analysing the skin barrier from down under

E-Mail [email protected]

Skin Pharmacol Physiol 2013;26:254–262 DOI: 10.1159/000351933

Analysing the Skin Barrier from Down Under

J. Grice a H.A.E. Benson b

a Therapeutics Research Centre, School of Medicine, University of Queensland, Brisbane, Qld. , and b School of Pharmacy, Curtin Health Innovation Research Institute, Curtin University, Perth, W.A. , Australia

both for analysing the skin barrier and influencing perme-ation across it. His fundamental work in the area of iontopho-resis provided models that defined the parameters influenc-ing its permeation enhancement. Mike’s research has been translated into improved clinical outcomes, reduced toxico-logical risk and changes to the regulation of skin products. This article provides an insight into Mike Roberts and the Australian contribution to skin science.

© 2013 S. Karger AG, Basel

Introduction

The perspective of the skin from Down Under has con-tributed substantially to our fundamental understanding of the skin barrier, factors that influence its permeability and their application to improve therapeutic, cosmetic and toxicological outcomes. The Aussies have been led by Michael Roberts, whose presence in the skin science field has been large in all perspectives. Mike has led a multicul-tural team from beginnings in Tasmania, to a brief stint in New Zealand, and to the groups best known and more permanent home in Brisbane, Qld., now with a sister lab-oratory in Mike’s home town of Adelaide. Individual con-tributions from around the globe have included Heather Benson (North Ireland), Sheree Cross (England), Yuri Anissimov (Kyrgyzstan), Tarl Prow (USA), Beatrice

Key Words

Skin barrier · Australia · Membrane permeation

Abstract

Over the past 40 years the Australian contribution to the field of skin science has been led by Michael Roberts. One of his earliest papers on membrane permeation was published in Nature, setting the scene for his huge contribution to both the fundamental understanding of skin permeability and the application of that knowledge to improved clinical out-comes, new delivery technologies and minimizing toxico-logical risk. His work has been characterized by a mechanis-tic, mathematical approach to defining skin permeation. He defined the parameters important to skin permeation, es-tablished structure-penetration relationships and demon-strated the importance of maximum flux from a clinical and toxicological viewpoint. Through his systematic approach, Mike showed a parabolic relationship between maximum flux and lipophilicity, and established that this is driven mainly by variations in solubility of the solute in the stratum corneum. One of the significant strengths of Mike’s work is the ability to express biological concepts in mathematical terms. He has developed mathematical models that en-hance our understanding of epidermal, dermal, deep tissue permeation and follicular transport. Throughout his career Mike has been involved in pioneering new technologies

Received: November 15, 2012 Accepted after revision: February 20, 2013 Published online: July 29, 2013

Dr. Heather Benson School of Pharmacy, Curtin University GPO Box U1987 Perth, WA 6845 (Australia) E-Mail h.benson @ curtin.edu.au

© 2013 S. Karger AG, Basel1660–5527/13/0266–0254$38.00/0

www.karger.com/spp

Dow

nloa

ded

by:

Uni

vers

ity o

f Hon

g K

ong

14

7.8.

204.

164

- 9/

6/20

13 1

1:36

:22

PM

http://dx.doi.org/10.1159%2F000351933


Skin Pharmacol Physiol 2013;26:254–262DOI: 10.1159/000351933

255

Magnusson (Sweden), Parminder Singh (India), Nagahi-ro Yoshida (Japan) and the occasional Aussie such as Jeff Grice and Chris Anderson ( fig. 1 ). All have been attracted to Mike’s group by his passion for skin science and of course the beautiful Queensland weather.

This article summarizes Mike’s major contributions in the area of defining skin penetration, developing new technologies to analyse the skin barrier and enhance per-meation, and his application of science in clinical out-comes and influence of regulatory bodies to reduce risk.

Defining Skin Penetration

Historical Perspective The 1960s and 1970s were times of unprecedented

growth in drug discovery. This led to an expansion of the number of drugs available for topical delivery and drove the awareness of the need to develop rapid yet meaningful screening methods. In order to do this, an understanding of the pathways and mechanisms of percutaneous ab-sorption was needed, and this began to develop with the work of Scheuplein, Blank, Tregear, Higuchi and many other pioneers. By this time, the importance of solute, ve-hicle and skin properties had become widely recognized, with solute properties such as lipophilicity, usually mea-sured by octanol-water partition coefficients, molecular size, aqueous solubility or melting point becoming re-garded as major determinants of skin permeability [1, 2] .

Michael Roberts was part of a group of scientists, in-cluding Peter Elias, Jonathan Hadgraft and Gordon Fly-nn, who came to the fore in the 1970s. These were later to be joined by others such as Russ Potts and Richard Guy in the 1980s. Their work can be seen as a natural progres-sion of earlier studies and was characterized by a mecha-nistic, mathematical approach to skin permeation. Nota-bly, however, a major goal of this mechanistic under-standing remains the application to clinical therapy.

Maximum Flux The concept of maximum flux is fundamental to

Mike’s work. While the permeability coefficient (units of distance/time) is more commonly used, maximum flux ( J max , units of amount/area/time) may be a more relevant parameter from a clinical or toxicological point of view, as it represents the maximum dose that can be delivered over a particular time interval. J max is defined as the rate of penetration of pure substances or saturated solutions and is independent of the vehicle (unless the vehicle or solute alters the properties of the skin), whereas the per-

meability coefficient is dependent on the vehicle used. In addition, if the concentration (C v ) and the solubility (S v ) of the solute in the vehicle are known, J max may be esti-mated from the experimental steady-state flux (J ss ) ob-tained from dilute solutions by the relationship:

J max = S v / C v × J ss .

Conversely, J ss may be estimated from J max . Thus, com-parisons between different literature data sets can be eas-ily made.

Skin Penetration Relationships A key goal since the early work of Scheuplein and

Blank [2] has been to define the properties that determine the skin permeation of solutes. To this end, many predic-tive algorithms have been developed using the physico-chemical properties of solutes, often in the form of quan-titative structure-activity relationships [3] . Properties such as lipid-water partition coefficients, aqueous solu-bility, melting point and molecular size were regarded as important [1, 2, 4] . Along with others [5] , Mike and his group, including Beatrice Magnusson, analysed large lit-erature data sets to conclude that molecular weight was the major determinant of skin permeability, defined as J max [6] . Melting point and hydrogen bond acceptor ca-pability, parameters related to molecular association, were also shown to be less important predictors of J max .

Mike’s early work had centred on the percutaneous penetration of phenolic compounds. One of the key ob-servations was a parabolic relationship between epider-mal penetration (as the log of the permeability coeffi-cient) and lipophilicity, with a deviation from linearity towards reduced permeability for higher log P com-pounds [4] . They went on to show that in homologous series of compounds, where log P increases with increas-ing carbon chain length, there was a progressive decrease in J max , which they regarded as a general phenomenon. Subsequent reports from the Roberts group [7, 8] and others [9–11] in human and animal skin have confirmed a parabolic relationship between J max and log P. Mike ar-gued at the time that the reduced penetration of the more lipophilic solutes was a lipophilicity problem, as they could be hindered by the presence of more polar, aque-ous layers. Potts and Guy [5] suggested that it was most likely related to the size of the more lipophilic solutes. The difficulty in interpreting these data is that, as Potts and Guy [5] have pointed out, log P and molecular size (molecular volume or weight) are codependent; larger compounds are also more lipophilic. This issue was ex-amined in some more recent research coming from

Dow

nloa

ded

by:

Uni

vers

ity o

f Hon

g K

ong

14

7.8.

204.

164

- 9/

6/20

13 1

1:36

:22

PM

http://dx.doi.org/10.1159%2F000351933

Grice /Benson


256

a b c

d e f

g

h

i j

Colo

r ver

sion

ava

ilabl

e on

line

1

Dow

nloa

ded

by:

Uni

vers

ity o

f Hon

g K

ong

14

7.8.

204.

164

- 9/

6/20

13 1

1:36

:22

PM

http://dx.doi.org/10.1159%2F000351933



257

Mike’s two laboratories in Brisbane and Adelaide, which used a series of phenolic compounds of similar molecular weight, but varying in lipophilicity, with experimental log P values ranging from 1.95 to 3.5 [12] . Importantly, the parabolic relationship between J max and lipophilicity was retained in this series of compounds where molecu-lar size was no longer a variable. A key conclusion here was that this relationship is due mainly to variations in solubility of the solute in the stratum corneum, rather than interface barriers, confirming Mike’s earlier conclu-sions.

Modelling Perhaps one of the great strengths of Mike’s work has

been the ability to express biological concepts in terms of mathematical models, and being aware of the appropriate level of sophistication required of the model for the ex-perimental data. His ‘KISS principle’ is exemplified by a saying attributed to Einstein, quoted in a recent model-ling review [13] by Mike and Yuri Anissimov: ‘Everything must be made as simple as possible, but not one bit sim-pler.’

Mike was introduced to the world of mathematical modelling using tools such as non-linear regression and Laplace transforms during his PhD studies in the 1970s. He credits this early interest and training for the subse-quent prolific series of publications, including a paper in Nature where membrane permeation was studied using a desorption technique [14] .

Mike’s contribution to mathematical modelling of epi-dermal and dermal transport in particular has been ac-complished with a long line of protégés and colleagues, including Ovais Siddiqui, Parminder Singh, Sheree Cross, Yuri Anissimov, Owen Jepps, Xin Liu, Yuri Dancik and John Pugh.

In a series of papers spanning over a decade, Mike and Yuri Anissimov tackled a number of problems on the per-cutaneous absorption of a solute through the skin, using a diffusion modelling approach [6, 15–17] . Numerical in-version of the Laplace domain solutions was used for sim-ulations of kinetic parameters and to model particular ex-amples. They began by examining the case of a constant donor concentration with a finite rate of removal from the receptor [15] , to define J ss , lag time and receptor concen-tration. Subsequently, they considered the effect of a finite donor volume and the case of solvent-deposited solids [16] , with the solutions successfully taking into account finite receptor clearance, viable epidermal resistance and unstirred donor layer resistance. The third paper in the series examined stratum corneum heterogeneity using stratum corneum desorption and epidermal penetration studies [6] . Modelling of stratum corneum heterogeneity, in terms of the partition coefficient decreasing exponen-tially for half of the stratum corneum thickness, then re-maining constant, was supported by tape stripping data for clobetasol propionate and other solutes. The final pa-per once again dealt with stratum corneum heterogeneity, by modelling the desorption of water from the stratum corneum [17] . The derived model was more consistent with a heterogeneous membrane, with a slow equilibra-tion/slow binding phase, as well as permeation through the stratum corneum. Stratum corneum water desorption data obtained by Scheuplein and Morgan [18] in 1967 showed a good fit to the Roberts and Anissimov model.

Follicular transport is another skin penetration route of interest. An example is the work of Xin Liu, who mod-elled caffeine penetration data generated by Mike’s col-laborator, Jürgen Lademann [19] . Using either a Wagner-Nelson method, or a model recognizing the stratum cor-neum and the hair follicle as separate donor compartments with first-order elimination, the follicular route was shown to be important (up to 33% of total skin penetra-tion) only for the first 30 min, after which absorption through the stratum corneum became dominant.

After work with Parminder Singh in the 1990s [20, 21] , Mike has recently revisited the modelling of dermal and deeper tissue permeation with Yuri Anissimov, Yuri Dancik and Owen Jepps. A new model was developed to encompass blood and lymphatic flow, blood protein binding and dermal binding [22] . By taking blood and/or lymphatic transport into account, the model was shown to be consistent with in vivo human literature data ob-tained from biopsies following topical application of 6 different compounds. Yuri Dancik published a further model of deep tissue penetration of drugs, based on blood,

Fig. 1. A pictorial history. a Mike in Hobart, with his children (clockwise from right) Jason, Gareth, Darren and Natasha. b Matt, Mike’s youngest son, born in New Zealand. c Mike with Les Benet in San Francisco, en route to take up his position at the University of Otago – Les taught Mike how to apply Laplace transforms in pharmacokinetics as a PhD student. d Sheree Cross, a long-stand-ing colleague of Mike in the 1990s. e PhD students Nagahiro Yo-shida and Parminder (Bobby) Singh in 1991. f Davoud Hassan-Zadeh (as a postdoc, from Iran) and Pam Lai as a PhD student in 1995. g Mike being microdialysed in Linköping, Sweden, by an expat Australian, Chris Anderson (1995). h Mike and Norman Bowery (University of London, Pharmacology) in India in 1995. i Modellers and mathematicians, Frank Burcynski (Canada), MR, Ludvik Bass and Tony Bracken (about 1995). j Mike giving a little support to Heather Benson’s son Tom in Canada (1999).

Dow

nloa

ded

by:

Uni

vers

ity o

f Hon

g K

ong

14

7.8.

204.

164

- 9/

6/20

13 1

1:36

:22

PM

http://dx.doi.org/10.1159%2F000351933

Grice /Benson


258

lymphatic and interstitial transport, tissue diffusion and drug properties and used it to interpret published human microdialysis data [23] . Convective transport was shown to be far more important for highly protein-bound drugs than poorly bound ones.

Mike recently reported a new method to determine lo-cal diffusion coefficients in the stratum corneum [24] . This involves determination of fluorescence recovery af-ter photobleaching as determined by fluorescence to-mography, coupled with mathematical models to yield the diffusion coefficients. The method was validated with the fluorescent dye rhodamine B and shown to permit detailed analysis of localized diffusion coefficients in the stratum corneum lipid domains, with the potential to be extended to the determination of diffusion in both the lipid and corneocyte phases.

Penetration into Deeper Tissues In the 1980s, the Roberts group began to look beyond

the skin barrier, to consider the role of dermal transport in the absorption of solutes to deeper tissues and beyond. Using an experimental apparatus similar to that used for subcutaneous absorption by Levy and Rowland [25] in 1974, Siddiqui et al. applied aqueous solutions of metho-trexate [26] or steroids [27] directly to the exposed dermis of anaesthetized or sacrificed rats. Dermal transport was estimated by measuring the rate of disappearance of the solute from the donor solution applied to the dermis. From these studies, they showed that dermal clearance was dependent upon diffusion in the dermis and elimina-tion by dermal blood flow. In addition, vasoconstriction in the skin was seen as a means of inhibiting clearance by restricting blood flow, a topic that was revisited in later work by Singh and Roberts [20] . Siddiqui et al. [27] were able to show that steady-state unbound steroid concen-trations in the viable epidermis tended to increase with the lipophilicity of the solute. This can be understood by considering the relative contributions of epidermal pen-etration and dermal clearance, since epidermal penetra-tion generally increased with lipophilicity, whereas der-mal clearance was relatively insensitive to it.

Using the same experimental system, Singh and Rob-erts [20] used the vasoconstrictor phenylephrine to in-hibit dermal blood flow in anaesthetized rats and thereby significantly increase the delivery of lidocaine, salicylic acid and water to local tissues after dermal application. For this and other work [21] , Singh and Roberts devel-oped a series compartmental model to describe solute dis-tribution in underlying tissues.

In contrast to the importance of blood flow in dermal clearance, Mike showed with Sheree Cross that the clear-ance of large molecules, like interferon injected subcuta-neously, was insensitive to vasoconstrictor action and probably occurred mostly by the lymphatics [28] .

Further mechanisms of uptake were investigated with synovial fluid, where half of the clearance of intra-artic-ular diclofenac was due to binding of synovial protein [29] .

In human clinical studies, Cross et al. [30] used cuta-neous microdialysis, introduced to the group by fellow Aussie and long term Swedish resident Chris Anderson, to monitor tissue concentrations of topically applied sa-licylate and methyl salicylate. This technique has an im-portant advantage over the measurement of plasma con-centrations and urinary excretion in that it allows in vivo pharmacokinetics to be studied directly at the local tissue level.

Pioneering New Technologies

Mike has also turned his attention to skin permeation enhancement technologies such as iontophoresis and, in particular, developing structure-penetration relation-ships that provide an understanding of the factors influ-encing iontophoretic enhancement of skin permeation. He showed that factors affecting transdermal iontopho-resis include current density, pH, ionic strength, concen-tration of drug, molecular size and method of current ap-plication (continuous or pulse current) [31] . The pH of the applied vehicle is an important factor in iontopho-retic delivery as it affects the degree of solute ionization and the permselectivity of the skin [32, 33] . Yoshida and Roberts [34, 35] showed the importance of the ionic con-tent of the applied vehicle and the potential to use con-ductivity as a predictor of iontophoretic flux. The addi-tion of buffer ions led to ion competition for the applied iontophoretic current. In the case of uncharged solutes their transport is enhanced by iontophoresis due to the process of electro-osmosis. The transport of ionic solutes results from a combination of electro-osmosis and elec-trorepulsion. The anodal iontophoretic transport of cat-ionic solutes is favoured because the skin carries a nega-tive charge at physiological pH and is thus permselective to cations under the influence of an electric field [35] . Yoshida and Roberts showed that a linear relationship existed between the reciprocal of the iontophoretic flux and overall conductivity of the donor solution for both the anionic salicylic acid [34] and cationic phenylethyl-

Dow

nloa

ded

by:

Uni

vers

ity o

f Hon

g K

ong

14

7.8.

204.

164

- 9/

6/20

13 1

1:36

:22

PM

http://dx.doi.org/10.1159%2F000351933



259

amine [35] . They suggested that conductivity of solutes in vehicle solutions is one means of predicting the ionto-phoretic flux of solutes from different vehicle composi-tions.

Roberts et al. [36] then developed the integrated ionic mobility-pore iontophoretic model to describe a range of determinants of iontophoretic flux in a single unifying equation. This model takes into account solute size (de-fined by molecular volume, molecular weight or radius), solute mobility, solute shape, solute charge, solute con-centration, fraction ionized, presence of extraneous ions (defined by solute conductivity), Debye layer thickness, total current applied and epidermal permselectivity, as the determinants of iontophoretic flux [36] . It incorpo-rates partitioning rates to account for the interaction of unionized and ionized lipophilic solutes with the wall of the pore, as well as electro-osmosis. The ionic mobility-pore model was applied to the iontophoretic transport of a range of local anaesthetics (both ester and amide type) [37] . Good correlations were obtained between predicted fluxes, obtained from solute size and conductivity, and observed iontophoretic fluxes obtained in vitro across human epidermis under varying pH donor and receptor solutions. The ionic mobility-pore model was found to be applicable for a range of solute molecules based on an ex-amination of previously published iontophoretic data [38] .

Passive permeation of solutes across the skin occurs by the transcellular, intercellular and transappendageal routes. In the case of iontophoretic transport, the trans-appendageal route via the hair follicles and sweat ducts predominates [39] . Lai and Roberts [38] showed that most small solutes are transported through a pore with radii of 6.8–17 Å, which is consistent with transport through the polar intercellular and transappendageal pathway.

Applied Science – Influencing Regulatory

Perspectives and Industry Interactions

Sunscreens Australia has the highest rate of skin cancer in the

world thanks to an almost perfect climate and the Aussie love of sport, the beach and the great outdoors. Conse-quently sunscreen usage is very high with most schools and sporting clubs providing sunscreen and insisting all children are protected prior to outdoor activities. A re-port from Mike’s group published in the Lancet in 1997 [40] changed the perspective on topical sunscreen prod-

ucts and brought about increased scrutiny by regulatory authorities. Ultimately this led to the reformulation of products to improve safety. The Lancet report showed that a sunscreen chemical (oxybenzone or benzophe-none-3) present in many sunscreen products readily per-meated the skin, with substantial systemic absorption, and was ultimately excreted in the urine as both un-changed chemical and metabolites. Whilst there was no evidence that this sunscreen chemical was toxic [41] , its systemic absorption is an unnecessary risk, given that its desired activity is at the skin surface [42, 43] . Mike’s group also showed that sunscreen products designed and mar-keted for application to children did not reduce sunscreen permeation compared to general-purpose sunscreen for-mulations [44] . These reports led to increased interest in the reformulation of sunscreen products to optimize ac-tivity at the skin surface and minimize skin penetration and systemic absorption both by Mike’s group [45–48] and others [48–51] .

Nanotechnology Mike’s research in sunscreens led to an interest in nan-

otechnology, initiated by investigations of the potential for skin permeation by zinc oxide nanoparticles [48] . This occurred at a time when there was considerable contro-versy and public debate but very little scientific investiga-tion to establish the safety of nanotechnology-based for-mulations despite their increased use in topical products, particularly cosmetics and sunscreens. Using techniques to quantify nanoparticle permeation into follicles pio-neered by Jürgen Lademann, and spectroscopic and mi-croscopy techniques developed in his laboratory, Mike sought to address the paucity of scientific information on nanoparticle skin permeation [52–54] . He quickly moved to reduce concern by stating that the current body of lit-erature shows that there is a lack of evidence that nanopar-ticles such as zinc oxide and titanium dioxide, widely used in sunscreen products, permeate intact skin [55, 56] . At the same time his research supported this view. Following the application of a zinc oxide nanoparticle sunscreen product to human skin in vitro less than 0.03% of the ap-plied zinc content penetrated the epidermis [48] . This value was not significantly more than the zinc detected in the receptor phase following application of a placebo for-mulation. No nanoparticles were detected in the lower stratum corneum or viable epidermis by electron micros-copy, suggesting that minimal nanoparticle penetration occurs through the human epidermis. In a subsequent study he used a non-invasive imaging technique (time-correlated single-photon counting) to provide simultane-

Dow

nloa

ded

by:

Uni

vers

ity o

f Hon

g K

ong

14

7.8.

204.

164

- 9/

6/20

13 1

1:36

:22

PM

http://dx.doi.org/10.1159%2F000351933

Grice /Benson


260

ous real-time quantification of zinc oxide nanoparticle concentration and nicotinamide adenine dinucleotide phosphate [NAD(P)H: a measure of the metabolic state of the skin] in the stratum corneum and viable epidermis of volunteers [54] . In this study there was no zinc oxide nanoparticle permeation into the viable skin following application to subjects with altered barrier function, in-cluding tape-stripped skin and in psoriasis or atopic der-matitis lesions. The free NAD(P)H signal significantly in-creased in tape-stripped viable epidermis but not lesional skin treated with zinc oxide nanoparticles for 4 h com-pared to vehicle control. This reiterated the advice Mike had provided to regulatory bodies, that the benefits of us-ing topical sunscreens outweigh any potential or per-ceived risks.

Mike then sought to explore the potential for utilizing nanotechnology to deliver and target drugs to the skin. This involved advanced skin visualization techniques such as dermoscopy, reflectance confocal microscopy, multiphoton tomography (first developed within the group by postdoctoral scientist Gareth Winckle [57] ), with fluorescence lifetime imaging microscopy to permit real-time imaging of permeation and metabolic ef-fects within the skin layers [58, 59] . Imaging techniques have been further refined with the application of the emerging luminescence nanomaterial, called upconver-sion nanoparticles [60] . This technique allows complete suppression of background due to the excitation light back-scattering and biological tissue autofluorescence as-sociated with other optical images of the skin. Systematic studies were performed to better understand the influ-ence of nanoparticle physicochemical properties on skin permeation. Nanoparticle size was shown to be a critical factor with 15-nm gold nanoparticles applied in aqueous solution shown to associate on the skin surface with no evidence of permeation into intact skin. Permeation of smaller nanoparticles (6 nm) applied in toluene was de-tected [58] . Quantum dot (QD) fluorescent nanoparticles with three surface modifications [polyethylene glycol (PEG) at pH 7.0, 8.3 and 9.0, PEG-amine (PEG-NH 2 ) at pH 8.3 and PEG-carboxyl (PEG-COOH) at pH 9.0] were evaluated for human skin penetration [61] . Only the PEG-QD at pH 8.3 showed any permeation into theviable epidermis. However, following tape-stripping 30 strips of stratum corneum, all QDs penetrated through the viable epidermis and into the upper dermis within24 h. In a comprehensive review of the literature, Mike concluded that nanoparticles greater than 10 nm in diam-eter are unlikely to penetrate through the stratum cor-neum into viable human skin but will accumulate in the

hair follicle openings, especially when applied with mas-sage. However, significant uptake does occur in damaged and in certain diseased skin.

Tea Tree Oil Another area of particular interest to Australian in-

dustry is tea tree oil, extracted from the leaves of Mela-leuca alternifolia , a tree which is native to south-east Queensland and the north-east coast of New South Wales, Australia. It has been used as a traditional medicine by the indigenous Bundjalung people of eastern Australia who use ‘tea trees’ as a traditional medicine for a range of pur-poses including coughs, colds, sore throat, skin condi-tions and wounds. Commercial production was initiated in the 1920s and grew into a major industry involving plantations with the increased interest in natural prod-ucts. Mike’s group, in association with the Department of Primary Industries and the Tea Tree Growers Associa-tion, made an important contribution to the fate of Aus-tralian tea tree oil in overseas markets by defining the skin penetration of a number of the major chemical ingredi-ents of the oil [62] . This led to the submission of docu-mentation to the European agencies to allow its contin-ued sale in overseas markets. This work was extremely important to the local industry and was also recognized by the Australian Society of Cosmetic Chemists who awarded the research the best Australian research paper award at its annual conference.

Industry Collaborations Mike’s group has actively collaborated with many oth-

er manufacturers of pharmaceutical, cosmetic and per-sonal care products both in Australia and globally. These include: Pfizer Global Research & Development (Ann Ar-bor, Mich.) in the area of minoxidil formulation develop-ment and modelling follicular and intercellular transport; investigating the effect of melanosomes on skin colour with Johnson & Johnson (Chong-Jin Loy in Singapore); non-steroidal anti-inflammatory drug formulation de-velopment with Futura Medical Pty Ltd. (UK); sebum analysis and development of anti-acne products with Mi-metica Pty Ltd. (Australia); transdermal formulation de-velopment with Leo Pharma A/S (Denmark) – transder-mal formulations, vitamin C/vitamin A formulations for Ultraceuticals Pty Ltd. (Australia), and testing effects of veterinary treatments on collagen synthesis with Hatchtech Pty Ltd. (Australia). Mike shared his passion for skin product formulation with Johann Weichers, who was a honorary professor with Mike’s group and a fre-quent visitor to Australia.

Dow

nloa

ded

by:

Uni

vers

ity o

f Hon

g K

ong

14

7.8.

204.

164

- 9/

6/20

13 1

1:36

:22

PM

http://dx.doi.org/10.1159%2F000351933



261

Conclusion

This article provides an insight into Mike Roberts’ contribution to the field of skin science, but we believe a story that Adrian William’s shared a few years ago, sum-marizes Mike particularly well. At a Gordon Conference some years ago, Mike suggested a game of tennis to Adri-an Williams (a young 30 something at the time). Adrian was rather dubious given the age and size difference, but did not want to be impolite, so decided he would just go easy on Mike. Some time later a soundly defeated and ex-hausted Adrian limped off court whilst Mike had barely

warmed up. Adrian described how every winner that pinged past him was accompanied by a word of encour-agement from Mike. This story paints a very accurate pic-ture of Mike to those of us who have worked closely with him. A huge talent, who has greatly contributed to devel-oping our understanding of the skin and its permeability. But an equally important legacy is the hundreds of young scientists he has mentored and in whom he has encour-aged a passion for skin science. In so many respects Mike has been the head of a big family, many of whom have gone on to make important contributions to the field.

References

1 Lien EJ, Tong GL: Physicochemical proper-ties and percutaneous absorption of drugs. J Soc Cosmet Chem 1973; 24: 371–384.

2 Scheuplein RJ, Blank IH: Permeability of the skin. Physiol Rev 1971; 51: 702–747.

3 Grice JE, Cross SE, Brownlie C, Roberts MS: The application of molecular structural pre-dictors of intestinal absorption to screening of compounds for transdermal penetration. J Pharm Pharmacol 2010; 62: 750–755.

4 Roberts MS, Anderson RA, Swarbrick J: Per-meability of human epidermis to phenolic compounds. J Pharm Pharmacol 1977; 29: 677–683.

5 Potts RO, Guy RH: Predicting skin permea-bility. Pharm Res 1992; 9: 663–669.

6 Magnusson BM, Anissimov YG, Cross SE, Roberts MS: Molecular size as the main deter-minant of solute maximum flux across the skin. J Invest Dermatol 2004; 122: 993–999.

7 Cross SE, Magnusson BM, Winckle G, Anis-simov Y, Roberts MS: Determination of the effect of lipophilicity on the in vitro permea-bility and tissue reservoir characteristics of topically applied solutes in human skin layers. J Invest Dermatol 2003; 120: 759–764.

8 Magnusson BM, Cross SE, Winckle G, Rob-erts MS: Percutaneous absorption of steroids: Determination of in vitro permeability and tissue reservoir characteristics in human skin layers. Skin Pharmacol Physiol 2006; 19: 336–342.

9 Diez I, Colom H, Moreno J, Obach R, Peraire C, Domenech J: A comparative in vitro study of transdermal absorption of a series of cal-cium channel antagonists. J Pharm Sci 1991; 80: 931–934.

10 Hinz RS, Lorence CR, Hodson CD, Hansch C, Hall LL, Guy RH: Percutaneous penetration of para-substituted phenols in vitro. Fundam Appl Toxicol 1991; 17: 575–583.

11 Yano T, Nakagawa A, Tsuji M, Noda K: Skin permeability of various non-steroidal anti-in-flammatory drugs in man. Life Sci 1986; 39: 1043–1050.

12 Zhang Q, Grice JE, Li P, Jepps OG, Wang GJ, Roberts MS: Skin solubility determines maxi-mum transepidermal flux for similar size molecules. Pharm Res 2009; 26: 1974–1985.

13 Anissimov YG, Jepps OG, Dancik Y, Roberts MS: Mathematical and pharmacokinetic modelling of epidermal and dermal transport processes. Adv Drug Deliv Rev 2013; 65: 169–190.

14 Roberts MS, Triggs EJ, Anderson RA: Perme-ability of solutes through biological mem-branes measured by a desorption technique. Nature 1975; 257: 225–227.

15 Anissimov YG, Roberts MS: Diffusion model-ing of percutaneous absorption kinetics. 1. Ef-fects of flow rate, receptor sampling rate, and viable epidermal resistance for a constant do-nor concentration. J Pharm Sci 1999; 88: 1201–1209.

16 Anissimov YG, Roberts MS: Diffusion model-ing of percutaneous absorption kinetics. 2. Fi-nite vehicle volume and solvent deposited sol-ids. J Pharm Sci 2001; 90: 504–520.

17 Anissimov YG, Roberts MS: Diffusion model-ling of percutaneous absorption kinetics. 4. Effects of a slow equilibration process within stratum corneum on absorption and desorp-tion kinetics. J Pharm Sci 2009; 98: 772–781.

18 Scheuplein RJ, Morgan LJ: ‘Bound water’ in keratin membranes measured by a microbal-ance technique. Nature 1967; 214: 456–458.

19 Liu X, Grice JE, Lademann J, Otberg N, Trau-er S, Patzelt A, Roberts MS: Hair follicles con-tribute significantly to penetration through human skin only at times soon after applica-tion as a solvent deposited solid in man. Br J Clin Pharmacol 2011; 72: 768–774.

20 Singh P, Roberts MS: Effects of vasoconstric-tion on dermal pharmacokinetics and local tissue distribution of compounds. J Pharm Sci 1994; 83: 783–791.

21 Singh P, Roberts MS: Blood flow measure-ments in skin and underlying tissues by mi-crosphere method: application to dermal pharmacokinetics of polar nonelectrolytes. J Pharm Sci 1993; 82: 873–879.

22 Anissimov YG, Roberts MS: Modelling der-mal drug distribution after topical application in human. Pharm Res 2011; 28: 2119–2129.

23 Dancik Y, Anissimov YG, Jepps OG, Roberts MS: Convective transport of highly plasma protein bound drugs facilitates direct pene-tration into deep tissues after topical applica-tion. Br J Clin Pharmacol 2012; 73: 564–578.

24 Anissimov YG, Zhao X, Roberts MS, Zvyagin AV: Fluorescence recovery after photo-bleaching as a method to determine local dif-fusion coefficient in the stratum corneum. Int J Pharm 2012; 435: 93–97.

25 Levy RH, Rowland M: Absorption kinetics of a series of local anesthetics from rat subcuta-neous tissue. Part I. J Pharmacokinet Bio-pharm 1974; 2: 313–335.

26 Siddiqui O, Roberts MS, Polack AE: Topical absorption of methotrexate – role of dermal transport. Int J Pharm 1985; 27: 193–203.

27 Siddiqui O, Roberts MS, Polack AE: Percuta-neous absorption of steroids: relative contri-butions of epidermal penetration and dermal clearance. J Pharmacokinet Biopharm 1989; 17: 405–424.

28 Cross SE, Roberts MS: Subcutaneous absorp-tion kinetics and local tissue distribution of interferon and other solutes. J Pharm Phar-macol 1993; 45: 606–609.

29 Owen SG, Francis HW, Roberts MS: Disap-pearance kinetics of solutes from synovial flu-id after intra-articular injection. Br J Clin Pharmacol 1994; 38: 349–355.

30 Cross SE, Anderson C, Roberts MS: Topical penetration of commercial salicylate esters and salts using human isolated skin and clini-cal microdialysis studies. Br J Clin Pharmacol 1998; 46: 29–35.

31 Singh J, Roberts MS: Transdermal delivery of drugs by iontophoresis: a review. Drug Des Deliv 1989; 4: 1–12.

32 Siddiqui O, Roberts MS, Polack AE: The effect of iontophoresis and vehicle pH on the in-vi-tro permeation of lignocaine through human stratum corneum. J Pharm Pharmacol 1985; 37: 732–735.

Dow

nloa

ded

by:

Uni

vers

ity o

f Hon

g K

ong

14

7.8.

204.

164

- 9/

6/20

13 1

1:36

:22

PM

http://dx.doi.org/10.1159%2F000351933

Grice /Benson


262

33 Siddiqui O, Roberts MS, Polack AE: Ion-tophoretic transport of weak electrolytes through the excised human stratum corneum. J Pharm Pharmacol 1989; 41: 430–432.

34 Yoshida NH, Roberts MS: Prediction of cath-odal iontophoretic transport of various an-ions across excised skin from different vehi-cles using conductivity measurements. J Pharm Pharmacol 1995; 47: 883–890.

35 Yoshida NH, Roberts MS: Role of conductiv-ity in iontophoresis. 2. Anodal iontophoretic transport of phenylethylamine and sodium across excised human skin. J Pharm Sci 1994; 83: 344–350.

36 Roberts MS, Lai PM, Anissimov YG: Epider-mal iontophoresis. I. Development of the ion-ic mobility-pore model. Pharm Res 1998; 15: 1569–1578.

37 Lai PM, Roberts MS: Epidermal iontophore-sis. II. Application of the ionic mobility-pore model to the transport of local anesthetics. Pharm Res 1998; 15: 1579–1588.

38 Lai PM, Roberts MS: An analysis of solute structure-human epidermal transport rela-tionships in epidermal iontophoresis using the ionic mobility: pore model. J Control Rel 1999; 58: 323–333.

39 Lee RD, White HS, Scott ER: Visualization of iontophoretic transport paths in cultured and animal skin models. J Pharm Sci 1996; 85: 1186–1190.

40 Hayden CG, Roberts MS, Benson HA: Sys-temic absorption of sunscreen after topical application. Lancet 1997; 350: 863–864.

41 Hayden CG, Cross SE, Anderson C, Saunders NA, Roberts MS: Sunscreen penetration of human skin and related keratinocyte toxicity after topical application. Skin Pharmacol Physiol 2005; 18: 170–174.

42 Hayden CGJ, Roberts MS, Benson HAE: Sun-screens: are Australians getting the good oil? Aus NZ J Med 1998; 28: 639–646.

43 Benson HAE: Assessment and clinical impli-cations of absorption of sunscreens across skin. Am J Clin Dermatol 2000; 1: 217–224.

44 Jiang R, Roberts MS, Collins DM, Benson HA: Absorption of sunscreens across human skin: an evaluation of commercial products for children and adults. Br J Clin Pharmacol 1999; 48: 635–637.

45 Cross SE, Jiang R, Benson HA, Roberts MS: Can increasing the viscosity of formulations be used to reduce the human skin penetration of the sunscreen oxybenzone? J Invest Der-matol 2001; 117: 147–150.

46 Jiang R, Benson HA, Cross SE, Roberts MS: In vitro human epidermal and polyethylene membrane penetration and retention of the sunscreen benzophenone-3 from a range of solvents. Pharm Res 1998; 15: 1863–1868.

47 Jiang R, Roberts MS, Prankerd RJ, Benson HAE: Percutaneous absorption of sunscreen agents from liquid paraffin: self-association of octyl salicylate and effects on skin flux. J Pharm Sci 1997; 86: 791–796.

48 Cross SE, Innes B, Roberts MS, Tsuzuki T, Robertson TA, McCormick P: Human skin penetration of sunscreen nanoparticles: in-vitro assessment of a novel micronized zinc oxide formulation. Skin Pharmacol Physiol 2007; 20: 148–154.

49 Scalia S, Mezzena M, Ramaccini D: Encapsu-lation of the UV filters ethylhexyl methoxy-cinnamate and butyl methoxydibenzoyl-methane in lipid microparticles: effect on in vivo human skin permeation. Skin Pharmacol Physiol 2011; 24: 182–189.

50 Touitou E, Godin B: Skin nonpenetrating sunscreens for cosmetic and pharmaceutical formulations. Clin Dermatol 2008; 26: 375–379.

51 Simeoni S, Scalia S, Benson HA: Influence of cyclodextrins on in vitro human skin absorp-tion of the sunscreen, butyl-methoxydibenzo-ylmethane. Int J Pharm 2004; 280: 163–171.

52 Song Z, Kelf TA, Sanchez WH, Roberts MS, Ricka J, Frenz M, Zvyagin AV: Characteriza-tion of optical properties of ZnO nanoparti-cles for quantitative imaging of transdermal transport. Biomed Opt Express 2011; 2: 3321–3333.

53 Zvyagin AV, Zhao X, Gierden A, Sanchez W, Ross JA, Roberts MS: Imaging of zinc oxide nanoparticle penetration in human skin in vi-tro and in vivo. J Biomed Opt 2008; 13: 064031.

54 Lin LL, Grice JE, Butler MK, Zvyagin AV, Becker W, Robertson TA, Soyer HP, Roberts MS, Prow TW: Time-correlated single pho-ton counting for simultaneous monitoring of zinc oxide nanoparticles and NAD(P)H in in-tact and barrier-disrupted volunteer skin. Pharm Res 2011; 28: 2920–2930.

55 Nohynek GJ, Dufour EK, Roberts MS: Nano-technology, cosmetics and the skin: is there a health risk? Skin Pharmacol Physiol 2008; 21: 136–149.

56 Nohynek GJ, Lademann J, Ribaud C, Roberts MS: Grey goo on the skin? Nanotechnology, cosmetic and sunscreen safety. Crit Rev Toxi-col 2007; 37: 251–277.

57 Winckle G, Anissimov YG, Cross SE, Wise G, Roberts MS: An integrated pharmacokinetic and imaging evaluation of vehicle effects on solute human epidermal flux and retention characteristics. Pharm Res 2008; 25: 158–166.

58 Labouta HI, Liu DC, Lin LL, Butler MK, Grice JE, Raphael AP, Kraus T, El-Khordagui LK, Soyer HP, Roberts MS, Schneider M, Prow TW: Gold nanoparticle penetration and re-duced metabolism in human skin by toluene. Pharm Res 2011; 28: 2931–2944.

59 Roberts MS, Dancik Y, Prow TW, Thorling CA, Lin LL, Grice JE, Robertson TA, Konig K, Becker W: Non-invasive imaging of skin physiology and percutaneous penetration us-ing fluorescence spectral and lifetime imaging with multiphoton and confocal microscopy. Eur J Pharm Biopharm 2011; 77: 469–488.

60 Song Z, Anissimov YG, Zhao J, Nechaev AV, Nadort A, Jin D, Prow TW, Roberts MS, Zvy-agin AV: Background free imaging of upcon-version nanoparticle distribution in human skin. J Biomed Opt 2013; 18: 061215.

61 Prow TW, Monteiro-Riviere NA, Inman AO, Grice JE, Chen X, Zhao X, Sanchez WH, Gi-erden A, Kendall MA, Zvyagin AV, Erdmann D, Riviere JE, Roberts MS: Quantum dot pen-etration into viable human skin. Nanotoxicol-ogy 2012; 6: 173–185.

62 Cross SE, Russell M, Southwell I, Roberts MS: Human skin penetration of the major compo-nents of Australian tea tree oil applied in its pure form and as a 20% solution in vitro. Eur J Pharm Biopharm 2008; 69: 214–222.

Dow

nloa

ded

by:

Uni

vers

ity o

f Hon

g K

ong

14

7.8.

204.

164

- 9/

6/20

13 1

1:36

:22

PM

http://dx.doi.org/10.1159%2F000351933

analysing the skin barrier from down under

Documents