Skin Barriers in Dermal Drug Delivery: Which Barriers Have to Be Overcome and How Can We Measure Them?

Abstract

Although, drugs are required in the various skin compartments such as viable epidermis, dermis, or hair follicles, to efficiently treat skin diseases, drug delivery into and across the skin is still challenging. An improved understanding of skin barrier physiology is mandatory to optimize drug penetration and permeation. The various barriers of the skin have to be known in detail, which means methods are needed to measure their functionality and outside-in or inside-out passage of molecules through the various barriers. In this review, we summarize our current knowledge about mechanical barriers, i.e., stratum corneum and tight junctions, in interfollicular epidermis, hair follicles and glands. Furthermore, we discuss the barrier properties of the basement membrane and dermal blood vessels. Barrier alterations found in skin of patients with atopic dermatitis are described. Finally, we critically compare the up-to-date applicability of several physical, biochemical and microscopic methods such as transepidermal water loss, impedance spectroscopy, Raman spectroscopy, immunohistochemical stainings, optical coherence microscopy and multiphoton microscopy to distinctly address the different barriers and to measure permeation through these barriers in vitro and in vivo.

Keywords: TEER; TEWL; atopic dermatitis; claudin; drug delivery; microscopy; skin barrier; spectroscopy; stratum corneum; tight junctions; tomography.

Figure 1

Overview of epidermal molecular structures important for skin barrier function. (A) Schematic drawing of the uppermost stratum granulosum and stratum corneum. KHG: keratohyalin granula KLK: kallikrein-like kinases, LB: lamellar body, TGN: trans-golgi-net TJ: tight junctions. (B) Schematic drawing of the cornified envelope. SPR: small proline-rich. (C) Organization of the intercellular lipids of the stratum corneum. LPP: long periodicity phase, SPP: short periodicity phase. (D) Tight Junction (TJ) structure and TJ proteins in the epidermis. Cldn: claudin, JAM: junctional adhesion molecule, Ocln: Occludin, ZO: zonola occludens protein. (E) Composition and structure of the basement membrane, (A) modified from [19], (B) from [20], (C) from [21], (D) from [2], (E) from [22].

Figure 3

Localization of mechanical barriers in the skin (light red: stratum corneum (SC), Tight junctions (TJs) and basement membrane (BM)) and graphical representations of the various physical/chemical methods described in this review: (A) Fourier transform infrared (FTIR) spectroscopy (B) electrical impedance spectroscopy, (C) chemical analyses, (D) transepidermal water loss (TEWL), (E) transepithelial electrical resistances (TEER), (F) dye permeation analyses, and (G) Raman spectroscopy. Brackets denote the area measured by a specific method not discriminating between the different barriers within this area. (B) and (C) can be used to measure complete skin but can also be used—by additional preparation steps or more sophisticated methods—to discriminate between SC, viable epidermis and dermis. BV: blood vessel, HF: hair follicle, SbG: sebaceous gland, SwG: sweat gland. (A) from [122], (C) from [123], (F) from [74], (G) from [124].

Figure 4

Localization of mechanical barriers in the skin (light red: stratum corneum (SC), Tight junctions (TJs) and basement membrane (BM)) and graphical presentation of the various microscopical methods described in this review: (A1–A6) Multiphoton microscopy with fluorescence lifetime imaging and second harmonic generation signal detection (B) transmission electron microscopy (C) confocal laser scanning microscopy of immunofluorescence stainings (D) Immunohistochemical analysis of a biotinylation assay (E) optical coherence tomography (OCT). The bracket denotes the area measured by OCT not discriminating between the different barriers within this area. Dotted black lines denote specific barriers/skin areas addressed by a method. Arrow in E denotes a hair follicle. BV: blood vessel, HF: hair follicle, SbG: sebaceous gland, SwG: sweat gland, (B) from [172], (C) from [170], (E) from [173].

Figure 2

Schematic drawing of a hair follicle in anagen (A) and catagen (B) phase denoting stratum corneum (SC) and tight junction (TJ) barriers. More intense color of SC denotes SC similar to epidermal SC. Less intense color denotes infundibular SC with slightly different properties. Please note that the companion cell layer in the central and isthmus region is overlayed by the TJ barrier labelling.