j. Soc.Cosmet. (5hem., 47, 41-48 (January/February1996) Petrolalumis not comedogenicin rabbitsor humans: A critical reappraisalof the rabbit ear assayand the conceptof "acne cosmetica" ALBERT M. KLIGMAN, Departmentof Dermatology,University of PennsylvaniaSchool of Medicine,Philadelphia, PA 19104. AcceptedMarch 8, •996. Synopsis The objectivewas to find out whetherpetrolatum, previously reported to be comedogenicin the rabbit ear model,would worsenacne. Vaseline petrolatum jelly (Vaseline©, Chesebrough-Ponds)and Vaselinepe- trolatumjelly cream(Creamy Vaseline ©, Chesebrough-Ponds)were applied to separategroups of ten acne patientseach twice daily for eight weeks. No evidenceof comedogenicpotential was found by the follicular biopsytechnique. Papulo-pustules significantlydecreased in both groups. Greasinesscannot be equatedwith comedogenicity.The admonitionfor acnepatients to seek"oil-free" skin careproducts has no scientificmerit with regardto comedogenicity.Cosmetics are an unlikelycause of the relativelyhigh prevalenceof post-adolescentfemale acne. INTRODUCTION In a 1972 paperentitled "Acnecosmetica," Kligman and Mills reportedthat asmany as 50% of commercialcosmetics were comedogenic in the rabbit earmodel (1). Sincethen, this assayhas been extensively used by producersof drugsand cosmeticsto assesstheir potentialfor inducingacneform eruptions (2). Recently,manufacturers have added the human back test to appraiseacnegenicity. Reputable manufacturers of cosmeticsrou- tinely screentheir productsfor comedogenicityin order to satisfythe consumerthat their productis "non-comedogenic,"a usefulclaim in a highly competitivemarket. However, neither the rabbit nor the human assayis entirely satisfactory.Serious con- troversieshave sprung up regardingthe reliability andrelevance of the rabbit earmodel. A voluminousliterature reveals remarkably divergent results among laboratories and investigators.Substances deemed to be stronglycomedogenic by someare declaredby othersto be innocuous.Reviewers have not failed to notice the high frequencyof strikingly contradictoryresults. Long ago Frank questionedwhether the rabbit ear modelhad any relevancefor humans(3). The mostextreme example of the controversies generatedby the rabbitear assay is the publicationsof Fulton,a prominentacneologist 41 42 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS (4). He has classifieda large number of substancesaccording to their comedogenic potential.He statescategorically that manufacturersshould automatically exclude in- gredientsthat arecomedogenic at anyconcentration. This caveathas brought to the fore a vexing problemfor manufacturers,namely, the substantialnumber of desirableand seeminglysafe substances that havebeen "blacklisted" by Fulton. Nelsonand Rumsfield, too, havepublished lists of "unacceptableingredients" (5). They go so far as to recom- mendproducts that do not contain"unacceptable ingredients." Draelos, too, hassharply commentedon the complexitiesand uncertainties of evaluatingcosmetic components by the rabbit ear model (6). Especiallynotable in Fulton'smultitudinous list of come- dogenicsubstances are D & C red dyes and a large variety of fatty substancesthat contributeto the aestheticand functionalproperties of cosmetics.These have not been demonstratedto be acnegenicin humans.Because of theseconflicts, I updatedthe rabbit ear assayin a 1990 publication,adding specifications that, hopefully,would substan- tially reduceinter-laboratory disagreements (7). REASSESSMENT OF THE COMEDOGENICITY OF OILS This wordy introductionbrings us to the issuethat is centralto the focusof this paper. Using the originalmodel, Kligman and Mills statedthat petrolatumsand mineraloils from different sourceswere uniformly comedogenic(1). With the updatedmodel, it turnsout that thesewere "falsepositives" (see below). This is far morethan an academic controversysince it stronglyimpacts on the credibility of safetyclaims for skin care products. Seborrhoea,the excessiveproduction of sebum,is a prerequisitefor the developmentof acne(8). Personswith seborrhoeaare also greatly discomforted and try to removeexcess oil by frequentwashing or by oil-absorbingpapers. It is understandablethat acnepatients intuitively wish to avoid oily and greasycos- metics. Indeed, it has become the universal mantra for cosmeticmanufacturers to claim that their products are "oil-free." Dermatologists, too, routinely proscribe oil- containingfacial products for acnesufferers. Patients are advisedto readthe labelsand avoid medicamentsand cosmeticsthat contain "oils." The assumptionis that oily substancesare intrinsically comedogenic. A justificationfor this beliefstemmed from an earlierpublication that describedpomade acne in Afro-Americanmen (9). Adult black men usea varietyof greasyproducts daily to groomcurly, kinky hair. Densecrops of open and closedcomedones sometimes develop on the glabrousskin adjacentto the scalp. The casewould seemto have been made for a generalwarning againstproducts that containgreases and oils. As a matter of fact, the shelvesare now loadedwith a wide varietyof productsthat claim that theyare "oil-free.""Oils," however,comprise a great variety of chemicallyunrelated materials. The implicationis that viscosity(greasiness) alonedetermines comedogenicity. Morris and Kwan have also becomevexed by contradictoryreports and questionthe usefulnessof the rabbit ear test for formulatingnon-comedogenic cosmetics (10). The presentstudy was undertaken to seeka resolutionof thesedisputes. Petrolatum was selectedas the centerpieceof this investigationfor two reasons:(1) Petrolatumis the PETROLATUM AND COMEDOGENICITY 43 archetypicalhydrophobic grease, and (2) it is widely usedin a great variety of topical productsfor varied purposes.It behoovesus to have accurateinformation about its potential for producingadverse effects. METHODS SUBJECTSAND TREATMENTS Paid volunteerswere recruitedfrom a singledistrict in SouthPhiladelphia. Each par- ticipant had persistent,moderate acne of at least two yearsduration. The criteria for entry wasat leastten papulo-pustulesand 15 comedones.Both mild and severecases of acne were excluded. There were two groupsof ten each.In eachgroup therewere six malesand four females, with an averageage of about 18 (range14 to 22). A varietyof OTC productshad been usedby most subjectswith varyingefficacy. Anti-acne medications were stoppedone month beforestarting the study. One groupreceived a productconsisting of a highly refinedpetrolatum, twice daily, to the entire facefor eight weeks.The secondgroup receiveda creamproduct that is an aqueousemulsion containing 30% petrolatum. The identity of the productswas masked to the subjectsby putting the contentsinto newunmarked jars. The volunteerswere told that the treatments,though greasy, might be beneficialin soothingacne. We indicatedthat the useof oily formulationsin acnewas a new approachand that this wasa preliminarystudy to evaluateefficacy. The subjects were paid a modestfee for their participation. We showedeach subject how to applya pea-sizedamount to both sidesof the forehead, followedby spreadingwith the fingersover the entire face. The facewas washedwith a mild soapprior to eachapplication. The studywas carried out in the winter months, when the greasinessof the productswould be lessdiscomforting. OBSERVATIONS Lesioncounts of the entire facewere madeat baselineand after eight weeks.A derma- tologistexperienced in the methodcounted all comedones,open and closed,recording thesetogether as a singlecategory. Likewise, papules were not separatedfrom pustules, and were simply categorizedas papulo-pustules. Microcomedoneswere estimatedby the cyanoacrylatefollicular biopsy technique at 0 and 8 weeks, following the quantitative image analysismethodology of Pagnoni and Kligman (11,12). Briefly, a drop of cyanoacrylate(Krazy Glue) was applied to the mid-foreheadand spreadout under a plasticslide. After hardeningof the polymer in abouttwo minutes, the slidewas gently lifted off, carryingwith it a thin sheetof the outerhorny layer with attachedveilus hairs. Those hairs that wereencased by hornycasts as observedunder the stereomicroscopewere consideredto be microcomedones.The densityof microcomedoneswas expressedas the numberper squarecentimeter. Global estimatesof improvementwere made by the subjectsand by the dermatologist 44 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS at 0, 4, and 8 weeks, using the followinggrading system:worse, no change,slight improvement,moderate improvement. STATISTICS Changesin lesioncounts from baselinewere assessedby Student'st-test. RESULTS Changesfrom baselineare shownin Figure 1 for microcomedones,visible comedones, andpapulo-pustules. A slightbut insignificantdecrease in visiblecomedones was noted in both groups.Likewise, the slightdecrease in the densityof microcomedoneswas not statisticallysignificant. By contrast, in both groupspapulo-pustules decreased significantly (p = <0.05). Global self-assessmentsyielded the followingdata: PETROLEUM JELLY: ß Three recordedno change. ß Six recordedslight improvement. ß One recordedmoderate improvement. 20 Week0 T Week8 Mean 10 Comedones Papulo-pustules Microcomedones 20- Vaseline Petroleum • Week0 15- • Week8 Mean 10- 5 T 0 Comedones Papulo-pustules Microcomedones 'Jr P=<.05 Figure 1. Effect of petrolatumson acne lesion counts after twice-daily applicationsfor eight weeks. Coinedoneswere not affected.Both productssignificantly reduced papulo-pustules. PETROLATUM AND COMEDOGENICITY 45 PETROLEUM JELLY CREAM: ß Two recordedno change. ß Six recordedmild