Achiral LC-MS/MS and chiral SFC-MS methods for quantification of methoxphenidine and O-desmethyl-methoxphenidine metabolite in rat serum and brain

Methoxphenidine (MXP), a dissociative anaesthetic derivative, has garnered the attention of toxicologists for their increasing abuse and associated toxicity. Despite that there is limited forensic and clinical toxicology data on MXP, especially regarding its metabolism and enantiomers. To fill this gap, we developed, validated and applied achiral liquid chromatography-tandem mass spectrometry (LC-MS/MS) and chiral supercritical fluid chromatography-MS (SFC-MS) methods to quantify MXP and its primary metabolite, O-desmethyl-methoxphenidine (dmMXP), in rat serum and brain samples collected after a single subcutaneous dosing of racemic MXP. Serum samples were extracted by protein precipitation with 0.1 % formic acid in acetonitrile, and salting-out-assisted liquid-liquid extraction was utilised for brain extraction. The samples were analysed by a reversed-phase LC-MS/MS method equipped with a Poroshell 120 phenyl-hexyl column, and the enantioselective SFC-MS method was equipped with an Alcyon Amylose-SA column. Both methods were fully validated according to the European Medicines Agency guidelines. The LC-MS/MS method had a total run time of 4.8 min with a linear response up to 400 ng/mL in serum and 2400 ng/g in brain, and the limits of quantification were 1.00 ng/mL and 6.00 ng/g for MXP and 1.00 ng/mL and 1.50 ng/g for dmMXP. The enantioselective SFC-MS method had a total run time of 15 min, showing linear ranges up to 1000 ng/mL for individual enantiomers in serum and 7200 ng/g in brain. The limits of quantification of (R,S)-MXP were 12.5 ng/mL and 30.0 ng/g, while those of (R,S)-dmMXP were 25.0 ng/mL and 60.0 ng/g. The achiral LC-MS/MS method enabled quantification of racemic MXP and dmMXP, while the chiral SFC-MS method was used only for MXP enantiomers, as its higher lower limit of quantification did not allow for enantioselective quantification of dmMXP. Serum MXP peaked at 1600 ng/mL (0.5 h) and decreased to 5.87 ng/mL at 24 h, while dmMXP peaked at 11.8 ng/mL (1 h) and was below the limit of quantification at 24 h. In brain, MXP peaked at 13200 ng/g (0.5 h) and decreased to 36.1 ng/g (24 h), while dmMXP reached 67.1 ng/g (1 h) and decreased to 1.63 ng/g (24 h). Moreover, it was found that the (S)-MXP concentrations in the brain appeared to be higher than the (R)-enantiomer concentrations. The validated methods allow for the generation of pharmacokinetic curves for MXP within a behavioural study and provide a valuable tool for forensic and clinical toxicology for the study of the dissociative anaesthetic MXP and its metabolite in rat serum and brain.