SEMINARS IN THROMBOSIS AND HEMOSTASIS—VOLUME 18, NO. 2, 1992
Clinical Application of the Synthetic Thrombin Inhibitor, Argatroban (MD-805)
Synthetic thrombin inhibitors, which are very selective for thrombin, are expected to show promise as thrombolytic agents. Among the various synthetic thrombin inhibitors, including those under development at present, argatroban (MD-805) is an arginine derivative that has been found to show a potent antithrombin effect (Ki = 0.019 µmol) and has no antifibrinolytic activity. In comparative experiments using heparin, it was found to be effective in treating and preventing experimental arterial thrombosis and disseminated intravascular coagulation (DIC).1 Although its efficacy is difficult to compare directly to that of heparin due to the different mechanisms of their anticoagulant action, argatroban offers some advantages, including no requirement for antithrombin III (AT III) as a cofactor and avoidance to the effects of heparin-neutralizing substances such as platelet factor 4 (PF4) and histidine-rich glycoprotein or heparinase enzymes. Synthetic thrombin inhibitors, which very selectively inhibit thrombin, are also less likely to cause hemorrhage than unfractionated heparin. The incidence of hemorrhage due to low molecular weight heparin, which mainly shows an anti-Xa effect, has been found to be lower than that of hemorrhage due to unfractionated heparin, which acts on both factors Xa and IIa. Thrombin strongly induces platelet aggregation through binding with glycoprotein Ib (GPI) on the platelet membrane, and argatroban has been shown to inhibit this action of thrombin completely.2 Thrombin affects the vascular smooth muscle and causes strong vasoconstriction at optimal concentrations. Argatroban has also been reported to decrease dose-dependently experimental constriction of the
From the Hyogo Prefectural Awaji Hospital, Sumoto, Japan, and the Kobe Saiseikai General Hospital, Kobe, Japan. Reprint requests: Dr. Matsuo, Hyogo Prefectural Awaji Hospital, Sumoto 656, Japan.
cerebral arteries induced by thrombin in an animal model.3 Because some synthetic thrombin inhibitors having more or less antifibrinolytic action, these drugs may interfere with resolution of thrombi and induce multiple organ failure in DIC with an impaired fibrinolysis. Argatroban, which is more selective for thrombin than the other drugs, may be classified as second-generation thrombolytic agents. Since argatroban exerts an antithrombin effect in the presence of AT III deficiency, we have clinically used it for the anticoagulation in AT Ill-deficient patients. This article reports the results obtained at our hospital so far. It also presents recent evidence related to platelet antibodies, which suggest that argatroban is useful not only for the treatment of heparin-induced thrombocytopenia, a severe thrombotic side effect of heparin, but is also a viable alternative anticoagulant to heparin.
ANTITHROMBIN III DEFICIENCY Substitution therapy using AT III concentrate is easily conducted for the management of congenital AT III deficiency associated with deep vein thrombosis. The agent has even enabled AT Ill-deficient women to have a normal pregnancy and delivery. AT III concentrate is often administered together with heparin to AT Ill-deficient patients with thrombosis. However, the risk of hemorrhage is increased due to the development of the ordinary action of heparin as blood AT III levels return to normal. Therefore, the use of a synthetic thrombin inhibitor has been proposed as an alternative to AT III concentrate. In animal models of AT III deficiency produced with anti-AT III gamma globulin, thrombosis is prone to
Copyright © 1992 by Thieme Medical Publishers, Inc., 381 Park Avenue South, New York, NY 10016. All rights reserved.
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TAKEFUMI MATSUO, M.D., KAZUOMI KARIO, M.D., KAZUYA KODAMA, M.D., and SHOUSUKE OKAMOTO, M.D.
SEMINARS IN THROMBOSIS AND HEMOSTASIS—VOLUME 18, NO. 2, 1992
occur due to a decrease in the AT III level, and argatroban has been found to be effective in preventing such thrombosis.4 Furthermore, argatroban appears to minimize the consumption of AT III in the blood, since twodimensional Immunoelectrophoresis shows a decrease in the peak representing AT III-protease complexes after argatroban therapy. We administered argatroban to a patient with congenital AT III deficiency who required hemodialysis due to renal failure resulting from thrombosis in both renal veins subsequent to recurrent deep vein thrombosis. Clot formation in the extracorporeal circuit was a frequent problem, although full-dose heparinization was used in an attempt to prevent blood coagulation during hemodialysis. However, hemodialysis could be carried out without any problems when heparin was used in combination with AT III concentrate. Antithrombin therapy by the continuous infusion of argatroban during hemodialysis also allowed a smooth session of dialysis without clot formation in the circuit. The antithrombin activity in the blood during argatroban hemodialysis was determined using a synthetic substrate (S-2238) and was found to be comparable to that determined during the use of AT III concentrate. This result suggests that argatroban is equivalent in effect to AT III concentrate when used to prevent clot formation during hemodialysis in AT III-deficient patients.5 Argatroban has been found to inhibit the production of thrombin-AT III complexes6 (TAT), perhaps due to competitive binding of argatroban to the AT III-binding site on the active center of thrombin because argatroban binds strongly to thrombin at the same site as AT III. This reaction results in the formation of a thrombin-argatroban complex and consequently reduces the production of TAT. Figure 1 shows the concentration of TAT in plasma allowed to stand at room temperature after the addition of argatroban to whole blood. Production of TAT was reduced in a dose-dependent manner by argatroban, with complete inhibition by argatroban at a final concentration of 10 µg/ml. This suggested that AT III was protected from consumption in the formation of TAT by argatroban. Therefore this drug can be used for the normalization of pathologically high levels of TAT, which are often associated with a hypercoagulable state. Figure 2 illustrates changes in the level of TAT in an AT Illdeficient patient on hemodialysis who received either argatroban or heparin with AT III concentrate as the anticoagulant for the extracorporeal circulation. The decrease in TAT induced by argatroban was comparable to that produced by AT III concentrate plus heparin, suggesting that argatroban is effective in reducing the high plasma levels of TAT produced by hypercoagulable states.
FIG. 1. Prevention of thrombin-antithrombin III (TAT) com plex formation by addition of argatroban to whole blood. Argatroban concentration :□—□; 1.25 µg/ml; ■—■ : 2.5 µg/ ml;○—○: 5 µg/ml; •—• : 10 µg/ml.
HEMODIALYSIS AND ARGATROBAN Heparin is the most popular anticoagulant used during hemodialysis. Heparin stimulates platelets, and a bolus of heparin has been shown to increase transiently both PF4 and beta-thromboglobulin (BTG) release. Figure 3 illustrates the transient release of PF4 observed after a bolus of heparin in normal volunteers. This transient PF4 release is not inhibited by preincubation with aspirin. In contrast, a bolus of argatroban did not induce PF4 release. Argatroban also failed to release BTG, although a bolus of heparin induced its transient release.7 Therefore heparin can aggravate hypercoagulability by stimulating platelets and consequently increasing PF4, which has a heparin-neutralizing effect, especially in patients with sepsis who already have activated platelets. In addition, the platelet-stimulating effect of heparin is believed to contribute to the hyperaggregation shown by platelet function tests analyzed during hemodialysis.8 Kumon et al9 administered argatroban for postoperative DIC after open heart surgery, and reported that the drug was as effective as heparin in promptly prolonging the activated partial thromboplastin time (APTT) and improving the platelet count.
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FIG. 2. Thrombin-antithrombin III (AT III) complex (TAT) levels before (□) and after ( ) hemodialysis with five anticoagulant regimens in an AT III deficient patient.5
The blood argatroban level needed to increase the APTT to an extent similar to heparin (1.5- to 2.5-fold) has been found to range between 0.23 and 0.75 µg/ml by an in vitro comparative study. Since a recovery test after the addition of an estimated dose of argatroban has suggested the probability of its nonspecific inactivation in the blood, periodic monitoring of the anticoagulant activity of this drug is recommended during anticoagulation with argatroban.10 The plasma level as an equivalent of the antithrombin activity was determined using a synthetic substrate (Tos-Gly-Pro-Arg-pNA) during hemodialysis with the continuous infusion of argatroban. The plasma level of argatroban was 2.2 ± 0.5 µg/ml
FIG. 3. Differences of the platelet factor 4 (PF4) release in duced by heparin (•—•), and argatroban (○—○) (mean ± SD).
(mean ± SD) and APTT increased 2.4 ± 0.4 times at 240 minutes after the administration of 10 mg as a bolus plus 20 mg/hr as a continuous infusion.11 These findings also indicated that larger doses were required in vivo than would be expected from in vitro estimates. We have been routinely using the previously mentioned argatroban regimen for anticoagulation instead of heparin when hemodialysis is required in patients with heparin-induced thrombocytopenia.12
ARGATROBAN AND HEPARIN-INDUCED THROMBOCYTOPENIA Heparin-dependent antiplatelet antibodies (heparin antibodies) are found in the plasma during heparininduced thrombocytopenia (HIT). In vivo platelet aggregation and destruction occur following the binding of heparin with such heparin antibodies, resulting in thrombocytopenia that is sometimes associated with severe arterial thrombosis. Heparin antibodies are mainly thought to exist in the immunoglobulin G (IgG) fraction. Platelet aggregation, the release of serotonin and BTG, and an increase of thromboxane A2 from normal platelet-rich plasma are observed when heparin is added with patient IgG. In patients with HIT, platelet-binding IgG (PBIgG) increases more markedly than platelet-associated IgG. The increase of PBIgG is an important early laboratory finding in HIT, when a 20-fold increase has been observed following the addition of heparin to patient plasma.13 Figure 4 shows the increase of the PBIgG level observed in HIT. The increase remains slight without the addition of heparin, but a rather marked increase is seen following the addition of heparin from the same lot as used previously for the patient. Also, an increase of the
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SEMINARS IN THROMBOSIS AND HEMOSTASIS—VOLUME 18, NO. 2, 1992
FIG. 4. Increase of platelet binding immunoglobulin G (PBIgG) induced by two kinds of heparin, and the inhibitory effect by argatroban after its addition to a mixture of heparin-induced thrombocytopenia plasma and heparin.
PBIgG level was found to be produced by low molecular weight heparin. The increase of the PBIgG level due to heparin was clearly suppressed when argatroban was previously added to plasma from the HIT patient at a final concentration of 2.2 µg/ml. Although a Fc receptor-mediated mechanism has been proposed to explain the heparin antibody-binding sites on the platelet membrane, a study has shown that the von Willebrand factor-binding site on GPIb is closely related to the heparin antibody-binding site. Since the binding of heparin antibodies to the von Willebrand factor site does not in itself cause platelet activation, multiple heparin antibody binding sites on the platelet membrane, including the Fc receptor site, have been postulated to be involved when heparin induces platelet aggregation.14 Since it was observed in one case of HIT that actual heparin-induced aggregation was completely inhibited by the pretreatment with anti-GPIb antibody, GPIb is thought to be closely related to the induction of platelet aggregation in HIT. Argatroban administered immediately after the onset of HIT was a very effective replacement for heparin. Heparin-induced platelet aggregation was also inhibited after the in vitro addition of argatroban.15 Argatroban was found to inhibit heparininduced aggregation in a dose-dependent manner in the IgG fraction from a HIT patient, and the ex vivo increase
of PBIgG due to the addition of heparin was completely abolished by the infusion of argatroban.16 Although the exact mechanism of the inhibitory effect of argatroban on the function of GPIb remains unknown, the drug may competitively interfere with thrombin to the thrombinbinding site in GPIb. Also, hirudin is an antithrombin derivative that has been found to inhibit heparin-induced platelet aggregation, suggesting that local thrombin formation on the platelet membrane during platelet activation plays an important role in the induction of such heparin-induced aggregation.17 These findings suggest that small molecular antithrombin derivatives prevent heparin-induced platelet aggregation in HIT.
TREATMENT OF HEPARIN-INDUCED THROMBOCYTOPENIA WITH ARGATROBAN No other effective therapy for HIT except the immediate discontinuation of heparin has been recommended so far. Although oral anticoagulants such as warfarin have been used in replacement of heparin, these anticoagulants require a relatively long time before the initiation of their action and so cannot be used for patients requiring immediate anticoagulation, such as HIT pa-
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monomer complex, a fibrinogen degradation level of 1280 µg/ml, and platelet aggregation induced by heparin were observed in the 8th session, although the dose of heparin was increased to overcome clot formation. The 9th dialysis session, in which argatroban was continuously infused at 25 mg/hr instead of heparin, could be completed without clot formation in the circuit. Although, the PBIgG level in the 8th session increased to 229.3 ng/107 platelets (normal: 83.1 ± 29.7), no significant release of both PF4 and BTG were observed during argatroban. Thus, argatroban was effective as an alternative anticoagulant for the treatment of HIT in a patient requiring immediate anticoagulation. Discontinuation of heparin must be considered as the first line of treatment for HIT. However, when immediate anticoagulation is absolutely necessary despite the presence of HIT, the replacement of heparin with argatroban is recommended.
REFERENCES
FIG. 5. Immediate resolution of heparin-induced thrombo cytopenia after replacement of heparin with argatroban dur ing the 9th hemodialysis session.
tients need with hemodialysis. In the hemodialysis patient with HIT, the combined use of aspirin with heparin may be effective in preventing the exacerbation of this complication. In a HIT hemodialysis patient for whom 1.0 gm of oral aspirin was effective, the high PBIgG level at the onset of HIT was decreased by aspirin therapy and heparin could be continued with no problems.18 When one HIT patient required cardiac surgery with extracorporeal circulation, the combined use of aspirin and dipyridamole allowed full-dose heparin to be given.19 However, aspirin is not effective in all HIT patients. There are a few reports of cases in which HIT did not respond, even though aspirin inhibited heparin-induced platelet aggregation ex vivo.20 Other drugs that are under study as therapeutic agents for HIT include prostacyclin, low molecular weight heparin, and low molecular weight heparinoid, but none of these drugs are yet established for the clinical resolution of this complication. There is much controversy over the effect of low molecular weight heparin, and it cannot be generally recommended for HIT.21 Figure 5 shows the clinical course of a HIT hemodialysis patient with acute renal failure, in whom thrombocytopenia and clot formation in the hemodialysis circuit occurred during the 7th dialysis session, although the APTT was sufficiently prolonged. Clot formation in the circuit, thrombocytopenia (from 27.1 x 104/µl on admission to 22.7 x 104/µl), a positive soluble fibrin
1. Hara H, Y Tamao, R Kikumoto, S Okamoto: Effect of a synthetic thrombin inhibitor MCI-9038 on experimental models of disseminated intravascular coagulation in rabbits. Thromb Haemost 57:165-170, 1987. 2. Nakamura K, Y Hatano, K Mori: Thrombin-induced vasoconstriction in isolated cerebral arteries and the influence of a synthetic thrombin inhibitor. Thromb Res 40:715-720, 1985. 3. Czervionke RL, JC Hoak, DL Haycraft, GL Fry: Inhibition of thrombin-induced platelet adherence to vascular cells by synthetic thrombin inhibitor No. 805. Trans Assoc Am Physicians 96:271277, 1983. 4. Kumada T, Y Abiko: Comparative study on heparin and a synthetic thrombin inhibitor No. 805 (MD-805) in experimental antithrombin Ill-deficient animals. Thromb Res 24:285-298, 1981. 5. Matsuo T, T Yamada, T Yamanashi, K Kodama: Choice of anticoagulant in a congenital antithrombin III (AT-III)-deficient patient with chronic renal failure undergoing regular haemodialysis. Clin Lab Haematol 11:213-219, 1989. 6. Hijikata-Okunomiya A, S Okamoto, K Wanaka: Effect of a synthetic thrombin-inhibitor MD805 on the reaction between thrombin and plasma antithrombin-III. Thromb Res 59:967-977, 1990. 7. Matsuo T, T Yamada, K Nakao: Effects of a new synthetic anticoagulant (MD805) on platelets. Acta Haematol Jpn 50:654-657, 1987. 8. Charvat J, J Konig, J Blaha: Is heparin responsible for enhanced platelet aggregation after haemodialysis? Nephron 44:89-91, 1986. 9. Kumon K, K Tanaka, N Nakajima, Y Naito: Anticoagulant with a synthetic thrombin inhibitor after cardiovascular surgery and for treatment of disseminated intravascular coagulation. Crit Care Med 12:1039-1043, 1984. 10. Matsuo T, C Shimano, Y Ohoki: Monitoring of a synthetic thrombin inhibitor (MD805). J Jpn Soc Dial Ther 19:1005-1008, 1986. 11. Matsuo T, K Nakao: Plasma antithrombin activity of MD805 determined by chromogenic substrate during hemodialysis. Blood Vessel 18:378-380, 1987. 12. Matsuo T, K Nakao, T Yamada, O Matsuo: A new thrombin inhibitor MD805 and thrombocytopenia encountered with heparin hemodialysis. Thromb Res 44:247-251, 1986.
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13. Lynch DM, SE Howe: Heparin-associated thrombocytopenia: Antibody binding specificity to platelet antigens. Blood 66:1176— 1181, 1985. 14. Adelman B, M Sobel, Y Fujimura, ZM Rugeri, TS Zimmerman: Heparin-associated thrombocytopenia: Observations on the mechanism of platelet aggregation. J Lab Clin Med 113:204-210, 1989. 15. Matsuo T, T Yamada, T Yamanashi, R Ryo: Anticoagulant therapy with MD805 of a hemodialysis patient with heparin-induced thrombocytopenia. Thromb Res 58:663-666, 1990. 16. Matsuo T, Y Chikahira, T Yamada, K Nakao, S Ueshima, O Matsuo: Effect of synthetic thrombin inhibitor (MD805) as an alternative drug on heparin induced thrombocytopenia during hemodialysis. Thromb Res 52:165-171, 1988. 17. Brace LD, J Fareed, J Walenga: Heparin-induced platelet aggregation (H-IPA) is not observed in hirudin anticoagulated platelet-rich plasma. (Abst.) Thromb Haemost 62:416, 1989.
18. Matsuo T, T Yamada, Y Chikahira, T Kadowaki: Effect of aspirin on heparin-induced thrombocytopenia (HIT) in a patient requiring hemodialysis. Blut 59:393-395, 1989. 19. Makhoul RG, RL McCann, EH Austin, CS Greenberg, JE Lowe: Management of patients with heparin-associated thrombocytopenia and thrombosis requiring cardiac surgery. Ann Thorac Surg 43:617-621, 1987. 20. Kappa JR, CA Fisher, HD Berkowitz, ED Cottrell, VP Addonizio: Heparin-induced platelet activation in sixteen surgical patients: Diagnosis and management. J Vasc Surg 5:101-107, 1987. 21. Gouault-Heilmann M, Y Hent, S Adnot, G Contant, F Bonnet, L Intrador, D Payen, M Levent: Low molecular weight heparin fractions as an alternative therapy in heparin-induced thrombocytopenia. Haemostasis 17:134-140, 1987.
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Clinical Application of the Synthetic Thrombin Inhibitor, Argatroban (MD-805)
Synthetic thrombin inhibitors, which are very selective for thrombin, are expected to show promise as thrombolytic agents. Among the various synthetic thrombin inhibitors, including those under development at present, argatroban (MD-805) is an arginine derivative that has been found to show a potent antithrombin effect (Ki = 0.019 µmol) and has no antifibrinolytic activity. In comparative experiments using heparin, it was found to be effective in treating and preventing experimental arterial thrombosis and disseminated intravascular coagulation (DIC).1 Although its efficacy is difficult to compare directly to that of heparin due to the different mechanisms of their anticoagulant action, argatroban offers some advantages, including no requirement for antithrombin III (AT III) as a cofactor and avoidance to the effects of heparin-neutralizing substances such as platelet factor 4 (PF4) and histidine-rich glycoprotein or heparinase enzymes. Synthetic thrombin inhibitors, which very selectively inhibit thrombin, are also less likely to cause hemorrhage than unfractionated heparin. The incidence of hemorrhage due to low molecular weight heparin, which mainly shows an anti-Xa effect, has been found to be lower than that of hemorrhage due to unfractionated heparin, which acts on both factors Xa and IIa. Thrombin strongly induces platelet aggregation through binding with glycoprotein Ib (GPI) on the platelet membrane, and argatroban has been shown to inhibit this action of thrombin completely.2 Thrombin affects the vascular smooth muscle and causes strong vasoconstriction at optimal concentrations. Argatroban has also been reported to decrease dose-dependently experimental constriction of the
From the Hyogo Prefectural Awaji Hospital, Sumoto, Japan, and the Kobe Saiseikai General Hospital, Kobe, Japan. Reprint requests: Dr. Matsuo, Hyogo Prefectural Awaji Hospital, Sumoto 656, Japan.
cerebral arteries induced by thrombin in an animal model.3 Because some synthetic thrombin inhibitors having more or less antifibrinolytic action, these drugs may interfere with resolution of thrombi and induce multiple organ failure in DIC with an impaired fibrinolysis. Argatroban, which is more selective for thrombin than the other drugs, may be classified as second-generation thrombolytic agents. Since argatroban exerts an antithrombin effect in the presence of AT III deficiency, we have clinically used it for the anticoagulation in AT Ill-deficient patients. This article reports the results obtained at our hospital so far. It also presents recent evidence related to platelet antibodies, which suggest that argatroban is useful not only for the treatment of heparin-induced thrombocytopenia, a severe thrombotic side effect of heparin, but is also a viable alternative anticoagulant to heparin.
ANTITHROMBIN III DEFICIENCY Substitution therapy using AT III concentrate is easily conducted for the management of congenital AT III deficiency associated with deep vein thrombosis. The agent has even enabled AT Ill-deficient women to have a normal pregnancy and delivery. AT III concentrate is often administered together with heparin to AT Ill-deficient patients with thrombosis. However, the risk of hemorrhage is increased due to the development of the ordinary action of heparin as blood AT III levels return to normal. Therefore, the use of a synthetic thrombin inhibitor has been proposed as an alternative to AT III concentrate. In animal models of AT III deficiency produced with anti-AT III gamma globulin, thrombosis is prone to
Copyright © 1992 by Thieme Medical Publishers, Inc., 381 Park Avenue South, New York, NY 10016. All rights reserved.
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SEMINARS IN THROMBOSIS AND HEMOSTASIS—VOLUME 18, NO. 2, 1992
occur due to a decrease in the AT III level, and argatroban has been found to be effective in preventing such thrombosis.4 Furthermore, argatroban appears to minimize the consumption of AT III in the blood, since twodimensional Immunoelectrophoresis shows a decrease in the peak representing AT III-protease complexes after argatroban therapy. We administered argatroban to a patient with congenital AT III deficiency who required hemodialysis due to renal failure resulting from thrombosis in both renal veins subsequent to recurrent deep vein thrombosis. Clot formation in the extracorporeal circuit was a frequent problem, although full-dose heparinization was used in an attempt to prevent blood coagulation during hemodialysis. However, hemodialysis could be carried out without any problems when heparin was used in combination with AT III concentrate. Antithrombin therapy by the continuous infusion of argatroban during hemodialysis also allowed a smooth session of dialysis without clot formation in the circuit. The antithrombin activity in the blood during argatroban hemodialysis was determined using a synthetic substrate (S-2238) and was found to be comparable to that determined during the use of AT III concentrate. This result suggests that argatroban is equivalent in effect to AT III concentrate when used to prevent clot formation during hemodialysis in AT III-deficient patients.5 Argatroban has been found to inhibit the production of thrombin-AT III complexes6 (TAT), perhaps due to competitive binding of argatroban to the AT III-binding site on the active center of thrombin because argatroban binds strongly to thrombin at the same site as AT III. This reaction results in the formation of a thrombin-argatroban complex and consequently reduces the production of TAT. Figure 1 shows the concentration of TAT in plasma allowed to stand at room temperature after the addition of argatroban to whole blood. Production of TAT was reduced in a dose-dependent manner by argatroban, with complete inhibition by argatroban at a final concentration of 10 µg/ml. This suggested that AT III was protected from consumption in the formation of TAT by argatroban. Therefore this drug can be used for the normalization of pathologically high levels of TAT, which are often associated with a hypercoagulable state. Figure 2 illustrates changes in the level of TAT in an AT Illdeficient patient on hemodialysis who received either argatroban or heparin with AT III concentrate as the anticoagulant for the extracorporeal circulation. The decrease in TAT induced by argatroban was comparable to that produced by AT III concentrate plus heparin, suggesting that argatroban is effective in reducing the high plasma levels of TAT produced by hypercoagulable states.
FIG. 1. Prevention of thrombin-antithrombin III (TAT) com plex formation by addition of argatroban to whole blood. Argatroban concentration :□—□; 1.25 µg/ml; ■—■ : 2.5 µg/ ml;○—○: 5 µg/ml; •—• : 10 µg/ml.
HEMODIALYSIS AND ARGATROBAN Heparin is the most popular anticoagulant used during hemodialysis. Heparin stimulates platelets, and a bolus of heparin has been shown to increase transiently both PF4 and beta-thromboglobulin (BTG) release. Figure 3 illustrates the transient release of PF4 observed after a bolus of heparin in normal volunteers. This transient PF4 release is not inhibited by preincubation with aspirin. In contrast, a bolus of argatroban did not induce PF4 release. Argatroban also failed to release BTG, although a bolus of heparin induced its transient release.7 Therefore heparin can aggravate hypercoagulability by stimulating platelets and consequently increasing PF4, which has a heparin-neutralizing effect, especially in patients with sepsis who already have activated platelets. In addition, the platelet-stimulating effect of heparin is believed to contribute to the hyperaggregation shown by platelet function tests analyzed during hemodialysis.8 Kumon et al9 administered argatroban for postoperative DIC after open heart surgery, and reported that the drug was as effective as heparin in promptly prolonging the activated partial thromboplastin time (APTT) and improving the platelet count.
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FIG. 2. Thrombin-antithrombin III (AT III) complex (TAT) levels before (□) and after ( ) hemodialysis with five anticoagulant regimens in an AT III deficient patient.5
The blood argatroban level needed to increase the APTT to an extent similar to heparin (1.5- to 2.5-fold) has been found to range between 0.23 and 0.75 µg/ml by an in vitro comparative study. Since a recovery test after the addition of an estimated dose of argatroban has suggested the probability of its nonspecific inactivation in the blood, periodic monitoring of the anticoagulant activity of this drug is recommended during anticoagulation with argatroban.10 The plasma level as an equivalent of the antithrombin activity was determined using a synthetic substrate (Tos-Gly-Pro-Arg-pNA) during hemodialysis with the continuous infusion of argatroban. The plasma level of argatroban was 2.2 ± 0.5 µg/ml
FIG. 3. Differences of the platelet factor 4 (PF4) release in duced by heparin (•—•), and argatroban (○—○) (mean ± SD).
(mean ± SD) and APTT increased 2.4 ± 0.4 times at 240 minutes after the administration of 10 mg as a bolus plus 20 mg/hr as a continuous infusion.11 These findings also indicated that larger doses were required in vivo than would be expected from in vitro estimates. We have been routinely using the previously mentioned argatroban regimen for anticoagulation instead of heparin when hemodialysis is required in patients with heparin-induced thrombocytopenia.12
ARGATROBAN AND HEPARIN-INDUCED THROMBOCYTOPENIA Heparin-dependent antiplatelet antibodies (heparin antibodies) are found in the plasma during heparininduced thrombocytopenia (HIT). In vivo platelet aggregation and destruction occur following the binding of heparin with such heparin antibodies, resulting in thrombocytopenia that is sometimes associated with severe arterial thrombosis. Heparin antibodies are mainly thought to exist in the immunoglobulin G (IgG) fraction. Platelet aggregation, the release of serotonin and BTG, and an increase of thromboxane A2 from normal platelet-rich plasma are observed when heparin is added with patient IgG. In patients with HIT, platelet-binding IgG (PBIgG) increases more markedly than platelet-associated IgG. The increase of PBIgG is an important early laboratory finding in HIT, when a 20-fold increase has been observed following the addition of heparin to patient plasma.13 Figure 4 shows the increase of the PBIgG level observed in HIT. The increase remains slight without the addition of heparin, but a rather marked increase is seen following the addition of heparin from the same lot as used previously for the patient. Also, an increase of the
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FIG. 4. Increase of platelet binding immunoglobulin G (PBIgG) induced by two kinds of heparin, and the inhibitory effect by argatroban after its addition to a mixture of heparin-induced thrombocytopenia plasma and heparin.
PBIgG level was found to be produced by low molecular weight heparin. The increase of the PBIgG level due to heparin was clearly suppressed when argatroban was previously added to plasma from the HIT patient at a final concentration of 2.2 µg/ml. Although a Fc receptor-mediated mechanism has been proposed to explain the heparin antibody-binding sites on the platelet membrane, a study has shown that the von Willebrand factor-binding site on GPIb is closely related to the heparin antibody-binding site. Since the binding of heparin antibodies to the von Willebrand factor site does not in itself cause platelet activation, multiple heparin antibody binding sites on the platelet membrane, including the Fc receptor site, have been postulated to be involved when heparin induces platelet aggregation.14 Since it was observed in one case of HIT that actual heparin-induced aggregation was completely inhibited by the pretreatment with anti-GPIb antibody, GPIb is thought to be closely related to the induction of platelet aggregation in HIT. Argatroban administered immediately after the onset of HIT was a very effective replacement for heparin. Heparin-induced platelet aggregation was also inhibited after the in vitro addition of argatroban.15 Argatroban was found to inhibit heparininduced aggregation in a dose-dependent manner in the IgG fraction from a HIT patient, and the ex vivo increase
of PBIgG due to the addition of heparin was completely abolished by the infusion of argatroban.16 Although the exact mechanism of the inhibitory effect of argatroban on the function of GPIb remains unknown, the drug may competitively interfere with thrombin to the thrombinbinding site in GPIb. Also, hirudin is an antithrombin derivative that has been found to inhibit heparin-induced platelet aggregation, suggesting that local thrombin formation on the platelet membrane during platelet activation plays an important role in the induction of such heparin-induced aggregation.17 These findings suggest that small molecular antithrombin derivatives prevent heparin-induced platelet aggregation in HIT.
TREATMENT OF HEPARIN-INDUCED THROMBOCYTOPENIA WITH ARGATROBAN No other effective therapy for HIT except the immediate discontinuation of heparin has been recommended so far. Although oral anticoagulants such as warfarin have been used in replacement of heparin, these anticoagulants require a relatively long time before the initiation of their action and so cannot be used for patients requiring immediate anticoagulation, such as HIT pa-
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monomer complex, a fibrinogen degradation level of 1280 µg/ml, and platelet aggregation induced by heparin were observed in the 8th session, although the dose of heparin was increased to overcome clot formation. The 9th dialysis session, in which argatroban was continuously infused at 25 mg/hr instead of heparin, could be completed without clot formation in the circuit. Although, the PBIgG level in the 8th session increased to 229.3 ng/107 platelets (normal: 83.1 ± 29.7), no significant release of both PF4 and BTG were observed during argatroban. Thus, argatroban was effective as an alternative anticoagulant for the treatment of HIT in a patient requiring immediate anticoagulation. Discontinuation of heparin must be considered as the first line of treatment for HIT. However, when immediate anticoagulation is absolutely necessary despite the presence of HIT, the replacement of heparin with argatroban is recommended.
REFERENCES
FIG. 5. Immediate resolution of heparin-induced thrombo cytopenia after replacement of heparin with argatroban dur ing the 9th hemodialysis session.
tients need with hemodialysis. In the hemodialysis patient with HIT, the combined use of aspirin with heparin may be effective in preventing the exacerbation of this complication. In a HIT hemodialysis patient for whom 1.0 gm of oral aspirin was effective, the high PBIgG level at the onset of HIT was decreased by aspirin therapy and heparin could be continued with no problems.18 When one HIT patient required cardiac surgery with extracorporeal circulation, the combined use of aspirin and dipyridamole allowed full-dose heparin to be given.19 However, aspirin is not effective in all HIT patients. There are a few reports of cases in which HIT did not respond, even though aspirin inhibited heparin-induced platelet aggregation ex vivo.20 Other drugs that are under study as therapeutic agents for HIT include prostacyclin, low molecular weight heparin, and low molecular weight heparinoid, but none of these drugs are yet established for the clinical resolution of this complication. There is much controversy over the effect of low molecular weight heparin, and it cannot be generally recommended for HIT.21 Figure 5 shows the clinical course of a HIT hemodialysis patient with acute renal failure, in whom thrombocytopenia and clot formation in the hemodialysis circuit occurred during the 7th dialysis session, although the APTT was sufficiently prolonged. Clot formation in the circuit, thrombocytopenia (from 27.1 x 104/µl on admission to 22.7 x 104/µl), a positive soluble fibrin
1. Hara H, Y Tamao, R Kikumoto, S Okamoto: Effect of a synthetic thrombin inhibitor MCI-9038 on experimental models of disseminated intravascular coagulation in rabbits. Thromb Haemost 57:165-170, 1987. 2. Nakamura K, Y Hatano, K Mori: Thrombin-induced vasoconstriction in isolated cerebral arteries and the influence of a synthetic thrombin inhibitor. Thromb Res 40:715-720, 1985. 3. Czervionke RL, JC Hoak, DL Haycraft, GL Fry: Inhibition of thrombin-induced platelet adherence to vascular cells by synthetic thrombin inhibitor No. 805. Trans Assoc Am Physicians 96:271277, 1983. 4. Kumada T, Y Abiko: Comparative study on heparin and a synthetic thrombin inhibitor No. 805 (MD-805) in experimental antithrombin Ill-deficient animals. Thromb Res 24:285-298, 1981. 5. Matsuo T, T Yamada, T Yamanashi, K Kodama: Choice of anticoagulant in a congenital antithrombin III (AT-III)-deficient patient with chronic renal failure undergoing regular haemodialysis. Clin Lab Haematol 11:213-219, 1989. 6. Hijikata-Okunomiya A, S Okamoto, K Wanaka: Effect of a synthetic thrombin-inhibitor MD805 on the reaction between thrombin and plasma antithrombin-III. Thromb Res 59:967-977, 1990. 7. Matsuo T, T Yamada, K Nakao: Effects of a new synthetic anticoagulant (MD805) on platelets. Acta Haematol Jpn 50:654-657, 1987. 8. Charvat J, J Konig, J Blaha: Is heparin responsible for enhanced platelet aggregation after haemodialysis? Nephron 44:89-91, 1986. 9. Kumon K, K Tanaka, N Nakajima, Y Naito: Anticoagulant with a synthetic thrombin inhibitor after cardiovascular surgery and for treatment of disseminated intravascular coagulation. Crit Care Med 12:1039-1043, 1984. 10. Matsuo T, C Shimano, Y Ohoki: Monitoring of a synthetic thrombin inhibitor (MD805). J Jpn Soc Dial Ther 19:1005-1008, 1986. 11. Matsuo T, K Nakao: Plasma antithrombin activity of MD805 determined by chromogenic substrate during hemodialysis. Blood Vessel 18:378-380, 1987. 12. Matsuo T, K Nakao, T Yamada, O Matsuo: A new thrombin inhibitor MD805 and thrombocytopenia encountered with heparin hemodialysis. Thromb Res 44:247-251, 1986.
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13. Lynch DM, SE Howe: Heparin-associated thrombocytopenia: Antibody binding specificity to platelet antigens. Blood 66:1176— 1181, 1985. 14. Adelman B, M Sobel, Y Fujimura, ZM Rugeri, TS Zimmerman: Heparin-associated thrombocytopenia: Observations on the mechanism of platelet aggregation. J Lab Clin Med 113:204-210, 1989. 15. Matsuo T, T Yamada, T Yamanashi, R Ryo: Anticoagulant therapy with MD805 of a hemodialysis patient with heparin-induced thrombocytopenia. Thromb Res 58:663-666, 1990. 16. Matsuo T, Y Chikahira, T Yamada, K Nakao, S Ueshima, O Matsuo: Effect of synthetic thrombin inhibitor (MD805) as an alternative drug on heparin induced thrombocytopenia during hemodialysis. Thromb Res 52:165-171, 1988. 17. Brace LD, J Fareed, J Walenga: Heparin-induced platelet aggregation (H-IPA) is not observed in hirudin anticoagulated platelet-rich plasma. (Abst.) Thromb Haemost 62:416, 1989.
18. Matsuo T, T Yamada, Y Chikahira, T Kadowaki: Effect of aspirin on heparin-induced thrombocytopenia (HIT) in a patient requiring hemodialysis. Blut 59:393-395, 1989. 19. Makhoul RG, RL McCann, EH Austin, CS Greenberg, JE Lowe: Management of patients with heparin-associated thrombocytopenia and thrombosis requiring cardiac surgery. Ann Thorac Surg 43:617-621, 1987. 20. Kappa JR, CA Fisher, HD Berkowitz, ED Cottrell, VP Addonizio: Heparin-induced platelet activation in sixteen surgical patients: Diagnosis and management. J Vasc Surg 5:101-107, 1987. 21. Gouault-Heilmann M, Y Hent, S Adnot, G Contant, F Bonnet, L Intrador, D Payen, M Levent: Low molecular weight heparin fractions as an alternative therapy in heparin-induced thrombocytopenia. Haemostasis 17:134-140, 1987.
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