Acquired Disorders of Platelet Function

Adenosine Diphosphate (ADP) Receptor Antagonists

The thienopyridines selectively and irreversibly inhibit ADP-mediated platelet activation and aggregation. ADP activates platelets by raising the concentration of cytoplasmic Ca ²⁺ through influx from the extracellular fluid and mobilization from internal stores, and by decreasing the concentration of intracellular cyclic adenosine monophosphate (cAMP) coupled to inhibition of adenylyl cyclase. ADP receptors can be divided into two groups: the G protein–coupled receptors, termed P2Y, and the ionotropic receptors, P2X. Platelets contain two P2Y receptors, P2Y ₁ and P2Y ₁₂ , complexed to the heterotrimeric G proteins G _q and G _i2 , respectively. ADP binding to P2Y ₁ is necessary for platelet aggregation but is not sufficient. Instead, P2Y ₁ is responsible for ADP-induced platelet shape change, and its engagement by ADP triggers a transient aggregatory response. P2Y ₁₂ , coupled to inhibition of adenylyl cyclase, mediates the amplification of the aggregation response. The thienopyridine derivatives have no effects on arachidonic acid metabolism and hence act synergistically with ASA to inhibit platelet function.

The thienopyridines are all prodrugs of similar structure that produce active metabolites that bind irreversibly to P2Y ₁₂ , inhibiting ADP-mediated aggregation for up to 10 days after withdrawal of the drug, paralleling the platelet life span. Because most platelet agonists require ADP for their full activity, the thienopyridines inhibit platelet activation by all agonists except strong agonists at high concentrations. Another hypothetical advantage over ASA is their inhibition of shear-induced platelet aggregation.

There are three generations of thienopyridine drugs, represented by ticlopidine, clopidogrel, and prasugrel. Ticlopidine is rarely used because of potentially serious side effects including agranulocytosis, thrombocytopenia, and thrombotic thrombocytopenic purpura. The non-thienopyridine P2Y ₁₂ inhibitors, which include ticagrelor and cangrelor, are not prodrugs and serve as reversible inhibitors.

Direct-Acting Agents

Thrombin, the most potent platelet agonist, activates platelets via protease activated receptor (PAR)-1 and PAR-2. Vorapaxar, a tricyclic himbacine derivative, is a PAR-1 antagonist with a long half-life (up to 8 days) and a slow off-rate. When used in combination with ASA, vorapaxar increased the risk of major bleeding compared with placebo. On its own, vorapaxar reduced the risk of major adverse cardiovascular events in patients with peripheral artery disease compared with placebo, but was associated with a small increase in the risk of bleeding. Vorapaxar is contraindicated in patients with a history of stroke, transient ischemic attack, or intracranial hemorrhage.

Indirect Agents

The COMPASS trial showed that a combination of ASA plus low dose rivaroxaban reduced the risk of major adverse cardiovascular and limb events compared with ASA or rivaroxaban alone. The mechanism responsible for the synergistic effect of ASA and rivaroxaban is unclear but likely reflects the capacity of rivaroxaban to attenuate thrombin generation and to inhibit PAR-1 activation by factor Xa. Although these findings suggest that any anticoagulant that reduces factor Xa generation would have antiplatelet effects, when full dose apixaban was compared with placebo on top of antiplatelet therapy in patients with acute coronary syndrome, there was excessive bleeding with apixaban and no reduction in major adverse cardiovascular events. Therefore, synergism depends on using the right dose of the anticoagulant in combination with antiplatelet therapy.

Thromboxane Receptor Antagonists

Thromboxane synthesis inhibitors and thromboxane receptor antagonists were developed to circumvent the non-platelet effects of cyclooxygenase (COX) inhibitors. Development was halted when clinical trials revealed no advantages over ASA.

GPVI Antagonists

Genetic deficiency of glycoprotein (GP) VI, a collagen receptor on platelet, is associated with a mild bleeding diathesis. GPVI antagonists include revacept, a fusion protein containing the extracellular domain of GPVI linked to an Fc fragment, antibodies against GPVI and peptide fragments of the Tropidolaemus wagleri venom trowaglerix .

GP I/IX/V Antagonists

The GP I/IX/V complex serves as the primary platelet receptor for von Willebrand factor (vWF). Caplacizumab, which binds vWF and blocks its interaction with the receptor, is approved for the treatment of thrombotic thrombocytopenic purpura (see Chapter 132 ). Other agents that target the same interaction include ARC1779, an aptamer that binds vWF, and anfibatide, which binds GPIb.

Adenyl Cyclase Stimulators or Prostacyclin Analogues

Epoprostenol (the synthetic salt of prostacyclin), prostacyclin (iloprost), and an orally active stable analogue (beraprost) are prostacyclin-like agents that are used for the treatment of pulmonary arterial hypertension and peripheral artery disease. Despite the potent inhibition of platelet aggregation induced by prostacyclin in vitro, it has little or no effect on the bleeding time.

Parenteral α _IIb β ₃ (GPIIb/IIIa) Inhibitors

The final common step in platelet aggregation is activation of the major platelet integrin α _IIb β ₃ to a ligand-competent form, which binds multivalent ligands such as fibrinogen or vWF, thereby cross-linking platelets into aggregates. Conventional antiplatelet agents such as ASA and clopidogrel each inhibit only one pathway leading to platelet aggregation, ASA preventing thromboxane production, and clopidogrel blocking the major ADP receptor. Dual therapy with ASA and clopidogrel improves the clinical benefit of antiplatelet therapy, but aggregation is still able to proceed through the action of other agonists such as thrombin. Because α _IIb β ₃ engagement is required for aggregation through all pathways, blocking this receptor inhibits platelet aggregation more effectively.

Inhibitors of α _IIb β ₃ work selectively and competitively to inhibit platelet aggregation. These agents are intended to produce a transient effect akin to the defect in Glanzmann thrombasthenia, but there are important differences between the drug effect and the disease.

Although these agents share many similarities in their antithrombotic properties, they also have several clinically important pharmacokinetic and off-target effect differences. Three drugs are available: eptifibatide, tirofiban, and abciximab.

Abciximab

The first α _IIb β ₃ inhibitor approved for clinical use, abciximab (ReoPro), is a human-murine chimeric Fab fragment of a monoclonal antibody that targets the β ₃ subunit of α _IIb β ₃ and therefore also reacts with another integrin that shares the β ₃ subunit, α _V β ₃ . Although α _IIb β ₃ expression is restricted to megakaryocytes and platelets, α _V β ₃ is expressed on platelets; at low levels; and in cells of the vascular wall, including endothelial cells and fibroblasts. In addition to its antithrombotic properties, abciximab may also reduce infarct size, prevent stent thrombosis, and modulate inflammatory responses by virtue of β ₃ subunit inhibition in other tissues.

Abciximab administered intravenously as a standard 0.25 mg/kg bolus blocks approximately 80% of surface α _IIb β ₃ and inhibits platelet aggregation to a similar extent, although the extent of inhibition is variable. This level of blockade increases the bleeding time only mildly. The bleeding time only becomes markedly prolonged when receptor blockade exceeds 90%. After the bolus dose, abciximab is infused at a dose of 0.125 µg/kg/min for 12 hours. Several trials have established this as a clinically effective regimen. Plasma levels of free abciximab drop rapidly after its administration, with an initial half-life of approximately 30 minutes. Most of the drug is bound to platelets. This explains why patients with thrombocytosis may require larger weight-adjusted doses of abciximab to attain a therapeutic antiplatelet effect and why patients with lower platelet counts treated with the usual dose of abciximab display more profound platelet inhibition. Abciximab is not excreted in the urine and is probably metabolized by the reticuloendothelial system when platelets carrying bound abciximab are cleared from the circulation. The dose of abciximab does not need adjustment in patients with renal impairment. Although a daily platelet turnover rate of 10% would predict that no abciximab would be detected in blood after 10 days of its administration, platelet-bound abciximab has been observed up to 3 weeks after its initial administration, suggesting platelet-to-platelet redistribution of the drug or release of new platelets from megakaryocytes that had previously bound the drug. Because of the low plasma levels of unbound abciximab, the drug’s inhibitory effect can be rapidly reversed by platelet transfusion, and hemostasis should be normal when the concentration of infused platelets exceeds 50,000/µL, as it is in normal individuals. However, because of the ability of abciximab to redistribute to the newly transfused platelets, the platelet-bound abciximab may produce a gradual inhibitory effect on the infused platelets; in patients with severe or refractory bleeding, it may be necessary to infuse a very large dose of platelets, sufficient to leave 50% of α _IIb β ₃ receptors free, a number shown to be sufficient for normal hemostasis in studies of Glanzmann thrombasthenia heterozygotes. Without platelet transfusion, platelet aggregation generally returns to baseline levels within 12 to 24 hours after discontinuing abciximab.

Eptifibatide

Eptifibatide (Integrilin) is a cyclic heptapeptide based on the Lys-Gly-Asp (KGD) sequence found in barbourin, a platelet-inhibitory disintegrin from the venom of the Southeastern Pigmy rattlesnake, Sistrurus miliarius barbourin . Eptifibatide differs pharmacokinetically from abciximab in several important ways. For example, unlike abciximab, intravenous eptifibatide infusion produces high levels of unbound drug because of its lower affinity for the receptor. Platelet transfusion, therefore, is not a good method for acutely reversing the antiplatelet effect of eptifibatide because the newly transfused platelets are rapidly inhibited. However, its short plasma half-life (≈2.5 hours) allows for rapid clearance of eptifibatide and the reversal of its antiplatelet effect when administration is discontinued. Platelet aggregation returns to normal in approximately 4 hours, with the bleeding time normalizing within 1 hour.

Tirofiban

Tirofiban (Aggrastat) is a peptidomimetic agent based on the Arg-Gly-Asp (RGD) sequence, with a pharmacokinetic profile similar to that of eptifibatide; neither cross-reacts with α _V β ₃ . As with eptifibatide, the maximum antiplatelet effect is seen at concentrations that leave a high concentration of unbound drug in the plasma. Thus, platelet transfusions are not often effective for reversal of the antiplatelet effect of tirofiban. However, its half-life is even shorter than that of eptifibatide (≈2 hours), and its antiplatelet effect diminishes rapidly after discontinuation of the infusion.

The major adverse effects of α _IIb β ₃ inhibitors are thrombocytopenia and bleeding complications. Up to 5% of patients receiving abciximab experience mild thrombocytopenia (<100,000/µL), and 1% develop profound thrombocytopenia (<20,000/µL). The incidence of severe thrombocytopenia is lower with tirofiban and eptifibatide, being approximately 0.2%. A decrease in the platelet count is appreciable within the first hours after administration. It is therefore essential that a platelet count be obtained 2 to 4 hours after initiating treatment. Most patients with severe thrombocytopenia respond well to platelet transfusions and their platelet counts usually recover within 5 days, although they may take more than 1 week to do so.

Bleeding complications with α _IIb β ₃ inhibitors are quite common, especially after femoral artery catheterization. Rates of major bleeding vary between 0.7% and 5.2% in different randomized controlled studies. Intracranial bleeding is very rare (0.09%). The risk of bleeding increases in patients older than 65 years of age; in patients weighing less than 50 kg; in patients taking anticoagulant, antiplatelet, or thrombolytic therapies; and in patients with inherited or acquired bleeding disorders such as hemophilia and uremia, respectively.

Oral α _IIb β ₃ Inhibitors

Although several oral αIIbβ3 inhibitors were developed, they failed to prevent major adverse cardiovascular events and some were associated with increased mortality that was not related to bleeding. A plausible but unproven explanation for the increased mortality is that these agents produce a prothrombotic state because they block platelet aggregation but not platelet activation, and the activated platelets can then promote thrombin generation.