Enantioselective determination of ibuprofen in saliva by liquid chromatography/tandem mass spectrometry with chiral electrospray ionization-enhancing and stable isotope-coded derivatization

• A method was developed for the enantioselective determination of ibuprofen in saliva.
• The DAPAP-derivatization enabled the sensitive detection and enantiomeric separation.
• The use of 2H4-DAPAP significantly improved the assay precision and accuracy.
• Saliva-based noninvasive pharmacokinetic analyses of IBU enantiomers were performed.

A method was developed and validated for the enantioselective determination of trace ibuprofen (IBU) in saliva using liquid chromatography/electrospray ionization-tandem mass spectrometry (LC/ESI-MS/MS) combined with the derivatization using a chiral ESI-enhancing reagent, (S)-1-(4-dimethylaminophenylcarbonyl)-3-aminopyrrolidine (DAPAP), and its isotope-coded analog, 2H4-DAPAP (d-DAPAP). The DAPAP-derivatization enabled the highly sensitive detection [detection limit, 0.15 fmol (equivalent to 30 fg of intact IBU) on the column] and complete separation (resolution 3.1) of the IBU enantiomers. The use of d-DAPAP significantly improved the assay precision and accuracy; the intra- (n = 5) and inter-assay (n = 5) relative standard deviations did not exceed 6.2%, and good accuracy (101.3–106.1%) was obtained. The developed method was successfully applied to the quantitative analysis of IBU in saliva. Using this method, salivary concentration–time profiles of each enantiomer after a single oral administration of the racemic IBU to healthy subjects were obtained. The area under the salivary concentration-time curve of the (S)-enantiomer was ca. twice that of the (R)-enantiomer due to the unidirectional chiral inversion of the (R)- to (S)-enantiomer in vivo. Thus, saliva-based noninvasive pharmacokinetic analyses of IBU enantiomers were achieved by this method.

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