Contact

Daria Pašalić
Editor-in-Chief
Department of Medical Chemistry, Biochemistry and Clinical Chemistry
Zagreb University School of Medicine
Šalata ul 2.
10 000 Zagreb, Croatia
Phone +385 (1) 4590 205; +385 (1) 4566 940
E-mail: dariapasalic [at] gmail [dot] com

Useful links

S2-1

Why do we need new oral anticoagulant drugs?

Zdravka Poljaković. Why do we need new oral anticoagulant drugs? Biochemia Medica 2015;25(Suppl 1):S16-S17.

Neurological Intensive Care Unit, University Hospital Centre Zagreb, Zagreb, Croatia

 

Anticoagulant therapy is a golden standard for treatment and prevention of embolic stroke. According to recent epidemiologic data, at least 20% of ischemic strokes are cardioembolic. Further 30% are so called “cryptogenic“strokes, meaning that they are of unknown cause, however, most of them are embolic as well. Finally, we should also add all ischemic strokes which developed due to cerebral sinus thrombosis, where anticoagulant therapy also should be used. Bearing all this in mind, in primary or secondary prevention of nearly half of all strokes, this kind of antithrombotic therapy is indicated.

Until recently, the first treatment choice for patients which had to take anticoagulant therapy was warfarin. Warfarin is indeed a highly effective drug, but with some serious disadvantages. Again, according to statistic data, after introducing warfarin therapy the risk of ischemic stroke became 20 times lower in patients with atrial fibrillation (which is the most frequent cause of cardioembolic stroke). However, only in patients who had warfarin in a narrow therapeutic dose dependent range. Nevertheless, considering interindividual metabolic differences in warfarin pharmacogenetics and metabolism, as well as known fear from adverse events, only about 50% of all patients who have indication for warfarin really take the medication, and of them, only 60% are in needed therapeutic range. Furthermore, warfarin significantly increases the risk of intracerebral and gastrointestinal bleeding, requires frequent and regular laboratory check-ups and has proven interactions with other drugs and food.

New oral anticoagulant drugs (NOACs) which came to market recently (dabigatran, rivaroxaban and apixaban) have some different characteristics, on the first place stable pharmacokinetics and pharmacodynamics. Therefore, there is no need for laboratory check-ups, they can be taken continuously in the same dose, having, according to clinical studies and experience, same effectiveness and significantly less risk for adverse events (on the first place for intracranial and systemic bleeding) than warfarin. They also have significantly less interactions with other drugs and food. According to European and American guidelines, they became the drug of the first choice for prevention of cardioembolic stroke in patients with non-valvular atrial fibrillation.

However, we still do not have a long-time experience in using NOACs, and those drugs still have some unrevealed facts. Due to this, there are, at the moment, a lot of clinical trials focusing to the use of NOACs in some other indications (prevention of embolic stroke of some other origin), studying their pharmacokinetics and anticoagulant effect, finding the antidote and discovering their characteristics and risk in some special populations or special situations (like thrombolytic therapy or use after intracranial bleeding).

e-mail: zdravka [dot] po [at] gmail [dot] com

S2-2

Laboratory aspects on anticoagulant drugs

Andreas Hillarp. Laboratory aspects on anticoagulant drugs. Biochemia Medica 2015;25(Suppl 1):S17-S19.

Department of Clinical Chemistry and Transfusion Medicine, Halland Hospital, Halmstad, Sweden

 

Introduction: A new generation of drugs for treatment and prophylaxis of thromboembolic disorders, known as Non-vitamin K dependent oral anticoagulants (NOAC), have been introduced in recent years. These provide direct inhibition of either thrombin (dabigatran) or factor Xa (rivaroxaban or apixaban) and offer an alternative to the traditional vitamin K antagonists for many indications. Laboratory monitoring is possible but not necessary however, NOACs interfere with many common coagulation assays and knowledge of these effects may be clinically valuable.

Materials and Methods: In order to systematically evaluate the interfering effects of NOACs plasma from 10 healthy individuals were spiked with dabigatran, rivaroxaban or apixaban in the concentration range 0 – 1000 µg/L and analysed using different reagents for activated thromboplastin time (APTT), prothrombin time (PT), fibrinogen, antithrombin, activated protein C resistance, lupus anticoagulans, protein C and protein S assays. In order to measure the drug concentration we evaluated drug-specific assays, based on inhibition of thrombin or factor Xa.

Results: All NOACs display a curve-linear dose-response with APTT where dabigatran had a slightly greater effect compared to rivaroxaban. At an expected peak concentration (200 µg/L) of dabigatran the measured APTT was invariably above the upper reference range for all tested APTT reagents. The effect of rivaroxaban was slightly less compared to dabigatran but most samples resulted in abnormal APTT at peak concentration. Apixaban had weak effects and at the simulated peak concentration (200 µg/L) almost all samples resulted in values within the normal reference ranges. Similar effects on the PT assays were also observed but rivaroxaban displayed a linear dose-response although there were marked differences in sensitivities between PT-reagents. Again, apixaban had weak effects on the PT-assays and few samples with 200 µg/L differed from samples without addition of apixaban. Other coagulation assays revealed more predictive effects. Thus, antithrombin assays based on thrombin were affected by dabigatran and assays based on Xa were affected by rivaroxaban and apixaban. Moreover, functional assays for APC resistance, protein C or S cannot be used during therapy with NOACs as these assay types are greatly influenced by the drugs.

Conclusion: The thrombin-inhibitor dabigatran and the Xa-inhibitor rivaroxaban will affect most coagulation assays at therapeutic drug concentrations. However, apixaban had surprisingly weak responses on the APTT and PT assays compared to the other two drugs. Many assays involved in a laboratory investigation of venous thrombosis are also affected by all three NOACs and it’s not advisable to perform such an investigation during therapy. The reason for the observed in vitro difference between NOACs is not elucidated but makes it difficult to give generalizable recommendations about the effects on common coagulation assays. In cases when urgent and reliable measurement of NOACs is needed it’s recommended to use drug-specific assays. Dabigatran can accurately be determined with dilute thrombin time assays or assays based on ecarin-activation of coagulation. Rivaroxaban and apixaban can be measured with direct Xa-inhibition assays.

e-mail: Andreas [dot] Hillarp [at] regionhalland [dot] se

S2-3

 

Measuring the anticoagulant effect of new oral anticoagulants – practical aspects

Désirée Coen Herak. Measuring the anticoagulant effect of new oral anticoagulants – practical aspects.Biochemia Medica 2015;25(Suppl 1):S19-S21.

Department of Laboratory Diagnostics, University Hospital Centre Zagreb, Zagreb, Croatia

Implementation of new oral anticoagulants (NOACs) in clinical practice has presented new challenges for coagulation laboratories in finding appropriate laboratory methods for the measurement of their anticoagulant effects. Recently published recommendations from the ISTH SSC on Control of Anticoagulation propose two types of coagulation assays for each NOAC: semiquantitative and quantitative assays. Semiquantitative assays, such as PT and APTT, can only be used for estimation of the relative intensity of anticoagulation, and should be considered as indicators of supratherapeutic, therapeutic or subtherapeutic anticoagulation status. Furthermore, it has been confirmed that TT is suitable for exclusion of supratherapeutic dabigatran concentrations due to its strong sensitivity to dabigatran, so TT result within the reference interval indicate undetectable or very low level of dabigatran.

The anticoagulant effect of each NOAC on PT and APTT has been demonstrated In a number of studies. In general, thrombin inhibitors (dabigatran) exibit a more pronounced influence on APTT, while PT is more affected by FXa inhibitors (rivaroxaban, apixaban). However, as remarkable between-reagent variability has been observed, it has been proposed that every laboratory should explore the relative sensitivity of their local APTT reagent to dabigatran and local PT reagent to rivaroxaban and apixaban by using NOAC-specific calibrators or plasma samples from patients receiving a specific NOAC.

Quantitative measurement of NOAC concentrations can be performed either directly by LC-tandem mass spectrometry, or indirectly by using specific coagulation assays that are based on methods closely related to their anticoagulant targets and mechanism of action. The most suitable coagulation assays for the measurement of dabigatran concentrations are diluted TT, ecarin clotting time and ecarin chromogenic assay calibrated with dabigatran calibrators, whereas quantitative measurement of rivaroxaban and apixaban concentrations should be performed by a chromogenic anti-Xa assay calibrated with specific rivaroxaban or apixaban calibrators.

In vitro anticoagulant effect of rivaroxaban on local PT and APTT reagents was investigated by analyzing commercial rivaroxaban calibrators (STA-Rivaroxaban Calibrator, Diagnostica Stago, France) with concentrations of 0, 100, 254 and 494 µg/L and in pooled normal plasma spiked with increasing rivaroxaban concentrations (29, 121, 346 and 734 µg/L), corresponding to through, peak and supratherapeutic levels. A dose-dependent prolongaton of both PT and APTT results was observed, with PT and APTT ratios above the reference interval at 121 and 100 µg/L, respectively.

Furthermore, ex-vivo effect of rivaroxaban was investigated in plasma samples of 21 patients with nonvalvular atrial fibrilation (creatinine clearence >50 mL/min) receiving 20 mg of rivaroxaban once daily. Rivaroxaban concentrations were measured in plasma samples taken before (through concentration) and 3h after oral administration (peak concentration) using a chromogenic anti-Xa assay (Berichrom Heparin) calibrated with STA-Rivaroxaban Calibrators. The obtained through (mean: 13.0 μg/L, range: 0-72.2 μg/L) and especially peak concentrations (mean: 105.1 μg/L, range: 1.9-221.3 μg/L) differed cosiderably between patients, as well as PT peak and through values (2-45%) and APTT ratios (1.01-1.43).

In conclusion, the possibility to assess NOACs with reliable coagulation assays can help clinicians, if needed, in elucidating whether an active agent is present or not.

e-mail: dcoen [at] kbc-zagreb [dot] hr