Organon Scientific Development Group, Oss, The Netherlands
Orgaran (Org 10172): Its Pharmacological Profile in Experimental Models
KeyWords
Abstract
Orgaran Org 10172 Heparin Factor Xa Thrombin Thrombosis models Bleeding models
Orgaran is a mixture of glycosaminoglycans extracted from animal muco sa. It consists of heparan, dermatan and chondroitin sulfate; a small pro portion of heparan sulfate (4%) has high affinity for antithrombin III (AT III). Orgaran is devoid of heparin or heparin fragments. Orgaran catalyses the inactivation of factor Xa and thrombin. Compared to heparin and most low-molecular-weight heparins, Orgaran has a much higher antiXa/anti-IIa ratio. The inactivation of factor Xa is mediated by AT III and that of thrombin by both AT III and heparin cofactor II. Compared to heparin, which is a strong inhibitor of thrombin generation. Orgaran has only moderate inhibitory effects on thrombin generation. Orgaran shows minimal or no effects on platelet function in vitro or in vivo. It inhibits the formation of various types of thrombi (clot-like and mixed thrombi) with approximately the same potency as heparin. Both the high- and low-affin ity fraction for AT III contribute to the antithrombotic activity. In con trast to heparin, Orgaran does not inhibit platelet deposition in experi mental mixed thrombi unless very high doses of the heparinoid are used. Orgaran is more efficacious than heparin in preventing the extension of established venous thrombosis. Orgaran promotes less bleeding-enhanc ing activity than heparin in various experimental models. In addition, compared to heparin, it has only minimal effects on platelet degranulation during hemostatic plug formation. In vivo studies in experimental animals have demonstrated that at equivalent anti-Xa doses, the effect of Orgaran on thrombosis inhibition lasts considerably longer than that of heparin; in contrast, the effect of Orgaran on bleeding is less marked and of much shorter duration than that seen with heparin. Based on these observations, Orgaran has a significantly better risk/benefit ratio than heparin.
D G. Meuleman
Introduction
The new antithrombotic agent Orgaran (Org 10172) is a mixture of sulfated glycos aminoglycans extracted from animal mucosa.
Although isolated from the same starting ma terial as heparin and low-molecular-weight (LMW) heparins, Orgaran has chemical and pharmacological properties different from these other glycosaminoglycans [1]. Orgaran
D.G. Meuleman Organon Scientific DevelopmentGroup PO Box 20 NL-5340 BH Oss (The Netherlands)
© 1992 S. Karger AG. Basel 0301-0147/92/ 0222-0058S2.75/0
Downloaded by: King's College London 137.73.144.138 - 1/15/2018 1:54:28 AM
Haemostasis 1992;22:58-65
Orgaran
Heparin
Fig. 1. Major repeating units of heparin and heparan sulfate, the major constituent of Orgaran [1],
Percentage in Orgaran Constituents Heparan sulfate HA heparan sulfate LA heparan sulfate Dermatan sulfate Chondroitin sulfate Specific antifactor Xa activity Specific antithrombin activity (in AT III buffer) Mean molecular mass
84 4 80 12
4 14 U/mg < 0.5 U/mg 6,000 D
HA = Fraction with high affinity for AT III; LA = fraction with low affinity for AT III.
is composed of heparan sulfate (the major component), dermatan sulfate (a relatively minor component), and very small amounts of chondroitin sulfate (table 1). Orgaran is de void of heparin and heparin fragments: this difference is reflected in a fundamentally dif ferent chemical composition of the repeating saccharide units of Orgaran (fig. 1).
Orgaran was developed as an improved antithrombotic agent with less hemorrhagic properties than heparin for an equivalent an tithrombotic effect. The aim of this review is to summarize the anticoagulant properties of Orgaran and the results of experiments in which the antithrombotic and hemorrhagic effects of Orgaran are compared with heparin, including the relationship between the plasma clearance of these glycosaminoglycans and their effects on thrombosis and bleeding. These animal experiments provide a basis for understanding the antithrombotic properties of Orgaran in man.
Anticoagulant Profile of Orgaran
The properties of Orgaran are summarized in table 1. Orgaran has minimal inhibitory activity on the standard screening coagulation tests such as the partial thromboplastin time, prothrombin time, and thrombin time. It does, however, inhibit factor Xa and to a lesser extent thrombin. Like heparin, Orgaran exerts its anticoagu lant effect by catalyzing serine protease inhib
59
Downloaded by: King's College London 137.73.144.138 - 1/15/2018 1:54:28 AM
Table 1. Product characteristics of Orgaran
60
Mculcman
arin and between 2 and 4 for LMW heparins. Orgaran exhibits a concentration-dependent inhibition of the generation of thrombin in duced by a factor Xa/phospholipid complex, the concentration-response curve of Orgaran is linear over a broader concentration range than that of heparin [15]. The heparan sulfate fraction with low affinity for AT III lacks sig nificant effects on coagulation factors Xa and thrombin, nevertheless, it contributes sub stantially to the antithrombotic activity by an unknown mechanism.
Antithrombotic Effects of Orgaran
Orgaran exerts antithrombotic effects in a variety of experimental thrombosis models (table 2). The so-called ‘stasis models’, in which ex perimental thrombosis is induced by a combi nation of venous stasis and a hypercoagulable state, simulate the pathogenesis of deep vein thrombosis. The resulting thrombi have a clot-like appearance, in which blood cells are trapped at random in a fibrin network. Orga ran has been shown to inhibit the formation of these experimental venous thrombi in jugu lar veins of rats and rabbits, the I D 5o (the dose required for 50% inhibition of thrombosis) being 15-25 aXa U/kg i.v. The relative effec tiveness of Orgaran in these models is similar to that of heparin. In the arteriovenous shunt model in rats [21], experimental thrombosis is induced by contact of the circulating blood with a nonhematocompatible surface in the shunt. This results in the formation of thrombi which consist of a platelet core adhering to the for eign surface and a red tail extending down stream (mixed thrombi). Orgaran prevents the formation of these thrombi in a dose-dependent manner. Orga ran is as potent as heparin on an anti-Xa basis
Pharmacological Profile of Orgaran
Downloaded by: King's College London 137.73.144.138 - 1/15/2018 1:54:28 AM
itors (serpins), which are the endogenous in hibitors of activated coagulation factors. The anticoagulant effect of heparin is mediated through its binding to the serpin antithrom bin III (AT III) [2-5]. Binding to AT III requires a unique pentasaccharide [6, 7]. Hep arin can accelerate the inhibition of thrombin and factor Xa by AT III. Heparin fractions with molecular weights less than 5,000 retain their ability to poten tiate the inactivation of factor Xa by AT III, but are not able to enhance the inactivation of thrombin [8, 9]. For the catalysis of the factor Xa inactivation, only the unique pentasac charide sequence is required [10], whereas for the catalysis of thrombin inactivation by AT III, a monosaccharide length of at least 18 units is needed. Like LMW heparins, Orgaran exerts a stronger catalytic effect on the inactivation of factor Xa by AT III than on the inactivation of thrombin by AT III [11], but the ratio of factor Xa to thrombin inhibition is even greater than that of LMW heparins. The antifactor Xa activity of Orgaran re sides in the high-affinity heparan sulfate frac tion. The specific activity of Orgaran with respect to factor Xa inactivation by AT III is approximately 10% of that of heparin. Orga ran also enhances the inactivation of throm bin but with a much lower potency than towards factor Xa. Orgaran catalyzes the for mation of both thrombin-AT III and throm bin-heparin cofactor II (HC II) [12, 13]. The catalytic effect on the thrombin-AT III forma tion is due to the high-affinity heparan sulfate fraction, while dermatan sulfate is primarily responsible for catalyzing the formation of thrombin-HC II [14], The specific activity of Orgaran with respect to thrombin inactiva tion is less than 1% of that of heparin, conse quently, Orgaran has a high antifactor Xa/ antithrombin ratio. The overall aXa/alla ra tio is > 28 as compared to a ratio of 1 for hep
Table 2. Antithrombotic activity of Orgaran in ex perimental models
Model
ID 5 0 , aXa U/kg i.v. Orgaran
heparin
Venous stasis model (rat) Vogel et al. [16]
15
15
Venous stasis model (rabbit) Ockelford et al. [ 17]
25
5
Arterio-venous shunt model (rat) Meuleman et al. [11]
40
40
0
3301
Orgaran (Org 10172): Its Pharmacological Profile in Experimental Models
KeyWords
Abstract
Orgaran Org 10172 Heparin Factor Xa Thrombin Thrombosis models Bleeding models
Orgaran is a mixture of glycosaminoglycans extracted from animal muco sa. It consists of heparan, dermatan and chondroitin sulfate; a small pro portion of heparan sulfate (4%) has high affinity for antithrombin III (AT III). Orgaran is devoid of heparin or heparin fragments. Orgaran catalyses the inactivation of factor Xa and thrombin. Compared to heparin and most low-molecular-weight heparins, Orgaran has a much higher antiXa/anti-IIa ratio. The inactivation of factor Xa is mediated by AT III and that of thrombin by both AT III and heparin cofactor II. Compared to heparin, which is a strong inhibitor of thrombin generation. Orgaran has only moderate inhibitory effects on thrombin generation. Orgaran shows minimal or no effects on platelet function in vitro or in vivo. It inhibits the formation of various types of thrombi (clot-like and mixed thrombi) with approximately the same potency as heparin. Both the high- and low-affin ity fraction for AT III contribute to the antithrombotic activity. In con trast to heparin, Orgaran does not inhibit platelet deposition in experi mental mixed thrombi unless very high doses of the heparinoid are used. Orgaran is more efficacious than heparin in preventing the extension of established venous thrombosis. Orgaran promotes less bleeding-enhanc ing activity than heparin in various experimental models. In addition, compared to heparin, it has only minimal effects on platelet degranulation during hemostatic plug formation. In vivo studies in experimental animals have demonstrated that at equivalent anti-Xa doses, the effect of Orgaran on thrombosis inhibition lasts considerably longer than that of heparin; in contrast, the effect of Orgaran on bleeding is less marked and of much shorter duration than that seen with heparin. Based on these observations, Orgaran has a significantly better risk/benefit ratio than heparin.
D G. Meuleman
Introduction
The new antithrombotic agent Orgaran (Org 10172) is a mixture of sulfated glycos aminoglycans extracted from animal mucosa.
Although isolated from the same starting ma terial as heparin and low-molecular-weight (LMW) heparins, Orgaran has chemical and pharmacological properties different from these other glycosaminoglycans [1]. Orgaran
D.G. Meuleman Organon Scientific DevelopmentGroup PO Box 20 NL-5340 BH Oss (The Netherlands)
© 1992 S. Karger AG. Basel 0301-0147/92/ 0222-0058S2.75/0
Downloaded by: King's College London 137.73.144.138 - 1/15/2018 1:54:28 AM
Haemostasis 1992;22:58-65
Orgaran
Heparin
Fig. 1. Major repeating units of heparin and heparan sulfate, the major constituent of Orgaran [1],
Percentage in Orgaran Constituents Heparan sulfate HA heparan sulfate LA heparan sulfate Dermatan sulfate Chondroitin sulfate Specific antifactor Xa activity Specific antithrombin activity (in AT III buffer) Mean molecular mass
84 4 80 12
4 14 U/mg < 0.5 U/mg 6,000 D
HA = Fraction with high affinity for AT III; LA = fraction with low affinity for AT III.
is composed of heparan sulfate (the major component), dermatan sulfate (a relatively minor component), and very small amounts of chondroitin sulfate (table 1). Orgaran is de void of heparin and heparin fragments: this difference is reflected in a fundamentally dif ferent chemical composition of the repeating saccharide units of Orgaran (fig. 1).
Orgaran was developed as an improved antithrombotic agent with less hemorrhagic properties than heparin for an equivalent an tithrombotic effect. The aim of this review is to summarize the anticoagulant properties of Orgaran and the results of experiments in which the antithrombotic and hemorrhagic effects of Orgaran are compared with heparin, including the relationship between the plasma clearance of these glycosaminoglycans and their effects on thrombosis and bleeding. These animal experiments provide a basis for understanding the antithrombotic properties of Orgaran in man.
Anticoagulant Profile of Orgaran
The properties of Orgaran are summarized in table 1. Orgaran has minimal inhibitory activity on the standard screening coagulation tests such as the partial thromboplastin time, prothrombin time, and thrombin time. It does, however, inhibit factor Xa and to a lesser extent thrombin. Like heparin, Orgaran exerts its anticoagu lant effect by catalyzing serine protease inhib
59
Downloaded by: King's College London 137.73.144.138 - 1/15/2018 1:54:28 AM
Table 1. Product characteristics of Orgaran
60
Mculcman
arin and between 2 and 4 for LMW heparins. Orgaran exhibits a concentration-dependent inhibition of the generation of thrombin in duced by a factor Xa/phospholipid complex, the concentration-response curve of Orgaran is linear over a broader concentration range than that of heparin [15]. The heparan sulfate fraction with low affinity for AT III lacks sig nificant effects on coagulation factors Xa and thrombin, nevertheless, it contributes sub stantially to the antithrombotic activity by an unknown mechanism.
Antithrombotic Effects of Orgaran
Orgaran exerts antithrombotic effects in a variety of experimental thrombosis models (table 2). The so-called ‘stasis models’, in which ex perimental thrombosis is induced by a combi nation of venous stasis and a hypercoagulable state, simulate the pathogenesis of deep vein thrombosis. The resulting thrombi have a clot-like appearance, in which blood cells are trapped at random in a fibrin network. Orga ran has been shown to inhibit the formation of these experimental venous thrombi in jugu lar veins of rats and rabbits, the I D 5o (the dose required for 50% inhibition of thrombosis) being 15-25 aXa U/kg i.v. The relative effec tiveness of Orgaran in these models is similar to that of heparin. In the arteriovenous shunt model in rats [21], experimental thrombosis is induced by contact of the circulating blood with a nonhematocompatible surface in the shunt. This results in the formation of thrombi which consist of a platelet core adhering to the for eign surface and a red tail extending down stream (mixed thrombi). Orgaran prevents the formation of these thrombi in a dose-dependent manner. Orga ran is as potent as heparin on an anti-Xa basis
Pharmacological Profile of Orgaran
Downloaded by: King's College London 137.73.144.138 - 1/15/2018 1:54:28 AM
itors (serpins), which are the endogenous in hibitors of activated coagulation factors. The anticoagulant effect of heparin is mediated through its binding to the serpin antithrom bin III (AT III) [2-5]. Binding to AT III requires a unique pentasaccharide [6, 7]. Hep arin can accelerate the inhibition of thrombin and factor Xa by AT III. Heparin fractions with molecular weights less than 5,000 retain their ability to poten tiate the inactivation of factor Xa by AT III, but are not able to enhance the inactivation of thrombin [8, 9]. For the catalysis of the factor Xa inactivation, only the unique pentasac charide sequence is required [10], whereas for the catalysis of thrombin inactivation by AT III, a monosaccharide length of at least 18 units is needed. Like LMW heparins, Orgaran exerts a stronger catalytic effect on the inactivation of factor Xa by AT III than on the inactivation of thrombin by AT III [11], but the ratio of factor Xa to thrombin inhibition is even greater than that of LMW heparins. The antifactor Xa activity of Orgaran re sides in the high-affinity heparan sulfate frac tion. The specific activity of Orgaran with respect to factor Xa inactivation by AT III is approximately 10% of that of heparin. Orga ran also enhances the inactivation of throm bin but with a much lower potency than towards factor Xa. Orgaran catalyzes the for mation of both thrombin-AT III and throm bin-heparin cofactor II (HC II) [12, 13]. The catalytic effect on the thrombin-AT III forma tion is due to the high-affinity heparan sulfate fraction, while dermatan sulfate is primarily responsible for catalyzing the formation of thrombin-HC II [14], The specific activity of Orgaran with respect to thrombin inactiva tion is less than 1% of that of heparin, conse quently, Orgaran has a high antifactor Xa/ antithrombin ratio. The overall aXa/alla ra tio is > 28 as compared to a ratio of 1 for hep
Table 2. Antithrombotic activity of Orgaran in ex perimental models
Model
ID 5 0 , aXa U/kg i.v. Orgaran
heparin
Venous stasis model (rat) Vogel et al. [16]
15
15
Venous stasis model (rabbit) Ockelford et al. [ 17]
25
5
Arterio-venous shunt model (rat) Meuleman et al. [11]
40
40
0
3301
