The implications of lipid mobility, drug-enhancers (surfactants)-skin interaction, and TRPV1 activation on licorice flavonoid permeability.

Abstract

Licorice flavonoids (LFs) are derived from perennial herb licorice and have been attaining a considerable interest in cosmetic and skin ailment treatments. However, some LFs compounds exhibited poor permeation and retention capability, which restricted their application. In this paper, we systematically investigated and compared the enhancement efficacy and mechanisms of different penetration enhancers (surfactants) with distinct lipophilicity or "heat and cool" characteristics on ten LFs compounds. Herein, the aim was to unveil how seven different enhancers modified the stratum corneum (SC) surface and influence the drug-enhancers-skin interaction, and to relate these effects to permeation enhancing effects of ten LFs compounds. The enhancing efficacy was evaluated by enhancement ratio (ER)permeation, ERretention, and ERcom, which was conducted on the porcine skin. It was summarized that heat capsaicin (CaP) and lipophilic Plurol® Oleique CC 497 (POCC) caused the most significance of SC lipid fluidity, SC water loss, and surface structure alterations, thereby resulting in a higher permeation enhancing effects than other enhancers. CaP could completely occupied drug-skin interaction sites in the SC, while POCC only occupied most drug-skin interactions. Moreover, the enhancing efficacy of both POCC and CaP was dependent on the log P values of LFs. For impervious LFs with low drug solubility, enhancing their drug solubility could help them permeate into the SC. For high-permeation LFs, their permeation was inhibited ascribed to the strong drug-enhancer-skin strength in the SC. More importantly, drug-surfactant-skin energy possessed a good negative correlation with the LFs permeation amount for most LFs molecules. Additionally, the activation of transient receptor potential vanilloid 1 (TRPV1) could enhance LFs permeation by CaP. The study provided novel insights for drug permeation enhancement from the viewpoint of molecular pharmaceutics, as well as the scientific utilization of different enhancers in topical or transdermal formulations.