Quantification of a potent 5-HT2a antagonist and an active metabolite in rat plasma and brain microdialysate by liquid chromatography-tandem mass spectrometry

A method based on liquid chromatography-tandem mass spectrometry and microbore column separation was developed for the quantification of a potent 5-HT_2a receptor antagonist (R)-(+)-α-(2,3-dimethoxyphenyl)-1-[2-(4-fluorophenyl)etnyl]-4-piperidine-methanol (I) and the desmethyl metabolite (II) in rat brain extracellular fluid (ECF) following microdialysis sampling. The analytical method was also applied to determining plasma concentrations of these compounds. The lower limit of quantification (LLQ) for each compound in microdialysis perfusate is 500 pg/mL, which translates to <7 fmol (injected). The recovery of I and II for the microdialysis probe in brain ECF was 18.5 and 22.7%, respectively. The LLQ for each compound in plasma is 1 ng/mL. The inherent selectivity offered by tandem mass spectrometry eliminated chemical noise, thereby improving the detectability of these compounds. These methods were used to confirm that I and II penetrated the blood-brain barrier following administration of I to rats and enabled comparison of plasma and brain ECF concentrations.