Ecarin clotting time but not aPTT correlates with PEG-hirudin plasma activity

Anticoagulative strategies in reconstructive surgery--clinical significance and applicability

Advanced strategies in reconstructive microsurgery and especially free tissue transfer with advanced microvascular techniques have been routinely applied and continously refined for more than three decades in day-to-day clinical work. Bearing in mind the success rates of more than 95%, the value of these techniques in patient care and comfort (one-step reconstruction of even the most complex tissue defects) cannot be underestimated.

However, anticoagulative protocols and practices are far from general acceptance and – most importantly – lack the benchmark of evidence basis while the reconstructive and microsurgical methods are mostly standardized.

Therefore, the aim of our work was to review the actual literature and synoptically lay out the mechanisms of action of the plethora of anticoagulative substances.

The pharmacologic prevention and the surgical intervention of thrombembolic events represent an established and essential part of microsurgery. The high success rates of microvascular free tissue transfer as of today are due to treatment of patients in reconstructive centers where proper patient selection, excellent microsurgical technique, tissue transfer to adequate recipient vessels, and early anastomotic revision in case of thrombosis is provided. Whether the choice of antithrombotic agents is a factor of success remains still unclear. Undoubtedly however the lack of microsurgical experience and bad technique can never be compensated by any regimen of antithrombotic therapy. All the more, the development of consistent standards and algorithms in reconstructive microsurgery is absolutely essential to optimize clinical outcomes and increase multicentric and international comparability of postoperative results and complications.

Direct thrombin inhibitors: pharmacology and application in intensive care medicine

PURPOSE

Anticoagulation is part of the daily routine of intensive care physicians. As the possibilities of pharmacological anticoagulation are becoming more numerous and diverse, intensive care physicians have to be familiar with indications, contraindications, dosing, and reversal of many different substances. This paper presents an overview of the substance group of direct thrombin inhibitors (DTI) indicated for alternative anticoagulation in intensive care medicine.

METHODS

The review is a synopsis of scientific evidence, expert opinion, open forum commentary, and clinical feasibility data.

RESULTS AND CONCLUSIONS

Due to their antithrombotic potential without direct activation of platelets, DTI could offer potential advantages over heparins and vitamin K antagonists in critically ill patients, especially regarding heparin-induced thrombocytopenia. Because of multiple organ dysfunction, organ failure, and comedications, simple extrapolation of results of medical to critically ill patients is not permissible. The fine line between thrombosis and bleeding in intensive care patients requires cautious dosing and close drug monitoring. Studies dealing with DTI in the intensive care setting are of utmost clinical interest.

[Direct thrombin inhibitors: pharmacology and application in cardiovascular anesthesia]

The options for drug-controlled anticoagulation are becoming noticeably more manifold. In the area of anaesthesiology and intensive care, there are furthermore special disease patterns, such as heparin-induced thrombocytopenia (HIT) to be known, diagnosed and treated. This article gives a review of the substance groups of the direct thrombin inhibitors (DTI) as alternative anticoagulants for HIT in combination with cardiovascular diseases. For the administration of DTIs, experience and the correct dose are the keys to success and are the deciding factors for the two sides of haemostasis: thrombosis and haemorrhage.

Lepirudin in the management of patients with heparin-induced thrombocytopenia

Lepirudin, a recombinant hirudin, is a direct irreversible thrombin inhibitor by binding to both free and clot-bound thrombin. It is approved for treatment of heparin-induced thrombocytopenia (HIT), which is a serious antibody-mediated drug reaction mostly associated with the use of unfractionated heparin. Clinical experience during the last 10 years has proved the efficacy of lepirudin in the management of HIT. The major route of elimination of lepirudin is the kidney, accounting for approximately 90% of its systemic clearance. The most important adverse reactions are bleeding and the induction of immunologic reactions. The risk of bleeding can be reduced by implementing an optimal monitoring and dose adjustment strategy, particularly in patients undergoing cardiopulmonary bypass surgery and in those with impaired renal function. Development of antihirudin antibodies may enhance the anticoagulant effect of lepirudin. Anaphylactic reactions associated with lepirudin therapy are rare. The lack of an antidote against lepirudin is still a concern, particularly during cardiopulmonary bypass surgery with a heart-lung machine and during artificial renal support. Currently, hemofiltration using high-flux filter systems is the only available and valid means to manage hirudin overdose. Nevertheless, the drug can be safely used if meticulous monitoring strategy is installed.

Effects of melagatran on activated partial thromboplastin time and on ecarin clotting time in cord versus adult plasma

Melagatran is the active form of the oral direct thrombin inhibitor ximelagatran. Melagatran does not require antithrombin as a cofactor. Its administration is therefore of special interest in neonatal patients, whose plasma is relatively deficient in antithrombin. We investigated the effects of increasing amounts of melagatran (0.05-1 micromol/l) on the activated partial thromboplastin time (APTT) and ecarin clotting time (ECT) in cord versus adult plasma. Both the APTT and ECT were dose-dependently prolonged in the presence of increasing amounts of melagatran. Furthermore, the ECT revealed a higher susceptibility of cord plasma to addition of melagatran than adult plasma. Whereas similar amounts of melagatran were required in cord and adult plasma samples to double the APTT (IC(50), 0.47 vs 0.46 micromol/l), significantly less melagatran was required in cord versus adult plasma to double the ECT (IC(50), 0.26 vs 0.56 micromol/l). Based on APTT measurements, similar plasma levels of melagatran might be required in neonates and in adults to treat thromboembolic complications. The APTT, however, is relatively insensitive to plasma melagatran concentrations. When the sensitive indicator ECT is used, results suggest that lower amounts of melagatran might be required in neonates than in adults. This has to be scrutinized in future clinical studies.

Comparison of Two Different Ecarin Clotting Time Methods

BACKGROUND

Ecarin Clotting Time (ECT) assay specifically determines the inhibition of meizothrombin by direct thrombin inhibitors (DTI). Blood coagulation factor levels lowered by vitamin K antagonists (VKA) may prolong ECT. Concomitant treatment of VKA with DTI may influence differently the two published ECT methods.

METHODS

Lepirudin (100-3,000 ng/ml), argatroban (300--3,000 ng/ml) and melagatran (30--1000 ng/ml) were added to normal plasma (NP; n=12) samples and to plasma of patients on stable vitamin K antagonist therapy with warfarin (VKAP; n=12). ECT assays were performed according to [5] (method 1) and according to [6] (method 2). Data were subjected to multifactorial variance analysis.

RESULTS

Normal ranges were 35.5+/-2.8 s in NP versus 31.8+/-1.2 s in VKAP with method 1 (p< 0.001) and 44.3+/-3.9 s in NP vs. 51.4+/-8.3 s in VKAP with method 2 (p< 0.004). Besides the inhibitors (p<0.0001), the method used (p<0.0001) and the group (NP vs. VKAP, p=0.003) had an influence on the ECT. Inhibitors (p< 0.02) or method used (p< 0.03) and the group (NP vs. VKAP, p=0.0001) influenced also the ECT ratio.

DISCUSSION

Both ECT methods are suitable for monitoring different DTIs over a large linear range with both methods during concomitant treatments with vitamin K antagonists. The ECT ratio improves but not abolishes the differences between the methods. Additive effects of vitamin K antagonists on ECT methods have to be taken into consideration in clinical routine.

Practical applications of snake venom toxins in haemostasis

Snake venom toxins affecting haemostasis have facilitated extensively the routine assays of haemostatic parameters in the coagulation laboratory. Snake venom thrombin-like enzymes (SVTLE) are used for fibrinogen/fibrinogen breakdown product assay and for the detection of fibrinogen dysfunction. SVTLE are not inhibited by heparin and can thus can be used for assaying antithrombin III and other haemostatic variables in heparin-containing samples. Snake venoms are a rich source of prothrombin activators and these are utilised in prothrombin assays, for studying dysprothrombinaemias and for preparing meizothrombin and non-enzymic forms of prothrombin. Russell's viper (Daboia russelli) venom (RVV) contains toxins which have been used to assay blood clotting factors V, VII, X, platelet factor 3 and, importantly, lupus anticoagulants (LA). Other prothrombin activators (from the taipan, Australian brown snake and saw-scaled viper) have now been used to assay LA. Protein C and activated protein C resistance can be measured by means of RVV and Protac, a fast acting inhibitor from Southern copperhead snake venom and von Willebrand factor can be studied with botrocetin from Bothrops jararaca venom. The disintegrins, a large family of Arg-Gly-Asp (RGD)-containing snake venom proteins, show potential for studying platelet glycoprotein receptors, notably, GPIIb/IIIa and Ib. Snake venom toxins affecting haemostasis are also used in the therapeutic setting: Ancrod (from the Malayan pit viper, Calloselasma rhodostoma), in particular, has been used as an anticoagulant to achieve 'therapeutic defibrination'. Other snake venom proteins show promise in the treatment of a range of haemostatic disorders.

Comparing Direct Thrombin Inhibitors Using aPTT, Ecarin Clotting Times, and Thrombin Inhibitor Management Testing

BACKGROUND:

Patients with heparin-induced thrombocytopenia and thrombosis may be acutely anticoagulated with direct thrombin inhibitors (DTIs). The anticoagulation is typically monitored using the activated partial thromboplastin time (aPTT) or ecarin clotting time (ECT).

OBJECTIVE:

To compare 14 methods for measuring aPTT, as well as ECT and thrombin inhibitor management test (TIM), in samples containing DTIs.

METHODS:

DTIs were added to pooled normal plasma to achieve low (0.1–1.2 μg/mL) and high (1.5–8.0 μg/mL) drug concentrations. Each low-concentration DTI sample was tested using all aPTT reagents, while each low- and high-concentration DTI was tested using the ECT and TIM.

RESULTS:

All aPTT reagents had a significant dose-dependent correlation with drug concentration. Only Actin FSL and APTT-S demonstrated equivalent aPTT ratios obtained from any DTI. The TAS-aPTT was the most sensitive aPTT reagent to argatroban, with the aPTT ranging from 52.7 to 121.2 seconds corresponding to 0.1 to 1.2 μg/mL of drug concentration. The TAS-aPTT and Pathromtin were the most sensitive aPTT reagents to bivalirudin, with aPTTs of 87.4 seconds and 101.5 seconds, respectively, at 1.2 μg/mL of drug. Pathromtin was the most sensitive aPTT reagent to lepirudin, with a maximum aPTT of 108.9 seconds at 1.2 μg/mL of drug. There was no statistically significant difference between the TIM and ECT clotting times for each DTI. Lepirudin and bivalirudin ECT and TIM clotting times were equivalent.

CONCLUSIONS:

There are unique differences between reagent manufacturers in the monitoring of DTIs. Acceptable alternatives to aPTT monitoring of DTI anticoagulation include the ECT and TIM.

Efficiency in clinical research: assessment in vitro of potential anti-thrombotic drug interactions

OBJECTIVE

To determine whether testing in vitro of combinations of anti-thrombotic agents can identify potentially important interactions, we evaluated the combination of rNAPc2 with antagonists of platelet GP IIb-IIIa to identify potentially altered anticoagulant properties, antiplatelet effects, or both.

METHODS

Blood was obtained from healthy subjects who were taking aspirin (325 mg/day). Selected concentrations of rNAPc2, enoxaparin, and GP IIb-IIIa inhibitors were added in vitro. Platelet function was assessed with the use of flow cytometry.

RESULTS

No effect on clotting or platelet inhibition was apparent when abciximab was added to the combination of aspirin, enoxaparin, and rNAPc2 at concentrations up to 250 ng/ml. A modest (less than 10%, P <0.02) effect on the time to clot assessed with the activated clotting time was demonstrated when either eptifibatide or tirofiban was combined with aspirin and enoxaparin plus rNAPc2. rNAPc2 did not alter antiplatelet effects of eptifibatide. By contrast, a modest, approximately 10%, increase in the inhibition of fibrinogen binding (P <0.01) was seen when rNAPc2 was added to the combination of aspirin, enoxaparin, and tirofiban.

CONCLUSIONS

The lack of an exaggerated effect on clotting and platelet function when GP IIb-IIIa inhibitors were combined with rNAPc2, aspirin, and enoxaparin suggests that no substantial increment in the incidence of bleeding would be observed when concentrations of rNAPc2 up to 250 ng/ml were to be used in clinical studies. More extensive use of testing in vitro in advance of large-scale clinical trials of anti-thrombotic agents and regimens is likely to enhance their design and implementation.

Are coagulation times biomarkers? Data from a phase I study of the oral thrombin inhibitor LB-30057 (CI-1028)

Ecarin clotting time and activated partial thromboplastin time are coagulation tests that meet the definition of a biomarker. Prolongation of these coagulation times closely correlated with blood concentrations of the oral thrombin inhibitor LB-30057 (CI-1028) during a phase 1 study. But this simply reflects their functioning as enzyme inhibition assays of drug concentration. Directly adding the drug to blood results in the same concentration-response relationship. Changes in coagulation tests only demonstrate that ex vivo clot formation has been altered, not that an in vivo process has been affected. To be most informative in drug development, biomarker assays should measure in vivo drug effects, not drug concentrations.

Effects of lepirudin, argatroban and melagatran and additional influence of phenprocoumon on ecarin clotting time

INTRODUCTION

Direct thrombin inhibitors (DTI) prolong the ecarin clotting time (ECT). Oral anticoagulants (OA) decrease prothrombin levels and thus interact with actions of DTIs on the ECT method during concomitant therapy.

MATERIALS AND METHODS

Actions of lepirudin, argatroban and melagatran on ECT were investigated in normal plasma (NP) and in plasma of patients (n=23 each) on stable therapy with phenprocoumon (OACP). Individual line characteristics were tested statistically.

RESULTS

Control ECT in OACP was prolonged compared to NP (50.1+/-0.9 vs. 45.7+/-0.8 s; p<0.001). Lepirudin prolonged the ECT linearly. Argatroban and melagatran delivered biphasic dose-response curves. OA showed additive effects on the ECT of lepirudin but not of argatroban and melagatran. Both in NP and OACP, the first and second slopes of melagatran were steeper compared to argatroban (primary analysis; p<0.001). When using the same drug, slopes in OACP were steeper than in NP (secondary analysis; p<0.001). At similar molar concentrations, the crossing points of both slopes were significantly higher with melagatran (323.1+/-11.0 s in NP and 333.2+/-8.2 s in OACP) than with argatroban (219.6+/-14.7 and 248.4+/-15.2 s) corresponding to ratios of 7.1+/-0.2 and 6.7+/-0.2 (melagatran) vs. 4.8+/-0.3 and 4.9+/-03 with argatroban (p<0.0001).

DISCUSSION

The patterns of interactions between vitamin K antagonists and DTI effects are different for bivalent (increase of slope without affecting linearity) and monovalent inhibitors (slight increase or alteration of nonlinear slopes), but there are also differences between the two monovalent inhibitors on thrombin inhibition as determined by ECT.

Effect of phenprocoumon on monitoring of lepirudin, argatroban, melagatran and unfractionated heparin with the PiCT method

Prothrombinase-induced clotting time (PiCT) is a clotting-time test for heparins and direct thrombin inhibitors to reduce drawbacks of aPTT. Effects of the direct thrombin inhibitors lepirudin, argatroban, melagatran and of unfractionated heparin (UFH) were investigated in normal and oral anticoagulant plasma samples. Lepirudin showed potentiating interferences with phenprocoumon effects. Melagatran, argatroban and UFH delivered distinct linear additive effects in both plasma sample groups. PiCT ratio reduces differences between both groups with UFH, and argatroban inhibitor-receptor-binding mode plays a role in interaction patterns.