Low-molecular-weight heparin in thrombosis and cancer

Venous thromboembolism (VTE) is a recognized complication of malignant disease and multiple risk factors contribute to the hypercoagulability that commonly accompanies malignancy. Thromboprophylaxis with antithrombotic drugs such as the low-molecular-weight heparins (LMWHs) can be used to control the hypercoagulable state and to reduce the incidence of VTE in patients with cancer. Clinical and biochemical data suggest that LMWHs may also inhibit tumor growth, leading to a survival benefit in these patients. Many cancer patients reportedly have a hypercoaguable state, with recurrent thrombosis due to the impact of cancer cells and chemotherapy or radiotherapy on the coagulation cascade. Studies have demonstrated that unfractionated heparin (UFH) or its low-molecular-weight fractions interfere with various processes involved in tumor growth and metastasis. Clinical trials in cancer patients with thromboembolic disorders have suggested a clinically relevant effect of LMWHs (as compared with UFH) on the survival of cancer patients with deep vein thrombosis. Similarly, the impact of warfarin on the survival of cancer patients with thromboembolic disorders was demonstrated in certain tumor types. Studies from our laboratory demonstrated a significant role for LMWH, warfarin, anti-VIIa, and LMWH releasable tissue factor pathway inhibitor on the regulation of angiogenesis, tumor growth, and tumor metastasis. However, a direct anticancer effect for heparin in cancer patients without thrombosis still remains to be clinically documented.     

Blocking inhibition of prothrombinase by tissue factor pathway inhibitor alpha: a procoagulant property of heparins

Unfractionated heparin (UFH) has procoagulant activity in antithrombin/heparin cofactor II (HCII)‐depleted plasma. UFH prevents tissue factor pathway inhibitor alpha (TFPIα) from inhibiting the procoagulant enzyme complex, prothrombinase, providing a possible mechanism for its procoagulant activity. The procoagulant potential of UFH and various low molecular weight heparins (LMWHs) were characterized for TFPIα dependence, using thrombin generation assays performed with antithrombin/HCII‐depleted plasma. UFH, the LMWHs enoxaparin and dalteparin, and the low anticoagulant LMWH 2‐O, 3‐O desulphated heparin (ODSH) all promoted thrombin generation, but fondaparinux did not, and this activity was blocked by a TFPIα antibody. UFH, enoxaparin, and dalteparin were anticoagulant in reactions containing 1–2% normal plasma. In prothrombinase activity assays, UFH, enoxaparin, dalteparin and ODSH blocked prothrombinase inhibition by TFPIα, while again fondaparinux did not. In both the plasma and purified assays, LMWHs displayed greater procoagulant potential than UFH, even when normalized to saccharide concentration. These biochemical data reveal that UFH and LMWHs, but not fondaparinux, block prothrombinase inhibition by TFPIα, thereby producing their paradoxical procoagulant activity observed in the absence of antithrombin/HCII. The findings may help to understand the complex pathophysiology and treatment of patients that are simultaneously bleeding and clotting, such as those with disseminated intravascular coagulation.     

Physiological function of tissue factor pathway inhibitor and interaction with heparins

Tissue factor pathway inhibitor (TFPI) is now recognized as a major physiological anticoagulant. Its main role is to modulate factor VIIa/tissue factor catalytic activity. Another important role is to potentiate the effect of heparins. TFPI is released from the vascular endothelium after injection of either unfractionated heparin (UFH) or low-molecular-weight heparins (LMWHs), which may then provide high concentrations of TFPI at sites of tissue damage and ongoing thrombosis. In dilute prothrombin-time-based assays, released TFPI contributes approximately one-third to the anticoagulant effect of heparin, the remaining being accounted for by antithrombin. Released TFPI, but not plasma TFPI, contains the basic carboxy-terminal tail which is important for the anticoagulant effect. UFH and LMWH exert differential effects on intravascular TFPI. UFH, but not LMWH, given in therapeutic doses, is associated with a progressive depletion of TFPI, which is associated with a strong rebound activation of coagulation after cessation of treatment. Such depletion may explain the apparent superior efficacy of LMWH observed in clinical trials.     

Partial depletion of tissue factor pathway inhibitor during subcutaneous administration of unfractionated heparin, but not with two low molecular weight heparins

Tissue factor pathway inhibitor (TFPI) is released to circulating blood after intravenous (i.v.) and subcutaneous (s.c.) injections of heparins, and may thus contribute to the antithrombotic effect of heparins. We have recently shown that total TFPI activity, plasma free TFPI antigen, and heparin releasable TFPI were partially depleted during repeated and continuous i.v. infusion of unfractionated heparin (UFH), but not during s.c. treatment with a low molecular weight heparin (LMWH). The difference may be attributed to a different mode of action or the different mode of administration. In the present randomized cross-over study, s.c. administration of therapeutic doses of UFH was compared with s.c. administration of two LMWHs. 12 healthy male volunteers were treated for 3 d with UFH, 250 U/kg twice daily, dalteparin, 200 U/kg once daily, and enoxaparin, 1.5 mg/kg once daily. Six participants were also treated with UFH, 300 U/kg once daily. On day 5 a single dose of either drug was given. Peak levels of total TFPI activity and free TFPI antigen were detected 1 h after injection, whereas maximal prolongation of activated partial thromboplastin time (APTT) and peak levels of anti-factor Xa activity and anti-factor IIa activity were detected after 4 h. On UFH administered twice daily, free TFPI antigen decreased by 44% from baseline level before the first injection on day 1 to pre-injection level on day 5. On UFH administered once daily, basal free TFPI antigen decreased by 50%, 56% and 27% on day 2, 3 and 5 respectively, compared with day 1. Minimal depletion of TFPI was detected during treatment with LMWHs. The study demonstrates the different modes of action of LMWHs and UFH and may help to explain the superior antithrombotic efficacy of LMWHs.     

Differential effect of unfractionated heparin and low molecular weight heparin on intravascular tissue factor pathway inhibitor: evidence for a difference in antithrombotic action

Tissue factor pathway inhibitor (TFPI) is a potent inhibitor of tissue factor (TF)-induced blood coagulation, which is increased several-fold in post-heparin plasma and thought to contribute significantly to the antithrombotic action of heparin. In the present study we investigated whether subcutaneous (s.c.) administration of a low molecular weight heparin (LMWH), enoxaparin, had a different effect on intravascular pools of TFPI compared with continuous i.v. infusion of unfractionated heparin (UFH). 18 healthy male volunteers were randomly assigned to continuous i.v. infusion with UFH (initially 450 U/kg/24 h, n  = 6) or to s.c. treatment with LMWH once daily (enoxaparin, 1.5 mg/kg, n  = 12) for 72 h. A bolus injection of 5000 IU UFH i.v. caused an 8–13-fold increase in plasma-free TFPI antigen (TFPI Ag), followed by a progressive decrease (81 ± 4%, P  < 0.001) during the 72 h infusion with UFH. 4 h after discontinuation of the infusion, basal free TFPI Ag and heparin-releasable TFPI were significantly decreased compared with the concentrations before the infusion (30 ± 9% and 27 ± 7%, respectively). In contrast, LMWH treatment did not reduce either basal or heparin-releasable TFPI Ag. The changes in plasma TFPI Ag by UFH and LMWH were statistically different between groups both in pre- ( P  < 0.001) and post-heparin ( P  < 0.0001) plasma. The differential effect of UFH and LMWH on intravascular pools of TFPI may contribute to the understanding of the apparent superior efficacy of LMWHs in the treatment of both arterial and venous thrombosis.     

Anticoagulants in thrombosis and cancer: the missing link

Many cancer patients reportedly have a hypercoagulable state, with recurrent thrombosis due to the impact of cancer cells and chemotherapy on the coagulation cascade. Studies have demonstrated that unfractionated heparin (UFH) or low-molecular-weight heparin (LMWH) interferes with various processes involved in tumor growth and metastasis. These processes might include fibrin formation, binding of heparin to angiogenic growth factors such as basic fibroblast growth factor and vascular endothelial growth factor, modulation of tissue factor, and other mechanisms. Clinical trials have indicated a clinically relevant effect of LMWH as compared with UFH on the survival of cancer patients with deep vein thrombosis. Similarly, the impact of warfarin on the survival of cancer patients with thromboembolic disorders was demonstrated. Recent studies from our laboratory defined the role of an LMWH (tinzaparin), warfarin, anti-factor VIIa, and recombinant tissue factor pathway inhibitor in the modulation of angiogenesis, tumor growth, and tumor metastasis.     

Low-molecular-weight heparin and cancer

Heparin is a familiar anticoagulant drug with properties that may impede tumor growth; it modifies properties of cells that contribute to malignant dissemination such as angiogenesis, growth factor and protease activity, immune function, proliferation, and gene expression. Heparin has antitumor effects in animal models of malignancy, and studies in human malignancy show improved cancer outcome with heparin treatment. Meta-analyses comparing unfractionated heparin (UFH) and low-molecular-weight heparin (LMWH) for treatment of deep-vein thrombosis have shown apparent substantial improvement in cancer outcome in the subset of patients with malignancy who were randomly assigned to receive LMWH. This experience, together with the favorable pharmacokinetic properties of LMWH, provides a rationale for prospective clinical trials of LMWH in patients with cancer. Such trials should provide (a) definitive data on possible antitumor effects of this treatment, (b) insight into possible heterogeneous responses to heparin treatment among different histological types and stages of malignancy, and (c) a setting for exploring mechanisms of antineoplastic effect in human malignancy.     

New perspectives for treatment of pulmonary embolism

Subcutaneous low-molecular-weight heparins (LMWHs) have been shown to be as safe and effective as intravenous unfractionated heparin (UFH) for the initial treatment of patients with deep vein thrombosis and acute symptomatic pulmonary embolism. In comparison with UFH, LMWHs have a longer half-life, greater bioavailability and more predictable antithrombotic effect when administered in fixed doses, thus obviating the need for laboratory monitoring. It is therefore feasible that LMWH preparations may replace UFH for the treatment of pulmonary embolism in the future. It is recommended that LMWH should be administered for 5-10 days and should overlap with oral anticoagulant therapy by at least 4 days. Surgical pulmonary embolectomy should only be considered in patients with life-threatening pulmonary embolism who have failed to respond during the first 3 h of thrombolytic therapy. The systematic use of vena cava filters is not recommended in patients presumed to be at high risk for pulmonary embolism.     

Pharmacodynamic considerations in the selection of dosage of tinzaparin for various indications: experimental studies in primates

Like unfractionated heparin (UFH), low-molecular-weight heparins (LMWHs) are polypharmacologic agents that can modulate the hemostatic system at multiple points. Thus, to select an optimal dose of LMWH for a given indication, it is necessary to consider multiple actions of the drug. In this study, nonhuman primates were treated with intravenous or subcutaneous boluses of the LMWH tinzaparin or UFH. Doses were selected on the basis of the expected prophylactic (75 U/kg) and therapeutic (175 U/kg) dosing of tinzaparin. Blood samples were drawn periodically up to 24 hours after administration. Circulating anti-Xa and anti-thrombin (anti-IIa) activities determined using amidolytic assays were used to estimate plasma tinzaparin (heparin) concentrations. In addition, total tissue factor pathway inhibitor (TFPI) levels were measured in these primates. Subcutaneous administration of 75 U/kg tinzaparin resulted in plasma levels of approximately 0.2 to 0.3 U/mL, concentrations sufficient for DVT prophylaxis. Such drug levels were not associated with a significant release of TFPI. Intravenous administration of the same dose resulted in a peak drug level of approximately 1.5 anti-Xa U/mL. The elimination half-life was approximately 1 hour. Thus, intravenously administered tinzaparin may be useful for providing anticoagulation during coronary interventions. Subcutaneous administration of 175 U/kg resulted in tinzaparin levels of approximately 0.7 anti-Xa U/mL and a significant increase in TFPI levels. Interestingly, the increase in TFPI levels occurred over a different time frame than anticoagulant activity. Intravenous administration of 175 U/kg resulted in peak drug concentrations of almost 5 anti-Xa U/mL. The pharmacokinetic behavior of intravenously administered tinzaparin was comparable to that of UFH. The data show that the pharmacokinetic and pharmacodynamic effects measured using different assays widely differ. For a proper pharmacodynamic analysis, multiple assays should be considered, given that both UFH and LMWHs are polycomponent in nature.     

Unfractionated heparin and low-molecular-weight heparin for the initial treatment of acute venous thromboembolism

Anticoagulant drugs represent the therapy of choice for the initial treatment of venous thromboembolism. Unfractionated heparin (UFH) in adjusted doses and low-molecular-weight heparin (LMWH) in fixed doses are equally as effective and safe. Proper use of UFH requires considerable expertise, can cause inconvenience and has limitations. The use of LMWHs has multiple advantages over UFH including a more predictable dose-response and fixed administration dose. These properties make the treatment of suitable patients feasible in an outpatient setting. In two major clinical trials addressing the treatment of deep vein thrombosis, outpatient management with LMWH was associated with a substantial cost reduction compared with inpatient treatment using UFH. Recent studies have also shown that LMWHs are at least as effective and safe as UFH for the treatment of non-critical patients with pulmonary embolism. Whether or not home treatment of pulmonary embolism is feasible and safe remains to be demonstrated.     

M118, a novel low-molecular weight heparin with decreased polydispersity leads to enhanced anticoagulant activity and thrombotic occlusion in ApoE knockout mice

Heparin and low-molecular weight heparin (LMWH) are complex, heterogeneous polysaccharides used in the treatment of arterial and venous thrombosis. M118 is a novel LMWH with low polydispersity and pronounced anti-Xa and anti-thrombin (IIa) activity as compared to current LMWHs. To determine if M118 is effective in preventing thrombosis in the setting of a vascular plaque, apolipoprotein E knockout mice fed a high fat diet were injected with M118, enoxaparin, unfractionated heparin, or saline control and examined for arterial thrombosis using a rose bengal laser induced carotid artery injury model. M118 significantly increased the time to occlusion as compared to control and unfractionated heparin but not compared to enoxaparin although fewer M118 treated animals had any vascular occlusion present at the time of protocol completion. Platelet-neutrophil aggregates were studied by flow cytometry and were found to be decreased with M118 as compared to enoxaparin. This is the first published report examining M118, a novel LMWH designed to have low polydispersity and enhanced anticoagulant activity. In an animal model of vascular plaque, M118 is a potent inhibitor of arterial thrombosis and, despite lower in vivo anti-Xa and anti-IIa activity levels, M118 was superior to UFH in the prevention of arterial thrombosis.     

Decreased prevalence of heparin-induced thrombocytopenia with low-molecular-weight heparin and related drugs

Heparin-induced thrombocytopenia (HIT) Type II represents a disease spectrum associated with a high risk of thrombosis leading to limb loss and death. The pathophysiology of HIT is based on the development of antibodies to the heparin-platelet factor 4 (PF4) complex. Unfractionated heparin (UFH) is heterogeneous in molecular chain length and degree of sulfation accounting in part, for, the heterogeneity of HIT antibodies. Because of its smaller size, low-molecular-weight heparin (LMWH) does not interact with PF4 and platelets as efficiently as does UFH. This translates into a lower risk of immune sensitization with LMWH than with UFH treatment. LMWH is less likely than UFH to cause antibody generation and thus patients do not develop clinical HIT at the same frequency with LMWH as with UFH treatment. The antibodies generated by LMWH treatment are more often immunoglobulin A (IgA) and IgM as opposed to IgG antibodies, which are associated with symptomatic clinical HIT generated by exposure to UFH. However, platelet activation/aggregation can occur from LMWHs in the presence of most pre-existing HIT antibodies that had been generated from UFH exposure, although the response is less than that caused by UFH plus HIT antibody. With the expanded use of LMWH, the frequency of clinical HIT may naturally decline, given that LMWHs are less likely to generate HIT antibody.     

Orally active heparin and low-molecular-weight heparin

The heparins, (unfractionated heparin (UFH) and low-molecular-weight heparins (LMWH)) given by subcutaneous or intravenous injection have been used extensively in the prevention and treatment of both venous and arterial thromboembolic disorders. The increasing use of the heparins, LMWHs in particular, in the out of hospital setting has stimulated interest in the development of orally absorbable antithrombotic agents that require little or no monitoring, and this includes the heparins. UFH or LMWH delivered orally has been shown to have an antithrombotic effect in animal thrombosis models although there is little change in plasma coagulation tests. The addition of a simple organic chemical N -(8-(2-hydroxybenzoyl)amino)caprylate (SNAC) to UFH markedly enhances its absorption. A phase II study in patients undergoing total hip replacement indicated that SNAC heparin in two different doses was as effective and safe as UFH given subcutaneously. A phase III clinical trial comparing two doses of SNAC heparin given orally with LMWH by subcutaneous injection for the prevention of venous thromboembolism in patients undergoing total hip replacement is currently underway.     

Efficacy of unfractionated heparin, low molecular weight heparin and both combined for releasing total and free tissue factor pathway inhibitor.

Unfractionated heparin (UFH) exerts its anticoagulant properties by increasing the inactivation of thrombin and activated factor X by antithrombin III. Apart from this main action release of tissue factor pathway inhibitor (TFPI) from endothelial cells could also be important for the antithrombotic activity of heparins. Four different heparin preparations were injected subcutaneously into 5 healthy volunteers 1 week apart: (1) UFH 2,500 IU fix dose (FixUFH), (2) 1 mg/kg body weight of low molecular weight heparin (LMWH), (3) the combined LMWH-adjusted dose plus UFH 2,500 IU fix dose (ComHep) and (4) UFH 2,500 IU/10 kg body weight (UFHvar). Plasma samples were drawn before and 1, 2, 4, 6, 12 and 24 h afterwards. FixUFH did not affect the concentration of total and free TFPI. Total TFPI increased in the 1st hour after LMWH injection from 74 to 124 ng/ml (p < 0.01), after ComHep from 82 to 144 ng/ml (p < 0.01), and after UFHvar from 91 to 113 ng/ml (p < 0.05). All observed elevations were significant at the peak value (+/- 2 h, p < 0.01 compared with baselines). The increase of free TFPI produced by UFHvar (74.5 and 70.5 ng/ml) was significantly higher than with LMWH (42.8 and 38.0 ng/ml) at 2 and 4 h (p < 0.001 and p < 0.01, respectively). UFHvar and ComHep but not LMWH produced a statistically significant increase of free TFPI compared with FixUFH at 2, 4 and 6 h (p < 0. 01). We concluded that at comparable therapeutic doses, subcutaneous UFHvar released more free TFPI than LMWH and ComHep. A synergism between LMWH and low dose of UFH was found in 4-, 6- and 12-hour blood samples.     

Low-molecular weight heparin for deep vein thrombosis prophylaxis in hospitalized medical patients: results from a cost-effectiveness analysis.

Multiple options exist for the prevention of deep vein thrombosis (DVT) in medical inpatients. We sought to determine the cost-effectiveness of low-molecular-weight heparin (LMWH) relative to unfractionated heparin (UFH) for DVT prevention in this setting. We conducted a cost-effectiveness analysis from the perspective of a third-party payer employing a decision model and literature-based estimates for inputs. In the base-case analysis, LMWH had little impact on the rate of DVT. Despite higher acquisition costs, however, LMWHs resulted in net savings. Routine use of LMWH saves approximately US$89 per patient. The lower rate of heparin-induced thrombocytopenia (HIT) with LMWH accounted for this differential. Univariate sensitivity analysis revealed the model was moderately sensitive to the odds ratio of HIT with LMWH and the cost of HIT. Multivariate sensitivity analysis confirmed the LMWH approach dominated financially. 'Worst-case' scenario modeling, where LMWH actually increased the risk for DVT, had little effect on the rate of HIT, and was substantially more costly than UFH, still demonstrated that LMWHs were economically superior. Monte-Carlo simulation indicated the 95% confidence interval around the estimate for savings with LMWH ranged from US$7 to US$373. We conclude that, despite their higher cost, LMWHs for thromboprophylaxis in medical patients result in savings.     

Low molecular weight heparin in pregnancy: current issues

Low molecular weight heparin (LMWH) is now the most commonly used anticoagulant for prophylaxis and treatment of venous thromboembolism in pregnancy and the puerperium in the UK. The reliable pharmacokinetics of LMWHs and their long half-life, resulting in the need for less frequent injections than unfractionated heparin (UFH), makes them attractive for practical use in the 9 months of pregnancy. Widespread use over the last 10 years has shown that LMWHs are safer than UFH in pregnancy. There is, however, poor consensus and wide disparity of views among experts with regard to the appropriate dose for the varying indications, the duration of treatment, and whether and how LMWH should be monitored because of the lack of an evidence base. These areas of uncertainty reflect the fact that clinical practice has grown largely through the publication of small trials, observational studies, personal experience and anecdote. Good clinical trials are urgently required.     

Low molecular weight heparins

Low molecular weight heparins (LMWHs) differ from unfractionated heparin (UFH) in a number of characteristics, which is probably due to differences in molecular weight distribution. From a clinical point of view the better subcutaneous bioavailability and longer biological half-life are important, making it sufficient to inject LMWHs once-daily only. For practical purposes it is also important that LMWHs be used without monitoring. They are effective as prophylaxis against postoperative venous thromboembolism after all types of surgery; in most studies, more effective than UFH. In most studies, this effect can be obtained safely and with less bleeding than with UFH. LMWHs compare favourably with UFH for starting treatment of deep vein thrombosis, as well as an anticoagulant during haemodialysis. Adverse effects such as thrombocytopenia and osteoporosis are more common with UFH than with LMWHs. Studies evaluating whether or not LMWHs can replace UFH in arterial diseases are still few with small sample sizes. Thus further systematic research is needed.     

Low molecular weight heparins and coronary artery disease

Coronary artery disease encompasses a wide spectrum of conditions including silent ischemia, exertional angina, unstable angina, and myocardial infarction. Acute coronary syndromes (unstable angina and myocardial infarction) are caused by the rupture of an atherosclerotic plaque, platelet activation, and fibrin deposition resulting in thrombosis. Aspirin and unfractionated heparin (UFH) have traditionally been the treatment of choice in patients with acute coronary syndromes; however, low molecular weight heparins (LMWHs) offer potential advantages over UFH. Available evidence indicates that LMWH is superior to UFH in reducing ischemic events or death in the acute phase of unstable angina or non-Q-wave myocardial infarction. Long-term therapy with lower doses of LMWH may not offer any advantage to aspirin in the prevention of coronary events or death. Major bleeding complications are similar for LMWH and UFH although minor bleeding complications are more common with LMWH, primarily due to injection-site hematomas. Finally, use of LMWH appears to be costeffective compared with UFH. The available evidence supports improved clinical outcomes, favorable safety profile, and cost savings associated with LMWH use in the management of unstable angina and non-Q-wave myocardial infarction and should be favored over UFH.