Coagulation & Anticoagulant Drugs

Key Terms:

• Thrombosis refers to vessel occlusion.
• Hemostasis refers to the regulation of the coagulation system.
• Injury exposes collagen and von Willebrand factors, causing platelets to adhere, activate, and form a clot.
• Platelets synthesize thromboxane A2 from arachidonic acid, leading to platelet activation and vasoconstriction.
• Activated platelets release serotonin and adenosine diphosphate (ADP), further inducing aggregation and vasoconstriction.
• Platelet activation causes conformational changes in integrin IIb/IIIa, binding fibrinogen and forming a platelet plug.

Blood Coagulation Cascade:

• Prothrombin, a protein, is converted by Factor Xa, an enzyme, into thrombin, also an enzyme.
• Thrombin, an enzyme, converts fibrinogen, a protein, into fibrin, a blood clot.
• Factor Xa is a protease enzyme targetted by antithrombin III (ATIII).
• Antithrombin III (ATIII) binds to and inactivates factors IXa. Xia, Xa, preventing the conversion of soluble fibrinogen into insoluble fibrin.
• Antithrombin III (ATIII) inhibits Factor Xa, slowing down the clotting cascade.
• Thrombin is an enzyme that converts fibrinogen into insoluble fibrin to form a blood clot.
• Many of the enzymes in the clotting cascade do not use "-ase" nomenclature and are given Roman numerals (X, XII).

Anticoagulant Drugs: Indirect Thrombin Inhibitors:

• These drugs interact with Antithrombin III, a protein in the blood.
• These drugs inactivate Factor Xa.
• Activated antithrombin blocks Factor Xa and thrombin, preventing fibrin clot formation.
• Types include unfractionated heparin (UFH), low molecular weight heparin (LMWH), and fondaparinux (a synthetic pentasaccharide).
• These drugs enhance the inactivation of factor Xa and thrombin.

Heparin:

• Heparin, a mixture of sulfated mucopolysaccharides, accelerates antithrombin action by 1000x.
• It binds to antithrombin, causing a conformational change.
• High molecular weight heparin (HMWH) has a molecular weight of 5000-30000 and is administered via injection.
• Low molecular weight heparins (LMWH) like enoxaparin, dalteparin and tinzaparin, act similarly with equal efficacy and end in "parin."
• LMWH are now preferred due to easier control of absorption, subcutaneous administration, and no need for aPTT blood test monitoring.
• Commercial heparins are sourced from bovine lung or porcine intestines.
• Unfractionated heparin (UFH) is a large molecule and can only be injected intravenously (IV) or subcutaneously (SC), not intramuscularly (IM) to avoid hematoma.
• Activated partial thromboplastin time (aPTT or PTT) is used to monitor unfractionated heparin.
• A major adverse drug reaction (ADR) of unfractionated heparin is bleeding.
• Heparin-induced thrombocytopenia (HIT), a hypercoagulable state, develops in 1-4% of patients on Unfractionated Heparin (UFH).
• HIT occurs more frequently with heparin and less with fractionated LMWH; it never occurs with fondaparinux.
• HIT is caused by an idiosyncratic immune response, where the patient develops heparin antibodies.
• HIT occurs more frequently with UFH derived from bovine sources compared to pig sources.
• Heparin is discovered in the liver.
• Heparin’s clinical uses include the prevention and treatment of venous thromboembolic disease, arterial thrombosis, and prevention of thrombosis in arterial or cardiac surgery.
• Heparin is the medication of choice during pregnancy and will not cross the placental barrier.
• Heparin antibody complexes activate platelets, leading to possible clots.
• Thrombocytopenia results from loss of platelets, as macrophages remove the heparin antibody-platelet complex which can cause platelet counts to drop from 500,000 /uL to 20,000 /uL or less.
• The remaining antibodies activate the remaining platelets leading to systemic clot formation.
• Argatroban, a direct thrombin binder, helps stop coagulation and is used as a treatment.
• Protamine sulfate neutralizes heparin and reverses excessive bleeding from overdose.
• Protamine is the drug of choice for Unfractionated Heparin (UFH).
• Protamine may partially neutralize enoxaparin.
• Protamine will not reverse fondaparinux.
• Plasmapheresis is required to reverse other low molecular weight heparins.
• Unfractionated Heparin comes from pig or cow intestines and may cause allergic reactions.

Fondaparinux:

• Fondaparinux is not a heparin, but is still too large and must be injected subcutaneously (SC).
• It is synthetically made in a lab and does not come from animal intestines unlike heparin which comes from pigs and cows.
• It binds to antithrombin which inactivates Factor Xa, similarly to heparins.
• It has a long half-life of 15 hours, allowing once-daily subcutaneous dosing.
• Protamine is ineffective with fondaparinux.

Direct Thrombin Inhibitors:

• These directly bind to thrombin and inhibit its effects.
• Heparin and LMWHs bind to antithrombin, not thrombin.
• Hirudin and bivalirudin bind to two sites on thrombin.
• Argatroban and melagatran, bind only to the thrombin active site.

Parenteral Direct Thrombin Inhibitors:

• Hirudin is derived from medical leeches (Hirudo medicinalis), specifically from the saliva of the leech.
• Lepirudin is the recombinant form of hirudin.
• Administered subcutaneously like heparin and monitored with the aPTT test.
• Bivalirudin is administered intravenously and binds to two sites on thrombin.
• Argatroban is administered intravenously monitored with aPTT.
• Argatroban is often used to treat HIT, which is a key exam question.

Oral Direct Thrombin Inhibitors:

• Warfarin and coumarin anticoagulants are a major advancement.
• Cows that ate spoiled clover feed bled to death and the spoiled feed contained bishydroxycoumarin
• Bishydroxycoumarin is found in spoiled sweet grass and is a powerful anticoagulant.
• Dicumarol and Warfarin were derived from Bishydroxycoumarin.
• Warfarin is derived from coumarin and first sold under the brand name "Coumadin".
• Warfarin and dicumarol were first sold as rat poison ("d-Con" sold and renamed "coumadin").
• Warfarin blocks prothrombin and factors VII, IX, and X, inactivating the coagulation cascade
• It blocks vitamin K epoxidase, which activates vitamin K required for clot formation.
• Coumarin anticoagulants interfere with a large number of coagulation factors all use Vitamin K for normal use.
• Warfarin is not used in pregnancy because it crosses the placental barrier (use heparins instead).
• It is dosed orally at 5-10 mg/day and monitored with prothrombin time (PT), now called International Normalized Ratio (INR).
• The target therapeutic range for INR is 2-3 and ISI is engineered to be 1
• INR is a Prothrombin Time blood test that is standardized among the world's labs.
• Clinical effects of of Warfarin may take a week or more to kick in; heparin acts immediately
• The clinical effects of Warfarin can be reversed with Vitamin K.

Fibrinolytic Drugs:

• These drugs lyse thrombi by catalyzing the formation of plasmin, a serine protease enzyme.
• Plasminogen is the inactive precursor of plasmin.
• Plasmin precursors are used instead of plasmin because it is inactivated by inhibitors in human plasma.
• Streptokinase, a protein from bacteria, binds to plasminogen and activates it to form plasmin.
• No inactivators of bacterial streptokinase exist in human plasma.
• Urokinase is a human enzyme synthesized by the kidney that converts plasminogen to plasmin and has no natural inhibitors.
• Anistreplase is a human plasminogen + bacterial streptokinase combination.
• Anistreplase is acylated and was removed from the market in the US.
• Tissue plasminogen activators (t-PAs) activate only clot-bound plasminogen.
• Alteplase, reteplase, and tenecteplase are produced by recombinant technology and are types of human t-PA.

Antiplatelet Agents:

• Agents from outside the platelet that induce platelet activation include catecholamines, collagen, thrombin, and prostacyclin.
• Agents generated inside the platelet that induce activation include ADP, prostaglandin D2, prostaglandin E2, and serotonin.
• Agents generated within the platelet that induce activation include prostaglandin endoperoxides, thromboxane A2, cAMP/cGMP, and calcium ions.
• Drugs to stop platelet activation block these factors. Drug targets are either within or on the platelet.
  • Aspirin inhibits prostaglandin synthesis.
  • Clopidogrel and ticlopidine inhibit ADP-induced platelet aggregation.
  • Abciximab, tirofiban, and eptifibatide block IIb/IIIa receptors on platelets.

Aspirin:

• Thromboxane A2 produced within platelets is responsible for platelet aggregation.
• Aspirin inhibits the synthesis of thromboxane A2 in platelets.
• Aspirin inhibits the prostaglandin thromboxane A in the platelet. Aspirin inhibits COX in the platelet and is irreversible.
• Aspirin is an irreversible inhibitor, meaning the enzyme is inhibited even after the drug is cleared.
• It takes about 11 days for the platelet to be replaced with new, uninhibited platelets.
• Platelets have no nucleus and cannot synthesize new enzyme (COX).

Clopidogrel & Ticlopidine:

• Clopidogrel, prasugrel, & ticlopidine inhibit platelet aggregation by inhibiting the ADP pathway within platelets.
• They inhibit platelet aggregation by blocking ADP receptors on platelets.
• They have no effect on prostaglandin synthesis so are useful in patients allergic to aspirin.

Block Platelet Glycoprotein IIb/IIIa Receptors:

• Each platelet has approximately 50,000 of these receptors.
• Fibrinogen, vitronectin, fibronectin, and von Willebrand factor bind to the receptor and complete aggregation.
• Abciximab is a monoclonal antibody that binds to the receptor and blocks fibronectin binding etc.
• Eptifibatide is an analog of the carboxyl terminal of fibronectin that binds to the receptor and blocks fibronectin etc. binding.
• Tirofiban is similar in function and structure to eptifibatide.
• All three drugs are given parenterally.

Preventing Venous Thrombosis:

• Thrombophilia is the tendency to form thrombi.
• Causes of thrombophilia include inherited defects in the clotting cascade, prolonged bed rest, mechanical heart valves, atrial fibrillation, and cancer.
• Heparin and warfarin are used to prevent venous thrombosis.
• LMWH and fondaparinux are often used and given daily as subcutaneous injections.

Drugs Used for Bleeding Disorders:

• Vitamin K activates prothrombin and is found in leafy green vegetables.
• Vitamin K reverses excess warfarin dosing.
• K1 comes from green vegetables, while K2 is synthesized by bacteria in human GI.
• Desmopressin acetate increases factor VIII activity and can be given intranasally.

Oral Direct Factor Xa Inhibitors

• Examples include rivaroxaban, apixaban, edoxaban, and betrixaban.
• Direct Factor Xa Inhibitors all end with "xaban".
• Oral Direct Factor Xa Inhibitors are drugs used to prevent clot formation in a wide variety of conditions from hip and knee replacement to prevention of emboli in patients with atrial fibrillation.
• Blood test monitoring is not required for Direct Factor Xa Inhibitors.
• The clinical effects of the Oral Direct Factor Xa Inhibitors can be reversed with injections of andexanet alpha that binds to Factor Xa and removes all of the inhibitor drugs.

Oral Direct Thrombin Inhibitors:

• Dabigatran is administered orally, with no blood test monitoring required but aPTT will be prolonged which can be used to test for drug effect, if necessary.
• Idarucizumab is the antidote for excess dabigatran.

Required Drugs:

• Indirect Thrombin Inhibitors: heparin
• Low Molecular Weight Heparins: enoxaparin, dalteparin, tinzaparin
• Synthetic Anticoagulant: fondaparinux
• Heparin Antidote: protamine
• Coumarin Anticoagulants: warfarin
• Direct Factor Xa Inhibitors: rivaroxaban, apixaban, edoxaban
• Direct Thrombin Inhibitors: hirudin, lepirudin, bivalirudin, dabigatran (po)

Other information:

• Fibrinolytic Drugs: streptokinase, urokinase
• Antiplatelet Agents: aspirin (low dose), clopidogrel
• Block IIb/IIIa Receptors: abciximab
• Bleeding Disorder Drugs: Vitamin K

Heparin Mechanism of Action

• Heparin promotes the activity of Antithrombin, a polypeptide.
• Antithrombin, the polypeptide, inhibits many different coagulation factors (enzymes) that are all protease enzymes.
• Antithrombin inhibits factor enzymes, slowing down the coagulation cascade.
• Heparin binds to Antithrombin, conformational change in Antithrombin, makes Antithrombin a more potent inhibitor of thrombin, Factor Xa and Factor IXa.
• This happens in heparin, all the low molecular weight heparins (LMW heparins) and fondaparinux.
• Heparin is a secondary inhibitor of thrombin (an enzyme) by making the thrombin inhibitor, Antithrombin, more active.
• Unfractionated Heparin enhance antithrombin which helps inhibit many protease enzymes required to form a clot.

Warfarin Mechanism of Action

• With Warfarin the coagulation Factors II, VII, IX, X and the anticoagulant proteins C and S are "activated” by reduced (active) Vitamin K.
• If there is no “active” Vitamin K then these factors do not work and coagulation is reduced and clots will not form.
• Warfarin inhibits the enzyme that converts food ingested inactive Vitamin K into active reduced Vitamin K.
• This enzyme that it inhibits is named, gamma glutamyl carboxylase (GGC).
• Warfarin inhibits GGC, so Warfarin inhibits Vitamin K activation.