Arbutin is a natural glycoside compound in various plants [1], such as pear tree [2], bilberry [3], etc., and it is easily soluble in water and methanol. Previous literature has uncovered that arbutin molecules contain two functional groups: a hydrophilic glucose residue and a phenolic group. Arbutin could effectively inhibit the activity of tyrosinase and the synthesis of melanin [4,5], thereby achieving the skin-whitening effect, which has been widely added to skin care products. Compared with traditional whitening additives such as azelaic acid and niacinamide, arbutin has a better whitening effect and fewer toxic side effects. In addition, arbutin's anti-inflammatory and antibacterial properties make it the preferred choice in SHISEIDO, WINONA, and The-Ordinary. Arbutin is classified into α-arbutin and β-arbutin based on their difference in spatial configuration. α-arbutin mainly reduces cellular melanin synthesis predominantly by competitively inhibiting tyrosinase activity, while β-arbutin exerts the whitening effects by inhibiting tyrosine expression and cell division [6]. As previously demonstrated, α-arbutin possessed higher safety and lower effective doses than β-arbutin [7]. In particular, the former harbors a significantly higher drug permeability through the porcine skin than the latter, accompanied by a higher drug accumulation in the subcutaneous tissue. However, α-arbutin still demonstrated a poor drug permeation amount of 2.35 ± 0.93 μg/cm2 and a low subcutaneous retention amount of 58.50 ± 12.82 μg/g tissue within 12 h, which makes it ineffective at low doses [8].

At present, permeation enhancers are commonly utilized to promote the transdermal penetration of whitening compounds [[9], [10], [11]]. The permeation enhancers can interact with the lipids in the stratum corneum and reduce the orderliness of the lipid arrangement [12,13], denature the keratin by changing its conformation, and disrupt the tight junction of keratin [14,15], thereby promoting the topical/transdermal absorption of the drug. In addition, they increase the solubility of the drug in the stratum corneum (SC), further improving their distribution in the SC [10]. Ideal topical/transdermal permeation enhancers should own the advantages of low cost, easy production, high affinity with drug and matrix, as well as good biocompatibility [[16], [17], [18]]. In this work, to enhance the whitening ability of α-arbutin, a selection of compounds with remarkable whitening efficacy were selected to screen compounds that could promote the penetration of α-arbutin. In the meanwhile, synergistic whitening effects with α-arbutin were expected to be found.

Glabridin (Gla) [19], Butein [20], glucosyl glycerol (Glu), 3-O-Ethyl-L-ascorbic acid (3-OEA) [21], tranexamic acid (TA) [22], Adenosine (ADE) [23], Raspberry ketone glucoside [24], 4-(1-Phenylethyl)-1,3-benzenediol (PB) [25], GLUCOSYL HESPERIDIN (Glu), Chlorhexidine digluconate (CD), and undecylenoyl Phenylalanine (UP) [26] were reported to possess a remarkable skin-whitening effect in literatures or are granted by patents.The physicochemical properties, such as the molecular weight (MW), log P, polar surface area, and polarizability of the enhancers pose a vital effect on their enhancement effect [27,28]. These compounds ranged in their log P from −3.26 to 5.05, polar surface area from 55.8 to 157, and molecular weight from 157.21 to 897.76. Therefore, the structure−permeation enhancement relationships of these compounds were also quantitatively analyzed. However, their enhancing effects and mechanisms on the permeation of α-arbutin were unexplored.

Gla is a natural flavonoid ingredient extracted from the roots of Glycyrrhiza glabra L., a leguminous plant. It has anti-inflammatory, antiviral, and antioxidant properties and is used for the treatment of viral hepatitis and AIDS. It was documented that Gla could inhibit the proliferation of skin cancer cells. Currently, its skin-lightning effects are the most well-known, earning it the nickname “the golden ingredient for skin lightning.” Its mechanisms of skin whitening includes as follows. First, it effectively inhibits the tyrosinase activity to suppress melanin synthesis in the skin. Second, it suppresses the activity of dopa pigment interconversion enzyme (TYP-2) and DHICA oxidase to block enzymatic reactions and reduce melanin synthesis. Third, it acts as an endothelin antagonist to reduce endothelin's regulatory effect on melanocyte proliferation [38,39]. Lastly, it enhances antioxidant capacity, inhibiting the production of reactive oxygen species (ROS), preventing DNA oxidative damage and the activation of the mitochondrial apoptosis pathway, which eventually inhibits UVB radiation-induced damage to human keratinocytes [40].

In the present study, the enhancement efficacy of 10 enhancers was evaluated on the porcine skin, and Gla was finally selected as the osmotic enhancer. Next, the enhancing mechanisms were elucidated by scanning electron microscopy (SEM) and attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR) spectra from the perspectives of molecular pharmaceutics. Since α-arbutin exhibited higher miscibility and interaction with keratin rather than SC lipids, molecular docking studies were performed to understand the interaction properties between keratin and different enhancers or α-arbutin. Lastly, the synergistic whitening effects of α-arbutin and enhancers were examined by α-melanocyte-stimulating hormone-induced B16 model. The study endeavored to find topical enhancer agents that could simultaneously promote the penetration and dermal retention, and increase the anti-melanogenic efficacy of α-arbutin, which provided the scientific basis for expanding the application of arbutin in whitening cosmetics.