Available onllne at wwaeieneedlreetoom
`
` Advanced Drug Delivery Reviews 59 (2007}1152—1161
`
`. _ _-__ ’ -'
`
`ScienceDirect
`
`
`
`Advanced
`DRUG DELIVERY
`Reviews
`
`melscvieeeomflmw’addr
`
`Transdermal skin delivery: Predictions for humans
`from in viva, ex vivo and animal models“5r
`
`Biana Godin, E11611 Touitou’
`
`Esperanza! 4W, School 151'”ram Faculty affledieine, me Hebrew University (grim-sucker, Jammie-m Mill}, lime!
`Received 10 My 2007: located 20 July 2007
`Available airline 16 August 2130'!
`
`Alan-111:1
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`Themafpemumenuspermeanenofmnlemlesismeoffliemainstepsiniheinifialdeeipmdlaterinflieevsiuxfienefdemulor
`menisdefiveryeyslems. Thefiteeannelwcmsnnmemusarvim fisvienendinmmndelamdmdeterminedmgskinpenneafim
`profiles and kinetic parameters, some studies focusing on file correlation of file data obtained using these models with flie dermalfuansdennnl
`absurpfioninhumane.mmmmwflmmfimmmmmefimmmmmfingwfimmmm
`medhdmmflfkansdmflmsmhhehflngmeuseofemiaedmmmaeimalskm.culnuedsleinequivalentaandanimalmsmdiesfawsing
`one-madam} nbsorptiauofaeaiesofdmgmoleemes andvm‘ieusdeliverysystemsaswellmmflnemaficalmodels forakinabwipiien are
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`
`COME
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`Inkeduefinn ............................................................. 1152
`1.
`[5m related in in vibe and in W skin penneatinn studies .................................... 1153
`2.
`Skin mm: 11111111111 vs. animal models .............................................. 1153
`3.
`Inwbepmefioumhmanskinvs.miumlmodels ..................................... 1154
`4.
`The use of time emcee-deified skin equivalents in 1.111116613111131 research ............................. 1155
`5.
`In vim; akin peaneeiien simliea focusingen delhl‘ery systems ................................... 1155
`6.
`'1'. meme forevaluafion ofskin absorption mhmms: mnlecules .............................. 1156
`8. Animnlmndelsforevaluafionofskinabsorpfioninhimianmdeliverysysnems ........................... 1157
`9. Mummies] models ofab'n 11119011111011 .............................................. 1158
`10.
`Conclusions ............................................................. 1159
`Acknowledgment ............................................................. 1160
`Referenws ................................................................. 1160
`
`
`
`1. Introducflon
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`

`

`B. Godot, E. Taoism [Advanced Drug Delivery Reviews 59 (20039 1152—1161‘
`
`1153
`
`factors that detennine a good in viva pa‘formance. Certainly, the
`most reliable skin absorption data are collected in human studies;
`however, such studies are generally not feasible dining the initial
`development of a novel pharmaceutical dosage form or consid-
`eration ofa new drug candidate. Thus, one of“the main challenges
`ofbiophannacemical research is finding a con-elation between 2::
`viva, animal and human studies for prediction of percutaneous
`absorption in hmnsns. It is practically impossible to assess the
`skin pmcability of materials using in viva experiments alone.
`Consequently, numerous as vim and in vino models are fre-
`quently employed to assess drug skin permeation profiles and
`kinetic parameters. Hence, a method that can consistently ccr~
`relsteexvr'vosndr'n vivodatatoshortenandeconomizeflre
`
`process of drug development and minimize the number ofhumrm
`studies is critically needed.
`This article begins with a short overview of various aspects
`aswellasprossndccnsofinvinoendinfimanimalmodels
`for skin permeation. Further, studies evaluating per-emanation
`absorption ofvar'io‘us drugs with or without permeation enhance-
`menttechniques arecovesed. Andfinally,theuseofdatafiom
`experiments in skin cultures and malhemaficaliphmacokinetie
`models for predicting transdermal absorption are critically
`discussed.
`
`2. Issues related to la vine and in viva skin permeation
`studies
`
`Despite ethical ccncerns,theuse ofanirnals orisolatedanimal
`skin models to assess percutaneous absorption of molecules is
`frequently reported. These models, generally mme available than
`humansktmareofprime importanceinbssicresesrchtoimprove
`our understanding of the processes. pathways and driving forces
`ofvarious agents moss the skinbarrier. However, due tothe large
`numbercfsnimal species deemibed inthelitenetlne, itisquite
`difiicult to compare the data in the field ofdermal and transdermal
`drug delivery. Variations inmelhodclcgy used with a specific skin
`model, such as type of difl’usion cells, skin ternperatrue, receiver
`media, application dose and difiiision area, can all significantly
`afi‘ectdstaU]. Yet, itisimponanttocmphaaizetharorvr‘rmand
`animal models provide important tools for screening a series of
`drug fornsrlslions, evaluation of skin pameation enhancing
`properties and mechanism of action of the carrier systems and
`estimation ofrank ofskin transport for a series ofdrug molecules.
`
`3. Skin structure: human vs. animal models
`
`Skin is the largest body organ, weighing approximately 5 kg
`with a surface area of about two square meters in adult humans
`[2—4]. This mlfltilsyered organ has an essential function of
`protecting the body from the surrounding environment, thus
`being an efiicient pameation obstacle for exogenous molecules.
`The barrierproperties ofthe skin lie mainlywithin itsuppermost
`shuts, the strattnn comeum (SC). This highly hydrophobic layer
`is composed of differentiated non-nucleated cells, comeocytes,
`which are filled with keratins and embedded in the lipid domain.
`Since the rate limiting step for skin absorption ofmost molecules
`is considered to be this non-viable layer, percutnneous per-
`0002
`
`mention of molecules is believed to be governed by difl‘usion
`laws [2]. The extent of skin permeation of a compound may
`depend on the route of absorption. There are three pathways
`which can be involved in the h‘ansdcrmal permeation of che-
`micals: (I) through the intercellular lipid domains in SC; (2)
`throughthe slcin appendages; and (3) through the keratin bundles
`in SC [2,5].
`The lack ofcorrelation in transdermal permeation ofmolecules
`etn'oss species or from difibrmt application sites in the same
`anhnalmodelisduemainlytovariationsinsldn(chC)thickness,
`in the composition ofintercellular SC lipids and in the number of
`skin shaits. Netzlafi'et a]. [6] have shown that the amount of free
`fatty acids and triglycerides and the density of hair follicles are
`inmortrmt factors cursing differences between the skin barriers
`among species. As the majority ofmoleculss applied onto the skin
`permeatealongmeSChpiddomammeorganizationofthese
`regionsisveryimportantforthebanierfimcfionoftheskin.
`The SC lipid composition and organization difi‘tn' from that of
`other biological membranes, with long chain ceramides, free fatty
`acids, cholesterol and cholesteryl esters being the main lipid
`classes [2—433].
`To evaluate nansdermal absorption of a molecule, the most
`relevant mmnbrsnc is human skin. Skin from various sources,
`including cosmetic surgery and amputations, has been used for
`the in vino assessment of percutaneous penetration [9,10].
`However, its availability is limited and animal slrin is therefin'e
`fiequently used. A wide range of animal models has been sug-
`gestedas asuitsblereplaccment forhuman skinand hasbeen used
`to evaluate percutaneous permeation ofmolecules These include
`primates, porcine, mouse, rat, guinea pig and snake models
`Sincetheuse ofprirnstssin researchrs highlyrestrieued, the
`most relevant animal model for human skin is the pig. Porcine
`skin is readily obtainable horn abattoirs and its histological and
`biochemical properties have been repeatedly shown to be
`similar to human skin [11—15]. Porcine ear skin is particularly
`well-suited for permeation studies and gives comparable results
`to human skin. Studies examining tldckness of various skin
`layers have shown that the SC thickness in pigs is 21—26 pm
`[10,12] which is comparable to human skin [10,16]. The viable
`epidermis in porcine ear skin is 66—72. pm thick [10,12], which
`is very similar to the human epidermal thickness of70 um
`(shoulder) [17]. The follicular structtn'e of pig skin also resem-
`bles that of humans, with hairs and infimdr‘bula extending
`cepl; into the dermis. An average of 20 hairs are present per
`1 cm ofporcine our skin as compared 11314—32 hairs (except
`the forehead area) in humans [12]. Moreover, the vascular
`anatomy and collagen fiber arrangement in me dermis, as well
`as the contents of SC glycosphingolipids and ccramiden are
`similar in man and in the domestic pig [18].
`Due to its availability, skin of rodents (mice, rats and guinea
`pigs) is the most commonly used in in vim: and in vii-in per-
`cutancons permeation studies. The advantages of these animals
`are their small size, uncomplicated handling and relatively low
`cost. There are a number of hairless species (nude mice, hairless
`rats) in which the absence of hair coat mimics the human skin
`betterthanhairyslrin [l9]. [ndrese animalsthereisnoneedfor
`hair removal (clipping or shaving) prior to the expu'irnent, thus
`
`

`

`1154
`
`B. Godin, E. Ibur'l‘ou / Advanced Drug Delivery Reviews 59 (2007) 1152—1161
`
`avoiding the risk ofinjury to cutaneous tissue Other models have
`a disadvantage of an extremely high density ofhair follicles and
`require hair removal. Since both issues may meet percutaneous
`absorption ofmolecules, hairyrodent skinis usually not used in in
`vie-o permeation studies, although in vivo studies are still
`performed on these species. Among rodents, rat skin has more
`ah‘trdhnal similarities to human tissue (Table 1).
`Except for rat skin, rodent skin generally shows higher
`permeation rates than human skin [20—21]. Regarding the rat
`skin, permeation kinetic parametms are fi'equently comparable
`with hlnnsn skin.
`
`Snakesldnwasalsoproposedasamemhraneinskinper-
`meation experiments. Difl'erential scanning calorimetry (DSC)
`thermogramsandinfia—redm) spectrashowedthattheSCof
`snake, porcine and human skins have some similarities in
`structure andcornponents [2.2]. The distinguishing festureof‘die
`shed snake membrane is its lack of follicles.
`
`4. In vim: permeation across human skin vs. animal models
`
`Various studies have been carried out in. an attempt to
`correlate in vitro permeation data in animal and human skin.
`Some of them are reviewed here. Most of reports substantiate
`the value of the pig as an animal model for man in skin
`permeation studies. Singh et al. [23] evaluated skin permeability
`coefficients (K13) and SC reservoir of three hydrocarbons in
`porcine ear compared to hummi skin. They reported that pig
`skin was slightly more permeable to the substances with the
`ratiostporcineskinflCphturranskinofl.71,128 and HG for
`heptane, hexadecane and xylene, respectively. The permeation
`profiles ofheptane across human and porcine skin are presented
`in Fig. 1. SC binding ofthe hydrocarbons toporcine and human
`skins was also comparable. The skin pemreability (Kp) of
`nicorandil was investigated by Sato and co-authors [21] using
`excised skin samples from hairless mouse, hairless rat, guinea-
`pig. dog, pig, and human. Among the tested skins, the Kp values
`of nicorandil in pigs and humans were in good agreement. The
`authors also found that comparable porcine and human skin
`permeation could be attributed to similar surface lipids, barrier
`thickness, and morphological aspects of the excised pig skin
`samples and human tissue. In another series ofexperiments, the
`in vitro permeability ofpig ear skin was compared with human
`(abdominal) skin and rat (dorsal) skin using both hydrophilic
`(water, mannitol, paraquat) and lipophilic (aldrin, carbaryl,
`fluazifop-butyl) penetrants [13]. Pig skin was found to have a
`closer permeability character than rat skin to human skin,
`particularly for lipophilic penetrants. The authors suggested that
`electrical conductivity measurements across pig skin mern-
`branes could be a valuable tool for evaluating the integrity of
`
`Thole 1
`Thickness ofskinstrstsinnt. misc mdhtonsns [10]
`
`Rat
`Mouse
`Brunei:
`
`SC, tun
`lit
`9
`1‘?
`
`Epidemris, urn
`32
`29
`4?
`
`Whole akin, nun
`2.09
`0.10
`2.97
`
`I200
`
`§§§§
`
`§
`
`
`
`Manon:ofhepaticpenoratcd
`
`{enchant}
`
`0
`
`5
`
`It:
`
`Is
`
`2a
`
`is
`
`Time to]
`
`Fig. 1. In vitro permeation profiles of heptane stapes brunet: (squares) md
`porcine (months) skin (reproduced with permission from Ref [23]).
`
`membranes. Sekkat et al. [24] reported that difl‘erentially tape-
`stripped, porcine skin could serve as an in firm model for the
`evaluation of transdermal drug delivery to pranature neonates.
`In this study the passive permeation of caffeine, phenobarbital,
`and lidocaine and the iontophoretic delivery of lidocaine across
`tape-stripped porcine skin barriers were tested. The barrier
`function of the tissue was monitored by measuring the trans-
`epidermal water loss (TEWL). For all tested drugs, the per-
`meation behavior consisted well with the skin barrier fimction
`
`[24]. The results were sustained by a study on dismorphine is
`we've absorption inprernature neonates [25]. Iontophoretic lido-
`caine delivery was precisely controlled,
`independent of the
`barrier capability. Lin et al [22] compared in via-o penetration of
`theophylline, sodium diclofenac and benzoic acid through
`artificial cellulose membrane, animal skin (frog, snake with or
`without scales, nude mice, Sprsgue—Dawley rat and porcine)
`and human skin. The fastest permeation of subsmces was
`observed through cellulose membrane and frog skin and the
`slowest through human skin, with bmzoic acid being the fastest
`penchant
`through all skin types.
`In the case of sodium
`diclofenac the transderrnal permeation flux in porcine SC was
`33 times higherthan inintact skin, but in snake andhuman skin.
`the rate through SC was only 2.2 and 1.6 times higher than
`through intact ones.
`Afocusofsevemlreportswastocomparetransdennsl
`petmestionkinetics betweenrodent-andhumanskianashtdy
`byRoy et al. [26] permeability coefficients ofmorphine, fentanyl,
`and sufmtanil across full-thickness hairless mouse skin were in an
`
`order ofmagnitude higher than those found for human epidermis.
`There was no correlation between the enhancement in percuta—
`neous transport caused by SC removal in hairless mice and human
`epidermis. Another study examined permeation characteristics of
`hmnansk‘infiomvarious sites comparedto anirnalskins,and
`foundthatshedsnakesndhairlessratsldnshowedshnilm'
`
`pumeabflhymhummbresstmdthighskimwhfle Wistmratsnd
`nude mouse perfonned similarly to human cheek, neck, and
`inguinal skin [27]. Ravenzwaay at al [28] evaluated transport of
`compounds with various lipophilicities across rat and human
`sldnsinvirmandinfivoinratslnallcasestheinvr'oodermal
`
`penehnfionthroughrstskinwashigherthon invivaandratskin
`was approximately 11-fold more permeable than human skin.
`These outliers suggested the use of the following equation
`0003
`
`

`

`B. Good», E. Twines I driver-Iced Dngelr‘vay Reviews 59 (20039 1152—116!
`
`1155
`
`(Eq. (l))to estimatetransdennaltransportthroughhuman skin,
`basedonthecombincduseofinvimandinvirmdala:
`
`% Percutaneous absorptionm
`= %Percrnaneous absorption,“ X (Jhm/Jra)
`
`0)
`
`where Jis the percutaneous permeation flint.
`Inaseparate suldyevaluatinginvr'oopercutaneousabsorption
`of four antihypertensive drugs in mice and human cadaver skin,
`Ghoshctal. rcportcdthatthepcrmcationratcinmice skinwas
`much higher than that inhuman skin [29]. Van de Sandt et a]. [30]
`reported a mold-center skin permeation trial. comparing the to
`vice absorption of benzoic acid, cafi'eine, and testosterone com-
`pormds through human skin (nine laboratories) and rat skin (one
`laboratory) in ten European laboratories. All laboratories ranked
`the absorpdon ofbenzoic acid through human skin as the highest
`of the three molecules (overall mean flux of 16.54zk11.87 pg!
`cmth), whiletbe absomtion ofcaffeine and twtosteroue through
`human skin was comparable (2.24s: 1.43 and 1.63s 1 .94 pg!
`m2 X h, respectively). In this study, no difi‘erences were observed
`betweenthemeanabsorptionthroughhumansldnandtheonerat
`study for benzoic acid and testosterone, however for caffeine, the
`flmrvahreandthetotal quantitypermeetedacrosstheratsldnwere
`higher-thanthecorreqsondentvalues inhuman skin.
`
`5. The use of tissue culture-derived skin equivalents in
`nansdermal research
`
`A number of tissue culture derived skin equivalents such as
`living skin equivalent models (LSEs) and human reconstructed
`epidermis (HIRE) have been used to measure per-cutaneous
`absorption. These models generally are comprised of human
`cells grown as tissue culture and man-ht equivalents normally
`press! in skin, and are utilized as alternatives to animal skins.
`LSEs resemble human skin, having a dermis, epidermis and
`partially-difi'erentiated stratum corneum, but are deficient in skin
`appendages including pilosebaceous units, hair follicles and
`sweat glands [31]. These tissues provide much lower barrier
`properties than the whole skin due to their structure and lipid
`composition. For this reason, the kinetic parameters of skin
`permeation obtained when using LSEs usually highly overes-
`timatefluxacrosshumanskin.Forexample,inashrdyby
`Schmoolr et al., the permeation characteristics ofhuman, porcine
`andratskinswiththe Graflskin" LSEandtheSkinethic 9
`
`HRE models were compared using four low molecular weight
`dermatological drugs with various hydrophilicities [32]. The
`penneation of more hydrophobic compounds (clotrimazole and
`terbinafine) through the skin equivalents resulted in an 800—900
`fold higher flux than through split-thickness human skin. 0n the
`other hand, transdermal flux. of a less hydrophobic compound,
`salicylic acid, was in the same order of magnitude as fluxes
`obtained svith human skin. In this study porcine skin pater-med
`as the most appropriate model for human skin and they
`concluded that reconstituted skin models are not suitable for in
`
`vino penetration smdies [32]. A similar conclusion was drawn
`from results of another study in which Roy et a1. [33] evaluated
`the in vino permeabilities of alkyl p-aminobcnzoates through
`0004
`
`LSEandhumancadaverskin. lnthecaseofcadaver sldn, the
`permeability coefficient increased as the carbon chain length
`increased. However, this relationship was not observed in the
`permeability coeficients of these esters across LSE. Moreover,
`LSE showed very low resistance to that compared to cadaver
`skin as the permeability coefficients ofthese esters tinough LSE
`were an order of magnitude hith than through cadaver skin.
`Ontheodinhmdnummousmportssuppofiflieuseofsldn
`equivalents for evaluation of skin irritation [31,34]. In a study by
`Monteiro-Riviere and colleagues [35], EpiDerm LEE 3 was
`found to be morphologically and biochemically comparable to
`nouns] human epidermis, providing a model in toxicological and
`skin metabolism studies. Ponce and Kempenaar [36] reportedthat
`architecture, homeostasis and lipid composition of reconstructed
`lnnnanskinmodelsIIEpiDeerSkinEdiic @,Fpiskin®sndRE-
`[JED in) were comparable to native human tissue his noteworthy
`that Colipa, the European I‘rade Assocation for cosmetic and
`toiletry industry, recommends the use of in vim) reconstructed
`skin equivalents as the preferred testing model for slrin irritation
`studies [34]. However, the overall use of skin cultures is likely
`tobelimitedduetoquestionableperl‘onnanceasabarrierin
`skinpermeation studies, aswell asdue totheircostsnddata
`reproducibility.
`
`6. In vitro skin permeation studies focusing on delivery
`systems
`
`Correlation of permeation between animal and human skin
`studies fi'om drug delivery systems and pharmaceutical dosage
`format has sweeter! significant attention from the pharmaceu-
`tical industry, academia, and regulatory sectors. Design and
`optimization of carriers for active rag-ts is a time- and resotuce-
`consuming process that is an integral part ofthe development of
`any drug delivery system. In vim) tests reflecting bioavailability
`data are required to prove that a new delivery carrier is bio-
`equivalent with or superior to the standard. Mechanistic studies
`with sophisticawd carriers are performed in animal and human
`skin to try to predict the firture performance ofthe drug delivery
`systems inhumans from in vino data.
`Among the drug delivery systems tested were carriers based
`on chi-cal skin permeation enhancers, specially designed
`vesicles, physical and niicroinvssive techniques. Touitou et al.
`[37] tested transport oftetrahydrocannabinol from an enhancing
`carrier containing 10% wiw oleic acidfpropylene glycol!
`polyethylene glycol 4000!ethanol mixture. In this study drug
`permeation across Sabra-strain rat skin was found to be about
`12.8-fold higher than across human skin. Difi'ering lag times,
`11.5 vs 8.5 h for the rat and human skin, respectively,rnay point
`toward difi'erent diffusion pathways for this drug across the skin
`ofthese two species. Priborsky and Muhlbacllova [38] assessed
`the effect of chemical permeation enhancers on the in-vr‘tro
`transport across human skin as compared to animal models. Rat
`skin was ~3.3—4 times more permeable than human tissue.
`Using rat skin, the least potent enhancer was dimcthylsulph-
`oxide and the maximum permeation enhancement was observed
`with sodium latn'ylsulphate. In contrast all the tested enhancers
`performed comparably to human skin. In this study, human and
`
`

`

`1156
`
`B. Godin, 5'. Ibnirou / Advanced Drug Delivery Retreats 59 [2007) 1152—1161
`
`guinea-pig skins were not significantly different in the per-
`meation of Numethyl-2-pyrrolidone. In another study, transder-
`mal delivery of 6-beta—naltrexol,
`the active metabolite of
`naltrexone, across human slain and guinea pig skin in vitro
`and in hairless guinea pigs in viva was assessed fi'om a
`propylene glycol} bufi'ermixture [391.111 w‘rm flux ofnsltrexone
`Was about 2.3 and 5.6 times higher than 6-beta-naltrexol across
`guinea pig and human skin, respectively, and 6-beta-naltreatol
`lag times were longer in both skin types (Fig. 2). In vivo studies
`in guinea pigs showed that the steady-state plasma level of
`naltrexone was twofold greater than 6-betar-nalh‘exol, which
`correlated well with in vitro data in guinea pig skin. Rigg and
`Barry [40] investigated the skin permeability of two species of
`snake (Elaphe obsolete, Bikes mallow) compared to is vice
`experimental results for human skin and for hairless mouse.
`The effect of typical enhancers on the permeabilities of the
`membranes to a model penetrant 5-fluorouracil (S-FU) was
`evaluated. The studied enhancers were 3% Arcane in Tween 20!
`
`humestingly, the amount oftirnolol transported drning iontopho-
`reels (2 h) was significantly difl'm'eut among the various skin
`species, but the final quantity of Iirnolol crossing the slain during
`24 h (2 h iontophoresis and 22 h post-iontophorelic passive
`diffusion) was comparable'to the difi'crent species. According to
`this data, iontophoresis may diminish into-species variafions in in
`vitro skin permeation studies. Micruinvasive techniques (micro-
`needles, RF skin ablation, etc.) represent another means of skin
`permeation enhancem-t. Recently Wang et al.
`[42] imaged
`infusion of dye molecules, insulin, polymer microparticles, and
`cells into the skin by brigbtfield and fluorescence nficroscopy
`following the insertion of hollow glass micronwdles into hairless
`rat skin in vivo and human cadaver skin is vice. Studying the
`flow mechanism the authors reported that using both models,
`partial moon ofme needle by withdrawing 100—300 1.1 or
`vibrating the microneedle array dramatically increased infirsion
`flow rate.
`
`saline, propyI-e glycol (PG), 2% Azone in PG, and 5% oleic
`acid in PG. The data from snake membranes showed minor
`
`7. Animal models for evaluation of skin absorption in
`humans: molecules
`
`cfiects of the enhancers, while for hairless mouSe skin, the
`
`enhancer effects were significant. None ofthe membranes was a
`completely reliable model for human percutaneous absorption
`in assessing the effect of skin permeation enhancers. The
`authors concluded that human sldn should be used in skin
`
`permeation studies and not hairless mouse or snake skin;
`otherwise, misleading results may be obtained
`Kanilrlrannan and colleagues [41] evaluated the effect of
`species variation (rat, rabbit, mouse, guinea pig and human) on
`the transdeflual iontophoretic permeation of timolol maleate.
`
`In studies conducted in the 1970s and 1980s, nansdermsl
`absorption of various radio-labeled molecules in human volun-
`teers and animals was assessed [43—45]. In these studies, the same
`concentration ofsubstance (4 pg/cm’) was applied on the forearm
`of subjects in an attempt to standardize the application conditions,
`and percutaneous absorption was quantified by following the
`excretion ofthc nacerforS days. Barteketal. [45] undertooka
`comparative study of percutancous absorption of halopmgin,
`acetylcysteirr, cortisone, cafi'eine and testosterone in m in
`various animal species (rats, rabbits, miniature swine) and
`humans. The highest extent of percutaneous absorption was
`observed with haloprogin, with complete absorption in rats and
`rabbits but not in humans and pigs. In rats and rabbitsthe absorbed
`fraction of applied dose followed the order: acetylcys-
`teinécmfisone<caffeine=testostemne<halopmgm 15: WW data
`fi'ommanandpigsindicstedthatthe orderofabsorptiouwas:
`acetylcystein<corlisone<haloprogin<testosterone<cafi'eine.
`The authors concluded that the transdermal absorption in rats and
`rabbits was not predictive for human data, while results obtained
`in porcine model and humus were comparable.
`Using the same mohnique, Wester and Marbach [46,47] com-
`pared the percutaneous absorption of various molecules between
`rhesus monkey and humans. They found that the in viva
`perentaneous absorption of hydrocortisone,
`testosterone and
`benzoic acidwas similar furthesus monkeyandman. For exarrqale,
`when hydrocottisone, testosterone sndbenzoic acid were applied at
`a dose of4 rig/c1112, the absorbed dose was 2.9, 18.4 and 59.2% vs.
`1.9, 13.2 and42.6% inmonkeyvsmmnami, respectivelyflrrmaugh
`sndMaihach[48]messuredthepercutaneous absmption extentof
`five nitroaromatic compounds (p-nitrornu'line, 4-amino-2-nilruphe-
`no], 2,4—dinitrochlorobenzene, 2—nitro-p-phenylenediamine, nitro—
`benzme) inhumans andmonloeysushrghofirr‘nvion andr'n vim
`mohfiqumhwasfoundmatexceptformehiglflyvolafilc
`nih’cbenzene, no significant difl‘crmces were observed in the four
`groupsofdata. Andersenerai.,usedthesamemethodology
`to evaluate percutaneous absorption of 1“C ring-labelled
`0005
`
`'
`
`40
`
`50
`
`A g
`
`500
`400
`
`Em
`E .—
`3-2 300
`5.5:, 200
`II
`.5
`100
`D
`
`E3
`
`5
`
`o
`
`10
`
`30
`20
`11rne{I1}
`
`
`
`0
`
`1o
`
`so
`20
`Time (b)
`
`40
`
`so
`
`B'U
`
`s E
`
`500
`400
`g:
`= ‘- 3m
`3'3
`E5. 200
`E
`100
`.E
`
`g
`3
`
`o
`
`Fig. 2. mm mnount of attitudes (squares. n-7) and 6-bets-naltrexol
`(rhomha,n=fl) permeatedflrmugbthebrnnanslrin(A)andglnneapigsldn(B)
`(reproduced with pumission fi'om Ref. [39]).
`
`

`

`3. MI, E. Tacitus [Warned Drug Delivery Review 59 (2007) 1152—116!
`
`1157
`
`
`
`hydrocortiaonefimtostcroneandhenzoicacidinvmainguineapigs
`andcomparetheobtainedtesul‘tsmpreviousbtunan data[49].'Ihe
`absorpfionofhydroeortisoneandbenzoicaeidwas similartothe
`pubfishedhummabsomfimdmbutmostemnewasabsosbedto
`agreatcrextentinguineapigsthanmmm Interestinglyginmis
`work a thioglycollate based depilatmy cream significantly
`inmeased the extmt of transdennal permeation oftestosterone.
`Although the above studies [43—49] used mdiolabeled molecules
`(Whose Weakness is the accurate detection of the original
`cmnpmind), the clear advantage of these early works was their
`abilitytocompareskinabsorptionofalarge series ofmolecules
`using the same expaimeotal protocol.
`later reports used more advanced analytical methods for
`evaluation and comparison of percutaneons absorption in
`animals and humans. Wester et a]. [50] employed inductively
`coupled plasma—mass spectrometry for quantitation of biolog-
`ical samples of boric acid, horas and disodium ocmborate
`tetrahydrate after-their applicationonthe skin. They compared
`the usefulness of finite and infinite dose permeation method-
`ologies across human skin to absorption data in humans. The
`results from the finite dose model were much closer to the in
`
`viva absorption data, while the infinite dose methodology
`difi’ered by 10-fold from the in viva results. Cnubben and
`colleagues [5]] measured the pet-cutaneous absorption of ortho-
`phenylphenol, a fimgicide, in rats, humans and a perfilsed pig
`ear model. The drug was applied in a hydroethanolic vehicle
`and samples from in viva studies were evaluated using capillary
`gas chromatography with MS detector. In viva results indicated
`that in human volunteers, approximately 2?% of the applied
`dose was excreted with urine within 48 h versus 40% excreted
`
`inrats.Amongtheinvinaparameterstested.thefiaetionof
`applied dose most accurately predicted human in viva
`percutaneous absorption of the drug (Fig. 3). With respect to
`the other parameters studied, considerable differences were
`observed between the various in vitro models.
`
`Skin permeation studies using inadequate protocols will
`generate inaccurate data. Currently used sunfilters are lipophilic
`substances with relatively low molecular weight, thus posses-
`sing a good potential to be systemically absorbed across the
`skin. In fact, fora long period oftime scientists have been aware
`of the issues of potential toxicity caused by the percutaneous
`absorption of chemical sunscreens. Recently these cone-s
`have been confirmed in numerous reports [52—54]. However,
`the experimental conditions, such as a hydrophilic receiver fluid
`thatisusedinmanyr’nvia-oskinpenneationexperimentswflh
`sunscreens, generally do not permit a good clearance of these
`molecules from the sldn. For example, one study compared the
`skin penetration of henzophenone-3 (EPI-l), ethylhexyl meth-
`oxycinnamate, butyl memoxydibenaayl methane, ethylhexyl
`salicylate and homoaalate, from two vehicles, an oil-in-warer
`(OIW) emulsion gel and petrolatum jelly, both in vitra and in
`viva. The receptor fluid used in in ultra experiments was saline
`containing 1.5% BSA and at these conditions none oftihe filter
`agents permeated through the skin andnegligihle amounts were
`detectedin various skin layers after 6 h of product application
`Also the effect of the vehicle was minimal in the in vitro
`
`permeation experimental setup. 0!: the other hand,'in humans
`0006
`
`B
`
`8
`
`warrant] 8
`
`CUMUMWpenetration
`
`
`
`2
`
`
`he
`
`«I
`
`IDQ’
`
`+Ratepl¢mis
`
`+Humanapidom|is
`
`'
`
`Pmmantulskin
`
`+ Rululskln
`
`+ Musad 9b oar
`
`I Hot [LII sun
`
`II
`
`Flat splnanrls
`
`0 Human lull skln
`
`n Human solder-mks
`
`‘3
`
`a
`
`o Mummers:
`
`
`sauna
`9AM“ assume-um»
`a;
`m
`
`7 Hal
`
`Fig. 3. Translation] absorption at ["Clortho—phenylphmol, in vino-in viva
`correlation:
`(A) Cumulative amount. of [“Cjtortho-phmylphenol
`(11:6)
`permeated in vino through human viable skin, rat viable skin, hum epidermal
`membrmm,ratepidermalmembrenesandperfirsedpigesrs;(BJFactorof
`elem (POD) between in mm and in viva skin absorption of ['tcmo.
`phenylphenol based on the systerrlieally available (3A.) moron tit-4, B, 24, and
`48 haflara4—hmrpostneperiod ofl20uglern2,drepermeshility coeficient
`(Kp),and1hepotentiallyebsmheddose fPA)inhumsnsandrats(repr-oduced
`with permission thorn Ref. [Sl]).
`
`the amount of sun filtering agents accumulated in the SC was
`significantly higher (aromd 3 times) with the OIW emulsion gel
`than with the petroleum jelly, which was reflected also in SPF
`measured in viva 30 min after application offhe products [55].
`Yetwhenmappropfiamreceiverflrfidwasusedalarge
`amount. [9% from the applied dose] of oetyhnethoxycinnamate
`(0M0) permeated the skin [56]. In this study, the OMC skin
`plneation flux was 27 uglcnrzh. It is importantto keep inniind
`that sunscreen formulations are applied to a large skin area
`(>15 m2) and for a long period, producing a constant and high
`input of the chemical into the viable skin strata and to the
`systemic circulation. These in vim; results are supported by data
`from a number of human and animal in viva studies. Hayden
`et al. [57] reported that BPH has been detected in human mine
`and in breast milk, with up to 1-2% ofthe applied arnmmt
`estimated to be absorbed into the body. 'I‘reflhl and Gabard [58]
`have shown significant mounts of BPH, OMC, and octylsa-
`licylste recovered from tape-stripped strat