Quantification of lipoic acid from skin samples by HPLC using ultraviolet, electrochemical and evaporative light scattering detectors

• We developed extraction method for lipoic acid from skin samples.
• HPLC/ELS first time used for lipoic acid due to characteristic detection.
• HPLC/EC is useful to LA assays as in vitro permeation and skin bioavailability studies.
• HPLC/UV can be used for stability studies and formulation design of lipoic acid.
• We reported the validation of three different HPLC assays for lipoic acid.

Lipoic acid (LA) is an endogenous organosulfur compound with potent antioxidant property. LA is often used as a drug for the treatment of skin disorders. For the accomplishment of topical applications of LA appropriate drug quantification methods are essential. Thus far, no HPLC methods have been reported for the measurement of LA extracted from skin. In this article we report on the development and validation of three sensitive and specific HPLC methods for LA and dihydrolipoic acid (DHLA) using ultraviolet (UV), electrochemical (EC) or evaporative light scattering (ELS) detection. These methods demonstrate different linearity ranges. The chromatographic separations were performed by RP-HPLC (250 × 4 mm, 5 μm) with isocratic elution using an acidic mobile phase for the three detection techniques. The lower limits of detection and quantification were 0.04 and 0.08 ng LA, respectively, for HPLC coupled to ELS, an innovative detector for LA with high sensitivity. The extraction of LA from skin samples showed recoveries greater than 71%. The recovered LA concentrations from stratum corneum and epidermis+dermis layers were: 5.41 ± 0.56 and 4.92 ± 0.33 μg/mL, respectively for HPLC/UV and 6.52 ± 0.49 and 5.01 ± 0.41 μg/mL, respectively, for HPLC/EC for the added LA concentration (6.67 μg/mL), and 8.88 ± 0.46 and 8.95 ± 0.08 μg/mL, respectively, for HPLC/ELS for the added LA concentration (10 μg/mL). These three optimized HPLC methods allowed for a simple, rapid and reliable determination of LA in human skin. They should be useful for the development of drug delivery systems for topical applications of LA.