Determination of unbound ticagrelor and its active metabolite (AR-C124910XX) in human plasma by equilibrium dialysis and LC–MS/MS

Ticagrelor is the first direct acting reversibly binding oral platelet P2Y12 receptor antagonist. The parent molecule and the main metabolite (AR-C124910XX) are both able to block adenosine diphosphate-induced receptor signaling with similar potency. Drug binding to plasma proteins reduces free drug available for pharmacologic activity. Therefore, assessing unbound drug is important for interpretation of pharmacokinetic/pharmacodynamic findings. This paper describes the development and validation of an equilibrium dialysis/LC–MS/MS method for measuring unbound ticagrelor and AR-C124910XX in human plasma. Plasma samples (200 μl) were dialysed against phosphate buffered saline (37 °C, 24 h) in 96-well dialysis plates to separate unbound analytes. Drug–protein binding alterations during dialysis were minimized by maintaining physiologic conditions (pH 7.4, 37 °C). Ticagrelor and AR-C124910XX were quantified in dialysates (unbound fraction), retentates and plasma (total concentration) using liquid chromatography–mass spectrometry/mass spectrometry (LC–MS/MS) methods. Calibration curves were established for the retentate and plasma (total concentration) in the ranges 5–5000 ng/ml (ticagrelor) and 2.5–2500 ng/ml (AR-C124910XX), and for the dialysate in the range 0.25–100 ng/ml (both analytes). Both ticagrelor and AR-C124910XX were highly protein bound (>99.8%), i.e. unbound fraction <0.2%. Yet, the methodology was successfully applied to determine unbound concentrations of ticagrelor and AR-C124910XX in clinical samples.