PA without heparin. Data from these studies are encouraging with regard to the posslbility of developlng effective and relatively safe thrombolytic regimens that shorten the time to thrombolysis and delay or prevent coronary artery reocclusion. (Am J Cardiol 1991;67:12A-18A)

oronary thrombolysis has been evaluated extensively as a meansto prevent or attenuate pathophysiologic eventsoccurring with acute myocardial infarction. Studies in experimental animals and patients show that the administration of thrombolytic agentsearly in the courseof myocardial infarction reducesinfarct sizeand preservesventricular function.1-3However, the time required for thrombolysis with currently available agentsremainslonger than desirable,and there is an important risk of coronary artery reocclusionafter successfulthrombolytic therapy. Thus, it should be useful to identify adjunctive therapy that shortensthe time to thrombolysis and prevents coronary artery reocclusion following thrombolytic therapy. Platelets are activated in responseto a variety of substances.Studies from our laboratory as well as others have demonstratedthat thromboxane A2 and serotonin are important mediatorsof intracoronary platelet activation in experimental preparations of concentrically stenosedcanine coronary arteries with endothelial injury.4-s Others haveshownthat the platelet glycoprotein IIb/IIIa platelet receptors are important in platelet binding to fibrin and the developmentof a thrombus; a monoclonal antibody directed against the platelet glycoprotein IIb/ IIIa receptorsenhancesthrombolysis in experimental animal models.9JoIn addition, clinical studiesin which aspirin has been combined with selectedthrombolytic interventions suggest enhanced thrombolytic protection.” This review summarizescurrent information related to experimental animal and clinical studies that have been performed in an attempt to alter platelet activation and shorten the time to thrombolysis and prevent or delay coronary artery reocclusion after thrombolytic therapy.

C

STUDIES IN EXPERIMENTAL ANIMAL MODELS

We evaluatedwhether single or simultaneousadministration of thromboxane and serotonin receptor antagonists combinedwith tissue plasminogenactivator (t-PA) and heparin shortenthe time to thrombolysis and prevent or delay coronary artery reocclusion after thrombolytic therapy.i2J3 In thesestudies,coronary thrombi were induced in 26 anesthetized,open-chestdogsby inserting a copper coil into the left anterior descending coronary From the Departmentsof Internal Medicine and Pathology,University of Texas Medical School,Texas Heart Institute, Houston,Texas. This artery (LAD) (Fig. 1). Coronary artery blood flow velocistudy was supported in part by Specialized Center of Research in ty was monitored throughout the experiment with a Ischemic Heart Disease,Grant HL17669 from the National Heart, Doppler flow probe placed proximally to the coil. The Lung, and Blood Institute, Bethesda,Maryland. Address for reprints: James T. WilIerson, MD, Cardiology Re- presenceof a thrombus was documentedfor 30 minutes. search,1-191,Texas Heart Institute, P.O. Box 20345,Houston,Texas Dogs were assignedto 1 of 4 groups: group 1 (n = 8) 77225. served as control subjects,receiving a bolus of heparin

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Aortic Pressure (mmHg)

LAD Phasic Flow (KHZ)

4

0 LAD Mean FIOW (KHZ)

FIGURE 1. ReprewnM ve tracing of hendynamic data obtained frem a dog treated with heparin, tissue plasminogen activator and thremboxane remptor antagonist SQ29548 and serotonin receptar antagonisl LY53857 in combination. A, bassline mea~~~,25mmh).B,attarplacementofthecoppercoilintothekft~derrcen&lg C--Y artary WW and Uuumbus fonnath (paper speed, 25 nnn/min). C, administration of tissue pbminogen activator caused lysis of ths ttwomkuMd~~ltknofMoodflawthat~weda~a~aml~ntpattsrnthrwghart fhe experimentel period WP- wed, WithpenniSSiOllOftlWAmerican 25 mm/min). (See text for details.) (Repro&cd Heart Aswxlation, Cimutation.lz)

(200 U/kg) and of t-PA (80 pg/kg), followed by a continuous infusion (8 pg/kgmin) for 190 minutes or until reperfusion was achieved. Group 2 (n = 10) received, immediately before heparin and t-PA, an intravenous bolusof 0.4 mg/kg of a thromboxane receptorantagonist (SQ29548, Squibb Pharmaceuticals, Princeton, New Jersey) and 0.2 mg/kg of a serotonin receptor antagonist (LY 53857,Eli Lilly Pharmaceuticals,Indianapolis, Indiana). Group 3 (n = 7) received,beforeheparin and t-PA, an intravenous bolus of the thromboxane receptor antagonist alone (0.4 mg/kg). Group 4 (n = 7) received,before heparin and t-PA, an intravenous bolus of the serotonin receptor antagonist alone (0.2 mg/kg). After thrombolysis, all dogs were monitored for 90 minutes or until a persistent reocclusion occurred. Treatment with a combination of the thromboxane and serotonin receptor antagonists markedly shortened the time required to lyse coronary artery thrombi, from 46 f 7 to 15 f 3 minutes in groups 1 and 2 animals, respectively (p
the thrombolytic effect of t-PA, probably by preventing further platelet activation and by their incorporation into the thrombus during lytic therapy. The combination of these 2 receptor antagonists appears to be effective in preventing or delaying reocclusion after discontinuation of t-PA in this experimental model. EFFICACY OF A COMBINED THROWBOXANE SYNTHESIS INHIBITOR AND RECEPTOR ANTAGONIST IN ENHANCING THROMBOLYSIS AND DELAYING OR PREVENTING REOCCLUSION Seventyanesthetized,open-chestdogsin whom coronary artery thrombi were induced with a copper coil, as describedearlier, were evaluated.14In these studies, the efficacy of a combined thromboxane synthesisinhibitor and receptor antagonist (ridogrel, JanssenPharmaceutical, Beerse,Belgium) in combination with t-PA and heparin were evaluated and compared to that of t-PA and heparin alonein enhancingthrombolysis.In thesestudies, ridogrel added to t-PA and heparin markedly shortened the time to thrombolysis in treated dogs from approximately 45 to 12 minutes (Fig. 4).14In addition, coronary artery reocclusionafter discontinuation of the t-PA was either prevented or markedly delayed by combined thromboxane synthesis inhibition and receptor antagonism (Fig. 4). In thesestudies,weidentified an increasein distal coronary artery prostacyclin concentration after the administration of ridogrel, presumably as a result of shunting cyclic endoperoxidestoward prostacyclin after the administration of the combinedthromboxanesynthesis inhibitor and receptor antagonist. Increasesin thromboxane were prevented by ridogrel, and in vivo assess-

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IGURE 2. A, plot of time of recombinant tissue-type plasminogen activator (rt-PA) infuion (i.e., time to achii Wombolysis) in dogs with coronary thrombosis receiving heparin (group 1); heparin and thromboxane recepW antagoNist SQ2S!XS (SQ) and serotonin receptor antagonist LYS3SS7 (LY) in combination (group 2); heparln and !5Q alone (gro~Q 3); and hoparin and LY toachllefalone (group 4). Only ths athdnistration of SQ and LY in combination significantly m the time v fective thmnblysls (defined as left anterior m coronary artery blood flow 270% of baserme value). *p
4

Q-3

A

7GURE 3. Rspresentative tracing of hsmodynamic data obtained from d dog treated with heparin and tissue plasminogen activator only (group 1). A, before placement of ths coppsr coil into ths left anterior &xxending eoro~ly artery &AD), coronary bloodflowshowsaaableandcons~~pattem(papw~,2Smm/s).B,afterpo~ingofthe~,LAD#oodIkwckcreased to almost mwewrdable values (paper speed, 25 mmhin). C, dving infurion of tissus pJasminogen activatff, UK intratoconary thumbus was progressively lyssd and blood flow thraugh the LAD was restored (c/osed arrow). The arrow indkates the time when the infusion of tissue plasminogen activator was discontinusd. DespRe the adnrinistration of 200 U/kg of heparin, LAD blood flow showed a typkal pattern of gradual decrease of flow (open arrowsJ, followed by Ipontanaour restoration of blood flow @/osed arrow) until a reoc&&m OccuTBd (arrowheed) (paper speed, 25 mm/ml@. (Reproduced with permission from the American Heart Association, Circu/afion.13

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1

TABLE I Reocclusion Time and Activated Coagulation Time (ACT) in Dogs After Thrombolysis With Tissue Plasminogen Activator

Group No. 1 2 3

Reocclusion Time 17f5 (0)’ >90 (g/w+ 31 f29

Reperfusion

90 Minutes Reperfuslon or Reocclusion

96 f 6

207 f 15

199 f 12

107 f 7

199 f 14

193 f 19

104f7

212 f 21

220 f 23

227f24

204f

Base

T

(l/W* 4

27f31 * Numbers in parentheses

lOOf lndlcate dogs

if ,:

19

in whom the treatment prevented reocclu-

0 ISQ 29548

SIO~ after thrombolysle. t p <0.005 versus groups 1, 3, and 4 by Fisher’s exact test Reocclus~on tune (minutes) was defined as the ttme elapsed between reperfus~on and the occurrence of a persistent reocclusion. ACT (seconds) was measured at baseline, at the moment of reperfus~on. and after 90 minutes of reperfwon or at the moment of reocclus~on. See text for details. (Reproduced wth permission from the Amencan Heart Association, C,rculatmn 12)

ments of platelet aggregation demonstratedthat platelet aggregation in response to a thromboxane mimetic (U466 19) was preventedby the in vivo administration of ridogrel. I4 COMPARATIVE EFFECTS OF ASPIRIN, A SYNTHETIC THROMBIN INHIBITOR, AND A MONOCLONAL ANTIPLATELET GLYCOPROTEIN Ilb/llla ANTIBODY The comparative effects of intravenous aspirin, the synthetic thrombin inhibitor, (2R, 4R)-Cmethyl-1-[N2(3-methyl-1,2,3,4 - tetrahydro-8-quinolinesulfonyl)-Larginyll-2-piperidinecarboxylic acid monohydrate (argatroban), and F(ab’)Z fragments of the monoclonal antibody, 7E3, against platelet glycoprotein IIb/IIIa receptors (7E3-F[ab’]2) on thrombolysis, reocclusion and bleeding associatedwith 0.45 mg/kg of body weight bolus injections of recombinant t-PA were studied in a canine coronary artery thrombosismodel.l 5In thesestudies, coronary artery thrombosiswas induced by mechanical damage to the endothelium. Total coronary artery occlusionwasconfirmed by coronary arteriography. Four groups of animals were studied: group 1 animals were pretreated with 17 mg/kg of intravenous aspirin (n = 6); group 2 was pretreated with 200 kg/kg per minute of intravenous argatroban for 60 minutes (n = 5); group 3 was pretreated with aspirin and argatroban (n = 5); and group 4 was pretreated with 0.8 mg/kg of intravenous 7E3-F(ab’)2 (n = 5). In group 1, reperfusion occurred in 4 of 6 dogs,but did not persist.Reperfusionoccurred in 4 of 5 dogsin group 2, persisting in 1. In group 3, reperfusion occurred in all 5 dogsand persistedin 4; and in group 4, reperfusion occurred in 4 of 5 dogs and persistedin 2 (Fig. 5, Table II). Reperfusion in group 3 animals was significantly higher than in combined groups 1 and 2, whereasthe time to reperfusion was significantly shorter in the groups receiving argatroban than in the aspirintreated group (25 vs 55 minutes, p = 0.04) (Fig. 5). There were significant differencesbetweengroup 3 and 4 animals in the frequency of reperfusion. Bleeding times

R 68070

$ 3

30

l l

8

l l

8

e

0 Reocclusion o No F&occlusion

e

0 SQ 29548

R 68070

IGURE 4. Top, the reduction in the time re@-ed for thrombolysis when tissm plasminogem activator (t-PA) is given to anesthe&ed, open-dmt dogs with a copper coilinduced cafwith a combhod thromboxans synthoonaryartwythmbm sis inhibitor and receqtor antagonist (RGBO70). The animals were treated with t-PA, hepadn, and either a tfmmboxme receptor antagonist (gQ2954B) or a combined thromboxane synhsls inhibitor and receptor antagonist (RGBO70). *p <0.01.Bottom,theadminiiationof acombinedthmboxan0 synthesis inhbitor and receptor dagonist (RGBO70) aftor tissue plasmlnogen activator and heparin delayed or preventedcomnaryarteryfeoddoninanerthetized,open-chest dogswhenacoppercoilh8dbeenusedtoimbcethrombosis. In contrast, the administration of a thromboxane receptor antagonist (S@BS4B) by itself after tissue plasminogen activator and heparin failed to delay or prevemt -J-Y aewy reocchl!3loll.

wereprolongedfrom 4 f 1 to 5 f 3 minutesat 60 minutes in group 1; 9 f 11 minutes in group 2; 12 f 11 minutes in group 3; and 27 f 8 minutes in group 4 animals. One hour after the infusion, bleedingtimes were 5.0 f 0.9,5.2 f 1.3, 14 f 9.2, and 26 f 9 minutes, respectively. Thesedata suggestthat argatroban acceleratescoronary thrombolysis with t-PA and that the combination of aspirin and argatroban delaysor preventscoronary artery reocclusion. These effects were achieved with less pronouncedprolongation of the bleeding time than was ob served with a potent antiplatelet glycoprotein IIb/IIIa receptor antibody, and suggestthat inhibition of platelet activation with aspirin in combination with a short-lived synthetic thrombin inhibitor may constitute an alterna-

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FIGURE 5. Sdmmatk reprermtatian of ~eoronarY=@YpaencY~~in receiving bdus injectiom of 0.45 mg/kg of tissus plasminogen activator in associatium with athninistrstion of aspirin, argatrobm or 7U-F(abq2. open box, continuorn lntravenous infusion of agatroban. Open bars I.a!pmd patency and bdcbed bars occlu~ofthe coronaryartery.lheh& open bar repreads partial rellow. (Re~9vith pemlssion from J Am Co/l .

(09mglkglmhl

TABLE II Effects of Aspirin, Argatroban Tissue-Type Plasminogen Activator

and 7E3-F(ab’)2 on Coronary Artery Recanalization and Reocclusion With Recombinant Reperfusiont

Blood Flow (ml/min) Experimental Group I: aspirin II: argatroban III aspirin + argatroban IV: 7E3-F(ab’)2

Coronary Patency* (frequency)

Time (min)

No.

Baseline

Poststenotic

PO

RR

CR

PP

Total No.

Mean f SD

Median

Range

6 5 5

16f5 15f3 12f4

6.3 f 1.1 6.0 f 1.7 5.1 f 0.5

2

2

0

1

0

2 3

0

1

45

4O*20 23f14 24rt 11

55 28 21

14-120 7-120

0

4/6 4/5 5/5

5

13 f 5

5.0 f 1.8

1

1

1

211

4/5

27f 17

33

9-120

1s

14-39

* p - 0.036 for significant differences by Kruska-Walk nonparametric analysis of variance. t Values are mean i standard dewation (SD) in responsive animals, and the median and range are values for the total group. * p = not sipificant versus aspirin. 5 p = 0.009 versus pooled aspirin and argatroban groups. p = 0.064 versus argatroban and p = 0.008 versus aspirin. ‘1Difference not significant versus pooled aspirin and argatroban and versus aspinn + argatroban. Unless indicated. values are mean f SD. CR = cyclic reflow: PU = persistent occlusion; PP = pewstent patency; RR = reflow and reocclusion. (Reproduced with permission from JAm Co/l Cardiof.‘5)

tive approachto improving coronary artery patencywith thrombolytic therapy.15 CLINICAL EVALUATIONS Second International Study of Infarct Survival (ISIS-2): A multicenter, randomized trial of intravenous

streptokinase,oral aspirin, both, or neither among 17,187 patients with suspectedmyocardial infarction showed that intravenous streptokinasealone, aspirin alone, and, even more prfoundly, the combination of intravenous streptokinase and aspirin reduce mortality when given within the first 24 hours of symptom onset to patients with suspectedmyocardial infarction (Fig. 6).” Thesestudiesidentify the importance of the administration of a platelet-active agent,suchasaspirin, in potentiating the thrombolytic effect of streptokinasewith regard to its ability to reducemortality. The doseof aspirin usedwas 160mg/day, and it wasgiven immediately after the patient was identified. Bleeding risks were more frequent in patients receiving intravenous streptokinase. Thesedata have servedto encouragethe administration of aspirin with thrombolytic intervention, and the dose used is usually one-half to 1 aspirin tablet per day. The Heparin-Aspirin Reperfueion Trial: Recently, the Heparin-Aspirin Reperfusion Trial (HART) was completed.In this study, patients with acute myocardial 16A

THE AMERICAN JOURNAL OF CARDIOLOGY VOLUME 67

infarction presenting within 6 hours of symptom onset receivedt-PA and were randomized to immediate additional therapy with intravenous heparin or oral aspirin.16 Patients receiving t-PA and heparin had an infarct-related artery patency rate of 82% on initial angiography performedat a meanof 18 hours. In contrast, only 52%of patients receiving t-PA and aspirin had a patent infarctrelated artery (p
These studies indicate that the thrombolytic efficacy of the available thrombolytic interventions may be enhanced in experimental animals by a combined thromboxane synthesisinhibitor and receptor antagonist and with theseagentsand a serotonin receptor antagonist. A

Placebo lnlusion and tablets. 568/4300 (13.2%) 49lj4295 (to.hq Asphn: Stre toklnaw: 446 P QOO

FIGURE 6. Renefkial affect of combining streptoklnase md aspirin In redwing mortaHyduhga36-daypcsiodaftertheadministrationof~thsrapytopatii with suspecW myocanlsal infarctii. Asp&in and streptokinase atone also eflect, but the magniexerted a bedfdzd tude of the effect was potentiated by combining streptokinass and aspirin. (Reproduced with permission from Lancetzz)

treptoklnase 34314292

14

(10.4~

and Aspirin: ( 8.0%)

21

29

Days tram randomlsation

monoclonalantibody to the platelet glycoprotein IIb/IIIa receptor (platelet-fibrinogen receptor) exerts the same beneficial effects in experimental animals. Concomitant administration of aspirin and heparin potentiates the thrombolytic effects of t-PA and streptokinase in humans. Theseadjunctive strategiescombined with thrombolytic therapy appear to enhancereperfusion frequency and delay or prevent coronary artery reocclusion.In canine models, selectedplatelet-active therapies markedly shorten the time to thrombolysis when given in combination with t-PA, and they will most likely exert similar effectswith at least selectedother thrombolytic interventions as well. These results are not surprising, in that platelets adhere at sites of endothelial injury when the subendothelium is exposed, such as occurs with atherosclerotic plaque fissuring and ulceration. The accumulation of platelets at such sites is associatedwith local increasesin thromboxane A2 and serotonin concentrations,and, most likely, adenosinediphosphateand platelet activating factor.6J7-19In addition, Fitzgerald demonstratedthat the administration of the thrombolytic interventions, streptokinase and t-PA, stimulates platelet activation and thromboxane AZ release.20Thus, one would anticipate that therapeutic strategiesthat interfere with the effects of thromboxane A2 and serotonin would enhancethrombolysis and delay or prevent coronary artery reocclusion. The clinical observation that aspirin potentiates the thrombolytic effect of streptokinase in reducing subsequent mortality in patients is not unexpectedbasedon the

experimentaldata that are available.4s,’1,17-20 It remains to be determined whether combined thromboxane synthesisinhibition and receptorantagonismand a serotonin receptor antagonist, the monoclonal antibody to the platelet glycoprotein IIb/IIIa receptoror other antiplatelet antibodies, antagonists, or synthetic peptides that block glycoprotein IIb/IIIa or lb platelet receptorswill shorten the time to thrombolysis in patients. An antithrombin intervention should be coupledwith a thrombolytic intervention; heparin appearsto be acceptablebut, in the future, more specific thrombin antagonists might be usedwith a thrombolytic agent alone or coupled with heparin to enhance the thrombolytic effect. Additional data related to thrombin inhibitors, alone and in combination, will be forthcoming from clinical studies performed in the near future. REFERENCES 1. Sheehan FH. Braunwald E. Canner I’, and co-investigators. The effects of intravenous thrombolytic therapy on left ventricular function: a report on tissuetype plasminogen activator and streptokinase from the Thrombolysis in Myocardial Infarction (TIM1 Phase I) Trial. Circu/o/ion I987;75:R 17-829. 2. Bush LR, Buja LM, Tilton G, Watson M, Apprill P, Ashton JH, WilIerson JT. Effects of propranolol and diltiazem alone and in combination on the recovery of left ventricular segmental function after long-term reperfusion following temporary coronary occlusion in conscious dogs. Circularion 1985:72:413-430. 3. Collen D, Top01 EJ, Tiefenbrunn AJ, Gold WK. Weisfeldt ML, Sobel BE, Leinbach RC. Brinker JA. Ludbrook PA, Yasuda T, Bulkley BH, Robinson AK, Hotter AM, Bell WR, Spadaro JJ. Khaw BA, Grossbard EB. Coronary thrombolysis with recombinant human tissue-type plasminogen activator: a prospective, randomized-placebo controlled trial. Circularion 1984;70:1012-1017. 4. Bush LR, Campbell W B, Kern K, Tilton CD, Apprill P. Ashton JH, Schmitz J. Buja IM, Willerson JT. The effects of alpha? adrenergic and scrotonergic receptor

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blockade on platelet aggregation in stenosed canine coronary arteries. Circ Res 1984;55:642-652. 5. Bush LR, Campbell WB, Tilton CD, Buja LM, WilIerson JT. Effects of the selective thromboxane synthase inhibitor dazoxiben on variations in cyclic blood flow in stenosed canine coronary arteries. Circulation 1984;69:1161-I 170. 6. Ashton JH, Benedict CR, Fitzgerald C, Raheja S, Taylor A, Campbell W, Buja LM, WilIerson JT. Serotonin is a mediator of cyclic flow variations in steno& canine coronary arteries. Circularion 1986;73:572-578. 7. Ashton JH, Schmitz JM, Campbell WB, Ogletree ML, Raheja S, Taylor AL, Fitzgerald C, Buja LM, Willerson JT. Inhibition of cyclic flow variations in stenosed canine coronary arteries by thromboxane Az/prostaglandin H2 receptor antagonists. Circ Res 1986;59:568-578. 6. Ashton JH, Ogletree ML, Michel IM, Golino P, McNatt J, Taylor AL, Raheja S, Schmitz J, Buja LM, Campbell WB, Willerson JT. Serotonin S+hromLwxane A,/prostaglandin Hz receptor activation cooperatively mediate cyclic flow variations in dogs with severe coronary artery stenosis. Circulation 1987;76:952-960. 9. Caller BS, Scudder LE. Inhibition of dog platelet function by in viva infusion of F(ab’)Z fragments of a monoclonal antibody. Blood 1986;66:1456-1459. 10. Caller BS. A new murine monoclonal antibody reports on activation-dependent change in the conformation and/or micro environment of the platelet glycoprotein Ilb/IIIa complex. .I C/in Inrest 1985;76:101-108. 11. ISIS-2 (Second International Study of Infarct Survival) Collaborative Group. Randomized trial of intravenous streptokinase, oral aspirin, both or neither among 17,187 cases of suspected acute myocardial infarction: ISIS-2. Lancet 1988;2:349-360. 12. Golino P, Ashton JH, McNatt J, Glas-Greenwalt P, Sheng-Kun Y, O’Brien RA, Buja LM, Willerson JT. Simultaneous administration of thromboxane Aland serotonin Sz-receptor antagonists markedly enhances thrombolysis and prevents or delays reccclusion after tissue-type plasminogen activator in a canine model of coronary thrombosis. Circulation 1989,79:91 l-919.

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13. Golino P, Ashton JH, Glas-Greenwalt P, McNatt J, Buja LM, WilIerson JT. Mediation of reocclusion by thromboxane AZ and serotonin after thrombolysis with tissue-type plasminogen activator in a canine preparation of coronary thrombosis. Circulation 1988;77:678-684. 14. Golino P, Rosolowsky M, Yao S, McNatt J, De Clerck F, Buja LM, Willerson JT. Endogenous prostaglandin endoperoxides and prostacyclin modulate the thrombolytic activity of tissue plasminogen activator. J Chin Imesf 1990;86:10951102. 15. Yasuda T, Gold HK, Yaoita H, Leinbach RC, Guerrero JL, Jang IK, Holt R, Fallon JT, Collen D. Comparative effects of aspirin, a synthetic thrombin inhibitor and a monoclonal antiplatelet glycoprotein IIb/IIIa antibody on coronary artery reperfusion, reocclusion and bleeding with recombinant tissue-type plasminogen activator in a canine preparation. J Am Coil Cardiol 1990;16:714-722. 16. Hsia J, Hamilton WP, Kleiman N, Roberts R, Chaitman BR, Ross AM, for the Heparin-Aspirin Reperfusion Trial (HART) investigators. A comparison between heparin and low-dose aspirin as adjunctive therapy with tissue plasminogen activator for acute myocardial infarction. N Eng/ J .&fed 1990,323:1433-1437. 17. Schmitz JM, Apprill PG. Buja LM, Willerwn JT, Campbell WB. Vascular prostaglandin and thromboxane production in a canine model of myocardial &hernia. Circ Res 1985;57:223-231. 16. Hirsh PD, Hillis LD, Campbell WB, Firth BG, Willerson JT. Release of prostaglandins and thromboxane into the coronary circulation in patients with ischemic heart disease. N Engl J Med 198 1;304:685-69 I 19. van den Berg EK, Schmitz JM, Benedict CR, Malloy CR, Willerson JT, Dehmer GJ. Transcardiac serotonin concentration is increased in selected patients with limiting angina and complex coronary lesion morphology. Circularion 1989;49:116-124. 20. Fitzgerald DJ, Catella F, Roy L, FitzGerald GA. Marked platelet activation in viva after intravenous streptokinase in patients with acute myocardial infarction Circulation 1988;77:142-I 54.