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The metabolism of skin and how to test it.

Metabolism refers to the entire cascade of enzymatic reactions that a compound undergoes when it interacts with body biology. Typically, metabolism refers to the activity of liver enzymes aimed to convert compounds to such that the body is able to excrete. However, since our body--each and every organ, skin included--is constantly renewing itself, it contains a variety of metabolic enzymes. These enzymes are meant to act on substrates that are present in the tissue to maintain health and homeostasis.

When a foreign substrate is introduced to the enzymes, if its structure resembles the natural substrate, they are likely to convert it. In pharmaceutical product development, metabolism is a key part of the pharmacokinetic profile abbreviated as ADME (Absorption, Distribution, Metabolism and Excretion). ADME defines relevant aspects of the fate of a compound in an organism. Establishing an understanding of the metabolism of a substance in the body is critical to the understanding of its potential toxicity. Metabolism of drugs and actives usually involves two phases; the first phase includes oxidation, reduction and hydrolysis, introducing or exposing nucleophilic groups on the compound. The second phase is aimed at generating more hydrophilic chemistries that will allow easier excretion. This phase involves conjugation with endogenous groups such as glucuronic acid or glutathione. The scheme above describes the potential impact of metabolism on activity.


Metabolism's Impact on Activity

As the focus of this column is skin care products, it will emphasize skin metabolism only and not on systemic metabolism. As such, it will not describe liver metabolism that may occur when a compound is absorbed systemically into blood, although this is definitely possible with skin applications if the compound of interest is absorbed through the skin into the circulatory system. When a significant amount of a compound applied to skin is absorbed into the blood, it must find efficient ways to leave the body, usually through the digestive system or the kidney; liver metabolism assists in this process. If such excretion is not possible or delayed, the compound may accumulate in the body and cause illness. This accumulation is termed "bioaccumulation."

If the compound applied to the skin concentrates in the skin tissues and does not penetrate significantly to the circulation, even if it is not metabolized and does not accumulate in skin, it may eventually, within 30 days of total skin renewal in healthy individuals, leave the skin via exfoliation. However, during this period of time, it may be toxic to skin.

Metabolic Activity

Although the skin is not considered an organ that specializes in metabolism, it has an ability to activate or detoxify compounds applied to it. The skin contains metabolic paths similar to the Cytochrome P450 (CYP450) en zymes contained in the liver. It expresses functional phase 1 oxidative, reductive and hydrolytic reactions as well as phase 2 conjugation capabilities. It is known, for example, that the skin can metabolize steroids, that this metabolism is concentrated in the follicle area and that kertainocytes can metabolize steroids, too. It is generally thought that the skin exhibits metabolic activity that is 10% or less of the activity of the liver; but such metabolism should be studied on a case-by-case basis and may differ greatly from one compound to another. In addition, since many of the cosmetic formulations are aimed at applying to large skin areas, if metabolism occurs, it can be significant to the overall biotransformation of the compound of interest.


Skin contains a variety of enzyme families aimed at maintaining its health. For example, it has lipases for lipid conversions to build and orchestrate stratum corneum intercellular lipids and proteases to metabolize proteins to build and break the dermal fiber network. The skin also contains enzymes secreted from its commensal biota. These enzymes can identify and metabolize compounds applied to skin. For example, Corynebacterium, which resides in moist skin areas, releases enzymes that convert sweat secretions into malodor compounds, while scalp-residing Malassezia species convert sebum components into fatty acids.

Key considerations in the decision to conduct skin metabolism studies include:

The compound applied to skin is identical, similar to or resembles skin natural substrate;

* The compound of interest demonstrated clear specific activity in a cell culture model that has not been shown, or is very different when applied to a whole tissue or in vivo;

* The compound or its chemical family is known to undergo metabolism; and

* General QSAR analysis point toward potential metabolites, especially if the metabolites are identified as toxic.

To date there is no validated method to follow for assessment of skin metabolism but a few options exist. Skin metabolism can be conducted in conjunction with skin absorption study (see previous column describing skin absorption methods). If the skin used in this assay is fresh human skin, enzymatic conversion may occur and metabolites can be analyzed by sensitive analytics. These can be quantified and if significant amounts are present, they should be identified for their structure and potential activity. For this, the skin mounted on the diffusion cells is to be homogenated at the end of the experiment and its extract analyzed while compared to the appropriate standards. Generally, metabolism will not occur in cadaver skin or frozen skin tissues so the skin tested must be fresh and from a live donor.

Animal testing models for metabolism are known to be limited due to significant differences in the types and activities of the enzymes involved in metabolism and this is an area where human based in vitro models are critical even in industries that permit animal testing such as the pharmaceutical industry.

The European Centre for the Validation of Alternative Methods (ECVAM) proposed few options to establish metabolism related data. These methods all refer to liver related metabolism:

* Microsomes-cellular organelles containing metabolic enzymes;

* Liver suspended cells; and

* Three-dimensional human hepatocytes model.


A study describing testosterone metabolism in vitro using fresh and frozen pigskin as well as human skin microsomes was published in 2014. In this paper, scientists concluded that the absorption of the steroid through frozen skin was lower when compared to fresh skin. Using fresh skin more than 95% of the applied dose has been recovered in the study and corresponded to the compound metabolites. The metabolites were indicative of oxidative enzymes. Microsomes are vesicles that are formed in the lab from broken endoplasmic reticulum of debris of mechanically broken cells. This process separates cytochrome P450 enzymes and fragmented endoplasmic reticulum. Microsomes are considered a valuable tool for investigating compound metabolism in vitro.

Insufficient Models

It is clear that existing experimental models and methodologies for the assessment of skin metabolism are insufficiently validated. An attempt to assess the utilization of fresh full-thickness human skin explants as a model was conducted and published in The American Society for Pharmacology and Experimental Therapeutics (ASPET) Journal in 2015. Here, too, scientists determined that the whole-skin metabolic activity was significantly lower when compared to wholeliver, but certainly relevant and important.

They demonstrated several metabolic reactions in skin including glucuronidation, sulfation, N-acetylation, catechol methylation, and glutathione conjugation. When frozen, the skin lost its glucuronidation activity.

Compounds that demonstrated metabolism in this study were triclosan, indomethacin, diclofenac sodium, minoxidil and estradiol. A limiting factor in using skin explants from human donors to study skin metabolism is the great variability between them, which can be affected by age, lifestyle, ethnicity and other factors. In this specific study variability was from 1.4- to 13.0-fold depending on the test substrate.

A joint 2010 publication by MatTek and Procter & Gamble describes gene expression analysis of metabolisms in the 3D Epiderm in vitro model. The team conducted a comparison of the expression of 139 genes encoding for xenobiotic metabolizing enzymes in this model that is composed of human-derived skin cells.

In this analysis, 87% of the genes were consistent when compared to human skin. This observation points toward some similarities in metabolic function. The study was conducted comparing Epiderm models derived from four donors and demonstrated consistency in the model with donors' variability. The enzymes expressed in this model were mainly phase 2 metabolic enzymes that are characteristic to skin metabolism.

In summary, while the scientific community agrees that skin metabolism is of importance and can, in some cases, significantly affect product performance, there is no clear assessment path. Although there are testing options, none has been validated or extensively studied. In addition, such studies require intense, potentially costly, sensitive analytical capabilities. A key question to be answered in the chain of product development is the amount of compound penetrated into and through the skin as reflected in skin absorption studies. If a significant amount penetrates the circulation, liver metabolism may be a relevant endpoint to be tested. If the compound resides in skin living tissue, skin metabolism should be considered. One of the reasons for finding no correlation between in vitro and in vivo assays may be skin metabolism since if the compound is metabolized in the skin it may exhibit an activity that is very different from that observed in vitro.

As always, the need to conduct skin metabolism should be determined on a case by case basis in the overall assessment of risk.


* Jacques C. et al. Effect of skin metabolism on dermal delivery of testosterone: qualitative assessment using a new short-term skin model. Skin Pharmacol Physiol (2014) 188-200.

* Manevski N. et al. Phase 2 metabolism in human skin: skin explants show full coverage of glucuronidation, sulfation, n-acetylation, catechol methylation and glutathione conjugation. ASPET. 43 (2015) 26-139.

* Hu T. et al. Xenobiotic metabolism gene expression in the Epiderm in vitro human epidermis model compared to human skin. Toxicology In Vitro 24 (2010) 1450-1463.

Nava Dayan


Dr. Nava Dayan LLC

Nava Dayan Ph.D, is the owner of Dr. Nava Dayan L.L.C, a skin science and research consultancy and serving the pharmaceutical, cosmetic, and personal care industries. She has 25 years of experience in the skin care segment, and more than 150 publication credits.

Tel: 201-206-7341

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Title Annotation:Safety Assessment in Skin Care
Author:Dayan, Nava
Publication:Household & Personal Products Industry
Date:Sep 1, 2016
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