Enantiomeric determination of α-lipoic acid in urine by LC/MS/MS

An analytical method for the enantiomeric determination of α-lipoic acid in human urine was developed to evaluate the pharmacokinetics of α-lipoic acid, an ingredient in health food. Urine samples were collected over time after administering α-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-α-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 α-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 C_max values of d-α-lipoic acid were statistically significantly higher than those of l-α-lipoic acid (p <  0.05). It was suggested that l-α-lipoic acid is more bioavailable than d-α-lipoic acid despite individual differences in excretion rate.