5 Biological factors influencing skin absorption

Dermal absorption of pesticides - evaluation of variability and prevention

Many factors are known to influence dermal absorption. First of all the site of application/exposure is very important when it comes to skin penetration, just as the age of the person exposed has an effect on the amount of substance penetrating the skin.

As quite a few studies have been made using skin from different animals, the knowledge that there is a significant difference in absorption when it comes to animals and humans has led to the necessity of a thorough interpretation, if adapting data from animal studies to be used in relation to humans.

Also the state of the skin is important when using it for experimental research. It is fundamental to evaluate the barrier function as the integrity of this parameter, e.g. the hydration of the skin, is very essential to the experiment. Since it is known that the skin has its own metabolism even though it is very low compared to metabolism in the liver, this must also be considered when making skin penetration studies where the absorption of a specific substance is being explored.

5.1 Anatomical site

The site of exposure has proved to be of significance in the penetration of many substances and there is not a complete pattern of regional absorption variation that accounts for all substances. Yet there is a general pattern shown by Feldman and Maibach (1967) in a penetration study of hydrocortisone. Here the skin on the scrotum had the highest permeability and the increasing rate over the areas was as follows: plantar < palmar < back < scalp < axilla < forehead < scrotum. The penetration rate from the foot to the scrotum varied 42-fold (Feldmann & Maibach, 1967). Another study demonstrated a lower permeability across abdominal skin than leg skin. The different order of absorption in this study demonstrates that the variation between areas is unaffected by the thickness of the skin in the particular site (Elias, 1981). The explanation is not quite clear. Other influencing factors are the number of follicles, the thickness of the stratum corneum, the sebum composition as well as the distance of capillaries to the surface of the skin (Rougier et al., 1999).

5.2 Age

Age has an influence on the skin and also on the penetration through the skin. The skin structure changes with increasing age. The stratum corneum becomes drier as the activity of the sebaceous glands decreases and the surface lipids diminish. Some have pointed towards a marked age-related decrease in skin lipids, at least up to age 50 years (Rogers et al., 1996), although others indicated sparse or no relationship (Cua et al., 1995;Schreiner et al., 2000). The amount of collagen decreases and becomes less soluble in chronologically/intrinsically aged skin, but becomes thickened and more soluble in photoaged areas. Intrinsic aging also slowly degrades elastin which accumulates as damaged elastin. Increased synthesis of abnormally structured elastin occurs in photoexposed areas. In general, age leads to increased folding and decreased interaction of proteins with water. Thus, although aged skin holds an increased amount of water, the majority of this is tied to itself in tetrahedral form, rather than being bound to proteins (Waller & Maibach, 2006). Also the blood supply is reduced as the capillary network degenerates. This has shown to be most effective on hydrophilic substances whereas very lipid-soluble substances are able to dissolve into the stratum corneum even when the accessible surface lipids are reduced (Roskos et al., 1989).

5.3 Barrier function

The barrier functions depend mainly on the integrity of the stratum corneum. Changing or damaging the skin structure increases the permeability. The permeability can be affected chemically (detergents, solvents), physically (weather, occlusion, sunlight) or pathologically (mechanical damage, disease). A number of detergents, alcohols and solvents have been shown to alter the barrier integrity by changing the properties of the barrier (Nielsen & Nielsen, 2000;Dias et al., 2008;Rosado & Rodrigues, 2003;Kezic et al., 2001). Several studies have been made where the skin was damaged in different grades. Thus, Bronaugh and Stewart (1985) used abraded, UV-radiated and tape-stripped skin to demonstrate an increasing absorption from < 2 to > 100-fold, depending on the degree of damage being done to the skin (Bronaugh & Stewart, 1985b). Tape-stripping is a mechanical method that is used to remove the stratum corneum. After tape-stripping the permeability coefficient of morphine is seen to increase several hundred fold compared to intact mouse skin. Absorption of fentanyl and sufentanil is increased more than 40 times (Roy et al., 1994). In an in vivo study using volunteers and microdialysis, the absorption of salicylic acid was highly enhanced (150 times) in a tape-stripped skin (Benfeldt et al., 1999).

Also diseased skin can cause an inherent skin barrier defect and studies have shown that patients suffering from skin diseases like atopic dermatitis or lamellar ichthyosis have reduced or altered lipid contents in their stratum corneum (Imokawa et al., 1991;Yamamoto et al., 1991). The changed lipid composition causes abnormal lipid organization in the stratum corneum (Pilgram et al., 2001). Lately Jakasa et al. have shown an altered penetration profile of SLS and polyethylene glycol into the stratum corneum (SC) of patients with atopic dermatitis (AD) compared to control subjects. This indicates that even non-involved skin in patients with AD has altered barrier characteristics, emphasizing the importance of skin protection and prevention of skin contact with chemicals (Jakasa et al., 2006a;Jakasa et al., 2007). Another recent study reports that about 10% of people of European ethnicity are carriers of loss-of-function mutations in the filaggrin gene (Palmer et al., 2006). Filaggrin is a key protein of the SC that assists terminal differentiation of the epidermis and creation of the skin barrier. Different types of mutations in the filaggrin gene lead to damaged barrier formation, which manifests as altering degrees of dry skin (Kezic et al., 2008), ichthyosis (Chen et al., 2008), and/or dermatitis (Nomura et al., 2007; de Jongh et al., 2008). Additionally, as a precursor of amino acids and derivatives that act as a “natural moisturizing substance” filaggrin is largely responsible for the ability of SC of the skin to stay hydrated at low environmental humidity (Rawlings & Harding, 2004; Kezic et al., 2008). The above results do not give quite enough quantitative information and more specific research is needed. This will be described later.

5.4 Species

There are significant differences in the dermal absorption in animals and in humans. Differences in the lipid content, structure and thickness of the stratum corneum are significant factors (Walters & Roberts, 1993). Further, laboratory animal skin has more appendages than human skin which can be the reason for increased transdermal absorption. A range of experimental studies in vitro as well as in vivo have been published. Most of them have an acceptable internal validity, but clearly need an interpretation before being used for human risk assessment. A recent study has also questioned the reliability of converting percutaneous absorption data from rats to humans due to the mentioned differences in species as they studied the absorption of hazardous substances (Korinth et al., 2007a).

5.5 Metabolism

The primary metabolic organ of the human body is the liver. The skin, however, also maintains a certain metabolic capacity. It contains enzymes which can be very active in degradation of penetrating substances (Denyer S.P et al., 1985). Enzymes can catalyse both endogenous agents such as steroids and hormones, and xenobiotics such as pharmaceuticals and environmental chemicals. Most substances that are absorbed across the skin barrier have a reasonable lipophilicity. The role of the enzymes is to detoxify and to increase polarity and thereby produce more water-soluble products that are more easily eliminated from the body. But the enzymes can also activate the molecules to more toxic metabolites as was shown by Liu et al. where carbosulfan and furathiocarb were metabolized to the more toxic carbofuran (Liu et al., 2002;Liu & Kim, 2003). The balance between cutaneous activation and detoxification is a critical determinant of systemic exposure in humans (Hotchkiss SAM, 1998).

The activity of skin metabolism is very low compared to the hepatic activity. Skin metabolism may, however, be important if large surface areas are exposed. The degree of metabolism largely depends on the enzymes involved. Esterase is very active in the skin whereas cytochrome P450 enzymes are not. Therefore metabolism of chemicals, which are primarily metabolized by P450 enzymes will hardly be affected by skin metabolism (Sartorelli et al., 1997).

5.6 Hydration

To give the skin a good barrier function the hydration of the skin needs to be balanced, and a certain quantity of water is needed. If the hydration increases the permeability may be enhanced manyfold. Increased skin hydration is often seen in occlusive environments, such as in the use of protective gloves or working in a humid environment like dishwashers, hairdressers, cleaners etc. These occupations are associated with high prevalence of contact dermatitis which has been associated with enhanced penetration of skin irritants through hydrated skin. Since occlusion has proved to be the single factor which increases skin penetration the most it is of great significance to avoid chemicals inside a glove or other equipment (Wester & Maibach, 1983). In a study of medical drugs the occlusion of the application area resulted in hydration of the tissue. Consequently, the skin got swollen and wrinkled. The temperature increased at the same time and thereby increased the permeability with up to 300-fold (Varvel et al., 1989). See section 10.4.3.