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R-/S-2-(2-hydroxypropanamido)-5-trifluoromethyl benzoic acid (R-/S-HFBA) is a novel COX inhibitor with remarkable anti-inflammatory and antiplatelet aggregation activities, but no gastrointestinal toxicity. In our previous study, the different pharmacokinetic profiles of the two enantiomers in rats were observed after administration of R-HFBA and S-HFBA. Stereoselective protein binding of the two enantiomers may be a reason for the different pharmacokinetic behaviors. In this study, we developed and validated an UPLC-MS/MS method for determining stereoselective binding of HFBA enantiomers to rat, dog, and human plasma in vitro. Chromatographic separation was achieved by gradient elution with a flow rate of 0.4 mL/min. MS/MS detection was operated in positive electrospray using multiple reaction monitoring (MRM) mode. The method was proved to be linear over the concentration range of 0.005 to 10 μg/mL with a lower limit of quantification of 0.005 μg/mL. The developed method was successfully employed to the plasma protein binding study of HFBA enantiomers. Equilibrium dialysis method was applied to assess drug-plasma protein interactions.The results showed that the enantiomers were both extensively bound to three species plasma and protein binding of R-/S-HFBA was concentration dependent.R-HFBA and S-HFBA showed significant species difference among rat, dog, and human plasma and stereoselective plasma protein binding. K E Y W O R D Splasma protein binding, R-/S-2-(2-hydroxypropanamido)-5-trifluoromethyl benzoic acid, species difference, stereoselectivity, UPLC-MS/MS

R-/S-2-(2-Hydroxypropanamido)-5-trifluoromethyl benzoic acid (R-/S-HFBA), as a novel COX inhibitor, was firstly reported to have remarkable anti-inflammatory and antiplatelet aggregation activities by our group.