The Mode of Action of Low Molecular Weight Heparin Preparation (PK10169) and Two of its Major Components on Thrombin Generation in Plasma

Heparins: A Shift of Paradigm

Heparins inhibit the thrombin forming capacity of plasma, i. e., the endogenous thrombin potential (ETP), by their anti-thrombin (aIIa) activity, the anti-factor Xa (aXa) activity is of minimal importance. This holds for both unfractionated heparin (UFH) and low molecular weight heparin (LMWH) at aXa/aIIa ratios < 25. Clinical experience and epidemiological evidence show a direct relationship between the ETP and the risk of thrombosis and bleeding. Consequently, the therapeutic potency of a heparin is determined by its aIIa activity, i.e., the concentration of a domain in which 12 sugar flank the high affinity antithrombin-binding pentasaccharide (HA5) at one side. The response of individual plasmas to a fixed dose of any heparin is highly variable. This suggests that individualization of heparin dosage, on basis of the ETP, might reduce bleeding or re-thrombosis. There exist simple laboratory methods for both the ETP and the concentration of the active domain. These methods can be used both for unequivocally characterization of a heparin preparation and for controlling heparin therapy and allow arbitrary units relative to a standard to be abandoned. These tests are as robust as any hematological routine test but not yet routinely available, which severely encumbers progress in the field.

The study of the thrombin generation mechanism and the effect of low molecular weight heparin as thromboprophylaxis in patients undergoing total knee and hip replacement

INTRODUCTION

The recommended duration of post-operative Low-Molecular-Weight-Heparins (LMWHs) thromboprophylaxis in Total-Hip-Replacement (THR) and Total-Knee-Replacement (TKR) surgery is controversial. Our aim is to study the thrombin generation (TG) modifications induced by surgery and to evaluate the effect of LMWH on TG during and after the recommended duration.

PATIENTS/METHODS

Thirty-one patients received 4000IU anti-Xa/day of enoxaparin, 8-hours post-operatively (15 THR for 30-days and 16 TKR for 15-days). TG assay sensitive to enoxaparin was performed, pre-operatively (D0), 7-hours post-surgery (D1), 8-days post-surgery (D8), and 2-days after thromboprophylaxis withdrawal (D32 and D17), evaluating: lag-time, endogenous thrombin potential (ETP), peak amount of generated thrombin (Peak), time-to-Peak (tt-Peak), and the Mean-Rate-Index [MRI=Peak/(tt-Peak-lag-time)].

RESULTS

TKR surgery decreased lag-time and tt-Peak and increased MRI on D1 vs D0 (p<0.05). In contrast, THR did not significantly modify TG. Enoxaparin effectively reduced thrombin generation in both groups. Thromboprophylaxis withdrawal resulted in rebound increase of TG in the TKR patients (ETP, Peak & MRI significantly increased on D17 vs D0; p<0.05, and vs. D1; p<0.05) but not in THR patients. Variability in the response to enoxaparin was observed among patients of the same group.

CONCLUSIONS

TKR surgery is more thrombogenic than THR surgery. In THR patients TG was efficiently inhibited by 30-day thromboprophylaxis, whereas, in TKR patients treated for 15-days TG was not effectively inhibited. Individual variability of the response to enoxaparin was observed in both groups revealing some form of biological resistance to enoxaparin. TG assay may represent the breakthrough step to efficient antithrombotic strategy in clinical settings with high thrombotic risk.

Data management in thrombin generation

To obtain a thrombin generation (TG) curve from the conversion of added fluorogenic substrate, thrombin concentrations are to be derived from the observed velocity of increase of fluorescence (dF/dt). The relation between velocity and thrombin concentration varies during the experiment because substrate is consumed and because fluorescence is not linear with the concentration of product. Here we review the techniques that we developed to:

Antithrombin-Independent Effects of Heparins on Fibrin Clot Nanostructure

Objective—

Because of the widespread clinical use of heparins, their effects on the enzymatic cascade are very well known. In contrast, little is known about the direct effect of heparins on the nanostructure of fibrin fibers, even though this nanostructure plays a major role in the mechanical strength and lysis of clots. This lack of reliable data can be correlated with the lack of a nonintrusive, quantitative method to determine this structure. We recently developed such a method that allows the simultaneous determination of the average fiber radius and the protein content using spectrometric data. In this study, we assessed the nanostructure of fibrin in a system composed of human thrombin and fibrinogen.

Methods and Results—

We provide quantitative evidence showing that both unfractionated heparin and low molecular weight heparin directly alter the nanostructure of fibrin fibers independent of their other actions on the coagulation cascade; as expected, the pentasaccharide fondaparinux has no effect.

Conclusion—

Our results show that in addition to the effect of heparin on the coagulation cascade, modifications of the fibrin nanostructure may also contribute to improved fibrinolysis.

Recollections on thrombin generation

Against an autobiographic background, a historical sketch is given of the development of the technique of thrombin generation, from subsampling to duly calibrated continuous measurement with fluorogenic substrates. Its application to various problems in the pathophysiology of hemostasis and thrombosis is discussed.

Fixed dosage of low-molecular-weight heparins causes large individual variation in coagulability, only partly correlated to body weight

BACKGROUND

Low-molecular-weight heparins (LMWHs) are routinely given without the control of their effect on coagulation. The endogenous thrombin potential (ETP) is a sensitive detector of the heparin effect.

QUESTION

What is the interindividual variation in TG after a fixed dose of LMWH in normal volunteers, is it explained by variation in weight?

METHODS

Subcutaneous (s.c.) injection, in 12 healthy volunteers, of 9000 aXa-units of unfractionated heparin (UFH) and of three heparins with narrow MW distribution around 10.5, 6.0 and 4.5 kD. Measurement of anti-thrombin (aIIa) and antifactor Xa (aXa)-activities and ETP at 11 time points over 24 h.

RESULTS

The coefficient of variation (CV) of the AUCs of aXa- and aIIa-activities is 50% for UFH and 22-37% for LMWHs. Because of the hyperbolic form of the dose-response curve, the CV of the inhibition of the ETP is lower: 32% for UFH and 13-21% for the LMWHs. Fixed dosage of LMWH caused under-dosage in 10-13% of the samples and over-dosage in 5-11%. High or low response is an individual property independent of the type of heparin injected and only partially explained by variation in body weight.

CONCLUSION

Optimized individual dosage of LMWH is possible through recognition of high and low responders, which requires one measurement of the heparin concentration or, preferably, the heparin effect on the ETP, 2-5 h after a first injection.

Pharmacodynamic and pharmacokinetic properties of enoxaparin : implications for clinical practice

Enoxaparin is a low-molecular-weight heparin (LMWH) that differs substantially from unfractionated heparin (UFH) in its pharmacodynamic and pharmacokinetic properties. Some of the pharmacodynamic features of enoxaparin that distinguish it from UFH are a higher ratio of anti-Xa to anti-IIa activity, more consistent release of tissue factor pathway inhibitor, weaker interactions with platelets and less inhibition of bone formation. Enoxaparin has a higher and more consistent bioavailability after subcutaneous administration than UFH, a longer plasma half-life and is less strongly bound to plasma proteins. These properties mean that enoxaparin provides a more reliable anticoagulant effect without the need for laboratory monitoring, and also offers the convenience of once-daily administration. Clinical studies have confirmed that these pharmacological advantages translate into improved outcomes. There are important pharmacokinetic and pharmacodynamic differences between enoxaparin, other LMWHs and UFH, and therefore these molecules cannot be regarded as interchangeable.

Cost-effectiveness analysis of enoxaparin versus unfractionated heparin in patients with acute coronary syndrome in Poland: modelling study from the hospital perspective

AIM AND PERSPECTIVE: To estimate the cost effectiveness of enoxaparin versus unfractionated heparin (UFH) in patients with acute coronary syndrome (ACS) from a Polish hospital perspective. This was intended to facilitate the decisionmaking process in selecting the most cost-effective treatment for ACS.

METHOD

A decision model was used to quantify costs and effectiveness of alternative treatments. Published results from the Efficacy and Safety of Subcutaneous Enoxaparin in Non-Q-Wave Coronary Events (ESSENCE) study were used to estimate the probability for clinical endpoints (death, myocardial infarction or recurrent angina) at 30 days. Probabilities of patients undergoing revascularisation procedures were obtained from the Global Registry of Acute Coronary Events (GRACE) which included data from 961 patients at six centres in Poland. The analysis assessed only direct medical costs, determined from actual resource consumption on a patient-specific basis (6-month observational study) and estimated using Polish data on unit costs. One- and two-way sensitivity analyses and threshold analysis were performed.

RESULTS

At 30 days, 19.8% of patients receiving enoxaparin compared with 23.3% of those receiving UFH reached a composite endpoint consisting of death, myocardial infarction and recurrent angina (p = 0.02). The average costs (in zloty ; 1 US dollar = 4 zloty [2000 values]) were 1,085 per patient receiving enoxaparin compared with 1,097 per patient receiving UFH. Therefore, for every 29 patients treated, not only would enoxaparin therapy avoid one event of the composite endpoint, it would also save 348 zloty. The threshold analysis suggests that enoxaparin would lose dominance when the cost of enoxaparin increased by 10%, the cost of monitoring UFH therapy decreased by 12%, the probability of reaching the composite endpoint in the enoxaparin arm increased to 0.22 or decreased to 0.21 in the UFH arm.

CONCLUSION

According to our model enoxaparin was more effective at a lower cost than UFH, therefore this treatment was shown to be dominant for patients with ACS in Poland.

Antithrombotic therapy in acute coronary syndromes: key notes from ESSENCE and TIMI 11B

Platelet activation and thrombus formation are key events in the pathogenesis of acute coronary syndromes (unstable angina and non-Q-wave myocardial infarction). Therefore, current management of these conditions consists of antithrombotic and antiplatelet therapy, principally heparin and oral aspirin. However, such treatment is unsuccessful in a significant proportion of patients. Two recent large studies with the low-molecular-weight heparin (LMWH) enoxaparin have shown that this agent significantly reduces the risk of major ischemic events compared with unfractionated heparin. In the Efficacy and Safety of Subcutaneous Enoxaparin in Non-Q-Wave Coronary Events (ESSENCE) study, treatment with enoxaparin for 2 to 8 days reduced the risk of death, myocardial infarction, or recurrent angina by 20% at 14 days compared with treatment with unfractionated heparin. In the Thrombolysis in Myocardial Infarction (TIMI) 11B study, enoxaparin was associated with a significant reduction in the risk of death, myocardial infarction, or urgent revascularization compared with unfractionated heparin, which became apparent within 48 hours; this benefit was maintained during outpatient treatment for 5 weeks. A meta-analysis of these two studies showed that the risk of death or myocardial infarction was consistently approximately 20% lower in enoxaparin-treated patients than in heparin-treated patients. In contrast, studies with other LMWHs have not shown consistent superiority over unfractionated heparin. This may reflect the pharmacologic heterogeneity of LMWH and/or differences in trial design.

Unfractionated heparin and low-molecular-weight heparin in acute coronary syndrome without ST elevation: a meta-analysis

BACKGROUND

In acute coronary syndrome without ST elevation, the role of unfractionated and low-molecular-weight heparin in aspirin-treated patients remains unclear, and there is conflicting evidence regarding the efficacy and safety of low-molecular-weight heparin (LMWH) relative to unfractionated heparin. We did a systematic overview of the randomised trials to assess the effect of unfractionated heparin and LMWH on death, myocardial infarction, and major bleeding.

METHODS

Randomised trials comparing unfractionated heparin or LMWH with placebo or untreated control, or comparing unfractionated heparin with LMWH, for the short-term and long-term management of patients with acute coronary syndrome without ST elevation, were identified by electronic and manual searches and through contact with experts and industry representatives. Odds ratios for death, myocardial infarction, and major bleeding were calculated for each trial, and results for the individual trials were combined by a modification of the Mantel-Haenszel method.

FINDINGS

12 trials, involving a total of 17157 patients, were included. The summary odds ratio (OR) for myocardial infarction or death during short-term (up to 7 days) unfractionated heparin or LMWH compared with placebo or untreated control was 0.53 (95% CI 0.38-0.73; p=0.0001) or 29 events prevented per 1000 patients treated; during short-term LMWH compared with unfractionated heparin was 0.88 (0.69-1.12; p=0.34); and during long-term LMWH (up to 3 months) compared with placebo or untreated control was 0.98 (0.81-1.17; p=0.80). Long-term LMWH was associated with a significantly increased risk of major bleeding (OR 2.26, [95% CI 1.63-3.14], p<0.0001), which is equivalent to 12 major bleeds per 1000 patients treated.

INTERPRETATION

In aspirin-treated patients with acute coronary syndrome without ST elevation, short-term unfractionated heparin or LMWH halves the risk of myocardial infarction or death. There is no convincing difference in efficacy or safety between LMWH and unfractionated heparin. Long-term LMWH has not been proven to confer benefit additional to aspirin and there is no evidence to support its use after the first 7 days.

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 cost- effective 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.

The emerging role of low-molecular-weight heparin and antiplatelet therapies in the cardiac catheterization laboratory

The advent of new pharmacotherapies and interventional devices has exponentially increased the breadth of coronary procedures in a variety of clinical settings. The low-molecular-weight heparins, a new class of antithrombins, offer several advantages over unfractionated heparin as anticoagulants. New antiplatelet agents have also been developed that block components of the platelet aggregation pathway not inhibited by aspirin. The use of these new therapies has the potential to significantly improve the outcome of percutaneous coronary interventions. One low-molecular-weight heparin, enoxaparin, was shown in the ESSENCE trial to be significantly superior to unfractionated heparin in the medical management of unstable angina. Evidence from ESSENCE suggests that enoxaparin used in conjunction with percutaneous revascularization and stenting does not cause increased bleeding. Trials directly comparing the safety and efficacy of heparin and enoxaparin as adjunctive therapies in percutaneous interventions are in progress. In addition, intramural delivery of enoxaparin to achieve a locally high concentration is being investigated for the prevention of restenosis after coronary stenting. Aspirin together with ticlopidine, which inhibits adenosine diphosphate-induced platelet activation, has been shown to be superior to aspirin plus anticoagulation in trials of patients undergoing percutaneous revascularization with stenting. Clopidogrel has emerged as a possible alternative to ticlopidine. Antiplatelet therapies directed against the glycoprotein IIb/IIIa receptor, which plays a critical role in aggregation, have been tested in several clinical trials in patients undergoing percutaneous intervention. The combination of new antiplatelet and new anticoagulant therapies may offer added benefit not seen with the individual agents alone. The safety and effectiveness of such new regimens is currently being investigated in a number of clinical trials.

Assessment of the treatment effect of enoxaparin for unstable angina/non-Q-wave myocardial infarction. TIMI 11B-ESSENCE meta-analysis

BACKGROUND

Two phase III trials of enoxaparin for unstable angina/non-Q-wave myocardial infarction have shown it to be superior to unfractionated heparin for preventing a composite of death and cardiac ischemic events. A prospectively planned meta-analysis was performed to provide a more precise estimate of the effects of enoxaparin on multiple end points.

METHODS AND RESULTS

Event rates for death, the composite end points of death/nonfatal myocardial infarction and death/nonfatal myocardial infarction/urgent revascularization, and major hemorrhage were extracted from the TIMI 11B and ESSENCE databases. Treatment effects at days 2, 8, 14, and 43 were expressed as the OR (and 95% CI) for enoxaparin versus unfractionated heparin. All heterogeneity tests for efficacy end points were negative, which suggests comparability of the findings in TIMI 11B and ESSENCE. Enoxaparin was associated with a 20% reduction in death and serious cardiac ischemic events that appeared within the first few days of treatment, and this benefit was sustained through 43 days. Enoxaparin's treatment benefit was not associated with an increase in major hemorrhage during the acute phase of therapy, but there was an increase in the rate of minor hemorrhage.

CONCLUSIONS

The accumulated evidence, coupled with the simplicity of subcutaneous administration and elimination of the need for anticoagulation monitoring, indicates that enoxaparin should be considered as a replacement for unfractionated heparin as the antithrombin for the acute phase of management of patients with high-risk unstable angina/non-Q-wave myocardial infarction.

Early increase of von Willebrand factor predicts adverse outcome in unstable coronary artery disease: beneficial effects of enoxaparin. French Investigators of the ESSENCE Trial

BACKGROUND

The pathogenesis of unstable angina and non-Q-wave myocardial infarction is still poorly understood, and early evaluation of prognosis remains difficult. We therefore studied the predictive value of 5 biological indicators of inflammation, thrombogenesis, vasoconstriction, and myocardial necrosis, and we examined the effects of enoxaparin and unfractionated heparin on these markers after 48 hours of treatment.

METHODS AND RESULTS

Sixty-eight patients with unstable angina or non-Q-wave myocardial infarction randomized in the international ESSENCE trial participated in this French substudy. C-reactive protein, fibrinogen, von Willebrand factor antigen, endothelin-1 and troponin I were measured on admission and 48 hours later. The composite end point of death, myocardial infarction, recurrent angina, or revascularization was significantly lower at 14 and 30 days of follow-up in patients allocated to enoxaparin compared with unfractionated heparin. All acute-phase reactant proteins were elevated on admission and increased further at 48 hours. Multivariate analysis demonstrated that the rise of von Willebrand factor over 48 hours was a significant and independent predictor of the composite end point at both 14 days and 30 days. Moreover the early increase of von Willebrand factor was more frequent and more severe with unfractionated heparin than with enoxaparin (mean change was +8.7+/-8.8% with enoxaparin versus +93.9+/-11.7% with unfractionated heparin, P<0.0001). The other clinical and biological variables did not predict outcome.

CONCLUSIONS

In patients with unstable angina or non-Q-wave myocardial infarction, the acute-phase proteins increase over the first 2 days despite medical treatment. The early rise of von Willebrand factor is an independent predictor of adverse clinical outcome at 14 days and at 30 days. Enoxaparin provides protection as evidenced by the reduced release of von Willebrand factor, which represents a favorable prognostic finding.

Differential effect of a low-molecular-weight heparin (dalteparin) and unfractionated heparin on platelet interaction with the subendothelium under flow conditions

Effects of heparins on platelet function are controversial, but perfusion studies, that simulate physiological conditions, might help to explain their clinical behaviour. We used a perfusion system to test the effect unfractionated heparin and a low-molecular-weight heparin (dalteparin, Fragmin) on platelet adhesion promoted by a damaged vascular surface. Normal blood samples containing increasing concentrations of unfractionated heparin and dalteparin were perfused through annular chambers containing enzymatically denuded rabbit aorta segments (5 min at 800 sec-1). Segments were evaluated morphometrically measuring the total surface covered by platelets and by large aggregates (thrombi). Unfractionated heparin inhibited platelet interaction with subendothelium in a lesser extent than dalteparin in both covered surface and thrombi (p < 0.01). At heparin doses that caused maximal inhibition covered surface was 10.0% +/- 3.1% with dalteparin and 18.3% +/- 4.2% with unfractionated heparin, and thrombi were 5.6% +/- 3.2% and 12.3% +/- 3.0%, respectively. These results may collaborate to explain some differences observed in the clinical results obtained with unfractionated heparin and with dalteparin.

Effect of nonspecific binding to plasma proteins on the antithrombin activities of unfractionated heparin, low-molecular-weight heparin, and dermatan sulfate

BACKGROUND

Nonspecific binding to plasma proteins decreases the anti-factor Xa (anti-Xa) activity of unfractionated heparin (UFH) but not that of low-molecular-weight heparin (LMWH). However, plasma proteins could influence the anti-thrombin (anti-IIa) activity of LMWH. To explore this possibility, we compared the effects of plasma proteins on the anti-IIa activities of UFH and LMWH. We also examined their effects on the anti-IIa activity of dermatan sulfate (DS) because, like UFH, DS binds to plasma proteins.

METHODS AND RESULTS

There was almost complete recovery of anti-IIa activity when UFH, LMWH, or DS was added to plasma from each of 20 healthy volunteers. The addition of a chemically modified heparin with low affinity for antithrombin III to plasma containing UFH increased the anti-IIa activity in a concentration-dependent fashion by displacing UFH from plasma proteins. In contrast, addition of low-affinity heparin had no effect on the anti-IIa activity of LMWH. LMWH does not bind to plasma proteins because the bulk of the LMWH chains are < 6000 D, and only heparin fractions > 6000 D bind nonspecifically to plasma proteins. As further evidence that plasma proteins do not influence the anti-IIa activity of LMWH, the rate of thrombin inhibition in plasma in the presence of LMWH is virtually identical to that in buffer containing physiological amounts of the major antithrombins. In contrast, with UFH or DS, the rate of thrombin inhibition is twofold slower in plasma than in buffer.

CONCLUSIONS

Nonspecific binding of UFH to plasma proteins most likely contributes to the variable anti-IIa response to UFH in patients with thromboembolic disease. Although DS also binds to plasma proteins, the clinical significance of this finding is unclear. In contrast, because LMWH does not bind to plasma proteins, the anti-IIa activity of LMWH should be just as predictable as its anti-Xa activity.

Comparative effects of enoxaparin and unfractionated heparin in healthy volunteers on prothrombin consumption in whole blood during coagulation, and release of tissue factor pathway inhibitor

In a randomized crossover study twelve healthy male volunteers (23.5 +/- of 4.8 years, 73.0 +/- 6.4 kg, 180.8 +/- 5.7 cm) received one subcutaneous injection of either enoxaparin (EN) at 40 mg or 1 mg kg-1, or unfractionated heparin (UH) at 5,000 IU at one week intervals. Area under curves (AUC) of Anti-Xa and Anti-IIa activities correlated with EN dose. The relative effectiveness of EN versus UH 5,000 U as assessed by AUC ratio (EN/UH) was 7 and 15 for Anti-Xa activity, 1.3 and 3.1 for Anti-IIa activity after sc injection of EN 40 mg (4,000 Anti-Xa IU and 1,200 Anti-IIa U) and 1 mg kg-1 (7,300 +/- 640 Anti-Xa IU and 2,190 +/- 290 Anti-IIa IU) respectively. In volunteers receiving EN, a dose dependent inhibition of thrombin generation rate in platelet depleted plasma (PDP), measured with a new and simple chromogenic thrombin generation assay, was observed when compared with baseline values. Similarly, intrinsic prothrombin activation in whole blood, evidenced by measuring residual factor II in serum 2 hours after clotting (prothrombin consumption test: PC), was inhibited in a dose dependent manner. In UH treated volunteers, although the inhibition of thrombin generation rate in PDP was similar to that observed with EN 40 mg, prothrombin consumption in whole blood was not significantly modified. Tissue factor pathway inhibitor (TFPI) activity release was increased similarly for UH and EN 40 (1.4 fold increase above baseline values) and 1.9 fold for the higher dose of EN. The discrepancy between prothrombin consumption in whole blood and inhibition of thrombin generation rate in PDP in the UH and not in the EN group strongly suggests that UH and not EN is influenced by the presence of a platelet component. This could be formed during thrombin induced platelet activation. Platelet factor 4 is a possible candidate. Another hypothesis involves the role of TFPI-UH complex anticoagulant activity which might be inhibited more during whole blood coagulation than the TFPI-EN complex.

Impaired fibrinolysis and postoperative thromboembolism in orthopedic patients

The incidence of deep vein thrombosis (DVT) and pulmonary embolism was studied prospectively in patients undergoing elective total hip replacement. 96 patients were randomly allocated to receive either low molecular weight heparin (LMWH) or unfractionated heparin (UFH). All patients had bilateral phlebography and pulmonary perfusion/ventilation scintigraphy 10-12 days after surgery. The following fibrinolytic variables were analysed in plasma and related to thromboembolism: tissue plasminogen activator (t-PA) activity, t-PA antigen (t-PA Ag), plasminogen activator inhibitor (PAI-1) activity and PAI-1 antigen (PAI-1 Ag). No significant difference was found, regarding the fibrinolytic response to surgery, between patients treated with LMWH and UFH. The level of PAI-1 activity was significantly increased before operation in patients developing DVT as compared to non-DVT patients (p less than 0.03). Immediately after surgery and in the morning the first postoperative day the levels of PAI-1 activity, PAI-1 Ag and t-PA Ag were positively correlated to thromboembolism. PAI-1 activity was the only preoperative fibrinolytic variable correlated to thromboembolism.

Rationale for development of low-molecular-weight heparins and their clinical potential in the prevention of postoperative venous thrombosis

Interest in low-molecular-weight heparins (LMWHs) as potential antithrombotic agents was stimulated by two observations in the mid-1970s and early 1980s. The first was finding that LMWH fractions prepared from unfractionated heparin (UFH) progressively lost their ability to prolong the activated partial thromboplastin time (APTT) while retaining their ability to inhibit Factor Xa. The second was the observation that LMWHs prepared by chemical depolarization of UFH are antithrombotic in experimental animal models but produce less microvascular bleeding in experimental models for an equivalent antithrombotic effect than the UFH from which they are derived. Subsequently, it was shown that LMWHs inhibit platelet function and impair vascular permeability less than standard heparin and that LMWHs have a longer biological half-life than standard heparin. A number of LMWHs have been evaluated in clinical trials in general and orthopedic surgery and in the treatment of venous thrombosis. LMWHs are highly effective in orthopedic surgery, where they appear to be more effective than standard heparin. LMWHs have also been shown to be either as effective or more effective than UFH in preventing postoperative thrombosis following general surgery. In preliminary studies, LMWHs appear to be as effective as standard heparin in the treatment of venous thrombosis, but larger studies are required using clinically relevant outcome measures.