Viable epidermis

Products that are allowed to remain on the skin are differentiated from those that are meant to be rinsed off. Components of products left on the skin can be expected to penetrate the viable epidermis and to be systematically absorbed. Products that are rinsed off shordy after skin contact, such as shampoos, can, if propedy labeled, contain preservatives that might eUcit adverse reactions if left on the skin. Typical examples of such preservatives are formaldehyde, formaldehyde releasers such as Quatemium 15 or MDM hydantoin, and the blend of methylchloroisothia2olinone and methylisothia olinone. 

Patients with photodamage can apply a lotion containing 25% glycolic acid for 6 months. In such cases an increase in total skin thickness of approximately 25% was reported, accompanied by an increased thickness of viable epidermis and dermis, an increased content of acid mucopolysaccharides, a greater collagen density and an improved quality of the elastic fibers. This could be defined as self-treatment. 

Fig. 2.3.8 Lower GARField profiles of a human skin sample sandwiched between two glass slides, recorded immediately after the sample was floated onto the first slide and again approximately 90 min later. Upper increasing the pulse gap T from 150 to 500 ps increases mobility contrast and allows discrimination between the stratum comeum (right) and viable epidermis (left). Again two profiles are shown, recorded approximately 90 min apart.

Fig. 2.3.9 A time series of profiles showing the is shown by the lowest trace. The inset shows ingress from right (stratum corneum) to left the advance of the glycerine front against the (viable epidermis) of glycerine into human skin square root of time from which Fickian in vitro. The skin before application of glycerine diffusion is inferred.

Preliminary studies, which have repeated many of the in vitro experiments in vivo, reported that the differentiation between stratum corneum and viable epidermis is at least as good, if not better in vivo and that many of the other experiments are similarly reproduced [16]. 

First, consider the transepidermal route. The fractional area of this route is virtually 1.0, meaning the route constitutes the bulk of the area available for transport. Molecules passing through this route encounter the stratum corneum and then the viable tissues located above the capillary bed. As a practical matter, the total stratum corneum is considered a singular diffusional resistance. Because the histologically definable layers of the viable tissues are also physicochemically indistinct, the set of strata represented by viable epidermis and dermis is handled comparably and treated as a second diffusional resistance in series. 

It is possible to identify various factors that confer on chemicals the ability to induce skin sensitization and allergic contact dermatitis. These include the capacity to gain access to the viable epidermis across the stratum corneum, to associate stably with host proteins, to provoke a certain degree of proinflammatory cytokine production by skin cells, and to be recognized by specific T lymphocytes. The effectiveness with which these requirements are met, and possibly other properties of the chemical that influence the vigor of induced immune responses, together with the extent of exposure, will dictate the degree to which sensitization is achieved. 

Because of practical reasons, the human epidermis can be generally divided into two main layers, the stratum corneum and the viable epidermis. 

Generally, the stratum corneum is considered to be the rate limiting layer of the skin with regard to transdermal drug absorption. However, for the invasion of very lipophilic compounds, the bottleneck moves from the stratum corneum down to the viable, very hydrophilic layer of the epidermis, due to substances reduced solubility in this rather aqueous layer [14],... 

This may be achieved by complete immersion of full-thickness skin in trypsin solution or by placing the heat-separated epidermis for 24 h at 37°C on a filter paper soaked with the enzyme preparation [64, 83]. Other techniques, such as vacuum or chemically induced blistering, stretching, application of staphylococcal exfoliatin, or proteolytic digestion of viable cells, are seldom reported [74, 87-89],... 

The human skin model assay involves measuring the effects of corrosives on viable cells in a reconstituted human skin equivalent. To be accepted as a valid human skin model, several criteria must be met. The artificial skin must comprise a functional stratum corneum with an underlying layer of viable cells. Furthermore, the barrier function of the stratum corneum, as well as the viability of the epidermis, must be verified with appropriate experimental setups. The chemicals to be tested are applied up to 4 h as a liquid or a wet powder onto the skin model. Afterwards, careful washing has to be performed, followed by investigation of the cell viability [e.g., with a (MTT)] reduction assay). 

It has been established that in analogy to Ohm s law the overall resistance to diffusion of a multilayer laminate is given simply by the sum of the separate resistances of the layers (Figure 20.2). For example, the total resistance of skin being composed of stratum corneum, viable epidermis, and dermis may be expressed as... 

Wenkers BP, Lippold BC (1999) Skin penetration of nonsteroidal antiinflammatory drugs out of a lipophilic vehicle influence of the viable epidermis. J Pharm Sci 88 1326-1331. 

The skin barrier properties and effect of hand hygiene practices are known to be important in protecting the body. The average adult has a skin area of about 1.75 m2. The superficial part of the skin, the epidermis, has five layers. The stratum corneum, the outermost layer, is composed of flattened dead cells (comeocytes or squames) attached to each other to form a tough, homy layer of keratin mixed with several lipids, which help maintain the hydration, pliability, and barrier effectiveness of the skin. This part of skin has been compared to a wall of bricks (comeocytes) and mortar (lipids) and serves as the primary protective barrier. Approximately 15 layers make up the stratum corneum, which is completely replaced every 2 weeks a new layer is formed almost daily. From healthy skin, approximately 107 particles are disseminated into the air each day, and 10% of these skin squames contain viable bacteria. This is a source of major dirt inside the house and contributes to many interactions. 

In the context of skin sensitization bioavailability can be seen as the capacity of the compound to reach the viable epidermis, where it interacts with keratinocytes and Langerhans cells. This capacity is dependent on its molecular weight and solubility in polar and apolar solvents [115]. Importantly, potency prediction solely on the basis of cell culture models (steps 3 and 4) does not account for skin penetration rate and may thus wrongly predict potency in vivo. Possible in vitro approaches to detect allergic capacity of chemicals/pharmaceuticals are presented in Table 18.5. 

The pharmacokinetics and metabolism of tazarotene (2) is especially worth noting. Topical gel application provides the direct delivery of tazarotene into the skin. Ten hours after a topical application of 0.1% tazarotene gel to the skin, approximately 4-6% of the dose resides in the stratum comeum and 2% of the dose is distributed to the viable epidermis and dermis. As depicted in Scheme 3, both tazarotene (2) and tazarotenic acid (11) undergo further metabolism to their corresponding sulfoxides 12 and 13, respectively. Sulfoxides 12 and 13, in turn, are even further oxidized to sulfones 14 and 15, respectively. These very polar metabolites do not accumulate in adipose tissue, but are rapidly eliminated via both urinary and fecal pathways with a terminal half-life of approximately 18 h. A lesson learned here is that installation of a sulfide moiety promotes clearance because it is oxidized to polar metabolites that are rapidly cleared. As the consequence, the systemic exposure is minimized. Percutaneous absorption of tazarotene (2) led to a plasma concentration below 1 gg/L. The systemic... 

Another striking difference between normal and cultured skin is shown in Fig. 15.6. As discussed above (see Fig. 15.3c, factor 2), cholesterol-rich pockets containing highly ordered lipid chains are occasionally detected in human skin and are characterized by a Raman-active mode of cholesterol near 700 cm-1 and an intense lipid C-C stretch near 1130 cm-1 in Fig. 15.4a and b, respectively. The intensity of the cholesterol mode is normalized to a Phe vibration near 620 cm-1 and imaged in Fig. 15.6b. As is evident there are many such pockets in the cultured skin model, in contrast to human skin where they are only rarely observed (Fig. 15.3c, factor 2), and usually in the viable epidermis rather than in the SC (as in the cultured skin). These measurements illustrate the power of confocal Raman microscopy for combining spatial measurements with molecular structure characterization. 

Due to the relative ease of oxidation of the parent compound, common delivery forms in cosmetic formulations and clinical trials are vitamin E acetate (a-TAc, structure in Fig. 15.7a) and vitamin E phosphate. These forms are expected to permeate and to regenerate free active a-TH through enzyme-catalyzed hydrolysis activities in skin. Although a-TAc is readily hydrolyzed by esterase action to vitamin E upon oral ingestion, no consensus as to the extent of bioconversion of topically applied a-TAc has been reached. Two published studies demonstrate bioconversion up to 10-15% in the viable epidermis [35] including the basal layer [36]. These and other studies show no detectable metabolism of a-TAc in stratum corneum [37]. 

To distinguish the SC from the underlying epidermis and hence to define the site within the skin to which the a-TAc had permeated, a confocal Raman image was acquired from an untreated section taken from the same piece of skin whose spatial distribution of factor scores is depicted in Fig. 15.3c. A comparison of Fig. 15.3c (factors 1 (SC) and 4 (epidermis)) with the map of a-TAc permeation (Fig. 15.8) clearly reveals that a-TAc remains localized mostly in the stratum corneum, with little reaching the viable epidermis. The... 

Fig. 15.11. a Factor loadings constructed from Raman spectra specific to the general area of the wound. The amide I and III contours of factors 1 and 2 are characteristic of high collagen and high keratin levels, respectively, b Factor score images reveal that collagen (factor 1) in the wounded area is pressed up from the dermis, while the keratin (factor 2) in the non-wounded area arises from the SC and from the viable epidermis... 

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