Low molecular weight heparins: differences and similarities in approved preparations in the United States

Generic Forms of Low-Molecular-Weight Heparins: Some Practical Considerations

This commentary briefly reviews the controversies of therapeutic and generic interchangeability, as they apply to the antithrombotic drug class called low-molecular-weight heparin (LMWH). Recommendations are prepared for the generic LMWH approval process by various regulatory bodies.

Comparison of the efficacy and safety of low molecular weight heparins for venous thromboembolism prophylaxis in medically ill patients

OBJECTIVE

To conduct a systematic review and mixed-treatment comparison (MTC) meta-analysis to compare the efficacy and safety of low molecular weight heparins (LMWHs) for venous thromboembolism (VTE) prophylaxis in hospitalized medically ill patients. As a secondary objective we compared all therapies within the network to each other.

METHODS

We conducted a systematic literature search for randomized trials that evaluated pharmacologic VTE prophylaxis in hospitalized medically ill patients. We conducted a traditional meta-analysis for all pairwise comparisons using a random effects model, reporting relative risks (RRs) and 95% confidence intervals for each outcome. To determine the relative efficacy and safety of included therapies we conducted a MTC meta-analysis using a Bayesian framework, reporting odds ratios (OR) and 95% credible intervals.

RESULTS

Twenty trials met inclusion criteria. Enoxaparin, dalteparin, nadroparin and certoparin were the LMWHs evaluated although none in direct comparative trials. Upon MTC, the relative efficacy of all LMWHs was similar in preventing mortality and VTE as well as in the odds of major and minor bleeding. Dalteparin was not included in the network to evaluate deep vein thrombosis (DVT) and pulmonary embolism (PE) due to lack of reported data and the remaining LMWHs were found to be similar in relative efficacy in preventing these outcomes.

LIMITATIONS

Traditional meta-analysis was not possible for many drug comparisons made within the MTC. Heterogeneity was observed in several of the traditional meta-analyses although this may be an inherent limitation of the studied population. Overall rarity of events contributed to imprecise estimates demonstrated by the wide confidence intervals.

CONCLUSIONS

Enoxaparin, dalteparin, nadroparin and certoparin are similar in relative efficacy for the prevention of mortality and VTE and in the odds of major or minor bleeding while enoxaparin, nadroparin and certoparin are similar in relative efficacy for the prevention of PE and DVT in hospitalized medical patients.

Are all low molecular weight heparins equivalent in the management of venous thromboembolism?

Low molecular weight heparins are replacing unfractionated heparin in a number of clinical indications because of their improved subcutaneous bioavailability and more predictable antithrombotic response. Clinical trials have demonstrated that low molecular weight heparins are at least as safe and effective as unfractionated heparin for the initial treatment of venous thromboembolism, and unfractionated heparin and warfarin for primary and secondary thromboprophylaxis. The mechanism behind the antithrombotic action of low molecular weight heparins is not fully understood but is likely to involve inhibition of coagulation factors Xa and IIa (thrombin), release of tissue-factor-pathway inhibitor, and inhibition of thrombin activatable fibrinolytic inhibitor. Different low molecular weight heparins have been shown to have various effects on coagulation parameters. Seven low molecular weight heparins are currently marketed worldwide, each demonstrated distinct chemical entities with unique pharmacokinetic and pharmacodynamic profiles. Each low molecular weight heparin is approved for specific indications based on the available efficacy and safety data for that product. The relative efficacy and safety of the low molecular weight heparins are unclear because there have been very few direct comparisons in randomized clinical trials. While recommending low molecular weight heparins for the prevention and treatment of venous thromboembolism, clinical guidelines have not specified individual agents. National and international organizations recognize that low molecular weight heparins are distinct entities and that they should not be used interchangeably in clinical practice. Each low molecular weight heparin should be used at the recommended dose when efficacy and safety data exist for the condition being treated. When these data are not available, the dosing and administration of low molecular weight heparins must be adapted from existing data and recommendations.

Effect of the anti-factor Xa and anti-factor IIa activities of low-molecular-weight heparins upon the phases of thrombin generation

BACKGROUND

Low-molecular-weight heparins (LMWHs), derived from unfractionated heparin (UFH) by different depolymerization procedures, vary in both their relative abilities to enhance the inhibition of FXa (anti-FXa) and thrombin (anti-FIIa), and in their physicochemical properties.

OBJECTIVE

We aimed to profile the inhibition of thrombin generation induced by bemiparin, enoxaparin, nadroparin, dalteparin and tinzaparin in platelet-rich plasma (PRP), and to compare them with UFH and fondaparinux (a synthetic pentasaccharide that specifically enhances FXa inhibition).

METHODS

Different LMWHs, UFH or fondaparinux were added to normal PRP. Thereafter, tissue factor-triggered thrombin generation was assessed using the Thrombogram-Thrombinoscope assay.

RESULTS

At equivalent anti-FIIa activity concentrations, LMWHs and UFH exhibited similar inhibitory effects upon thrombin generation. However, when used at equivalent anti-FXa activity concentrations, tinzaparin was significantly more active than the other LMWHs at inhibiting thrombin generation, and had similar activity to that of UFH. Enoxaparin, nadroparin and dalteparin all showed similar inhibitory activities. In these experiments, bemiparin exhibited the lowest inhibitory effect on thrombin generation of all the LMWHs. At 0.1 microg mL(-1) (0.093 anti-FXa IU mL(-1)), fondaparinux inhibited the rate of thrombin generation by 50%. A 7-fold higher concentration of fondaparinux was required to inhibit the endogenous thrombin potential by 50%.

CONCLUSIONS

LMWHs have a variable inhibitory effect on thrombin generation in vitro when compared by anti-FXa activity, but are similar when compared by their anti-FIIa activities. The rate of thrombin generation during the propagation phase, rather than the endogenous thrombin potential, is more sensitive to the anticoagulant activity of fondaparinux and the polysaccharide chains of LMWHs possessing only anti-FXa activity.

The Effect of Low-Molecular-Weight Heparin on Microvenous Thrombosis in a Rat Model

OBJECTIVE

To assess the impact of a low-molecular-weight heparin sodium, dalteparin sodium, on a thrombogenic microvenous anastomosis, using a randomized, blinded animal model.

METHODS

Using male Sprague-Dawley rats, 70 IU/kg of dalteparin sodium (for the treatment group) or isotonic sodium chloride solution (for the control group) were administered subcutaneously in a blinded randomized fashion. Using microsurgical techniques, the femoral venous pedicle was isolated bilaterally. A tuck anastomosis was then performed on each side. Vessel patency was assessed periodically for 3 hours using a strip and refill test. Patency or thrombosis was confirmed by cutting the vessel proximal to the anastomosis and examining the lumen for thrombus.

RESULTS

A total of 58 venous tuck anastomoses were performed. There was no difference in bleeding complications between the treatment and control groups. The control group had a thrombosis rate of 50%, and the treatment group had a thrombosis rate of 60%. The chi2 analysis does not indicate a statistical difference between these 2 groups (P = .59).

CONCLUSION

Low-molecular-weight heparin, at standard therapeutic dosing, may not provide an adequate antithrombotic effect to prevent anastomotic thrombosis in free tissue transfer.

Low-molecular-weight heparin (dalteparin) effectively prevents thrombosis in a rat model of deep arterial injury

Unfractionated heparin is often used to prevent thrombosis in microvascular surgery, but a major drawback of heparin therapy is increased bleeding. Low-molecular-weight heparins prevent venous thrombosis as effectively as heparin and have better bioavailability and a longer plasma half-life, which explains the increased use of low-molecular-weight heparins as substitutes for heparin in clinical practice. However, the ability of low-molecular-weight heparins to prevent arterial thrombosis has been debated. In this study, the authors compared the antithrombotic and antihemostatic effects of heparin and the low-molecular-weight heparin dalteparin in a rat model of arterial thrombosis. A segment of the left common carotid artery was isolated between vascular clamps and opened longitudinally. An endarterectomy was performed and the arteriotomy was closed with a running suture. The antithrombotic effect (vascular patency 31 minutes after reperfusion) and the surgical bleeding were measured. Groups of 10 rats were treated in a blind, random fashion with intravenous injection of one of the following substances 1 minute before clamp release. Three groups received a bolus of heparin (20, 60, or 180 IU anti-factor Xa/kg), three groups received dalteparin (60, 180, or 540 IU anti-factor Xa/kg), and one group was treated with vehicle (saline). Heparin 180 IU/kg produced a distinct antithrombotic effect compared with the control group (p = 0.03), but it also significantly increased the surgical bleeding to 2.0 g compared with 1.5 g in the control group (medians, p = 0.01). Dalteparin 180 and 540 IU/kg also produced a powerful antithrombotic effect (p = 0.01 and p = 0.03, respectively). In contrast to heparin, 180 IU/kg dalteparin did not increase the surgical bleeding (median, 1.5 g; p = 0.37 versus controls). Dalteparin 540 IU/kg increased the median surgical bleeding to 2.6 g (p = 0.06 versus controls). The nonsignificant difference may be explained by the great interindividual variation of surgical bleeding in the high-dose dalteparin group. Dalteparin prevented arterial thrombosis as effectively as unfractionated heparin. In contrast to heparin, dalteparin did not increase the surgical bleeding, which indicates that dalteparin instead of heparin can be used to prevent thrombosis in microvascular surgery.

Management of thrombotic and cardiovascular disorders in the new millenium

Anticoagulants and antithrombotic drugs have played a key role in the prophylaxis, treatment and surgica/interventional management of thrombotic and cardiovascular disorders. There are several newer drugs which are currently developed for the anticoagulant management of cardiovascular diseases in both the medical and surgical indications. These include the low molecular weight heparins (LMWHs), antithrombin agents such as the Hirudin, Hirulog and Argatroban and indirect and direct anti-Xa drugs, represented by Pentasaccharide (Arixtra) and DX 9065a, respectively. Several other agents such as the natural and recombinant anti-Xa drugs and anti-tissue factor agents are also developed. The antiplatelet agents include Clopidogrel, Cilostazol, Anplag and GP IIb/IIIa inhibitors. For the subcutaneous indications, unfractionated heparin is gradually replaced by the low molecular weight heparins (LMWHs). LMWHs such as the Enoxaparin and Dalteparin are commonly used for the management of acute coronary syndrome. These drugs have been approved for the treatment of unstable angina and are currently undergoing rigorous trials for interventional indications. Arixtra is also developed for various subcutaneous indications. However, it exhibits lower anticoagulant effects and may not be optimal for intravenous and interventional purposes. At a higher dosage when administered intravenously the LMWHs produce varying degrees of anticoagulation at relatively lower activated clotting times (150-200). Several studies in vascular and cardiovascular interventions have shown that even at a relatively lower anticoagulant level the LMWHs are as effective as unfractionated heparin at the recommended dosages which produce a relatively higher level of anticoagulation (ACT > 200 secs.). Thus, these agents are currently developed for interventional and surgical indications. It should be emphasized that different LMWHs produce different degrees of anticoagulation and should therefore be individually optimized for a given interventional or surgical purposes. At a relatively high dosage the levels of LMWHs can be measured by using the ACT and APTT. When administered with such GP IIb/IIIa inhibitors as the Abciximab, Aggrastat or Eptifibratide, these drugs may require dosage adjustment However, since the introduction of the front loading of Clopidogrel, the unqualified use of GP IIb/IIIa is debated. LMWHs will find expanded indications in both the medical and surgical management of patients with cardiovascular disorders including atrial fibrillation and congestive heart failure. The only approved anti-Xa drug is represented by a synthetic heparinomimetic, namely, Arixtra. This drug is given for the prophylaxis of post orthopedic indications. This agent is undergoing additional clinical trials in the management of coronary artery diseases. Because of the dependence on antithrombin III (AT) and the sole anti-Xa effects, it has a narrow therapeutic index and its efficacy in this indication may be limited. Additional clinical trials are needed at this time to validate the clinical potential of this drug. The antithrombin agents (Hirudin, Hirulog and Argatroban) were initially developed for arterial indications. However, these agents are approved as a substitute anticoagulant in patients with heparin induced thrombocytopenia (HIT) and PCI. Currently an of these agents are being developed for surgical and interventional use. However, since there is no available antidote at this time, the development is somewhat limited. The antithrombin agents may be useful in patients with HIT which require further clinical validation. Many other anti-Xa agents are also developed. Most of these can be given parenterally. However, the clinical data is somewhat limited. Similarly, several of the new antiplatelet drugs can be administered parenterally and may be useful in CAD. Since most of these newer anticoagulant and antithrombotic drugs are mono-therapeutic their therapeutic index is rather limited. Only in combination these agents can mimic heparins. At this time it is safe to state that heparin and its LMW derivatives will remain the anticoagulant of choice for cardiovascular indications until these newer agents have been validated in extended clinical trials in polytherapeutic settings.

New antithrombin-based anticoagulants

Clinically used anticoagulants are inhibitors of enzymes involved in the coagulation pathway, primarily thrombin and factor Xa. These agents can be either direct or indirect inhibitors of clotting enzymes. Heparin-based anticoagulants are indirect inhibitors that enhance the proteinase inhibitory activity of a natural anticoagulant, antithrombin. Despite its phenomenal success, current anticoagulation therapy suffers from the risk of serious bleeding. The need for safer and more effective antithrombotic agents clearly exists. The past decade has seen enormous effort directed toward discovering and/or designing new molecules with anticoagulant activity. These new molecules can be classified into (a). antithrombin and its mutants, (b). natural polysaccharides, (c). synthetic modified heparins and heparin-mimics, (d). synthetic oligosaccharides, and (e). synthetic non-sugar antithrombin activators. This review focuses on these efforts in designing or discovering new molecules that act through the antithrombin pathway of anticoagulation.

Anticoagulation therapy

Despite refinements and standardization in the use of anticoagulants, many problems remain for clinicians. Dr. Crowther describes appropriate starting and maintenance doses of warfarin, factors accounting for inter- and intra-observer variability and importantly, the management of the over-anticoagulated patients and bleeding patients. Dr. White compares unfractionated heparin (UFH) and low molecular weight heparin (LMWH) and addresses whether there truly are differences in the efficacy and safety of different LMWH's for both arterial and venous indications. Dr. Ortel discusses the management of the problem patient who requires anticoagulants, the management of heparin-induced thrombocytopenia, the pregnant patient, the obese patient, patients who have renal insufficiency and/or liver disease, patients with malignant disease, and other challenging patient populations.

Therapeutic equivalency of low-molecular-weight heparins

OBJECTIVE

To review the recent literature on the approved uses of enoxaparin, dalteparin, ardeparin, and tinzaparin and the evidence for therapeutic equivalence.

DATA SOURCES

A MEDLINE search (1993-January 2001) was conducted to identify English-language literature available on enoxaparin, dalteparin, ardeparin, and tinzaparin.

STUDY SELECTION

All controlled trials evaluating low-molecular-weight heparins (LMWHs) versus standard therapy powered to detect a significant difference were reviewed.

DATA EXTRACTION

Agents were reviewed with regard to safety and efficacy.

DATA SYNTHESIS

As a class, LMWHs have chemical, physical, and clinical similarities. LMWHs have greater bioavailability, longer half-lives, a more predictable pharmacologic response, possible improved safety, and similar or greater efficacy compared with unfractionated heparin (UFH). Because of this, enoxaparin, dalteparin, ardeparin, and tinzaparin are being considered as alternatives to UFH or warfarin, and there is potential for therapeutic interchange. Evaluation of clinical trials is limited because of differing diagnostic methods, drug administration times, dose equivalencies, and outcome measurements.

CONCLUSIONS

Only 1 trial has evaluated 2 LMWHs in a direct comparison in the same study. There is insufficient evidence for determining the therapeutic equivalence of LMWHs.

Stasis ulcers refractory to therapy--accelerated healing by treatment with clopidogrel +/- dalteparin: a preliminary report

Stasis ulcers are commonly the result of chronic venous insufficiency. We have recently assessed 15 patients with stasis ulcers that failed to heal after one year of routine wound care. All patients demonstrated a defect in hemostasis, and a biopsy revealed livido vasculitis. Eleven of fifteen patients were treated with clopidogrel and dalteparin, and 4 of 15 patients were treated with clopidogrel alone. Thirteen of fifteen patients (86.6%) completely healed within three months of starting antithrombotic therapy. Patients with stasis ulcers and chronic venous insufficiency who fail to heal with routine wound care should be subjected to biopsy, a procoagulant defect evaluation, and initiation of clopidogrel and dalteparin therapy if a defect is found.