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Chapter: Clinical Pharmacology: Hematologic drugs

Antiplatelet drugs

Antiplatelet drugs are used to prevent arterial thromboembolism,particularly in patients at risk for MI, stroke, and arteriosclerosis (hardening of the arteries).

Antiplatelet drugs

 

Antiplatelet drugs are used to prevent arterial thromboembolism,particularly in patients at risk for MI, stroke, and arteriosclerosis (hardening of the arteries).

 

Aspirin, clopidogrel, dipyridamole, sulfinpyrazone, and ticlopi-dine are examples of oral antiplatelet drugs. Antiplatelet drugs ad-ministered I.V. include abciximab, eptifibatide, and tirofiban.

Pharmacokinetics

When taken orally, antiplatelet drugs are absorbed very quickly and reach peak concentration in 1 to 2 hours. Aspirin maintains its antiplatelet effect for about 10 days, or as long as platelets normal-ly survive. The effects of clopidogrel last about 5 days. Sulfinpyra-zone may require several days of administration before its anti-platelet effects occur.

 

After I.V. administration, antiplatelet drugs are quickly distrib-uted throughout the body. They’re minimally metabolized and ex-creted unchanged in urine. The effects of these drugs occur within 15 to 20 minutes of administration and last about 6 to 8 hours.

 

Elderly patients and patients with renal failure may have de-creased clearance of antiplatelet drugs, which would prolong the antiplatelet effect.

Pharmacodynamics

 

Antiplatelet drugs interfere with platelet activity in different drug-specific and dose-related ways.

 

§    Low doses of aspirin inhibit clot formation by blocking the syn-thesis of prostaglandin, which in turn prevents formation of the platelet-aggregating substance thromboxane A2.

§    Clopidogrel inhibits platelet aggregation by inhibiting platelet-fibrinogen binding.

§    I.V. antiplatelet drugs inhibit the glycoprotein IIa-IIIb receptor, which is the major receptor involved in platelet aggregation.

§    Dipyridamole may inhibit platelet aggregation because it in-creases adenosine, a coronary vasodilator and platelet aggrega-tion inhibitor.

§    Ticlopidine inhibits the binding of fibrinogen to platelets during the first stage of the clotting cascade.

§    Sulfinpyrazone inhibits several platelet functions. It lengthens platelet survival and prolongs the patency of arteriovenous shunts used for hemodialysis. A single dose rapidly inhibits platelet ag-gregation.

Pharmacotherapeutics

 

Antiplatelet drugs have many different uses.

 

Managing MIs

 

Aspirin is used in patients who have had a previous MI or who have unstable angina to reduce the risk of death in pa-tients at high risk for CAD. It’s also prescribed to reduce the risk of transient ischemic attacks (TIAs) (temporary reduction in circulation to the brain).

 

Clopidogrel is used to reduce the risk of stroke or vas-cular death in patients with a history of a recent MI, stroke, or established peripheral artery disease. Clopido-grel is also used to help treat acute coronary syndromes, especially in patients undergoing percutaneous translumi-nal coronary angioplasty (PTCA) or coronary artery by-pass graft.

 

Eptifibatide may be used for patients with acute coronary syn-drome and for those undergoing percutaneous coronary interven-tion (PCI). Abciximab may also be used in combination with PCI. Tirofiban may be used to treat acute coronary syndrome.

Pharmacodynamics

 

Antiplatelet drugs interfere with platelet activity in different drug-specific and dose-related ways.

 

Low doses of aspirin inhibit clot formation by blocking the syn-thesis of prostaglandin, which in turn prevents formation of the platelet-aggregating substance thromboxane A2.

Clopidogrel inhibits platelet aggregation by inhibiting platelet-fibrinogen binding.

 

I.V. antiplatelet drugs inhibit the glycoprotein IIa-IIIb receptor, which is the major receptor involved in platelet aggregation.

Dipyridamole may inhibit platelet aggregation because it in-creases adenosine, a coronary vasodilator and platelet aggrega-tion inhibitor.

 

Ticlopidine inhibits the binding of fibrinogen to platelets during the first stage of the clotting cascade.

 

Sulfinpyrazone inhibits several platelet functions. It lengthens platelet survival and prolongs the patency of arteriovenous shunts used for hemodialysis. A single dose rapidly inhibits platelet ag-gregation.

Pharmacotherapeutics

 

Antiplatelet drugs have many different uses.

 

Managing MIs

 

Aspirin is used in patients who have had a previous MI or who have unstable angina to reduce the risk of death in pa-tients at high risk for CAD. It’s also prescribed to reduce the risk of transient ischemic attacks (TIAs) (temporary reduction in circulation to the brain).

 

Clopidogrel is used to reduce the risk of stroke or vas-cular death in patients with a history of a recent MI, stroke, or established peripheral artery disease. Clopido-grel is also used to help treat acute coronary syndromes, especially in patients undergoing percutaneous translumi-nal coronary angioplasty (PTCA) or coronary artery by-pass graft.

 

Eptifibatide may be used for patients with acute coronary syn-drome and for those undergoing percutaneous coronary interven-tion (PCI). Abciximab may also be used in combination with PCI. Tirofiban may be used to treat acute coronary syndrome.

Salve for surgery

Dipyridamole is used with a coumarin compound to prevent thrombus formation after cardiac valve replacement. Dipyri-damole may be administered with aspirin to prevent blood clots in Hypersensitivity reactions, particularly anaphylaxis, can occur. Bleeding is the most common ad-verse reaction when I.V. antiplatelet drugs are administered.

Circumventing stroke

 

Ticlopidine is used to reduce the risk of thrombotic stroke in high-risk patients, such as those with a history of frequent TIAs or a previous thrombotic stroke.

Drug interactions

 

Antiplatelet medications taken with NSAIDs, heparin, oral anti-coagulants, or another antiplatelet medication increase the risk of bleeding.

 

Sulfinpyrazone taken with aspirin and oral anticoagulants in-creases the risk of bleeding.

Tales of toxicity

 

§    Aspirin increases the risk of toxicity of methotrexate and val-proic acid.

 

§    Aspirin and ticlopidine may reduce the effectiveness of sulfin-pyrazone to relieve signs and symptoms of gout.

 

§    Antacids may reduce the plasma levels of ticlopidine.


§    Cimetidine increases the risk of ticlopidine toxicity and bleed-ing.

 

You just don’t know

 

Because guidelines haven’t been established for administrating ticlopidine with heparin, oral anticoagulants, aspirin, or fibrinolyt-ic drugs, these drugs should be discontinued before ticlopidine therapy begins. (See Adverse reactions to antiplatelet drugs.)

 

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