Chapter 30: Blood Coagulation and Anticoagulant, Fibrinolytic, and Antiplatelet Drugs

Conversion of Fibrinogen to Fibrin. Fibrinogen, a 340,000-Da protein, is a dimer, each half of which consists of three pairs of polypeptide chains (designated Aα, Bβ, and γ). Disulfide bonds covalently link the chains and the two halves of the molecule together. Thrombin converts fibrinogen to fibrin monomers by releasing fibrinopeptide A (a 16–amino acid fragment) and fibrinopeptide B (a 14–amino acid fragment) from the amino termini of the Aα and Bβ chains, respectively. Removal of the fibrinopeptides creates new amino termini, which fit into pre-formed holes on other fibrin monomers to form a fibrin gel, which is the end point of in vitro tests of coagulation (see "Coagulation in vitro"). Initially, the fibrin monomers are bound to each other non-covalently. Subsequently, factor XIII, a transglutaminase that is activated by thrombin, catalyzes interchain covalent cross-links between adjacent fibrin monomers, which enhance the strength of the clot.

Structure of Coagulation Factors. In addition to factor XIII, the coagulation factors include factors II (prothrombin), VII, IX, X, XI, XII, and prekallikrein. A stretch of about 200 amino acid residues at the carboxyl-termini of each of these zymogens exhibits homology to trypsin and contains the active site of the proteases. In addition, 9-12 glutamate residues near the amino termini of factors II, VII, IX, and X are converted to γ-carboxyglutamate (Gla) residues during their biosynthesis in the liver. The Gla residues bind Ca²⁺ and are necessary for the coagulant activities of these proteins.

Nonenzymatic Protein Cofactors. Factors V and VIII, which are homologous 350,000-Da proteins, serve as cofactors. Factor VIII circulates in plasma bound to von Willebrand factor, while factor V not only circulates in plasma but also is stored in platelets in a partially activated form. Platelets release this stored factor V when they are activated. Thrombin cleaves factors V and VIII to yield activated cofactors (factors Va and VIIIa) that have at least 50 times greater procoagulant activity than their nonactivated counterparts.

Factors Va and VIIIa serve as cofactors by binding to the surface of activated platelets, where they act as receptors for factors Xa and IXa, respectively. The activated cofactors also help localize prothrombin and factor X, the respective substrates for these enzymes, on the activated platelet surface. Assembly of these coagulation factor complexes increases the catalytic efficiency of factors Xa and IXa by ∼10⁹-fold.

TF is a non-enzymatic lipoprotein cofactor; it initiates coagulation by enhancing the catalytic efficiency of VIIa. Not normally present on blood-contacting cells, TF is constitutively expressed on the surface of subendothlial smooth muscle cells and fibro-blasts, which are exposed when the vessel wall is damaged. Lipid-rich cores of atherosclerotic plaques also are rich in TF, which explains why atherosclerotic plaque disruption often triggers the formation of a superimposed thrombus that can occlude the lumen of the vessel.

When exposed to stimuli such as endotoxin, tumor necrosis factor, or interleukin-1, monocytes also express TF on their surface. Circulating TF-bearing monocytes and membrane fragments derived from them, known as microparticles, contribute to thrombosis. In response to inflammatory stimuli or products of coagulation, such as thrombin, endothelial cells express adhesion molecules, which tether these monocytes and microparticles onto their surface. This augments coagulation by delivering more TF to sites of injury. TF-bearing monocytes and microparticles also home to thrombi by binding to P-selectin expressed on the surface of activated platelets. Additional TF from these sources maintains coagulation and promotes thrombus expansion.

Another plasma protein, high-molecular-weight kininogen, also serves as a cofactor. Negatively charged surfaces, such as catheters, stents, and other blood-contacting medical devices, activate factor XII in a reaction enhanced by high-molecular-weight kininogen. Factor XIIa then propagates coagulation by activating factor XI, a reaction also enhanced by high-molecular-weight kininogen. In the presence of activated platelets, thrombin also activates factor XI, thereby bypassing factor XIIa.

Activation of Prothrombin. By cleaving two peptide bonds on prothrombin, factor Xa converts it to thrombin. In the presence of factor Va, a negatively charged phospholipid surface, and Ca²⁺, factor Xa activates prothrombin with 10⁹-fold greater efficiency. The maximal rate of activation only occurs when prothrombin and factor Xa contain Gla residues, which endow them with the capacity to bind to phospholipids. Artificial mixtures of anionic phospholipids substitute for activated platelets in laboratory tests of coagulation. However, activated platelets not only provide a surface for coagulation factor assembly but also release factor Va, which may be more important than circulating factor Va for thrombin generation.

Activation of factor XII is not essential for hemostasis, as evidenced by the fact that patients deficient in factor XII, prekallikrein, or high-molecular-weight kininogen do not have excessive bleeding. Factor XI deficiency is associated with a variable and usually mild bleeding disorder. The mechanism responsible for factor XI activation in vivo is unknown. Activation may occur through feedback activation of factor XI by thrombin. Alternatively, factor XIIa may activate factor XI after injury to the vessel wall because there is evidence that RNA released from damaged cells activates factor XII and administration of RNA-degrading enzymes to mice attenuates thrombus formation after arterial injury.

Coagulation in vitro. Whole blood normally clots in 4-8 minutes when placed in a glass tube. Clotting is prevented if a chelating agent such as ethylenediaminetetraacetic acid (EDTA) or citrate is added to bind Ca²⁺. Recalcified plasma normally clots in 2-4 minutes. The clotting time after recalcification is shortened to 26-33 seconds by the addition of negatively-charged phospholipids and a particulate substance, such as kaolin (aluminum silicate) or celite (diatomaceous earth), which activates factor XII; the measurement of this is termed the activated partial thromboplastin time (aPTT). Alternatively, recalcified plasma clots in 12-14 seconds after addition of "thromboplastin" (a mixture of TF and phospholipids); the measurement of this is termed the prothrombin time (PT).

Two pathways of coagulation are recognized. An individual with a prolonged aPTT and a normal PT is considered to have a defect in the intrinsic coagulation pathway because all of the components of the aPTT test (except kaolin or celite) are intrinsic to the plasma. A patient with a prolonged PT and a normal aPTT has a defect in the extrinsic coagulation pathway because thromboplastin is extrinsic to the plasma. Prolongation of both the aPTT and the PT suggests a defect in a common pathway.

About 10-15 glycosaminoglycan chains, each containing 200-300 saccharide units, are attached to a core protein and yield a proteoglycan with a molecular mass of 750-1,000 kDa. The glycosaminoglycan then undergoes a series of modifications, which include the following: N-deacetylation and N-sulfation of glucosamine residues, epimerization of D-glucuronic acid to L-iduronic acid, O-sulfation of iduronic and glucuronic acid residues at the C2 position, and O-sulfation of glucosamine residues at the C3 and C6 positions. Each of these modifications is incomplete, yielding a variety of oligosaccharide structures. After transport of the heparin proteoglycan to mast cell granules, an endo-β-D-glucuronidase slowly degrades the glycosaminoglycan chains to fragments of 5-30 kDa (mean, ∼15 kDa, corresponding to ∼40 saccharide units).

Source. Heparin is commonly extracted from porcine intestinal mucosa, which is rich in mast cells, and preparations may contain small amounts of other glycosaminoglycans. Despite the heterogeneity in composition among different commercial preparations of heparin, their biological activities are similar (∼150 USP units/mg). A USP unit reflects the quantity of heparin that prevents 1 mL of citrated sheep plasma from clotting for 1 hour after the addition of 0.2 mL of 1% CaCl₂. Although North American heparin manufacturers have traditionally quantified heparin potency in USP units, European manufacturers measure potency with an anti-factor Xa assay. This assay involves monitoring the activity of factor Xa added to human citrated plasma with a synthetic factor Xa–directed substrate that changes color when cleaved by the enzyme. The higher the heparin concentration in the sample, the less the residual factor Xa activity detected. To determine heparin potency, residual factor Xa activity in the sample is compared with that detected in controls containing known concentrations of an international heparin standard. When assessed this way, heparin potency is expressed in international units per mg.

Effective October 1, 2009, the new USP unit dose has been harmonized to the international unit dose. Contamination of heparin with oversulfated chondroitin sulfate, a non-heparin glycosaminoglycan, prompted this shift because this contaminant eludes detection with the USP assay but not with the anti-factor Xa assay. As a result, the new USP unit dose is less potent than the old USP unit dose by ∼10%, and heparin doses using the new USP units will have to increase slightly to achieve the same level of anticoagulation. This likely is of little or no clinical consequence for subcutaneous administration, due to the low and variable bioavailability of heparin when administered by this route. However, for intravenous administration, dosing adjustments and more frequent monitoring of aPTT may be necessary.

Heparin Derivatives. Derivatives of heparin in current use include low-molecular-weight heparins (LMWHs) and fondaparinux. The features that distinguish these derivatives from heparin are outlined in Table 30–1. Several LMWH preparations are marketed (e.g., daltaparin [FRAGMIN], enoxaparin [LOVENOX], tinzaparin [INNOHEP]), but all are fragments of heparin ranging in molecular weight from 1-10 kDa (mean ∼5 kDa, ∼17 saccharide units). LMWH preparations differ from heparin and, to a lesser extent, from each other in their pharmacokinetic properties. The potency of LMWH is assessed with anti-factor Xa assays, which use an international LMWH standard for reference purposes.

In contrast to heparin and LMWHs, which are biologicals derived from animal tissues, fondaparinux (arixtra) is a synthetic five-saccharide analog of a natural pentasaccharide sequence that is found in heparin and LMWHs and mediates their interaction with antithrombin. Fondaparinux has unique pharmacokinetic properties that distinguish it from LMWH. The potency of fondaparinux also is assessed with an anti-Xa assay.

This conformational change accelerates the rate of factor Xa inhibition by at least two orders of magnitude but has no effect on the rate of thrombin inhibition. To enhance the rate of thrombin inhibition by antithrombin, heparin serves as a catalytic template to which both the inhibitor and the protease bind. Only heparin molecules composed of 18 or more saccharide units (molecular weight >5400 Da) are of sufficient length to bridge antithrombin and thrombin together. With a mean molecular weight of 15,000 Da, most of the chains of heparin are long enough to serve this role. Consequently, by definition, heparin catalyzes the rates of factor Xa and thrombin to a similar extent, as expressed by an anti-factor Xa to anti-factor IIa (thrombin) ratio of 1:1. In contrast, at least half of LMWH molecules (mean molecular weight of 5000 Da, ∼17 saccharide units) are too short to provide this bridging function and have no effect on the rate of thrombin inhibition by antithrombin. Because these shorter molecules still induce the conformational change in antithrombin that accelerates inhibition of factor Xa, LMWHs have greater anti-factor Xa activity than anti-IIa activity, and the ratio ranges from 3:1 to 2:1 depending on the preparation. Fondaparinux, an analog of the pentasaccharide sequence in heparin or LMWHs that mediates their interaction with antithrombin, has only anti-factor Xa activity because it is too short to bridge antithrombin to thrombin (Figure 30–5).

When the concentration of heparin in plasma is 0.1-1 units/mL, thrombin, factor IXa, and factor Xa are inhibited rapidly (t_1/2 <0.1 second) by antithrombin. This effect prolongs both the aPTT (discussed earlier) and the thrombin time (i.e., the time required for plasma to clot when exogenous thrombin is added); the PT is affected to a lesser degree. Factor Xa bound to platelets in the prothrombinase complex and thrombin bound to fibrin are both protected from inhibition by antithrombin in the presence of heparin or LMWH. Thus, heparin and LMWHs may promote the inhibition of factor Xa and thrombin only after they have diffused away from these binding sites.

Platelet factor 4, a cationic protein released from the α granules during platelet activation, binds heparin and prevents it from interacting with antithrombin. This phenomenon may limit the activity of heparin in the vicinity of platelet-rich thrombi. Because LMWH and fondaparinux have a lower affinity for platelet factor 4, these agents may retain their activity in the vicinity of such thrombi to a greater extent than heparin.

Miscellaneous Pharmacological Effects. High doses of heparin can interfere with platelet aggregation and thereby prolong the bleeding time. In contrast, LMWHs and fondaparinux have little effect on platelets. It is unclear to what extent the antiplatelet effect of heparin contributes to the hemorrhagic complications of treatment with the drug. Heparin "clears" lipemic plasma in vivo by causing the release of lipoprotein lipase into the circulation. Lipoprotein lipase hydrolyzes triglycerides to glycerol and free fatty acids. The clearing of lipemic plasma may occur at concentrations of heparin below those necessary to produce an anticoagulant effect. Rebound hyperlipemia may occur after heparin administration is stopped.

Because they do not require routine laboratory monitoring, LMWHs or fondaparinux can be used for out-of-hospital management of patients with venous thrombosis or pulmonary embolism, an approach that reduces healthcare expenditures. Patients who develop these disorders due to cancer often are treated with long-term LMWH instead of warfarin because nausea and vomiting from chemotherapy, involvement of the liver with cancer, and poor venous access can render therapy with warfarin problematic.

Heparin and LMWH are used during coronary balloon angioplasty with or without stent placement to prevent thrombosis. Fondaparinux is not used in this setting because of the risk of catheter thrombosis, a complication caused by catheter-induced activation of factor XII; longer heparin molecules are better than shorter ones for blocking this process. Cardiopulmonary bypass circuits also activate factor XII, which can cause clotting of the oxygenator. Heparin remains the agent of choice for surgery requiring cardiopulmonary bypass because it blocks this process and because the heparin can rapidly be neutralized with protamine sulfate after the procedure. Heparin also is used to treat selected patients with disseminated intravascular coagulation. While low-dose heparin, LMWH, or fondaparinux regimens all are effective, subcutaneous administration of low-dose heparin remains the recommended regimen for the prevention of post-operative deep venous thrombosis (DVT) and pulmonary embolism in patients undergoing major abdominothoracic surgery or who are at risk of developing thromboembolic disease. Specific recommendations for heparin use have been reviewed (Geerts et al., 2008).

In contrast to warfarin, heparin, LMWH, and fondaparinux do not cross the placenta and have not been associated with fetal malformations; therefore, these are the drugs of choice for anticoagulation during pregnancy. LMWH or fondaparinux is used most often in this setting because these agents need only be given once daily by subcutaneous injection. In addition, the risk of heparin-induced thrombocytopenia or osteoporosis is lower with LMWHs or fondaparinux than with heparin. Heparin, LMWH, and fondaparinux do not appear to increase fetal mortality or prematurity. If possible, the drugs should be discontinued 24 hours before delivery to minimize the risk of postpartum bleeding.

The t_1/2 of heparin in plasma depends on the dose administered. When doses of 100, 400, or 800 units/kg of heparin are injected intravenously, the half-lives of the anticoagulant activities are approximately 1, 2.5, and 5 hours, respectively (see Appendix II for pharmacokinetic data). Heparin appears to be cleared and degraded primarily by the reticuloendothelial system; a small amount of undegraded heparin also appears in the urine. LMWHs and fondaparinux have longer biological half-lives than heparin, 4-6 hours and ∼17 hours, respectively. Because these smaller heparin fragments are cleared almost exclusively by the kidneys, the drugs can accumulate in patients with renal impairment, which can lead to bleeding. Both LMWH and fondaparinux are contraindicated in patients with a creatinine clearance <30 mL/min. In addition, fondaparinux is contraindicated in patients with body weight <50 kg undergoing hip fracture, hip replacement, knee replacement surgery, or abdominal surgery.

Administration and Monitoring. Full-dose heparin therapy usually is administered by continuous intravenous infusion. Treatment of venous thromboembolism is initiated with a fixed-dose bolus injection of 5000 units or with a weight-adjusted bolus, followed by 800-1600 units/hour delivered by an infusion pump. Therapy routinely is monitored by measuring the aPTT. The therapeutic range for heparin is considered to be that which is equivalent to a plasma heparin level of 0.3-0.7 units/mL, as determined with an anti-factor Xa assay (Hirsh et al., 2001). The aPTT value that corresponds to this range varies depending on the reagent and instrument used to perform the assay. An aPTT two to three times the normal mean aPTT value generally is assumed to be therapeutic; however, values in this range obtained with some aPTT assays may overestimate the amount of circulating heparin and therefore be subtherapeutic. The risk of recurrence of thromboembolism is greater in patients who do not achieve a therapeutic level of anticoagulation within the first 24 hours. Initially, the aPTT should be measured and the infusion rate adjusted every 6 hours; dose adjustments may be aided by use of nomograms; weight-based nomograms appear to outperform those that used fixed doses (Hirsh et al., 2001). Once a steady dosage schedule has been established in a stable patient, daily laboratory monitoring usually is sufficient.

Very high doses of heparin are required to prevent coagulation during cardiopulmonary bypass. The aPTT is infinitely prolonged over the dosage range used. A less sensitive coagulation test, such as the activated clotting time, is employed to monitor therapy in this situation. Because the activated clotting time can be performed in a point-of-care fashion, patients undergoing coronary angioplasty also typically have their heparin therapy monitored this way.

For therapeutic purposes, heparin also can be administered subcutaneously on a twice-daily basis. A total daily dose of ∼35,000 units administered as divided doses every 8-12 hours usually is sufficient to achieve an aPTT of twice the control value (measured midway between doses). Monitoring generally is unnecessary once a steady dosage schedule is established. For low-dose heparin therapy (to prevent DVT and thromboembolism in hospitalized medical or surgical patients), a subcutaneous dose of 5000 units is given two to three times daily. Laboratory monitoring of heparin administered in this way usually is unnecessary because low-dose regimens have negligible effects on the aPTT.

LMWH preparations include Enoxaparin (lovenox), dalteparin (fragmin), tinzaparin (innohep, others), ardeparin (normiflo), nadroparin (fraxiparine, others), and reviparin (clivarine) (the latter three are not available in the U.S. currently). These agents differ considerably in composition, and one cannot assume that two preparations that have similar anti-factor Xa activity will have equivalent anti-thrombotic effects. The more predictable pharmacokinetic properties of LMWH, however, permit administration in a fixed or weight-adjusted dosage regimen once or twice daily by subcutaneous injection.

Because LMWHs produce a relatively predictable anticoagulant response, monitoring is not done routinely. Patients with renal impairment may require monitoring with an anti-factor Xa assay because this condition may prolong the t_1/2 and slow the elimination of LMWHs. Obese patients and children given LMWHs also may require monitoring. Specific dosage recommendations for various LMWH preparations may be obtained from the manufacturer's literature.

Fondaparinux (arixtra) is administered by subcutaneous injection, reaches peak plasma levels in 2 hours, and is excreted in the urine (t_1/2 ∼17 h). It should not be used in patients with renal failure. Because it does not interact significantly with blood cells or plasma proteins other than antithrombin, fondaparinux can be given once a day at a fixed dose without coagulation monitoring. Fondaparinux appears to be much less likely than heparin or LMWH to trigger the syndrome of heparin-induced thrombocytopenia (see later in the chapter). Fondaparinux is approved for thromboprophylaxis in patients undergoing hip or knee surgery or surgery for hip fracture (Buller et al., 2003) and in general medical or surgical patients. It also can be used for initial therapy in patients with pulmonary embolism or DVT.

Idraparinux, a hypermethylated version of fondaparinux, underwent phase III clinical testing. This drug has a t_1/2 of80 hours and is given subcutaneously on a once-weekly basis. To overcome the lack of an antidote, a biotin moiety was added to idraparinux to generate idrabiotaparinux, which can be neutralized with intravenous avidin. Ongoing phase III clinical trials are comparing idrabiotaparinux with warfarin for treatment of pulmonary embolism or for stroke prevention in patients with atrial fibrillation. Idraparinux, idrabiotaparinus, and avidin are not available for routine clinical use.

Heparin Resistance. The dose of heparin required to produce a therapeutic aPTT varies due to differences in the concentrations of heparin-binding proteins in plasma, such as histidine-rich glycoprotein, vitronectin, and large multimers of von Willebrand factor and platelet factor 4; these proteins competitively inhibit binding of heparin to antithrombin. Some patients do not achieve a therapeutic aPTT unless very high doses of heparin (>50,000 units/day) are administered. Such patients may have "therapeutic" concentrations of heparin in plasma at the usual dose when measured using an anti-factor Xa assay. This "pseudo" heparin resistance occurs because these patients have short aPTT values prior to treatment, as a result of increased concentrations of factor VIII. Other patients may require large doses of heparin because of accelerated clearance of the drug, as may occur with massive pulmonary embolism. Patients with inherited antithrombin deficiency ordinarily have 40-60% of the usual plasma concentration of this inhibitor and respond normally to intravenous heparin. However, acquired antithrombin deficiency, where concentrations may be <25% of normal, may occur in patients with hepatic cirrhosis, nephrotic syndrome, or disseminated intravascular coagulation; large doses of heparin may not prolong the aPTT in these individuals.

Because LMWHs and fondaparinux exhibit reduced binding to plasma proteins other than antithrombin, heparin resistance rarely occurs with these agents. For this reason, routine coagulation monitoring is unnecessary. Occasional patients, particularly those with underlying cancer, develop recurrent thrombosis despite therapeutic doses of a LMWH. Anti-factor Xa assays often are subtherapeutic in these patients, and higher doses of LMWH are needed to achieve a therapeutic response.

The anticoagulant effect of heparin disappears within hours of discontinuation of the drug. Mild bleeding due to heparin usually can be controlled without the administration of an antagonist. If life-threatening hemorrhage occurs, the effect of heparin can be reversed quickly by the intravenous infusion of protamine sulfate, a mixture of basic polypeptides isolated from salmon sperm. Protamine binds tightly to heparin and thereby neutralizes its anticoagulant effect. Protamine also interacts with platelets, fibrinogen, and other plasma proteins and may cause an anticoagulant effect of its own. Therefore, one should give the minimal amount of protamine required to neutralize the heparin present in the plasma. This amount is approximately 1 mg of protamine for every 100 units of heparin remaining in the patient; protamine (up to a maximum of 50 mg) is given intravenously at a slow rate (over 10 minutes).

Protamine is used routinely to reverse the anticoagulant effect of heparin after cardiac surgery and other vascular procedures. Anaphylactic reactions occur in about 1% of patients with diabetes mellitus who have received protamine-containing insulin (NPH insulin or protamine zinc insulin), but untoward reactions are not limited to this group. A less common reaction consisting of pulmonary vasoconstriction, right ventricular dysfunction, systemic hypotension, and transient neutropenia also may occur after protamine administration.

Protamine only binds long heparin molecules. Therefore, protamine only partially reverses the anticoagulant activity of LMWHs and has no effect on that of fondaparinux. Because the long heparin molecules account for the anti-factor IIa activity of LMWH, protamine completely reverses this activity. In contrast, protamine only partially reverses the anti-factor Xa activity of LMWH, which is mediated by shorter heparin molecules. The very short molecules of fondaparinux do not bind protamine. Therefore, fondaparinux lacks a specific antidote.

Venous thromboembolism occurs most commonly, but arterial thromboses causing limb ischemia, myocardial infarction, and stroke also occur. Bilateral adrenal hemorrhage, skin lesions at the site of subcutaneous heparin injection, and a variety of systemic reactions may accompany heparin-induced thrombocytopenia. The development of IgG antibodies against complexes of heparin with platelet factor 4 (or, rarely, other chemokines) appears to cause all of these reactions. These complexes activate platelets by binding to FcγIIa receptors, which results in platelet aggregation, release of more platelet factor 4, and thrombin generation. The antibodies also may trigger vascular injury by binding to platelet factor 4 attached to endogenous heparan sulfate on the endothelium.

Heparin, LMWH, and fondaparinux should be discontinued immediately if unexplained thrombocytopenia or any of the clinical manifestations mentioned above occur 5 or more days after beginning therapy, regardless of the dose or route of administration. The onset of heparin-induced thrombocytopenia may occur earlier in patients who have received heparin within the previous 3-4 months and have residual circulating antibodies. The diagnosis of heparin-induced thrombocytopenia can be confirmed by a heparin-dependent platelet activation assay or an assay for antibodies that react with heparin/platelet factor 4 complexes. Because thrombotic complications may occur after cessation of therapy, an alternative anticoagulant such as lepirudin, argatroban (see the next section), or fondaparinux should be administered to patients with heparin-induced thrombocytopenia. LMWH preparations should be avoided, because these drugs often cross-react with heparin in heparin-dependent antibody assays. In contrast, fondaparinux does not cross-react with these heparin-dependent antibodies, and, although large clinical trials are lacking, fondaparinux has been used successfully in patients with heparin-induced thrombocytopenia. Warfarin may precipitate venous limb gangrene or multicentric skin necrosis in patients with heparin-induced thrombocytopenia and should not be used until the thrombocytopenia has resolved and the patient is adequately anticoagulated with another agent.

Other Toxicities. Abnormalities of hepatic function tests occur frequently in patients who are receiving heparin or LMWHs. Mild elevations of the activities of hepatic transaminases in plasma occur without an increase in bilirubin levels or alkaline phosphatase activity. Osteoporosis resulting in spontaneous vertebral fractures can occur, albeit infrequently, in patients who have received full therapeutic doses of heparin (>20,000 units/day) for extended periods (e.g., 3-6 months). The risk of osteoporosis is lower with LMWHs or fondaparinux than it is with heparin. Heparin can inhibit the synthesis of aldosterone by the adrenal glands and occasionally causes hyperkalemia, even when low doses are given. Allergic reactions to heparin (other than thrombocytopenia) are rare.

Lepirudin is administered intravenously at a dose adjusted to maintain the aPTT at 1.5-2.5 times the median of the laboratory's normal range for aPTT. The drug is excreted by the kidneys and has a t_1/2 of ∼1.3 hours. Lepirudin should be used cautiously in patients with renal failure because it can accumulate and cause bleeding in these patients. Patients may develop antibodies against hirudin that occasionally prolong the t_1/2 and cause a paradoxical increase in the aPTT; therefore, daily monitoring of the aPTT is recommended. There is no antidote for lepirudin.

Desirudin is indicated for the prophylaxis of DVT in patients undergoing elective hip replacement surgery. The recommended dose is 15 mg every 12 hours by subcutaneous injection. It is eliminated by the kidney; the t_1/2 is ∼2 hours following subcutaneous administration. Similar to lepirudin, the drug should be used cautiously in patients with decreased renal function, and serum creatinine and aPTT should be monitored daily.

Bivalirudin contains the sequence Phe¹–Pro²–Arg³–Pro⁴, which occupies the catalytic site of thrombin, followed by a tetraglycine linker and a hirudin-like sequence that binds to exosite I. Thrombin slowly cleaves the Arg³–Pro⁴ peptide bond and thus regains activity.

Bivalirudin is administered intravenously and is used as an alternative to heparin in patients undergoing coronary angioplasty or cardiopulmonary bypass surgery. Patients with heparin-induced thrombocytopenia or a history of this disorder also can be given bivalirudin instead of heparin during coronary angioplasty. The t_1/2of bivalirudin in patients with normal renal function is 25 minutes; dosage reductions are recommended for patients with moderate or severe renal impairment.

Argatroban is administered intravenously and has an immediate onset of action. Its t_1/2 is 40-50 minutes. Argatroban is metabolized by hepatic CYPs and is excreted in the bile; therefore, dosage reduction is required for patients with hepatic insufficiency. The dosage is adjusted to maintain an aPTT of 1.5-3 times the baseline value. Argatroban can be used as an alternative to lepirudin for prophylaxis or treatment of patients with, or at risk of developing, heparin-induced thrombocytopenia. In addition to prolonging the aPTT, argatroban also prolongs the PT, which can complicate the transitioning of patients from argatroban to warfarin. A chromogenic factor X assay can be used instead of the PT to monitor warfarin in these patients.

A 96-hour continuous infusion of drotrecogin alfa decreases mortality in adult patients who are at high risk for death from severe sepsis if given within 48 hours of the onset of organ dysfunction (e.g., shock, hypoxemia, oliguria). The major adverse effect is bleeding.

History. Following the report of a hemorrhagic disorder in cattle that resulted from the ingestion of spoiled sweet clover silage, Campbell and Link, in 1939, identified the hemorrhagic agent as bishydroxycoumarin (dicoumarol). In 1948, a more potent synthetic congener was introduced as an extremely effective rodenticide; the compound was named warfarin as an acronym derived from the name of the patent holder, Wisconsin Alumni Research Foundation (WARF). Warfarin's potential as a therapeutic anticoagulant was recognized but not widely accepted, partly due to fear of unacceptable toxicity. However, in 1951, an army inductee uneventfully survived an attempted suicide with massive doses of a preparation of warfarin intended for rodent control. Since then, these anticoagulants have become a mainstay for prevention of thromboembolic disease.

Chemistry. Numerous anticoagulants have been synthesized as derivatives of 4-hydroxycoumarin and of the related compound, indan-1,3-dione (Figure 30–6). Only the coumarin derivatives are widely used; the 4-hydroxycoumarin residue, with a nonpolar carbon substituent at the position 3, is the minimal structural requirement for activity. This carbon is asymmetrical in warfarin (and in phenprocoumon and acenocoumarol). The R- and S-enantiomers differ in anticoagulant potency, metabolism, elimination, and interactions with other drugs. Commercial preparations of these anticoagulants are racemic mixtures. No advantage of administering a single enantiomer has been established.

Reduced vitamin K must be regenerated from the epoxide for sustained carboxylation and synthesis of biologically competent proteins. The enzyme that catalyzes this, vitamin K epoxide reductase (VKOR), is inhibited by therapeutic doses of warfarin. Vitamin K (but not vitamin K epoxide) also can be converted to the corresponding hydroquinone by a second reductase, DT-diaphorase. This enzyme requires high concentrations of vitamin K and is less sensitive to coumarin drugs, which may explain why administration of sufficient vitamin K can counteract even large doses of oral anticoagulants.

Therapeutic doses of warfarin decrease by 30-50% the total amount of each vitamin K-dependent coagulation factor made by the liver; in addition, the secreted molecules are undercarboxylated, resulting in diminished biological activity (10-40% of normal). Congenital deficiencies of the procoagulant proteins to these levels cause mild bleeding disorders. Vitamin K antagonists have no effect on the activity of fully carboxylated molecules in the circulation. Thus, the time required for the activity of each factor in plasma to reach a new steady state after therapy is initiated or adjusted depends on its individual rate of clearance. The approximate t_1/2 (in hours) are as follows: factor VII, 6; factor IX, 24; factor X, 36; factor II, 50; protein C, 8; and protein S, 30. Because of the long t_1/2 of some of the coagulation factors, in particular factor II, the full anti-thrombotic effect of warfarin is not achieved for several days, even though the PT may be prolonged soon after administration due to the more rapid reduction of factors with a shorter t_1/2, in particular factor VII. There is no obvious selectivity of the effect of warfarin on any particular vitamin K-dependent coagulation factor, although the anti-thrombotic benefit and the hemorrhagic risk of therapy may be correlated with the functional level of prothrombin, and to a lesser extent, factor X (Zivelin et al., 1993).

Polymorphisms in two genes, CYP2C9and VKORC1(vitamin K epoxide reductase complex, subunit 1) account for most of the genetic contribution to the variability in warfarin response. CYP2C9variants affect warfarin pharmacokinetics, whereas VKORC1variants affect warfarin pharmacodynamics. Common variations in the CYP2C9gene (designated CYP2C9*2 and *3), encode an enzyme with decreased activity, and thus are associated with higher drug concentrations and reduced warfarin dose requirements. At least one variant allele of CYP2C9*2 or CYP2C9*3 is present in ∼25% of European-Americans, but these variants are relatively uncommon in African-American and Asian populations (Table 30–2). Heterozygosity for CYP2C9*2 or *3 decreases the dose of warfarin required for anticoagulation by approximately 20-30% compared with "wild type" individuals (CYP2C9*1/*1). Homozygosity for CYP2C9*2 or *3 can decrease the warfarin dose requirement by approximately 50-70%. Generally, the *3 allele has a greater effect than the *2 allele.

Consistent with decreased warfarin dose requirements, subjects who carry at least one copy of a CYP2C9 variant allele appear to have an increased risk of bleeding. Thus, compared with individuals with no variant alleles, carriers of CYP2C9*2 and CYP2C9*3 have relative risks of bleeding of 1.91 and 1.77, respectively.

VKORC1 reduces vitamin K epoxide to vitamin K hydroquinone (see Figure 30–7) and is the target of coumarin anticoagulants, such as warfarin. Several genetic variations in VKORC1 are in strong linkage disequilibrium and have been designated haplotypes A and B (or non-A). VKORC1variants are more prevalent than those of CYP2C9. The prevalence of VKORC1 genetic variants is higher in Asians, followed by European-Americans and African-Americans. Polymorphism in VKORC1 explains ∼30% of the variability in warfarin dose requirements. Compared with VKORC1 non-A/non-A homozygotes, the warfarin dose requirement is decreased by ∼25% in heterozygotes and ∼50% in A/A homozygotes.

The clinical relevance of these genetic polymorphisms remains uncertain. The goal of warfarin therapy is to maintain a patient within a target INR range, most often an INR value between 2 and 3. The risk of serious bleeding increases with INR values >4 and is highest during initiation of warfarin therapy. Variations in VKORC1 have a greater effect than CYP2C9variants on warfarin responses early in therapy. Patients with VKORC1haplotype A have significantly higher INR values in the first week of warfarin therapy than non-A homozygotes; those with one or two VKORC1 haplotype A alleles achieve a therapeutic INR more rapidly and are more likely to have an INR >4 than patients with two non-A alleles. Both the VKORC1 haplotype and CYP2C9genotype have a significant effect on the warfarin dose after the first 2 weeks of therapy.

Based on evidence that genetic variations affect warfarin dose requirements and responses to therapy, the FDA amended the prescribing information for warfarin in 2007 to indicate that lower warfarin initiation doses be considered for patients with CYP2C9 and VKORC1 genetic variations. Efforts to facilitate the rational incorporation of genetic information into patient care have included the development of a warfarin dosing algorithm and point-of-care methods for CYP2C9 and VKORC1 genotyping. In a study of more than 4000 patients, the International Warfarin Pharmacogenetics Consortium (2009) compared the accuracy of a pharmacogenetic algorithm that included VKORC1 and CYP2C9 genotypes with two conventional clinical approaches: one based on clinical information to adjust the initial dose, and the other using a fixed-dose approach. The pharmacogenetic algorithm predicted the warfarin dose significantly better than the other two approaches. Moreover, the pharmacogenetic algorithm significantly improved the dose prediction for patients who required either high or low doses of warfarin (<21 mg/week or >49 mg/week).

Genetic variation clearly affects warfarin dose requirements and predilection to toxicity. Table 30–2 summarizes the effect of currently known genetic factors on warfarin dose requirements. However, it is not clear whether genotyping patients and modifying warfarin therapy accordingly will improve the quality of anticoagulation as determined by such clinically important outcomes as time to therapeutic INR, time spent within the target INR range, and rates of serious bleeding. Large randomized controlled trials comparing genotype-based therapy with standard care are under way to define the clinical impact of incorporating genetic information into clinical practice.

Frequently cited interactions that enhance the risk of hemorrhage in patients taking oral anticoagulants include decreased metabolism due to CYP2C9 inhibition by amiodarone, azole antifungals, cimetidine, clopidogrel, cotrimoxazole, disulfiram, fluoxetine, isoniazid, metronidazole, sulfinpyrazone, tolcapone, or zafirlukast, and displacement from protein binding sites caused by loop diuretics or valproate. Relative deficiency of vitamin K may result from inadequate diet (e.g., postoperative patients on parenteral fluids), especially when coupled with the elimination of intestinal flora by antimicrobial agents. Gut bacteria synthesize vitamin K and are an important source of this vitamin. Consequently, antibiotics can cause excessive PT prolongation in patients adequately controlled on warfarin. In addition to an effect on reducing intestinal flora, cephalosporins containing heterocyclic side chains also inhibit steps in the vitamin K cycle. Low concentrations of coagulation factors may result from impaired hepatic function, congestive heart failure, or hypermetabolic states, such as hyperthyroidism; generally, these conditions increase the prolongation of the PT. Serious interactions that do not alter the PT include inhibition of platelet function by agents such as aspirin and gastritis or frank ulceration induced by anti-inflammatory drugs. Agents may have more than one effect; e.g., clofibrate increases the rate of turnover of coagulation factors and inhibits platelet function. Elderly patients are more sensitive to oral anticoagulants.

Resistance to Warfarin. Some patients require >20 mg/day of warfarin to achieve a therapeutic INR. These patients often have excessive vitamin K intake from the diet or parenteral supplementation. Noncompliance and laboratory error are other causes of apparent warfarin resistance. There have been reported a few patients with hereditary warfarin resistance, in whom very high plasma concentrations of warfarin are associated with minimal depression of vitamin K-dependent coagulation factor biosynthesis; mutations in the VKORC1 gene have been identified in some of these patients (Rost et al., 2004).

Sensitivity to Warfarin. Approximately 10% of patients require <1.5 mg/day of warfarin to achieve an INR of 2-3. As indicated earlier, these patients often possess variant alleles of CYP2C9 or variant VKORC1 haplotypes, which affect the pharmacokinetics or pharmacodynamics of warfarin, respectively (Daly and King, 2003).

Although the reported incidence of major bleeding episodes varies considerably, it is generally <3% per year in patients treated with a target INR of 2-3. The risk of intracranial hemorrhage increases dramatically with an INR >4, especially in older patients. In a large outpatient anticoagulation clinic, the most common factors associated with a transient elevation of the INR to a value >6 were use of a new medication known to potentiate warfarin (e.g., acetaminophen), advanced malignancy, recent diarrheal illness, decreased oral intake, and taking more warfarin than prescribed (Hylek et al., 1998). Patients must be informed of the signs and symptoms of bleeding, and laboratory monitoring should be done at frequent intervals during intercurrent illnesses or any changes of medication or diet.

If the INR is above the therapeutic range but <5 and the patient is not bleeding or in need of a surgical procedure, warfarin can be discontinued temporarily and restarted at a lower dose once the INR is within the therapeutic range (Hirsh et al., 2003). If the INR is ≥5, vitamin K₁ (phytonadione, MEPHYTON, AQUAMEPHYTON) can be given orally at a dose of 1-2.5 mg (for 5≤ INR ≤9) or 3-5 mg (for INR >9). These doses of oral vitamin K₁ generally cause the INR to fall substantially within 24-48 hours without rendering the patient resistant to further warfarin therapy. Higher doses or parenteral administration may be required if more rapid correction of the INR is necessary. The effect of vitamin K₁ is delayed for at least several hours because reversal of anticoagulation requires synthesis of fully carboxylated coagulation factors. If immediate hemostatic competence is necessary because of serious bleeding or profound warfarin overdosage (INR >20), adequate concentrations of vitamin K-dependent coagulation factors can be restored by transfusion of fresh frozen plasma (10-20 mL/kg), supplemented with 10 mg of vitamin K₁, given by slow intravenous infusion. Transfusion of plasma may need to be repeated because the transfused factors (particularly factor VII) are cleared from the circulation more rapidly than the residual oral anticoagulant. Concentrates containing three or four of the vitamin K-dependent clotting factors are available in some countries; these rapidly restore the INR to normal. Vitamin K₁ administered intravenously carries the risk of anaphylactoid reactions and therefore should be used cautiously and administered slowly. Patients who receive high doses of vitamin K₁ may become unresponsive to warfarin for several days, but heparin can be used if continued anticoagulation is required.

Birth Defects. Administration of warfarin during pregnancy causes birth defects and abortion. A syndrome characterized by nasal hypoplasia and stippled epiphyseal calcifications that resemble chondrodysplasia punctata may result from maternal ingestion of warfarin during the first trimester. Central nervous system abnormalities have been reported following exposure during the second and third trimesters. Fetal or neonatal hemorrhage and intrauterine death may occur, even when maternal PT values are in the therapeutic range. Vitamin K antagonists should not be used during pregnancy, but as indicated in the previous section, heparin, LMWH, or fondaparinux can be used safely in this circumstance.

Skin Necrosis. Warfarin-induced skin necrosis is a rare complication characterized by the appearance of skin lesions 3-10 days after treatment is initiated. The lesions typically are on the extremities, but adipose tissue, the penis, and the female breast also may be involved. Lesions are characterized by widespread thrombosis of the microvasculature and can spread rapidly, sometimes becoming necrotic and requiring disfiguring débridement or occasionally amputation. Because protein C has a shorter t_1/2 than do the other vitamin K-dependent coagulation factors (except factor VII), its functional activity falls more rapidly in response to the initial dose of vitamin K antagonist. It has been proposed that the dermal necrosis is a manifestation of a temporal imbalance between the anticoagulant protein C and one or more of the procoagulant factors and is exaggerated in patients who are partially deficient in protein C or protein S. However, not all patients with heterozygous deficiency of protein C or protein S develop skin necrosis when treated with warfarin, and patients with normal activities of these proteins also can be affected. Morphologically similar lesions can occur in patients with vitamin K deficiency.

Other Toxicities. A reversible, sometimes painful, blue-tinged discoloration of the plantar surfaces and sides of the toes that blanches with pressure and fades with elevation of the legs (purple toe syndrome) may develop 3-8 weeks after initiation of therapy with warfarin; cholesterol emboli released from atheromatous plaques have been implicated as the cause. Other infrequent reactions include alopecia, urticaria, dermatitis, fever, nausea, diarrhea, abdominal cramps, and anorexia.

Warfarin can precipitate the syndromes of venous limb gangrene and multicentric skin necrosis when given to patients with heparin-induced thrombocytopenia who are not receiving a parenteral anticoagulant (Warkentin, 2003). Other anticoagulant agents, such as lepirudin, argatroban, or fondaparinux should be continued until the heparin-induced thrombocytopenia has resolved (see "Heparin toxicities," earlier in the chapter).

Prior to initiation of therapy, laboratory tests are used in conjunction with the history and physical examination to uncover hemostatic defects that might make the use of vitamin K antagonists more dangerous (e.g., congenital coagulation factor deficiency, thrombocytopenia, hepatic or renal insufficiency, vascular abnormalities). Thereafter, the INR calculated from the patient's PT is used to monitor the extent of anticoagulation and compliance. Therapeutic INR ranges for various clinical indications have been established empirically and reflect dosages that reduce the morbidity from thromboembolic disease while minimally increasing the risk of serious hemorrhage. For most indications, the target INR is 2-3. A higher target INR (e.g., 2.5-3.5) generally is recommended for patients with high-risk mechanical prosthetic heart valves (Hirsh et al., 2003).

For treatment of acute venous thromboembolism, heparin, LMWH, or fondaparinux usually is continued for at least 5 days after warfarin therapy is begun and until the INR is in the therapeutic range on 2 consecutive days. This overlap allows for adequate depletion of the vitamin K-dependent coagulation factors with long t_1/2, especially factor II. Frequent INR measurements are indicated at the onset of therapy to guard against excessive anticoagulation in the unusually sensitive patient. The testing interval can be lengthened gradually to weekly and then monthly for patients on long-term therapy whose test results have been stable.

Monitoring Anticoagulant Therapy: The INR (International Normalized Ratio). To monitor therapy, a blood sample is obtained, and the PT is determined along with that of a sample of normal pooled plasma. Formerly, the results were reported as a simple ratio of the two PT values. However, this ratio can vary widely depending on the thromboplastin reagent and the instrument used to initiate and detect clot formation. The PT is prolonged when the functional levels of fibrinogen, factor V, or the vitamin K-dependent factors II, VII, or X are decreased. Reduced levels of factor IX or proteins C or S have no effect on the PT. PT measurements are converted to INR measurements by the following equation:

The ISI value, supplied by the manufacturer of the test regent, indicates the relative sensitivity of the PT (determined from a given batch of thromboplastin) to decreases in the vitamin K-dependent coagulation factors in comparison with a World Health Organization human thromboplastin standard. Reagents with lower ISI values are more sensitive to the effects of vitamin K antagonists (i.e., the PT is prolonged to a greater extent in comparison with that obtained with a less-sensitive reagent having a higher ISI). Ideally, the ISI value of each batch of thromboplastin should be confirmed in each clinical laboratory using a set of reference plasmas to control for local variables of sample handling and instrumentation.

The INR does not provide a reliable indication of the degree of anticoagulation in patients with the lupus anticoagulant, in whom the PT and other phospholipid-dependent coagulation tests are prolonged at baseline. In these patients, a chromogenic factor X assay or the prothrombin-proconvertin time assay may be used to monitor therapy (Moll and Ortel, 1997).

Phenprocoumon (marcumar) has a longer plasma t_1/2 (5 days) than warfarin, as well as a somewhat slower onset of action and a longer duration of action (7-14 days). It is administered in daily maintenance doses of 0.75-6 mg. By contrast, acenocoumarol (sinthrome) has a shorter t_1/2 (10-24 hours), a more rapid effect on the PT, and a shorter duration of action (2 days). The maintenance dose is 1-8 mg daily.

Indandione Derivatives. Anisindione (miradon) is available for clinical use in some countries. It is similar to warfarin in its kinetics of action but offers no clear advantages and may have a higher frequency of untoward effects. Phenindione (dindevan) still is available in some countries. Serious hypersensitivity reactions, occasionally fatal, can occur within a few weeks of starting therapy with this drug, and its use can no longer be recommended.

Rodenticides. Bromadiolone, brodifacoum, diphenadione, chlorophacinone, and pindone are long-acting agents (prolongation of the PT may persist for weeks). They are of interest because they sometimes are agents of accidental or intentional poisoning. In this setting, reversal of the coagulopathy can require very large doses of vitamin K (i.e., >100 mg/day) for weeks or months.

The drug has oral bioavailability of ∼6%, a peak onset of action in 2 hours, and a plasma t_1/2 of 12-14 hours. When given in fixed doses, dabigatran etexilate produces such a predictable anticoagulant response that routine coagulation monitoring is unnecessary. Dabigatran etexilate is approved in the E.U. and Canada for prevention of venous thromboembolism after elective hip or knee replacement surgery. It is not yet available in the U.S. In phase III trials, dabigatran etexilate was non-inferior to warfarin for treatment of patients with venous thromboembolism (Schulman et al., 2009) and superior to warfarin for stroke prevention in patients with atrial fibrillation (Connolly et al., 2009). Therefore, this drug represents a promising alternative to warfarin for patients who require long-term anticoagulation.

About one-third of the drug is excreted unchanged in the urine, the remainder is metabolized by the liver, and inactive metabolites are excreted in the urine or feces. This drug is given in fixed doses and does not require coagulation monitoring. Like dabigatran etexilate, rivaroxaban also is approved in the E.U. and Canada for thromboprophylaxis after hip or knee replacement surgery. Rivaroxaban is not available in the U.S. Ongoing trials are comparing rivaroxaban with warfarin for treatment of venous thromboembolism and stroke prevention in patients with atrial fibrillation.

Other New Agents. Other new oral anticoagulants include apixaban, edoxaban, betrixaban, YM150, and TAK-442, which are oral factor Xa inhibitors, and AZD0837, which is an oral thrombin inhibitor. Apixaban and edoxaban are currently undergoing phase III clinical evaluation.

Plasminogen's five kringle domains mediate the binding of plasminogen (or plasmin) to carboxyl-terminal lysine residues in partially degraded fibrin; this enhances fibrinolysis. A plasma carboxypeptidase termed thrombin-activatable fibrinolysis inhibitor (TAFI) can remove these lysine residues and thereby attenuate fibrinolysis. The lysine binding kringle domains of plasminogen are located between amino acids 80 and 165, and they also promote formation of complexes of plasmin with α₂-antiplasmin, the major physiological plasmin inhibitor. Plasminogen concentrations in human plasma average 2 μM. A plasmin-degraded form of plasminogen termed lys-plasminogen binds to fibrin with higher affinity than intact plasminogen.

Plasma concentrations of α₂-antiplasmin (1 μM) are sufficient to inhibit about 50% of potential plasmin. When massive activation of plasminogen occurs, the inhibitor is depleted, and free plasmin causes a "systemic lytic state" in which hemostasis is impaired. In this state, fibrinogen is degraded and fibrinogen degradation products impair fibrin polymerization and therefore increase bleeding from wounds. α₂-Antiplasmin inactivates plasmin nearly instantaneously, as long as the first kringle domain on plasmin is unoccupied by fibrin or other antagonists, such as aminocaproic acid (see "Inhibitors of Fibrinolysis" section).

Since the advent of newer agents, streptokinase is rarely used clinically for fibrinolysis. It currently is not marketed in the U.S.

Because it has little activity except in the presence of fibrin, physiological t-PA concentrations of 5-10 ng/mL do not induce systemic plasmin generation. During therapeutic infusions of t-PA, however, when concentrations rise to 300-3000 ng/mL, a systemic lytic state can occur. Clearance of t-PA primarily occurs by hepatic metabolism, and its t_1/2 is ∼5 min. t-PA is effective in lysing thrombi during treatment of acute myocardial infarction or acute ischemic stroke.

t-PA (alteplase, ACTIVASE) is produced by recombinant DNA technology. The currently recommended ("accelerated") regimen for coronary thrombolysis is a 15-mg intravenous bolus, followed by 0.75 mg/kg of body weight over 30 minutes (not to exceed 50 mg) and 0.5 mg/kg (up to 35 mg accumulated dose) over the following hour. Recombinant variants of t-PA now are available (reteplase, RETAVASE and tenecteplase, TNKASE). They differ from native t-PA by having longer plasma half-lives that allow convenient bolus dosing; reteplase is administered in two bolus doses given 30 minutes apart, while tenecteplase requires only a single bolus. In contrast to t-PA and reteplase, tenecteplase is relatively resistant to inhibition by PAI-1. Despite these apparent advantages, these agents are similar to t-PA in efficacy and toxicity (GUSTO III Investigators, 1997).

If heparin is used concurrently with t-PA, serious hemorrhage will occur in 2-4% of patients. Intracranial hemorrhage is by far the most serious problem. Hemorrhagic stroke occurs with all regimens and is more common when heparin is used. In several large studies, t-PA was associated with an excess of hemorrhagic strokes of about 0.3% of treated patients. Based on the data of three large trials involving almost 100,000 patients, the efficacies of t-PA and streptokinase in treating myocardial infarction are essentially identical. Both agents reduce death and reinfarction by ∼30% in regimens containing aspirin. With good evidence that primary coronary angioplasty with or without stent placement, when feasible, is superior to thrombolytic therapy, the use of fibrinolytic therapy has decreased. Fibrinolytic therapy remains the treatment of choice for patients with acute ischemic stroke who present within 3 hours of symptom onset.

Aminocaproic acid is absorbed rapidly after oral administration, and 50% is excreted unchanged in the urine within 12 hours. For intravenous use, a loading dose of 4-5 g is given over 1 hour, followed by an infusion of 1-1.25 g/hour until bleeding is controlled. No more than 30 g should be given in a 24-hour period. Rarely, the drug causes myopathy and muscle necrosis.

Like aminocaproic acid, tranexamic acid is excreted in the urine and dose reductions are necessary in patients with renal impairment. The FDA approved oral tranexamic acid tablets for treatment of heavy menstrual bleeding in 2009. When used for this indication, tranexamic acid usually is given at a dose of 1 g four times a day for 4 days.

Complete inactivation of platelet COX-1 is achieved with a daily aspirin dose of 75 mg. Therefore, aspirin is maximally effective as an anti-thrombotic agent at doses much lower than those required for other actions of the drug. Numerous trials indicate that aspirin, when used as an anti-thrombotic drug, is maximally effective at doses of 50-320 mg/day (Antithrombotic Trialists' Collaboration, 2002; Patrono et al., 2004). Higher doses do not improve efficacy; moreover, they potentially are less efficacious because of inhibition of prostacyclin production, which can be largely spared by using lower doses of aspirin. Higher doses also increase toxicity, especially bleeding.

Other NSAIDs that are reversible inhibitors of COX-1 have not been shown to have anti-thrombotic efficacy and in fact may even interfere with low-dose aspirin regimens (see Chapters 33 and 34 for details).

In trials in which a regimen of dipyridamole plus aspirin was compared with aspirin alone, dipyridamole provided no additional beneficial effect (Antithrombotic Trialists' Collaboration, 2002). Two studies suggest that dipyridamole plus aspirin reduces strokes in patients with prior strokes or transient ischemic attacks (Diener et al., 1996). A formulation containing 200 mg of dipyridamole, in an extended-release form, and 25 mg of aspirin (aggrenox) is available. A recent study comparing this combination with clopidogrel for secondary prevention in patients with stroke or transient ischemic attacks showed no advantage of dipyridamole plus aspirin.

Ticlopidine is converted to the active thiol metabolite by a hepatic CYP (Savi et al., 2000). It is rapidly absorbed and highly bioavailable. It permanently inhibits the P2Y₁₂ receptor by forming a disulfide bridge between the thiol on the drug and a free cysteine residue in the extracellular region of the receptor, and thus has a prolonged effect. Like aspirin, it has a short t_1/2 but a long duration of action, which has been termed "hit-and-run pharmacology" (Hollopeter et al., 2001). Maximal inhibition of platelet aggregation is not seen until 8-11 days after starting therapy. The usual dose is 250 mg twice daily. Loading doses of 500 mg sometimes are given to achieve a more rapid onset of action. Inhibition of platelet aggregation persists for a few days after the drug is stopped.

Adverse Effects. The most common side effects are nausea, vomiting, and diarrhea. The most serious is severe neutropenia (absolute neutrophil count <500/μL), which occurred in 2.4% of stroke patients given the drug during premarketing clinical trials. Fatal agranulocytosis with thrombopenia has occurred within the first 3 months of therapy; therefore, frequent blood counts should be obtained during the first few months of therapy, with immediate discontinuation of therapy should cell counts decline. Platelet counts also should be monitored, as thrombocytopenia has been reported. Rare cases of thrombotic thrombocytopenic purpura-hemolytic uremic syndrome (TTP-HUS) have been associated with ticlopidine, with a reported incidence of 1 in 1600-4800 patients when the drug is used after cardiac stenting; the mortality associated with these cases is reported to be as high as 18-57% (Bennett et al., 2000). Remission of TTP has been reported when the drug is stopped (Quinn and Fitzgerald, 1999).

Therapeutic Uses. Ticlopidine has been shown to prevent cerebrovascular events in secondary prevention of stroke and is at least as good as aspirin in this regard (Patrono et al., 1998). Because ticlopidine is associated with life-threatening blood dyscrasias and a relatively high rate of TTP, it has largely been replaced by clopidogrel.

The usual dose is 75 mg/day with or without an initial loading dose of 300 or 600 mg. The drug is somewhat better than aspirin in the secondary prevention of stroke, and the combination of clopidogrel plus aspirin is superior to aspirin alone for prevention of recurrent ischemia in patients with unstable angina. The superiority of the combination suggests that the actions of the two drugs are synergistic, as might be expected from their distinct mechanisms of action. Clopidogrel is used with aspirin after angioplasty and coronary stent implantation, and this combination should be continued for at least 4-6 weeks in patients with a bare metal stent and for at least 1 year in those with a drug-eluting stent. The FDA-approved indications for clopidogrel are to reduce the rate of stroke, myocardial infarction, and death in patients with recent myocardial infarction or stroke, established peripheral arterial disease, or acute coronary syndrome.

There is wide inter-individual variability in the capacity of clopidogrel to inhibit ADP-induced platelet aggregation, and some patients are designated resistant to the antiplatelet effects of the drug. This variability reflects, at least in part, genetic polymorphisms in the CYPs involved in the metabolic activation of clopidogrel, most importantly CYP2C19.Clopidogrel-treated patients with the loss-of-function CYP2C19*2allele exhibit reduced platelet inhibition compared with those with the wild-type CYP2C19*1allele and experience a higher rate of cardiovascular events (see Table 30–2). Even patients with the reduced-function CYP2C19*3, *4,or *5alleles may derive less benefit from clopidogrel than those with the full-function CYP2C19*1allele. Concomitant administration of proton pump inhibitors, which are inhibitors of CYP2C19, with clopidogrel, produces a small reduction in the inhibitory effects of clopidogrel on ADP-induced platelet aggregation. The extent to which this interaction increases the risk of cardiovascular events remains controversial. Although CYP3A4 also contributes to the metabolic activation of clopidogrel, polymorphisms in this enzyme do not appear to influence clopidogrel responsiveness. However, a small study in patients undergoing percutaneous coronary interventions revealed that atorvastatin, a competitive inhibitor of CYP3A4, reduced the inhibitory effect of clopidogrel on ADP-induced platelet aggregation. The impact of this interaction on clinical outcomes is unknown.

The observation that genetic polymorphisms that affect clopidogrel metabolism influence clinical outcomes raises the possibilities that pharmacogenetic profiling may be useful to identify clopidogrel-resistant patients and that point-of-care assessment of the extent of clopidogrel-induced platelet inhibition may help detect patients at higher risk for subsequent cardiovascular events. It is unknown whether administration of higher doses of clopidogrel to such patients will overcome this resistance.

These characteristics reflect the rapid and complete absorption of prasugrel from the gut and its more efficient activation pathways. Virtually all of the absorbed prasugrel undergoes activation; by comparison, only 15% of absorbed clopidogrel undergoes metabolic activation, with the remainder inactivated by esterases.

Because the active metabolites of prasugrel and the other thienopyridines bind irreversibly to the P2Y₁₂ receptor, these drugs have a prolonged effect after discontinuation. This can be problematic if patients require urgent surgery. Such patients are at increased risk for bleeding unless the thienopyridine is stopped at least 5 days prior to the procedure.

Prasugrel was compared with clopidogrel in patients with acute coronary syndromes scheduled to undergo a coronary intervention. The incidence of cardiovascular death, myocardial infarction, and stroke was significantly lower with prasugrel than with clopidogrel mainly reflecting a reduction in the incidence of nonfatal myocardial infarction. The incidence of stent thrombosis also was lower with prasugrel than with clopidogrel. However, these advantages were at the expense of significantly higher rates of fatal and life-threatening bleeding. Because patients with a history of a prior stroke or transient ischemic attack are at particularly high risk of bleeding, the drug is contraindicated in those with a history of cerebrovascular disease. Caution is required if prasugrel is used in patients weighing <60 kg or in those with renal impairment. After a loading dose of 60 mg, prasugrel is given once daily at a dose of 10 mg. Patients >75 years of age or weighing <60 kg may do better with a daily prasugrel dose of 5 mg.

In contrast to their effect on the bioactivation of clopidogrel, CYP2C19polymorphisms appear to be less important determinants of the activation of prasugrel. There is no association between the loss-of-function allele and decreased platelet inhibition or increased rates of cardiovascular events with prasugrel. Therefore, prasugrel may be a reasonable alternative to clopidogrel in patients with the loss-of-function CYP2C19 allele.

The antibody is administered to patients undergoing percutaneous angioplasty for coronary thromboses, and when used in conjunction with aspirin and heparin, has been shown to prevent restenosis, recurrent myocardial infarction, and death. The unbound antibody is cleared from the circulation with a t_1/2 of about 30 minutes, but antibody remains bound to the α_IIbβ₃ receptor and inhibits platelet aggregation as measured in vitro for 18-24 hours after infusion is stopped. It is given as a 0.25-mg/kg bolus followed by 0.125 μg/kg/min for 12 hours or longer.

Adverse Effects. The major side effect of abciximab is bleeding, and the contraindications to its use are similar to those for the fibrinolytic agents listed in Table 30–3. The frequency of major hemorrhage in clinical trials varies from 1-10%, depending on the intensity of anticoagulation with heparin. Thrombocytopenia with a platelet count <50,000 occurs in about 2% of patients and may be due to development of neo-epitopes induced by bound antibody. Since the duration of action is long, if major bleeding or emergent surgery occurs, platelet transfusions can reverse the aggregation defect because free antibody concentrations fall rapidly after cessation of infusion. Readministration of antibody has been performed in a small number of patients without evidence of decreased efficacy or allergic reactions. The expense of the antibody limits its use.

Eptifibatide is given as a bolus of 180 μg/kg followed by 2 μg/kg/min for up to 96 hours. It is used to treat acute coronary syndrome and for angioplastic coronary interventions. In the latter case, myocardial infarction and death have been reduced by ∼20%. Although the drug has not been compared directly to abciximab, it appears that its benefit is somewhat less than that obtained with the antibody, perhaps because eptifibatide is specific for α_IIbβ₃ and does not react with the vitronectin receptor. The duration of action of the drug is relatively short, and platelet aggregation is restored within 6-12 hours after cessation of infusion. Eptifibatide generally is administered in conjunction with aspirin and heparin.

Adverse Effects. The major side effect is bleeding, as is the case with abciximab. The frequency of major bleeding in trials was about 10%, compared with ∼9% in a placebo group, which included heparin. Thrombocytopenia has been seen in 0.5-1% of patients.

Side effects also are similar to those of eptifibatide. The agent is specific to α_IIbβ₃ and does not react with the vitronectin receptor. Meta-analysis of trials using α_IIbβ₃ inhibitors suggests that their value in antiplatelet therapy after acute myocardial infarction is limited (Boersma et al., 2002). Tirofiban is administered intravenously at an initial rate of 0.4 μg/kg/min for 30 minutes, and then continued at 0.1 mg/kg/min for 12-24 hours after angioplasty or atherectomy. It is used in conjunction with heparin.

Cangrelor. Cangrelor is an adenosine analog that binds reversibly to P2Y₁₂and inhibits its activity. The drug has a t_1/2 of 3-6 minutes and is given intravenously as a bolus followed by an infusion. When stopped, platelet function recovers within 60 minutes. Recent trials comparing cangrelor with placebo during coronary interventions or comparing cangrelor with clopidogrel after such procedures revealed little or no advantages of cangrelor.

Ticagrelor. Ticagrelor is an orally active, reversible inhibitor of P2Y₁₂. The drug is given twice daily and not only has a more rapid onset and offset of action than clopidogrel, but also produces greater and more predictable inhibition of ADP-induced platelet aggregation. When compared with clopidogrel in patients with acute coronary syndromes (Wallentin et al., 2009), ticagrelor produced a greater reduction in cardiovascular death, myocardial infarction, and stroke at 1 year than clopidogrel did. This difference reflected a significant reduction in both cardiovascular death and myocardial infarction with ticagrelor. Rates of stroke were similar with ticagrelor and clopidogrel, and there were no differences in rates of major bleeding. When minor bleeding was added to the major bleeding results, however, ticagrelor showed an increase relative to clopidogrel. Although not yet approved, ticagrelor is the first new antiplatelet drug to demonstrate a reduction in cardiovascular death compared with clopidogrel in patients with acute coronary syndromes.

SCH530348. SCH530348 an orally active inhibitor of PAR-1, is under investigation as an adjunct to aspirin or aspirin plus clopidogrel. Two large phase III trials are under way. E5555, a second oral PAR-1 antagonist, is earlier in development.

History. In 1929, Dam observed that chickens fed inadequate diets developed a deficiency disease characterized by spontaneous bleeding and reduced prothrombin in the blood. Subsequently, Dam and coworkers (1935, 1936) found that the condition could be alleviated rapidly by feeding an unidentified fat-soluble substance, named vitamin K (Koagulation vitamin) by Dam. Independently, Almquist and Stokstad (1935) performed similar work. Quick and coworkers (1935) observed a coagulation defect in jaundiced individuals that was due to a decrease in the plasma concentration of prothrombin. In the same year, Hawkins and Whipple reported that animals with biliary fistulas were likely to develop excessive bleeding. Subsequently, Hawkins and Brinkhous (1936) showed that this, too, was due to a deficiency in prothrombin and that the condition could be relieved by the feeding of bile salts. The culmination of these studies came with the demonstration by Butt and coworkers (1938) and Warner and associates (1938) that combination therapy with vitamin K and bile salts was effective in the treatment of the hemorrhagic diathesis of jaundice. Thus, the relationship between vitamin K, adequate hepatic function, and the physiological mechanisms operating in the normal clotting of blood was established.

Chemistry and Occurrence. Vitamin K activity is associated with at least two distinct natural substances, designated as vitamin K₁ and vitamin K₂. Vitamin K₁, or phytonadione(also referred to as phylloquinone), is 2-methyl-3-phytyl-1,4-naphthoquinone; it is found in plants and is the only natural vitamin K available for therapeutic use. Vitamin K₂ actually is a series of compounds (the menaquinones) in which the phytyl side chain of phytonadione has been replaced by a side chain built up of 2-13 prenyl units. Considerable synthesis of menaquinones occurs in Gram-positive bacteria; indeed, intestinal flora synthesize the large amounts of vitamin K contained in human and animal feces (Bentley and Meganathan, 1982). Depending on the bioassay system used, menadione is at least as active on a molar basis as phytonadione.

Figure 30–7 summarizes the coupling of the vitamin K cycle with glutamate carboxylation. Vitamin K, as KH₂, the reduced vitamin K hydroquinone, is an essential cofactor for γ-glutamyl carboxylase (Rishavy et al., 2004). Using KH₂, O₂, CO₂, and the glutamate-containing substrate, the enzyme forms a Gla protein and concomitantly, the 2,3-epoxide of vitamin K. A coumarin-sensitive 2,3-epoxide reductase regenerates KH₂. The γ-glutamyl carboxylase and epoxide reductase are integral membrane proteins of the endoplasmic reticulum and seem to function as a multicomponent system that may include the chaperone protein, calumenin, which reportedly inhibits γ-carboxylation (Wajih et al., 2004). Two natural mutations in γ-glutamyl carboxylase lead to bleeding disorders (Mutucumarana et al., 2003). With respect to proteins affecting blood coagulation, these reactions occur in the liver, but γ-carboxylation of glutamate also occurs in lung, bone, and other cell types. Vitamin K epoxide reductase recently has been cloned (Rost et al., 2004; Li et al., 2004).

The γ-carboxylation reaction is aided by the grouping of glutamate residues in a Gla domain (∼45 residues in clotting factors) near the amino terminus of the nascent substrate protein; the reaction is guided by an adjacent propeptide (18-28 amino acids) that interacts with the carboxylase. In the case of the vitamin K-dependent clotting proteins, the Gla domain is ∼45 residues long, and 9-13 glutamates are γ-carboxylated in the primary sequence of the Gla domain, C-terminal to the propeptide. The Glu→Gla conversion enables the factors to interact well with Ca²⁺ and thence with phospholipids of platelet membranes, positioning the factors in the proper conformations for interacting with their substrates and modulators. In the absence of vitamin K (or in the presence of a coumarin derivative), newly synthesized vitamin K-dependent clotting factors lack Gla residues and are inactive because the molecular conformations needed for their interactions are not achieved.

Human Requirements. The human requirement for vitamin K has not been defined precisely. In patients made vitamin K deficient by a starvation diet and antibiotic therapy for 3-4 weeks, the minimum daily requirement is estimated to be 0.03 μg/kg of body weight (Frick, 1967) and possibly as high as 1 μg/kg, which is approximately the recommended intake for adults (70 μg/day).

Symptoms of Deficiency. The chief clinical manifestation of vitamin K deficiency is an increased tendency to bleed (see the discussion of hypoprothrombinemia in the section on "Oral Anticoagulants," earlier in the chapter). Ecchymoses, epistaxis, hematuria, GI bleeding, and postoperative hemorrhage are common; intracranial hemorrhage may occur. Hemoptysis is uncommon. The discovery of a vitamin K-dependent protein in bone suggests that the fetal bone abnormalities associated with the administration of oral anticoagulants during the first trimester of pregnancy ("fetal warfarin syndrome") may be related to a deficiency of the vitamin.

Considerable evidence indicates a role for vitamin K in adult skeletal maintenance and the prevention of osteoporosis. Low concentrations of the vitamin are associated with deficits in bone mineral density and fractures; vitamin K supplementation increases the carboxylation state of osteocalcin and also improves bone mineral density, but the relationship of these two effects is unclear (Feskanich et al., 1999). Bone mineral density in adults is not changed by therapeutic use of vitamin K antagonists (Rosen et al., 1993), but new bone formation may be affected.

Toxicity. Phytonadione and the menaquinones are nontoxic to animals, even when given at 500 times the recommended daily allowance. However, menadione and its derivatives (synthetic forms of vitamin K) have been implicated in producing hemolytic anemia and kernicterus in neonates, especially in premature infants (Diploma and Ritchie, 1997). For this reason, menadione should not be used as a therapeutic form of vitamin K.

Absorption, Fate, and Excretion. The mechanism of intestinal absorption of compounds with vitamin K activity varies with their solubility. In the presence of bile salts, phytonadione and the menaquinones are adequately absorbed from the intestine, almost entirely by way of the lymph. Phytonadione is absorbed by an energy-dependent, saturable process in proximal portions of the small intestine; menaquinones are absorbed by diffusion in the distal portions of the small intestine and in the colon. Following absorption, phytonadione is incorporated into chylomicrons in close association with triglycerides and lipoproteins. In a large survey, plasma phytonadione and triglyceride concentration were well correlated (Sadowski et al., 1989). The extremely low phytonadione levels in newborns may be partly related to very low plasma lipoprotein concentrations at birth and may lead to an underestimation of vitamin K tissue stores. After absorption, phytonadione and menaquinones are concentrated in the liver, but the concentration of phytonadione declines rapidly. Menaquinones, produced in the lower bowel, are less biologically active than phytonadione due to their long side chain. Very little vitamin K accumulates in other tissues.

Phytonadione is metabolized rapidly to more polar metabolites, which are excreted in the bile and urine. The major urinary metabolites result from shortening of the side chain to five or seven carbon atoms, yielding carboxylic acids that are conjugated with glucuronate prior to excretion.

Apparently, there is only modest storage of vitamin K in the body. Under circumstances in which lack of bile interferes with absorption of vitamin K, hypoprothrombinemia develops slowly over several weeks.

Phytonadione (aquamephyton, konakion, mephyton, generic) is available as tablets and in a dispersion with buffered polysorbate and propylene glycol (konakion) or polyoxyethylated fatty acid derivatives and dextrose (aquamephyton). konakion is administered only intramuscularly. aquamephyton may be given by any route; however, oral or subcutaneous injection is preferred because severe reactions resembling anaphylaxis have followed its intravenous administration.

Inadequate Intake. After infancy, hypoprothrombinemia due to dietary deficiency of vitamin K is extremely rare: The vitamin is present in many foods and also is synthesized by intestinal bacteria. Occasionally, the use of a broad-spectrum antibiotic may itself produce a hypoprothrombinemia that responds readily to small doses of vitamin K and reestablishment of normal bowel flora. Hypoprothrombinemia can occur in patients receiving prolonged intravenous alimentation. It is recommended to give 1 mg of phytonadione per week (the equivalent of about 150 μg/day) to patients on total parenteral nutrition.

Hypoprothrombinemia of the Newborn. Healthy newborn infants show decreased plasma concentrations of vitamin K-dependent clotting factors for a few days after birth, the time required to obtain an adequate dietary intake of the vitamin and to establish a normal intestinal flora. In premature infants and in infants with hemorrhagic disease of the newborn, the concentrations of clotting factors are particularly depressed. The degree to which these changes reflect true vitamin K deficiency is controversial. Measurements of non-γ-carboxylated prothrombin suggest that vitamin K deficiency occurs in about 3% of live births (Shapiro et al., 1986).

Hemorrhagic disease of the newborn has been associated with breast-feeding; human milk has low concentrations of vitamin K (Haroon et al., 1982). In addition, the intestinal flora of breast-fed infants may lack microorganisms that synthesize the vitamin (Keenan et al., 1971). Commercial infant formulas are supplemented with vitamin K.

In the neonate with hemorrhagic disease of the newborn, the administration of vitamin K raises the concentration of these clotting factors to the level normal for the newborn infant and controls the bleeding tendency within about 6 hours. The routine administration of 1 mg phytonadione intramuscularly at birth is required by law in the U.S. This dose may have to be increased or repeated if the mother has received anticoagulant or anticonvulsant drug therapy or if the infant develops bleeding tendencies. Alternatively, some clinicians treat mothers who are receiving anticonvulsants with oral vitamin K prior to delivery (20 mg/day for 2 weeks) (Vert and Deblay, 1982).

Inadequate Absorption. Vitamin K is poorly absorbed in the absence of bile. Thus, hypoprothrombinemia may be associated with either intrahepatic or extrahepatic biliary obstruction or a severe defect in the intestinal absorption of fat from other causes.

Biliary Obstruction or Fistula. Bleeding that accompanies obstructive jaundice or biliary fistula responds promptly to the administration of vitamin K. Oral phytonadione administered with bile salts is both safe and effective and should be used in the care of the jaundiced patient, both preoperatively and postoperatively. In the absence of significant hepatocellular disease, the prothrombin activity of the blood rapidly returns to normal. If oral administration is not feasible, a parenteral preparation should be used. The usual dose is 10 mg/day of vitamin K.

The treatment of a patient during hemorrhage requires transfusion of fresh blood or reconstituted fresh plasma. Vitamin K also should be given. If biliary obstruction has caused hepatic injury, the response to vitamin K may be poor.

Malabsorption Syndromes. Among the disorders that result in inadequate absorption of vitamin K from the intestinal tract are cystic fibrosis, sprue, Crohn's disease and enterocolitis, ulcerative colitis, dysentery, and extensive resection of bowel. Because drugs that greatly reduce the bacterial population of the bowel are used frequently in many of these disorders, the availability of the vitamin may be further reduced. Moreover, dietary restrictions also may limit the availability of the vitamin. For immediate correction of the deficiency, parenteral therapy should be used.

Inadequate Utilization. Hepatocellular disease may be accompanied or followed by hypoprothrombinemia. Hepatocellular damage also may be secondary to long-lasting biliary obstruction. In these conditions, the damaged parenchymal cells may not be able to produce the vitamin K-dependent clotting factors, even if excess vitamin is available. However, if an inadequate secretion of bile salts is contributing to the syndrome, some benefit may be obtained from the parenteral administration of 10 mg of phytonadione daily. Paradoxically, the administration of large doses of vitamin K or its analogs in an attempt to correct the hypoprothrombinemia associated with severe hepatitis or cirrhosis actually may result in a further depression of the concentration of prothrombin. The mechanism for this action is unknown.

Drug- and Venom-Induced Hypoprothrombinemia. Anticoagulant drugs such as warfarin and its congeners act as competitive antagonists of vitamin K and interfere with the hepatic biosynthesis of Gla-containing clotting factors. The treatment of bleeding caused by oral anticoagulants is discussed earlier in this chapter. Vitamin K may be of help in combating the bleeding and hypoprothrombinemia that follow the bite of the tropical American pit viper or other species whose venom destroys or inactivates prothrombin.