Objectives. Understand the coagulation and thrombosis process. Explore the different anticoagulants treatments. available.

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1 Assistant Professor of Molecular Toxicology College of Pharmacy, KSU Objectives Understand the coagulation and thrombosis process. Explore the different anticoagulants treatments available. Shed the light on the molecular mechanisms of action of these agents 2 1

2 Definitions Anticoagulants prevent clot formation and extension Antiplatelets interfere with platelet activity Thrombolytic agent dissolve existing thrombi 3 Mechanisms of Blood Coagulation The vascular endothelial cell layer lining blood vessels has an anticoagulant phenotype, and circulating blood platelets and clotting factors do not normally adhere to it. In the setting of vascular injury, the endothelial cell layer rapidly undergoes a series of changes resulting in a more procoagulant phenotype. Injury exposes reactive subendothelial matrix proteins such as collagen and von Willebrand factor, which results in platelet adherence and activation, and secretion and synthesis of vasoconstrictors and platelet-recruiting and activating molecules. 4 2

3 Mechanisms of Blood Coagulation thromboxane A2 is synthesized from arachadonic acid within platelets working as potent vasoconstrictor. Adenosine diphosphate is secreted from platelet granules to induce platelet aggregation, and serotonin (5-HT), stimulates aggregation and vasoconstriction. Activation of platelets results in a conformational change in the IIb III integrin (IIb/IIIa) receptor, enabling it to bind fibrinogen, which cross-links adjacent platelets, resulting in aggregation and formation of a platelet plug. Simultaneously, the coagulation system cascade is activated, resulting in thrombin generation and a fibrin clot, Hesham M. Korashy, PhD PHL platelet plug. 5 which stabilizes the Mechanisms of Blood Coagulation Thrombus formation at the site of the damaged vascular wall. Platelet membrane receptors include the glycoprotein (GP) Ia receptor, binding to collagen (C); GP Ib receptor binding von Willebrand factor (vwf), and GP IIb/IIIa, which binds fibrinogen and other macromolecules. Antiplatelet prostacyclin (PGI2) is released from the endothelium. Aggregating substances released from the degranulating platelet include adenosine diphosphate (ADP), thromboxane A2 (TXA2), and serotonin (5-HT). Production of factor Xa. 6 3

4 Mechanisms of Blood Coagulation 7 Clotting Process A) Physical Process Wound B) aspirin Chemical Process Wound antithrombins platelet activation factor Xa fibrinogen release of ADP A2 thrombin generation release of thromboxane thrombin fibrin (monomers) Further activation of platelets ADP receptor antagonists GP IIb-IIIa receptor antagonists fibrin (polymer) Platelet aggregation prothrombin vitamin K-dependent γ-carboxylation of Glu residues coumarins CLOT 8 4

5 Extrinsic Pathway Tissue trauma Intrinsic Pathway Blood trauma/ contact with collagen Activation of factor XII, IX, VIII Leakage of Tissue Factor Ca+2, factor VII X Xa X Xa Ca+2 Ca+2 Prothrombin activator Prothrombin (factor II) Ca+2 Thrombin Fibrinogen Prothrombin activator Thrombin Prothrombin (factor II) Fibrin Monomers Ca+2, factor XIII 9 Fibrin threads Coagulation Pathways The coagulation process that generates thrombin consists of two interrelated pathways: extrinsic pathway: is initiated by the activation of clotting factor VII, and the release of tissue thromboplastin a phospholipid and protein mixture. intrinsic pathway: is triggered when blood comes in contact with exposed collagen fibres in the subendothelium of damaged blood vessels. 10 5

6 Coagulation cascade 11 Classes of Anti-Coagulants Warfarin Heparin Aspirin 12 6

7 Clinical indications Prophylaxis of DVT (INR 2 2.5) Rx of DVT and PE, AF, cardioversion,dilated cardiomyopathy, mural thrombus following MI, rheumatic mitral valve disease ( INR 2.5) Recurrent DVT and PE, mechanical prosthetic heart valve ( INR 3.5) 13 Chemistry & Pharmacokinetics A chemist at the University of Wisconsin identified the toxic agent as bishydroxycoumarin. Warfarin (Wisconsin Alumni Research Foundation, with "arin" from coumarin added. In the 1950s warfarin (under the brand name Coumadin) was introduced as an antithrombotic agent in humans. Warfarin is one of the most commonly prescribed drugs, used by approximately 1.5 million individuals, and several studies have indicated that the drug is significantly underused in clinical situations where it has proven benefit. 14 7

8 Chemistry & Pharmacokinetics Warfarin is generally administered as the sodium salt and has 100% bioavailability. Over 99% of racemic warfarin is bound to plasma albumin, which may contribute to its small volume of distribution. Has long half-life in plasma (36 hours), and the lack of urinary excretion of unchanged drug. Warfarin used clinically is a racemic mixture composed of equal amounts of two enantiomorphs. 15 Chemistry & Pharmacokinetics The levorotatory S-warfarin is four times more potent than the dextrorotatory R-warfarin. This observation is useful in understanding the stereoselective nature of several drug interactions involving warfarin. 16 8

9 Monitoring of Warfarin Therapy Prothrombin time PT ratio INR (International Normalized Ratio) 17 Monitoring of Warfarin Therapy PROTHROMBIN TIME (PT) Time required for blood to coagulate is called PT Performed by adding a mixture of calcium and thromboplastin to citrated plasma Patient s PT PT ratio (PTR) = Control PT Thromboplastins from various manufacturers differ in their sensitivity to prolong PT May result in erratic control of anticoagulant therapy 18 9

10 Monitoring of Warfarin Therapy INTERNATIONAL NORMALISED RATIO (INR) INR = ([PTpt]/[PTRef]) ISI PTpt prothrombin time of patient PTRef prothrombin time of normal pooled sample ISI International Sensitivity Index 19 MOA Coumarin anticoagulants block the -carboxylation of several glutamate residues in prothrombin and factors VII, IX, and X as well as the endogenous anticoagulant proteins C and S. The blockade results in incomplete coagulation factor molecules that are biologically inactive. The protein carboxylation reaction is coupled to the oxidation of vitamin K. The vitamin must then be reduced to reactivate it. Warfarin prevents reductive metabolism of the inactive vitamin K epoxide back to its active hydroquinonephl form

11 MOA Mutational change of the responsible enzyme, vitamin K epoxide reductase, can give rise to genetic resistance to warfarin in humans and especially in rats. There is an 8- to 12-hour delay in the action of warfarin. Its anticoagulant effect results from a balance between partially inhibited synthesis and unaltered degradation of the four vitamin K-dependent clotting factors. The resulting inhibition of coagulation is dependent on their degradation half-lives in the circulation. These halflives are 6, 24, 40, and 60 hours for factors VII, IX, X, and II, respectively 21 Plasma T1/2 of Vitamin K-Dependent Clotting Factors Factor II Factor VII 72h 6h Factor IX 24h Factor X 36h Peak anticoagulant effect may be delayed by 72 to 96 hours 22 11

12 MOA Vitamin K cycle metabolic interconversions of vitamin K associated with the synthesis of vitamin K dependent clotting factors. Vitamin K1 or K2 is activated by reduction to the hydroquinone form (KH2). Stepwise oxidation to vitamin K epoxide (KO) is coupled to prothrombin carboxylation by the enzyme carboxylase. The reactivation of vitamin K epoxide is the warfarinsensitive step (warfarin). 23 Although prescribed for over 60 years the molecular mechanism remained unknown. Recently a new gene (VKORC1) which represents the target of coumarin (has been identified in

13 Factors Influencing Anticoagulant Response concomitant therapy with drugs or dietary or herbal supplements. genetic variations in warfarin metabolism and/or sensitivity. Concomitant diseases: vitamin K deficiency caused by decreased dietary intake, alterations in intestinal flora, or malabsorption states; hepatic dysfunction; renal impairment; and hyperthyroidism. 25 Pharmacogenomics Variations in the genes responsible for warfarin metabolism or pharmacodynamic response may affect warfarin dosage requirements. Over 30% of European and Caucasian populations have one or more variant alleles encoding CYP2C9, and such alleles are associated with reduced clearance of warfarin. Patients with one or more variant CYP2C9 alleles (e.g., CYP2C9*2, CYP2C9*3) are at increased risk of excessive bleeding (e.g., INR >3) and hence require lower dosages of warfarin

14 Pharmacogenomics 27 Toxicities Hemorrhage: the most common adverse effect of coumarin-derivative anticoagulants which occasionally result in death. Massive hemorrhage involves the GI tract or genitourinary sites but may involve the spinal cord or cerebral, pericardial, pulmonary, adrenal, or hepatic sites. Treatment: Minor hemorrhage (INR less than 5) decrease in dosage or to withholding one or more doses of the drug. 4 Severe hemorrhage (INR >5) phytonadione (1 2.5 mg) may be administered after omitting the next dose of warfarin

15 Toxicities Necrosis: Potentially fatal necrosis and/or gangrene of skin or other tissues with subcutaneous infarction. necrotic reactions occur because initiation of warfarin therapy causes plasma concentrations of protein C to decrease more rapidly than plasma concentrations of factors II, IX, and X. Occurs if warfarin therapy is discontinued and heparin therapy initiated. initiation of anticoagulant therapy with heparin for 4 5 days before initiation of warfarin therapy with the 2 drugs for 5 6 days (overlapping) may minimize the risk of warfarin-induced necrosis. 29 Toxicities GI disturbances: nausea, vomiting, anorexia, flatulence, abdominal cramps

16 Drug Interactions Drugs may alter patient sensitivity to warfarin: decreasing intestinal synthesis or absorption of vitamin K. affecting distribution or metabolism of the vitamin;. decreasing the rate of anticoagulant metabolism. competing for sites of metabolism or inhibiting the function or synthesis of metabolic enzymes;. increasing affinity of the anticoagulant for receptor sites;. decreasing synthesis and/or increasing catabolism of functional blood coagulation factors II, VII, IX, and X. Interfering competitively or noncompetitively with protein binding of warfarin 31 Heparin: Introduction An anionic, sulfated glycosaminoglycan anticoagulant present in mast cells. Heparin is a heterogeneous molecule with an average molecular weight of about 12,000. Heparin is commercially available as the sodium salt. Heparin sodium is prepared from either porcine intestinal mucosa or bovine lung tissue

17 Heparin: Clinical indications Prophylaxis of DVT and PE Rx of DVT and PE, AF, cardioversion,dilated cardiomyopathy, mural thrombus following MI, rheumatic mitral valve disease. Recurrent DVT and PE, mechanical prosthetic heart valve. Heparin is used as an in vitro anticoagulant in blood transfusions, extracorporeal circulation, dialysis procedures, and in blood samples drawn for laboratory purposes. 33 Heparin : Chemistry & Pharmacokinetics Heparin is not absorbed from the GI tract and must be administered parenterally. The onset of anticoagulant activity is immediate (20 60 minutes). Heparin appears to be extensively bound to low-density lipoprotein, globulins, and fibrinogen. Heparin does not cross the placenta and is not distributed into milk. The plasma half-life of heparin averages 1 2 hours in healthy adults

18 Heparin : Low-Molecular-Weight Heparins Low-molecular-weight heparins (LMWHs) are derived from depolymerization of unfractionated heparin. yields fragments approximately one third the size of the parent compound. These lower-molecular-weight fractions have several properties that differentiate them from unfractionated heparin. 35 Heparin : Chemistry & Pharmacokinetics Heparin may be partially metabolized in the liver to uroheparin, which is partially desulfated heparin. A small fraction of each dose of heparin excreted in urine as unchanged drug. Heparin is not removed by hemodialysis

19 Heparin : Dosage and Administration heparin is administered by IV infusion, intermittent IV injection, or deep subcutaneous injection. fixed low-dose therapy, heparin usually is administered by deep SC injection. Heparin should not be administered IM because of the frequency of irritation, pain, and hematoma at the injection site. 37 Heparin : Laboratory Monitoring of Therapy The activated partial thromboplastin time (aptt): is the most commonly used laboratory method for monitoring full-dose heparin therapy. The target of appt is bwtween times the control value in seconds. Activated coagulation time (ACT) may also be used 38 19

20 Heparin : MOA Heparin inhibits clotting of blood in vitro and in vivo by enhancing the action of antithrombin III. Antithrombin III inhibits the activity of activated clotting factors including thrombin (factor IIa) and activated factor X (factor Xa). The inhibition of thrombin blocks the conversion of fibrinogen to fibrin, and the inhibition of factor Xa blocks the conversion of prothrombin to thrombin. Heparin also has some effect on platelet function, inhibits the formation of a stable fibrin clot, Heparin : MOA 20

21 LMWH : MOA LMWHs inhibit factor Xa and augmenting tissuefactor-pathway inhibitor. but minimally affect thrombin, or factor IIa. Thus, the aptt, a measure of antithrombin (anti-factor IIa) activity, is not used to measure the activity of LMWHs 41 Heparin: Toxicities Hemorrhage: the most common adverse effect of heparin-derivative anticoagulants which occasionally result in death. complications occur in approximately % of patients receiving heparin Massive hemorrhage involves the GI tract or genitourinary sites but may involve the spinal cord or cerebral, pericardial, pulmonary, adrenal, or hepatic sites. Patients with renal failure or with a history of recent surgery or trauma may be at increased risk of bleeding complications during heparin therapy Treatment: Protamin sulfate (1 mg per 100 Units of Heparin that had been given over 4 hours) can be given to counteract the action Hesham M. Korashy, PhD of heparin

22 Heparin: Toxicities Thrombocytopenia: thrombocytopenia has occurred in about 15% of patients treated with heparin sodium. Two forms of acute, reversible thrombocytopenia have been reported. thrombocytopenia appears to be caused by a direct, nonimmunologic effect on circulating platelets. In others, the reaction appears to be caused by the presence of a heparin-dependent IgG plateletaggregating antibody. Thrombocytopenia has been reported with both low-dose and fulldose heparin therapy and does not appear to be dose related. Thrombocytopenia usually develops 1 20 days (average 5 9 days) after initiation of therapy. However, heparin-induced thrombocytopenia or thrombocytopenia and thrombosis can occur up to several weeks after discontinuance of heparin therapy. 43 Heparin: Drug Interactions Drugs Affecting Platelet Function : Aspirin and other NSAIDs may increase the risk of hemorrhage and should be used with caution in patients receiving heparin. decreasing the rate of anticoagulant metabolism. Dihydroergotamine Mesylate: potentiate the antithrombogenic effects of heparin by helping to reduce factors that contribute to venous thrombus formation increasing affinity of the anticoagulant for receptor sites