Antithrombotics: new drugs, new doses? (Proceedings)

Thromboembolic agents are indicated for a variety of conditions, ranging from disseminated intravascular coagulation to pulmonary thromboembolism in both dogs and cats. The selection of appropriate thromboembolic therapy will depend on the underlying condition and the goal of treatment. For example, if a thromboembolic obstruction has been diagnosed, then the goal of therapy is generally dissolution of the thrombus. However, a second goal of therapy for a primary thrombus is the prevention of additional thromboembolism due to fragmentation of the original lesion with subsequent blockage of the smaller, down-stream vessels. Similarly, prophylaxis of thromboembolic disease is frequently the primary goal of anticoagulant therapy in patients that are predisposed to thrombus formation, such as cats with hypertrophic cardiomyopathy. Thromboembolic agents can be broadly classified into agents that are thrombolytic and those that are anticoagulant. It is important to recognize that while anticoagulant therapy can indirectly support the lysis of pre-existing thrombi, anticoagulants cannot directly lyse a clot.

Thrombolytic agents include urokinase, streptokinase, and recombinant tissue plasminogen activators (rt-PA). The present thrombolytic agents share a common mechanism of action, serving to stimulate the endogenous system of thrombus removal. Normally, the initiation of thrombus degradation is by release of t-PA from endothelial cells that respond to signals such as vessel occlusion. The t-PA first binds to fibrin in the clot, than converts free plasminogen to the enzyme plasmin, which in turn will digest fibrin. The action of the thrombolytic agents tends to be nonspecific, such that both pathological fibrin clots and those at the sites of vascular injury are lysed. In addition, plasmin can degrade other plasma proteins, including coagulation cofactors, worsening the possibility of hemorrhage. As a consequence, hemorrhage is the most worrisome side effect associated with the use of thrombolytic agents and can occur at sites of catheter placement and trauma. For this reason, recent surgery, gastrointestinal bleeding, and hemorrhagic disorders are all contraindications for thrombolytic therapy. However, the thrombolytic agents do differ with respect to their specificity for fibrin. Streptokinase is the oldest member of this class of drugs and is produced from beta-hemolytic Streptococcus. Streptokinase facilitates the cleavage of plasminogen to plasmin. Because Streptococcus spp. exposure generally produces antibodies to streptokinase, streptokinase is immunogenic. High doses may be necessary to bind streptokinase antibody and allow high enough circulating concentrations of streptokinase for drug efficacy. In addition to the risk of hemorrhage due to the lack of specificity for fibrin degradation, streptokinase may also be associated with hypersensitivity reactions. Only the low molecular weight form of urokinase is presently available in the U.S., as an injectable drug approved for the treatment of pulmonary emboli in humans. Urokinase is derived from human donor neonatal kidney cells, making it an expensive preparation with major supply issues. Like streptokinase, urokinase is also not selective for fibrin, making it unattractive from the standpoint of both side effects and cost. The use of t-PA has largely supplanted that of streptokinase and urokinase in human medicine. By acting similarly to endogenous t-PA, rt-PA products (such as alteplase and reteplase) provide a greater measure of safety as compared to the streptokinase and urokinase. Because endogenous t-PA only converts plasminogen to plasmin in the presence of fibrin, systemic lysis of proteins does not occur. However, physiological thrombi at the site of vascular injury will still be affected by low concentrations of t-PA. In addition, supra-physiological concentrations of rt-PA occur during drug therapy, such that hemorrhage is still the major side effect associated with the administration of rt-PA. Therapy with rt-PA is also very expensive. Alteplase has been used in companion animals with thromboembolic disease,1,2 although evidence for its safety and efficacy in presently incomplete.3 Similarly, retrospective studies of streptokinase administration in companion animals with arterial thromboembolic disease have not provided strong evidence of increase survival.3,4 Overall, clinical evidence for the efficacy of thrombolytic therapy for the treatment of thromboembolic disease in veterinary species is primarily retrospective and inconclusive, but does not support a robust treatment effect.

Anticoagulant, antiplatelet agents include aspirin, ticlopidine, and clopidogrel. These agents interfere the formation of thrombi and as such may be used both to keep thrombi from forming or worsening. Clot formation requires numerous factors and co-factors, presenting numerous pharmacological targets for an anticoagulant effect. Upon exposure of collagen by endothelial cell injury, activation of von Willebrand factor allows binding of collagen to platelet adhesive glycoprotein receptors (e.g., GPIa/IIa, GPIb) which produces several downstream events, such as the formation of thromboxane (TXA2) and adenosine diphosphate (ADP). In addition to other release factors, the localized actions of TXA2 and ADP amplify hemostasis by recruiting and activating additional platelets to the growing thrombus. The eicosanoids TXA2 and prostacyclin (PGI2) are eicosanoid products of COX-1 that are balanced against one another to affect appropriate homeostasis. Platelets generate TXA2 for a pro-coagulant effect, whereas endothelial cells produce PGI2 for an anti-coagulant effect. Although other nonselective nonsteroidal anti-inflammatory drugs (NSAIDs) besides aspirin also inhibit the formation of eicosanoid products, aspirin is distinct in its irreversible inhibition of cyclooxygenase-1 (COX-1). As platelets do not synthesize new proteins, COX-1 is effectively inhibited for the lifespan of the platelet, thus losing its ability to produce TXA2. This inhibition of platelet COX-1 occurs at low aspirin concentrations. In contrast, the endothelial cell COX-1 is not similarly inhibited or is able to be rapidly turned over, such that the vasodilatory and anticoagulant eicosanoid PGI2 is still active. It is the relative difference in TXA2 and PGI2 concentrations associated with low-doses of aspirin that appears to be responsible for aspirin's anticoagulant effect. This effect may be decreased by high doses of aspirin or by the concomitant administration of additional NSAIDs. Although aspirin has been used with apparent safety in cats with a history of thrombotic episodes, its efficacy at preventing future episodes has been questionable in several retrospective studies.5,6 A second class of antiplatelet drugs act by inhibiting purinergic receptors (P2Y1/P2Y12) that respond to localized ADP release by activated platelets. Inhibition of these purinergic receptors by ticlopidine and clopidogrel prevents the activation and recruitment of additional platelets. Like aspirin, the biological half-lives of ticlopidine and clopidogrel are longer than their pharmacokinetic half-lives, due to irreversible inhibition of the purinergic receptor. Clopidogrel appears to inhibit platelet aggregation more strongly than does aspirin, but the combination of clopidogrel and aspirin produces a synergistic effect.7 This synergy has been utilized to prevent thromboembolic disease in high-risk populations of humans with heart disease, but is also more likely to cause hemorrhage and is detrimental in some disease states.8 Whether such a synergistic effect would be beneficial in veterinary species at high risk of thromboembolic disease, such as cats with hypertrophic cardiomyopathy and thrombi, has not been determined. Whereas, preliminary studies of clopidogrel and ticlopidine efficacy have been encouraging in veterinary species, ticlopidine also appears to be associated with unacceptable gastrointestinal toxicity in cats.9-11 A more definitive clinical trial of the efficacy of clopidogrel versus that of aspirin in the prevention of second thromboembolic events in cats is currently underway.

Other anticoagulant agents include unfractionated heparin, low molecular weight heparin, and warfarin. Like the antiplatelet anticoagulants, these agents are used for the therapy of both existing thromembolism and for the prophylaxis of thrombi formation. However, the rationale for using these drugs to treat an existing thrombus is the prevention of further thrombus expansion and the formation of new thrombi downstream from a fragmented thrombus. In this case, limitation of thrombus expansion may allow endogenous thrombolytic mechanisms to dissolve the existing clot. Both unfractionated heparin and low molecular weight heparin are used for this purpose. Unfractionated heparin is derived from porcine tissue and is a mixture of sulfated, anionic, and polysufated glycosaminoglycans. This heterogeneous mixture consists of glycosaminoglycans with differing molecular weights, action, and pharmacokinetic properties. Because there may be batch to batch variation in heparin composition, the response to heparin administration may also be unpredictable. Due to this heterogeneity, only a fraction of the heparin may bind to antithrombin III (ATIII). It is by binding to and activating ATIII that heparin acts as an anticoagulant, as ATIII inactivates thrombin (Factor IIa) and Factor Xa. Intact heparin is poorly absorbed from the gastrointestinal tract, so it is given parenterally by intravenous of subcutaneaous injection. The major adverse effect associated with heparin administration is hemorrhage, although protamine can be given as an antidote to heparin overdose. Both the activated partial thromboplastin time (APTT) and activated clotting time (ACT) test the intrinsic and common pathway, and so can be used to monitor the anticoagulant activity of heparin therapy. However, the dose of unfractionated heparin is best monitored using the APTT. In addition to the predictable increase in bleeding and hemorrhage expected from heparin administration, recent recalls due to anaphylaxis from unfractionated heparin preparations are particularly troubling. Hypersensitivity appeared to be associated with oversulphation of the related glycosaminoglycan, chondroitin sulfate, which contaminated the preparations.12 The FDA responded to numerous anaphylactoid adverse events in humans by increasing the inspection of the overseas suppliers of heparin, but this incident has exposed some of the safety issues involved in the remote manufacture and importation of drugs. An alternative to unfractionated heparin is the use of low molecular weight heparin (LMWH). LMWH is purified from unfractionated heparin to produce a heparin with more uniform molecular size. This uniformity is associated with more predictable pharmacokinetic behavior and activity than unfractionated heparin, although LMWH has also been contaminated with oversulphated chondroitin sulfate. LMWH primarily inactivates factor Xa, such that the APTT test is not accurate for monitoring response to therapy. Factor Xa inhibitory activity (anti-Xa activity) can instead be measured to assess the response to LMWH therapy.13 Although LMWH products are uniform within preparations, these agents differ considerably between different manufacturers and trade names, so direct substitution requires monitoring and dose adjustment. In addition the shorter biological half-life of LMWH as compared to unfractionated heparin necessitates more frequent dosing of LMWH to cats.13 Warfarin is used as an anticoagulant that is similar to products used as rodenticides because it is capable of producing a profound anticoagulant effect at high enough doses. Warfarin acts by inhibiting the hepatic synthesis of vitamin K-dependent clotting factors (II, VII, IX, X). Vitamin K is necessary for postribosomal carboxylation of precursors to active clotting factors. Warfarin is orally administered and highly protein bound (>90%). It is eliminated by hepatic cytochrome P450 enzymes, and is subject to potentially dangerous drug interactions.14 Hemorrhage is the major adverse effect associated with warfarin administration and its activity can be monitored using prothrombin time (PT) or international normalized ratio (INR).15 The target PT is 1.5-2 times normal and the typical onset of action occurs at 8-12 hours after administration. If an overdose of warfarin is given, vitamin K1 can be used as an antidote.

REFERENCES

1. Clare AC, Kraje BJ. Use of recombinant tissue-plasminogen activator for aortic thrombolysis in a hypoproteinemic dog. J Am Vet Med Assoc 1998;212(4):539-43.

2. Pion PD. Feline aortic thromboemboli and the potential utility of thrombolytic therapy with tissue plasminogen activator. Vet Clin North Am Small Anim Pract 1988;18(1):79-86.

3. Lunsford KV, Mackin AJ. Thromboembolic therapies in dogs and cats: an evidence-based approach. Vet Clin North Am Small Anim Pract 2007;37(3):579-609.

4. Moore K, Morris N, Dhupa N, et al. Retrospective study of streptokinase administration in 46 cats with arterial thromboembolism. J Vet Emerg Crit Care 2000;10(4):245-257.

5. Fox PR. Evidence for or against efficacy of beta-blockers and aspirin for management of feline cardiomyopathies. Vet Clin North Am Small Anim Pract 1991;21(5):1011-22.

6. Schoeman JP. Feline distal aortic thromboembolism: a review of 44 cases (1990-1998). J Feline Med Surg 1999;1(4):221-31.

7. Moshfegh K, Redondo M, Julmy F, et al. Antiplatelet effects of clopidogrel compared with aspirin after myocardial infarction: enhanced inhibitory effects of combination therapy. J Am Coll Cardiol 2000;36(3):699-705.

8. Fisher M, Folland E. Acute ischemic coronary artery disease and ischemic stroke: similarities and differences. Am J Ther 2008;15(2):137-49.

9. Hogan DF, Andrews DA, Green HW, et al. Antiplatelet effects and pharmacodynamics of clopidogrel in cats. J Am Vet Med Assoc 2004;225(9):1406-11.

10. Hogan DF, Ward MP. Effect of clopidogrel on tissue-plasminogen activator-induced in vitro thrombolysis of feline whole blood thrombi. Am J Vet Res 2004;65(6):715-9.

11. Hogan DF, Andrews DA, Talbott KK, et al. Evaluation of antiplatelet effects of ticlopidine in cats. Am J Vet Res 2004;65(3):327-32.

12. Kishimoto TK, Viswanathan K, Ganguly T, et al. Contaminated heparin associated with adverse clinical events and activation of the contact system. N Engl J Med 2008;358(23):2457-67.

13. Alwood AJ, Downend AB, Brooks MB, et al. Anticoagulant effects of low-molecular-weight heparins in healthy cats. J Vet Intern Med 2007;21(3):378-87.

14. Trepanier LA. Cytochrome P450 and its role in veterinary drug interactions. Veterinary Clinics of North America-Small Animal Practice 2006;36(5):975-+.

15. Monnet E, Morgan MR. Effect of three loading doses of warfarin on the international normalized ratio for dogs. Am J Vet Res 2000;61(1):48-50.