Transdermal behaviour of the N-alkylamide spilanthol (affinin) from Spilanthes acmella (Compositae) extracts

Aim of the studyN-Alkylamides are a large group of bioactive molecules found in several plants from the genera Echinacea, Xanthoxylum and Spilanthes. Extracts and formulations derived from these plants are not only orally used, but also applied on the skin as well. However, there is currently no specific information available about the intrinsic local pharmacokinetics of N-alkylamides after topical application on human skin, questioning the role of this mode of administration.The present study investigates the transdermal behaviour of spilanthol, a prominent N-alkylamide.Materials and methodsTwo pharmaceutically accepted dose solutions (ethanol and propylene glycol based aqueous donor vehicles), combined with three different receptor fluids (PBS, PBS + 0.5% HPβCD, EtOH/H2O (30:70, v/v)), were applied on split-thickness human skin in a Franz diffusion cell (FDC) system. Fundamental permeation characteristics of spilanthol in a solvent-independent way (100% aqueous dose solution) were also obtained using an extrapolation approach with different organic solvent/H2O ratios.Results and conclusionsWe demonstrated for the first time that spilanthol permeates the skin. The following aqueous-extrapolated primary transdermal parameters were obtained (mean ± SEM): Kp,aq = 3.31 (±0.29) × 10−3 cm/h, Dm,aq = 1.86 (±0.09) × 10−4 cm2/h and Km,aq = 7.28 (±1.59) × 10−1. Partitioning (Km) was strongly dependent on the donor solution composition, while diffusion (Dm) was mainly influenced by the receptor fluid composition.

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