Anti-Xa and Anti-IIa Drugs Alter International Normalized Ratio Measurements: Potential Problems in the Monitoring of Oral Anticoagulants

Experiences from six years of quality assured Model of End Stage Liver Disease (MELD) diagnostics

BACKGROUND

The model of end-stage liver disease (MELD) score was established for the allocation of liver transplants. The score is based on the medical laboratory parameters: bilirubin, creatinine and the international normalized ratio (INR). A verification algorithm for the laboratory MELD diagnostic was established, and the results from the first six years were analyzed.

METHODS

We systematically investigated the validity of 7,270 MELD scores during a six-year period. The MELD score was electronically requested by the clinical physician using the laboratory system and calculated and specifically validated by the laboratory physician in the context of previous and additional diagnostics.

RESULTS

In 2.7% (193 of 7,270) of the cases, MELD diagnostics did not fulfill the specified quality criteria. After consultation with the sender, 2.0% (145) of the MELD scores remained invalid for different reasons and could not be reported to the transplant organization. No cases of deliberate misreporting were identified. In 34 cases the dialysis status had to be corrected and there were 24 cases of oral anticoagulation with impact on MELD diagnostics.

CONCLUSION

Our verification algorithm for MELD diagnostics effectively prevented invalid MELD results and could be adopted by transplant centers to prevent diagnostic errors with possible adverse effects on organ allocation.

Multipurpose HTS Coagulation Analysis: Assay Development and Assessment of Coagulopathic Snake Venoms

Coagulation assays currently employed are often low throughput, require specialized equipment and/or require large blood/plasma samples. This study describes the development, optimization and early application of a generic low-volume and high-throughput screening (HTS) assay for coagulation activity. The assay is a time-course spectrophotometric measurement which kinetically measures the clotting profile of bovine or human plasma incubated with Ca²⁺ and a test compound. The HTS assay can be a valuable new tool for coagulation diagnostics in hospitals, for research in coagulation disorders, for drug discovery and for venom research. A major effect following envenomation by many venomous snakes is perturbation of blood coagulation caused by haemotoxic compounds present in the venom. These compounds, such as anticoagulants, are potential leads in drug discovery for cardiovascular diseases. The assay was implemented in an integrated analytical approach consisting of reversed-phase liquid chromatography (LC) for separation of crude venom components in combination with parallel post-column coagulation screening and mass spectrometry (MS). The approach was applied for the rapid assessment and identification of profiles of haemotoxic compounds in snake venoms. Procoagulant and anticoagulant activities were correlated with accurate masses from the parallel MS measurements, facilitating the detection of peptides showing strong anticoagulant activity.

Population Pharmacokinetic and Pharmacodynamic Modeling Analysis of GCC-4401C, a Novel Direct Factor Xa Inhibitor, in Healthy Volunteers

GCC‐4401C, an orally active direct factor Xa inhibitor that is similar to rivaroxaban, is currently under development for venous thromboembolic disease (VTE). The purpose of this study was to characterize the pharmacokinetics (PKs) and pharmacodynamics (PDs) of GCC‐4401C by population modeling analysis and to predict proper dosage regimens compared to rivaroxaban using data from two phase I clinical studies. Plasma GCC‐4401C concentrations over time were best described by a two‐compartment linear model and body weight was associated with central volume of distribution. Relevant PD markers generally changed in a dose‐dependent manner and were described well with sigmoid, simple maximum effect, or linear models. GCC‐4401C was absorbed more rapidly than rivaroxaban. Comparisons based on simulations of PD marker changes over time suggest that 20 mg and 40 mg of GCC‐4401C administered under fasted status are comparable to 10 mg and 20 mg of rivaroxaban under fed status.

New oral anticoagulants - a practical guide

Oral direct inhibitors of thrombin and activated factor Xa are approved as new anticoagulant drugs. In contrast to vitamin K antagonists (VKA) and heparins, the new agents have single targets in the coagulation cascade and more predictable pharmacokinetics, but they lack validated and available antidotes. Unlike VKA, they do not require routine monitoring of coagulation. However, the measurement of their pharmacologic effects might be of value in selected patients. They interfere with the routine coagulation tests, which should be interpreted with caution. Specific tests exist and can be used in case of emergencies. Adequate supportive care and temporary removal of all antithrombotic agents constitute the basis for management of serious bleeding complications. The administration of coagulation factors, such as fresh frozen plasma, prothrombin complex concentrates or recombinant activated FVII, can benefit in life-threatening bleeding or emergency surgery. Specific antidotes for non-vitamin K oral anticoagulants are in clinical development.

This review aims at answering in a brief and simplified manner some clinical questions.

Measuring the anticoagulant effects of target specific oral anticoagulants-reasons, methods and current limitations

To simplify and optimize oral anticoagulation, new target-specific oral anticoagulants (TSOAs) have been developed. The direct thrombin-inhibitor dabigatran and the direct factor Xa inhibitors rivaroxaban, apixaban and edoxaban are the first such compounds to receive approval in certain countries for various indications. Due to the predictable pharmacokinetic and pharmacodynamic profiles of these drugs, routine monitoring of patients receiving TSOA therapy has not been considered necessary. However, it has now been realized that in routine clinical settings, there are several situations where it may be prudent to assess the level of TSOA anticoagulation. Several studies evaluating the influence of TSOAs on various coagulation assays have been performed to identify systems that can be used to monitor these drugs. With a particular focus on dabigatran and rivaroxaban, we will describe and discuss the potential of several methods for measuring the anticoagulant effect of TSOAs, as well as their methodological limitations and the restrictions in transferring their results into clinical context.

Fabricating a novel porous releaser of heparin

Reduced graphene oxide (rGO) could adsorb heparin of 112 mg g⁻¹ and released 80% of them within 30 days.

Probing structural selectivity of synthetic heparin binding to Stabilin protein receptors

As one of the most widely used drugs worldwide, heparin is an essential anticoagulant required for surgery, dialysis, treatment of thrombosis, cancer, and general circulatory management. Stabilin-2 is a scavenger clearance receptor with high expression in the sinusoidal endothelium of liver. It is believed that Stabilin-2 is the primary receptor for the clearance of unfractionated and low molecular weight heparins in the liver. Here, we identify the modifications and length of the heparin polymer that are required for binding and endocytosis by both human Stabilin receptors: Stabilin-2 and its homolog Stabilin-1 (also found in liver endothelium). Using enzymatically synthesized (35)S-labeled heparan sulfate oligomers, we identified that sulfation of the 3-OH position of N-sulfated glucosamine (GlcNS) is the most beneficial modification for binding and endocytosis via both Stabilin receptors. In addition, our data suggest that a decasaccharide is the minimal size for binding to the Stabilin receptors. These findings define the physical parameters of the heparin structure required for efficient clearance from blood circulation. These results will also aid in the design of synthetic heparins with desired clearance rates.

A Novel Prothrombin Time Assay for Assessing the Anticoagulant Activity of Oral Factor Xa Inhibitors

Conventional prothrombin time (PT) assays have limited sensitivity and dynamic range in monitoring the anticoagulant activity of direct factor Xa inhibitors. Hence, new assays are needed. We modified a PT assay by adding calcium chloride (CaCl2) to the thromboplastin reagent to increase assay dynamic range and improve sensitivity. Effects of calcium and sodium ion concentrations, and sample handling, were evaluated to optimize assay performance. Increasing concentrations of calcium ions produced progressive increases in PT across the factor Xa inhibitor concentrations of 0 to 2500 nmol/L for razaxaban and apixaban. The greatest effect was seen when the thromboplastin reagent was diluted 1:2.25 with 100 mmol/L CaCl2 (thus selected for routine use). The optimized assay showed an interassay precision of 1.5 to 9.3 percentage coefficient of variation (%CV) for razaxaban and 3.1 to 4.6 %CV for apixaban. We conclude that the modified PT assay is likely to be suitable as a pharmacodynamic marker for activity at therapeutic concentrations of factor Xa inhibitors.

Chemoenzymatic synthesis of homogeneous ultralow molecular weight heparins

Ultralow molecular weight (ULMW) heparins are sulfated glycans that are clinically used to treat thrombotic disorders. ULMW heparins range from 1500 to 3000 daltons, corresponding from 5 to 10 saccharide units. The commercial drug Arixtra (fondaparinux sodium) is a structurally homogeneous ULMW heparin pentasaccharide that is synthesized through a lengthy chemical process. Here, we report 10- and 12-step chemoenzymatic syntheses of two structurally homogeneous ULMW heparins (MW = 1778.5 and 1816.5) in 45 and 37% overall yield, respectively, starting from a simple disaccharide. These ULMW heparins display excellent in vitro anticoagulant activity and comparable pharmacokinetic properties to Arixtra, as demonstrated in a rabbit model. The chemoenzymatic approach is scalable and shows promise for a more efficient route to synthesize this important class of medicinal agent.

Assays for measuring rivaroxaban: their suitability and limitations

Several new oral anticoagulants such as rivaroxaban (which targets Factor Xa) and dabigatran etexilate (which targets thrombin) are in advanced stages of clinical development and are already available for clinical use in some countries. Although these agents do not require routine coagulation monitoring, assays to assess the level of anticoagulation may be of assistance in certain circumstances such as in case of overdose, in patients with a hemorrhagic or thromboembolic event during treatment, or to assess compliance. Moreover, the influence of the new oral anticoagulants on routine coagulation tests must be recognized. The prothrombin time is not suitable for rivaroxaban measurement for several reasons, and the routinely used international normalized ratio for monitoring the vitamin K antagonists cannot be applied to rivaroxaban. Development of universal assays is challenging because the new oral anticoagulants have different targets, and even those with the same target have variable effects on routine coagulation assays. Focusing on rivaroxaban, there is emerging evidence that an anti-Factor Xa assay that uses rivaroxaban-containing plasma calibrators may provide the optimal method for determining plasma rivaroxaban concentrations.

Anticoagulation with argatroban in a parturient with heparin-induced thrombocytopenia

Unfractionated heparin and low-molecular-weight heparin are currently the anticoagulants of choice for the prevention of recurrent thromboembolic disease during pregnancy. However, heparin-induced thrombocytopenia contraindicates the use of unfractionated heparin and low-molecular-weight heparin. We describe a patient who was admitted to our hospital with deep vein thrombosis at 18 weeks of gestation and who developed heparin-induced thrombocytopenia during her antenatal care. Therapeutic anticoagulation was initially achieved with argatroban, then changed to fondaparinux. During early labor, fondaparinux was discontinued and intravenous argatroban was substituted. Argatroban was discontinued during transition to active labor. After return of a normal partial thromboplastin time, combined spinal-epidural analgesia was induced for routine completion of labor and vaginal delivery. We discuss the decisions made in the maintenance of this patient's anticoagulation during the peripartum period as well as timing of her neuraxial labor analgesia.

[Rivaroxaban: mode of action]

Rivaroxaban is the first oral anticoagulant with a direct anti-Xa activity to be registered (approval). As for all first comers in a class, it should be assessed both for itself and for the class. The targeting of factor-Xa factor, key component in the coagulation cascade, has the theoretical benefit of being an effective antithrombotic and a potential risk for hemorrhage, both highly dose-dependent. Experience has shown us that the representativeness and predictiveness of in vitro tests and preclinical models are only partial and sometimes even misleading. This is why the responses can only come from clinical trials and rigorous research testing doses, which should be conducted specifically in all the indications foreseen, with no extrapolations. The oral anticoagulant drugs are developed in the prevention of arterial thromboembolic events caused by atrial fibrillation too, where the vitamin K antagonists (VKAs) are the current standard of care. The well-known problems of monitoring and adaptation doses with VKAs have led to developing new replacement classes without the need for control or biological adaptation. However, in certain conditions there is a need to monitor the patient. The advantage for the direct anti-Xa inhibitors such as rivaroxaban is that the prothrombin time, a routine test is sensitive and provides a prolonged response that is proportional to the plasma concentration within a wide range of concentrations. This test is potentially usable provided that the indispensable standardization is forthcoming.

Inhibition of extrinsic hemostasis activation by low-molecular-weight heparin

Low-molecular-weight heparins (LMWHs) are very important drugs; unfortunately, the routine global hemostasis assays activated partial thromboplastin time and prothrombin time are not sensitive to LMWHs. Here the 50% inhibitory concentration (IC(50)) values of heparin and LMWHs on extrinsic thrombin generation are determined. Pooled normal plasma was supplemented with 0-2 IU/ml unfractionated heparin, 0-2 IU/ml LMWH dalteparin, or 0-20 microg/ml pentosanpolysulfate in 5-ml polystyrole tubes (23 degrees C) and tested in the tissue-factor-triggered extrinsic coagulation activity assay (EXCA): 50 microl plasma + 5 microl tissue factor/CaCl(2), 1 and 2 min incubation time at 37 degrees C (coagulation reaction time for EXCA-1 and EXCA-2); + 100 microl of 2.5 mol/l arginine (pH 8.6), 20 min at room temperature; + 50 microl of 1 mmol/l CHG-Ala-Arg-pNA, 1.25 mol/l arginine; increase in absorbance/time at 23 degrees C; calibrator = 1 IU/ml bovine thrombin in 6.7% human albumin replacing the plasma sample; in EXCA-1, about 1 IU/ml thrombin is generated in pooled unfrozen normal citrated plasma. The IC(50) values in EXCA-1 are 0.1 IU/ml heparin, 0.02 IU/ml LMWH, and 4.7 microg/ml pentosanpolysulfate. In ECXA-2 the IC(50) values are 0.07 IU/ml, 0.01 IU/ml, and 4.6 microg/ml, respectively. The EXCA reflects the efficiency of anticoagulants on plasmatic coagulation. It is suggested to adjust the dosage of LMWH according to the EXCA value; about 30% of normal extrinsic thrombin generation might be the correct dose for prophylactic anticoagulation.

Pharmacokinetics and pharmacodynamics of BIBT 986, a novel small molecule dual inhibitor of thrombin and factor Xa

BACKGROUND

Current anticoagulant development focuses on agents with predictable pharmacokinetic and pharmacodynamic (PD) properties. BIBT 986 is a novel potent anticoagulant with a dual mechanism of action: it competitively inhibits factor (F) Xa and FIIa.

AIMS

To determine the safety, tolerability, pharmacokinetics (PK) and PD of BIBT 986 following intravenous infusion in healthy male volunteers.

METHODS

In three randomized, double-blind, placebo-controlled trials, subjects were administered by intravenous infusion escalating doses of BIBT 986 for up to 32 h. BIBT 986 concentrations were determined in plasma and urine samples by high pressure liquid chromatography tandem mass spectrometry. Pharmacodynamic response was assessed by measuring the changes in blood coagulation times. Activated partial thromboplastin time, International Normalized Ratio, thrombin time and ecarin clotting time were determined and compared with baseline results.

RESULTS

In all three studies, intravenous infusion of BIBT 986 was safe and well tolerated. BIBT 986 exhibited linear PK over the dose range tested. Clearance was about 8 L h(-1) and V(ss) about 50 L. Apparent steady state concentrations were reached within 24 h, indicating a dominant half-life of about 6 h. The terminal half-life of BIBT 986 was approximately 12 h. Renal excretion contributes approximately 50% to total elimination. Overall interindividual variability in pharmacokinetic and PD parameters was < 40%. There was a linear correlation between plasma concentrations and PD responses, suggesting excellent predictability.

CONCLUSION

BIBT 986 is the first small molecule of a novel class of anticoagulants that potently and directly inhibits both coagulation FXa and thrombin. It has predictable pharmacokinetic and PD characteristics.