Clinical pharmacology of the adenosine diphosphate (ADP) receptor antagonist, clopidogrel

Clinical pharmacology of the adenosine diphosphate (ADP) receptor antagonist, clopidogrel Karsten Schrör Abstract: Antiplatelet compounds interfere with the platelet activation cascade at different levels. The antiplatelet effect of the thienopyridine, clopidogrel, results from antagonism of a platelet ADP receptor, P 2T, resulting in inhibition of platelet activation. This antagonism is non-competitive, irreversible, and results in a 50 70% inhibition of platelet fibrinogen binding. Additionally, clopidogrel may also antagonize the ADP-induced inhibition of adenylate cyclase, possibly resulting in an elevated platelet cyclic adenosine monophosphate level after stimulation by an appropriate agonist, such as prostacyclin. This spectrum of antiplatelet activities is different from that of aspirin. Further, clopidogrel is associated with a reduction in gastrointestinal hemorrhage, making it a valuable therapeutic alternative to aspirin in oral, long-term prevention of atherothrombotic vascular occlusion. Key words: ADP; atherosclerosis; clopidogrel; platelet; thrombosis Atherosclerosis and atherothrombosis Atherosclerosis is a chronic vascular disease that develops over years and may remain asymptomatic for long periods of time. 1 Morphologically, there is a progressive narrowing of the arterial lumen, eventually resulting in regional ischemia, such as transient ischemic attacks (TIA), effort angina, or intermittent claudication. Atherosclerotic vessels may potentially undergo a catastrophic event acute thrombotic occlusion with the corresponding clinical manifestations of myocardial infarction, sudden death, or ischemic cerebral infarction. Each of these life-threatening events are initiated by platelet-dependent thrombus formation, most commonly at the thrombogenic surface of an atherosclerotic plaque. Antiplatelet therapy is a symptomatic and effective approach 2 to preventing the consequences of endothelial dysfunction in atherosclerotic vascular disease, which could otherwise result in uncontrolled intravascular platelet activation. Pathophysiology of platelet-dependent thrombus formation Thrombus formation at a site of arterial vascular injury is initiated by the adhesion of resting platelets to the vessel wall. This response is mediated by the expression of platelet adhesion molecules, such as GpIb/IX and GpIIb/IIIa, which bind to von Willebrand factor and other components of the vascular subendothelium. 3 The binding of these adhesion molecules, along with other integrins, is regulated by accessibility of the subendothelial matrix, with its Institut für Pharmakologie, Heinrich-Heine-Universität Düsseldorf, Düsseldorf, Germany Address for correspondence: Karsten Schrör, Institut für Pharmakologie, Heinrich-Heine-Universität Düsseldorf, Moorenstr. 5, D-40225, Düsseldorf, Germany. adhesive surfaces, to circulating blood. 4 Soluble agonists, such as adenosine diphosphate (ADP) and thromboxane A 2 (TxA 2 ) synergistically accelerate platelet activation. This results in platelet shape change, granule secretion, 5 and exposure of platelet phospholipids to circulating blood, which accelerates local thrombin formation. Finally, the platelet integrin, GpIIb/IIIa, is activated, undergoing a conformational change at the surface of the platelet allowing it to bind soluble fibrinogen. This results in the cross-linking of platelets via fibrinogen bridges, termed platelet aggregation. The significance of ADP in platelet activation Almost 40 years ago, ADP was found to cause red blood cell-dependent sticking (adhesion) of platelets to glass. 6,7 Shortly after that, it was also observed that ADP could induce platelet-to-platelet adhesion (aggregation). This action was dependent on the presence of Ca ++ and fibrinogen. 8,9 Further, platelet-dependent hemostatic plug formation was inhibited in the presence of ADP-metabolizing enzymes. 10 Shear stress induced platelet activation is largely dependent on ADP 11 and is resistant to aspirin, 12 as is ADP-induced platelet activation in vivo, i.e. in the presence of physiological Ca ++ concentrations. These data and a number of follow-up studies led to the view that ADP is one of the most important mediators of platelet activation in vivo. 13 There are several rich sources of extracellular nucleotides in the blood, such as ADP. These include red blood cells, endothelial cells and platelet-dense granules. 14 16 In the platelet aggregates formed after vascular injury, local extracellular nucleotide concentrations may temporarily exceed 100 M. 17 Thus, ADP appears to be a potent, broad-spectrum, endogenous platelet-stimulatory agent in vivo. Arnold 1998 1358-863X(98)VM244MP

248 K Schrör Mechanisms of ADP-induced platelet activation ADP receptor(s) Biochemical and pharmacologic findings suggest the existence of at least two different ADP receptors on the platelet membrane belonging to the P 2X1 and P 2T subtypes of purinergic receptors (Figure 1). The P 2X1 receptor is an ADP ligand associated with an ion channel. The P 2T receptor is a heptahelical membrane receptor that is coupled to intracellular signaling pathways via G-proteins. The P 2T receptor subtype mediates other, still unknown platelet-stimulating actions of ADP. There are about 400 1000 ADP receptors per platelet, a number comparable with those of other G-protein coupled platelet receptors, including vasopressin, -agonists and TxA 2. 18 ADP receptor signal transduction There are at least three different signal transduction pathways that may become activated after ligand binding to the platelet ADP receptor: activation of GpIIb/IIIa; inhibition of adenylate cyclase activation; and stimulation of phospholipase C (PLC ). The significance of an ADP-stimulated increase in cytosolic Ca ++ concentration via activation of PLC and the subsequent formation of inositol triphosphate and release of Ca ++ from intracellular storage sites has not been fully elucidated. 19 It has been shown, however, that increases in cytosolic Ca ++ levels are a consequence of PLC stimulation by TxA 2. Inhibition of TxA 2 formation by aspirin may enhance the antiaggregatory nature of ADP antagonists on platelets via this mechanism (Figure 1). The most important cellular signal transduction pathway of ADP, however, may result from activation of the platelet GpIIb/IIIa complex, the final common pathway of platelet aggregation used by all platelet agonists. Irreversible platelet aggregation requires the binding of fibrinogen to this receptor. As with other agonists, such as collagen and thrombin, ADP initiates the activation of fibrinogen binding sites on platelet membranes. This occurs via conformational changes in the GpIIb/IIIa complex that are prerequisite to stable platelet thrombus formation. Mode of action of clopidogrel Although most evidence suggests that the activity of clopidogrel is a direct result of its antagonism of the ADP receptor, it may also interfere with a more distal step in the platelet activation pathway, such as a -protein response or tyrosine kinase phosphorylation. 20 This mode of action is different from aspirin, which does not directly affect ADPinduced platelet functions, including platelet adhesion and -granule secretion. In contrast to aspirin, clopidogrel does not directly interfere with arachidonic acid metabolism. Thus, the inhibition of platelet aggregation brought about by clopidogrel is completely different from that of aspirin. Figure 1 Signal transduction by ADP in platelets and its modification by clopidogrel. (Abbreviations: ADP, adenosine diphosphate; P 2X1, principal platelet ADP receptor (ADP-liganded ionic channel); AC, adenylate cyclase; camp, cyclic adenosine- 3 5 -monophosphate; IP 3, inositol triphosphate; DAG, diacylglycerol; PLC, phospholipase C; Ca ++, calcium; PIP 2, phosphatidylinositol 4,5-bisphosphate; TP, thromboxane receptor; IP, prostacyclin receptor; TxA 2, thromboxane A 2 ;G q,g s,g i,g proteins; P 2T, purinergic platelet ADP receptor (high-affinity, G-protein-coupled heptahelical receptor); GP IIb/IIIa, glycoprotein IIb/IIIa; ATP, adenosine triphosphate; PGI 2, prostacyclin.)

Pharmacology of clopidogrel 249 ADP receptors Clopidogrel markedly reduces high-affinity ADP binding to a subset of ADP receptors (P 2T ) at the platelet membrane (Figure 1). 21,22 The affinity of these receptors for ADP remains unchanged during treatment with clopidogrel, which causes a 60 70% reduction in ADP binding. 18,22 A similar, 70% reduction in ADP binding was observed in one patient with a congenitally defective ADP response (Figure 2). 21 Clopidogrel s inhibition of ADP binding is non-competitive and irreversible, 23 although 20 30% of receptors are not affected. Since clopidogrel does not affect the shape change associated with platelet activation, it has been suggested that the population of binding sites that remain unaffected after treatment might be specifically involved in the platelet shape change. 23 It has also been suggested that clopidogrel has effects other than on platelets, such as the modulation of vascular tone. 24 GpIIb/IIIa complex As a consequence of reduced agonist binding at the P 2T ADP receptor, there is a marked inhibition of ADP-induced conformational changes in the GpIIb/IIIa complex. This activation is essential for the binding of soluble agonists, such as fibrinogen, and subsequent platelet aggregate formation. 20,25 Insufficient fibrinogen bridging resulting from reduced numbers of platelet GpIIb/IIIa receptors will also result in less stable platelet aggregates and more rapid clot resolution. 25 Fibrinogen binding after clopidogrel treatment is reduced by about 70%. 25 The clinical result is a doubling of the bleeding time. In contrast with ADP receptor antagonists, selective GpIIb/IIIa antagonists, such as abciximab, integrelin and tirofiban, bind directly to the integrin and prevent the access of ligands, such as soluble fibrinogen, to the activated GpIIb/IIIa complex through competitive antagonism. Platelet secretion Studies in non-anticoagulated, ex vivo human blood have shown that clopidogrel profoundly inhibits both the Figure 2 ADP binding sites in human platelets after oral clopidogrel (75 mg/day) compared with a patient with a congenital defect in ADP response. ADP binding was determined with the stable ligand 2MeS-ADP. (Source: data from refs 21 and 22.) degranulation of platelets in contact with a collagen surface and the subsequent formation of a thrombus. Since clopidogrel reduces fibrin deposition and thrombus formation, it has been suggested that clopidogrel interferes with plateletto-platelet interactions 26 through its effects on ADP-dependent platelet activation. Adenylate cyclase activity Exposure of platelets to ADP causes inhibition of adenylate cyclase at the platelet membrane. 27 Clopidogrel suppresses this inhibition of adenylate cyclase and the subsequent fall in camp levels. 28,29 This action might be less significant in ex vivo conditions because of the short half-life of compounds that increase camp levels such as prostacyclin (PGI 2 ). However, this response may contribute to clopidogrel s inhibition of platelet aggregation in vivo, particularly in ischemic areas, where PGI 2 production is increased. 30 Active principle(s) of clopidogrel Clopidogrel is inactive in vitro in conventional assays of platelet activation. Ex vivo data suggest that the irreversible inhibition of platelet aggregation by clopidogrel is not mediated by metabolites detected in the plasma. More detailed studies of the mechanisms involved in clopidogrel bioactivation were conducted in animals, particularly the rat. In this species, the antiaggregating potency of clopidogrel was similar after intravenous or oral administration, but was markedly reduced after functional hepatectomy. 31 It was also seen that inhibition of cytochrome P 450 (CYP) by drugs or antibodies decreased the antiaggregating effect of clopidogrel, while stimulators of the CYP1A1 enzyme subfamily had the opposite effect. 23 Based on these data, it was suggested that the active principle of clopidogrel is generated in the liver by an unknown metabolic change that activates the native drug. 31 Time-dependency of antiplatelet action Clinical inhibition of platelet aggregation does not occur immediately with administration of clopidogrel. A detectable antiplatelet effect can be observed within 2 h of treatment; 32 it becomes significant after 2 days of treatment and maximum inhibition is achieved after 4 7 days. 33 The velocity of the initial response may be increased with higher doses. 32 Interestingly, one preliminary report has suggested that administration of a loading dose of clopidogrel (375 mg) will result in a 55% inhibition of ADP-induced platelet aggregation after 1 h and 80% inhibition after 5 h. A maximal prolongation of bleeding time approximately twofold was seen after 3 days. 34 The half-life of clopidogrel (assessed with the metabolite SR 26334) is 7 8 h after a single administration; full recovery of platelet activity is seen about 1 week after treatment is ended. This suggests normalization of platelet function through platelet turnover. 21,33,35 The efficacy of clopidogrel (75 mg/day) was comparable with that of ticlopidine (500 mg/day). 36 The ability of clopidogrel to inhibit platelet function and prolong bleeding time was unchanged between weeks 1 and 4 of treatment. 36 A study in healthy volunteers found that the antiplatelet effects of clopidogrel (75 mg/day) increased by less than 10% after 3 months of therapy, compared with

250 K Schrör the level of steady-state inhibition achieved after 8 12 days of therapy. There was no further prolongation of the bleeding time. Similarly, no significant differences in antiplatelet efficacy were seen when clopidogrel was administered to young and elderly patients with and without atherosclerosis. 37 Collectively, these data suggest that the prolonged half-life of clopidogrel after regular daily intake for several weeks does not result in a potentiation of antiplatelet effects or additional prolongation of bleeding time. Clopidogrel versus aspirin In dose-ranging studies, clopidogrel (75 mg) was found to have the same effect as ticlopidine (500 mg). 36 The clinical efficacy of clopidogrel has been demonstrated in the CAP- RIE trial 38 in which clopidogrel (75 mg/day) was found to cause a significant 8.7% relative risk reduction (p = 0.043) in the combined vascular endpoints of myocardial infarction, ischemic stroke and vascular death compared with aspirin (350 mg/day). The mean duration of treatment was about 2 years. The percentage of patients who dropped out of the study early because of adverse events was 11.4%: 6.01% of these came from the aspirin group and 5.38% came from the clopidogrel group. 38 Not surprisingly, the side-effects associated with aspirin and clopidogrel were different. The most common side-effects observed during clopidogrel therapy were diarrhea and rash. Upper gastrointestinal discomfort and bleeding occurred more frequently during aspirin therapy. Importantly, the CAPRIE trial showed that the rate of neutropenia was similar in both groups. Because of the different potencies, mechanisms of action and adverse effect spectra, clopidogrel is considered to be a valuable therapeutic alternative to aspirin. An interesting approach to antiplatelet therapy is the combined use of clopidogrel and aspirin for prevention of thromboembolic stent occlusion. Several controlled clinical trials have indicated that the combined use of ticlopidine and aspirin is more effective and has fewer side-effects than conventional therapy with anticoagulants. 39 Similar beneficial results were obtained in patients who received stents as a bail-out procedure for failed angioplasty or suboptimal PTCA results. 40 However, to date these uses have not been studied with clopidogrel in humans. Animal experiments clearly suggest that clopidogrel prevents thrombotic reocclusion of coronary, 41 femoral, 41,42 and carotid arteries after endothelial injury under conditions in which aspirin is ineffective. In addition, clopidogrel prevented coronary artery reocclusion more effectively than aspirin. 43 Clopidogrel pharmacology Drug interactions Data in animal experiments (rats) suggest that clopidogrel might interact with at least two components of the hepatic P 450 system. Therefore, it is important to know whether similar events occur in humans, since this could eventually prolong the drug s half-life. Alternatively, ADP receptor antagonists may interfere with the metabolism of other compounds that are subject to extensive hepatic clearance, such as phenytoin, theophylline and antipyrin. To examine the potential in vivo inhibition and/or induction of hepatic enzymes, clopidogrel use was studied in healthy male volunteers. After clopidogrel administration (75 mg/day for 10 days), no statistically significant difference in the elimination half-life of antipyrine (non-specific isoenzyme substrate) was observed. Plasma levels of gamma-glutamyl transpeptidase ( -GT) and cortisol were not affected by this treatment, nor was urinary excretion of 6- OH-cortisol (CYP3A substrate). This led the authors to conclude that clopidogrel, under the conditions of this study, does not cause hepatic enzyme induction nor does it modify the activity of hepatic oxidative enzyme systems. 44 No liver dysfunction was reported in the CAPRIE trial. 38 It should be noted, however, that the CAPRIE trial was a phase III study with numerous exclusionary criteria for patient selection, including a history of drug-induced hepatic abnormalities. Other studies have indicated that pharmacologic interactions do not occur during co-administration of clopidogrel and aspirin, 45 digoxin, 46 heparin, 47 atenolol, 48 or nifedipine. 48 The results of studies performed with clopidogrel show that it has antiplatelet activity and reduces the occurrence of thrombotic occlusive vascular disease. Clopidogrel has a greater efficacy than aspirin (8.7% relative risk reduction compared with aspirin; p = 0.043), the current gold standard for antiplatelet activity, and a reduced association with gastrointestinal bleeding and discomfort; in addition, clopidogrel does not appear to cause neutropenia. Based on results of the CAPRIE trial, clopidogrel appears to be an efficacious and safe new agent for patients at risk of occlusive vascular events. Note added in proof Subsequent to the original submission of this paper, Weber and co-workers 49 have reported that clopidogrel specifically inhibits ADP-induced aggregation of washed human platelets in vitro without hepatic bioactivation. References 1 Ross R, Fuster V. The pathogenesis of atherosclerosis. In: Fuster V, Ross R, Topol EJ eds. Atherosclerosis and coronary artery disease. Philadelphia: Lippincott-Raven, 1996: 411 60. 2 Antiplatelet Trialists Collaboration. Collaborative overview of randomized trials of antiplatelet therapy. I. Prevention of death, myocardial infarction and stroke by prolonged antiplatelet therapy in various categories of patients. Br Med J 1994; 308: 81 106. 3 Ruggeri ZM. Mechanisms initiating platelet thrombus formation. Thromb Haemost 1997; 78: 611 16. 4 Du X, Ginsberg MH. Integrin IIb 3 and platelet function. Thromb Haemost 1997; 78: 96 100. 5 Schafer AL. Antiplatelet therapy. Am J Med 1996; 101: 199 209. 6 Hellem A. The adhesiveness of human blood platelets in vitro. Scand J Clin Invest 1960; 12: 1 17. 7 Gaarder A, Jonsen J, Laaland S, Hellem A, Owren PA. Adenosine diphosphate in red cells as a factor in the adhesiveness of human blood platelets. Nature 1961; 192: 531 32. 8 Born GVR. Platelet aggregation and its reversal. Nature 1962; 194: 927 29. 9 O Brien JR. Platelet aggregation. Part II. Some results from a new method of study. J Clin Pathol 1962; 15: 446 51. 10 Zawilska KM, Born GVR, Begent N. Effect of ADP-utilizing enzymes on the arterial bleeding time in rats and rabbits. Br J Haematol 1982; 50: 317 25.

Pharmacology of clopidogrel 251 11 Moritz MW, Reimers RC, Baker RK, Sutera SP, Joist JH. Role of cytoplasmic and releasable ADP in platelet aggregation induced by laminar shear stress. J Lab Clin Med 1983; 101: 537 44. 12 Moake JL, Turner NA, Stathopoulos NA, Nolasco L, Hellums JD. Shear-induced platelet aggregation can be mediated by vwf released from platelets, as well as by exogenous large or unusually large vwf multimers, requires adenosine diphosphate, and is resistant to aspirin. Blood 1988; 71: 1366 74. 13 Born GVR. Adenosine diphosphate is a mediator of platelet aggregation in vivo: an editorial view. Circulation 1985; 72: 741 46. 14 Rao AK, Willis J, Holmsen H. A major role of ADP in thromboxane transfer experiments: studies in patients with platelet secretion defects. J Lab Clin Med 1984; 104: 116 26. 15 Valles J, Santos MT, Aznar J et al. Erythrocytes metabolically enhance collagen-induced platelet responsiveness via increased thromboxane production, adenosine diphosphate release and recruitment. Blood 1991; 78: 154 62. 16 Santos MT, Valles J, Marcus AJ et al. Enhancement of platelet reactivity and modulation of eicosanoid production by intact erythrocytes: a new approach to platelet activation and recruitment. J Clin Invest 1991; 87: 571 80. 17 Coade SB, Pearson JD. Metabolism of adenine nucleotides in human blood. Circ Res 1989; 65: 531 37. 18 Mills DC. ADP receptors on platelets. Thromb Haemost 1996; 76: 835 55. 19 Gachet C, Hechler B, Léon C, Vial C, Ohlmann P, Cazenave J-P. Purinergic receptors on blood platelets. Platelets 1996; 7: 261 67. 20 Nurden AT. New thoughts on strategies for modulating platelet function through the inhibition of surface receptors. Haemostasis 1996; 26 (suppl 4): 78 88. 21 Mills DCB, Puri RN, Hu CJ et al. Clopidogrel inhibits the binding of ADP analogues to the receptor mediating inhibition of platelet adenylate cyclase. Arterioscler Thromb 1992; 12: 430 36. 22 Gachet C, Cattaneo M, Ohlmann PO et al. Purinoceptors on blood platelets: further pharmacological and clinical evidence to suggest the presence of two ADP receptors. Br J Haematol 1995; 91: 434 44. 23 Savi P, Combalbert J, Graich C et al. The antiaggregating activity of clopidogrel is due to metabolic activation by the hepatic cytochrome P450 1A. Thromb Haemost 1994; 72: 313 17. 24 Yang LH, Fareed J. Vasomodulatory action of clopidogrel and ticlopidine. Thromb Res 1997; 86: 479 91. 25 Humbert M, Nurden P, Bihour C et al. Ultrastructural studies of platelet aggregates from human subjects receiving clopidogrel and from a patient with an inherited defect of an ADP-dependent pathway of platelet activation. Arterioscler Thromb Vasc Biol 1996; 16: 1532 43. 26 Roald HE, Barstad M, Kierulf P et al. Clopidogrel a platelet inhibitor which inhibits thrombogenesis in non-anticoagulated human blood independently of the blood flow conditions. Thromb Haemost 1994; 71: 655 62. 27 Cooper DMF, Rodbell M. ADP is a potent inhibitor of human platelet plasma membrane adenylate cyclase. Nature 1979; 282: 517 18. 28 Gachet C, Stierle A, Cazenave JP et al. The thienopyridine PCR 4099 selectively inhibits ADP-induced platelet aggregation and fibrinogen binding without modifying the membrane glycoprotein IIb/IIIa complex in rat and man. Biochem Pharmacol 1990; 40: 229 38. 29 Defreyn G, Gachet C, Savi P, Driot F, Cazenave JP, Maffrand JP. Ticlopidine and clopidogrel [SR 25990C] selectively neutralize ADP inhibition of PGE 1 -activated platelet adenylate cyclase in rats and rabbits. Thromb Haemost 1991; 65: 186 90. 30 Schrör K. The basic pharmacology of ticlopidine and clopidogrel. Platelets 1993; 4: 252 61. 31 Savi P, Herbert J-M, Pflieger AM et al. Importance of hepatic metabolism in the antiaggregating activity of the thienopyridine clopidogrel. Biochem Pharmacol 1992; 44: 527 32. 32 Kieffer G, Caplain H, Thiercelin JF, Thebault JJ. Tolerance and pharmacological activity of a new antiplatelet agent clopidogrel [SR 25990 C] in normal healthy volunteers after single increasing administrations. Thromb Haemost 1989; 62: 411 [abstract]. 33 Caplain H, Kieffer G, Thiercelin JF, Thebault JJ. Tolerance and clinical pharmacology of repeated administration of clopidogrel [SR 25990 C], a new antiplatelet agent, at three dose levels in normal healthy volunteers. Thromb Haemost 1989; 62: 410 [abstract]. 34 Bachmann F, Savcic M, Hauert J, Geudelin B, Kieffer G, Cariou R. Rapid onset of inhibition of ADP-induced platelet aggregation by a loading dose of clopidogrel. Eur Heart J 1996; 17 (suppl): 263. 35 Herbért JM, Frekel D, Vallee E et al. Clopidogrel, a novel antiplatelet and antithrombotic agent. Cardiovasc Drug Rev 1993; 11: 180 98. 36 Boneu B, Destelle G on behalf of the Study Group. Platelet anti-aggregating activity and tolerance of clopidogrel in atherosclerotic patients. Thromb Haemost 1996; 76: 939 43. 37 Guillin M-C, Bonnet G, Sissmann J, Neccian J, Dickinson JP. Pharmacodynamics and pharmacokinetics of the novel antiplatelet agent, clopidogrel, in the young and the elderly with and without symptomatic atherosclerosis. Eur Heart J 1996; 17 (suppl): 161. 38 Gent M, CAPRIE Steering Committee. A randomised, blinded trial of clopidogrel versus aspirin in patients at risk of ischaemic events [CAPRIE]. Lancet 1996; 348: 1329 39. 39 Schömig A, Neumann F-J, Kastrati A et al. A randomized comparison of antiplatelet and anticoagulant therapy after the placement of coronary-artery stents. N Engl J Med 1996; 334: 1084 89. 40 Lablanche J-M, McFadden EP, Bonnet J-L et al. Combined antiplatelet therapy with ticlopidine and aspirin. A simplified approach on intracoronary stent management. Eur Heart J 1996; 17: 1373 80. 41 Yao S-K, Ober JC, McNatt J et al. ADP plays an important role in mediating platelet aggregation and cyclic flow variations in vivo in stenosed and endothelium-injured canine coronary arteries. Circ Res 1992; 70: 39 48. 42 Samama CM, Bonnin P, Bonneau M et al. Comparative arterial antithrombotic activity of clopidogrel and acetyl salicylic acid in the pig. Thromb Haemost 1992; 68: 500 505. 43 Yao S-K, Ober JC, Ferguson JJ et al. Clopidogrel is more effective than aspirin as adjuvant treatment to prevent reocclusion after thrombolysis. Am J Physiol 1994; 267: H488-H93. 44 Pierce CH, Houle J-M, Kieffer G, Dickinson JP. Absence of effect of clopidogrel on human hepatic P-450 activities. Eur Heart J 1996; 17 (suppl): 160 [abstract]. 45 Caplain H, D Honneur G, Cariou R. Lack of interaction of aspirin (100 mg) with chronic clopidogrel in volunteers. Haemostasis 1996; 26 (suppl 3): 557. 46 Peeters PAM, Crijns WJ, Tamminga WJ et al. Absence of pharmacokinetic interaction between the novel antiplatelet agent, clopidogrel, and digoxin. Eur Heart J 1996; 17 (suppl): 160 [abstract]. 47 D Honneur G, Caplain H, Cariou R, Brouard R. Interaction study between clopidogrel and prolonged intravenous heparin administration in young healthy volunteers. Haemostasis 1996; 26 (suppl 3): 554. 48 Forbes CD, Belch JJ, Bridges AB et al. Pharmacodynamic compatability of clopidogrel with atenolol and nifedipine co-medication in patients with atherosclerotic disease. Eur Heart J 1996; 17 (suppl): 160 [abstract]. 49 Weber A-A, Reimann S, Schrör K. Specific inhibition of ADP-induced platelet aggregation by clopidogrel in vitro. Br J Pharmacol 1998 (in press).