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INDIRECT THROMBIN INHIBITORS
The indirect thrombin inhibitors are so-named because their anti-thrombotic effect is exerted by their interaction with a separate protein, antithrombin. Unfractionated heparin (UFH), low-molecular-weight heparin (LMWH), and the synthetic penta-saccharide fondaparinux bind to antithrombin and enhance its inactivation of factor Xa (Figure 34–4). Unfractionated heparin and to a lesser extent LMWH also enhance antithrombin’s inacti-vation of thrombin.
Heparin is a heterogeneous mixture of sulfated mucopolysaccha-rides. It binds to endothelial cell surfaces and a variety of plasma proteins. Its biologic activity is dependent upon the endogenous anticoagulant antithrombin. Antithrombin inhibits clotting fac-tor proteases, especially thrombin (IIa), IXa, and Xa, by forming equimolar stable complexes with them. In the absence of heparin, these reactions are slow; in the presence of heparin, they are accelerated 1000-fold.
Only about a third of the molecules in commercial heparin preparations have an accelerating effect because the remainder lack the unique pentasaccharide sequence needed for high-affinity binding to antithrombin. The active heparin molecules bind tightly to antithrombin and cause a con-formational change in this inhibitor. The conformational change of antithrombin exposes its active site for more rapid interaction with the proteases (the activated clotting factors). Heparin func-tions as a cofactor for the antithrombin-protease reaction without being consumed. Once the antithrombin-protease complex is formed, heparin is released intact for renewed binding to more antithrombin.
The antithrombin binding region of commercial unfraction-ated heparin consists of repeating sulfated disaccharide units composed of D-glucosamine-L-iduronic acid and D-glucosamine-D-glucuronic acid. High-molecular-weight (HMW), also knownas UFH, fractions of heparin with high affinity for antithrombin markedly inhibit blood coagulation by inhibiting all three factors, especially thrombin and factor Xa. Unfractionated heparin has a molecular weight range of 5000–30,000. In contrast, the shorter-chain low-molecular-weight (LMW) fractions of heparin inhibit activated factor X but have less effect on thrombin than the HMW species. Nevertheless, numerous studies have demonstrated that LMW heparins such as enoxaparin, dalteparin, and tinzaparin are effective in several thromboembolic conditions. In fact, these LMW heparins—in comparison with UFH—have equal efficacy, increased bioavailability from the subcutaneous site of injection, and less frequent dosing requirements (once or twice daily is sufficient).
Because commercial heparin consists of a family of molecules of different molecular weights extracted from porcine intestinal mucosa and bovine lung, the correlation between the concentra-tion of a given heparin preparation and its effect on coagulation often is poor. Therefore, UFH is standardized by bioassay. Heparin was reformulated in 2009 in response to heparin con-tamination events in 2007 and 2008. The contaminant was iden-tified as over-sulfated chondroitin sulfate and linked to more than150 adverse events in patients, most commonly hypotension, nausea, and dyspnea within 30 minutes of infusion. In response to this event, heparin sodium was reformulated with stricter qual-ity control measures and bioassays to make detection of contami-nants easier. This reformulation led to a decrease in potency of approximately 10% from the previous formulation. USP heparin is now harmonized to the World Health Organization International Standard (IS) unit dose. Enoxaparin is obtained from the same sources as regular unfractionated heparin, but doses are specified in milligrams. Dalteparin, tinzaparin, and danaparoid (an LMW heparanoid containing heparan sulfate, dermatan sulfate, and chondroitin sulfate), on the other hand, are specified in anti-factor Xa units.
Close monitoring of the activated partial thromboplastin time(aPTT or PTT) is necessary in patients receiving UFH. Levels ofUFH may also be determined by protamine titration (therapeu-tic levels 0.2–0.4 unit/mL) or anti-Xa units (therapeutic levels 0.3–0.7 unit/mL). Weight-based dosing of the LMW heparins results in predictable pharmacokinetics and plasma levels in patients with normal renal function. Therefore, LMW heparin levels are not generally measured except in the setting of renal insufficiency, obesity, and pregnancy. LMW heparin levels can be determined by anti-Xa units. Peak therapeutic levels should be 0.5–1 unit/mL for twice-daily dosing, determined 4 hours after administration, and approximately 1.5 units/mL for once-daily dosing.
The major adverse effect of heparin is bleeding. This risk can be decreased by scrupulous patient selection, careful control of dos-age, and close monitoring. Elderly women and patients with renal failure are more prone to hemorrhage. Heparin is of animal origin and should be used cautiously in patients with allergy. Increased loss of hair and reversible alopecia have been reported. Long-term heparin therapy is associated with osteoporosis and spontaneous fractures. Heparin accelerates the clearing of postprandial lipemia by causing the release of lipoprotein lipase from tissues, and long-term use is associated with mineralocorticoid deficiency.
Heparin-induced thrombocytopenia (HIT) is a systemic hyperco-agulable state that occurs in 1–4% of individuals treated with UFH for a minimum of 7 days. Surgical patients are at greatest risk. The reported incidence of HIT is lower in pediatric popula-tions outside the critical care setting and is relatively rare in preg-nant women. The risk of HIT may be higher in individuals treated with UFH of bovine origin compared with porcine heparin and is lower in those treated exclusively with LMWH.
Morbidity and mortality in HIT are related to thrombotic events. Venous thrombosis occurs most commonly, but occlusion of peripheral or central arteries is not infrequent. If an indwelling catheter is present, the risk of thrombosis is increased in that extremity. Skin necrosis has been described, particularly in individu-als treated with warfarin in the absence of a direct thrombin inhibi-tor, presumably due to acute depletion of the vitamin K-dependent anticoagulant protein C occurring in the presence of high levels of procoagulant proteins and an active hypercoagulable state.
The following points should be considered in all patients receiving heparin: Platelet counts should be performed frequently; thrombocytopenia appearing in a time frame consistent with an immune response to heparin should be considered suspicious for HIT; and any new thrombus occurring in a patient receiving heparin therapy should raise suspicion of HIT. Patients who develop HIT are treated by discontinuance of heparin and admin-istration of a direct thrombin inhibitor.
Heparin is contraindicated in patients with HIT, hypersensitivity to the drug, active bleeding, hemophilia, significant thrombocy-topenia, purpura, severe hypertension, intracranial hemorrhage, infective endocarditis, active tuberculosis, ulcerative lesions of the gastrointestinal tract, threatened abortion, visceral carcinoma, or advanced hepatic or renal disease. Heparin should be avoided in patients who have recently had surgery of the brain, spinal cord, or eye, and in patients who are undergoing lumbar puncture or regional anesthetic block. Despite the apparent lack of placental transfer, heparin should be used in pregnant women only when clearly indicated.
The indications for the use of heparin are described in the section on clinical pharmacology. A plasma concentration of heparin of 0.2–0.4 unit/mL (by protamine titration) or 0.3–0.7 unit/mL (anti-Xa units) usually prevents pulmonary emboli in patients with established venous thrombosis. This concentration generally corresponds to a PTT of 2–3 times baseline. However, the use of the PTT for heparin monitoring is problematic. There is no stan-dardization scheme for the PTT as there is for the prothrombin time (PT) and its international normalized ratio (INR). Currentlymore than 300 reagent-instrument combinations are in use, and the actual ratios required to obtain an anti-Xa activity of 0.3–0.7 units/mL are variable, ranging from 1.6 to 6 times control PTT. Thus, if the PTT is used for monitoring, the laboratory should determine the clotting time that corresponds to the thera-peutic range by protamine titration or anti-Xa activity, as listed above.
In addition, some patients have a prolonged baseline PTT due to factor deficiency or inhibitors (which could increase bleeding risk) or lupus anticoagulant (which is not associated with bleeding risk but may be associated with thrombosis risk). Using the PTT to assess heparin effect in such patients is very difficult. An alter-native is to use anti-Xa activity to assess heparin concentration, a test now widely available on automated coagulation instruments. This approach more accurately measures the heparin concentra-tion; however, it does not provide the global assessment of intrin-sic pathway integrity of the PTT.
The following strategy is recommended: prior to initiating anticoagulant therapy of any type, the integrity of the patient’s hemostatic system should be assessed by a careful history of prior bleeding events, and baseline PT and PTT. If there is a prolonged clotting time, the cause of this (deficiency or inhibitor) should be determined prior to initiating therapy, and treatment goals strati-fied to a risk-benefit assessment. In high-risk patients measuring both the PTT and anti-Xa activity may be useful. When intermit-tent heparin administration is used, the aPTT or anti-Xa activityshould be measured 6 hours after the administered dose to main-tain prolongation of the aPTT to 2–2.5 times that of the control value. However, LMW heparin therapy is the preferred option in this case, as no monitoring is required in most patients.
Continuous intravenous administration of heparin is accom-plished via an infusion pump. After an initial bolus injection of 80–100 units/kg, a continuous infusion of about 15–22 units/ kg/h is required to maintain the anti-Xa activity in the range of 0.3–0.7 units/mL. Low-dose prophylaxis is achieved with subcu-taneous administration of heparin, 5000 units every 8–12 hours. Because of the danger of hematoma formation at the injection site, heparin must never be administered intramuscularly.
Prophylactic enoxaparin is given subcutaneously in a dosage of 30 mg twice daily or 40 mg once daily. Full-dose enoxaparin therapy is 1 mg/kg subcutaneously every 12 hours. This corre-sponds to a therapeutic anti-factor Xa level of 0.5–1 unit/mL. Selected patients may be treated with enoxaparin 1.5 mg/kg once a day, with a target anti-Xa level of 1.5 units/mL. The prophylac-tic dose of dalteparin is 5000 units subcutaneously once a day; therapeutic dosing is 200 units/kg once a day for venous disease or 120 units/kg every 12 hours for acute coronary syndrome. LMWH should be used with caution in patients with renal insuf-ficiency or body weight greater than 150 kg. Measurement of the anti-Xa level is useful to guide dosing in these individuals.
The synthetic pentasaccharide molecule fondaparinux (Figure 34–4) avidly binds antithrombin with high specific activ-ity, resulting in efficient inactivation of factor Xa. Fondaparinux has a long half-life of 15 hours, allowing for once-daily dosing by subcutaneous administration. Fondaparinux is effective in the prevention and treatment of venous thromboembolism, and appears to not cross-react with pathologic HIT antibodies in most individuals. The use of fondaparinux as an alternative anticoagu-lant in HIT is currently being tested in clinical trials.
A major focus of drug development has been to develop orally active anticoagulants that do not require monitoring. Rivaroxaban is the first oral factor Xa inhibitor to reach phase III clinical trials. The safety and efficacy of rivaroxaban appears to be at least equivalent, and possibly superior, to LMW heparins .
Excessive anticoagulant action of heparin is treated by discontinu-ance of the drug. If bleeding occurs, administration of a specific antagonist such as protamine sulfate is indicated. Protamine is a highly basic, positively charged peptide that combines with negatively charged heparin as an ion pair to form a stable complex devoid of anticoagulant activity. For every 100 units of heparin remaining in the patient, 1 mg of protamine sulfate is given intra-venously; the rate of infusion should not exceed 50 mg in any 10-minute period. Excess protamine must be avoided; it also has an anticoagulant effect. Neutralization of LMW heparin by protamine is incomplete. Limited experience suggests that 1 mg of protamine sulfate may be used to partially neutralize 1 mg of enoxaparin. Protamine will not reverse the activity of fondaparinux. Excess dan-aparoid can be removed by plasmapheresis.
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