Enantiomer-selective pharmacokinetics, oral bioavailability, and sex effects of various alpha-lipoic acid dosage forms

The present study aimed to examine the enantiomer-selective pharmacokinetics (PK), relative bioavailability (F(rel)), and sex effects of various oral dosage forms of racemic alpha-lipoic acid (ALA). In an open-label, randomized, four-period, four-sequence crossover study, 24 healthy adult subjects (12 males and 12 females) received single doses of 600 mg of ALA in fasted state at four different occasions as follows: three 200 mg tablets (T 200); two 300 mg tablets (T 300); one 600 mg tablet (T 600); and a racemic ALA solution (OS). All tablet formulations (Thioctacid HR) were considered test treatments, while the OS (Thioctacid, 600 T) served as the reference treatment. Serial blood samples were collected over 8 hours postdose to quantify R-(+)- and S-(−)-ALA enantiomer plasma concentrations for the PK evaluation. The maximum observed plasma concentration (C(max)) and total exposure (area under the curve [AUC](0–t)) were compared between treatments by analysis of variance. Weight-normalized C(max) and the AUC data of male and female study subjects were applied to examine the presence of sex effects. All treatments displayed rapid absorption of both enantiomers with median time to maximum concentration (t(max)) values ranging from 0.33–0.5 hours. The F(rel) of all tablet formulations was comparable, with R-(+)-enantiomer C(max) test/reference ratios ranging from 36% (T 600) to 43% (T 200), and R-(+)-enantiomer AUC test/reference ratios ranging from 64% (T 600) to 79% (T 300), indicating a favorable F(rel) of all tablet formulations, especially in terms of the total extent of absorption (AUC). An examination of weight-normalized female/male C(max) and AUC sex ratios for both ALA enantiomers indicated the absence of a significant sex effect for C(max), as well as 20%–26% and 25%–32% higher R-(+)- and S-(−)-ALA enantiomer AUC outcomes in females when compared to males. The observed modest sex effect was comparable for both ALA enantiomers and across all formulations, and it did not appear to require a dose adjustment in clinical practice.