J. Soc. Cosmet.Chem. 26, 173-187 (1975) ¸ 1975 Societyof CosmeticChemistsof Great Britain
Exaggeratedexposurein topical
irritancyand sensitization
testing
N.J. VAN ABBI•,* P. NICHOLASI and
ELIZABETH
BOON*
Presented
on26-30thAugust1974in Londonat theIFSCC Vlllth
International Congress on 'Cosmetics--Quality and Safety'
organfled by the Societyof CosmeticChemistsof Great Britain.
Synopsis--The
conceptof a 'safetymargin'providesa convenientexpression
for the hazardof
adversereaction followingtopical administration.Exaggerationof exposureconditionsin
PREDICTIVE TESTING helpsto establishthe safetymargin,but reliabilityof any prediction
dependson limitingthe effectsof exaggeration
to quantitativeratherthan qualitativeenhancementof responses.
Grossexaggeration
oftenleadsto qualitativechangesdefyinginterpretation
in terms of hazard during normal use.
Techniquesfor safetyevaluationbaseduponcausingonly thresholdeffectsand comparison
of an unknown with a well-established
'control' preparation of similar type are suggested
as
most suitablefor relativelyinnocuouscosmetics.Human tolerancetestswould probably be
idealfor the purposebut extremelytime-consuming.
If animaltestsare usedto screenfor skin
and EYE IRRITANCY,
there shouldnot be any need for grosslyexaggeratedexposuresince
the species
mostlyusedapproximatequitecloselyto man in their susceptibility
to skinand eye
irritants.
The predictionof sensitizing
potentialby exaggerating
exposure
is unsatisfactory
owingto
insufficiency
of data on DOSE-RESPONSEbehaviourfor mild SENSITIZERS. Experience
in
normaluseof a cosmeticby graduallyincreasingnumbersof individualswould seemto be the
only availableway to establishSENSITIZING POTENTIAL for cosmeticformulations,
althougha GUINEA-PIG TEST may be usefulfor screening
new raw materials.
INTRODUCTION
The possiblehazard of adversereactionin responseto topical
administrationneedsto be assessed
by meansof suitablepredictivetests.
The extentof sucha hazardis conveniently
expressed
in termsof themargin
* BeechamProductsResearchDept., Leatherhead,England.
•' Presentaddress:ToxicolLaboratoriesLtd, Ledbury,Herefordshire.
173
174
JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS
between probable exposureof the skin or mucous membranesduring
normal use and the level of exposure which would produce an adverse
reaction.In order to err on the sideof safety,broad marginsof safetyare
sought by exaggeratingthe levels and conditionsof exposurein the test
procedures.However, injury may result from grosslyexcessivedirect contact of many tissueswith eventhe mostinnocuousenvironmentalchemicals
and so a rational approachto exaggerationis essential.
The degreeof chemicalinsult that the skin or mucousmembranesmight
be expectedto tolerate is somewhatproblematical.The extensivetesting
carriedout on the safetyof ingestedmaterials,suchasfood additives,offers
little guidanceas suchinvestigations
are principallyaimedat demonstrating
toxic effectsafter systemicabsorption.The aspectof safetyevaluationfor
ingestedsubstances
correspondingto topical administrationis the direct
effect, if any, of the test materials on the gastro-intestinallining. Severe
irritation of the lining would indeedbe observedunder conditionsof gross
exposureto many universally-accepted
food materialsand especiallycondimentssuchasvinegarand mustard.In otherwords,althoughthereshould
be a wide margin of toleranceoncea test material has beendiluted in the
body fluids following absorption,a narrower margin is to be expectedin
the case of a tissue in direct contact with the test material.
Whilst acknowledgingthat thereare likely to be considerabledifferences
betweendirect exposureand exposureafter absorption,it might be instructive to consider the postulateson which the safety evaluation of food
additivesis based.A hundredfoldsafetyfactor (1) is commonlyquoted;
this may be deemedto offer a tenfold allowancefor the greater susceptibility to systemictoxicantsof man comparedto laboratory animals, togetherwith a further tenfold allowancefor variationin susceptibility
between
individuals. In terms of systemictoxicity, such a tenfold allowancefor
inter-speciesdifferencesin metabolic transformationand excretionseems
reasonable. Effects on the skin and mucous membranes, however, are not
primarily dependenton species-specific
metabolicpathways.Indeed, the
skin of thosemartahalsmost often usedfor irritancy testingapproximates
quite closelyto human skin in its susceptibilityto irritation or even shows
greatersensitivityto someirritants (2, 3).
The epidermal horny layer of the skin is an important barrier to the
absorptionof foreign chemicals(4) and providesthe first line of defence
againstirritants.Thicknessof the horny layer variesacrossthe humanskin
and in someregionsit is thinner than the horny layer of other mammals.
However, human epidermis overall is much thicker (5) than in most
EXAGGERATED
EXPOSURE
IN
PREDICTIVE
TESTING
175
mammals,includingthe rabbit, rat and mouse;this probablyexplainswhy
appliedsubstances
do not easilypenetratehumanskin (6) and why, on the
whole,it is no more susceptible
to irritantsthan is the skin of thesespecies.
When irritancy testsare carried out on animal skin, it would thereforebe
irrational to allow a tenfoldmarginfor interspecies
differences.
The possibleirritancy of cosmeticmaterials in contact with tissuesof
the eye is usually studiedby instillation into the conjunctivalsac of the
rabbiteye.On thebasisof wideexperience
of suchtests,Davies(7) suggested
that the rabbit eyewasdecidedlymore sensitiveto irritantsthan the human
eye. Thus no allowance for interspeciesdifferencesseemsnecessaryfor
extrapolatingrabbit eye test resultsin termsof hazard to man.
A major factor involvedin selectingthe appropriatelevelsof exaggeration in irritancytestingis that a quantitativeenhancement
of responses
may
help to establisha meaningfulsafetymargin, whereasa qualitativechange
in the type of responsecould well renderthe findingsincapableof interpretation; qualitatively atypical responsesmight well result from gross
exaggerationof exposurelevelsin testsfor skin and eye irritancy (Fig. 1).
To ensurethat testingproceduresgive the information requiredfor safety
assessment,
a critical re-appraisalof currentmethodsis needed.
TEST METHODS
In the study of systemictoxicity, suitablyexaggerateddose-levelsare
administeredto laboratory animalsin the diet, by gavageor by injection.
Exposureof the skin or mucousmembraneto substantiallyexaggerated
quantitiesof test material is seldom practical in the study of irritancy.
Direct contact within a circumscribedarea is essential;an exaggerated
quantityappliedto a largerareawill not necessarily
intensifythe response.
Exposureto a raw materialmay often be exaggerated
by applyingconcentratedsolutions,but this would not be feasiblefor completeformulations. Even with raw materials,unrealisticeffectsmay occur,for example,
owing to hypertonicityor grosslyabnormalhydrogenion concentration;
suchexaggerationcouldwell produceeffectstotally irrelevantto the hazards
of ordinaryuse,by producingqualitativeratherthan quantitativedifferences
in response.
An alternativeway of exaggeratingtopical exposureis to lengthenthe
time of contactcomparedwith normal useor to make multiple applications.
This is helpfulif it influencesthe responsequantitativelywithout provoking
176
JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS
2'5
--
2.0
1.5
1.0
x
x
0.5x-•X'"
-••
0
X50 XIO0
X300
0
X50
XIOOX300
0
X 'm=-- - X
X50 XlOOX300
Estimated exaggerationof exposureto zinc pyridinethione •n shampoocompared
with normal human usage
Figure 1. Comparison of skin irritancy produced by repeated shampooing
(,brokenlines) and occlusivepatch testing (solid lines). Breakage of the skin
under occlusionsuggestsa qualitative changeindicating excessiveexaggeration. X, Erythema; O, breakage. *Scored according to Draize, J. H. (1959).
Appraisal of the Safety of Chemicalsin Foods, Drugs and Cosmetics.
a qualitativelydifferentskin reaction.An exampleof a qualitativelyaltered
responsesometimesoccurswhen multiple exposuresto a moderateirritant
lead to an enhanced'fatigue' response(8); this would be irrelevantin the
studyof short-termhazards.Thus fatiguemay be pertinentto the safetyof
a face creamfor daily usebut not to a hair-wavinglotion usedonly two or
threetimesper year. Exaggerationby meansof multipleapplicationsshould
thereforebe reservedfor testingsubstances
intendedfor frequentlyrepeated
topical use.
Another method of exaggeratingexposurefor irritancy testing is to
apply the test material to damagedskin, whichis more readilypenetrated
by irritants than intact skin (9). Damage may be artificially inducedby
abrasion,adhesivetape strippingor chemicalpre-treatment.Direct effects
EXAGGERATED
EXPOSURE
IN
PREDICTIVE
177
TESTING
due to contactwith the underlyingtissues,however,may provemisleading
if extrapolatedin termsof normalskin with an intacthornylayer. Certainly
it is helpfulto knowwhatwill happenwhena productis appliedto damaged
skin, but simple quantitativerelationshipsto irritancy for normal skin
cannotthus be establishedand the distinctionneedsto be recognized.
Chemicalpre-treatmentof the skin, for example,by applyingformaldehyde or sodium lauryl sulphate,may not producegrosslyvisible damage
but will in many casesenhancepenetration.Usually the degreeof enhancement cannotbe quantifiedin termsof relativeirritancy to normal skin and
the predictivevalue of a provocativetest usingchemicalpre-treatmentis
thereforequestionable.Furthermore, the intensity of adverseeffectsmay
be too severeto regard as reasonablyjustifiable for either animal or human
studies.
Sincethe variousmethodsusedfor exaggeratingexposureby inflicting
damageto the skin in one form or anotherso often producedifficultyin
interpretation, a rational conclusionis that such damage should never
exceedthe minimumnecessary
to ensurea detectableresponse;discrimination betweentestmaterialsin termsof skin irritancymay evenbe improved
by limiting the overallresponse,e.g. by testingafter dilution of the product
(Table I).
Techniquesinvolvinggrosslyexaggerated
exposurehave led to serious
problemsof interpretationnot only in skin irritancy testingbut also in
studiesof eye irritancy. For example,it has long beencustomaryto instil a
Table I. Improved discrimination between irritancy of shampoosapplied to rabbit
skin at 10% dilution
Irritancy* after
5 h
Type of shampoo
Baby--based on
amphoteric detergents
Normal--based
Neat
24h
10%
Neat
10%
7
1.1
4
0
Erythema
1.5
1
2
0
Oedema
7
1.5
15
1.5
Erythema
8.5
I
14
0.5
Oedema
10.5
6
17
7.5
Erythema
I
15.5
7
Oedema
on
anionic detergents
Medicated--based
ot•
0.5% Zn pyridinethione
and anionic detergents
6
* Scoresaccordingto Draize (Group meansfor six rabbits).
178
JOURNAL
OF THE SOCIETY OF COSMETIC CHEMISTS
fixed quantity of undiluted test material into the conjunctivalsac of the
rabbit eye without, as well as with, a subsequentrinse (10). The effects
produced by instillation of undiluted material without rinsing may be
qualitatively,as well as quantitatively,differentfrom the resultslikely to
occurduringnormal useof the material. Consequently,productsare rarely
if ever deemedunfit for human use becauseof severeeye irritation when
tested under these grossly exaggeratedconditions in rabbits. A safety
evaluationtest proceduregivingrise to severeeffectswhich are commonly
and quite properly ignored,obviouslyhas little predictivevalue and no
justificationin terms of the sufferingcausedto the experimentalanimals.
A more meaningfulway of designingstudiesconcernedwith eye irritancy,
as well as skin irritancy, is to employtest conditionsresultingin threshold
or minimal irritation and to includea material with known irritancy in the
studyto serveas a control.Wheneverpossible,the controlmaterial should
be closelysimilarin chemicalstructureand mode of useto the testmaterial.
Gaunt and Harper (11) reporteda procedurewherebyshampoodiluted
to 10•o concentrationwas instilledinto the rabbit eye with no subsequent
rinse. This techniqueavoidsgrosslyunrealisticexposurebut it would still
be expectedto give someenhancementof irritant effectsby eliminatingthe
rinsing procedure.As the authors acknowledgedand we have confirmed,
their techniquehas the apparent disadvantageof militating against the
recognitionof any tendencyto corneal or iridial injury (Table II), but it
may nevertheless
havebetterpredictivevaluethan a testin whichshampoo
is instilledat 100•o concentration.Improvementsmight be madeby varying
the shampooconcentrationto someextent or by giving duplicateinstillations(12).
Table II. Rabbit eye test findingsshowingeffect of dilution of shampooswith water. Results
at 10% dilution apparently gave better prediction of human response,no corneal or iridial
injury having been notified as consumercomplaints
100% shampoo
(no rinse)
10% shampoo
(no rinse)
Cornea
Iris
Conjunctiva
Cornea
Iris
Conjunctiva
0
0
68
0
0
4
35
10
108
0
0
12
25
10
90
0
0
62
Non-medicated
liquid shampoo
Non-medicated
cream shampoo
Medicated
cream
shampoo
Scoresaccording to Draize; each figure gives sum of scoresfor three rabbits
after 1, 2, 3, 4 and 7 days.
EXAGGERATED
EXPOSURE
IN
PREDICTIVE
TESTING
179
The questionablefeaturesof the more usual form of rabbit eye test,
apply particularlyto its usein the evaluationof detergentingredientsand
shampooformulations.For cosmeticproducts,however,normally used
away from a washbasinand without rinsing, the instillation of undiluted
productmay be more meaningfuland seldomleadsto grossinjury in the
rabbit eye.
A particulardifficultyin usinglaboratoryanimalsis the choiceof species
and testing site for productssuch as dentifricescoming in contact with
mucous membranesduring normal use. The hamster cheek pouch has
sometimesbeen used for predictivetesting although the presenceof a
cornifiedepitheliallining of the buccalmucosain this species
may limit the
sensitivityof the test. In view of the easeof handling of theseanimals and
the relativelylarge area of tissueavailablefor examinationand biopsy,
this seemsnevertheless
to be the test of choiceat the presenttime; possible
lack of sensitivitymay be overcomeby reasonableexaggerationof product
concentrationand durationof contactwith the cheek pouch.
The need to limit irritant effects in the course of tests on human volun-
teersis obvious,to avoidcausingharm andto ensurecontinuingavailability
of willingpanellists.It is alsohighlydesirableto maintainand, if possible,
to improvestandardsin the humanitariantreatmentof laboratoryanimals
and closeattention shouldbe given to the designof suitableprocedures
both for humanand animal testing.The testsshouldpreferablynot, however, be designedin sucha way that they mostly lead to purely negative
findings,sincetheseare as hard to interpretas grosslyabnormalpositive
results;this is an additional reasonfor favouring a thresholdirritancy
approach.
Designsfor skinirritancystudiesbasedon thresholdirritation havebeen
put forward by Kligrnan and Wooding (13). Theseauthorssuggested
that
findingsshouldbe recordedin terms of ID50 (concentration
to yield threshold irritant effectin 505/0of test subjects)or IT50 (time of exposurefor
thresholdirritation in 505/0of subjects).Choicebetweenthe two methods
of expressingresultswould depend on the feasibility of testing diluted
productand the level of irritancy of the test material.Whereasthe ID50 or
IT50 principlemay be suitablefor evaluatingnewraw materials,formulated
cosmetics
will often prove altogethertoo bland for suchan approachand
are more readilytestedby directcomparisonwith an appropriately-chosen
control product.
180
JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS
PATCH TESTING
Skin irritancy is usuallyinvestigatedby meansof a patch testprocedure
(!4); frequentlythis involvesthe application of test material to the skin
with relatively prolonged occlusion under an impermeable or semipermeable dressing(Table III). Occlusion itself will produce minimal
damage of the skin and a 'closed' patch test therefore embodiessome
degreeof exaggeratedexposure.
Table Ill. Occlusivity olr patch test materials expressedas drying time oœ
hydrated CoCI2 paper on glassbacking, exposedat 19.5-4-1ø, 49.5 4-1.5% RH,
beneath the patches,for pink ->- blue colour change
Material
Nil
Mean drying time
(min)
20
Gauze (only)
Gauze+ blenderre-backedlint square
Micropore+ lint square
Micropore (only)
Micropore+blenderm-backed lint square
Dermicel + blenderm-backedlint square
Dermicel (only)
Blenderm (only)
45
65
80
155
200
230
390
1145
n•
SD
7
6.3
6
6
6
7
7
6
7
7
7.8
8.4
6.3
6.3
33.0
21.0
8.4
43.0
Micropore: supplied by 3M Co, London.
Dermicel: supplied by Johnson& Johnson Ltd, Slough.
Blenderm: supplied by 3M Co, London.
In the handsof an experiencedclinician, the occlusivepatch test is a
valuabletechniquefor diagnosingthe causalagentsof pre-existingirritation
and sensitization.Using occlusivetestingin a propheticmanner,however,
involvesdifferent considerations.Some preparations(antiperspirants,for
example)are normally used under conditionstantamount to occlusion;
relevanceof an occlusivepatch test is then obvious.For many other products,e.g. face creamsand shampoos,occlusivepatch testsmay be somewhat lessrealistic.However, an occlusiveor partially occlusiveprophetic
patchtestis probablythe bestavailableprocedurefor predictingthe effects
of long-termexposureby meansof a relativelyshort-durationtest, taking
into accountthe fact that many toilettiesand cosmeticsare usedrepeatedly
over long periodsof time (Tables IV and V). Data showingquantitative
results for occlusivevs non-occlusiveexposurein 21-day human patch
tests(2) indicate that occlusiongivesa substantialenhancementof effect
EXAGGERATED
EXPOSURE
IN
PREDICTIVE
181
TESTING
for many irritants. Skin irritancy testing carried out under 'open' patch
test conditions(e.g. with the appliedmaterial under a loosely-wovengauze
covering)might thereforebe preferablein order to avoid too many 'false
positive' results. However, since toiletries and cosmeticsare invariably
formulatedto give minimal skin reaction, 'open' patch testingin practice
would nearly alwaysyield wholly negativeresultswhich would be hard to
interpret. 'Closed'patch testsresultingin thresholdirritation, preferably
including controls with known irritant potential, allow decisionson the
acceptabilityof a cosmeticingredientor productto be reachedwith greater
confidence.
Another important considerationin patch testing concernsthe form
and amount of test material applied.When a volatile solventis presentin
the formulation, this shouldobviouslybe permittedto evaporatebefore a
'closed'patchis appliedto the skin; if this precautionis not taken, irritant
effectsdue to the solventwill tend to give 'false positive'responses
in the
sensethat solventevaporationduringnormal usewould avoid any irritation
from this source.A further complicationis that sometoiletry productsare
Table IV.
Detection
of moderate
increases in skin
irritation, using partially occlusivehuman patch test
(Uttley, M. and Van Abb6, N.J. J. Soc. Cosmet.Chem.
24, 217 (1973))
Product
Irritancy score
Nil (blank lint patch)
6.6*
Lotion
6.6
base
Base+ DHA (aged)
Base+DHA (aged)+fresh DHA**
Base (fresh)+ fresh DHA (10%)
8.4
9.3
10.9'
DHA = Dihydroxyacetone.
* Difference significantat 1% level (Wilcoxon).
** Adjusted to 10 •o concentration.
Table V. Correlation betweenpartially-occlusivehuman patch test (Uttley and Van
Abb6) and consumerreports
Moisturizing cream
Cheek gloss no. 1
Cheek glossno. 2
Cheek gloss no. 3
Irritancy
Interpretation of
score
score
6.3'
11.0+
12.6*
18.1
Non-irritant
Slight irritant
Slight irritant
Moderate
•- severe irritant
* Significant at < 1% level (Wilcoxon).
+ Significant at < 2% level (Wilcoxon).
Consumer
reports
No irritation
No irritation
No irritation
Irritation
182
JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS
rinsed off the skin during normal use soon after application. In these
instances,loading a patch with the undiluted product and leaving it in
contact with the skin at full concentrationfor many hours is highly unrealisticand may well prove intolerablefor the volunteers.
For a shampoo,a more informativemethodof patchtestingis to apply
the product to the patchesat a dilution of 10-20•o with water. When a
physiologically-activeconstituent occurs in the formula (e.g. an antimicrobial agent) an alternative procedure is to load the patch with a
quantitybasedon an amountper unit area of skinequivalentto the residue
left on the scalpafter shampooing,as determinedby assay.
Where the assay of actual residuespresentsseriousdifficulty, open
patch testsmay be carried out by actually shampooinga small test area of
skin daily for severaldays; experiencesuggeststhat this proceduremay
evenhavebetterpredictivevaluethan a closedpatchtest basedon estimates
of residualquantitiesafter rinsing.
There shouldthus be no insuperabledifficultyin showingan adequate
safetymargin for the topical administrationof cosmeticsand toiletriesby
reasonableexaggerationof the exposureconditionsin testsfor irritancy.
Since,for the reasonsalready stated,the presentauthorscontendthat no
allowanceis usuallynecessary
for interspecies
variation,the choicebetween
using human volunteersor laboratory animals does not appear to have
great significancefrom the investigator'sstandpoint. If, however, the
irritant potential of the test material really is unknown, initial screeningis
certainly best carried out with laboratory animals. Moreover, if effectson
damagedskin are being examined,an animal screeningtest is obviously
desirablebefore extendingthe studyto human skin, as a reasonablesafeguard for the volunteers.Despite the similarity in responsesto irritants
generallyshown,if interspeciesdifferencesare discerniblewhen resultsfor
animal and human irritancy tests are compared, greater reliance should
obviouslybe placedon the human data.
To avoid uncertainty in extrapolation from animal responsesto man
whilst minimizing the risk of seriousharm to volunteers,human studies
may sometimestake the form of tolerancetests.The degreeof exposure
(with respectto amount of test material applied, its concentrationor the
duration of contact)is graduallyincreaseduntil a thresholdresponseoccurs.
The predictivevalue of such a test shouldbe satisfactoryprovidedthat
precautionsare takento avoidfatigueby carefulchoiceof siteof application
or interval between applications.Human eye irritancy testing should
generallyfollow this type of cautiousapproach(12). The drawbackto more
EXAGGERATED
EXPOSURE
IN
PREDICTIVE
TESTING
183
widespreaduseof the humantolerancetest, however,is its time-consuming
nature and the difficulty of recruitingvolunteers.
SENSITIZING
POTENTIAL
Sensitizationis an important type of possible adverse reaction to
toilettiesand cosmetics;it usuallyinvolvesthe risk of causingan allergic
contact dermatitis and needsto be consideredseparatelyfrom irritancy.
Allergensmay sensitizeoccasionalindividualsat concentrationswhich are
without any significanteffecton the majority of the population;this feature
of allergenicitycausesgreat difficultyin predictivetesting.Somedermatologistsnevertheless
take the viewthat exposinga groupof randomly-chosen
subjectsunder test conditionsto an exaggeratedconcentrationof a suspectedallergenincreases
the chanceof recognizingits sensitizing
potential.
If so,what degreeof exaggeration
is appropriate?
Marzulli and Maibach (15) recentlyreporteda detailedinvestigationof
sensitizingpropertiesusing exaggeratedconcentrationsof test materials.
For example,with sorbicacid testedat 20•o concentration,one of the 33
subjectsthey testedgavea positivereaction.The crucialquestionis what
proportionof users,if any, would be sensitizedat a typical use-concentration of about 0.1-0.2•o. Clearly this is unanswerablewithout knowing the
shapeof the dose-response
curvefor an allergenof the sameor a similar
type, extendingright down to normal use-levelconcentrations
of the test
substance.These authors did, in fact, conduct testsat severalconcentration
levelswith a number of compoundsbut the proportion of subjectswith
positive responsesdid not show a dose-response
pattern justifying any
broad conclusions;a graded dose-response
relationshipmay perhapsbe
inferred on theoreticalgroundsbut eventhe extensivework carried out by
Marzulli and Maibach (15) was evidently insufficientto demonstrateit
clearly.
Some toxicologistsconcernedwith toiletties and cosmeticsprefer to
conducttests for sensitizingpotential at normal use-concentration
of the
test materials, using human volunteers.If such tests involve multiple exposuresunderocclusivepatches,the resultingminor degreeof skin damage
shouldmarginallyincreasethe likelihoodof a positiveresponse.However,
sinceit is well known that toilettiesand cosmeticsin generalsensitizeless
than, say, 1 in 10 000 users,the predictivevalue of any use-concentration
test carried out with only a few hundredvolunteersmust be exceedingly
184
JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS
small. Selectionof atopic subjectsfor test panelsis sometimesconsidered
to improvepredictivevalue but the evidenceto indicatethat atopicsshow
enhancedsusceptibilityto topical allergensin generalis questionable(16).
Bearing in mind the objectionsto grosslyexaggeratedexposuretesting
and recognizingthat sensitizationtestingat normal use-concentration
often
yields negativeresultsthat cannot be interpretedor which may be unreliable,thereare certainattractionsin devisinga testprocedureto enhance
responsesto use-concentrations
and to ensure that positive results are
obtainedevenwith moderateor mild sensitizers.
Kligman (17) in proposing
his 'maximization'test,offereda proceduregiving24/24 positiveresponses
top-phenylenediamine
eventhoughhe wasstillunableto detectsomeknown
mild sensitizerssuchas lanolin. The value of this type of test for cosmetic
evaluationdoesnot thereforeyet seemto have been established.
At the present time, despitethe energeticattempts by a number of
highly-skilledinvestigators,thereis clearlyno satisfactoryway of predicting
the sensitizingpotential of cosmeticsand toiletries by means of human
volunteerstudies;nevertheless,
suchstudiescarry a definiterisk of sensitizing volunteers,possiblyfor someyears(18), and theirjustificationis therefore doubtful. As an alternative, a reliable test for sensitizingpotential
usinglaboratory animalswould obviouslybe helpful.
Whereas the responseof some other mammals closelyresemblesthe
human responseto irritants, there are marked interspeciesdifferencesin
allergicresponses.
Suitabilityof the guinea-pigfor sensitization
testinghas
beenvalidatedto someextent(19) but it would be unwiseto expectguineapig studiesto eliminateany but the most potent sensitizers.Thus, although
it is reasonablypracticalto testfor the sensitizing
potentialof raw materials
by conductingchallengetestsat elevatedconcentrationsusing animals or
man, no comparableprocedureis yet available for studying complete
formulationslikely to displayno more than mild sensitizingability. Rather
than applying maximizing proceduresof dubious predictive value, it is
probablybetterto allow a productto be usednormallyby graduallyincreasing numbersof individuals.This view takesfor granteda prior scrutinyof
the raw materialsto eliminateany potentknown sensitizers
and an adequate
schemefor monitoring adversereactionsif they are reported by usersof
the product.
EXAGGERATED
EXPOSURE
IN
PREDICTIVE
TESTING
185
CONCLUSION
Unreasonablecriteria for assessingthe potential hazards of topical
administration do not necessarilyhelp to protect the consumer. Thus,
although animal feedingtestson a proposedfood colour may well show
that the maximum no-untoward-effectlevel is severalhundredtimesgreater
than the expectedhuman intake, eventhe most harmlessmaterialsapplied
to the skin with suchexaggerationare likely to prove injurious.Frazer (1)
claimed that the acceptableusagelevel of a substance--hewas referring
specificallyto food additives,thoughothershave appliedhis conceptmore
widelymshouldbe regardedas one-hundredthof the level requiredto producesignificantmodificationof structureor functionin not more than 50•o
of a group of test animals.Further, at a dose-levelequal to one-tenthof
the ED50, no significantchangesof any kind should occur. Suchmargins,
however,could not be applied generallyto substances
coming in contact
with the skin or mucousmembranes.For example, none of the synthetic
anionicsurfactantswouldbe acceptableif a shampoohad to be formulated
with no more than one-hundredthof the detergentconcentrationgiving
threshold
irritationin a closedpatchtest.Nevertheless,
present-day
shampoos are used almost universallywith minimal known adverseeffect; a
differentbasisfor judging acceptabilityis thereforeneededand one of the
possibleapproachesmight be to seek a tenfold margin in relation to
experimentalfindingsto allow for individual differencesin susceptibility.
As already shown, no allowancefor interspeciesdifferencesneed usually
be made in irritancy testing.A tenfold safetycriterion on theselines may
prove quite helpful for the safety evaluation of raw materials but it will
seldom be a technicallyfeasible criterion for use in testing formulated
products.Direct comparisonof a newly-formulatedproductin a threshold
irritancy test with other formulationsof similar type, whoseeffectsduring
normal use are known, will be more appropriate.Such a comparisonwill
certainly give practical guidanceon probable safety-in-use.A study on
human volunteerswill clearly be the most reliable and ideally the study
shouldtake the form of a comparisonwith a known 'safe' and a known
'unsafe'material of similartype (i.e. with 'negative'and 'positive'controls).
Comparisonwith a 'positive'control(e.g. a known irritant) might facilitate
quantitativeexpressionof the findings,if a human tolerancetest is carried
out. In circumstances
where laboratory animal studiesare judged to be
required,it will be equallydesirableto conducttheseas thresholdirritancy
tests to forecast the onset of hazard to man in normal use.
186
JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS
Reasonablyexaggeratedexposurein cosmeticsafety evaluation may
theoreticallybe achievedby designing'in use' tests.Human volunteersuse
the test material for a few weeks with more frequent applicationsthan
would normally be made and subjectto repeatedexaminationfor adverse
effects.Such a proceduremay involve a risk that the investigatorwill be
unableto control the amount and frequencyof applicationeffectivelyand
that comparisonswith suitablecontrolsmay be difficult to arrange.Unless
the conditionsof testingprove suitablefor achievingthresholdresponses,
interpretationmay dependon negativefindingswhich will limit the reliability of the study.'In use'testingwarrantsseriousconsideration,however,
as an alternativeto the highly empirical, grosslyexaggeratedprocedures
currentlyfavoured by someinvestigators.
ACKNOWLEDGMENT
The authorsare indebtedto Mr J. Brown for the data given in Table11I.
(Received:loth April 1974)
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