Enantiomeric determination of &agr;‐lipoic acid in urine by LC/MS/MS

Abstract

&NA; An analytical method for the enantiomeric determination of &agr;‐lipoic acid in human urine was developed to evaluate the pharmacokinetics of &agr;‐lipoic acid, an ingredient in health food. Urine samples were collected over time after administering &agr;‐lipoic acid dietary supplement to healthy subjects. The samples were cleaned up by solid‐phase extraction using an Oasis® MAX cartridge. In the LC/MS/MS method, CHIRALPAK AD‐3R was used as the chiral separation column and acetonitrile‐methanol‐formic acid (10 mM) (25:25:50, v/v/v) was used as the mobile phase. 13C4 1,2,5,6‐d‐l‐&agr;‐Lipoic acid was used as the internal standard. MS/MS was performed by electrospray ionization (ESI) in the negative ion mode using two monitoring ion transitions (m/z 205.0 → 170.9 and m/z 209.0 → 174.9). A calibration curve was prepared in the concentration range of 0.5–100 ng/mL for each enantiomer (r > 0.9999). The limit of detection (LOD, S/N = 3) and the limit of quantification (LOQ, S/N > 10) were 0.1 ng/mL and 0.5 ng/mL, respectively. The intra‐day and inter‐day accuracy of &agr;‐lipoic acid enantiomers at the LOQ level (0.5 ng/mL), the low concentration level (5 ng/mL), the middle concentration level (50 ng/mL), and the high concentration level (100 ng/mL) ranged from 93.7 to 103.1%. The intra‐day and inter‐day precision were ≦ 6.94% and ≦ 7.05%, respectively. Calculating the mean values of pharmacokinetic parameters revealed that the AUC and Cmax values of d‐&agr;‐lipoic acid were statistically significantly higher than those of l‐&agr;‐lipoic acid (p < 0.05). It was suggested that l‐&agr;‐lipoic acid is more bioavailable than d‐&agr;‐lipoic acid despite individual differences in excretion rate.