Pharmacokinetic and pharmacodynamic drug monitoring of direct acting oral anticoagulants: Where do we stand?

For decades, oral anticoagulation has been based on vitamin K antagonist such as warfarin, which requires pharmacodynamic drug (PD) monitoring to guide the therapy. The drug effect is measured by the clotting test prothrombin time (PT) and expressed as international normalized ratio (INR). New direct oral anticoagulants (DOACs) are increasingly used in fixed dose regimens but are licensed without any therapy monitoring.However, extensive clinical experiences have demonstrated that inter-individual variations in the response to the therapy with DOACs do exist. In situations such as bleeding or thrombosis, therapeutic drug monitoring could be useful. Unfortunately, global coagulation assays such as the PT or the activated partial thrombin time (aPTT) are not suitable for this purpose. To measure drug concentrations, more specific coagulation test can be used if they are externally calibrated with the respective drugs. For the direct thrombin inhibitor dabigatran etexilate, a calibrated diluted thrombin time (dTT) or ecarin clotting time (ECT) can be used whereas for anti-factor Xa drugs such as rivaroxaban, apixaban, edoxaban, and betrixaban, calibrated anti-factor Xa assays are appropriate. However, the gold standard to measure drug concentrations is LC-MS/MS.The variation in bleeding and thrombotic events noted with both drug classes under fixed dose conditions suggests additional inter-individual pharmacodynamic differences. Therefore, PD monitoring to individualize the therapy may be an option. For dabigatran, this is the inhibition of thrombin formation and for anti-factor Xa drugs, the inhibition of factor Xa activity, which can be followed using the functional assays mentioned above but without calibration. Alternatively, thrombin generation assays (TGE) have been proposed for both drug classes.So far not many clinical data have been published about the potentially beneficial effects of PD monitoring for dose individualization. The assay platforms for PD monitoring are present in many clinical laboratories, but efforts are needed to validate and standardize available assays to perform appropriate clinical trials.