579
EDITORIALS
Hirudins: return of the leech? The medicinal leech (Hirudo medicinalis) has been used therapeutically for thousands of years. Its traditional role was in systemic removal of blood; newer applications include removal of local accumulations of blood in threatened skin necrosis (eg, in digital reimplantations, skin grafts, and other plastic surgery procedures1) or in purpura fulminans with diffuse skin microthrombosis associated with protein C deficiency. The apparent benefits of leeches in such circumstances may reflect not only mechanical removal of blood and increase in local blood flow but also local release of their salivary anticoagulant, hirudin. Hirudin is also being developed as a systemic anticoagulant for clinical trials. Hirudin was discovered in 1884,3a third of a century before heparin in 1916.4However, biological hirudin was not isolated until the late 1950s. When the medicinal leech was placed on the list of threatened species, the biotechnologists intervened and, after cloning and expression of a cDNA coding for hirudin, succeeded in producing recombinant hirudins (r-hirudins) in bacteria or yeast cells. Sufficient quantities of r-hirudins have since become available to allow experimental studies of their use as alternative therapeutic direct anticoagulants to heparin.5-7 They may also find future uses as in-vitro anticoagulants for blood samples, especially in studies of platelet function, for which all standard in-vitro anticoagulants (heparin, citrate, and edetate) have limitations because of their effects on platelet function. Native hirudin is a miniprotein containing 65 aminoacids with a molecular weight of about 7 kD;
recombinant hirudins differ in lacking a sulphate group on tyrosine 63. The anticoagulant effects of hirudins are due to rapid specific inhibition of thrombin by the formation of tight stoichiometric complexes, interacting simultaneously with the fibrinogen-binding exosite and with the catalytic site of thrombin.8 Why is there so much therapeutic interest in hirudins and in other direct thrombin inhibitors such as synthetic hirudin analogues (hirulogs), argatroban, and argidipine?9 First, thrombin plays a central part in thrombosis, a concept based partly on experimental studies with specific thrombin inhibitors including hirudins Second, despite the efficacy of heparin as a parenteral direct antithrombotic agent, this drug has several limitations.il Recent studies with low-molecularweight heparins have shown the ability of newer agents to improve on the antithrombotic efficacy of standard unfractionated heparin.12-14 What is this central role of thrombin in thrombosis? Thrombin does more than convert fibrinogen to fibrin: it also activates factor XIII, which stabilises fibrin; it activates factors V and VIII, resulting in positive amplification of thrombin and fibrin formation; it is a potent activator of platelets, contributing to platelet-fibrin thrombosis; and finally it stimulates vascular cells, including endothelial cells which express adhesion receptors and proliferate, smooth muscle cells which contract, and fibroblasts which proliferate. All these processes are potentially important in thrombosis and atherosclerosis, and all are inhibited by hirudins.5-7 What are the differences between hirudins and heparin that might render hirudins better antithrombotic drugs? First, hirudins are not bound to or inactivated by antiheparin proteins such as platelet factor 4, histidine-rich glycoprotein, or vitronectin; raised plasma concentrations of these substances can cause heparin resistance in patients with inflammatory or malignant disorders." Second, hirudins have no direct effects on platelets or endothelial cells, whereas such effects of heparin may contribute to the associated risk of bleeding; use of hirudin might also avoid the problem of heparininduced immune thrombocytopenia and associated thrombosis." Third, interaction of hirudins with thrombin does not require antithrombin III as a cofactor, in contrast to the heparin mechanism. Thus hirudins have an antithrombin-III-sparing effect, whereas heparin infusion results in progressive depletion of this natural anticoagulant. Moreover, hirudins might be used in patients with antithrombin III deficiency, for effective anticoagulation without the need for antithrombin III replacement therapy. Fourth, thrombin bound to vascular subendothelium (which remains active in fibrin formation and platelet activation, and which may promote thrombosis after spontaneous rupture of arterial plaques, angioplasty, or thrombolysis) is protected from inactivation by antithrombin III and heparin but not from
580
inactivation by hirudin.15 Hirudins may therefore be more effective anticoagulants than heparin in arterial thrombosis, including thrombosis following
angioplasty thrombolytic therapy. As with heparin, the anticoagulant effect of hirudins can be monitored by routine thrombin time and activated partial thromboplastin time assays; studies in volunteers have shown predictable anticoagulant or
effects and hirudin concentrations after both intravenous and subcutaneous injection.55 Renal excretion is the predominant route of clearance. Animal studies have shown that hirudins inhibit disseminated intravascular coagulation and venous, arterial, and artificial-surface-induced thrombosis, often with greater efficacy than heparin; higher doses of hirudin are required for the latter two types of thrombosis.5-7,10 Haemorrhagic effects seem to be less than with heparin, but still occur. common Approaches to hirudin-induced bleeding might of infusion include prothrombin complex concentrates or recombinant factor VIla concentrate, which results in thrombin formation and complexing with circulating hirudin; another possibility is administration of specific antibodies to hirudin. 16 However, infusion of thrombogenic concentrates would be potentially hazardous in patients with thrombosis and, as with heparin-induced bleeding, rapid clearance of r-hirudins may render discontinuation of therapy sufficient in most cases of bleeding. Antibodies to hirudin have been detected in sheep;16 although allergic reactions have not been encountered in animals or man, surveillance for antibody formation and immune reactions (including thrombocytopenia) will be important during clinical
development. Possible future indications for clinical use of hirudins include prevention and treatment of venous thromboembolism; prevention of thrombosis after thrombolytic therapy or angioplasty; and prevention of or thrombosis haemodialysis during with low-molecularcardiopulmonary bypass. As weight heparins, careful dose-ranging studies followed by large randomised controlled trials"" and meta-analyses14,17 will be required to establish the extent to which the ratio of anti-thrombotic efficacy to bleeding risk differs from that of conventional therapy. As the leech might say-let’s suck it and see. 1. Adams SL. The medicinal leech: a page from the annelids of internal medicine. Ann Intern Med 1988; 109: 399-405. 2. Lowe GDO. Vascular disease and vasculitis. In: Ratnoff OD, Forbes CD, eds. Disorders of hemostasis. 2nd ed. Philadelphia: WB Saunders, 1991: 532-49. 3. Haycraft JB. Über die Einwirkung eines Sekretes des officinellen Blutegels auf die Gerinnbarkeit des Blutes. Arch Exp Pathol Pharmakol 1884; 18: 209. 4. McLean J. The thromboplastic action of cephalin. Am J Physiol 1916; 41: 250-57. 5. Markwardt F. Hirudin and derivatives as anticoagulant agents. Thromb Haemostas 1991; 66: 141-52. 6. Markwardt F, ed. Hirudin. Semin Thromb Haemostas 1991; 17: 79-159. 7. Hoet B, Close P, Vermylen J, Verstraete M. Hirudo medicinalis and hirudin. In: Poller L, ed. Recent advances in blood coagulation, 5. Edinburgh: Churchill Livingstone, 1991: 223-44.
8.
Rydel TJ, Ravichandran KG, Tulinski A, et al. The structure of a complex of recombinant hirudin and human &agr;-thrombin. Science 1990;
249: 277-80. 9. Salzman EW.
Low-molecular-weight heparin and other new antithrombotic drugs. N Engl J Med 1992; 326: 1017-19. 10. Chesebro JH, Zoldhelyi P, Badimon L, Fuster V. Role of thrombin in arterial thrombosis: implications for therapy. Thromb Haemostas 1991; 66: 1-5. 11. Hirsh J. Heparin. N Engl J Med 1991; 324: 1565-74. 12. Hull RD, Raskob GE, Pineo GF, et al. Subcutaneous low-molecularweight heparin compared with continuous intravenous heparin in the treatment of proximal-vein thrombosis. N Engl J Med 1992; 326: 975-82. 13. Prandoni P, Lensing AWA, Buller HR, et al. Comparison of subcutaneous low-molecular-weight heparin with intravenous standard heparin in proximal deep-vein-thrombosis. Lancet 1992; 339: 441-45. 14. Nurmohamed MT, Rosendaal FR, Buller HR, et al. Low-molecular weight heparin versus standard heparin in general and orthopaedic surgery: a meta-analysis. Lancet 1992; 340: 152-56. 15. Bar-Shavit R, Eldora A, Vlodavsky I. Binding of thrombin to subendothelial extracellular matrix: protection and expression of functional properties. J Clin Invest 1989; 84: 1096-104. 16. Spinner S, Stöffler G, Fink E. Quantitative enzyme-linked immunosorbent assay (ELI SA) for hirudin. J Immunol Meth 1986; 87: 79-83. 17. Lau J, Antman EM, Jimenez-Silva J, Kupelnick B, Mosteller F, Chalmers TC. Cumulative meta-analysis of therapeutic trials for myocardial infarction. N Engl J Med 1992; 327: 248-54.
Postoperative hypoxaemia At the beginning of this century W. Pasteur,’ writing in The Lancet, described lobar atelectasis after abdominal operations; at that time the clinical significance of this complication was unknown. Many years later it was realised that atelectasis combines with an impaired ventilatory control to cause profound postoperative hypoxaemia.2 Impaired gas exchange is caused by atelectasis, which develops during general anaesthesia and persists for several days into the postoperative period. Abnormalities of ventilatory control are due to postoperative opioid
analgesia which, during sleep, causes intermittent airway obstruction-an important cause of hypoxaemia.3,4 Periods of obstructive apnoea may be accompanied by a considerable fall in oxygen saturation, to as low as 65%, in patients receiving morphine for postoperative analgesia but not in recipients of regional analgesia. Oxygen desaturation upper
hours postoperatively, especially night. Moreover, this severe degree of hypoxaemia is not confined to the first postoperative night.6-8 An important observation is that in some subjects the degree ofhypoxaemia may be most severe not during the night immediately following surgery but up to five nights later; this finding may be related to the return of rapid-eyemovement sleep which is abolished for several nights after surgery.66 Patients with coronary or cerebrovascular disease are likely to be at risk during lengthy hypoxaemia and it has long been standard practice to give oxygen in the immediate postoperative period. However, there is no agreement about why, how much, or for how long. It is very unusual to administer oxygen routinely for five nights postoperatively, although we now know that
to
below
85%
may
occur
at
for
many
EDITORIALS
Hirudins: return of the leech? The medicinal leech (Hirudo medicinalis) has been used therapeutically for thousands of years. Its traditional role was in systemic removal of blood; newer applications include removal of local accumulations of blood in threatened skin necrosis (eg, in digital reimplantations, skin grafts, and other plastic surgery procedures1) or in purpura fulminans with diffuse skin microthrombosis associated with protein C deficiency. The apparent benefits of leeches in such circumstances may reflect not only mechanical removal of blood and increase in local blood flow but also local release of their salivary anticoagulant, hirudin. Hirudin is also being developed as a systemic anticoagulant for clinical trials. Hirudin was discovered in 1884,3a third of a century before heparin in 1916.4However, biological hirudin was not isolated until the late 1950s. When the medicinal leech was placed on the list of threatened species, the biotechnologists intervened and, after cloning and expression of a cDNA coding for hirudin, succeeded in producing recombinant hirudins (r-hirudins) in bacteria or yeast cells. Sufficient quantities of r-hirudins have since become available to allow experimental studies of their use as alternative therapeutic direct anticoagulants to heparin.5-7 They may also find future uses as in-vitro anticoagulants for blood samples, especially in studies of platelet function, for which all standard in-vitro anticoagulants (heparin, citrate, and edetate) have limitations because of their effects on platelet function. Native hirudin is a miniprotein containing 65 aminoacids with a molecular weight of about 7 kD;
recombinant hirudins differ in lacking a sulphate group on tyrosine 63. The anticoagulant effects of hirudins are due to rapid specific inhibition of thrombin by the formation of tight stoichiometric complexes, interacting simultaneously with the fibrinogen-binding exosite and with the catalytic site of thrombin.8 Why is there so much therapeutic interest in hirudins and in other direct thrombin inhibitors such as synthetic hirudin analogues (hirulogs), argatroban, and argidipine?9 First, thrombin plays a central part in thrombosis, a concept based partly on experimental studies with specific thrombin inhibitors including hirudins Second, despite the efficacy of heparin as a parenteral direct antithrombotic agent, this drug has several limitations.il Recent studies with low-molecularweight heparins have shown the ability of newer agents to improve on the antithrombotic efficacy of standard unfractionated heparin.12-14 What is this central role of thrombin in thrombosis? Thrombin does more than convert fibrinogen to fibrin: it also activates factor XIII, which stabilises fibrin; it activates factors V and VIII, resulting in positive amplification of thrombin and fibrin formation; it is a potent activator of platelets, contributing to platelet-fibrin thrombosis; and finally it stimulates vascular cells, including endothelial cells which express adhesion receptors and proliferate, smooth muscle cells which contract, and fibroblasts which proliferate. All these processes are potentially important in thrombosis and atherosclerosis, and all are inhibited by hirudins.5-7 What are the differences between hirudins and heparin that might render hirudins better antithrombotic drugs? First, hirudins are not bound to or inactivated by antiheparin proteins such as platelet factor 4, histidine-rich glycoprotein, or vitronectin; raised plasma concentrations of these substances can cause heparin resistance in patients with inflammatory or malignant disorders." Second, hirudins have no direct effects on platelets or endothelial cells, whereas such effects of heparin may contribute to the associated risk of bleeding; use of hirudin might also avoid the problem of heparininduced immune thrombocytopenia and associated thrombosis." Third, interaction of hirudins with thrombin does not require antithrombin III as a cofactor, in contrast to the heparin mechanism. Thus hirudins have an antithrombin-III-sparing effect, whereas heparin infusion results in progressive depletion of this natural anticoagulant. Moreover, hirudins might be used in patients with antithrombin III deficiency, for effective anticoagulation without the need for antithrombin III replacement therapy. Fourth, thrombin bound to vascular subendothelium (which remains active in fibrin formation and platelet activation, and which may promote thrombosis after spontaneous rupture of arterial plaques, angioplasty, or thrombolysis) is protected from inactivation by antithrombin III and heparin but not from
580
inactivation by hirudin.15 Hirudins may therefore be more effective anticoagulants than heparin in arterial thrombosis, including thrombosis following
angioplasty thrombolytic therapy. As with heparin, the anticoagulant effect of hirudins can be monitored by routine thrombin time and activated partial thromboplastin time assays; studies in volunteers have shown predictable anticoagulant or
effects and hirudin concentrations after both intravenous and subcutaneous injection.55 Renal excretion is the predominant route of clearance. Animal studies have shown that hirudins inhibit disseminated intravascular coagulation and venous, arterial, and artificial-surface-induced thrombosis, often with greater efficacy than heparin; higher doses of hirudin are required for the latter two types of thrombosis.5-7,10 Haemorrhagic effects seem to be less than with heparin, but still occur. common Approaches to hirudin-induced bleeding might of infusion include prothrombin complex concentrates or recombinant factor VIla concentrate, which results in thrombin formation and complexing with circulating hirudin; another possibility is administration of specific antibodies to hirudin. 16 However, infusion of thrombogenic concentrates would be potentially hazardous in patients with thrombosis and, as with heparin-induced bleeding, rapid clearance of r-hirudins may render discontinuation of therapy sufficient in most cases of bleeding. Antibodies to hirudin have been detected in sheep;16 although allergic reactions have not been encountered in animals or man, surveillance for antibody formation and immune reactions (including thrombocytopenia) will be important during clinical
development. Possible future indications for clinical use of hirudins include prevention and treatment of venous thromboembolism; prevention of thrombosis after thrombolytic therapy or angioplasty; and prevention of or thrombosis haemodialysis during with low-molecularcardiopulmonary bypass. As weight heparins, careful dose-ranging studies followed by large randomised controlled trials"" and meta-analyses14,17 will be required to establish the extent to which the ratio of anti-thrombotic efficacy to bleeding risk differs from that of conventional therapy. As the leech might say-let’s suck it and see. 1. Adams SL. The medicinal leech: a page from the annelids of internal medicine. Ann Intern Med 1988; 109: 399-405. 2. Lowe GDO. Vascular disease and vasculitis. In: Ratnoff OD, Forbes CD, eds. Disorders of hemostasis. 2nd ed. Philadelphia: WB Saunders, 1991: 532-49. 3. Haycraft JB. Über die Einwirkung eines Sekretes des officinellen Blutegels auf die Gerinnbarkeit des Blutes. Arch Exp Pathol Pharmakol 1884; 18: 209. 4. McLean J. The thromboplastic action of cephalin. Am J Physiol 1916; 41: 250-57. 5. Markwardt F. Hirudin and derivatives as anticoagulant agents. Thromb Haemostas 1991; 66: 141-52. 6. Markwardt F, ed. Hirudin. Semin Thromb Haemostas 1991; 17: 79-159. 7. Hoet B, Close P, Vermylen J, Verstraete M. Hirudo medicinalis and hirudin. In: Poller L, ed. Recent advances in blood coagulation, 5. Edinburgh: Churchill Livingstone, 1991: 223-44.
8.
Rydel TJ, Ravichandran KG, Tulinski A, et al. The structure of a complex of recombinant hirudin and human &agr;-thrombin. Science 1990;
249: 277-80. 9. Salzman EW.
Low-molecular-weight heparin and other new antithrombotic drugs. N Engl J Med 1992; 326: 1017-19. 10. Chesebro JH, Zoldhelyi P, Badimon L, Fuster V. Role of thrombin in arterial thrombosis: implications for therapy. Thromb Haemostas 1991; 66: 1-5. 11. Hirsh J. Heparin. N Engl J Med 1991; 324: 1565-74. 12. Hull RD, Raskob GE, Pineo GF, et al. Subcutaneous low-molecularweight heparin compared with continuous intravenous heparin in the treatment of proximal-vein thrombosis. N Engl J Med 1992; 326: 975-82. 13. Prandoni P, Lensing AWA, Buller HR, et al. Comparison of subcutaneous low-molecular-weight heparin with intravenous standard heparin in proximal deep-vein-thrombosis. Lancet 1992; 339: 441-45. 14. Nurmohamed MT, Rosendaal FR, Buller HR, et al. Low-molecular weight heparin versus standard heparin in general and orthopaedic surgery: a meta-analysis. Lancet 1992; 340: 152-56. 15. Bar-Shavit R, Eldora A, Vlodavsky I. Binding of thrombin to subendothelial extracellular matrix: protection and expression of functional properties. J Clin Invest 1989; 84: 1096-104. 16. Spinner S, Stöffler G, Fink E. Quantitative enzyme-linked immunosorbent assay (ELI SA) for hirudin. J Immunol Meth 1986; 87: 79-83. 17. Lau J, Antman EM, Jimenez-Silva J, Kupelnick B, Mosteller F, Chalmers TC. Cumulative meta-analysis of therapeutic trials for myocardial infarction. N Engl J Med 1992; 327: 248-54.
Postoperative hypoxaemia At the beginning of this century W. Pasteur,’ writing in The Lancet, described lobar atelectasis after abdominal operations; at that time the clinical significance of this complication was unknown. Many years later it was realised that atelectasis combines with an impaired ventilatory control to cause profound postoperative hypoxaemia.2 Impaired gas exchange is caused by atelectasis, which develops during general anaesthesia and persists for several days into the postoperative period. Abnormalities of ventilatory control are due to postoperative opioid
analgesia which, during sleep, causes intermittent airway obstruction-an important cause of hypoxaemia.3,4 Periods of obstructive apnoea may be accompanied by a considerable fall in oxygen saturation, to as low as 65%, in patients receiving morphine for postoperative analgesia but not in recipients of regional analgesia. Oxygen desaturation upper
hours postoperatively, especially night. Moreover, this severe degree of hypoxaemia is not confined to the first postoperative night.6-8 An important observation is that in some subjects the degree ofhypoxaemia may be most severe not during the night immediately following surgery but up to five nights later; this finding may be related to the return of rapid-eyemovement sleep which is abolished for several nights after surgery.66 Patients with coronary or cerebrovascular disease are likely to be at risk during lengthy hypoxaemia and it has long been standard practice to give oxygen in the immediate postoperative period. However, there is no agreement about why, how much, or for how long. It is very unusual to administer oxygen routinely for five nights postoperatively, although we now know that
to
below
85%
may
occur
at
for
many
