Development and validation of a sensitive LC–MS/MS method for the determination of fenoterol in human plasma and urine samples

• Developed an LC–MS/MS method for the determination of fenoterol enantiomers.
• Improved quantitation limit from 2 ng/ml to 50 pg/ml over existing methods.
• Enantioselective competitive interaction affects fenoterol presystemic sulfation.

Due to the lack of sensitivity in current methods for the determination of fenoterol (Fen), a rapid LC–MS/MS method was developed for the determination of (R,R′)-Fen and (R,R′;S,S′)-Fen in plasma and urine. The method was fully validated and was linear from 50 pg/ml to 2000 pg/ml for plasma and from 2.500 ng/ml to 160 ng/ml for urine with a lower limit of quantitation of 52.8 pg/ml in plasma. The coefficient of variation was <15% for the high QC standards and <10% for the low QC standards in plasma and was <15% for the high and low QC standards in urine. The relative concentrations of (R,R′)-Fen and (S,S′)-Fen were determined using a chirobiotic T chiral stationary phase. The method was used to determine the concentration of (R,R′)-Fen in plasma and urine samples obtained in an oral cross-over study of (R,R′)-Fen and (R,R′;S,S′)-Fen formulations. The results demonstrated a potential pre-systemic enantioselective interaction in which the (S,S′)-Fen reduces the sulfation of the active (R,R′)-Fen. The data suggest that a non-racemic mixture of the Fen enantiomers may provide better bioavailability of the active (R,R′)-Fen for use in the treatment of cardiovascular disease.