Principles and Practice of
`
`
`
`WILEY
`
`0001
`
`Noven Pharmaceuticals, Inc.
`EX2008
`Mylan Tech., Inc. v. Noven Pharma., Inc.
`IPR2018-00173
`
`

`

`Principles and Practice
`of Skin Toxicology
`
`Editors
`
`Robert P. Chilcott
`
`Chemical Hazards and Poisons Division, Health Protection Agency, Chilton, UK
`
`and
`
`Shirley Price
`
`School of Biomedical and Molecular Sciences, University of Surrey, UK
`
`John Wiley & Sons, Ltd
`
`0002
`
`

`

`Copyright © 2008
`
`John Wiley & Sons Ltd, The Atrium, Southern Gate, Chichester,
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`Library ofCongress Cataloging-in-Publication Data
`
`Principles and practice of skin toxicology / editors, Robert P, Chilcott,
`Shirley Price.
`p-;cm.
`
`Includes bibliographical references and index.
`ISBN 978-0-470-51172-5
`
`I. Chilcott, Robert P. II. Price, Shirley, Dr.
`1, Dermatotoxicology.
`{[DNLM:1. Skin Physiology. 2. Skin Absorption, 3. Skin Diseases. WR
`102 P957 2008]
`RL803.P75 2008
`615'.778 —dc22
`
`2008002901
`
`British Library Cataloguing in Publication Data
`
`A catalogue record for this bookis available from the British Library
`
`ISBN 978-0-470-51172-5
`
`Typeset in 10/12 Minion by Laserwords Private Limited, Chennai, India
`Printed and boundin Singapore by MarkonoLtd
`This book is printed on acid-free paper responsibly manufactured from sustainable forestry
`in which at least twotrees are planted for each one used for paper production.
`
`0003
`
`

`

`Dedications
`
`(RC) For all my teachers, lecturers and professors. Especially the ones that
`were left in despair.
`
`For Emlyn Evans and Trefor Pedrick. True gentlemen of knowledge.
`
`For all of my family.
`
`For the young ladies in my life:
`
`Caroline, Florence Megan and Charlotte Rose.
`
`(SP) For my partner in crime, Rob Chilcott, the more verbose member of the
`partnership, and Carolyn, for her patience
`
`For Pete, my husband, and for Jessica and Jonathan for their patience
`during the editing of this document
`
`For my mentors who taught me the essence of Toxicology - I am still
`learning!!
`
`0004
`
`

`

`Contents
`
`Foreword
`
`Preface
`
`Acknowledgements
`
`List of contributors
`
`PART I Introduction
`
`1 Cutaneous anatomy and function
`Robert P. Chilcott
`
`1.1
`1.2
`1.3.
`1.4
`
`Introduction and scope
`Surface features
`Functional histology of the epidermis and associated structures
`Species differences
`Summary
`References
`
`2 Biochemistry of the skin
`Simon C. Wilkinson
`
`2.1
`2.2
`2.3
`2.4
`2.5
`
`2.6
`
`Introduction and scope
`Protein synthesis and organisation during epidermal differentiation
`Lipid synthesis and organisation during epidermal differentiation
`Lipid classes in the stratum corneum
`Stratum corneum turnover
`
`Biotransformations in skin
`
`Summary
`References
`
`0005
`
`xix
`
`xxi
`
`13
`
`15
`
`15
`
`17
`
`17
`
`18
`
`19
`
`20
`
`23
`
`24
`
`42
`
`42
`
`

`

`viii
`
`3
`
`CONTENTS
`
`Skin photobiology
`Mark A. Birch-Machin and Simon C. Wilkinson
`
`3.1
`3.2
`3.3
`
`Introduction and scope
`Photoprotection and melanogenesis
`Increased environmental ultraviolet radiation exposure and its link with
`photoageing and skin cancer
`3.4 Mitochondrial DNA as a biomarker of sun exposure in human skin
`3.5
`Apoptosis
`3.6
`Sun protection
`Summary
`References
`
`PART II Skin Absorption
`
`4
`
`Skin as a route of entry
`Simon C. Wilkinson
`
`4.1
`
`4.2
`4.3.
`4.4
`4.5
`4.6
`4.7
`4.8
`
`Salient anatomical features of the stratum corneum — the ‘brick and mortar
`model’
`
`Species and regional variation in skin structure
`Species and regional variation in skin permeability
`Intra- and inter-individual variation in percutaneous absorption
`Effect of age on skin barrier function
`Role of skin appendages
`The in vitro skin sandwich model
`Penetration of particles through appendages
`Summary
`References
`
`5
`
`Physicochemical Factors Affecting Skin Absorption
`Keith R. Brain and Robert P. Chilcott
`
`5.1
`5.2
`5.3.
`
`Introduction
`Physicochemical properties
`Exposure considerations
`Summary
`References
`
`6 Principles of Diffusion and Thermodynamics
`W. John Pugh and Robert P. Chilcott
`
`0006
`
`51
`
`51
`
`51
`
`55
`
`60
`
`61
`
`63
`
`65
`
`65
`
`69
`
`71
`
`71
`
`72
`
`74
`
`75
`
`76
`
`77
`
`78
`
`79
`
`80
`
`80
`
`83
`
`83
`
`84
`
`89
`
`91
`
`91
`
`93
`
`

`

`CONTENTS
`
`ix
`
`93
`Introduction and scope
`6.1
`94
`Some definitions pertaining to skin absorption kinetics
`6.2
`97
`Basic concepts of diffusion
`6.3.
`97
`Fick's Laws of diffusion
`6.4
`98
`Thermodynamic activity
`6.5
`99
`Skin absorption of a substance from two different vehicles
`6.6
`
`6.7._—Partitioning 101
`6.8
` Diffusivity
`102
`6.9
`Skin absorption data and risk assessments
`105
`Summary
`106
`References
`106
`
`7
`
`In vivo measurements of skin absorption
`James C. Wakefield and Robert P. Chilcott
`
`109
`
`7.1—Introduction and scope 109
`
`7.2 Why conductin vivo studies?
`110
`7.3.
`Ethics and legislation
`110
`7.4
`Standard methodology: OECD Guideline 427
`115
`7.5.
`Alternative in vivo methods
`119
`
`Summary
`References
`
`8
`
`Invitro percutaneous absorption measurements
`Ruth U. Pendlington
`
`8.1
`8.2
`8.3
`8.4
`8.5
`
`8.6
`
`Introduction and scope
`Regulatory guidelines
`Why assess percutaneous absorption in vitro?
`Basic principle of in vitro percutaneous absorption measurements
`Choice of diffusion cell
`
`Skin membrane considerations
`
`8.7
`Integrity measurements
`8.8
`Choice of receptor fluid and sampling considerations
`8.9
`Test material considerations
`8.10 Application of test preparation to the skin
`8.11
`Examples of results from in vitro skin absorption studies
`8.12 What is considered to be absorbed?
`
`8.13 Micro-autoradiography
`Summary
`References
`
`0007
`
`126
`126
`
`129
`
`129
`129
`130
`131
`131
`
`136
`
`137
`138
`139
`140
`142
`146
`
`147
`147
`147
`
`

`

`x
`
`CONTENTS
`
`PARTIII Toxicological Assessment
`
`9
`
`Skin immunology and sensitisation
`David A. Basketter
`
`9.1
`
`9.2
`
`9.3
`
`9.4
`
`9.5
`
`9.6
`
`9.7
`
`Introduction
`
`Definitions
`
`Skin sensitisation
`
`Identification of skin sensitisers
`
`Risk assessment
`
`Other types of allergic skin reaction
`Future prospects
`Summary
`References
`
`10 In vitro phototoxicity assays
`Penny Jones
`
`10.1
`
`10.2
`
`10.3
`
`10.4
`
`10.5
`
`10.6
`
`Introduction and scope
`In vitro strategies for phototoxicity testing
`The UV/visible absorption spectrum as a pre-screen for phototoxicity
`In vitro assays for phototoxicity using monolayer cultures
`In vitro assays for photoallergenicity
`In vitro assays for phototoxicity using human 3-D skin models
`Summary
`References
`
`149
`
`151
`
`151
`
`151
`
`152
`
`155
`
`160
`
`163
`
`164
`
`164
`
`165
`
`169
`
`169
`
`169
`
`171
`
`172
`
`174
`
`177
`
`181
`
`181
`
`11 In vitro alternatives for irritation and corrosion assessment
`
`185
`
`Penny Jones
`
`11.1
`
`T12
`
`13:3
`
`11.4
`
`11.5
`
`11.6
`
`Introduction and scope
`Acute dermal irritation/corrosion
`Validation/regulatory status of in vitro assays for skin corrosion
`In vitro tests for skin corrosion
`Validation/regulatory status of in vitro assays for skin irritation
`In vitro tests for skin irritation
`
`Summary
`References
`
`12 Instruments for measuring skin toxicity
`Helen Taylor
`
`12.1
`
`Introduction and scope
`
`185
`
`185
`
`186
`
`188
`
`194
`
`195
`
`197
`
`198
`
`201
`
`201
`
`0008
`
`

`

`CONTENTS
`
`Skin surface pH
`Biomechanical properties
`Sebum
`
`Skin surface contours
`
`Thickness
`
`Desquamation
`Applications and measurement of transepidermal water Loss
`Guidance for TEWL measurements
`Hydration measurement
`Guidance for hydration measurements
`Relationship between hydration and dermal toxicity
`Colour measurement
`
`12.2
`
`12.3
`
`12.4
`
`12.5
`
`12.6
`
`12.7
`
`12.8
`
`12.9
`
`12.10
`
`12.11
`
`12.12
`
`12.13
`
`12.14
`
`12.15
`
`Measurementof vascular perfusion
`A final word of caution
`
`Summary
`References
`
`PART IV Clinical Aspects
`
`13 Introduction to dermatology
`Manjunatha Kalavala and Alex Anstey
`
`13.1
`
`13.2
`
`13.3
`
`13.4
`
`Introduction and scope
`Clinical assessment of patient with skin disease
`Cutaneous manifestations of disease following exposure to chemicals and
`pharmaceutical formulations
`Overview of standard treatments
`
`Summary
`
`14 Clinical aspects of phototoxicity
`Anthony D. Pearse and Alex Anstey
`
`14.1
`
`14.2
`
`14.3
`
`14.4
`
`Introduction and scope
`UV-induced skin reactions
`Phototoxicity (photoirritancy) reactions
`Photosensitive reactions
`
`Summary
`References
`
`15 Occupational skin diseases
`Jon Spiro
`
`0009
`
`xi
`
`202
`
`204
`
`205
`
`205
`
`205
`
`205
`
`206
`
`208
`
`209
`
`212
`
`213
`
`213
`
`215
`
`216
`
`217
`
`217
`
`221
`
`223
`
`223
`
`224
`
`234
`
`241
`
`243
`
`245
`
`245
`
`247
`
`247
`
`251
`
`256
`
`256
`
`259
`
`

`

`xii
`
`15.1
`
`15.2
`
`15.3
`
`15.4
`
`15.5
`
`15.6
`
`15f
`
`15.8
`
`15.9
`
`15.10
`
`CONTENTS
`
`Introduction and scope
`Dermatitis
`
`Development of occupational dermatitis
`Patterns of occupational dermatitis
`Incidence of occupational dermatitis
`Effects of dermatitis on work
`
`The outlook in occupational dermatitis
`Identification of occupational dermatitis
`Other occupational skin disorders
`Investigation of a case of dermatitis at work
`Summary
`References
`
`16 Prevention of occupational skin disease
`Chris Packham
`
`16.1
`
`16.2
`
`16.3
`
`16.4
`
`16.5
`
`16.6
`
`16.7
`
`16.8
`
`16.9
`
`Prevention of occupational skin disease
`Defining the problem
`Material safety data sheets
`Chain of responsibility
`Managing dermal exposure
`Selection and use of personal protective equipment
`Protective or ‘barrier’ creams: do they have a role?
`The role of education and training
`Conclusions
`Summary
`References
`
`PART V Regulatory
`
`17 Occupational skin exposures: legal aspects
`Chris Packham
`
`17.1
`
`Li2
`
`17:3
`
`17.4
`
`17.5
`
`17.6
`
`Introduction and scope
`Brief overview of current United Kingdom legislation
`The employer's perspective
`Hazard identification
`
`Risk assessment
`
`Gloves: a note of caution
`
`Summary
`References
`
`0010
`
`259
`
`260
`
`263
`
`264
`
`265
`
`265
`
`266
`
`266
`
`267
`
`270
`
`276
`
`276
`
`279
`
`279
`
`280
`
`282
`
`283
`
`284
`
`289
`
`294
`
`294
`
`294
`
`294
`
`294
`
`297
`
`299
`
`299
`
`300
`
`303
`
`304
`
`306
`
`309
`
`310
`
`310
`
`

`

`CONTENTS
`
`18 Safety assessment of cosmetics: an EU perspective
`Jo Larner
`
`Introduction and scope
`18.1
`18.2 Overview and scope of Cosmetics Directive 76/768/EC
`18.3 Overview of the requirements of the EU Cosmetics Directive
`18.4 Scientific advice
`
`Influence of other legislation
`18.5
`18.6 Adverse effects from cosmetics
`18.7 Toxicity of cosmetic ingredients
`18.8 The safety assessment
`18.9
`A final consideration
`
`Summary
`References
`Appendix 18.1 Additional obligations for cosmetic suppliers
`
`19 Regulatory dermatotoxicology and international guidelines
`Adam Woolley
`
`19.1
`Introduction
`19.2 Regulatory context
`19.3
`Product groups and the human context
`19.4
`Dermal toxicology with the different product groups
`19.5
`Factors in dermal toxicity
`19.6 Repeat dose dermal toxicology
`19.7 Classic short-term dermal toxicity studies
`19.8
`Pragmatic considerations
`Summary
`References
`
`20 Glossary of main terms and abbreviations
`James C. Wakefield
`
`Index
`
`xiii
`
`311
`
`311
`
`312
`
`315
`
`316
`
`317
`
`318
`
`320
`
`326
`
`328
`
`329
`
`329
`
`330
`
`333
`
`333
`
`334
`
`335
`
`336
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`338
`
`339
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`341
`
`344
`
`345
`
`345
`
`347
`
`358
`
`0011
`
`

`

`5 Physicochemical Factors
`Affecting Skin Absorption
`
`Keith R. Brain’ and Robert P. Chilcott?
`
`‘Welsh School of Pharmacy, Cardiff University, CF10 3XF and An-eX, Capital Business Park,
`Cardiff, CF3 2PX, UK
`Chemical Hazards and Poisons Division, Centre for Radiation, Chemical and Environmental
`Hazards, Chilton, Oxfordshire OX11 ORQ, UK
`
`Primary Learning Objectives
`
`e Relating the concept of ‘dose’ to skin absorption.
`e Identifying the major physicochemical determinants ofskin absorption and a consideration of
`how other modulatingfactors (principally exposure conditions) can affect dermalabsorption.
`
`Introduction
`5.1
`
`
`The primaryfactor that dictates the percutaneoustoxicity of a chemicalis its ability to penetrate the
`skin. This dictum, whilst relatively straightforward, is complicated bythe fact that there is a myriad
`of factors which can affect skin absorption.
`
`The central dogmaoftoxicology was formulated by Paracelsus(a.k.a. TheophrastusPhillippus
`Aureolus Bombastus von Hohenheim; Box 5.1), who understood that it is the dose of a
`chemical which ultimately dictates its toxicity; “sola dosis facit venenum’ (Oser 1987). Thus,
`factors which influence skin absorption (and thus ‘dose’) are necessarily factors which affect
`percutaneoustoxicity: a toxic chemical which is unable to traverse normal skin will not be
`toxic via the percutaneousroute. For example, botulinum toxinis one of the most poisonous
`substances known. However,it does not penetrate healthy skin andsois essentially non-toxic
`following skin contact.
`Skin absorption of chemicals is a passive process. Unfortunately, this does not mean that
`the process of dermal absorption is simple and highly predictable, as there are a diverse range
`of factors that can affect the rate and extent to which a chemical is absorbed. These include
`(amongst others) vehicle effects (Hilton et al. 1994), ageing (Roskos and Maibach 1992), race
`(Kompaoreet al. 1993), gender (Bronaughetal. 1983), disease (Moon and Maibach 1991),
`
`Principles and Practice of Skin Taxicalogy Edited by Robert P, Chilcott and Shirley Price
`© 2008 John Wiley & Sons, Ltd
`
`0012
`
`

`

`
`
`84
`
`CHO5: PHYSICOCHEMICAL FACTORS AFFECTING SKIN ABSORPTION
`
`Box 5.1 Paracelsus
`
`Paracelsus, also known as Theophrastus Phillippus Aureolus
`Bombastus von Hohenheim (circa 1493 — 1541), was a largely
`self-taught polymath who recognised the dose-response
`relationship which is an underpinningprincipal of
`moderntoxicology.
`
`A somewhatinteresting character, he roamed Europe, north
`Africa and parts of Asia in his pursuit of alternative medical
`knowledge. His published works, personal activities and
`outspokencriticism of contemporaneous medical practices
`
`
`did not particularly endear him to his peers!
`
`A full-colourversion of this figure appearsin the colour plate section of this book
`
`chemical damage (Wahlberg 1972), lipid content (Elias 1981), hydration (Behl et al. 1980),
`pH (Allenby et al. 1969), stress (Denda et al. 2000) and physicochemical properties of the
`penetrant (Lien et al. 1973); one could even imagine that the services of an astrologer may
`be a useful adjunct to predicting skin absorption! However, the problem can be simplified
`by considering just a relatively small number of factors which exert the greatest influence
`over skin absorption. The mostpertinent to dermal toxicology include the physicochemical
`properties of the penetrant and the exposure conditions.
`Other important factors influencing percutaneous absorption(e.g. effects of metabolism,
`species and regional variation, thermodynamic factors and methodological considerations)
`are considered in Chapters 2, 4, 6 and 8, respectively. A number of authoritative texts can
`also be consulted (Bronaugh and Maibach 1999; Schaefer and Redelmeier 1996; Wester and
`Maibach 1983).
`
`Physicochemicalproperties
`5.2
`
`
`A major determinant of skin absorption relates to the physicochemical properties of the applied
`chemical, in particular, size, solubility, charge and hydrogen bonding capacity. An understanding
`of these factors can allow an approximation of the extent to which a given chemical will cross the
`stratum corneum.
`
`The primary factors affecting skin absorption are concerned with the physicochemical
`properties of the penetrant. The most important physicochemical parameters are arguably
`molecular weight, solubility, charge and hydrogen bonding capacity. A basic understand-
`ing of these relatively simple factors will enable even the least experienced toxicologist
`to be able to make a reasonable judgement as to the dermal bioavailability of a given
`chemical.
`
`0013
`
`

`

`5.2: PHYSICOCHEMICAL PROPERTIES
`
`85
`
`5.2.1 Molecular weight
`
`As a general rule of thumb, chemicals with a molecular weight greater than ~500 Da do not
`penetrate the skin. This is knownasthe ‘rule of 500’ (Bos and Meinardi 2000). This upperlimit
`on molecular size mainly results from the physical arrangementoflipids between adjacent
`corneocytes of the stratum corneum (see Chapters 1 and 2). However, there is some evidence
`to suggest that large, linear, ‘wiggly’ molecules (such as heparin and DNA) maybeable to
`traverse the stratum corneum, albeitin relatively small quantities.
`
`5.2.2 Solubility
`
`The solubility of a chemical is commonly quantified in terms of how it partitions between
`two immiscible liquids, such as water and ether. The more common measureofsolubility
`is the octanol—water partition coefficient (Log P, also known as Kow). The Log P value can
`be experimentally derived (Figure 5.1) or estimated using commercially available computer
`software'. Clearly, an experimentally derived value represents the gold standard. The value of
`Log P is calculated using Equation (5.1).
`
`Rio = bee i|
`
`[water]
`
`(5.1)
`
`Where [octanol] and [water] represent the concentration of a chemical in octanol and water,
`respectively.
`Being hydrophobic, octanol represents a lipophilic environment. In contrast, water is
`(perhaps rather obviously!) a hydrophilic environment. Thus, the tendency of a chemical
`to partition into octanol rather than wateris reflected in a positive Log P value, whereas
`preferential partitioning into the water phaseresults in a negative value (Table 5.1). An equally
`amphiphilic compound would have a LogP of 0.
`The relationship between solubility and the rate of skin absorption stems primarily from
`the ability of a chemical to partition into the stratum corneum. If a chemical is excessively
`hydrophilic, it will not partition into the predominantly lipid environment of the stratum
`corneum (Chapters 1 and 4). In contrast, if a chemical is too strongly lipophilic, it will
`readily partition into the stratum corneum butwill not partition out into the predominantly
`hydrophilic environmentof the underlying epidermal tissue. Put simply, it will remain stuck
`within stratum corneum. Thus, in order to penetrate the skin, the solubility of a chemical
`requires a balance between these two extremes. In general, a Log P of between 1 and 3 is
`considered to be optimalfor skin absorption (Figure 5.2).
`The importance of lipophilicity and molecular size on skin permeation has been well
`established and incorporated into a series of models, the best known of which is the ‘Potts
`and Guy’ equation (Potts and Guy 1992); Equation (5.2). In recent years, considerable efforts
`have been putinto refinement of these models, driven both by innatescientific curiosity and
`an increased requirement for cost-effective methods of generating dermal safety data on a
`large range of existing chemicals. However, it is important to appreciate that these models
`
`‘Tt should be noted that Log P can only be used to describe the solubility of uncharged species. Log D (distribution
`coefficient) should be used for charged molecules. However, as most charged molecules do not readily penetrate the
`skin Log P is the most commonly used parameter.
`
`0014
`
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`86
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`CHO5: PHYSICOCHEMICAL FACTORS AFFECTING SKIN ABSORPTION
`
`Sample of
`chemical
`
`®
`
`Octanol
`
`-——>
`
`Wate -
`
`Aliquot samples taken
`from octanol and water
`
`phases and concentration
`of chemical determinedin
`each
`
`—_—_—_—_—_—_—_—_
`
`Constantstirringte
`
`
`Centrifugation /settling
`
`
`
`
`Kow
`
`is introduced into a vial containing octanol and water (which are immiscible and so
`The chemical
`separate on standing into to distinct layers). The mixture is then stirred for a period before centrifugation
`(to separate out the octanol and water layers). Samples of the upper (octanol) and lower (water) phases
`are then carefully obtained and the concentration of chemical
`in each phase determined using an
`appropriate analytical method. The K,,, is then calculated by dividing the concentration of the chemical
`in the octanol phase by the concentration in the water (Equation (5.1)).
`
`Figure 5.1
`
`Summary of method to measure Log P (Koy) of a test compound
`
`are based on a limited data set on permeation from saturated aqueoussolutions and neither
`formulation effects nor physiological factors are considered. Obvious anomalies, such as
`differential permeation of stereo-isomers with identical MW and log P values, demonstrate
`that predictions generated by such models should be used with caution.
`
`Log Kp = 0.71 Log Kgw — 0.006.mw — 2.74
`
`(5.2)
`
`Where Kp = permeability coefficient (see Chapter 6), Koy is the octanol—water partition
`coefficient (Log P) and mwis molecular weight.
`
`5.2.3 Charge
`
`The presence of proteins (such as keratin) endowsthe stratum corneum with both positively
`and negatively charged groups. This characteristic, in combination with the lipophilic nature
`of the stratum corneum providesan effective barrier against charged molecules (ions). Thus,
`in general, ions are (at best) poorly absorbed across the stratum corneum. Indeed, there is
`
`0015
`
`

`

`5.2: PHYSICOCHEMICAL PROPERTIES
`
`87
`
`A small selection of chemicals and their associated Log P values obtained from the EDETOX
`Table 5.1
`database* andFlynn list (Reproduced from Flynn, G.L. (1990). Physicochemical determinants of skin
`absorption, in Principles of route to route extrapolation for risk assessment(eds Gerrity, T.R. and Henry,
`C.J.), Copyright © 1990, Elsevier Science)
`
`Chemical
`
`glucose
`sucrose
`water
`
`butanediol
`ethanol
`scopolamine
`Nicotine
`nicotine
`paroxon
`diethyltoluamide
`oestradiol
`meperidine
`testosterone
`fentanyl
`chlorpyrifos
`
`Log P
`
`—3.24
`—2.25
`—1.38
`
`—0,.92
`—0.31
`0.98
`1.17
`1.17
`1.98
`2.18
`2.69
`2.72
`3,32
`4.05
`4,96
`
`(Arbitrary)
`
`SkinAbsorptionRate
`
`Solubility (Log P)
`
`Figure 5.2 Representation of the theoretical effect of solubility (expressed as Log P) on the rate
`of skin absorption through skin (Note that this is an empirical generalisation! In reality, the actual
`relationship will vary according to the particular group of chemicals being studied (e.g. alkanols,
`phenols, esters etc))
`
`evidence to suggest that appendageal routes (Chapter 4) may be the predominantpathwayfor
`diffusion for charged molecules, especially hydrophilic ions.
`The presence of negatively charged groups outnumbersthose that are positive and so the
`stratum corneum carries a net negative charge. For this reason, the penetration of positively
`
`7http://edetox.ncl.ac.uk/index.html
`
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`CHO5: PHYSICOCHEMICAL FACTORS AFFECTING SKIN ABSORPTION
`
`(arbitrary)
`
`
`Relativepenetrationrate
`
`Figure 5.3 Theoretical effect of vehicle pH on the skin absorption of a weak acid (A) and a weak
`base (B) (This illustration assumes a pKa or pKb of 5 for the acid and base, respectively. The fraction
`of non-ionised acid increases at low pH, resulting in an increase in skin absorption. Conversely, the
`fraction of unionised base decreases at lower pH, leading to reduced penetration)
`
`charged molecules (cations) is generally faster than negatively charged molecules (anions). In
`other words, the stratum corneumis ‘cationselective’, which has implications for transdermal
`delivery of drugs (Walters 2002).
`Thecase is slightly more complicated for chemicals whoseionisation state is pH-dependent,
`suchas weakacids, bases and zwitterions (molecules which have both acid and base groups). In
`general, unionised moieties penetrate better than ionised (Figure 5.3) and soa vehicle pH that
`favours the formation of non-ionised moleculeswill result in more extensive skin absorption.
`Furthermore, unionised molecules tend to be more lipophilic than ionised forms and so a pH
`that favours the formation of non-ionised moieties may also promote skin absorption through
`a changein solubility.
`Giventhat the pH ofthe stratum corneum ranges from around4to 6, then molecules which
`are predominantly non-ionised will tend to be absorbed more extensively than chemicals
`which are predominantly ionised within this pH range.
`
`5.2.4 Hydrogen bonding
`
`The stratum corneum contains a wealth of hydrogen bonding groupsarising fromits lipid
`and protein composition. These can form reversible bonds with chemicals as they diffuse
`through the stratum corneum, provided that the penetrant has complementary hydrogen
`bonding groups. Diffusion of a chemical through the stratum corneum can beretarded if
`it undergoes hydrogen bonding within the stratum corneum. Put simply, hydrogen bonds
`between a penetrant and the componentsof the stratum corneum can be thoughtofas brief
`molecular handshakes.
`There are essentially two factors that affect the extent to which hydrogen bondingwill slow
`downdiffusion of a molecule through the stratum corneum. Thefirst is the potential strength
`
`0017
`
`

`

`1000
`
`ak oO o
`
`ek o
`
`5.3: EXPOSURE CONSIDERATIONS
`
`89
`
`Flux(gcm-*htx107%) 0.1
`
`A
`
`B
`
`Cc
`
`D
`
`E
`
`F
`
`G
`
`resorcinol; D: hydroquinone; E: pyrogallol;
`A: phenol; B: catechol; C:
`F: benzenetriol; G: phloroglucinol.
`
`The effect of the number and position of hydrogen bonding groupsfor a range of phenol
`Figure 5.4
`derivatives on penetration through a surrogate biological membrane (Du Plessis et al. 2001, page 7,
`Copyright 2001, reprinted with permission from Elsevier)
`
`of the hydrogen bond; some groupsinteract more strongly than others — the handshakingis
`firmer and longer. For example, hydrogen bonding between a nitrogen atom andanalcohol
`(OH) groupis roughly twice as strong as that between a nitrogen atom and an amine (NH2)
`group. Secondly, the number of hydrogen bonding groups(and their relative position on
`the penetrating molecule) is also important: more hands mean more handshakes! This is
`illustrated by the differential skin absorption of phenolderivatives (Figure 5.4). A quantitative
`consideration ofthe effects of hydrogen bonding on skin absorption is given in Chapter6.
`
`5.3 Exposure considerations
`
`The way in which a chemicalis presented to the skin can have a substantial impact on the subsequent
`rate of absorption and this must be accounted for in the experimental design of skin absorption
`
`studies.
`
`Whilst the physicochemical properties of a molecule can strongly influence skin absorption,
`the way in which skin exposure occursis also important. Of relevance to toxicological studies
`are such considerations as the solvent (vehicle) in which the chemical is dissolved, whether
`the exposuresite is covered (occluded)orleft open to the environment (unoccluded) and the
`general condition ofthe skin.
`
`5.3.1 Vehicle effects
`
`The influence of a vehicle on the skin absorption of a chemical cannot be overstated. Even
`apparently small changes to a topical formulation can have a profound influence on the
`
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`
`

`

`90
`
`CHO5: PHYSICOCHEMICAL FACTORS AFFECTING SKIN ABSORPTION
`
`rate and extent of skin absorption;this is reflected by international toxicological or cosmetic
`testing guidelines, which generally require that a topical formulation being assessed should be
`as close as possible to that intended to be marketed (see Chapters 7, 8 and 19). It is pointless,
`for example, to develop a margin of safety factor for a compoundthat is formulated for use
`in an aqueousbasedgel, based on skin permeation data of the test compound applied in an
`ethanolic solution. Cumulative permeation of the compoundwill probably betotally different
`over set exposure periods and the margin of safety may be completely over or underestimated.
`Vehicle effects are also of relevance to the pharmaceutical industry (transdermal drug
`delivery); whilst outside the remit of this book, a numberof goodtexts are available on this
`subject (Barry 2003; Delgado-Charro and Guy 2001; Walters 2002). One reason whyvehicles
`can so profoundly affect skin absorptionis that they can alter the thermodynamicactivity or
`fugacity of a penetrant. A more detailed overview of vehicle effects on the thermodynamic
`activity of a penetrantis given in Chapter6.
`
`5.3.2 Volatility
`
`Volatility can affect the duration over which a chemical remains in contact with the skin
`and this can have a considerable influence on the rate and extent of skin absorption (and
`thus percutaneoustoxicity). This is particularly apparent for highly toxic substances, such as
`chemical warfare agents, where systemic toxicity is directly proportionalto the volatility of a
`given substance within a chemicalseries (Chilcott 2007).
`
`5.3.3 Occlusion
`
`Occlusion can have twoeffects. The first (rather obvious) consequence of occlusion relates
`to volatile chemicals: preventing evaporative loss from the skin surface can enhance skin
`absorption and thus increase percutaneoustoxicity. For example, contamination of skin with
`benzeneresults in very little systemic absorption as the vast majority of the applied dose
`(>99.9%) is lost through evaporation under normal circumstances. However, occlusion (for
`example, with a plastic film) can significantly reduce vapour loss and consequently increases
`skin absorption (Figure 5.5), potentially resulting in greater local or systemic toxicity.
`A secondeffect of occlusion is to increase skin hydration by preventing the normal loss
`of water from the skin surface from sweating or transepidermal water loss (TEWL; see
`Chapter 12). Water is essential for the maintenanceofskin barrier function and, in normal
`skin, accounts for around 10% ofthe weight of the stratum corneum. Occlusion can increase
`the amount of water to 50% (w/w) and this excess hydration is generally associated with
`disruption of the normal structure of the stratum corneum with a corresponding loss of
`skin barrier function. However, the general rule that increased hydration leads to increased
`skin absorption has somenotable exceptions, especially hydrophilic chemicals, which do not
`appearto be affected by occlusion-induced hydration (Zhai and Maibach 2001).
`
`5.3.4 Skin treatments
`
`Many guidelines for dermal toxicity assessmentprescribe clipping, shaving or depilation as
`steps for preparing skin exposure sites (see Chapters 7 and 19). Such measures are necessary
`
`0019
`
`

`

`REFERENCES
`
`91
`
`
`
`
`
`PercentageDosePenetrated
`
`O occluded
`
`18
`
`B unoccluded
`
`
`
`10
`
`50
`
`100
`
`Amount Applied (pl)
`
`the amount
`Figure 5.5 Effect of occlusion on the evaporative loss of benzene from pig skin;
`penetrating occluded skin is consistently greater than unoccluded skin, regardless of the amount
`originally applied to the skin surface (Hattersley 2002)
`
`to provide an evensite for application of a test substance and application chamber (where
`applicable). It should be noted that depilatories and shaving (wet and dry) both cause
`considerable damageto the stratum corneum (Marti et al. 2003; Wahlberg 1972) and sotheir
`use should be avoided or accountedfor with suitable controls where appropriate.
`
`
`Summary
`
`e The ‘dose makes the poison’ and so the extent to which a chemical can penetrate the skin will
`largely dictate its percutaneoustoxicity.
`
`e Skin absorptionis largely influenced by the physicochemical properties of the penetrant:
`© Whilst biological factors can be a source of significant variation, physicochemical properties
`generally control the magnitudeof skin absorption.
`
`e The wayin which a chemicalis applied to the skin surface can also have a substantial effect on the
`rate or extent of dermal absorption (and hence percutaneoustoxicity).
`
`
`
`
`
`References
`
`Allenby, A.C., Fletcher,J., Schock, C. and Tees, T.F.S. (1969). The effect of heat, pH and organic solvents
`on theelectrical impedance and permeability of excised human skin. Br J Dermatol, 81(S4): 31-39.
`Barry, B.W. (2003). Trans