j. Soc. Cosmet. Chem., 40, 321-333 (November/December 1989) Comedogenicity and irritancy of commonly used ingredients in skin care products JAMES E. FULTON, JR., Acne Research Institute, 1236 Somerset, Newport Beach, CA 92660. Received September 3, 1989. Presented at the Southern California Section, California Chapter, Society of Cosmetic Chemists, Spring 1989. Synopsis A survey, using the rabbit ear, of the comedogenicity and irritancy of several groups of skin care products indicates that many contain follicular and surface epithelial irritating ingredients. These ingredients fall into several chemical classes. Certain generalizations can be deduced by examining the results: (1) me- dium-chain-length fatty acids are more potent than short- or long-chain fatty acids in producing follicular keratosis, (2) the comedogenicity and irritancy of an organic material can be reduced by combining the molecule with a polar sugar or a heavy metal, (3) increasing the degree of ethoxylation in a molecule tends to reduce the comedogenicity and irritancy of the chemical, and (4) the longer chain lipids, i.e., waxes, appear too large to produce a reaction. By following the guidelines developed in this study, it is possible to formulate nonirritating, noncomedogenic moisturizers, sunscreens, hair pomades, cosmetics, and condi- tioners. INTRODUCTION The possibility of comedogenicity and irritancy of facial skin care products has been well documented (1- 3). Because of this work and an increasing public awareness, facial products that are less comedogenic are now becoming available (4). However, other skin care products such as hair conditioners, hair pomades, moisturizers, sunscreens, and even acne treatment products may be a source of cosmetic acne. By taking these products apart, testing their ingredients, and putting them back together and retesting them, an extensive ingredient listing has been created. By studying this list, the cos- metic chemist can begin to be selective in developing formulas for less irritating and less comedogenic products. The rabbit ear assay has been used since the mid-1950s as a method of measuring follicular keratinization by externally applied compounds (5). The advantage of this rapid screening tool is that it takes only two weeks to develop follicular impactions in the rabbit ear, while it may take six months to develop similar reactions on human skin. The disadvantage of the model is its extreme sensitivity. The fragile, protected epithe- lium of the inner ear is extremely sensitive. Not everything that irritates this model will also irritate human skin. However, this extensive screening of cosmetic formula- 321
322 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS tions and their ingredients would not have been possible without the use of this animal model. We have now extended the model to include an index of surface skin irritancy as well as of follicular hyperkeratosis. METHODS Ingredients are mixed in propylene glycol at a 9 to 1 dilution for testing unless other- wise indicated (10% concentration). A colony of New Zealand albino rabbits that has genetically good ears and is free from mites is used. Three rabbits, weighing two to three kilograms, are used for each assay. Animals are housed singly in suspended cages and fed Purina Rabbit Chow and water ad libitum. Animals are maintained on a 12-hour light and 12-hour dark cycle. A dose of 1 ml of the test material is applied and spread once daily to the entire inner surface of one ear five days per week for two weeks. The opposite untreated ear of each animal serves as an untreated control. Follicular keratosis is judged both macroscopically (visually) and microscopically with a micrometer to measure the width of the follicular keratosis. The macroscopic response is determined by averaging the measurements of the width of six follicles using a Mitutoyo Dial Micrometer (#536-724). A similar microscopic micrometer measurement is obtained by averaging the width of six follicles under a magnification of 430 x after a 6-ram biopsy specimen is fixed in formalin, sectioned at six microns, and stained with hema- toxylin-eosin. The results are then combined on a scale of one to five: Micrometer reading Grade 0.009 in or less 0 No significant increase in follicular keratosis 0.010 in-.014 in 1 0.015 in-.019 in 2 A moderate increase in follicular keratosis 0.020 in-.025 in 3 0.025 in-.029 in 4 An extensive increase in follicular keratosis 0.030 in or more 5 Grade 5 is the presence of large comedones throughout the ear, similar to those induced by the application of our standard "positive" testing agent, isopropyl myristate. As reported in our previous studies, a minimal grade of 0 to 1 is not considered significant. Grade 2 to 3 is borderline. However, a grade of 4 to 5 is uniformally reproduceable and considered positive. The irritancy produced by the repeated application of a chemical or skin care product on the surface epidermis in the rabbit ear is also evaluated on a similar scale of 0 to 5. The grades are summarized as follows: 0 No irritation 1 Few scales, no erythema 2 Diffuse scaling, no erythema 3 Generalized scaling with erythema 4 Scaling, erythema, and edema 5 Epidermal necrosis and slough To study the effects of different vehicles on comedogenicity and irritancy, several fatty acids and the D&C red pigment #36 are reexamined in different solvents. The fatty
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