Dermal absorption of substances - KKT Expert Talk

The skin is the largest organ of our body and it has multiple functions. Among the most important functions of the skin is protection against dehydration and against the penetration of unwanted substances from the environment, which includes protection against cold, heat, UV radiation, dirt and pathogens. Although the skin can very effectively prevent the penetration of substances, in various situations it is desirable to transport substances into the skin. Examples of this are medicinal substances or cosmetic agents. In these cases, the goal is to overcome the skin barrier and channel the active substances specifically to where they are actually needed. In pharmacy, this is called “drug targeting”. In order for the active ingredient (drug substance) to actually reach the target, the destination and the route to it must be clearly defined–just like a journey by car. What is relatively easy to do when travelling by car, however, is not easy when it comes to the skin. The definition of the target, i.e. the consideration of whether the active substance should be transported into the viable dermis and from there into the systemic circulation, or only into the upper skin layers, is still the easiest task. It is more difficult to predict how the active substance will reach the desired target. The reason for this is that the currently known mechanisms of how active ingredients enter the skin seem to be too simplistic to precisely define the active ingredient penetration of an active ingredient from a formulation into the skin. 

The basic assumption today is that active ingredient molecules must first diffuse from the base to the surface of the skin and then penetrate from there into the skin. The basis for predicting the amount of active ingredient that penetrates the skin is Fick’s diffusion law, which states that a large amount of dissolved active ingredient molecules in the base causes good penetration. Other factors that lead to the expectation of good active ingredient penetration are the application of the formulation on the largest possible skin surfaces and on areas of thinner skin. The affinity of the active ingredient to the skin, i.e. its solubility in the skin, should be as high as possible than its affinity to the formulation and the active ingredient molecules should be able to diffuse easily from the formulation to the skin. Even with all of the above parameters in mind, there are always products in the formulation of active ingredients that do not allow sufficient active ingredient penetration. Possible causes for this have been systematically investigated at Philipps University in Marburg since 2020. 

The basis of the Marburg research is an ex-vivo pig ear model, which is used as a surrogate for human skin due to its similarity to human skin. The test formulations are applied to the pig skin under real application conditions (Figure 1). The dermal penetration is then determined from skin biopsies taken after a defined incubation period. For this purpose, the skin biopsies are cut vertically and the obtained sections are then photographed using inverted epifluorescence microscopy. The amount and depth of drug penetrated is then determined using digital image analysis (Figure 1). 

The method is highly sensitive and makes it possible to simultaneously test many formulations comparatively against each other in a time- and cost-saving manner and thus to determine which formulations are well suited and which are less suited for effective drug penetration. At the same time, changes in bio-physical skin properties (e.g. changes in transepidermal water loss, skin hydration, skin elasticity and stratum corneum thickness) can be measured, making it possible to understand the impact of the formulation or treatment on the skin. Thus, with the ex-vivo model, it is possible to simultaneously study drug penetration into the skin and the impact of the formulation on the skin1-6. This combined investigation of drug penetration and the change in bio-physical skin properties was previously only possible with in vivo studies, i.e. on living organisms. Since in vivo studies are extremely time-consuming and expensive, holistic studies of this kind have been very rare and systematic studies investigating the influence of the vehicle on drug penetration and skin properties have not been available. With the introduction of the Marburg ex-vivo model in 2020, this type of investigation has become possible. With the help of the model, numerous systematic studies have been conducted since then, which clearly prove that the penetration mechanisms into the skin are much more complex than previously assumed.

How does an active substance get from the formulation into the skin?

The Marburg ex-vivo model allows the influence of a formulation on drug penetration and skin properties to be investigated simultaneously. The investigations are carried out under realistic conditions, i.e. climate (e.g. temperature, humidity, air pressure, UV index) as well as the application conditions (with or without massage, application to untreated or treated skin) can be exactly modelled on reality according to the research question. The results obtained in this way therefore accurately reflect what happens to the skin and the active ingredient after application of a dermal formulation. The main findings from these studies in the period 2020-2022 can be summarised as follows: 

1. Liquids, such as water or thin-bodied oils, can penetrate the skin. Substances dissolved in these liquids are “dragged” into the skin with the solvents. The mechanism is therefore called the “solvent drag” mechanism7. Accordingly, with the help of this mechanism, lipophilic substances can be effectively drawn into the skin through the use of thin oils and hydrophilic substances with the help of water2.

2. Substances can penetrate intercellularly and/or transcellularly through the uppermost skin layer, the stratum corneum. If the pathway taken by a particular drug molecule is known, this pathway can be “enlarged” or reduced so that drug penetration is improved or worsened accordingly8. The manipulation of the “width of the penetration path” – similar to a road for cars – can be influenced relatively easily by the addition of auxiliary substances. Glycerol, for example, increases transcellular penetration, while urea reduces it. The reason for this is seen in the different localisation of the different excipients. Glycerol is more likely to be deposited in the lipid layer of the stratum corneum, where it “widens” the pathway for transcellular penetrating agents by attracting water. Urea is more likely to be deposited in the corneocytes, where it causes the corneocytes to swell by attracting water, thus reducing the path width for transcellular penetrating agents (Figure 2). 

3. If several active ingredients are incorporated into a formulation which penetrate the skin in the same way, competition occurs during penetration (competitive penetration), as only a limited total number of active ingredient molecules can penetrate the skin per time. This means that the GAW is a fixed constant and represents the sum of all molecules that can penetrate the skin per time. If a formulation contains only one active ingredient, the amount of active ingredient penetrated = GAW. If several substances are incorporated, all molecules penetrate, i.e. the GAW is the sum of all individual molecules. Since the GAW is constant, the quantity of penetrated individual active substances is correspondingly smaller. Effective active ingredient penetration is therefore achieved when the ingredients of a formulation do not interfere with each other’s penetration. This is usually best achieved when a formulation contains as few ingredients as possible. 

4. Emulsifiers can improve the penetration of active ingredients, but they can also worsen it. An increase occurs when the active ingredients can be trapped in the micelles. Since lipophilic active ingredients can be entrapped in O/W micelles, the penetration of lipophilic substances can be improved accordingly by O/W emulsifiers, whereas the penetration of hydrophilic substances can rather be improved by the addition of emulsifiers that form W/O micelles. The addition of O/W emulsifiers to formulations containing hydrophilic substances reduces their penetration and the addition of W/O micelles correspondingly reduces the uptake of lipophilic active ingredients. It can be assumed that micelles penetrate the skin and transport the trapped active ingredient into the skin. Empty micelles, on the other hand, displace the active ingredient and therefore reduce its penetration as they occupy part of the GAW (see 4.). Current studies are still investigating this mechanism of micellar assisted penetration in detail. 

5. The addition of solid particles to a dermal formulation increases the dermal penetration of dissolved active ingredients in the formulation (particle assisted penetration). How much the active ingredient penetration is increased depends on the size and quantity of the added particles. An optimal increase in penetration occurs with small particles and particle concentrations > 20% calculated on the total formulation. By varying the size and quantity of particles, the active ingredient penetration and the penetration depth can be precisely controlled9. The addition of particles to dermal formulations is thus a very simple and cost-effective formulation tool for the production of dermal formulations with a defined penetration profile. The reason for the increase in penetration is the formation of a liquid film (meniscus) between the particles and the skin when the liquid of the formulation dries out. The liquid film contains an increased concentration of dissolved active ingredient molecules. Due to the resulting increased concentration gradient and prolonged residence time on the skin, penetration is locally increased (Figure 3).