j. Soc.Cosmet. Chem., 40, 321-333 (November/December1989) Comedogenicityand irritancy of commonlyused ingredients in skin care products JAMESE. FULTON, JR., AcneResearch Institute, 1236 Somerset, NewportBeach, CA 92660. ReceivedSeptember 3, 1989. Presentedat the Southern California Section, CaliforniaChapter, Society of CosmeticChemists, Spring 1989. Synopsis A survey,using the rabbitear, of the comedogenicityand irritancyof severalgroups of skin careproducts indicatesthat manycontain follicular and surfaceepithelial irritating ingredients. These ingredients fall into severalchemical classes. Certain generalizations can be deducedby examiningthe results:(1) me- dium-chain-lengthfatty acidsare more potent than short- or long-chainfatty acidsin producingfollicular keratosis,(2) the comedogenicityand irritancyof an organicmaterial can be reducedby combiningthe moleculewith a polarsugar or a heavymetal, (3) increasingthe degreeof ethoxylationin a moleculetends to reducethe comedogenicityand irritancyof the chemical,and (4) the longerchain lipids, i.e., waxes, appeartoo large to producea reaction.By followingthe guidelinesdeveloped in thisstudy, it is possibleto formulatenonirritating, noncomedogenicmoisturizers, sunscreens, hair pomades,cosmetics, and condi- tioners. INTRODUCTION The possibilityof comedogenicityand irritancyof facialskin careproducts has been well documented(1- 3). Becauseof this work and an increasingpublic awareness, facial productsthat are lesscomedogenic are now becomingavailable (4). However, other skin careproducts such as hair conditioners,hair pomades,moisturizers, sunscreens, and evenacne treatment products may be a sourceof cosmeticacne. By taking these productsapart, testing their ingredients,and putting them back together and retesting them, an extensiveingredient listing hasbeen created. By studyingthis list, the cos- metic chemistcan begin to be selectivein developingformulas for lessirritating and lesscomedogenic products. The rabbit ear assayhas been used since the mid-1950sas a methodof measuring follicularkeratinization by externallyapplied compounds (5). The advantageof this rapidscreening tool is that it takesonly two weeksto developfollicular impactions in the rabbitear, whileit maytake six months to developsimilar reactions on humanskin. The disadvantageof the modelis its extremesensitivity. The fragile,protected epithe- lium of the inner ear is extremelysensitive. Not everythingthat irritatesthis model will alsoirritate humanskin. However,this extensivescreening of cosmeticformula- 321 322 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS tionsand their ingredientswould not havebeen possible without the useof this animal model.We havenow extended the modelto includean indexof surfaceskin irritancy as well as of follicularhyperkeratosis. METHODS Ingredientsare mixed in propyleneglycol at a 9 to 1 dilution for testingunless other- wise indicated(10% concentration).A colonyof New Zealandalbino rabbits that has geneticallygood ears and is free from mitesis used.Three rabbits,weighing two to threekilograms, are usedfor eachassay. Animals are housed singly in suspendedcages and fed Purina Rabbit Chow and water ad libitum. Animals are maintained on a 12-hour light and 12-hourdark cycle.A doseof 1 ml of the testmaterial is appliedand spread oncedaily to the entireinner surfaceof oneear five daysper weekfor two weeks.The oppositeuntreated ear of eachanimal serves as an untreatedcontrol. Follicular keratosis is judged both macroscopically(visually) and microscopicallywith a micrometerto measurethe width of the follicularkeratosis. The macroscopicresponse is determined by averagingthe measurementsof the width of six folliclesusing a Mitutoyo Dial Micrometer(#536-724). A similarmicroscopic micrometer measurement is obtained by averagingthe width of six folliclesunder a magnificationof 430 x after a 6-ram biopsyspecimen is fixed in formalin,sectioned at six microns,and stainedwith hema- toxylin-eosin.The resultsare then combinedon a scaleof one to five: Micrometerreading Grade 0.009 in or less 0 No significantincrease in follicularkeratosis 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 increasein follicular keratosis 0.030 in or more 5 Grade5 is the presenceof largecomedones throughout the ear, similarto thoseinduced by the applicationof our standard"positive" testing agent, isopropylmyristate. As reportedin our previousstudies, a minimal gradeof 0 to 1 is not consideredsignificant. Grade2 to 3 is borderline.However, a gradeof 4 to 5 is uniformallyreproduceable and consideredpositive. The irritancyproduced by the repeatedapplication of a chemicalor skincare product on the surfaceepidermis in the rabbit ear is alsoevaluated on a similar scaleof 0 to 5. The gradesare summarizedas follows: 0 No irritation 1 Few scales,no erythema 2 Diffusescaling, no erythema 3 Generalizedscaling with erythema 4 Scaling,erythema, and edema 5 Epidermalnecrosis and slough To studythe effectsof differentvehicles on comedogenicityand irritancy,several fatty acidsand the D&C red pigment#36 are reexaminedin differentsolvents. The fatty COMEDOGENICITY 323 acids are dissolved in either a volatile solvent or sunflower oil. The D&C red #36 pigmentis testedin mineraloil, propyleneglycol, polyethylene glycol 400, andpen- taerythritaltetra capra/caprylate. RESULTS AND DISCUSSION Cosmeticacne was first reportedby Frenchdermatologists in the mid-forties.They reportedon brilliantinesand hair pomadescausing flareups on the templeand forehead facial regions.They attributed the problemsto impuritiesin the brilliantines(6). In 1970, Kligman requestedthat Gerd Piewig and I examineover 700 men to find some with normal facial skin. Much to our chagrin, the majority had cosmeticacne (7). About 70% showedsome evidence of follicularkeratoses on the foreheadand temples. Occasionallythe eruptionswere noted on the cheeksdown to the jawline area. The lesionswere usually noninflammatory, closed comedones. A few lesionsdeveloped into small inflammatorypapules. However, there were no casesof severe,cystic inflamma- tory acne. Histologically, the comedonesfrom pomadeacne caseswere identical to biopsiestaken from comedonesof classicacne vulgaris patients. In surveyingthe hair carepreparations, we felt that the actualingredients and not tracecontaminants were offenders.Interestingly, very few of the subjectsattributed their folliculareruptions to their daily useof a hair pomade.This studystimulated us to examineother skin care productsand ingredients. In 1972 Kligman and Mills reportedon acnecosmetica in their surveyat the Acne Clinic at the Universityof Pennsylvania(1). Approximatelyone third of the adult womenhad a low-grade,persistent acne in the cheekarea, consistingof closedcomedones quite similar to thosefound in pomadeacne. This appearedmore frequently in womenafter agetwenty and may explainone of the reasonsfor epidemicadult acnein womenin the 1970sand 1980s.In 1976 and 1984, Fultonpublished results on actualcosmetic lines and on ingredients,and proposedthe developmentof noncomedogeniccosmetics using ingredientsthat werenonoffenders in the rabbitear assay (2,3). Severalmajor cosmetic manufacturershave now producedthese types of products.However, our screening indicatesthat work is still neededon many skin careformulations. It becameapparent during our researchinto potentialnoncomedogenic ingredients that severalhypotheses could be developed: (1) In orderfor an ingredientto be comedogenic, it must penetrateinto the follicle, and (2) once in the follicle, the chemicalmust producethe follicularreaction of "retentionhyperkeratosis" (8). In addition, the overall penetratibilityof the moleculemay be relatedto (1) the water/oilpartition coefficient of the compound(HLB balance)and (2) the relativemolecular weight of the ingredient. The ingredientappears to havethe mostpotential if it is fairly solublein bothwater and oil (HLB around 10 to 12) and hasa rangeof molecularweight between200 and 300. The comedogenicityof an ingredientmay be reducedby addinga large constituent (i.e., polymersof PEGs), by addinga chargedmolecule (i.e., sugars),or by addinga heavymetal (i.e., zinc or lithium). This often relatesto raisingthe HLB balanceto above 12. Examplesof this conceptof water/lipidsolubility and molecularweights are apparent in eachclass of chemicalsexamined (Table I). Amongthe lanolins,the classicanhydrous lanolinsare not as comedogenicas the moderatelyethoxylated derivatives (laneth 10). 324 JOURNAL OF THE SOCIETY OF COSMETICCHEMISTS Table I Ingredientsand Their Comedogenicityand Irritancy Grade (0- 5) Grade (0- 5) Ingredient Comedo.? Irrit. ½ Ingredient Comedo.? Irrit.½ I. Lanolins and derivatives Myristylalcohol 2 4 Acetylatedlanolin 0 0 Cetyl alcohol 2 2 Acetylatedlanolin alcohol 4 2 Isocetylalcohol 4 4 Anhydrouslanolin 0-1' 0 Cetearylalcohol 2 1 Lanolin alcohol 0-2* 0 Oleyl alcohol 4 2 Lanolin oil 0-1' 0 Stearylalcohol 2 2 PEG 16 lanolin (Solulan 16) 4 3 Cetearylalcohol q- PEG 75 lanolin 0 0 ceteareth 20 4 Laneth- 10 2 1 Ceteareth-20 2 3 PPG 12 PEG 65 lanolin oil 2 0 Propyleneglycol 0 0 Butyleneglycol 1 0 II. Fatty acidsand their derivatives Hexyleneglycol 0-2* 0- Caprylicacid 1 3 PG caprylate/caprate 2 2 Capricacid 2 2 Lauric acid 4 PG dicaprylate/caprate 1 0 PG dipelargonate 2 2 Myristic acid 3 0 PG laurate 0 3 Palmitic acid 2 0 PG monostearate 0- 3 0- Stearic acid 2 - 3' 0 Ethyleneglycol Eicosanoic acid 2 0 monostearate 0 0 Behenic acid 0 0 Glucoseglutamate 0 0 Ascorbylpalmitate