High and Low Affinity Heparin Compared with Unfractionated Heparin as Antithrombotic Drugs

Pharmacology of Heparin and Related Drugs: An Update

Heparin has been used extensively as an antithrombotic and anticoagulant for close to 100 years. This anticoagulant activity is attributed mainly to the pentasaccharide sequence, which potentiates the inhibitory action of antithrombin, a major inhibitor of the coagulation cascade. More recently it has been elucidated that heparin exhibits anti-inflammatory effect via interference of the formation of neutrophil extracellular traps and this may also contribute to heparin's antithrombotic activity. This illustrates that heparin interacts with a broad range of biomolecules, exerting both anticoagulant and nonanticoagulant actions. Since our previous review, there has been an increased interest in these nonanticoagulant effects of heparin, with the beneficial role in patients infected with SARS2-coronavirus a highly topical example. This article provides an update on our previous review with more recent developments and observations made for these novel uses of heparin and an overview of the development status of heparin-based drugs. SIGNIFICANCE STATEMENT: This state-of-the-art review covers recent developments in the use of heparin and heparin-like materials as anticoagulant, now including immunothrombosis observations, and as nonanticoagulant including a role in the treatment of SARS-coronavirus and inflammatory conditions.

The anticoagulant and antithrombotic mechanisms of heparin

The molecular basis for the anticoagulant action of heparin lies in its ability to bind to and enhance the inhibitory activity of the plasma protein antithrombin against several serine proteases of the coagulation system, most importantly factors IIa (thrombin), Xa and IXa. Two major mechanisms underlie heparin's potentiation of antithrombin. The conformational changes induced by heparin binding cause both expulsion of the reactive loop and exposure of exosites of the surface of antithrombin, which bind directly to the enzyme target; and a template mechanism exists in which both inhibitor and enzyme bind to the same heparin molecule. The relative importance of these two modes of action varies between enzymes. In addition, heparin can act through other serine protease inhibitors such as heparin co-factor II, protein C inhibitor and tissue factor plasminogen inhibitor. The antithrombotic action of heparin in vivo, though dominated by anticoagulant mechanisms, is more complex, and interactions with other plasma proteins and cells play significant roles in the living vasculature.

Mechanistic basis of heparin-induced thrombocytopenia

Heparin-induced thrombocytopenia (HIT) occurs in 1% to 5% of patients treated with unfractionated high-molecular weight heparin and often results in limb- and/or life-threatening thrombotic complications. This disorder involves the formation of antibodies to a complex of Platelet Factor 4 (PF4) with the heparin. We believe that the pathogenesis of this disease begins with having an excess of platelet PF4 release. The freed PF4 is bound to platelet and other vascular membrane surfaces and has three critical roles in the initiation of HIT. (1) Infused heparin neutralizes a portion of excess surface PF4, directly enhancing local thrombosis. (2) The excess PF4 is mobilized into PF4/heparin complexes that stimulates HIT antibody production. (3) The remaining PF4 complexed to heparanoids and heparin on the vascular surfaces now bind to these HIT antibodies and through surface Fc gammaRII receptors leads to more platelet activation and removal, thrombus formation, and inflammation. These events, in turn, further stimulate PF4 release, perpetuating repetitive cycle that results in the clinical manifestations of this disorder.

Study of low molecular weight heparin effect on the relation between anticoagulant activity and antithrombin III affinity

Low molecular weight heparin (FR-860), and conventional unfractionated heparin (UF-heparin) were fractionated by rabbit antithrombin III (AT III)-Sepharose, and the effects of each affinity fraction on the coagulation and fibrinolytic activities were investigated. FR-860 was fractionated to no-affinity, low-affinity (LA) and high-affinity (HA) fractions, and UF-heparin to LA and HA fractions. The HA fractions showed higher activities regarding the prolongation of activated partial thromboplastin time, anti-factor Xa activity and antithrombin activity compared with those of LA. The HA and LA fractions exhibited the enhancement of heparin cofactor II (HC II) activity and fibrinolytic activity in a dose-dependent manner. These results suggest that the antithrombotic activity of FR-860 is exerted through AT III and other mechanism such as HC II-mediated system.

Antithrombotic effects of heparin oligosaccharides

Size homogeneous heparin oligosaccharides were prepared from nitrous acid depolymerized heparin by means of repeated gel filtration chromatography. These oligosaccharides were then further separated with respect to affinity for antithrombin by means of affinity chromatography. All the high-affinity oligosaccharides thus obtained had a strong ability to potentiate factor Xa inhibition while their ability to inhibit factor IIa abruptly dropped below a chain length of 20 monosaccharides. In a rabbit stasis model, high-affinity oligosaccharides below a chain length of 20 units also showed a continuous decrease in antithrombotic effect with increasing degree of depolymerization. However, there was no distinct drop paralleling the thrombin inhibiting capacity. Low-affinity oligosaccharides also exhibited a weak antithrombotic effect, although they did not always contribute to an increased anti-factor Xa activity ex vivo. This was the case whether or not they were administered alone or in combination with high-affinity oligosaccharides. Low-affinity oligosaccharides may therefore exert an antithrombotic effect per se with a mechanism of action that is independent of antithrombin III.

Pharmacology and special clinical applications of low-molecular-weight heparins

In this overview, the rationale of the development of low-molecular-weight (LMW) heparins and their toxicological, anticoagulant, fibrinolytic, lipolytic, and protamine interactions are summarized. Clinical experiences are reviewed on the benefit of LMW heparin for anticoagulation in patients with bleeding and other complications on conventional anticoagulants and during pregnancy. It is concluded that animal experiments have demonstrated the safety of LMW heparins, that the pharmacologic profile is improved compared with normal heparin, and that the simple and safe applicability of LMW heparins gives rise to new indications for the long-term prophylaxis of thromboembolism.

Clinical efficacy of heparin fractions: issues and answers

The recent development of heparin fractions and fragments for clinical use has created the prospect of some new agents at our disposal for the treatment of thrombotic disorders. The development of a drug that will block thrombosis but will not impair hemostasis now appears to be a possibility. Due to lack of understanding of all of the mechanisms of the pathology of thrombosis, we are not certain what the properties of the ideal anticoagulant should be. Of the heparins and heparin fractions, fragments, and heparinoids now available to us, we have yet to fully understand the mechanism of their pharmacologic activity. It has been amply demonstrated that decreasing the average molecular weight decreases the antithrombin activity while retaining the anti-Xa activity of heparin derivatives. Studies on animal models have proven the antithrombotic potency of some of these low molecular weight heparins to be equal to that of unfractionated heparin. There has been some evidence that these fractions are less likely than unfractionated heparin to cause hemorrhage in animal models as well as in at least one human clinical trial. A recently published human clinical trial revealed an unexpected incidence of hemorrhage following major surgery when a certain heparin fraction was given as prophylaxis against thrombosis. We are desperately in need of heparin derivatives, heparinoids or other anticoagulants that can be used in place of standard heparin in patients who are allergic to heparin or who have heparin-induced thrombocytopenia. Patients with these problems not infrequently require cardiopulmonary bypass surgery in which the use of heparin has been mandatory. There is now evidence from animal and human studies that such a procedure is possible with a heparin fraction or a heparinoid. This is true for hemodialysis as well. Studies in progress offer hope that a low molecular weight fragment with potent anti-Xa activity will not cause thrombocytopenia in patients with heparin-induced thrombocytopenia. Whether this agent, a pentasaccharide, will have sufficient antithrombotic potency for clinical use remains an important question. An important property of some of the newer heparin fractions is a prolonged duration of action which may permit fewer doses, thus reducing the cost as well as patient discomfort and inconvenience.

The effect of heparin fragments of different molecular weights on experimental thrombosis and haemostasis

The effect of heparin fragments of different molecular weights has been compared with that of conventional sodium heparin on experimental thrombosis in vivo and ex vivo and experimental haemostasis in vivo. In the first part of the study fragments of different molecular weights were given (4,900, 6,500, 9,500 and 22,200 dalton). All preparations including the control gave a significant prolongation of the haemostatic plug formation time in the rabbit mesenteric microcirculation, and all except the fragment with the lowest molecular weight reduced the frequency of jugular vein thrombosis (induced by a combination of endothelial denudation and stasis). There was a correlation between the XaI activity of the different heparin fragments and frequency of thrombosis. Using an ex vivo method (modification of Chandler's model) a dose dependent lag phase until start of thrombus formation was found. In the second part of the study a dose response investigation was made comparing different doses of a fragment (6,500 dalton) with conventional heparin in the same XaI doses (10, 30 and 60 units/kg). Sodium heparin in the highest dose prolonged the haemostatic plug formation time whereas none of the fragment doses did. The lowest dose both of the fragment and conventional heparin did not reduce the frequency of thrombosis, whereas the two higher doses did. Thus it may be possible to obtain preventive effect on thrombus formation with a heparin fragment.

Effect of heparin and low molecular weight heparins on thrombin-induced blood platelet activation in the absence of antithrombin III

We have investigated the antithrombin III independent effect of crude heparin, two heparin fractions and a heparinoid on in vitro thrombin-induced platelet activation. Thrombin-induced platelet factor Va generation and thrombin plus collagen-induced platelet prothrombin converting activity were tested. Crude heparin was a more potent inhibitor of these reactions than the fractions or the heparinoid. The inhibitory action of the heparins was found to be the result of a direct effect on thrombin and not of an effect either on platelet activation functions or on the assembly or functioning of the prothrombinase complex. Probably this heparin inhibition is due to the masking of secondary macromolecular substrate binding sites on the thrombin molecule. We found no correlation between IC50 values and the antithrombin III-dependent antithrombin specific activities of the heparins. This supports the notion that heparin properties other than their affinity for antithrombin III may contribute to the action of this drug in blood coagulation.