The first section on the guidelines published in October 2012 by The Organization for Economic Co-operation and Development (OECD) for testing of chemicals includes a list of relevant physical-chemical properties.¹ Identification of the composition of interest is the first step in any chemistry-body interaction assessment since it equips the assessor with tools for prediction and drafting a plan for valuation.

Properties that do not change the chemical nature of a matter are termed “physical” while properties that do change the chemical nature of matter are termed “chemical.” The more detailed the information obtained at this early stage the better one can identify a substance and the better the knowledge of its characteristics. These properties can help formulators understand how it may behave under different conditions.

Assessing Safety
The essence in safety evaluation of compounds we use on a daily basis, such as personal care and cosmetics, lies in the interaction and impact of the composition on the body. The assessment is based on the postulation that at a given concentration, route of exposure and intensity, the chemistry (composition applied) introduced to biology (skin) may be recognized by the body as foreign or harmful and therefore may generate an unfavorable reaction. The assessor, therefore, should strive to outline the risks associated with predicted exposure and determine whether the predicted risk is acceptable and the benefit of use justifies the risk.

The skin is the largest and most complicated immune organ in the human body. It fills multiple roles, acquiring signals from the environment that are communicated via sophisticated cascades to the circulation and vice versa. Its structure and composition provides a very sensitive responsive flow of stimuli inside and out. In approximately 30 days, a normal healthy skin is completely renewed. This means that abundance of stems cells supplemented by nutrients, trace elements and specific peptides are synthesized de novo to comply with the constantly generated barrier. Nourished by the blood in the vasculated dermis, accompanied with environmental indicators and factors that are secreted, compartmentalized and affecting its acquired immunity cells, this process holds the premise for the difference between health and disease. 

To add to this complexity, the skin hosts an abundance of biota that is an essential contributor to its maintenance, as well as to the prevention of proliferation and pathogenicity of opportunistic bacteria and fungi. Certain treatments and practices foster biota imbalance leading to impaired biological barrier, potential disease and abnormal sensitivity.

Compound Characterization
The first and most important step in safety assessment of skin care ingredients and compositions is the characterization of the compound/s applied; i.e., their chemistry, physics, stability and overall nature. The collection of such information at the early stage of product development is essential to the project path and its success. The safety assessment of skin care products is determined by their detailed compositions; i.e. the individual raw materials it contains. Raw materials should be characterized for the following:

• Chemical structure molecular weight and its innate stability under a variety of conditions;
• Physical properties such as appearance, color, density, particles size, electrical charge, light absorption and, when relevant, melting point, boiling point, freezing point, flash point and minimum ignition temperature. When determining particle size it should be highlighted that the average particle size is not sufficient for the understanding of particulate properties and a size distribution should be established so one can assess what is the size of the particles composing the majority of the batch; and
• Level and nature of purity. These are of key importance since impurities, although usually at lower concentration than the central ingredient, can impart a toxicological effect that is irrelevant to the core compound and can be minimized or eliminated if purity is higher. A validated analytical method should be developed and utilized to determine purity and impurities levels to ensure batch-to-batch consistency.

Tracing Impurities
According to the EU Commission guidelines published in November, 2013 “when chemically well-defined substances are present, their quantity and molecular formula should be given together with their analytical specifications (degree of purity, identification of major impurities, criteria and test methods used).”² Further in this document it is outlined that “traces can originate from the following sources: impurities in the raw materials/substances; the manufacturing process; potential chemical evolution/interaction and/or migration of substances in the product that could occur under normal storage conditions and/or through contact with the packaging material.”

• With compounds that are extracted with solvents, levels of residual solvents should be determined;
• Heavy metals nature and contents should be specified;
• Solubility in a variety of relevant solvents; and
• The pH in water should be determined and recorded. The skin’s normal pH is around 5.5. If the pH generated by the compound is 2 or lower or 11.5 or higher the compound is considered as a skin corrosive and cannot be used in a formulation or should be neutralized.

The interplay between the varieties of physical-chemical properties may allow, in part, predicting the interaction of the compound with the skin and hence its toxic potential. This assessment, of course, does not release one from the need to conduct empirically a battery of customized safety assessment studies but should be part of the overall assessment and justification of theorized risk.
Composed of keratin-filled corneocytes surrounded by highly organized lamellar structures, a healthy intact upper skin layer (stratum corneum) is lipophilic and relatively dry (composed of 15% water). Therefore it is generally assumed that lipophilic compounds will partition easier into the stratum corneum. This means that a lipophilic compound, depending on other relevant properties such as molecular weight and electric charge, may either remain in the skin’s upper layer, generating a reservoir, or further partition or absorb into deeper layers, the live epidermis, the dermis and the blood circulation.

The lipophilicity of a compound is determined by its octanol/water partition coefficient or Log P. This value reflects the ratio of concentrations of a compound in a mixture of octanol and water that are immiscible at equilibrium. The coefficient reflects the measure of the difference in solubility in the two phases; e.g., the higher the solubility in octanol, the higher the lipophilicity and vice versa.

Penetration Levels
The general view for the threshold of molecular weight associated with penetration to the skin is 500 dalton. This means that compounds that are high in molecular weight such as polymers will remain on top of the skin and not penetrate. This, however, does not mean that they will not impart a biological effect, but that this effect will not be directly associated with the polymer chemistry. For example, hyaluronic acid with molecular weight of 2×106 dalton will not penetrate unbroken skin. It may, however, elevate the stratum corneum water content by capturing water that attempts to leave the body through the skin to the environment thereby reducing trans epidermal water loss (TEWL). Long term, such an effect can change the skin biota population and also allow higher solubility and skin partitioning of water soluble compounds in the base applied.

If the compound applied to the skin generates a reservoir in the stratum corneum and remains there for sufficient time not further partitioning into the epidermis, it is most likely to be removed with cells naturally exfoliated from the skin. If it does penetrate beyond the epidermis into the vasculated dermis, it is reasonable to assume that certain amounts will be absorbed into the blood system to be distributed into internal organs.

Since the skin is negatively charged, cationic molecules, may exhibit better adherence onto the stratum corneum. Here too, depending on other properties, cationic compounds may remain at the upper layer or partition deeper.

The solubility of the compound in the base applied to skin is a crucial aspect to study. Skin absorption is highly dependent on the concentration presented to it, as molecules will penetrate the skin following Fick’s law of simple diffusion. According to Fick’s law, flux of molecules will travel from regions of high concentration to regions of low concentration, with a magnitude that is proportional to the concentration gradient. Since the maximal concentration is directly associated to the solubility, the higher the solubility, the higher the penetration and therefore, the higher the potential exposure and risk. This law is applied only to compounds at their molecular form and not to compounds in their particulate form such as powders. The interaction of particles with the skin will be discussed in future column that will also address safety aspects of nanoparticles.

Phototoxicity Issues
If a compound absorbs at the UV or visible spectra it may exhibit a potential to be photo-toxic (photo-irritating, photo-sensitizing or photo-genotoxic). This means that by absorbing radiation, its structure and therefore interaction with the skin may change and impart a toxic effect. Photo-toxicity will be discussed in details in a future column but this is an example of the implications of a physical property on potential safety. Since photo-toxicity by definition required radiation absorption, a compound with no such property cannot be photo-toxic; therefore such effect is not of concern and should not be part of the testing battery.

When it comes to natural compounds extraction in which total composition characterization can be difficult and sometimes impossible, the approach should be to focus on the following:

• The main compound with the highest percentage in the composition;
• Identification of key chemistries that are thought or known to impart the biological activity, their nature and percentage;
• Residual solvents;
• Heavy metals;
• Microbiological contamination; and
• Batch to batch consistency of the above.

Finally, outlining and documenting the physical-chemical properties of the compound or composition of interest should serve for the utilization of in silico approaches to elucidate safety. Such is for example, structure-activity relationships (SAR). In silico methods are virtual screening and evaluations composed by computerized methods that use information embedded into software to compare new or unknown chemistries to an existing pool and drawing correlation and relevancy. Such tools are used for predicting toxic effects using a special algorithm.

It should be acknowledged that such systems may sometimes provide false information since the algorithm used may not be relevant or when biological or physical effects, such as metabolism or degradation respectively, are not taken into consideration.

In summary, whether it is a raw material, blend, natural extract, composition or formulations, the understanding and collection of physical chemical properties is the first and most essential part in product development in general and safety assessment in particular. This information should be utilized in the overall evaluation and be part of the product information package (PIF).
It should pave the path and drive sequence study decisions, starting from highest concern based on these properties, level of exposure and population of choice. 

References:

1. OECD http://www.oecd.org/chemicalsafety/testing/TG%20List%20EN%20Aug%202012.pdf
2. Annex  I EU Commission http://eur-lex.europa.eu/legal-content/EN/TXT/PDF/?uri=CELEX:32013D0674&qid=1395764232390&from=FR:PDF

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Nava Dayan
President
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
E-mail: nava.dayan@verizon.net