Standardization of Low Molecular Weight Heparins : A Collaborative Study

Comparative Studies on the Anticoagulant Profile of Branded Enoxaparin and a New Biosimilar Version

Low molecular weight heparins (LMWH) represent depolymerized heparin prepared by various methods that exhibit differential, biochemical and pharmacological profiles. Enoxaparin is prepared by benzylation followed by alkaline depolymerization of porcine heparin. Upon the expiration of its patent, several biosimilar versions of enoxaparin have become available. Heparinox (Sodic enoxaparine; Cristália Produtos Químicos Farmacêuticos LTDA, Sao Paulo, Brazil) is a new biosimilar form of enoxaparin. We assessed the molecular weight and the biochemical profile of Heparinox and compared its properties to the original branded enoxaparin (Lovenox; Sanofi, Paris, France). Clotting profiles compared included activated clotting time, activated partial thromboplastin time (aPTT), and thrombin time (TT). Anti-protease assays included anti-factor Xa and anti-factor IIa activities. Thrombin generation was measured using a calibrated automated thrombogram and thrombokinetic profile included peak thrombin, lag time and area under the curve. USP potency was determined using commercially available assay kits. Molecular weight profiling was determined using high performance liquid chromatography. We determined that Heparinox and Lovenox were comparable in their molecular weight profile. Th anticoagulant profile of the branded and biosimilar version were also similar in the clot based aPTT and TT. Similarly, the anti-Xa and anti-IIa activities were comparable in the products. No differences were noted in the thrombin generation inhibitory profile of the branded and biosimilar versions of enoxaparin. Our studies suggest that Heparinox is bioequivalent to the original branded enoxaparin based upon in vitro tests however will require further in vivo studies in animal models and humans to determine their clinical bioequivalence.

History of heparin

The history of heparin is described from its initial discovery in 1916 to recent developments in knowledge of its mechanism of action and clinical use. Commercial production started soon after its discovery, in the 1920s, and improved purification methods led to animal studies and the first clinical trials in the 1930s. Research into heparin's chemical structure proved difficult, with uncertainty about the uronic acid moiety and the N-acetyl content, but the structure of the basic disaccharide unit was established by the 1960s, though knowledge of the heterogeneity and fine structure of heparin chains continued to accumulate over the next 20 years. In 1976, it was found that only one third of heparin chains bound with high affinity to antithrombin, and subsequent studies identified a unique pentasaccharide sequence, which was essential for antithrombin binding and anticoagulant activity - this pentasaccharide was synthesised in 1983. Clinical usage of heparin continued to increase and two major developments were the use of low- dose heparin for prevention of deep vein thrombosis and pulmonary embolism, and the development of low-molecular-weight heparin as a separate drug.

Standardisation of unfractionated and low-molecular-weight heparin

Unfractionated and low-molecular-weight heparins are complex biologicals. Standardisation and global harmonisation of units and methods of measurement are essential for safety and efficacy of this important class of anticoagulants. This chapter describes the traceability of the international unit and current status of the relationship between the international and pharmacopoeial standards, together with a review on current pharmacopoeial assay methods.

New-generation anticoagulants: the low molecular weight heparins

Heparin has been the mainstay of acute anticoagulation therapy for decades. Within the past 20 years, several different heparin fractions-collectively known as low molecular weight heparins (LMWHs)-have been evaluated in various medical and surgical settings in which anticoagulation is routinely warranted. The LMWHs are efficacious, safe, cost-effective, and easier to administer and monitor than standard, unfractionated heparin. As LMWH use becomes more widespread, emergency physicians will use these new agents instead of unfractionated heparin for unstable angina, non-Q-wave myocardial infarction, or thromboembolic disease. This review focuses on the pharmacologic properties of unfractionated heparin and LMWH, associated complications, and the use of these agents in acute ischemic coronary syndromes, thromboembolic disease, and other selected clinical situations.

Pharmacology of the low-molecular-weight heparins

Unfractionated heparin (UFH) is widely used to both treat and prevent venous thromboembolism. More recently, UFH has been used to prevent death and myocardial infarction in patients with unstable angina or acute myocardial infarction and acute occlusion in those undergoing percutaneous coronary revascularization. However, its poor bioavailability (when administered in low doses subcutaneously), its mechanism of clearance, and its short half-life make its anticoagulant activity difficult to predict and maintain. To overcome these limitations, low-molecular-weight heparins (LMWHs) have been developed that have greater bioavailability and a longer half-life in plasma. Because LMWHs provide more predictable anticoagulant activity compared with subcutaneous UFH, it is not necessary to monitor the activated partial thromboplastin time during treatment. These newer agents are as effective as UFH in the prophylaxis and treatment of thromboembolic and cardiac disorders and, by allowing shorter hospital stays, are more cost effective. Thus LMWHs offer clear pharmacokinetic advantages over UFH. More studies are needed to determine the extent to which clinically available LMWHs can be used in place of UFH.

Determination of the anti-factor Xa activity of the synthetic pentasaccharide SR 90107A/ORG 31540 and of two structural analogues

The anti-factor Xa activity of the synthetic pentasaccharide SR 90107A/ORG 31540 was assayed by a chromogenic method at pH 8.4 and pH 7.35, comparatively to the 4th International Heparin Standard (IHS) or to the Ist International Low Molecular Weight Heparin Standard (LMWHS). At pH 8.4, SR 90107A/ORG 31540 was found to have a specific anti-factor Xa activity of 639 +/- 14 and 659 +/- 19 IU/mg (mean +/- sem, n = 6) when assayed in comparison with the 4th ISH and the Ist LMWHS respectively. At pH 7.35, the corresponding figures were 864 +/- 6 and 1160 +/- 51 IU/mg (mean +/- sem, n = 6) respectively. The dissociation constants of the ATIII-pentasaccharide complex formed by SR 90107A/ORG 31540 and by two close analogues: SR 80327A and SR 80027A in the presence of purified human ATIII were found to be 41 +/- 8, 96 +/- 1 and 3 +/- 1.4 nM (mean +/- sem, n = 3) respectively. For the three compounds, the pseudo-first order molar catalytic constants for factor Xa inactivation by the ATIII-pentasaccharide complex were shown to be statistically comparable, in the range of 7-8 x 10(7) min-1 per mole. It is concluded that the differences in specific anti-factor Xa activities between SR 90107A/ORG 31540 and its synthetic chemical analogues can be attributed to variations of the dissociation constants whereas the catalytic constants for factor Xa inactivation remain unchanged.

Enoxaparin: the low-molecular-weight heparin for prevention of postoperative thromboembolic complications

OBJECTIVE

To introduce readers to a new low-molecular-weight heparin (LMWH) product, enoxaparin. The chemistry, pharmacology, pharmacodynamics, clinical efficacy in thromboembolic prophylaxis following surgery, and adverse effects are reviewed.

DATA SOURCES

A MEDLINE search of the English-language literature was used to identify relevant literature.

STUDY SELECTION

A focus was placed on human clinical studies with well-accepted measures of antithrombotic efficacy endpoints, i.e., venography and ultrasonography. Emphasis was on pharmacologic and pharmacokinetic studies conducted in humans.

DATA EXTRACTION

Most data were extracted from double-blind, controlled clinical studies. Other study designs were accepted if the results were believed to be significant. Pharmacology and pharmacokinetic data were selected from studies with exceptional design conducted in humans.

DATA SYNTHESIS

Enoxaparin is a polysaccharide chain produced by the depolymerization of heparin. In comparison with heparin, which has an average molecular weight of 12,000-15,000 daltons, the average molecular weight of enoxaparin is approximately 4500 daltons. Enoxaparin does not form a complex with antithrombin III and thrombin as extensively as does heparin; however, the anti-Xa activity of enoxaparin is similar. The significance of this fact is an enhancement of antithrombotic activity and clinical efficacy. Trials comparing enoxaparin with other thromboembolic prophylaxis techniques are ongoing.

CONCLUSIONS

Thromboembolism remains one of the major complications of all surgical procedures. Attempts have been made throughout the last century to develop the most effective means to prevent this complication. Clinical studies performed throughout the world have shown that enoxaparin is superior or equivalent to other antithrombotic agents, including heparin, in preventing the formation of venous thromboembolism. In addition, enoxaparin appears to possess an equivalent or lower incidence of bleeding complications when compared with heparin prophylaxis. Enoxaparin is expected to be joined by other LMWH products in the future. As a result, the methods of providing effective prophylaxis against thromboembolic complications is expected to change in the coming years.

Usefulness of thrombelastography for dosage monitoring of low molecular weight heparin and unfractionated heparin during hemodialysis

Low molecular weight heparin (LMH) acts as an anticoagulation agent mainly through its anti-activated coagulation factor X (Xa) activity. Thrombelastography (TEG) is expected to be useful to monitor the dosage of LMH during hemodialysis because reaction time on TEG (TEG-r) is considered to reflect blood thromboplastin formation time, which depends on the formation of Xa. To test this possibility, we compared the usefulness of TEG, activated coagulation time (ACT), activated partial thromboplastin time (APTT), and anti-Xa activity in 28 hemodialysis patients using both conventional unfractionated heparin (UFH) and LMH on separate dialysis procedures. Anti-Xa activity of LMH was comparable to that of UFH when it was measured using both LMH and UFH as standards. Anti-Xa activity, which theoretically depended on the heparin concentration in blood samples, did not correlate with the degree of dialyzer clotting. The APTT correlated well with anti-Xa activity in patients using LMH (r = 0.686, p less than 0.01) and UFH (r = 0.906, p less than 0.01), but not with the degree of dialyzer clotting. The TEG-r correlated well with the degree of dialyzer clotting both in patients using LMH and those using UFH (measurements of samples obtained from the venous side of the extracorporeal circuit) and weakly correlated with anti-Xa activity in patients using LMH (r = 0.402, p less than 0.05). The ACT did not correlate with the degree of dialyzer clotting or anti-Xa activity. These results suggest that TEG-r reflects the efficacy of heparin in the extra-corporeal blood circuit, whereas APTT mainly reflects heparin concentration of the blood samples.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanism of heparin action

Heparin catalysis of clotting proteinase inactivation occurs most efficiently through the reaction of the proteinase with the antithrombin-heparin complex. The efficiency of a heparin molecule in this reaction depends on the presence of a specific pentasaccharide sequence in it, and its molecular weight. The mechanism by which such high affinity heparin acts when antithrombin III is the inhibitor is promotion of the formation of an intermediate proteinase-heparin-antithrombin complex. Heparin promotion of thrombin inactivation by heparin cofactor II may occur by a similar mechanism. The requirement for a specific oligosaccharide sequence within the heparin molecule does not, however, exist for heparin cofactor II. Binding of heparin to both thrombin and antithrombin III interferes with thrombin inactivation. This binding is very dependent on the ionic strength of the reaction mixture and may explain some of the discordant results and interpretations from early studies on the mechanism of heparin action. Low ionic strength in in vitro reactions also results in cleavage of antithrombin III by thrombin in the presence of heparin and effectively converts antithrombin III from an inhibitor to a substrate.

Release of lipoprotein lipase and hepatic lipase activities. Effects of heparin and a low molecular weight heparin fragment

Unfractionated heparin and a low molecular weight heparin fragment (LMWH, mean molecular weight 5000) were compared with respect to the ability to release lipoprotein lipase (LPL) and hepatic lipase (HL) from tissue binding sites. The investigations were carried out in vivo and in vitro in man and in the rat. The in vivo release of LPL activity was greater with heparin in both species. The release of HL activity was equal with both heparins in man, but greater with unfractionated heparin in the rat. In the in vitro studies the LMWH fragment consistently released more LPL activity from the tissues investigated, i.e. fat, skeletal muscle and heart muscle. With isolated adipocytes, however, we could not demonstrate any difference between the two heparins with respect to their lipase-releasing effect. In liver tissue homogenate the two heparins showed the same ability to release HL activity. It is concluded that not only biochemical differences between the two heparins are of importance for the difference in the lipolytic effect. The possibility of different rheological behaviours of the two heparins in the capillary lumen, which might partly explain the greater in vivo lipolytic effects of heparin, is discussed. It is suggested that LMWH enters the tissue preparation more readily in vitro, thereby exerting a greater lipase-releasing effect than heparin.

Treatment of acute venous thromboembolism with low molecular weight heparin (Fragmin). Results of a double-blind randomized study

We performed a prospective, randomized, double-blind trial in 194 unselected patients to determine the safety and efficacy of low molecular weight heparin (Fragmin) compared with standard heparin as the initial treatment of acute venous thromboembolism. Ninety-eight patients received continuous intravenous heparin, and 96 patients received Fragmin for 5-10 days. Doses were adjusted to maintain anti-Xa levels between 0.3 and 0.6 unit/ml for patients with a high risk for a bleeding complication and between 0.4 and 0.9 unit/ml for patients with a low risk for bleeding. Treatment was stopped when a therapeutic level of anticoagulation (International Normalized Ratio greater than 3.5) was reached with coumarins. Thirteen patients in the heparin group and 10 patients in the Fragmin group had a major bleeding complication. The incidence of major and minor bleeding complications combined decreased from 48.9% to 38.5% (95% confidence interval for the difference, -3.5% to +24.2%), corresponding with a relative bleeding risk reduction of 21.2%. There were no significant differences in efficacy as defined by new high-probability defects on repeat ventilation-perfusion scintigraphy of the lung in 80 patients: six of 46 patients in the heparin group and 3 of 34 patients in the Fragmin group had new defects (95% confidence interval for the difference, -9.4% to +17.8%). We conclude that low molecular weight heparin (Fragmin) given in adjusted, continuous, and intravenous doses is safe and effective as initial treatment of acute venous thromboembolism compared with heparin. There is a trend in risk reduction for bleeding in favor of low molecular weight heparin, a trend, however, that is smaller than expected compared with animal studies.

Biochemical and pharmacologic inequivalence of low molecular weight heparins

LMW heparin fractions obtained from various sources must not be considered as bioequivalent in both the in vitro and in vivo responses. Because of compositional variations, these agents exhibit individual behavior and should be considered as distinct drugs whose safety and efficacy profile must be determined separately. Currently, there is no valid LMW heparin standard available, however, different LMW heparins can be profiled in identical test systems. It is erroneous to assume that most LMW heparins will behave in a similar fashion in terms of safety and efficacy. The need for defined tests to characterized these agents is evident and efforts to profile their actions should be made at both the basic and applied levels. Needless to say, the true efficacy of LMW heparins can only be validated in well-designed randomized clinical trials. Optimization of LMW heparins in preclinical pharmacologic studies, as reported here, is a crucial factor in the development of these agents. The superior clinical efficacy/safety performance of some of the LMW heparins in contrast to other LMW heparins is a result of extensive preclinical pharmacologic investigations undertaken to optimize the therapeutic index of these agents. Such optimization studies have not been conducted during the development of many LMW heparins, resulting in decreased efficacy and undue complications.

The anticoagulant activity of heparin: measurement and relationship to chemical structure

For many years the anticoagulant activity of heparin has been estimated by coagulation assays, in which the prolongation of clotting times by heparin is measured under various conditions. More recently, assays have been developed which measure the inhibitory action of heparin on isolated coagulation enzymes, notably Factor Xa and thrombin, using specific amidolytic peptide substrates. The anticoagulant activity of heparin arises primarily from its ability to bind to antithrombin III (AT III), altering the conformation and enhancing the activity of this major protease inhibitor. Passage of heparin through an immobilised AT III column yields two fractions: a high affinity fraction with 300-350 iu mg-1 anticoagulant activity, comprising one-third of the total, and a low affinity fraction with an activity of less than 10 iu mg-1, comprising the remaining two-thirds. Studies in several laboratories have demonstrated that a specific pentasaccharide sequence is required for AT III binding. The authors have shown that the presence or absence of this sequence can be detected by high-field proton NMR, thus providing a semi-quantitative method for a functionally important group. A second major influence on anticoagulant activity is molecular weight distribution. Studies in the authors' laboratory on a series of fractions of 5000-35,000 showed that whereas anticoagulant activity in APTT clotting assays decreased with decreasing molecular weight (Mr), activity in anti-Xa assays was maintained or increased in the low Mr fractions. However, in vivo studies showed that high affinity fragments with anti-Xa activity only were poor antithrombotic agents. It appears that the presence of the AT III binding site alone is not sufficient for full antithrombotic activity; an extra length of polysaccharide chain of at least 15 residues is required. Molecular weight distribution is readily assessed by HPLC, although the lack of suitable reference materials hampers assignment of absolute molecular weights. Important determinants of anticoagulant activity can now be assessed by physicochemical techniques but, at present, these techniques are not precise enough to replace anticoagulant assays as predictors of in vivo behaviour.

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.

Thrombosis prophylaxis with low molecular weight heparin in total hip replacement

In a randomized prospective trial, the efficacy of low molecular weight heparin (LMWH) (Fragmin) and dextran 70 (Macrodex) in preventing deep vein thrombosis (DVT) in the legs was evaluated in 98 consecutive patients undergoing elective total hip replacement. The patients were randomly allocated to receive either 2500 anti-factor Xa units LMWH twice daily for 7 days, with the first dose given 2 h before surgery; or 500 ml dextran 70 twice during the day of operation, followed by a single infusion of 500 ml on the first and again on the third postoperative day. DVT was assessed by 125I-fibrinogen test for 2 weeks postoperatively, a positive test being followed by phlebography. DVT developed in 22 (45 per cent) of 49 patients receiving dextran 70 and in 10 (20 per cent) of 49 patients in the LMWH group (P less than 0.01). LMWH was thus statistically significantly better than dextran 70 in preventing DVT in the legs. It was not firmly established whether this benefit was also valid in the high ileofemoral region. Two patients with non-fatal pulmonary embolism were found in each group. Per- and postoperative blood loss and blood transfusion requirements were significantly lower in the LMWH group.

Low molecular weight heparin given the evening before surgery compared with conventional low-dose heparin in prevention of thrombosis

A prospective randomized double-blind trial was performed comparing conventional low-dose heparin with a low molecular weight heparin fragment for thromboprophylaxis in elective general abdominal surgical patients. The first dose of the heparin fragment was given the evening before surgery, and further doses were given thereafter every evening. There were 1002 analysable patients, 826 having received correct prophylaxis. Of these 1002 patients, 64 per cent were operated on for malignant disease. A total of 20 patients died, 10 in each group. The frequency of deep vein thrombosis was significantly reduced among patients with correct prophylaxis with the heparin fragment (9.2-5.0 per cent, P = 0.02) [corrected]. The frequency of bleeding was 6.7 per cent among the heparin fragment patients and 2.7 per cent among the patients given conventional heparin (P = 0.01), but all bleeds were of minor degree and there was no difference in the reoperation rate for bleeding, or in the transfusion requirements. Local pain at the injection site was reported significantly less often among patients given the heparin fragment.

Characterization of platelet binding of heparins and other glycosaminoglycans

The binding of glycosaminoglycans to intact washed human platelets was studied. The platelet binding of a 3H-labeled unfractionated heparin was saturable and reached equilibrium in 10-15 minutes. Heparin binding was specific: a 50-fold molar excess of an equivalent unlabeled heparin displaced up to 90% of labeled heparin, while chondroitin sulfate A and hyaluronic acid minimally displaced the binding of labeled heparin. Low molecular weight heparin fragments showed intermediate efficacy in displacing the binding of unfractionated [3H]heparin. Dextran sulfate (Mr 8,000, sulfation 17%) was as potent as unfractionated heparin in displacing binding, while neutral dextrans were ineffective. We observed that platelet activation by the calcium ionophore A23187 increased heparin binding by 2 to 3-fold, principally by enhancement of binding capacity not binding affinity. This process of heparin binding to the platelet surface may mediate some of the reported effects of heparin on platelet behavior.

Comparison between a low molecular weight and standard heparin for anticoagulation during extracorporeal CO2 removal in the dog

Anticoagulant properties towards an artificial surface of a chemically depolymerized low molecular weight heparin (LMWH) have been compared to those of a standard heparin (SH). The experimental model consisted in a seven hours extracorporeal veno-venous bypass for CO2 removal (EC-CO2R) using a membrane lung. Four animals received 150 anti-FXa U/kg followed by 40 anti-FXa U/kg/h of LMWH or 300 IU/kg followed by 100 IU/kg/h of SH. Mean Factor Xa inhibition was 49% in LMWH group and 28.5% in SH group. Mean Factor IIa inhibition was 31% and 49% respectively. After three hours of bypass fibrin deposition occurred in the reservoir in three out of four dogs receiving LMWH while none was observed under SH. No statistically significant difference between the two groups was found for any of the coagulation parameters tested (fibrinogen, factor V, antithrombin III, plasminogen, alpha 2-antiplasmin, platelet counts). At the end of bypass 5000 U protamine abolished both anti-FXa and anti-FIIa activities in the SH group but failed to neutralize more than half of the anti-FXa activity in the LMWH group. These results suggest that high anti-FIIa activities are required to prevent fibrin formation induced by artificial surfaces and that equivalent amounts of anti-FXa activities are ineffective for this purpose. In addition the use of LMWH may raise problems when emergency neutralization procedures are required.