Often, though, the correlation between the in vitro data and the in vivo manifestation is lacking. One key reason is an inappropriate study design and interpretation. Another may be a knowledge gap in the understanding of study limitations. For example, the fact that a gene is up-regulated or down-regulated may not be at all correlated to biological impact since not all mRNAs will be converted to proteins which are the actual acting entities. Another important reason is the availability of the compound to the site of action. For example, a large polymer such as hyaluronic acid that will not penetrate through healthy intact skin cannot impart pharmacological changes directly at the live epidermis or dermis layers since it will not reach these targets.
For marketing reasons, companies invest large funds in proving efficacy; however, in cosmetics, the most important aspect in exposure to humans is safety.
Cosmetics are not intended to treat disease, but are a luxury. While it is unethical to sell a product that does not deliver its premise in terms of elimination of wrinkles, it is much more problematic to expose the consumer to an unsafe chemistry, especially if used over periods of months and years.
A Variety of Environments
In the scientific community, the absorption of a compound into and through the skin is termed “partitioning.” This is since the skin is a complicated non-homogenous barrier. It is an extremely complex organ and when a chemical is penetrating through its layers it is exposed to different environments. For example, while the stratum corneum is relatively dry and holds between 10 to 15% water; the living epidermis is the sub-tissue that holds water in the skin (40-50%) and the vacuolated dermis can contain even more water. This creates a gradient of water level exposure for the chemical.
The profile of partitioning of compound into and through the skin is a key in its safety assessment and is highly recommended by all regulatory authorities. Many suppliers do not conduct those studies, due to the “double-edged sword” they present; i.e., if studies conducted on fibroblast in vitro demonstrates significant results, but in skin absorption studies the compound does not penetrate to the dermis (where fibroblasts exist), there is no validity to the study. Claiming that the compound is not partitioning deeply into the skin and therefore will not be found in the blood circulation may be a positive safety measure, but it will compromise the efficacy data and cause the product to lose its marketing edge.
More Study Is Necessary
While the industry chooses to behave as ostriches hiding their heads in the sand when faced with a problem, we all, sooner or later, may pay the price. If a compound penetrates through the skin in significant quantities and does impart a biological activity, it is carried via the blood circulation to internal organs. This effect may not be immediate and dramatic. Cosmetics and personal care products are being used for periods of months and years on a daily basis and often on large areas of the body.
How do we settle this issue? Reverting the industry from its aggressive marketing mode back to simply superficial makeup is not realistic. The solution is investing more, much more, in studying and understanding the safety profile of the compounds we use. It can be as simple as the following:
- Choose to work with compounds that their pharmacodynamics and pharmacokinetics to internal organs is known to be safe and well established; such as those that are present in healthy foods and edible plants. Make sure to add all aspects related to skin exposure that are different from oral toxicology endpoints.
- If a new chemical entity that is synthetic is of interest; know that investment in safety must be substantial, detailed and valid; especially if the compound is penetrating beyond the stratum corneum.
- Low molecular weight;
- Lipophilicity; and
- Skin barrier impairment properties.
Since in vitro studies have been demonstrated to provide reliable, reproducible data, for topically applied formulations when the skin is the target of action it is imperative that an understanding of penetration patterns be developed in order to allow for better product development as well as conservation of both resources and time in the process of product design.OECD TG 428 guidelines advise for the use of Franz diffusion cells for such experiment (See photo above). The principals of this method are as follow:
- A piece of excised skin (mostly human cadaver skin) is mounted between two compartments with the stratum corneum facing the environment;
- Formulation with the compound of interest is applied to the stratum corneum onto the “donor” compartment;
- In most cases the dermis is removed and only the epidermis is used since it is the rate limiting barrier to skin absorption;
- The “receiver” compartment is the compartment that mimics the blood circulation; if the compound is detected in this compartment; it means that it crossed the skin barrier.
- Amount of compound of interest present at the majority if the upper layer of the skin that is detected by analysis of sequential tape stripping of the stratum corneum;
- Analysis of compounds in the remaining epidermis;
- Analysis of compounds in the receiver compartment that is representing the blood circulation;
- Together with the amount left on the skin at the end of the experiment the total amount collected should sum close with certain accuracy level to the amount originally applied to the skin.
According to the updated Personal Care Product Council guidelines from 2014 ”test substance found in the receptor fluid is considered to be systemically available. Test substance remaining in viable layers of the skin at the end of the study is also generally considered to be systemically available. However, further experimentation may be undertaken to determine whether the material remaining in skin will ultimately diffuse through the skin, remain in the skin, or be removed through desquamation.
Guidelines differ as to the question of whether test substance found in the stratum corneum is considered to be bioavailable and thus included in the systemic exposure dose. Material in the stratum corneum is included according to OECD 428, but is excluded according to the SCCS and industry guidelines.”
A Battery of Measures
In summary, skin absorption, if assessed as required based on the physical-chemical properties of the compound of interest, should be conducted as part of safety tier to establish an understanding on the potential of a compound applied to skin to penetrate beyond the barrier into the systemic circulation and internal organs.
If the compound is absorbed into the body, the need for extensive expanded safety profile is highlighted. Therefore additional safety key endpoints such as genotoxicity and carcinogenicity, endocrine disruption, neurotoxicity and alike’ should be considered in the battery of safety assessments.
OECD guidelines TG 428
Personal Care Product Council Safety Evaluation Guidelines 2014
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.