Stephanie A. Smith, DVM, MS, DACVIM
Although hemostasis is a vital protective system, the excessive activation of coagulation or down-regulation of inhibitory pathways can result in a downward spiral of thrombin production, fibrin formation and lack of fibrinolysis, ultimately leading to inappropriate microvascular or macrovascular thrombosis. Heparin has been used clinically for prevention of thrombosis and thromboembolism for over 70 years. It has many advantages over other medications, but also many limitations. A variety of heparin protocols have been extensively studied in human venous and arterial thrombotic disorders. There are many specific circumstances where heparin is a proven efficacy with reasonable safety. Appropriate dose protocols and monitoring approaches are fairly well described for various specific disorders. In veterinary medicine, evidence-based medicine is extremely limited regarding indications for use of heparins, which heparins are indicated, and what are the appropriate dosing protocols. Unfortunately, we know very little about how to use heparin in veterinary species for best safety and efficacy.
How Heparin Works
Natural Inhibitors of Coagulation
The coagulation system is modulated by three major inhibitory pathways: The protein C pathway, tissue factor pathway inhibitor (TFPI), and antithrombin (AT). One of the most important natural inhibitors of coagulation is activated protein C (APC). APC is produced when thrombin binds to thrombomodulin (TM) on the endothelial surface. Once activated, APC dissociates and interacts with its cofactor protein S to inactivate FVa and then FVIIIa, preventing cofactor activity and therefore adequate amplification of thrombin production. TFPI is another natural anticoagulant protein that is an important inhibitor of the extrinsic pathway. It binds to FXa, and then to the TF/FVIIa complex, preventing further enzymatic activity of either FXa or FVIIa. The majority of TFPI is bound to the endothelial surface; only a small amount circulates in the plasma under normal physiologic conditions. Administration of heparin causes release of endothelial bound TFPI into the flowing blood.
AT acts as an anticoagulant by inactivating serine proteases such as FXa, thrombin, and others. The inhibition of serine proteases is very slow in the absence of heparins. When antithrombin is bound to heparin, it changes conformation in such a way that it becomes vastly more efficient at inhibiting serine proteases. Under physiologic circumstances the endothelial cells produce heparan sulfated proteoglycans (HSPGs), a small amount of which is expressed on the luminal surface in contact with the flowing blood. The HSPGs bind AT, which then is fully capable of inactivating thrombin which is produced in the vicinity of the HSPG. Thrombin is resistant to AT-heparin when the thrombin is bound to fibrin. Consequently, once fibrin is formed it can act as a protective reservoir for active thrombin.
Heparins are negatively-charged polymer molecules containing saccharide (sugar) residues. They vary widely in their size, structure, and charge, all of which impact their potential ability to enhance the activity AT against its target enzymes. In order to bind to AT (and therefore exert the structural change necessary for AT function), the molecule must contain an essential series of 5 particular residues. Only a small portion of molecules in most preparations of pharmaceutical heparin contain these residues. Short molecules containing this structure are also capable of binding to FXa, but are not adequately long to reach the heparin-binding site on thrombin. As a consequence, short heparin molecules are better at catalyzing AT to inhibit FXa than they are at catalyzing AT to inhibit thrombin. This is why "low molecular weight" heparins (LMWH) have much greater anti-FXa activity than anti-thrombin activity. LMWHs are preparations made from sorting more heterogeneous heparin preparations (containing large, medium, and small molecules) through depolymerization methods that eliminate the largest molecules and enriches the preparation for smaller molecules. Different manufacturers use different methods, so different brands of LMWH have differing ability to preferentially inhibit FXa over thrombin. The pentasaccharide fondaparinux is a designed synthetic molecule that is extremely specific for anti-Xa activity.
Specific Heparins and Heparin Derivatives
Unfractionated Heparin (UFH)
Unfractionated heparin is a heterogeneous (in both size and charge) mixture of heparin molecules derived from porcine intestinal mucosa. Depending on the preparation, the mean molecular weight is between 12,000 and 16,000 daltons. It can be administered either subcutaneously or intravenously. Intramuscular injection is not recommended due to local hematoma formation. It has immediate onset of activity when given intravenously, and is rapidly absorbed when given subcutaneously, resulting in rapid anticoagulation. Heparin molecules bind to a wide variety of proteins. As a result, the bioavailability of subcutaneous heparin can be quite variable, particularly in critical illness where acute phase and inflammatory protein levels can fluctuate. As a result UFH exhibits a variable dose-response relationship necessitating close monitoring by laboratory testing. Of all the types of heparins, use of UFH is the best characterized in both human and veterinary medicine. It is both widely available and extremely inexpensive, making it an attractive alternative. Protamine sulfate is a clinically available "antidote", allowing for immediate reversal of anticoagulant status when needed. Unfortunately, UFH has the highest variability in dose-response relationship, and consequently the greatest potential for over-anticoagulation. UFH also has the highest frequency of other adverse effects, in particular heparin induced thrombocytopenia (see below). The pharmacokinetics and dynamics of UFH have been studied in normal healthy cats and dogs at various doses. One study indicated that dogs with immune-mediated hemolytic anemia required much high (and more variable) doses of UFH subcutaneous to achieve a therapeutic plasma concentration.
Low Molecular Weight Heparins
LMWHs, like UFH, are chemically and functionally heterogenous in nature. All LMWHs are NOT created equal. They have distinct pharmacokinetic and pharmacodynamic profiles. They vary in potency, bioavailability, and tendency to accumulate. Consequently, one drug is not interchangeable for another. As a group, LMWHs all enhance AT inhibition of FXa more so than AT inhibition of thrombin. The relative effects against each serine protease are described by the ratio of anti-Xa activity/anti-thrombin activity. LMWHs, like UFH, induce the release of TFPI from endothelial surfaces, but to a lesser (and variable) degree than does UFH. In general LMWHs bind markedly less to proteins than does UFH. As a result, bioavailability and pharmacokinetics are more predictable. LMWHs are generally only administered subcutaneously. They tend to have longer half-lives so that less frequent dosing is required, making them more convenient. Protamine sulfate only partially reverses the anticoagulant effects of LMWHs. There is some investigation regarding use of a bacterially derived heparinase as an antidote. LMWHs tend to have a lower incidence of adverse effects than does UFH. Because these drugs are under patent and expensive to develop, they are extremely costly. Generic versions are likely to be available soon, but there are significant concerns regarding the likely equivalency of LMWH preparations made through different methods. The potential utility (if any) of generic LMWHs is an area of active debate in human medicine.
Enoxaparin is a LMWH preparation from Sonofi Aventis. It has a median molecular weight of 4800 daltons and an anti-Xa/anti-thrombin ratio of 3.3. It is supplied in prefilled syringes that contain no preservative as it is intended for single dose use. Pharmacokinetics and pharmacodynamics have been evaluated in normal healthy cats.
Dalteparin is a LMWH preparation from Pfizer. It has a median molecular weight of 5000 daltons and an anti-Xa/anti-thrombin ratio of 2.0. It is supplied in a multidose vial so is more practical for repeated administration to veterinary patients. Pharmacokinetics and pharmacodynamics have been evaluated in normal healthy cats and dogs.
Synthetic Heparin Derivatives
The development of synthetic derivatives arose out of the identification of the essential pentasaccharide sequence necessary for binding to AT. These drugs bind tightly to AT, resulting in slower elimination and consequently long biologic half-life. They have predictable pharmacokinetics and dose-response relationships. These designed agents are incapable of catalyzing the inhibition of thrombin by antithrombin. They also don't bind many of the other heparin binding proteins, resulting in elimination of many of the potential adverse effects of heparins. They are devoid of the ability to cause release of TFPI so there may be a difference in vivo mechanism of action as compared to heparins. These drugs are VERY expensive.
Fondaparinux is fairly new to the human market from Sanofi. Direct comparisons have indicated similar or better efficacy profiles to various LMWHs, but no reduction in the risk of bleeding.
Idraparinux (not yet available)
Idraparinux is currently in phase III clinical trials in humans. The primary potential advantage of idraparinux over other LMWHs is the favorable pharmacokinetic profile. It has an extremely long plasma half-life in humans, rats, dogs, and rabbits. This prolonged duration of effect enables use of a once weekly dosing protocol in humans, and the minimal inter-individual variation makes blood sample monitoring unnecessary in most people. An additional modification of a newer version of the drug will make it completely reversible.
Adverse Effects of Heparin Use
The most common adverse effect of heparin therapy is bleeding. In humans LMWHs are associated with less bleeding due to more predictable pharmacokinetics. Other potential adverse effects are a consequence of the ability of heparins to bind non-coagulation proteins. These include heparin-induced thrombocytopenia (HIT) and osteoporosis. These adverse effects are less common with LMWHs than with UFH.
The most feared adverse effect in humans is heparin-induced thrombocytopenia (HIT). The milder form of HIT is associated with a small (10-30%) reduction in platelet count that occurs within days of starting heparin in the heparin-naive patient, or within hours if previously exposed. The thrombocytopenia is of unclear mechanism and not generally clinically relevant. Although not specifically reported in the veterinary literature, many patients receiving heparin develop thrombocytopenia that could be associated with heparin therapy. Several studies in normal dogs have reported a mild decrease in platelet count. The more severe form of HIT is an immune-mediated event that occurs due to antibodies against a complex between heparin and platelet factor 4. It is associated with profound thrombocytopenia and thrombosis with severe morbidity and mortality. This form has not been specifically reported in veterinary patients.
Osteoporosis is generally a problem with very long-term heparin therapy (particularly in pregnant women). It has not been reported in the literature for veterinary patients, but the author observed pathologic fracture with loss of bone density in a cat receiving heparin for 18 months.
Monitoring Heparin Therapy
The traditional monitoring method is to evaluate the activated partial thromboplastin time (aPTT) for prolongation. Due to variations in the composition of aPTT reagents made by different manufacturers, there can be significant variability between the prolongation of aPTT as measured by different reagents, making it difficult to delineate specific target prolongation ranges as "therapeutic". Furthermore, the relationship between heparin concentration and prolongation of aPTT is not always linear. Lastly, because heparins cause release of TFPI from endothelial cells, measurement of in vitro heparin concentration may not adequately reflect degree of in vivo anticoagulation. Lastly, LMWHs and heparin derivatives do not reliably prolong the aPTT.
Heparin activity in plasma samples containing any type of heparin or heparin derivative can be measured by assessing the anti-FXa activity in a chromogenic assay. This assay is available commercially through the Cornell University Coagulation Laboratory. This test has reasonable performance characteristics, and a direct linear dose-response relationship between heparin concentration and inhibitory activity. While this test is highly specific for heparin activity against FXa, it does not necessarily reflect in vivo biologic activity for several reasons. Variation in patient antithrombin status can impact the biologic effect of a given dose of heparin. Additionally, since the test is specific for anti-FXa activity, it does not reflect any impact of in vivo activity against thrombin (especially for UFH) or any impact of TFPI release. However, because therapeutic plasma concentrations have been well defined based on studies of outcome in large numbers of human patients, this test is very helpful in targeting therapeutic plasma concentration in humans. The relationship between plasma concentration (as measured by anti-FXa activity) and outcome is less well-defined for veterinary species.
Thromboelastography is a real time dynamic method that evaluates the time course of clot formation in whole blood. It may have some utility in the future for monitoring heparin therapy.
Specific Data on Heparin Protocols For Use in Veterinary Medicine
Several studies have evaluated the pharmacokinetics of UFH, enoxaparin, and dalteparin in dogs and cats. Note however that, because all heparins and heparin derivatives are indirect inhibitors of coagulation (via their activity on AT), some variability in dose response is expected due to availability of AT. Due to the heparin binding capabilities of many other proteins that are increased in disease states, the dose-response relationships are likely much less predictable in sick animals than they would be in healthy individuals. No published studies have directly evaluated pharmacokinetics in sick populations.
Studies in Normal Dogs
UFH 200 U/kg as a single SC injection indicated adequate anticoagulation with a duration of up to 6 hours.
UFH after repeated subcutaneous injection of 500 U/kg either q8h or q12h suggested an initial dose of 500 U/kg followed by reduced doses q12h.
Dalteparin repeated subcutaneous injection at 150 U/kg q8h indicated adequate heparin levels.
UFH 250 U/kg q6h produced likely therapeutic anticoagulation
Enoxaparin 1 mg/kg SC q12 failed to induce sustainable anticoagulant activity
Dalteparin 100 U/kg SC q12h failed to induce sustainable anticoagulant activity
Thrombosis in Dogs with Immune-mediated Hemolytic Anemia (IMHA)
There is little agreement regarding therapy for prevention of coagulation activation in dogs. No outcome-based studies have been published to support any specific protocol for anticoagulation in canine patients. Prior descriptions of unfractionated heparin (UH) therapy in dogs with IMHA have reported doses of 50-200 U/kg SC, but higher doses may be needed. In one study, 49% of canine IMHA patients developed clinical signs consistent with TE, despite treatment with UH at 100-200 U/kg SC q8h. Thromboembolic events in spite of heparin therapy have been well described in human patients at risk, with sub-therapeutic heparin doses associated with a relative risk for TE of 6.0 to 22.2 (depending on the underlying disease process). Plasma heparin monitoring in dogs with IMHA has indicated that most of these patients required higher doses (up to 650 U/kg) to maintain therapeutic plasma levels.
Thrombosis in Cats with Arterial Thromboembolism (FATE)
No outcome-based studies have evaluated any heparin dose for cats with ATE, and recommendations are highly variable. Most cats receive unfractionated (UF) at either 50-100 U/kg ("low-dose") or 200-300 U/kg ("high-dose") q6-8h. In normal cats, an UF heparin dosage of 300 U/kg SQ q8h most consistently provides the plasma concentration associated with greatest clinical efficacy and least hemorrhagic complications in humans. However, in cats with ATE, there is wide individual variation in heparin pharmacokinetics with some cats requiring much higher dosages (up to 475 U/kg) to maintain plasma concentrations within the therapeutic range. The authors' current UF heparin dosage recommendation is 250-300 U/kg subcutaneously q8h. The first dose is administered IV in cats showing signs of shock.
Prospective, outcome based studies are needed to specifically assess the efficacy of appropriately dosed and monitored heparin therapy, as well as other anticoagulant options for prevention of thrombosis and thromboembolism in veterinary patients.
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