Nina Dragicevic - Percutaneous Penetration Enhancers Chemical Methods in Penetration Enhancement: Drug Manipulation Strategies and Vehicle Effects

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Part I
The Skin

Springer-Verlag Berlin Heidelberg 2015

Nina Dragicevic and Howard I. Maibach (eds.) Percutaneous Penetration Enhancers Chemical Methods in Penetration Enhancement 10.1007/978-3-662-45013-0_1

1. Skin Deep: The Basics of Human Skin Structure and Drug Penetration

Keng Wooi Ng 1

(1)

School of Pharmacy and Biomolecular Sciences, University of Brighton, Huxley Building, Lewes Road, Brighton, BN2 4GJ, UK

(2)

Reading School of Pharmacy, University of Reading, Whiteknights, Reading, RG6 6AP, UK

Keng Wooi Ng (Corresponding author)

Email:

Wing Man Lau

Email:

1.1

1.2

1.2.1

1.2.2

1.3

1.3.1

1.4

Keywords

Skin function Stratum corneum Lipid organisation Ceramide Permeation pathway

1.1 Introduction

The skin is the heaviest organ of the human body, on average accounting for 10 % of the body mass and covering nearly 2 m2 of the body surface area (McGrath et al. ). It defines the boundary between the body and its surroundings, thus allowing vital bodily functions to occur within a controlled physiological environment. However, the skin is more than just a physical partition; rather, it provides an important interface through which we interact with the world. One such interaction takes the form of substance exchange between the body and the surrounding environment. Substance exchange is usually finely regulated by the skin, which possesses some exceptional properties to enable it to carry out this function. As a result, the skin is highly selective as to what it lets into, or out of, the body and at what rate. This presents a challenge to drug delivery across the skin into the body, as the molecules in question are likely to be poorly absorbed due to low skin permeation.

The overarching subject of this book is on skin permeation enhancement. Two key concepts, i.e. the properties of the skin and molecular transport through the skin, are fundamental to a full understanding of the subject. In this introductory chapter, we describe the basic properties of the skin and its functions, as well as the mechanisms of skin penetration and permeation, and relate them to the challenges that may be encountered in attempts to deliver molecules through the skin. The coverage is intentionally brief, since the aim is to provide a basic understanding of the subject area to prepare the reader for more advanced discussions in later chapters of this book. Also, although this chapter describes human skin, it is worth noting that many structural and functional parallels can be drawn between human skin and that of certain other animals. This caveat underpins the use of certain animal skin as in vitro models for human skin research, as will be apparent in some of the later chapters.

1.2 The Skin

1.2.1 Structure of the Skin

Human skin is a stratified epithelium, each tissue layer consisting of different cell types that perform distinct functions. It can be broadly divided into the overlying epidermis, dermis and underlying hypodermis (or subcutis) (Fig. ). The epidermis can further be subdivided, from the outside to the inside, into the stratum corneum (horny layer), stratum granulosum (granular layer), stratum spinosum (prickle cell layer) and stratum basale (basal layer also called the stratum germinativum). The stratum basale and stratum spinosum are collectively known as the Malpighian layer. An additional layer, the stratum lucidum (clear layer) can be observed on parts of the body with thickened skin, such as the palm and sole of the foot. However, the stratum lucidum is often not considered a distinct epidermal layer but the lower part of the stratum corneum. In addition, there are appendageal features including hair follicles and sweat ducts that traverse various skin layers.

Fig. 1.1

A diagrammatic representation of the structure of human skin in cross section. The epidermis is composed of the stratum corneum and the viable epidermis. Diagram is not to scale

1.2.1.1 Stratum Corneum: The Primary Barrier

The stratum corneum is the outermost layer of the skin. It is typically 1020 m thick and composed of 1015 layers of corneocytes (Agache ). Corneocytes are connected by corneodesmosomes and are continuously shed from the skin surface via desquamation.

Since diffusion across the stratum corneum is considered as the major pathway of skin permeation (see Sect. ).

Through freeze-fracture electron microscopy, Breathnach et al. ().

Ceramides, cholesterol and free fatty acids are the main constituents of the extracellular matrix, present in approximately equimolar proportions (McGrath et al. ).

Fig. 1.2

Chemical structures of two stratum corneum ceramides and their possible conformations. Ceramide 2 (CER2) is shown as a typical ceramide consisting of a polar head group ( dotted circle ) and two hydrocarbon chains derived from a fatty acid and a sphingoid moiety, respectively. In CER2, the hydrocarbon chain derived from the fatty acid is 24 carbon atoms long, exemplifying the typical hydrocarbon chain length for this portion of ceramide molecules. Ceramide 1 (CER1) is shown to illustrate its exceptionally long fatty acid ester-derived hydrocarbon chain compared to the typical ceramide molecule (CER2). The inset shows two possible molecular conformations of ceramides, the hairpin or tuning fork conformation, where hydrocarbon chains point in the same direction, and the extended or splayed chain conformation, where the hydrocarbon chains point in opposite directions

The precise molecular arrangement of stratum corneum lipids within the extracellular matrix remains a subject of intense investigation. Under electron microscopy with ruthenium tetroxide fixation, the extracellular lipid matrix displays characteristic, alternating broad-narrow-broad lucent bands (Madison et al. ). This trilamellar motif, with a repeat distance (periodicity) of approximately 13 nm, is known as the long periodicity phase (LPP). This is in contrast with the short periodicity phase (SPP), which has a periodicity of approximately 6 nm but has not been observed in some animal species. For this reason, the LPP is considered to be more important for the skins barrier function and thus has been the focus of most investigations into stratum corneum lipid organisation. X-ray diffraction analysis has revealed that the extracellular lipids are packed into hexagonal or orthorhombic lattices along the plane parallel with the lamellae, orthorhombic packing being denser than hexagonal. Based on these and other supporting observations, a number of molecular models have been proposed to elucidate stratum corneum lipid organisation:

• The model proposed by Swartzendruber et al. () and, in so doing, contribute hydrocarbon chains to form a lipid monolayer between the bilayers that holds the two bilayers together.
• The domain mosaic model (Forslind ) describes an intracellular lipid matrix where polar lipids are segregated in crystalline domains surrounded by liquid crystalline grain borders. It is postulated that the liquid crystalline grain borders provide a diffusion pathway for hydrophobic molecules to penetrate the skin.

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