Permeation enhancers, as the generally used excipients in transdermal drug-delivery system (TDDS) formulations, can weaken the skin-barrier function and impact drug partitioning and diffusion in the various layers of the skin [1]. Permeation enhancers can be considered as molecules exerting “pharmacological effects” on opening skin-barrier function. However, the enhancement activity of permeation enhancers are not static. Some researchers have found that Azone, propylene glycol, and oleic acid show different enhancement effects during different treatment time [2], [3], indicating that a dynamic enhancing process of permeation enhancers exists. However, these studies still lack an integrated enhancing process. Accordingly, the skin pharmacokinetics (SPK) of permeation enhancers is crucial to in-depth research as it provides parameters to evaluate their fate and dynamic enhancing process after transdermal delivery into the skin.

The SPK of permeation enhancers can quantitatively describe their process of absorption, residence, and elimination of permeation enhancers in the skin, thereby directly representing their concentration change in the skin and dose–effect relationship on enhancing drug permeation. To evaluate the disposition of enhancers in the skin, the classic pharmacokinetic processes of absorption, distribution, metabolism, and excretion must obviously be considered. However, permeation enhancers primarily act on the opening of the skin barrier. Thus, the different absorption rate constants in the different layers of the skin, such as those of drugs, need not be considered [4]. Only the kinetics of permeation enhancers from the formulation into the stratum corneum (SC) require consideration, whereas the distribution of permeation enhancers into the deep layers of epidermis and dermis (EP) is negligible. Although metabolic activity in the skin is recognized, permeation enhancer biotransformation is at best a secondary phenomenon in general and may be significant in only rare cases [5]. Accordingly, the present study focused on the two major processes of absorption and elimination of permeation enhancers in the SC.

The SPK of permeation enhancers is a diffusion behavior in the skin. In addition to longitudinal diffusion to deeper layers, small-molecule substances may also have lateral-diffusion behavior in the skin [6]. The SPK behavior of permeation enhancers is not only related to their physicochemical properties, but also to the microenvironment of each layer of skin. The intercellular space of SC is a lipid bilayer structure formed by lipid molecules (mainly ceramide, cholesterol and fatty acid) [7]. This microenvironment may be suitable for diffusion of weakly polar molecules. However, the hydrophilic microenvironment of the EP layer is more suitable for the diffusion of polar molecules. The longitudinal and lateral diffusion behavior of permeation enhancers in the skin may be the fundamental cause of their different SPK. Thus, these diffusion behaviors warrant concern. Based on the different diffusion behaviors of permeation enhancers, different SPK parameters such as the maximum enhancer concentration (Cmax-enhancer), the time of maximum enhancer concentration (tmax-enhancer), elimination rate constant (ke-enhancer), and half-time period (t1/2-enhancer) can directly and quantitatively reflect the onset time, duration time and extent of the permeation enhancers’ barrier-opening function on SC. The dynamic opening states of the SC barrier function further influences the transdermal behaviors of drugs involving lag time, flux, and maximum permeation amount [8]. By establishing the correlations between the SPK variables of permeation enhancers and the transdermal behaviors of drugs, the dynamic permeation-enhancing effect of permeation enhancers on the drug can be predicted from its SPK parameters, thereby benefiting the wide-ranging application of permeation enhancers’ SPK. Knowing the SPK of permeation enhancers can elucidate their dynamic enhancing mechanism on drug permeation. On this basis, we can select or design synthetic permeation enhancers to provide efficient transdermal formulation and the optimum dosage regimen. Therefore, the demand for information concerning the SPK of permeation enhancers remains essential for the ongoing development and application of permeation enhancers in systemic TDDSs.

In the current work, the SPK parameters of permeation enhancers and their relationship with drug-permeation parameters in vivo were initially determined. Subsequently, the molecular mechanisms of permeation enhancers’ SPK processes on the transdermal behaviors of drug were investigated. Diclofenac (DIC) was selected as a model drug, and six permeation enhancers with different physicochemical properties and structures exerting significant permeation enhancement effect on DIC [9] were selected. They were Plurol Oleique CC (POCC), Span 80 (Span), Ascorbyl palmitate (AP), Lauroglycol FCC (FCC), Azone (AZ) and isopropyl myristate (IPM). Their structures are illustrated in Fig. 1. The SPK of permeation enhancers was determined through tissue-distribution test. The diffusion behaviors of permeation enhancers were evaluated with a specific parameter for permeation enhancers, diffusion ratio (DRSC-EP), which was the ratio of the diffusion depth of the permeation enhancers in the EP layer to the diffusion breadth in the SC layer. They were further characterized by molecular dynamics (MD) simulation and confocal laser scanning microscopy (CLSM). The influence on transdermal behaviors of DIC produced by the SPK of permeation enhancers was investigated by in vivo skin-permeation study. Correlation analyses were conducted to establish the relationship between the permeation enhancers’ SPK parameters and the drug’s permeation parameters. The molecular mechanisms of the dynamic effect of SPK process on drug transdermal behaviors were characterized utilizing modulated-temperature differential scanning calorimetry (MTDSC), dielectric spectroscopy, small-angle X-ray scattering (SAXS), and cross-polarization solid-state NMR (ssNMR).