Drugs are the most common cause of platelet dysfunction (Table 121–2). For example, in an analysis of 72 hospitalized patients with a prolonged bleeding time (a test no longer considered reliable), 54 percent were receiving large doses of antibiotics known to prolong the bleeding time and 10 percent were taking aspirin or other nonsteroidal antiinflammatory drugs.3 Some drugs can prolong the bleeding time and either cause or exacerbate a bleeding diathesis. Other drugs may prolong the bleeding time but not cause bleeding, while many only affect platelet function ex vivo or when added to platelets in vitro. It is important for the hematologist to understand the clinical significance of these distinctions.
Table 121–2.Drugs That Affect Platelet Function ||Download (.pdf) Table 121–2. Drugs That Affect Platelet Function
Nonsteroidal antiinflammatory drugs
Aspirin, ibuprofen, sulindac, naproxen, meclofenamic acid, mefenamic acid, diflunisal, piroxicam, tolmetin, zomepirac, sulfinpyrazone, indomethacin, phenylbutazone, celecoxib
PAR1 receptor antagonist
Integrin αIIbβ3 antagonists
Drugs that affect platelet cyclic adenosine monophosphate levels or function
Penicillin G, carbenicillin, ticarcillin, methicillin, ampicillin, piperacillin, azlocillin mezlocillin, sulbenicillin, temocillin
Anticoagulants, fibrinolytic agents, and antifibrinolytic agents
Psychotropic drugs and anesthetics
Foods and food additives
ω-3 Fatty acids, ethanol, Chinese black tree fungus, onion extract ajoene, cumin, turmeric
ASPIRIN AND OTHER NONSTEROIDAL ANTIINFLAMMATORY DRUGS
Aspirin irreversibly inactivates the enzyme cyclooxygenase (COX), also known as prostaglandin endoperoxide H synthase, by acetylating a serine residue at position 529.4 Two isoforms of COX have been identified (COX-1 and COX-2),5 as well as a splice variant of COX-1, COX-1b (COX-3), whose functional significance is uncertain.6 COX-1 is constitutively expressed by many tissues, including platelets, the gastric mucosa, and endothelial cells (Chap. 134 discusses the use of aspirin as an antithrombotic agent).5 COX-2 is undetectable in most tissues, but its synthesis is rapidly induced in cells such as endothelial cells, fibroblasts, and monocytes by growth factors, cytokines, endotoxin, and hormones.5 Platelets express only COX-1, whereas endothelial cells can express both COX-1 and COX-2.7,8 In the cardiovascular system, COX products regulate complex interactions between platelets and the vessel wall. The platelet product of COX-1 mediated prostaglandin synthesis, thromboxane A2 (TXA2), produces vasoconstriction and is a receptor-mediated agonist for platelet aggregation and secretion.4 Thus, inactivation of COX-1 by aspirin prevents platelet synthesis of TXA2, thereby inhibiting platelet responses that depend on this substance. Accordingly, platelet responses to adenosine diphosphate (ADP), epinephrine, low doses of collagen and thrombin, and arachidonic acid are affected (arachidonic acid completely), but there is almost no effect on the responses to higher doses of collagen or thrombin.9,10 On the other hand, the endothelial cell prostaglandin (PG) product, prostacyclin (PGI2), produces smooth muscle cell relaxation and vasodilation and increases the platelet content of cyclic adenosine monophosphate (AMP), thereby decreasing overall platelet reactivity.11
Platelet PG synthesis in an adult is nearly completely inhibited by a single 100-mg dose of aspirin or by 30 mg taken daily for 7 to 10 days.4 Although single doses of aspirin irreversibly inhibit platelet and endothelial cell COX,12 they have no lasting effect on PG synthesis by endothelial cells because of the ability of these cells to synthesize additional COX unaffected by aspirin.13 In vitro studies also suggest that the presence of erythrocytes contributes to agonist-stimulated platelet reactivity,14 an effect that can be inhibited by aspirin at doses greater than those required to inhibit platelet COX-1.15 A meta-analysis of clinical trials indicates that aspirin doses varying from 50 to 1500 mg daily are equally efficacious in preventing adverse cardiovascular and cerebrovascular events.16 This has led many to suggest that the lowest effective doses should be prescribed to minimize gastrointestinal toxicity. Nonetheless, even low doses of aspirin can be associated with significant gastrointestinal hemorrhage.17,18,19
Aspirin is one of the relatively few drugs that prolongs the bleeding time in humans and appears to do so by blocking aggregation rather than adhesion. In normal individuals, the effect on the bleeding time is slight (generally no more than 1.2 to 2.0 times the preaspirin bleeding time),20,21 observed in both males and females, and requires that almost all the COX in the circulating platelets be inhibited.11 The sensitivity of the bleeding time to aspirin is dependent on such technical variables as the direction of the incision on the forearm and the degree of hydrostatic pressure applied to the arm,22 and hence the current view that the test is unreliable. The bleeding time may remain prolonged for 1 to 4 days after aspirin has been discontinued, and platelet aggregation tests may remain abnormal for up to a week until platelets affected by aspirin are replaced as the result of thrombopoiesis.23
The significance of aspirin ingestion on the hemostatic competency of normal individuals appears to be minimal. Nevertheless, patients taking aspirin chronically report significant increases in bruising, epistaxis, and gastrointestinal blood loss.17,18,19 Gastrointestinal blood loss appears to be the result of a direct effect of aspirin on the gastric mucosa.24,25 Furthermore, there is an increase in the incidence of hemorrhagic stroke when aspirin is used in the primary and secondary prevention of vascular disease, as well as an increase in major gastrointestinal and other extracranial bleeding.26 Aspirin may also increase bleeding in the mother and the neonate during parturition.27 In addition, some studies show that aspirin taken preoperatively increases the amount of blood loss following cardiothoracic surgery.28,29 In contrast, a retrospective analysis has documented the safety of performing epidural and spinal anesthesia in patients who had ingested aspirin.30 Aspirin may increase the amount of blood loss following general surgery.31 The significance of aspirin ingestion in this setting was tested in the POISE-2 study32 in which patients at risk for vascular complications were randomized to aspirin or placebo prior to their noncardiac surgery. Although taking aspirin did not reduce the incidence of cardiovascular events, there was a small increase in hemorrhagic complications. This suggests that discontinuing aspirin prior to surgery is a useful practice, particularly prior to plastic or neurosurgical procedures in which the limits of tolerable bleeding are narrow.33 On the other hand, patients taking aspirin and other antiplatelet agents for severe cardiovascular disease may be at risk for thrombosis if these medications are discontinued. Thus, the clinician must thoroughly weigh the potential risks and benefits of discontinuing aspirin prior to noncardiac surgery. This is especially true in patients with other hemostatic disorders; for example, aspirin precipitates hemorrhage in individuals with von Willebrand disease, hemophilia A, warfarin ingestion, uremia, and disorders of platelet function.34,35,36 Infusion of desmopressin (DDAVP) has been effective in correcting a prolonged bleeding time caused by aspirin.37,38
Resistance to the antiplatelet effects of aspirin (“aspirin-resistance”) is a controversial topic and whether it exists depends to large extent on whether resistance is considered from a biochemical or clinical perspective.39 Biochemical resistance to the platelet inhibitory effects of aspirin, that is, the failure to achieve pharmacologic inhibition of TXA2 production, is uncommon.39 For example, when healthy subjects were given either standard or enteric-coated aspirin, 49 percent given a single dose of enteric-coated aspirin failed to inhibit TXA2 synthesis, whereas the failure to inhibit TXA2 synthesis was never seen in subjects given standard aspirin.40 Nevertheless, subjects given enteric-coated aspirin eventually responded when taking it daily, implying that although some patients absorb enteric-coated aspirin preparations poorly, they will ultimately absorb sufficient amounts of aspirin to prevent platelet TXA2 synthesis. Most commonly, aspirin-resistance occurs because patients are non-adherent with aspirin therapy, often because of gastrointestinal toxicity.41 Clinically, the term aspirin-resistance has been applied to patients who develop cardiovascular events despite taking aspirin. Given that aspirin treatment selectively inhibits platelet synthesis of only one endogenous platelet agonist, TXA2, it is not surprising that aspirin does not completely abolish platelet-mediated vascular events.
Traditional Nonsteroidal Antiinflammatory Drugs
Unlike aspirin, nonsteroidal antiinflammatory drugs (NSAIDs), such as ibuprofen, naproxen, diclofenac, sulindac, piroxicam, indomethacin, and sulfinpyrazone, reversibly inhibit COX enzymes.42 Although these drugs can cause a transient prolongation of the bleeding time when given in therapeutic doses, this is usually not clinically significant.43 Population studies have suggested that concurrent treatment with NSAIDs and anticoagulants increases the risk of bleeding complication, but many bleeding events were limited to the gastrointestinal tract where NSAIDs are known to induce gastritis and peptic ulcerations.44 As evidence of the modest effect of NSAIDs on platelet function, ibuprofen has been given safely to patients with hemophilia A.45,46 Nonetheless, care must be taken when ibuprofen is given to patients with hemophilia and HIV infection receiving zidovudine because increased bleeding has been reported in this circumstance.47 Because ibuprofen, and probably other NSAIDs, binds to COX-1, blocking its acetylation by aspirin,42 coadministration of NSAIDs and aspirin may impair the irreversible, antithrombotic effects of aspirin on platelets.48 For this reason, patients who require both medications should ingest aspirin at least 2 hours prior to the ingestion of traditional NSAIDs.
Coxibs (COX-2 Inhibitors)
COX-1 is present in the gastric mucosa where its products protect the integrity of the gastric lining cells. In inflammatory cells, COX-2 products such as PGE2 and PGI2 elicit an increased sense of pain and perpetuate the inflammatory process.40 Thus, the coxibs (COX inhibitors), designed to be relatively more specific for COX-2 versus COX-1, were intended to reduce pain and inflammation with fewer gastric side effects than traditional NSAIDs.40,42 However, clinical trials revealed that coxib administration was associated with cardiovascular toxicity (myocardial infarction, stroke, edema, exacerbation of hypertension), partly because of inhibiting PGI2 synthesis.11,49,50,51,52 On the basis of these results, rofecoxib and valdecoxib were withdrawn from the market (valdecoxib was also associated with cases of Stevens-Johnson syndrome) and a black box warning regarding serious cardiovascular events was added to prescribing information for celecoxib, the only coxib now available in the United States.50 Nonetheless, clinical evidence suggests there is no excess cardiovascular risk from daily doses of celecoxib of 200 mg or less.51 Traditional NSAIDs also inhibit COX-2 to a variable extent and several observational trials have revealed excess cardiovascular events associated with use of these drugs.50,53,54,55 Thus, a warning has also been added to their prescribing information. If indicated, analgesics such as acetaminophen, sodium or choline salicylate and narcotics may be substituted for aspirin and NSAIDs for treating musculoskeletal pain.50 One report suggests that acetaminophen can selectively inhibit COX-2,56 but the clinical significance of this observation is not clear.
Ticlopidine, clopidogrel, and prasugrel are thienopyridines that are used as antiplatelet agents in arterial diseases (Chap. 134) with results at least comparable to aspirin in the secondary prevention of cerebrovascular and cardiovascular events.16,57
Thienopyridines differ from aspirin in their mechanism of antiplatelet activity and their toxicity profile. All three thienopyridines are prodrugs that depend on oxidation by cytochrome P450 (CYP) enzymes in the liver (ticlopidine and clopidogrel) or in liver and intestine (prasugrel) to form the active metabolites that irreversibly inhibit the platelet P2Y12 ADP receptor.58,59,60,61 Ticlopidine at 250 mg twice a day, clopidogrel at 75 mg once per day, and prasugrel at 10 mg once a day inhibit platelet aggregation ex vivo in humans. The extent of this effect is equivalent to or greater than that of aspirin and the effect of thienopyridines and aspirin appears additive.62,63 When given at their usual oral doses, the effect of thienopyridines on platelet aggregation and the bleeding time can be seen within hours of the first dose, but are not maximal for 4 to 6 days. A 300-mg loading dose of clopidogrel or 60 mg of prasugrel, followed by their usual daily doses, shortens the time required for their maximal antiplatelet effect to a few hours.64,65 The common CYP polymorphism CYP2C19 results in lower levels of active clopidogrel and ticlopidine metabolites and has been reported to be associated with decreased platelet inhibition and an elevated risk for major adverse cardiovascular events.55,66,67 Because the enzyme CYP3A is present in the intestine and can oxidize prasugrel to its pharmacologically active metabolite, intestinal metabolism may account for the rapid appearance and higher levels of the active metabolite in plasma after an oral dose.61,68,69,70 Furthermore, prasugrel metabolism and inhibition of platelet function are not affected by CYP2C19 polymorphisms.61,68,69,70
The clinical efficacy of prasugrel has been compared to clopidogrel in patients with acute coronary syndrome scheduled for percutaneous coronary intervention in the Triton-TIMI 38 trial. Patients who received prasugrel had a significantly decreased incidence of ischemic events compared to patients who received clopidogrel (9.9 percent vs. 12.1 percent, p <0.001).69 However, major bleeding was also significantly increased in patients receiving prasugrel compared to clopidogrel (2.4 percent vs. 1.8 percent, p <0.03). Thus, although prasugrel appeared to be more efficacious than clopidogrel, this benefit was partially offset by a higher rate of hemorrhage.69
The platelet inhibitory effects of thienopyridines persist for 4 to 10 days after the drugs have been discontinued, either because of their extended half-life after multiple dosing or their irreversible effect on platelets.58 Ticlopidine administration is associated with potentially serious hematologic complications, including neutropenia (neutrophils <1200 × 109/L in 2.4 percent of individuals)58,71,72 and, less commonly, aplastic anemia, and thrombocytopenia.73,74 In addition, at least one in 5000 patients develop a thrombotic thrombocytopenic purpura (TTP)-like syndrome.75,76,77 Results from a large clinical trial suggest that hematologic complications may be less common with clopidogrel or prasugrel.57 Clopidogrel may also be rarely associated with a TTP-like syndrome (one in 270,000),78 although this rate is close to the TTP incidence in the general population. Because of its toxicity profile, ticlopidine has been replaced by the other thienopyridines in the United States.
Because aspirin and the thienopyridines inhibit platelet function by different mechanisms, their antithrombotic effects may be additive. In theory, this would be beneficial in the treatment of diseases associated with platelet activation such as ischemic heart disease, peripheral vascular disease, and ischemic strokes.62,79,80 This hypothesis was tested in the CURE trial of patients with acute coronary syndromes.62 Although clopidogrel plus aspirin decreased the combined incidence of cardiovascular deaths, myocardial infarctions, and strokes from 11.4 percent to 9.3 percent, the benefit was partially offset by an increase in severe bleeding from 2.7 to 3.7 percent. Similarly, in the CHARISMA trial of a broad population of patients at risk for cardiovascular events, there were 94 fewer ischemic events in patients treated with both clopidogrel and aspirin, but this occurred at the expense of 93 more moderate or severe bleeding events.81 Furthermore, a meta-analysis of seven randomized controlled trials involving more than 39,000 patients confirmed that intracranial hemorrhage was more frequent in patients who received both clopidogrel plus aspirin compared to clopidogrel alone.82 Thus, except for special circumstances such as coronary artery stenting, it appears that the added benefit of dual antiplatelet therapy is small and has the added risk of increased bleeding.83
OTHER ADENOSINE DIPHOSPHATE RECEPTOR ANTAGONISTS
Ticagrelor, cangrelor, and elinogrel are oral, reversible, nonthienopyridine P2Y12 receptor antagonists. Because they are not prodrugs and do not require metabolic activation, the onset of their inhibitory activity is more rapid than that of the thienopyridines. A novel, and as yet unexplained, side effect of treatment with this class of the P2Y12 antagonists is the occurrence of dyspnea which can complicate the management of patients with coronary artery disease.84
Ticagrelor, the first drug of the class, has been approved for use in acute coronary syndromes. Its efficacy versus clopidogrel was tested in the PLATO trial in which patients with an acute coronary syndrome were randomized to treatment with either ticagrelor or clopidogrel.85,86,87 At 1 year, the combined end point of death, myocardial infarction, and stroke was 9.8 percent in patients treated with ticagrelor compared to 11.7 percent in patients treated with clopidogrel.88 Although stent thrombosis was also decreased in the ticagrelor-treated group, major bleeding not associated with coronary artery bypass surgery was increased in this group. The incidence of fatal intracranial hemorrhage was also greater in the ticagrelor-treated patients, but it was a rare event (0.1 percent of treated patients). In the ATLANTIC trial, patients suffering from an ST-segment elevation myocardial infarction were randomized to receive ticagrelor in the ambulance or in the catheterization laboratory.89 Although initiating therapy before hospitalization was safe and lowered the incidence of stent thrombosis, there was no overall improvement in preventing major cardiovascular adverse events. Thus, ticagrelor, like prasugrel appears to be more efficacious than clopidogrel at preventing adverse cardiovascular events, but with more hemorrhagic complications.
THROMBIN RECEPTOR ANTAGONISTS
Thrombin is the most potent physiologic platelet agonist. Three G-protein–coupled thrombin receptors have been identified in humans (protease-activated receptors [PARs] 1, 3, and 4).90 Although human platelets express both PAR-1 and PAR-4, the major platelet thrombin receptor is PAR-1 and can be activated by nanomolar concentrations of thrombin. PAR-4 signaling appears to be unnecessary for platelet activation if PAR-1 signaling is intact.90 Vorapaxar is a potent, selective, long-acting, oral PAR-1 inhibitor generated from the naturally occurring muscarinic receptor antagonist himbacine.91 A high-resolution crystal structure of vorapaxar bound to PAR-1 revealed that the binding pocket for the drug is unusual for a peptide-activated G-protein–coupled receptor in that it consists of a superficial tunnel with little of the bound drug surface exposed to aqueous solvent, perhaps accounting for the very slow dissociation rate of vorapaxar from PAR-1.92
The efficacy of vorapaxar for the secondary prevention of arterial thrombosis was examined in the phase III TRA 2P–TIMI 50 trial in which patients with a history of myocardial infarction, stroke, or peripheral arterial disease were randomized between vorapaxar and placebo.93 Most patients were also taking either aspirin or a thienopyridine. Because of a high incidence of intracranial bleeding in the first years of the study, entry criteria were modified to eliminate patients with a history of a stroke. At 3 years, the incidence of the primary end point (cardiovascular death, myocardial infarction, and stroke) was significantly reduced in vorapaxar-treated patients (9.3 percent vs. 11.2 percent, p <0.001). However, moderate to severe bleeding, including intracranial bleeding, was significantly increased in the vorapaxar-treated patients (4.2 percent vs. 2.5 percent, p <0.001). Nonetheless, based on efficacy, vorapaxar received FDA approval in 2014. Atopaxar, a second PAR-1 antagonist, is currently being evaluated in clinical trials.94 Atopaxar has a shorter half-life than vorapaxar, suggesting that potential bleeding complications might be easier to manage.
INTEGRIN αIIbβ3 RECEPTOR ANTAGONISTS
Drugs that specifically impair the function of the major platelet integrin αIIbβ3 (GPIIb/IIIa) have been developed for short-term use as antithrombotic agents in the setting of ischemic coronary artery disease.95,96 Integrin αIIbβ3 mediates platelet–platelet cohesion by binding the divalent ligand fibrinogen, thereby crosslinking the integrin on adjacent platelets, causing the formation of platelet aggregates.97 Thus, integrin αIIbβ3 is a viable therapeutic target to prevent arterial thrombosis. Abciximab, eptifibatide, and tirofiban are three FDA-approved structurally dissimilar integrin αIIbβ3 inhibitors that rapidly impair platelet aggregation. Abciximab is a human-murine chimeric Fab fragment, eptifibatide is a cyclic heptapeptide based on the sequence Lys-Gly-Asp (KGD), and tirofiban is an Arg-Gly-Asp (RGD)-based peptidomimetic. All three drugs have demonstrated efficacy in the management of patients with acute coronary syndromes, particularly in the setting of percutaneous coronary interventions (PCI) where iatrogenic artery wall injury occurs.97
Inherited integrin αIIbβ3 abnormalities cause the bleeding disorder Glanzmann thrombasthenia (Chap. 120).98,99 Thus, it is not surprising that integrin αIIbβ3 antagonists can predispose to bleeding. In EPIC, a clinical trial of abciximab in patients undergoing PCI, 14 percent of patients given abciximab experienced major bleeding compared to 7 percent of patients given placebo.100 However, patients were also given aspirin and heparin. When the heparin dose was decreased in the subsequent EPILOG trial, the incidence of major bleeding in patients receiving abciximab decreased to 2.0 percent compared to 3.1 percent in the control group receiving heparin and aspirin alone.101 Nonetheless, in both EPIC and EPILOG, minor bleeding was significantly more frequent in patients given abciximab and standard-dose heparin compared to patients given standard-dose heparin alone, attesting to the ability of an integrin αIIbβ3 antagonist to impair normal hemostasis. In the PRISM-PLUS trial of tirofiban and the PURSUIT trial of eptifibatide, major and minor bleeding were slightly more frequent in patients receiving the study drug compared to controls.102,103 Similarly, patients receiving the oral integrin αIIbβ3 inhibitors xemilofiban and sibrafiban for 30 and 28 days, respectively, frequently experienced mucocutaneous bleeding similar to that experienced by patients with congenital thrombasthenia.104,105 Although short-term use of the parenteral integrin αIIbβ3 antagonists is often beneficial in patients with acute coronary syndrome of following PCI, paradoxically the long-term use of oral integrin αIIbβ3 inhibitors was associated with an increase in mortality.106 The cause of this paradoxical effect is not clear, but has been attributed by some to an antagonist-induced conformational change in integrin αIIbβ3 simulating the effect of physiologic platelet agonists.107
The risk of bleeding in patients undergoing PCI in the presence of integrin αIIbβ3 antagonists can be minimized by using heparin on a weight basis,101 by avoiding treatment of patients who are receiving warfarin at therapeutic doses, by early vascular sheath removal, and by meticulous care of vascular puncture sites.108 Platelet transfusions can rapidly reverse the platelet function defect in patients receiving abciximab, presumably by decreasing the overall extent of integrin blockade. The ability of platelet transfusion to reverse the effects of the other integrin αIIbβ3 antagonists is less clear, but these drugs have very short half-lives if renal and hepatic function are normal.
Thrombocytopenia occurring within 24 hours of initiating therapy has been observed in small numbers of patients following the administration of all integrin αIIbβ3 antagonists.102,105,108,109 In EPIC, the incidence of platelet counts of less than 100 × 109/L and of less than 50 × 109/L in patients receiving abciximab for the first time was 3.9 percent and 0.9 percent, respectively.109 Thrombocytopenia has also been reported in patients receiving eptifibatide, tirofiban, and a variety of small molecule RGD- and non–RGD-based integrin αIIbβ3 inhibitors with an incidence of up to 13 percent.102,105,109,110,111,112,113
The mechanism responsible for thrombocytopenia following the administration of these drugs is uncertain, but may be related to the presence of preexisting antiintegrin αIIbβ3 antibodies that recognize epitopes exposed by the antagonist, or, in the case of abciximab, to murine sequences incorporated into the abciximab Fab fragment.114 The thrombocytopenia usually reverses readily when the drug is stopped, but it may also be reversed by platelet transfusion if clinically indicated.108 Thrombocytopenia in patients receiving integrin αIIbβ3 antagonists must be differentiated from pseudothrombocytopenia as a result of drug-induced platelet clumping, from heparin-induced thrombocytopenia in patients receiving heparin concurrently, and from other causes of thrombocytopenia, depending on the clinical circumstances.115,116 It is important to identify thrombocytopenia early because integrin αIIbβ3 antagonists are administered as long infusions and the drug should be stopped as soon as true thrombocytopenia has been confirmed. In most cases of profound thrombocytopenia, a platelet count obtained 2 to 4 hours after initiating therapy will provide evidence of a significant decrease in platelet count, although cases of delayed thrombocytopenia have been observed after treatment with abciximab.114
DRUGS THAT AFFECT PLATELET CYCLIC NUCLEOTIDE LEVELS OR FUNCTION
The pyrimidopyrimidine derivative, dipyridamole, inhibits platelet cyclic nucleotide phosphodiesterase, resulting in the intraplatelet accumulation of the inhibitory cyclic nucleotide cyclic AMP (cAMP). Dipyridamole may also inhibit the breakdown of cyclic guanosine monophosphate (cGMP), resulting in potentiation of the platelet inhibitory effect of nitric oxide.117 Although the platelet inhibitory effects of dipyridamole are seen in vitro, the clinical utility of dipyridamole has been controversial.118,119 A meta-analysis failed to demonstrate the clinical benefit of adding dipyridamole to aspirin.16 However, many older dipyridamole trials used formulations with limited dipyridamole bioavailability.120 In the European Stroke Prevention Study 2 (ESPS 2), dipyridamole was beneficial in preventing stroke and transient ischemic attack, but there was no difference in mortality between patients taking dipyridamole and placebo or among patients taking dipyridamole plus aspirin compared to either dipyridamole or aspirin alone.121 The basis for the benefit of dipyridamole in the ESPS 2 trial is unclear, but could be from a higher dipyridamole dosage or to the sustained-release dipyridamole preparation used in the trial.
Intravenous infusions of PGE1, PGI2, or stable PGI2 analogues stimulate platelet adenylyl cyclase, causing an increase in platelet cAMP and a decrease in platelet responsiveness.122 These agents cause a transient inhibition of platelet shape change, aggregation, and secretion. However, their clinical utility is limited by their short half-life and side effects that include peripheral vasodilation.123 Cilostazol, a phosphodiesterase III inhibitor has been approved in the United States for the treatment of peripheral vascular disease124 and may have utility in the prevention of cardiac stent occlusion.125 Nitric oxide (NO) and organic nitrates such as nitroglycerin inhibit platelet function in vitro, probably by activating guanylyl cyclase, thereby increasing cGMP.126 Their effect on in vivo platelet function is uncertain. High concentrations of caffeine and theophylline also inhibit platelet phosphodiesterases in vitro.
Penicillins contain a β-lactam ring and a unique side chain. Most cause a dose-dependent prolongation of the bleeding time in normal volunteers.127 Because they reduce platelet aggregation and secretion, as well as ristocetin-induced platelet agglutination, they may affect both platelet adhesion and platelet activation. Tests of platelet aggregation are abnormal in 50 to 75 percent of individuals receiving large doses (at least several grams per day) of carbenicillin, penicillin G, ticarcillin, ampicillin, nafcillin, and azlocillin and in 25 to 50 percent of patients taking piperacillin, azlocillin, or mezlocillin.127,128,129 Differences in the antiplatelet effects of these antibiotics probably relate to differences in blood levels and drug potency. Their effect on platelets is maximal after 1 to 3 days of administration and may remain for several days after the antibiotic has been stopped, suggesting that the effect of these antibiotics on platelets in vivo is irreversible.
Penicillins can impair the interaction of agonists and von Willebrand factor (VWF) with the platelet membrane.130 Indeed, when many penicillins are incubated with washed platelets, albeit at concentrations higher than those attained in vivo, they inhibit the interaction of VWF and agonists, such as ADP and epinephrine, with their platelet receptors.131 The relative in vitro antiplatelet potency of the penicillins correlates well with their lipid solubility and with the inhibitory potency of the isolated side chains.132 Moreover, the inhibitory effect of penicillin G on platelet function in vitro is potentiated by the presence of probenecid.133 When platelet function was tested after intravenous administration of penicillin, oxacillin or mezlocillin for 3 to 17 days to patients or normal volunteers, irreversible inhibition of agonist-induced aggregation was noted, along with a 40 percent reduction in low-affinity TXA2 receptors.134 Thus, penicillins probably inhibit platelet function by binding to one or more membrane components necessary for adhesive interactions with the vessel wall or for stimulus-response coupling.
Although clinically significant bleeding is associated with the use of carbenicillin, penicillin G, ticarcillin, and nafcillin, it is far less common than prolongation of the bleeding time.127,135 Patients with coexisting hemostatic defects (e.g., thrombocytopenia, vitamin K deficiency, uremia) may be particularly prone to this complication. On the other hand, high doses of penicillin G did not increase gastrointestinal blood loss in a thrombocytopenic rabbit model.136 In our experience, bleeding attributable to antibiotic-induced platelet dysfunction is uncommon and unpredictable. Because β-lactam–induced platelet dysfunction resolves with time following cessation of the drug, this class of drugs should only be considered as a cause of bleeding in the appropriate clinical setting. A similar pattern of platelet dysfunction has been reported with some cephalosporins or related antibiotics, but not with others.127,137,138 Broad-spectrum antibiotics can also cause a bleeding diathesis attributable to killing of gut flora, resulting in vitamin K deficiency. Nitrofurantoin, a structurally unrelated antibiotic, may cause a mild prolongation of the bleeding time and impair platelet aggregation when blood levels of the drug are higher than 20 μM, as may occur in patients with renal insufficiency.139 Miconazole, an antifungal agent, inhibits human and rabbit platelet COX in vitro and rabbit platelet COX after intravenous infusion.140
ANTICOAGULANTS, FIBRINOLYTIC AGENTS, ANTIFIBRINOLYTIC AGENTS
Heparin predisposes to bleeding primarily through its anticoagulant effect, but it may also impair platelet function. For example, a bolus injection of heparin (100 U/kg) can cause a significant prolongation of the bleeding time in normal subjects and in patients prior to cardiopulmonary bypass, suggesting that therapeutic doses of heparin may impair platelet function.126 Heparin likely impairs platelet function by inhibiting the generation and action of the potent platelet agonist thrombin. On the other hand, in vitro studies suggest that heparin can enhance platelet aggregation induced by other platelet agonists.141 Heparin binds to a single class of high-affinity binding sites on resting platelets and to an additional class of lower-affinity binding sites on fully activated platelets.142 High heparin doses also impair VWF-dependent platelet function, possibly by binding to the heparin-binding domain of VWF.143 The contributions of these effects on platelet function to the bleeding complications of heparin therapy are uncertain.
Bleeding during fibrinolytic therapy is predominantly a result of the combined effects of structural lesions in blood vessels and the fibrin(ogen)olytic activity of the agent used. However, pharmacologic doses of streptokinase, urokinase, and tissue plasminogen activator (t-PA) can affect platelet function.144 High concentrations of plasmin ex vivo cause platelet aggregation.145 Moreover, marked increases in the urinary excretion of the TXA2 metabolite 2,3-dinor-TXB2 have been detected in patients receiving streptokinase or t-PA for coronary thrombolysis, suggesting that in vivo platelet activation had occurred during infusion of the drug.146,147 Nevertheless, several in vitro studies indicate that plasmin generation has an inhibitory effect on platelet function. First, very high levels of fibrin(ogen) degradation products, coupled with very low levels of fibrinogen, may impair platelet aggregation.148 Second, plasminogen can bind to platelets149 and after its conversion to plasmin, enzymatically degrade platelet glycoprotein (GP) Ib, impairing the interaction of platelets with VWF.150,151 Third, plasmin can inhibit platelet arachidonic acid metabolism.152 Fourth, t-PA promotes the disaggregation of platelet aggregates, presumably by inducing lysis of the fibrinogen that mediates aggregate formation.153 Finally, after initial activation, platelets incubated with plasmin and recombinant t-PA in vitro become refractory to activation by other agonists.154 Whether any of these in vitro and ex vivo observations apply to the in vivo situation and are clinically significant remains uncertain.155 The antifibrinolytic drug, ε-aminocaproic acid, can increase the bleeding time when administered for several days at doses 24 g/day or greater.150
Administration of nitroprusside, which increases platelet cGMP,156,157,158,159,160 nitroglycerine,161 and propranolol,162,163 can decrease platelet aggregation and secretion ex vivo. Nitroprusside can increase the bleeding time twofold when administered at infusion rates of 6 to 8 mcg/kg/min.156,164 Inhalation of NO advocated for the treatment of pulmonary hypertension and the adult respiratory distress syndrome, can impair agonist-induced platelet aggregation ex vivo, although the clinical significance of these observations is unclear.165,166,167 Calcium channel blockers such as verapamil, nifedipine, and diltiazem inhibit platelet aggregation when added at very high concentrations to washed platelets.123 This effect is seen primarily with epinephrine-induced aggregation and does not appear to be related to calcium channel blockade.168 At therapeutic doses, calcium channel blockers do not prolong the bleeding time, although one agent, nisoldipine, has been reported to inhibit agonist-induced calcium transients and platelet aggregation after ten days of oral administration.169 At high concentrations, the antiarrhythmic drug quinidine has been reported to cause a mild prolongation of the bleeding time and to potentiate the effect of aspirin.170
Dextran is a neutral polysaccharide that is heterogeneous in molecular size. Two preparations with average molecular weights of 40,000 and 70,000 are in clinical use. Although dextran infusions may prolong the bleeding time of normal subjects and patients with von Willebrand disease, this phenomenon has not been observed in most normal subjects.9,171,172 Infused dextran adsorbs to the platelet surface and can impair platelet aggregation, secretion, and procoagulant activity. The maximal effect of dextran may require several hours, suggesting that larger molecules with a slower rate of clearance are responsible.9 Curiously, the drug has no effect when added to platelet-rich plasma.9 Dextran infusion produces a modest reduction in plasma VWF antigen levels and ristocetin cofactor activity.171 Despite these effects on primary hemostasis, prospective studies indicate that dextran is not associated with significant postoperative bleeding, unless it is administered together with low-dose heparin.173,174 Hydroxyethyl starch, another volume expander, while generally safe, may prolong the bleeding time and predispose to hemorrhage, particularly if it is administered in doses exceeding 20 mL/kg of a 6-percent solution. Lower doses of hydroxyethyl starch may contribute to bleeding if administered simultaneously with low-dose heparin or if given to patients with preexistent hemostatic defects or after major cardiothoracic surgery.175,176,177,178 Different hydroxyethyl starch preparations vary in the average number of hydroxymethyl groups per glucose unit, and this may affect both intravascular survival and effects on hemostasis.179,180
PSYCHOTROPIC DRUGS, ANESTHETICS, AND COCAINE
Platelets from patients taking antidepressants or phenothiazines may exhibit impaired aggregation, but this is not associated with bleeding.181,182 The effect on aggregation has been attributed to inhibition of intracellular signaling molecules such as protein kinase C (PKC).183 Selective serotonin reuptake inhibitors such as paroxetine, have been shown to decrease platelet serotonin storage.184 Fluoxetine does not appear to impair platelet aggregation in vitro and has only rarely been associated with clinical bleeding.185,186 General anesthesia with halothane or propofol may cause a slight prolongation of the bleeding time, most likely the result of an effect on calcium signaling, but this has no adverse effect on surgical hemostasis.187,188 In addition to an association with thrombocytopenia, cocaine has been reported to either inhibit189,190 or stimulate platelet activation.191 It has been suggested that heroin decreases platelet NO production.192 The clinical relevance of these observations is unknown.
Administering mithramycin to a total dose of 6 to 21 mg decreases platelet aggregation and is associated with mucocutaneous bleeding.193 An ex vivo defect in platelet secretion and secondary aggregation has been reported in patients with solid tumors within 48 hours of receiving infusions of autologous marrow and high-dose chemotherapy consisting of cisplatin, cyclophosphamide, and either bis-chloroethylnitrosourea (BCNU) or melphelan.194 Both daunorubicin and BCNU can inhibit platelet aggregation and secretion when added to platelet-rich plasma, but they have not been shown to cause clinically significant platelet dysfunction.195,196,197 Administration of recombinant forms of thrombopoietin to thrombocytopenic patients with cancer results in the production of normally functioning platelets.198,199 Dasatinib, the broad-spectrum protein tyrosine kinase inhibitor, impairs collagen-induced platelet activation in vitro and increases tail bleeding times in mice, perhaps explaining some bleeding episodes in patients with chronic myelogenous leukemia who have been treated with the drug.200 Ibrutinib, a Bruton tyrosine kinase (BTK) inhibitor efficacious in a wide variety of lymphoid malignanies,201,202 is associated with hemorrhagic complications in up to half of patients, with significant hemorrhagic toxicity in 5 percent.201,202,203 Exposing platelets to ibrutinib ex vivo can produce defective platelet adhesion.204 Furthermore, humans or mice lacking BTK have impaired ex vivo platelet function, although the impairment is quite mild.205,206 Whether the hemorrhagic toxicity of ibrutinib is caused by platelet BTK inhibition or by an off-target effect remains to be determined.
The immunosuppressive drug cyclosporine has been reported to enhance ADP-stimulated platelet aggregation in vitro.207 It is unclear whether this contributes to the TTP-like syndrome associated with this drug. Antihistamines,208 the serotonin antagonist ketanserin,209 and certain radiographic contrast agents210,211 can impair platelet aggregation responses ex vivo by unknown mechanisms.
Certain foods and food additives affect platelet function in vitro, and it is conceivable that some may affect hemostasis, particularly in association with other hemostatic defects. For example, diets rich in fish oils containing ω-3 fatty acids (eicosapentaenoic acid; docosahexaenoic acid) cause a slight prolongation of the bleeding time.212 These fatty acids act by reducing the platelet content of arachidonic acid and by competing with arachidonic acid for COX.213,214 Easy bruising noted after eating Chinese food has been attributed to an antiplatelet effect of the black tree fungus.215 A component of extract of onion can inhibit platelet arachidonic acid metabolism.216 Ajoene, a component of garlic, is an inhibitor of fibrinogen binding and platelet aggregation.217 Extracts of two commonly used spices, cumin and turmeric, also inhibit platelet aggregation and eicosanoid biosynthesis.218