Natural moisturizing factors (NMFs) primarily consist of amino acids or their derivatives, such as pyrrolidone carboxylic acid (PCA) and urocanic acid, together with lactic acid, urea, citrate, and sugars. The importance of NMFs lie in the fact that the constituent chemicals, particularly PCA and lactic acid salts, are intensely hygroscopic. These salts absorb atmospheric water and dissolve in their own water of hydration, thereby acting as efficient humectants (Harding, 2006). After amino acids, PCA is the second most abundant constituent of NMFs, accounting for 12 % of NMFs. In the stratum corneum (SC) of atopic patients, there is a positive correlation between PCA level and SC hydration and a negative correlation between PCA level and trans-epidermal water loss (TEWL) (Jung, 2014). PCA applied to the SC after NMF extraction restores the mobility of keratin to a level comparable to that before extraction, indicating that supplementing dry SC with PCA is effective in regenerating SC function (Gunnarsson, 2021). Thus, PCA is important for maintaining the function of the SC and as a moisturizing ingredient for external formulations.

The permeability coefficient of NMFs in hairless mouse skin at neutral pH is 1–10 × 10–5 cm/h (Ruland, 1991). Moreover, a study using human cadaver skin showed that 5 % to 7 % of PCA is absorbed percutaneously within 24 h of application (Fiume, 2019). The permeability of PCA and other NMFs in the skin has been enhanced by modifying formulations, such as adding various skin permeation enhancers (Ruland, 1992), ion-pairing with fatty acids (Arct, 2002), and micro-emulsification (Kahsay, 2023). As mentioned above, PCA and other NMFs perform their moisturizing function in the SC of the skin because most of these NMFs are localized in corneocytes. Therefore, it is necessary to increase the concentrations of these NMFs in the SC and corneocytes to replenish PCA reduced from the SC by daily soap washing (Harding et al., 2000). In addition, approaches that inhibit the permeation of NMFs below the living cell layer are required for safety and irritation prevention. However, technologies developed to date mainly promote skin permeability below the dermis, not accumulating NMFs to the SC or corneocytes.

In recent years, various methods have been developed to control the skin permeability of active ingredients through the intermolecular interactions between active ingredients and additives (Moshikur et al., 2020, Piao et al., 2023). Sodium dilauramidoglutamide lysine (DLGL), an amphiphilic substance derived from amino acids, has been used as a skin moisturizer and biocompatible skin permeation enhancer (Fujii et al., 2017, Hikima et al., 2013, Terayama et al., 2023). DLGL has multiple amide groups capable of engaging in intermolecular interactions with NMFs. In addition, its large molecular weight may be effective in adjusting the solubility and increasing the apparent molecular weight of PCA, which affect the partition of the skin and the diffusion in the skin, through complexation. However, the specific molecular interactions between DLGL and PCA and the effects of DLGL on the skin permeability/accumulation of PCA are not clear.

In this study, we found that DLGL engages in intermolecular interactions with PCA in aqueous solutions, as well as enhances the partition of PCA in the SC and inhibits the diffusion of PCA in the reconstructed human epidermis model. In addition, the complex of DLGL and PCA readily accumulated in the SC and minimally increased the flexibility of an intercellular lipid model compared with PCA alone. Therefore, complexation of PCA with DLGL is an effective delivery method that increases the concentration of PCA in the SC without damaging the skin barrier nor increasing PCA below the dermis.