Photostability of sunscreens

Journal of Photochemistry and Photobiology C: Photochemistry Reviews 13 (2012) 91–110

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Journal of Photochemistry and Photobiology C: Photochemistry Reviews journal homepage: www.elsevier.com/locate/jphotochemrev

Review

Photostability of sunscreens Jutta Kockler, Michael Oelgemöller, Sherryl Robertson, Beverley D. Glass ∗ School of Pharmacy and Molecular Sciences, James Cook University, Townsville 4811, QLD, Australia

a r t i c l e

i n f o

Article history: Received 17 October 2011 Received in revised form 23 December 2011 Accepted 26 December 2011 Available online 6 January 2012 Keywords: Sunscreens Photostability Chemical UV-filters Physical UV-filters UV-protection

a b s t r a c t Sunscreens were originally designed to include mainly UVB-filters. Because of the deeper penetration of UVA light, causing photoaging and DNA damage, there has been a shift towards broad spectrum sunscreens. These broad spectrum sunscreens now include both UVA- and UVB-filters and other ingredients which possess antioxidant activity. Although sunscreens are regulated in most countries, photostability testing is not mandatory. Because of the ability of sunscreen ingredients to absorb UV-light and the complexity of most of these formulations, which may include more than one UV-filter, antioxidants and other formulation excipients, it is important that their photostability in combination is determined. © 2012 Elsevier B.V. All rights reserved.

Contents 1. 2.

3.

4.

5.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Photostability of chemical UV-filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Photostability of individual chemical UV-filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Combinations of chemical UV-filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Possible degradation products of chemical UV-filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.1. Identification of photodegradants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.2. Toxicity of degradation products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Effects of other active ingredients on photostability of the chemical UV-filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Addition of physical UV-filters (metal oxides) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.1. Coated metal oxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.2. Doped metal oxides. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Addition of antioxidants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Change of UV-protective performance by formulation excipients . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Different solvents or formulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Inclusion of chemical UV-filters in cyclodextrins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

∗ Corresponding author. Tel.: +61 7 4781 6423; fax: +61 7 4781 5356. E-mail address: [email protected] (B.D. Glass). 1389-5567/$20.00 © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jphotochemrev.2011.12.001

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J. Kockler et al. / Journal of Photochemistry and Photobiology C: Photochemistry Reviews 13 (2012) 91–110 Jutta Kockler completed her Pharmacy degree at the Goethe University in Frankfurt/Main (Germany) and spent six months as visiting researcher at James Cook University in Townsville (Australia) during her degree program. In 2010 she commenced her MPharm in photostability and formulation of sunscreens at James Cook University, Townsville (Australia).

Michael Oelgemöller received his Diploma from the University of Münster (Germany) in 1995 and his PhD from the University of Cologne (Germany) in 1999. He was a researcher at the Inoue Photochirogenesis project in Osaka and at Bayer CropScience Japan in Yuki. He held a position as a Lecturer in Organic and Medicinal Chemistry at Dublin City University (Ireland). Currently he is an Associate Professor in Organic Chemistry at James Cook University (Australia). His research activities include synthetic organic photochemistry, solar photochemistry, the development of new photochemical synthesis tools and photochemical water treatment. He received the KurtAlder award of the University of Cologne in 2000. Sherryl Robertson completed her BAppSc Honours degree in Chemistry at Central Queensland University in Rockhampton (Australia) and PhD from Murdoch University, Perth (Australia). After a research career in hydrometallurgy, she joined the School of Pharmacy and Molecular Sciences at James Cook University as a Senior Technical Officer. Beverley D. Glass obtained her BSc Honours degree in Chemistry from Nelson Mandela Metropolitan University in Port Elizabeth (South Africa) and her Bachelor of Pharmacy and PhD degrees from Rhodes University, Grahamstown (South Africa). Currently she is the Chair and Professor of Pharmacy in the School of Pharmacy and Molecular Sciences at James Cook University. Her research interests include the stability, especially photostability of drug substances and drug products with a focus on in-use stability and photosensitivity effects in patients.

1. Introduction Exposure to ultraviolet (UV)-light can be potentially dangerous and as such result in adverse health outcomes [1]. UV-light may be subdivided into the following regions: UVC (100–290 nm); UVB (290–320 nm); UVAI (320–340 nm) and UVAII (340–400 nm). While the UVC-light is filtered to a large extent by the atmosphere and thus does not reach the surface of the earth [2], UVB-light, although mainly restricted to penetration of the upper layers of the skin, can cause DNA-damage and sunburn. Since UVA is not absorbed by the DNA molecule to any great extent, damage caused by UVA-light is largely attributed to photosensitized oxidation, which may occur by two different mechanisms, Type 1 and 2: Type I process results in the substrate reacting with the excited state of the sensitizer to yield radicals or radical ions, by hydrogen atom or electron transfer, while the Type II process occurs when an excited sensitizer reacts with oxygen to form singlet molecular oxygen. UVA-light does penetrate to the deeper skin layers causing photoageing and DNA-damage mainly by generation of reactive oxygen species (ROS) [3,4]. However, DNA damage has also been shown to occur by a Type 1 mechanism involving an intramolecular electron transfer in folic acid and its photodegradation product, pterine-6-carboxylic acid [5]. Although skin cancer is the worst outcome of excessive exposure to UV-light [6,7], sunburn, eye conditions such as cataract or ocular melanoma, premature skin ageing such as wrinkles or irregular pigmentation of the skin and a compromised immune system may also result from this exposure [8]. It is noteworthy that Australia and New Zealand have the highest rates of skin melanoma in the world [9]. Sunscreens are photochemical systems containing UV-filters, which absorb or reflect light and can be divided into two types:

chemical and physical UV-filters. The chemical UV-filters can be further subdivided into UVA-filters which only absorb UVA-light, UVB-filters (only absorbing UVB-light) and broad spectrum filters which absorb both UVA- and UVB-light. The available physical UVfilters are broad spectrum filters, which absorb and reflect UV-light. Besides these UV-filters, sunscreens may contain other additives such as antioxidants, which are also thought to play a role in protecting the skin from the effects of exposure to UV-light [10]. Regular application of sunscreens to the skin is the most effective protection against the dangerous effects of UV-light. The first modern sunscreens merchandised in the 1930s [11] were characterized by the Sun Protection Factor (SPF), which is a laboratory measure of the effectiveness of a sunscreen. The higher the SPF, the more protection the sunscreen offers against UVB radiation [12]. The SPF-testing method is an in vivo method, where protected and unprotected skin-areas of subjects are exposed to artificial sunlight for various time periods. The SPF is defined as the minimum erythemal dose (MED = UVB-dose when redness of the skin is visible) on protected skin divided by the MED on unprotected skin (Eq. (1)) [13]. SPF =

MEDprotected MEDunprotected

(1)

Since the harmful and photoageing effects of UVA have been reported, UVA-filters are now included in sunscreens, which are classified as broad spectrum [6,14–17]. The UVA-protection factor (UVA-PF) can be measured in vivo or in vitro. The in vivo test-method, also called the Persistent-Pigment Darkening (PPD) method was developed by the Japanese industry and modified by the French health agency (Afssaps = Agence franc¸aise de sécurité sanitaire des produits de santé) and measures the minimal darkening effect of UVA-radiation on the skin before and after exposure to UVA-light. The UVA-PF is defined as minimal pigmenting dose (MPD = UVA-dose when darkening of the skin is visible) on protected skin divided by the MPD on unprotected skin (Eq. (2)) [18,19]. UVA − PF =

MPDprotected MPDunprotected

(2)

Because of their role in protecting the skin from the harmful effects of UV-light, sunscreens are required to be regulated. The regulation of sunscreens differs, depending on the particular country and/or the requirements of the relevant regulatory agency. These regulations include in general a list of approved UV-filters, the appropriate labelling of sunscreens and requirements for the measuring of SPF, UVA-PF and water resistance. In Australia, although most sunscreens are ‘listed’ medicines under the Therapeutic Goods Act 1989, some can be ‘exempt’ and others are required to be ‘registered’ [19]. In the USA, sunscreens are regulated as over-the-counter (OTC) drugs under the supervision of the FDA (Federal Drug Administration). The Federal Register of 1999 contains the final monograph with regulations for sunscreens, entitled: Sunscreen drug products for over-the-counter human use. Besides the list of allowed sunscreen active ingredients (UV-filters), their required maximum concentration and the allowed combinations of UV-filters, this monograph contains requirements for labelling and testing of the SPF and water resistance, but not for UVA-protection. In contrast to Australia and the USA, in Europe sunscreens are regarded as cosmetics. The European Commission publishes a Commission Recommendation on the efficacy of sunscreen products, where they provide advice regarding the hazards of UV-radiation, the need of sunscreens and recommendations about labelling and testing. An industry association, namely Colipa (European Cosmetic, Toiletry and Perfumery Association) was founded to develop the industry standards on testing, labelling and consumer education [18,20].

J. Kockler et al. / Journal of Photochemistry and Photobiology C: Photochemistry Reviews 13 (2012) 91–110

Because of the various regulatory requirements in different countries, especially with respect to the methods used to determine the UVA/UVB protection afforded by a sunscreen, it is important that these methods are known in order to compare products from different countries in terms of their labelled protection factors [21]. Although it has been proposed in the USA that a combined in vitro and in vivo test method for UVA-protection, which includes a photostability test [22] should be undertaken, photostability testing (Q1B) according to the International Conference on Harmonisation (ICH) guidelines is not mandatory in Europe, Australia and the USA [18,19,23]. Such testing is commonly applied to drug substances and products and this area has been extensively researched and summarized by Glass and co-workers [24–30]. The lack of required photostability testing of sunscreens by regulators has in fact attracted the interest of many researchers over recent years. In 2005, Bonda reviewed the photostability of organic sunscreen actives, referring to the light stability of chemical UV-filters. However, while Bonda’s review focused on the photostability of individual chemical UV-filters, the photostability of combinations of chemical UV-filters and the combination of chemical with physical UV-filters was not discussed in depth. In this review however, we will highlight the issue of combinations of UV-filters on the photostability, the potential photocatalytic effect

93

of including physical UV-filters on chemical UV-filters and how physical UV-filters can be modified, by coating or doping, to protect chemical UV-filters and the skin from ROS, generated by these filters. Although degradation products of chemical UV-filters are identified in Bonda’s review, no mention is made of their toxicity. Bonda also describes photostabilisation only by excited state quenching by other chemical UV-filters, whereas this review will show the effect of both antioxidants and cyclodextrins added to the formulation in increasing the photostability of sunscreens [31]. Based on the review by Bonda, Brewster reported on the photostability of UV-filters in 2006, focussing on the stabilisation of one UV-filter (Butyl methoxy dibenzoylmethane) and the effect of the solvent polarity of UV-filters. In addition, the possibility of complexing UV-filters with cyclodextrins or encapsulating them in nanoparticles made of poly-d,l-lactide-co-glycolide, to increase their photostability was reported [32]. Because broad spectrum sunscreens usually require the inclusion of a number of components and the fact that photostability testing is not a regulatory requirement, this review presents an account of not only the photostability of chemical and physical UV-filters and antioxidants, but also reports on the impact on photostability of these ingredients in combinations and in the presence of various formulation excipients, which may be included

Cinnamates: O

O O

O

O

Benzophenones:

OH

O

Ethylhexyl methoxycinnamate (1)

Isoamyl methoxycinnamate (2)

OH

O

O O O S O

Dibenzoylmethanes:

O

O O

O

O

O

Benzophenone-3 (9)

O

OH

Butyl methoxy dibenzoylmethane (3)

O

Na

Benzophenone-5 (10) O

O O

OH

Isopropyl dibenzoylmethane (4) N

O

Para-amino benzoic acid and its derivative: Diethylamino hydroxybenzoyl hexyl benzoate (11) O

O

Camphor derivative:

O OH N

H2N

Aminobenzoic acid (PABA) (5)

Ethylhexyl dimethyl PABA (6)

O

4-Methylbenzylidene camphor (13)

Triaminotriazine: O O

O

HN

N H O

HN

O

N N H

O

O N

N

HN

O

N N

N

O N H

N H O

Diethylhexyl butamido triazone (7)

O

Ethylhexyl triazone (8) Fig. 1. Chemical structures of chemical UV-filters.

Benzophenone-10 (12)

94

J. Kockler et al. / Journal of Photochemistry and Photobiology C: Photochemistry Reviews 13 (2012) 91–110

Salicylate:

O

O OH

Ethylhexyl salicylate (14)

N HO

Others:

N N

HO N N

N O HO

S

N

N H

O

Phenylbenzimidazole sulfonic acid (15)

Methylene bis-benzotriazolyl tetramethylbutylphenol (16)

O O

Na

O S O

HO N

O

N

HO

N O

HO

O

H N

N H

N

O S

OH O

O S O O

O

Bis-ethylhexyloxyphenol methoxyphenyl triazine (17)

O

S

N

Na

Disodium phenyl dibenzimidazole tetrasulfonate (18)

O

N

Octocrylene (19) Fig. 1. (Continued ).

in a sunscreen product, including mention of how this affects their ability to provide the required UV protection. The structures of the chemical UV-filters in this review are presented in Fig. 1, while their CAS numbers, INCI-Name, IUPAC-Name, UV-absorbance (UVA/UVB) and max (solvent) are listed in Table 1. 2. Photostability of chemical UV-filters Chemical UV-filters absorb UV-light and in this way protect the skin from hazardous UV-radiation and its damaging effects. Presently there are 28 chemical UV-filters available on the Australian, 26 on the European and 14 on the US-markets, including UVA-, UVB- and broad spectrum filters [19,22,41]. The photostability of sunscreens is an important consideration in their development and subsequent performance as these filters are designed to absorb UV-light. Absorption of UV-light leading to photochemical reactions in these molecules, such as trans-cis isomerisation (Scheme 1), where the trans form has a higher absorption coefficient than the cis form as in the case of UV-filter

1, can affect its UV-protective performance. Another possible photochemical reaction is keto-enol tautomerism (Scheme 2) shown to occur in UV-filter 3, where the enol-form absorbs in the UVA range, while the diketo-form absorbs in the UVC range. UV-filters also have the ability to react with other UV-filters or fragment and produce by-products after irradiation with light, as shown to occur with UV-filter 3 (Scheme 3) [38,42]. As a result of these reactions and subsequent degradation, the ability of these ingredients in sunscreens to absorb UVA/UVB-light or a combination may be compromised as well as their ability to function as a sunscreen. 2.1. Photostability of individual chemical UV-filters The photostability of 18 UVB- and broad spectrum filters was investigated by measuring changes in the SPF, which quantifies the effectiveness of the UV-filters. Each UV-filter was incorporated in an oil-in-water (O/W) emulsion and spread on a Polymethylmethacrylate (PMMA) plate before being irradiated in a solar

J. Kockler et al. / Journal of Photochemistry and Photobiology C: Photochemistry Reviews 13 (2012) 91–110

95

Table 1 Chemical UV-filters with CAS-Numbers, INCI-Name, IUPAC-Name, UV-absorbance (UVA/UVB), max with solvent and references for max . CAS-No.

INCI-Name

IUPAC-Name

UV-absorbance

max [nm] (Solvent)

Ref.

1

5466-77-3

308 (Methanol)

[33]

71617-10-2

UVB

310 (Ethanol)

[34]

3

70356-09-1

UVA

357 (Ethanol)

[35]

4

63250-25-9

UVA

341 (Cyclohexane)

[36]

5 6

150-13-0 21245-02-3

2-Ethylhexyl (2E)-3-(4-methoxyphenyl)prop-2-enoate 3-Methylbutyl (E)-3-(4-methoxyphenyl)prop-2-enoate 1-(4-Methoxyphenyl)-3-(4-tertbutylphenyl)propane-1,3-dione 1-Phenyl-3-(4-propan-2-ylphenyl)propane1,3-dione 4-Aminobenzoic acid 2-Ethylhexyl

UVB

2

UVB UVB

277 (Methanol) 311 (Methanol)

[33] [33]

7

154702-15-5

Ethylhexyl methoxycinnamate Isoamyl methoxycinnamate Butyl methoxy dibenzoylmethane Isopropyl dibenzoylmethane PABA Ethylhexyl dimethyl PABA Diethylhexyl butamido triazone

UVB

311 (Ethanol)

[34]

8

88122-99-0

Ethylhexyl triazone

UVB

313 (Ethanol)

[37]

9

131-57-7

Benzophenone-3

UVB/UVA

287, 325 (Methanol)

[33]

10

6628-37-1

Benzophenone-5

UVB/UVA

286, 323 (Not given)

[38]

11

302776-68-7

UVA

354 (Ethanol)

[35]

12

1641-17-4

Diethylamino hydroxybenzoyl hexyl benzoate Benzophenone-10

UVB/UVA

287, 325 (Methanol)

[39]

13

UVB

301 (Ethanol)

[37]

14 15

38102-624/36861-47-9 118-60-5 27503-81-7

UVB UVB

306 (Methanol) 304 (Methanol)

[34] [33]

16

103597-45-1

2,2 -Methanediylbis[6-(2H-benzotriazol-2-yl)4-(2,4,4-trimethylpentan-2-yl)phenol]

UVB/UVA

306, 359 (Water)

[34,40]

17

187393-00-6

UVB/UVA

310, 340 (Ethanol)

[35]

18

180898-37-7

2,2 -[6-(4-Methoxyphenyl)1,3,5-triazine-2,4-diyl] bis{5-[(2-ethylhexyl)oxy]phenol} Disodium 2,2 -(1,4-phenylene)bis(6-sulfo-1Hbenzimidazole-4-sulfonate)

UVA

335 (Not given)

[38]

19

6197-30-4

2-Ethylhexyl 2-cyano-3,3-diphenyl-2-propenoate

UVB

304 (Ethanol)

[34]

No.

4,4 -[[6-[[4-[[(1,1Dimethylethyl)amino]carbonyl]phenyl]amino]1,3,5-triazine-2,4-diyl]diimino]bis-, bis(2-ethylhexyl)benzoate 4-[[4,6-Bis[[4-(2-ethylhexoxyoxomethyl)phenyl]amino]-1,3,5-triazin-2yl]amino]benzoic acid 2-ethylhexyl ester (2-Hydroxy-4-methoxyphenyl)phenylmethanone Sodium 5-benzoyl-4-hydroxy-2methoxybenzenesulfonate Hexyl 2-[4-(diethylamino)-2hydroxybenzoyl]benzoate (2-Hydroxy-4-methoxyphenyl)-(4methylphenyl)methanone (3E/Z)-1,7,7-Trimethyl-3-[(4methylphenyl)methylene]-2-norbornanone 2-Ethylhexyl 2-hydroxybenzoate 2-Phenyl-3H-benzimidazole-5-sulfonic acid

4-Methyl-benzylidene camphor Ethylhexyl salicylate Phenylbenzimida-zole sulfonic acid Methylene bis-benzotriazolyl tetramethylbutylphenol Bis-ethylhexyloxyphenol methoxyphenyl triazine Disodium phenyl dibenzimidazole tetrasulfonate Octocrylene

O

hv

O

O

O

O

O

E1

Z1

Scheme 1. Trans- to cis-transformation of UV-filter 1 (E1) to Z1.

simulator ( > 290 nm) at 650 Wh/m2 for various time periods. The SPF was measured in vitro before and after the different irradiation times to calculate t90% (the time in minutes when 90% of the SPF value remained). A time period of 120 min of irradiation was used to distinguish between good and poor photostability, because of the recommendation that a sunscreen is applied every 2 h. The

O

UV-filters 5 and 7 showed superior photostability of all UV-filters tested with t90% of 1600 and 1520 min, respectively. However, although UV-filter 5 had the best photostability profile, its skin irritation potential was very high and therefore it is no longer used in sunscreen products. Four other UV-filters (9, 10, 15 and 16) also showed good photostability with a t90% between 180 and

O

O

OH

hv O

O Diketo tautomer

Enol tautomer

Scheme 2. Keto-enol-tautomerism of UV-filter 3.

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J. Kockler et al. / Journal of Photochemistry and Photobiology C: Photochemistry Reviews 13 (2012) 91–110

O

O

O

CH2

hv R1

O

3

.OC

.

3a

+

R2

3b

Scheme 3. Fragmentation of UV-filter 3 into a phenacyl radical (3a) and a benzoyl radical (3b) with R1 = –OCH3 and R2 = –C(CH3 )3 (or vice versa, depending of the position of the Norrish-cleavage).

Table 2 UVA-PF and UVA-PF (%) of UV-filters before and after irradiation [44]. UV-filter

UVA-PF t0 h

UVA-PF t2 h

UVA-PF (%)

3 9 10 11 16 17 18

2.76 2.50 2.46 9.80 5.34 15.63 5.03

1.63 2.42 2.34 4.60 4.85 5.89 4.81

−41 −3 −5 −53 −9 −62 −4

320 min, but t90% of the majority of twelve UV-filters was less than 120 min and therefore these sunscreens have demonstrated poor photostability [43]. A similar study by the same research group using the change in the UVA-PF of 7 UVA- and broad spectrum filters to determine their photostability was undertaken. Again each UV-filter was incorporated in an O/W emulsion, spread on a PMMA plate and irradiated in the same solar simulator. The UVA-PF was measured in vitro before and after an irradiation time of 120 min and the difference was calculated as UVA-PF (%). When the loss of the UVA-PF value was less than 10%, the UV-filter was regarded as photostable. UV-filter 9 showed the best photostability with a loss of only 3% of the UVAPF and was therefore regarded as photostable. The three UV-filters 3, 11 and 17 were regarded as not photostable with a loss between 41% and 62% reported (Table 2) [44].

maximum in the UVC range) after irradiation (i). Further irradiation may lead to a conversion of the exited singlet diketo form 1 3(k) to the triplet diketo form 3 3(k) (ii), which gives rise to photodegradation products (iii) or reacts with oxygen to form singlet oxygen (1 O2 ) (iv). This highly reactive oxygen species can then further react with the enol form of UV-filter 3 resulting in the formation of different degradation products (v). The stabilisation of 3(e) can occur in different ways: An added UV-filter can compete with 3(e) for light, preventing its transformation to the diketo form (i) and therefore prevent the subsequent reactions (ii–iv), or an energy transfer from 3 3(k) to other UV-filters (vi), which then can eventually undergo a photodegradation (vii), can occur and results in 1 3(k) and the excited 3 UV-filter. The UV-filter may also prevent the photodegradation of UV-filter 3 by quenching singlet oxygen (viii) [46]. The photostability profiles of four UV-filter combinations, each with three different UV-filters (A: 1, 9, 14; B: 1, 3, 13; C: 1, 9, 19; D: 1, 3, 19), were evaluated using high performance liquid chromatography (HPLC) and UV-spectrophotometry. The UV-filter combinations were incorporated in an emulsion, spread on a glass plate and irradiated for three different time periods (30, 60, 120 min) in a solar simulator (280–400 nm) at 20 mW/cm2 . The

2.2. Combinations of chemical UV-filters Since sunscreens nowadays usually contain more than one UV-filter, it is necessary to observe the photostability of combinations and the subsequent effect on sunscreen performance. It has however been shown that some chemical UV-filters have a photoprotective effect on other UV-filters [45–47]. The photostability of UV-filter 3, a UVA-filter, alone and in the presence of six different UV-filters (1, 7, 8, 11, 17 and 19) was studied by comparing the UV-filter-concentrations before and after irradiation in a solar simulator at 765 Wh/m2 for 4 h. The UV-filters were dissolved in Miglyol® 812N and applied on a quartz cell as a thin film before irradiation. Five UV-filters (7, 8, 11, 17 and 19) showed a good recovery after irradiation with recovery yields ranging from 92% to 100%, whereas the UV-filters 1 and 3 clearly degraded with recovery yields of only 72% and 44%, respectively (Fig. 2a). The photodegradation of UV-filter 3 (recovery of 44%) was reduced by 40% in the presence of UV-filter 19. The combination of the UV-filters 3 and 17 showed a recovery of 71% for UV-filter 3, while the recovery was only around 60% in the presence of the UV-filters 1, 7, 8 and 11 (Fig. 2b). The highest degradation of UV-filter 3 was observed in the presence of UV-filter 11 with a recovery of only 57% for UVfilter 3. Even UV-filter 11 degraded substantially in the presence of UV-filter 3 with a recovery of only 38% compared to 100%, when UV-filter 11 was irradiated alone. The mechanism of degradation of UV-filter 3 is proposed to occur by keto-enol tautomerism (shown in Scheme 4) where the active enol form 3(e) converts to the diketo form 1 3(k) (absorption

Fig. 2. Recovery of the UV-filters 1, 7, 8, 11, 17, 19 and 3 after 4 h of irradiation in a solar simulator at 765 Wh/m2 . (a) UV-filters irradiated separately and (b) 1, 7, 8, 11, 17 and 19 in combination with 3. Adapted from Ref. [46].

J. Kockler et al. / Journal of Photochemistry and Photobiology C: Photochemistry Reviews 13 (2012) 91–110

97

hν

1

(i)

hν

3

(ii)

Products

(iii)

1

(iv)

1

Products

(v)

3

1

3(k) + 3UV-filter

(vi)

Products

(vii)

Quenching

(viii)

3(e) 1

3(k)

3(k) 3(k)

3

3(k)

3

3(k)

O2

O2

O2 + 3(e) 3(k) + UV-filter

3

UV-filter

UV-filter + 1O2

Scheme 4. Processes involved in the photostability of the UV-filter 3. Adapted from Ref. [46].

Table 3 UV-filters 1, 3, 9, 13, 14 and 19 and their approximate recovery in % in formulations A, B, C and D [47]. Formulation

UV-filter and their approximate (exact data not given) recovery after 120 min of irradiation 1

A B C D a

50% 40% 70% 55%

3

9

13

90% 25%a

19

85% 75%

100% 40%

14

100% 85%

Irradiation time: 60 min.

recovery yields of every UV-filter (%) in each formulation were compared to each other and overall the two formulations containing UV-filter 19 were found to be most photostable (Table 3). This study suggests that UV-filter 19 has a photostabilising effect to the UV-filters 1, 3 and 9 in the formulations C and D [47]. Since chemical UV-filters have the ability to generate ROS [48–52], resulting in skin damage similarly to exposure to UVAradiation, it is important to study the capability of UV-filters and their combinations to generate these reactive species. It has been demonstrated that the UV-filters 1, 9 and 19 generate ROS after UV-irradiation. Each UV-filter was incorporated in a water-in-oil (W/O) emulsion and spread on model skin before being irradiated in a solar simulator with a complete dose of 20 mJ/cm2 . Before irradiation the skin containing the UV-filter emulsion was incubated for 0, 20 and 60 min at 5% CO2 and 37 ◦ C. ROS generation only increased after an incubation time of 20 min after irradiation, but still remained under that of the control emulsion (without UV-filters). After 60 min of incubation and irradiation the ROS generation increased above that of the control in the presence of all three UV-filters (about 20–60% more than the control) with UVfilter 9 showing the highest increase of about 60% (exact data not given) [51]. Several UV-filters (1, 3, 8, 11, 16, 17 and 19) were incorporated into phosphatidylcholine (PC)-based liposomes (to mimic the membrane lipids) alone and in combination before they were exposed to UVA-light with a dose of 275 kJ/m2 . To determine the ROS generation after irradiation, a modified thiobarbituric acid (TBA) assay was used and the concentration of the lipid peroxidation breakdown products, TBA reactive substances (TBARS), was measured. The UV-filters 3 and 8 showed three times the level of ROS generated compared to the irradiated control without UV-filters, while the ROS generation of other tested UV-filters (1, 11 and 19) remained in the same range as the control. A lower concentration of around 20% (exact data not given) was achieved with the UV-filters 16 and 17. The irradiation of all tested combinations including the UV-filter 3 resulted in high generation of

ROS, with the highest rate for the combination of UV-filter 3 with 8. The combinations of UV-filter 1 with other UV-filters showed no significant difference when compared to the control, while the combination of 1 and 8 showed more than double the generated ROS [52]. In addition the changes in the UV-absorbance spectra, as a result of generated ROS after irradiation, were measured to determine the photostability. The UV-absorbance maxima of the UV-filters 1 and 3 decreased by about 30% and 75%, respectively, after irradiation confirming their instability to light. On the other hand the UV-absorbance of 16 and 19 remained unchanged after UVA-irradiation, indicating photostability [52], which is consistent with the results of other studies [43,44,46,53–55]. However, Couteau et al. [43] contradicted these results reporting that the UV-filter 19 was unstable to light upon irradiation in a solar simulator at 650 Wh/m2 . The SPF was measured after various times and as early as after 95 min, the SPF value was 10% lower than before irradiation. UV-filter 19 was characterized as photounstable because the time period was less than 2 h [43]. In contrast, other authors reported the photostability of this UV-filter [46,53–55]. Lhiaubet-Vallet et al. [46] and Herzog et al. [53] measured the recovery of UV-filter 19 determined by HPLC after 4 h at 765 Wh/m2 [46] and a total dose of 300 J/cm2 [53], respectively, in a solar simulator. Nearly 100% recovery (exact data not given) of UV-filter 19 was achieved by Herzog et al. [53] and 100% by Lhiaubet-Vallet et al. [46]. The recovery of UV-filter 19 was measured by gas chromatography (GC) by Ricci et al. [54] and Rodil et al. [55] after an irradiation time of 20 h with a UVA lamp [54] and after 72 h with a halogen lamp (290–800 nm) [55], respectively. Ricci et al. reported a recovery of 100% in water and acetonitrile [54], whereas Rodil et al. describes a recovery of about 90% (exact data not given) of UV-filter 19 in water [55]. There have also been conflicting reports for the UV-filters 8, 11 and 17. Whereas UV-filter 8 is regarded as photostable measured by HPLC after 4 h of irradiation in a solar simulator at 765 Wh/m2 by Lhiaubet-Vallet et al. [46], it was not reported as photostable by Couteau et al. [43] and Damiani et al. [52]. Lhiaubet-Vallet et al.

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reported a recovery of 94% of UV-filter 8 after irradiation and classified it as photostable [46]. The SPF of UV-filter 8 lost 10% of its value after 35 min of irradiation in a solar simulator at 650 Wh/m2 . Since the time period was less than 2 h, this UV-filter was not regarded as photostable by Couteau et al. [43]. The instability of UV-filter 8 was confirmed by Damiani et al., who measured a three times higher ROS generation than the control after irradiation with UVA-light with a dose of 275 kJ/m2 [52]. Lhiaubet-Vallet et al. [46] and Damiani et al. [52] reported UVfilter 11 as photostable by measuring the recovery by HPLC after 4 h of irradiation in a solar simulator at 765 Wh/m2 [46] and by measuring the ROS generation after irradiation under UVA-light with a dose of 275 kJ/m2 [52], respectively. UV-filter 11 was regarded photostable by Lhiaubet-Vallet et al. [46] because it showed 100% recovery after irradiation. Damiani et al. [52] reported that the ROS generation by UV-filter 11 stayed in the same range as the control after irradiation and therefore classified this UV-filter as photostable. On the other hand Couteau et al. [44] showed the photoinstability of UV-filter 11 by measuring the UVA-PF before and after 2 h of irradiation in a solar simulator, due to the loss of the UVA-PF value (53%) being more than 10%. Studies on UV-filter 17 also present some contradictory results, with some authors regarding this UV-filter as being photostable [46,52,53] and others as not photostable [43,44]. The recovery of UV-filter 17, measured by HPLC after 4 h of irradiation at 765 Wh/m2 [46] and 300 J/cm2 [53], respectively, in a solar simulator, was reported by Lhiaubet-Vallet et al. [46] and Herzog et al. [53]. Lhiaubet-Vallet et al. reported a recovery of 92% [46] and Herzog et al. a recovery of around 100% (exact data not given) after irradiation [53]. With these results both authors classified UV-filter 17 as photostable [46,53]. Damiani et al. [52] showed its photostability by measuring the ROS generation after irradiation under UVA-light with a dose of 275 kJ/m2 . The author reported about 20% (exact data not given) less ROS generation in the presence of UV-filter 17 than for the control [52]. Couteau et al. reported that UV-filter 17 after 20 min of irradiation in a solar simulator at 650 Wh/m2 resulted in a 10% reduction of its SPF and classified it therefore as not photostable [43]. The UVA-PF lost 62% of its value after an irradiation time of 2 h in a solar simulator (650 Wh/m2 ). This loss was more than 10% and therefore UV-filter 17 was regarded as not photostable [44]. A reason for the stability of some UV-filters in the presence of each other is attributed to the ability of these UV-filters to act as singlet or triplet state quenchers. Since the mode of action of chemical UV-filters involves the absorption of UV-light (photons), an electron becomes excited and is promoted from the ground state to the singlet exited state. From there it either transfers to its triplet state or reverts back into the ground state releasing heat or resulting in fluorescence. In the ground state repeated absorption of photons may occur, while in the triplet state reversion to the ground state with phosphorescence or further photochemical reactions may occur. Further photochemical reactions cause UV-filters to loose photoprotective potential and they are thus regarded as being photolable. In the presence of another UV-filter an overlap of their electron clouds can result in a UV-filter acting as donor transferring the excited electron to the other UV-filter, the acceptor. This exchange usually occurs in the lower energy triplet state, with the acceptor UV-filter demonstrating greater stability in the triplet state compared to that of the donor UV-filter. The greater the overlap of the electron cloud the greater the quenching capacity [56]. Since these quenchers return the excited state of UV-filters to the ground state, they eliminate the destructive chemical reactions, which are most often initiated from the excited triplet state. However, the advantage of a singlet quencher is that in quenching the singlet excited state, it reduces the transition to the triplet state and can be used in lower concentrations compared with triplet quenchers. An example of the occurrence of triplet–triplet energy transfer is in the

combination of the UV-filters 1 and 3, where the enol form of UVfilter 3 acts as donor and UV-filter 1 as acceptor. This triplet–triplet energy transfer was confirmed by electron paramagnetic resonance (EPR) and time-resolved phosphorescence spectra recorded individually and together of both the UV-filters [57]. The stabilisation of UV-filter 3 by triplet–triplet energy transfer by the UV-filters 13, 17 and 19 has also been described [31,38,53]. In addition, the stabilisation of the UV-filter 3 by a singlet-singlet energy transfer was demonstrated by the photostabilizer, ethylhexyl methoxycrylene using several in vitro and in vivo methods. The fluorescencespectra, quantum yield and lifetime of UV-filter 3 were determined in the presence and absence of ethylhexyl methoxycrylene. Thin films of solutions were irradiated with a solar simulator to ascertain the recovery of UV-filter 3, and in vivo studies to determine the SPF and UVA-PF were also undertaken [58]. Even though the singlet excited state of UV-filter 3 is extremely short-lived, ethylhexyl methoxycrylene was successful in quenching this excited state. In addition, the inclusion of ethylhexyl methoxycrylene in sunscreens is advantageous as it results in improved levels of performance at lower concentrations of active ingredients. Stabilisation of compounds such as retinol, retinyl palmitate and trans-resveratrol by ethylhexyl methoxycrylene by singlet-singlet energy transfer was also demonstrated in further studies [59,60]. 2.3. Possible degradation products of chemical UV-filters The fact that some chemical UV-filters alone or in combination showed a greater extent of degradation upon UV-irradiation, may depend on the irradiation level, time and solvents, and thus different degradation products may occur, which may also be toxic. 2.3.1. Identification of photodegradants The degradation of the three UV-filters 3, 6 and 9 in cyclohexane was investigated. The UV-filters were irradiated for 70–140 h using a medium pressure mercury vapour lamp in a quartz immersion well reactor, purged with air. In contrast to the UV-filters 3 and 6, the UV-filter 9 remained unchanged even after an irradiation time of 100 h, with no degradation products detected by gas- or liquid-chromatography. After an irradiation time of 140 h, three degradation products (20, 21 and 22) of UV-filter 6 were detected by gas chromatography–mass spectroscopy (GC/MS) and identified by nuclear magnetic resonance (NMR) spectroscopy (Scheme 5). Total decomposition of UV-filter 3 in cyclohexane was observed by GC/MS after 100 h of irradiation resulting in several photodegradants. Although the cleavage of UV-filter 3 can occur on both sides of the methylene group, only three degradation products could be identified with two benzoic acid derivative (23 and 24) and t-butylbenzene (25) (Scheme 6), methoxybenzene not be detected. The amount of 23 formed was between two and three times higher than that of 24, whereas 10–15% of product 25 was formed. A possible mechanism for the fragmentation of UV-filter 3 is shown in Scheme 7. Other degradation products formed by a reaction with the solvent cyclohexane were identified including the cyclohexyl esters of 4-t-butyl benzoic acid and 4-methoxy benzoic acid. From the total percentage of degradation products of UV-filter 3, the amount of each cyclohexyl esters was less than 5% [61]. Isolation and identification by HPLC and GC/MS of degradation products of the UV-filters 3 and 4 was undertaken by Schwack and Rudolph and reported in detail [36]. Both UV-filters were irradiated with UV-light for 8 h in a solar simulator with two different glass filters (cut off at 260 nm: F1 and 320 nm: F2) and the photodegradation was measured by HPLC and GC/MS. Degradation was investigated in isopropyl alcohol, methanol, cyclohexane and isooctane purged with air. In the non-polar solvents cyclohexane and isooctane, both UV-filters degraded exponentially while they were stable in the polar solvents isopropyl alcohol and methanol.

J. Kockler et al. / Journal of Photochemistry and Photobiology C: Photochemistry Reviews 13 (2012) 91–110

O

O

O

O

O

hv

O

O

+

99

O

+

CH3

N

NH

NH2

N

6

20

21

22

Scheme 5. Proposed degradation products of UV-filter 6 after 140 h of irradiation in cyclohexane: 20 = 4-monomethylaminobenzoic acid 2-ethylhexyl ester (21%), 21 = 4aminobenzoic acid 2-ethylhexyl ester (<1%) and 22 = 4-dimethylamino-(2/3)-methylbenzoic acid 2-ethylhexyl ester (5%) [61].

O

O

O

+

OH

hv R

O

3

R = C(CH 3)3 (23) R = OCH 3 (24)

25

Scheme 6. Identified degradation products of UV-filter 3: 23 = 4-t-butyl benzoic acid, 24 = 4-methoxy benzoic acid and 25 = t-butylbenzene [61].

The degradation in cyclohexane was 14% for UV-filter 3 and 20% for UV-filter 4 using the filter F1, whereas it was about 8% for both UV-filters using filter F2. Data for the degradation in isooctane were not given. The first step of the degradation process was a cleavage of the UV-filters 3 and 4 resulting in a benzoyl and a phenacyl radical (Scheme 3) followed by several oxidation and/or recombination reactions. The 14 photoproducts were classified into the following seven groups: benzaldehydes, benzoic acids, phenylglyoxals, acetophenones, benzils, didenzoyl methanes and dibenzoyl ethanes as shown in Scheme 8. The photodegradation pathway may be affected by the ability of oxygen (in the air) to act as triplet quencher. A 1 H NMR analysis in cyclohexane-d12 and isopropyl alcohol-d8 identified the concentrations of enol and diketo forms of the UV-filters in the corresponding solvents. In cyclohexane-d12 UV-filter 4 showed a concentration of 1.7% of the O

O

diketo form and the UV-filter 3 a concentration of 3.5%, whereas in isopropyl alcohol-d8 the diketo form of the UV-filters was not detected. This result clearly shows that the degradation process depends on the formation of the diketo form [36]. The UV-filter 4 was removed from the market in 1993, because of photoallergic reactions [30,62]. The photodegradation products of UV-filter 3 in water were detected using liquid chromatography–mass spectroscopy (LC/MS). The UV-filter was irradiated in a solar simulator (an exact range of wavelengths was not given) in 4 min intervals at 250 W/m2 to a complete dose of 60 kJ/m2 . The degradation products 24, 27 and 33 detected by Schwack and Rudolph were present as well as two other degradation products, a hydroxypropenone (36) and a 1,4-diketone (37) (Scheme 9), due to the reaction with oxygen [63].

O

3 hv O

.

O

C

.

O

O

+O 2

O

O

+

O

.

O

O

solvent O

OH

OH O

24 O

O

C

H2C

solvent

O

hv

.. +

O OH

+ 25

Scheme 7. Proposed degradation mechanism of UV-filter 3 with possible degradation products after 100 h of irradiation in cyclohexane shown for the cleavage on only one side of the methylene group: 24 = 4-methoxy benzoic acid and 25 = t-butylbenzene. Adapted from Ref. [61].

100

J. Kockler et al. / Journal of Photochemistry and Photobiology C: Photochemistry Reviews 13 (2012) 91–110

O

O

O H

O OH

H

O

OH

O

26

23

24 O

O

O

O

O

O H

H

O

27

28

O

29

31

30

O

O

O

O

O

32

33

O

O

O

O

O

O

O

34

35

Scheme 8. Degradation products of UV-filter 3 after 8 h of irradiation in cyclohexane identified by GC–MS: 26 = 4-methoxy benzaldehyde, 23 = 4-t-butyl benzaldehyde, 24 = 4methoxy benzoic acid, 27 = 4-t-buthyl benzoic acid, 28 = 4-methoxy phenylglyoxal, 29 = 4-t-butyl phenylglyoxal, 30 = 4-methoxy acetophenone, 31 = 4-t-butyl acetophenone, 32 = 4,4-di-t-butyl benzil, 33 = 4-t-butyl-4-methoxy benzil, 34 = 4,4-dimethoxydibenzoyl methane, 35 = 4-t-butyl-4-methoxydibenzoyl ethane [36].

The degradation of UV-filter 3 in the three solvents cyclohexane, ethyl acetate and dimethyl sulfoxide (DMSO) was investigated after irradiation with a high-pressure mercury lamp using a pyrex filter (>300 nm), purged with air. The degradation products were detected by GC/MS and the results were categorised using the Wiley 275 library and compared to the findings by Schwack and Rudolph [36]. No degradation products could be identified in DMSO after 18 h of irradiation and the concentration of UV-filter 3 remained constant. In ethyl acetate, only two degradation products (29 and 33) were detected by GC/MS. HPLC analysis after 15 h of irradiation showed a decrease of 33% of the enol form of UV-filter 3 and an increase of the diketo form (exact data not given). Detailed chromatographic analyses showed that UV-filter 3 underwent photoisomeration and photodegradation to a similar extent in ethyl acetate, while in DMSO the main photoreaction was photoisomeration. In cyclohexane, previous results were confirmed and the products 23, 29, 32, 33, 35 (identified by Schwack and Rudolph [36]) and 37 (identified by Huong et al. [63]) were found. It was shown that in cyclohexane UV-filter 3 underwent photoisomeration from the enol to the diketo form, before the resulting diketo form underwent photodegradation [64]. The UV-protective capacity of seven UV-filters in petroleum jelly was investigated before and after UVB exposure followed by UVA

HO

O

O

exposure. UVB radiation was achieved by fluorescent light bulbs (peak at 313 nm) with a dose of 20 MED and UVA radiation of 100 J/cm2 by a solar simulator (320–400 nm). Possible degradation products were identified by GC/MS. Three UV-filters (9, 12 and 13) were regarded as photostable because their UV absorption spectra did not change significantly after UVA and UVB exposure. However, GC/MS showed a second peak for UV-filter 13, but with the same mass, therefore indicating a photoisomer. The cisand trans-form of UV-filter 13 have a similar absorption spectra and therefore both isomers have a photoprotective character. The absorption maximum of UV-filter 1 was reduced by about one third of its value after UVA and UVB exposure and was therefore regarded as not photostable. The degradation product was identified as the cis-isomer of the active trans-form of 1 (Scheme 1) which has a lower absorbance than that of the trans-form and therefore loses its photoprotective character in contrast to UV-filter 13. The UV absorbance spectra of UV-filter 6 did not change significantly after UVB exposure, however after UVA exposure two degradation products could be identified which are shown in Scheme 10. In addition to the methylamino benzoate (20), a formylmethylamino benzoate (38) was detected (percentage relations were not given). Prior to the photodegradation reaction, UV-filter 3 underwent an enol-diketo isomerisation as described in Scheme 2. The

O

36

O

O

O

O 37

Scheme 9. Possible degradation products of UV-filter 3 identified by LC/MS after an irradiation dose of 60 kJ/m2 in water: 36 = (Z)-1-(4-tert-butylphenyl)-3-hydroperoxy-3(4-methoxyphenyl)prop-2-en-1-one and 37 = 1,4-bis(4-methoxyphenyl)butane-1,4-dione [63].

J. Kockler et al. / Journal of Photochemistry and Photobiology C: Photochemistry Reviews 13 (2012) 91–110

O

O

O

O

O

O

101

O

O

NH

N

20

38

40

39

O

O

Scheme 10. Degradation products of UV-filter 6 identified by GC/MS after UVA exposure of 100 J/cm2 in petroleum jelly: 20 = 2-ethylhexyl 4-methylaminobenzoate and 38 = 4-(formylmethylamino) benzoate [42].

O

O

O

O

41 absorption maximum of UV-filter 3 shifted from the UVA range (enol-form) to the UVC range (diketo-form) and is therefore not useful as sunscreen. The additionally tested dibenzoylmethane, UVfilter 4, underwent a similar decomposition. Identified degradation products are shown in Scheme 11 [42]. The exposure of UV-filter 1 to natural sunlight in methanol resulted in only one degradation product detected by HPLC and identified by 1 H NMR. After one day of sun exposure the concentration of UV-filter 1 (1.000 ppm = 3.44 mM) was halved, while the degradation product increased to maintaining mass balance. This ratio remained the same for 30 days of sun exposure. A control sample was kept in the dark for one month and showed no decrease in the concentration of UV-filter 1 on HPLC analysis. Detailed investigations with online HPLC/MS and 1 H NMR concluded that the original UV-filter 1 was the trans-isomer and the degradation product its corresponding cis-isomer [65]. The molar absorption coefficient of both isomers of UV-filter 1 was measured in a methanol/water mixture (90/10), pure methanol, ethanol and hexane. In all four solvents the molar absorption coefficient of the trans-isomer was around twice that of the cis-isomer, which explains the reduction of UV absorbance

O

42

Scheme 11. Degradation products of UV-filter 4 identified by GC/MS after UVA exposure of 100 J/cm2 in petroleum jelly: 39 = benzil, 40 = isopropylbenzil, 41 = 1-phenyl-3-(4-iso-propylphenyl)-propane-1,2,3-trione and 42 = 1,3-di(4-isopropylphenyl)-propane-1,2,3-trione [42].

after UV irradiation of UV-filter 1. The ratio of both isomers strongly depends on the solvent type and the concentration. In methanol the trans/cis ratio for concentrations of 0.0344, 0.344 and 3.44 mM were 0.47, 0.75 and 1.1, respectively [66]. The degradation process of UV-filter 1 in water was different to other solvents (heptane dioxane, ethyl acetate, tetrahydrofuran, acetonitrile, isopropyl alcohol) after an irradiation time of 10 min at 250 W/m2 (= 150 kJ/m2 ) in a solar simulator. The sum of both isomers was only 71.7% in water, whereas the sum in the other solvents was around 100%, as determined by HPLC. This indicates the generation of other degradation products in water, but no suggestions for a particular reaction mechanism or the nature of the degradation products were given (see 4.1. for more details) [67].

O O

O

O

O

O

O

O

O

O

O

O

O

43

44 O

O

O

O

45

O

O

O

O

O

O

O

O

46

Scheme 12. Photocycloaddition products in cyclohexane: 43 = 2-ethylhexyl-5-[4-(2,2-dimethylethyl)phenyl]-3-(4-methoxyphenyl)-2-(4-methoxybenzoyl)-5oxopentanoate, 44 = 2-ethylhexyl-2-[4-(2,2-diethylethyl)benzoyl]-3,5-bis-(4-methoxyphenyl)-5-oxopentanoate, 45 = 2-ethylhexyl-4-[4-(2,2-dimethylethyl)phenyl]-3-(4methoxy-phenyl)-2-(4-methoxybenzoylmethyl)-4-oxobutanoate and 46 = 2-ethylhexyl-3,4-bis-(4-methoxyphenyl)-2-[4-(2,2-dimethylethyl)benzoylmethyl]-4-oxobutanoate [68].

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As soon as more than one UV-filter is incorporated in a sunscreen the photochemistry may be altered and this is dependent on the individual UV-filters, their concentration and the formulation. These factors may affect the rate of photodegradation and the degradation products. The photodegradation behaviour of the widely used UV-filter combination 1 and 3 was investigated in cyclohexane in a solar simulator (290–400 nm) for 10 h emitting a total UV irradiance of 27 W/m2 . Both underwent irreversible [2 + 2] cycloaddition resulting in a number of different diketones, as identified by HPLC/MS and NMR. The four major degradants are shown in Scheme 12 [68]. 2.3.2. Toxicity of degradation products Some UV-filters (5, 6 and 9) are known to cause photosensitizing reactions in patients [12,69,70]. Photosensitivity reactions may be classified as either phototoxic or photoallergic. A phototoxic reaction occurs after exposure to sunlight in the presence of a chemical (topical or systemic application) directly on the sun exposed area. A photoallergic reaction involves the immune system and can also occur on non-sun exposed areas after exposure to sunlight in the presence of chemicals. A prior exposure to sunlight and the particular chemical is required [69]. Besides the known photosensitivity reactions, it is also known that UV-filters can degrade into different degradation products. However, whether degradation products of UV-filters can cause toxic reactions is not well studied. To illustrate the importance of the photostability of a sunscreen product, an O/W emulsion containing a photostable UV-filter mixture A (3, 8 and 13) was tested against a photounstable O/W emulsion containing the UV-filters 1, 3 and 8 (mixture B), using biological markers. The photostability of these two UV-filter mixtures was defined by the author with no further explanation. Comet assays were used to investigate DNA damage in human keratinocyte cells such as direct DNA breakage, oxidative damage to purines or lemofloxacin-induced DNA damage caused by UV-light. The antibiotic lemofloxacin was chosen to simulate the exposure to a photosensitizing drug on the skin. Furthermore, the accumulation of the p53 protein in the cells was studied. Experiments were undertaken after exposure to UV-light in a solar simulator (␭ > 300 nm), but in the lemofloxacin studies only UVA-light was used (␭ > 320 nm). The average irradiance for UVA/UVB-light was 10 W/m2 for UVB and 90 W/m2 for UVA (90 kJ/m2 ), whereas irradiance for UVA-light alone was 70 W/m2 (60 kJ/m2 ). Before the UV-filter mixtures were added to the human cells and irradiated under the conditions above, they were preirradiated for 1 h (360 kJ/m2 ). The protective effect towards the keratinocyte cells was higher for the photostable UV-filter mixture A than for mixture B. The comet assay showed that keratinocyte nuclei maintained their spherical shape after UV-irradiation in the presence of mixture A whereas a tail could be observed in the presence of mixture B which indicates DNA cleavage in the nuclei. Western plots showed a two to three times stronger intensity of the protein p53-band in keratinocyte nuclei after 24 h post UV exposure in the UV-filter mixture B than in the stable mixture A. The comet assay in the presence of lemofloxacin showed a three times greater tail formation of the keratinocyte cells after UVA exposure in the presence of mixture B than that of mixture A which demonstrates more DNA cleavage in mixture B. These results show that reduced photostability has a negative impact on cell mechanisms such as DNA breakage and a higher p53 production. Whether the genotoxic events were due to the loss of UV-protecting character or to a toxic effect of degradation products was not determined [71]. The two UV-filters 1 and 3 were tested for their toxicity towards mouse cells in the dark compared to after UV-irradiation in a solar simulator. The UV-filter solutions were irradiated for 2 and 20 h then added to a mouse cell suspension and incubated for a total of

22 h. After the incubation time the percentage survival of mouse cells was determined. At concentrations of 2–10 ppm, the unexposed UV-filter 1 showed a decreased cell survival rate from about 95% (at 2 ppm) to nearly 0% (at 10 ppm). Unexposed UV-filter 3 maintained the high survival rate of about 95% (at 5 ppm) which then decreased to about 30% (at 10 ppm). After UV exposure of 2 h, the survival rate of UV-filter 1 decreased from nearly 100% to 90% in a 5 ␮l suspension and this percentage was maintained after 20 h of UV exposure. In a larger volume of 20 ␮l, the survival rate which was about 50% prior to irradiation, decreased to 40% after 2 h of irradiation and then increased again to 45% after 20 h of irradiation. The UV-irradiation of UV-filter 3 had no significant effect on cell survival compared to the unexposed sample. Both UV-filters showed toxicity towards mouse cells, by decreasing their survival rate. However, although the toxicity of UV-filter 1 increased after UV exposure, while in contrast UV-filter 3 showed no difference between dark toxicity and toxicity after UV exposure [72]. Algal toxicity of the UV-filters 1, 2 and 6 upon exposure to simulated sunlight from a halogen lamp (290–800 nm) in water was investigated. The reproduction inhibition of the algae S. vacuolatus was measured before and after irradiation of the UV-filter solutions and compared to the concentration of the UV-filters. For the UVfilters 1 and 2, the reproduction inhibition, which implies toxicity, decreased after UV exposure correlating to a decrease of the UVfilter concentration (Fig. 3a and b). This decrease suggested that the degradation products of 1 and 2 are less toxic to the tested algae than the parent UV-filters 1 and 2. Only the cis-isomers of both UV-filters and dimers of each UV-filter, which formed by [2 + 2] cycloaddition, were identified as degradation products. In the presence of UV-filter 6 however, the reproduction inhibition remained in the same range after 14 h of irradiation although half of UVfilter 6 was degraded (Fig. 3c). This implies that the degradation products of UV-filter 6 after 14 h have the same toxicity as the UVfilter. The three esters 20, 21 and 22 (Scheme 5) were identified as degradation products. Degradation product 20 showed the highest concentration after 14 h of irradiation and is thus proposed to be the reason for the sustained high toxicity. After a longer irradiation time, the toxicity decreased which indicates that the other subsequent degradation products were less toxic than UV-filter 6 [55]. As described in Section 2.3.1. UV-filter 3 was found to be unstable to light resulting in the formation of benzils and arylglyoxals in addition to other degradation products [36,63,64]. Four arylglyoxals (28, 29, 47 and 48) and four benzils (32, 33, 39 and 49) (Scheme 13) were investigated for their cytotoxic effect towards the acetyl-protected amino acid arginine (cell proliferation assay) and their photosensitizing effect using the local lymph node assay (LLNA). The cell proliferation assay results indicated a high toxicity to the compounds 32, 33 and 39, whereas it was not possible to determine the cytotoxicity of compound 49. It was concluded that all tested benzils were cytotoxic. In the LLNA, no effect was reported to the compounds 33 and 49, whereas compounds 32 and 39 gave a response, but detailed analysis led to the conclusion that this finding was due to their cytotoxicity and not photosensitivity. All tested arylglyoxals were shown to be strong sensitizers by LLNA. Although the chemical properties of the various arylglyoxal moieties are very different, no significant difference in their sensitizing capacity was detected. Because the compounds 29 and 47 are chemically similar, only the reactivity of 29, 47 and 48 towards acetyl-protected arginine was tested in the cell proliferation assay. All three arylglyoxals showed no difference in reactivity towards the arginine, which confirmed that the arylglyoxal moiety is not relevant to its allergic potential. The benzils are therefore cytotoxic rather than photosensitive, while the arylglyoxals were proved to be strong sensitizers [73].

J. Kockler et al. / Journal of Photochemistry and Photobiology C: Photochemistry Reviews 13 (2012) 91–110

Inhibition of reproduction (%)

a

O

30

H

20

R

14 h

28: R = OCH3 29: R = C(CH3)3 47: R = CH(CH3)2 48: R = NO2

10 28 h 42 h

5 77 h

0 0

50

100

150

200

Concentration of UV-filter 1 [ng/mL]

Inhibition of reproduction (%)

14 h

O

42 h

60 40 20 77 h

0 -20

32: R1 = C(CH3)3, R2 = C(CH3)3 33: R1 = C(CH3)3, R2 = OCH3 39: R1 = H, R2 = H 49: R1 = OCH3, R2 = OCH3

Scheme 13. Tested benzils (32, 33, 39 and 49) and arylglyoxals (28, 29, 47 and 48) for their cytotoxicity and photosensitive potential using a local lymph node assay (LLNA) and a cell proliferation assay.

0h

0

146 h

200

400

600

800

1000

Concentration of UV-filter 2 [ng/mL]

Inhibition of reproduction (%)

R1

Adapted from Ref. [73].

80

-40

c

O

15

-5

b

R2

O

0h

25

103

90

0h

14 h

85 80 75 42 h

70 65

72 h

60 55 50 0

100

200

300

400

500

Concentration of UV-filter 6 [ng/mL] Fig. 3. Algal toxicity of the mixture of UV-filters and their degradation products: 1 (3a), 2 (3b) and 6 (3c). Pictured as a function of the remaining concentrations of the UV-filter, after UV-irradiation, against the reproduction inhibition of the algae S. vacuolatus in %. Adapted from Ref. [55].

3. Effects of other active ingredients on photostability of the chemical UV-filters 3.1. Addition of physical UV-filters (metal oxides) Although there is a wide range of chemical UV-filters, there are only two physical UV-filters available: titanium dioxide (TiO2 ) and zinc oxide (ZnO), which are both broad spectrum filters. A review of 308 sunscreen products in the United Kingdom (UK) showed that 3.6% contained only a physical UV-filter (TiO2 and/or ZnO), 41.6%

contained a mixture of chemical and physical UV-filters and the remaining 54.8% contained only chemical UV-filters. It can therefore be concluded that nearly half of the sunscreen products contain metal oxides, with 90.2% of these containing TiO2 and 9.8% ZnO [74]. Depending on their particle size, they function by absorbing and reflecting UV-light. Nanoparticles of TiO2 and ZnO with particle sizes below 370 nm mainly absorb UV-light and practically no light is reflected. They are widely used as UV-filters, because they cause less skin irritation and therefore are more suitable for use on sensitive skin and for children [75]. Although there has been some concern that these particles are small enough to penetrate to the viable layers of the skin, it has been proven that this is not the case and that they remain on the surface or the stratum corneum [12,75–78]. The disadvantage of TiO2 and ZnO is that they may have an effect on the photostability of chemical UV-filters because of their ability to generate ROS by absorbing photon energy from the UV-light (Scheme 14). By absorbing light an electron migrates from the electron-filled valence band to the vacant conduction band thus creating an electron-hole-pair (i). ROS, mainly hydroxyl radicals OH• (ii) and superoxide radicals O2 •− (iii) are subsequently generated [79–83]. Other reactive species, such as hydrogen peroxide H2 O2 or singlet oxygen 1 O2 can also be formed (Scheme 15) [80,84]. These ROS may cause degradation of other sunscreen ingredients (by photocatalysis), skin damage or both [85]. TiO2 is available in two different polymorphic forms, the rutile and anastase. While the anastase form is generally more photoactive and is thus used as photocatalyst, the rutile form has lower photocatalytic activity [79,80,86,87]. The photocatalytic effect and the resulting possible degradation of UV-filters were studied by Dondi et al. [68]. A complete degradation of the chemical UV-filters 1 and 3 in the presence of TiO2 was shown. Both UV-filters were dissolved in acetonitrile with or without TiO2 and irradiated for 10 h in a solar simulator (total UV irradiance of 27 W/m2 ), before their concentration was determined by HPLC. The concentration of both chemical UV-filters in a suspension containing TiO2 was less than 10% that of the irradiated solution not including TiO2 [68]. In another study, it was reported that TiO2 induced mineralization of the UV-filters 1, 9, 14 and 19 in water after 20 h of irradiation in a multilamp UVA photoreactor, with a total exposure of 2.5 mJ/cm2 [54].

Scheme 14. Generation of selected ROS by physical UV-filters (TiO2 and ZnO).

104

J. Kockler et al. / Journal of Photochemistry and Photobiology C: Photochemistry Reviews 13 (2012) 91–110

Scheme 15. Generation of hydrogen peroxide (H2 O2 ) and singlet oxygen (1 O2 ). Adapted from Ref. [84].

3.1.1. Coated metal oxides To reduce this photocatalytic activity, the TiO2 - and ZnOnanoparticles used nowadays are mainly coated, for example with silica, aluminium or dimethicone. The photoreactivity of coated (with silica and dimethicone) and uncoated TiO2 and ZnO was studied by measuring the photocatalytic oxidation of isopropyl alcohol to propanone by GC. The metal oxides were irradiated in a batch photoreactor under oxygenated conditions (irradiation time not given). The photoreactivity is represented in terms of moles of isopropyl alcohol converted to propanone per gram of metal oxide per hour of irradiation. As clearly seen in Fig. 4 the photoreactivity of ZnO is much lower than that of TiO2 . The coated metal oxides were also less photoreactive than the uncoated forms, however, the decrease of photoreactivity of ZnO remained within the experimental error for this test [88]. TiO2 coated with silica, aluminium hydroxide, dimethicone/methicone copolymer and ZnO coated with diphenyl capryl methicone were used to investigate SPF changes in several UV-filter combinations containing chemical UV-filters and the physical UV-filters TiO2 and ZnO. The SPF for a combination of UV-filters was predicted and compared to the value achieved on measurement. The UV-filter combinations were incorporated in an O/W emulsion and spread on a PMMA plate before the SPF was measured in vitro. Overall the combinations with TiO2 were shown to be more effective than with ZnO, with a resulting higher SPF value. However only two combinations with TiO2 showed a higher SPF than predicted, but 11 combinations with ZnO showed this effect [89]. Given that there is a wide range of different coatings, the effectiveness of 10 uncoated and coated commercial nanoparticulate TiO2 formulations was studied by measuring the lipid peroxidation of linoleic acid after 120 min of irradiation with a UVB lamp at 2.4 W/m2 . Some of the TiO2 formulations were then further investigated by Electron Spin Resonance (ESR) spectroscopy and the spin trapping technique was used to evaluate the potential to generate ROS and the presence of surface active sites. It was concluded that

Rate of isopropyl alcohol oxidaon

200

197

180 160 140 120

109

100

TiO2

84

80

ZnO

60 40 20

16 3.5

1.5

0 no coang

Dimethicone

Silica

Fig. 4. Relative photoreactivity of ZnO and TiO2 , coated and uncoated, measured by the photocatalytic oxidation of isopropyl alcohol to propanone. Adapted from Ref. [88].

the TiO2 -nanoparticles coated with silica appeared to be the most effective in regards to the protection from ROS [90]. TiO2 , ZnO and their mixtures, extracted from commercial sunscreen products or obtained from manufacturers, were tested for their ability to degrade methylene blue dye [91,92]. The reduction in dye concentration was measured by UV-spectrophotometry and the half-life of methylene blue was determined. The three aluminium coated TiO2 samples and a manganese doped TiO2 showed the best results with half-lives of more than 120 min, whereas the TiO2 particles coated with aluminium and silica showed an intermediate half-life of 75 min. All five TiO2 powders were the pure rutile form of TiO2 , which appeared to be more photostable, because of longer half-lives, than the anastase/rutile mixtures described below. Two anastase/rutile mixtures, one coated with dimethicone and one with organosilane, with half-lives of 35 and 40 min, respectively, indicated moderate photoprotection. The uncoated TiO2 and two further organosilane coated TiO2 mixtures showed short half-lives of 12, 13 and 14 min, respectively. These two organosilane coated TiO2 mixtures contained 10–15% rutile TiO2 , whereas the more stable organosilane coated mixtures (halflife = 40 min) contained 25% rutile TiO2 , which is less reactive than the anastase TiO2 . The physical UV-filter ZnO was also tested for its ability to degrade methylene blue dye in its pure form and mixed with rutile TiO2 . An intermediate half-life of 75 and 76 min was obtained for two different ZnO particles coated with dimethicone, in contrast to the uncoated pure ZnO with a short half-life of 26 min. The uncoated ZnO/TiO2 (rutile) mixture and the two aluminium coated ZnO/TiO2 (rutile) mixtures also showed short half-lives of 11, 14 and 23 min, respectively [93].

3.1.2. Doped metal oxides A new innovation is the use of dopants such as manganese to reduce the ROS generation of the metal oxides [85,94,95]. Manganese doped TiO2 showed additional benefits to that of coated TiO2 [85,93]. These materials were tested comparatively in terms of their effect on UV-filter 3 degradation, determined by HPLC. The manganese doped TiO2 and two coated metal oxides (octylsilylated and silica-alumina coated) were incorporated in an O/W emulsion and irradiated in a solar simulator ( > 290 nm) for 2 h. In the presence of manganese doped TiO2 , 79% of UV-filter 3 remained, while in the formulation with UV-filter 3 alone, only 63% remained. No enhanced photostability of UV-filter 3 was observed with the two coated TiO2 samples. Furthermore, ROS formation for uncoated, undoped TiO2 was reduced by over 90% with the manganese doped TiO2 and only by 30% with the silica-alumina coated TiO2 measured by ESR after irradiation at 2 mW/cm2 . These results confirm that protection of the UV-filter from degradation afforded by manganese doped TiO2 is superior to that of the silica–alumina coated TiO2. [85]. Manganese doped TiO2 was tested against octylsilane coated rutile TiO2 in terms of their effect on the degradation of two UV-filters (1 and 3) and two antioxidants (vitamin E and C). They were incorporated in an emulsion and irradiated for 2 h in a solar simulator. After irradiation the recovery of UV-filter 3 was 20% without TiO2 , 36% with the coated metal oxide and 63% with the manganese doped TiO2 , respectively. The same trend was observed with UV-filter 1 which had a recovery of 24% without TiO2 , 49% with the coated TiO2 and 83% with the manganese doped metal oxide. These results showed the significant advantage of the manganese doped TiO2 over the coated TiO2 . Vitamin E and C evaluated at different concentrations, both showed a recovery of over 90% in a 10% emulsion in the presence of manganese doped TiO2 , whereas the control without TiO2 showed a recovery of 77–78% for both vitamins. The coated metal oxide did not protect the antioxidants and showed a lower recovery compared to the

J. Kockler et al. / Journal of Photochemistry and Photobiology C: Photochemistry Reviews 13 (2012) 91–110

control, with the remaining concentration of vitamin E even lower (24%) than that of vitamin C (about 60%) [96]. The photocatalytic effect upon UVA exposure of five modified TiO2 samples was investigated using different in vitro models, including cultured human skin fibroblasts. In this case manganese doped TiO2 and alumina coated TiO2 offered the most effective cell protection [97]. The photoactivity of undoped and doped ZnO was tested by a colorimetric test method which measures the photobleaching of the radical 1,1-diphenyl-2-picrylhydrazyl (DPPH). The reciprocal of the time it takes to transform the purple radical to its reduced form, which is yellow, is defined as the Photoactivity Index (PI) with units of min−1 . While all four tested dopants (iron, nickel, copper and manganese) showed a reduction in photoactivity compared to the undoped ZnO, nickel, copper and manganese were the most effective dopants [95]. A polymer film containing manganese doped ZnO was tested for its ability to improve the fading rate of a dyed polyester fabric exposed to UV-light compared to a film containing undoped ZnO. The colour change of two dyes (a benzopyran (D1) and an anthraquinone dye (D2)) tested was measured by a Datacolour spectrophotometer. The best protection of the fabric was achieved when the polymer films were separated from the fabric by a quartz slide, where the protection was 2.9 fold higher for D1 and 4.75 fold higher for D2 compared to the untreated fabric. When the ZnO containing film was in direct contact with the dyed fabric, the fading rate was dye dependent. Dye D2 was not protected by the undoped ZnO, whereas the fading rate of D1 decreased significantly (exact data not given). This difference can be explained by the photoreactivity of ZnO towards the different chromophores of the two dyes. The manganese doped ZnO showed a significant decrease of the fading rate in both dyes (exact data not given) [81]. Whereas TiO2 has a stronger absorption in the UVB range, ZnO offers better protection in the UVA range, especially in the UVA I range [89,98]. As reported in a study by El-Boury et al., the SPF of a 25% TiO2 cream was about 38, whereas it was only 7 for a 25% ZnO cream [89]. The UV-absorbance of ZnO is superior in the UVA range, while TiO2 loses its absorbance capacity at about 340 nm, ZnO remains stable until about 370 nm. In this range (340–370 nm), ZnO had a higher absorbance than TiO2 with maximum difference shown at 370 nm [98]. 3.2. Addition of antioxidants To protect the skin from the effects of ROS, antioxidants such as ␣-tocopherol (vitamin E) or ascorbic acid (vitamin C) can be added to the sunscreen product. These antioxidants scavenge the ROS before they reach the cells and thus prevent secondary molecular and cellular skin damage [10,79]. The protective nature of vitamin E in the presence of the UV-filters 1, 3 and 19 has been studied by measuring the reduction in lipid peroxidation via TBARS (thiobarbituric acid reactive substances) formation. The UV-filters alone (incorporated in multilamellar PC liposomes), vitamin E, vitamin E acetate and some combinations were dispersed in 5 mM phosphate buffer, 0.9% NaCl, 0.1 mM EDTA, pH 7.4 and irradiated with UVA-light (flux of

Fig. 5. ROS generation of substances, measured via TBARS concentration dissolved in acetonitrile and irradiated with UVA-light for 20 min. : no UVA exposure, : : UVA exposure in the presence of UV-filters 1, 3, 19, vitamin E UVA exposure, (VitE), vitamin E actate (VitEAc) alone and their combinations ( ). Adapted from Ref. [99].

23 mW/cm2 between 300 and 400 nm) for 20 min. The control sample (liposomal suspension without UV-filters and antioxidants), on UVA-light exposure showed a three times higher lipid peroxidation than the non-irradiated control sample. In the presence of UV-filter 3, the TBARS concentration was higher, compared to the irradiated control, while UV-filter 19 had no effect and 1 inhibited this process (Fig. 5). Vitamin E reduced the TBARS concentration almost to the level of the non-irradiated sample, whereas the lipid peroxidation in the presence of vitamin E acetate was similar to that of the irradiated control. This difference can be explained by the antioxidant activity of vitamin E (50), which is caused by a hydrogen donation to a radical from the hydroxyl group on the benzene ring, which is not present in the acetate (51) (Scheme 16). However, in experiments with viable human skin, it was shown that the vitamin E acetate bioconverts into its active form, vitamin E. The ROS generation by the combination of vitamin E and UV-filter 3 is about double that of vitamin E alone, but still significantly less than in the sample with UV-filter 3 alone. In a combination of the UV-filters 1, 3 and 19 with vitamin E the ROS generation was further decreased (Fig. 5). This outcome was attributed to the stabilising effect of UV-filter 19 on UV-filter 3 [99]. It has been demonstrated that topically applied vitamin C reduces the number of sunburn cells (dyskeratotic basal epidermal cells) on porcine skin after irradiation with a UVB lamp (emission centres around 311 nm) at a complete dose of 400 mJ/cm2 . In the control sample 33.1 sunburn cells could be identified in a 4 mm biopsy tissue, whereas the vitamin C treated sample showed only 20.5 sunburn cells. The difference between control and vitamin C treated skin was even greater when the skin was treated with 8-methoxypsoralen plus UVA (PUVA) from fluorescent lamps (emission peak around 360 nm) at a complete dose of 500 mJ/cm2 . The number of sunburn cells in the control was more than double that in the vitamin C treated skin (48.4 cells) with 114.5 sunburn cells in a 4 mm biopsy tissue [100].

O

HO O

O

105

O

50

51 Scheme 16. Vitamin E (50) and Vitamin E acetate (51).

106

J. Kockler et al. / Journal of Photochemistry and Photobiology C: Photochemistry Reviews 13 (2012) 91–110

Table 4 TiO2 samples with composition and crystal phase in % [103]. Commercial name

Composition (%)

Crystal phases (%)

Maxlight F-TS20 PW Covasil S-1

TiO2 = 75, SiO2 = 22 TiO2 > 95, trymethoxycaprylylsilane (TMCS) < 5 TiO2 > 50, SiO2 = 10–25, TMCS = 4.5 TiO2 = 84, Al(OH)3 = 7, Dimethicone = 4.5 TiO2 = 78, Al(OH)3 = 3.5, Silica = 7.5, Dimethicone = 5.5 TiO2 = 100

100 rutile 80 anastase, 20 rutile

Tego Sun TS plus T-Lite SF T-Lite SF-S

Aeroxide P 25

80 anastase, 20 rutile 100 rutile 100 rutile

80 anastase, 20 rutile

The protective effect of the oil-soluble vitamin E and the watersoluble vitamin C against lipid peroxidation has been proven in the presence of eicosapentaenoic acid (EPA) on porcine skin [101,102]. It was established that this acid protects against immunosuppression and photocarcinogenesis caused by UV-radiation, but this effect is limited due to oxidative degradation. EPA was applied in acetone onto the porcine skin initially and after a short drying period vitamin E, vitamin C or a combination of both in 20% (v/v) DMSO in ethanol was applied and dried for 60 min to allow the components to penetrate into the skin. After an irradiation time of 18.5 min using a 40 W/12 lamp light source (spectral region 260–390 nm; peak at 310 nm) the malondialdehyde (MDA)concentrations, an indicator of lipid peroxidation, in the viable epidermis, were measured by HPLC. Each vitamin alone protected the EPA treated skin from lipid peroxidation completely, which implies that the MDA-concentration was reduced to the level of the non-irradiated sample without an antioxidant. However, a 500fold higher concentration of vitamin C (5000 nmol/cm2 ) than that of vitamin E (10 nmol/cm2 ) was required. A total protection was also achieved by the combination of both vitamins, even with lower doses of 100 nmol/cm2 for vitamin C and 5 nmol/cm2 for vitamin E. The higher photostability of vitamin E in the presence of vitamin C is attributed to the combined action of these antioxidants. When vitamin E quenches ROS, it forms a radical which reverts to the non-radical vitamin E molecule in the presence of vitamin C. Thus vitamin C has a role in regenerating vitamin E [102]. Carlotti et al. [103] investigated the effect of the antioxidants phenylalanine, sodium ascorbyl phosphate and ascorbyl palmitate on the oxidation of linoleic acid and porcine skin in the absence and presence of TiO2 . The antioxidants were added separately to a sodium dodecyl sulphate (SDS) aqueous solution (pH 4.0) containing 1% linoleic acid or pieces of porcine skin. The mixture was irradiated for 2 h with a UVB lamp at 2.4 W/m2 . One sample was irradiated without TiO2 and six with different compositions of this photocatalyst (Table 4), whose catalytic quantities were investigated in a previous study [90]. The formation of the linoleic peroxidation product, MDA, was determined by a 2-thiobarbituric acid (TBA) assay. The amino acid phenylalanine did not protect against the peroxidation of linoleic acid in the absence of the photocatalyst, as well as in the presence of the three TiO2 variations (Maxlight F-TS20, Tego Sun TS plus, T-Lite SF-S) which have been found to be protective against peroxidation. Only in the presence of the three most active TiO2 nanoparticles (Aeroxide P 25, T-Lite SF, PW Covasil S-1) was phenylalanine protective against the formation of MDA. The two ascorbic acids, sodium ascorbyl phosphate and ascorbyl palmitate, behaved similarly in that both had a protective effect on linoleic acid, both without the photocatalyst as well as with all six TiO2 variations. The protective effect of all three antioxidants was dose dependent. When tested with porcine skin only the antioxidant concentration with the best protective effect was used both without TiO2 and with two different TiO2 compositions,

Fig. 6. MDA concentration produced from lipids of porcine skin by peroxidation in the absence and presence of TiO2 (PW Covasil S-1 and Tego Sun TS plus), with or without the antioxidants phenylalanine (Phe), sodium ascorbyl phosphate (SAPh) and ascorbyl palmitate (AP). Adapted from Ref. [103].

one with a high catalytic character (PW Covasil S-1) and one with a low catalytic character (Tego Sun TS plus) (Fig. 6). In contrast to the results achieved in the linoleic acid peroxidation experiments, phenylalanine showed a protective effect without and with both TiO2 compositions. Sodium ascorbyl phosphate showed about one third lower peroxidation of porcine skin in the absence of TiO2 compared to the sample without antioxidant. These findings stand in contrast to the peroxidation of linoleic acid, where the peroxidation was nearly zero. Ascorbyl palmitate showed a notable (one third to one fifth) and nearly equal reduction in peroxidation in all three samples (with and without TiO2 ), attributed to its lipophilic character, which allows a good penetration through the skin (Fig. 6). Ascorbyl palmitate appeared to be the best antioxidant because of its stability, lipophilicity and scavenging potential [103]. 4. Change of UV-protective performance by formulation excipients 4.1. Different solvents or formulations It has been reported in different studies, that solvents can increase or decrease the stability of UV-filters [33,36,63,64,104]. The photoreactivity of UV-filter 3 was investigated in dilute solvents of various polarities, namely dioxane, acetonitrile, ethyl acetate, tetrahydrofuran (THF), ethanol, isopropyl alcohol, hexane, heptane, cyclohexane and water. The UV-absorbance spectra were determined after irradiation in a solar simulator at a complete dose of 60 kJ/m2 (4 min interval at 250 W/m2 ). In the solvents dioxane, acetonitrile, ethyl acetate, THF, ethanol and isopropyl alcohol UVfilter 3 was regarded as stable because the loss of absorbance was in most cases only 1–2%. On the other hand, in the non-polar solvents hexane, heptane and cyclohexane, UV-filter 3 was unstable upon irradiation. The absorption maximum of UV-filter 3 lost about 75% of its value in heptane; in hexane and cyclohexane a reduction in the absorption maximum was recognised as well but exact data were not given. The instability of UV-filter 3 was reversible after 12 h in the darkness but completely inhibited by the addition of 1% isopropyl alcohol to the non-polar solvents. The phenomenon of the reversibility of the photodegradation was not detected in water. It is suggested that under the described conditions in water degradation of UV-filter 3 occurs [63]. The instability of UV-filter 3 to light in most non-polar solvents, and its photostability in most polar solvents, was confirmed by other authors [36,64] (Table 5). Schwack and Rudolph investigated UV-filter 3 in cyclohexane and isooctane, where it degraded upon irradiation, and in isopropyl alcohol and methanol, where it

J. Kockler et al. / Journal of Photochemistry and Photobiology C: Photochemistry Reviews 13 (2012) 91–110 Table 5 Photostability of the UV-filter 3 in different solvents in air [36,63,64]. Solvents

Huong et al. [63]

Dioxane Acetonitrile Ethyl acetate THF Ethanol Methanol Isopropyl alcohol Hexane Heptanes Cyclohexane Isooctane DMSO

Stable Stable Stable Stable Stable Stable Unstable Unstable Unstable

Schwack and Rudolph [36]

Mturi and Martincigh [64]

Unstable

Stable Stable

Stable

Unstable Unstable

Unstable

107

Table 6 Absorbance of 7 commercial sunscreen products before UV exposure and photoinactivation after increasing UV exposure in the UVA range [105]. Product

A1 A2 A3 B1 B2 B3 B4

Absorbance A (%)

97.7 99.9 99.9 99.4 98.4 99.9 95.5

Photoinactivation, A (%), by biologically effective UV exposure (SED) 5

12.5

25

50

5.3 0.1 0.0

13.8 0.0 0.1 15.7 13.2 0.1 18.2

26.5 0.0 0.6 28.1 24.3 0.5 31.8

32.7

7.7 0.0 8.3

29.7

Unstable

remained photostable. The photodegradation process was monitored by HPLC and GC/MS during irradiation with UV-light in a solar simulator purged with air. The solar simulator was used with two different glass-filters: filter F1 ( > 260 nm) resulting in an irradiance of 12.4 mW/cm2 UVA and 0.54 mW/cm2 UVB; and filter F2 ( > 320 nm) resulting in an irradiance of 11.1 mW/cm2 UVA. In cyclohexane the degradation was exponential with both filters, whereas the degradation was about 8% with filter F2, and 14% with filter F1. Data for the degradation in isooctane were not given [36]. Mturi and Martincigh [64] showed the photostability of UVfilter 3 in methanol and its instability to light in DMSO, ethyl acetate and cyclohexane (Table 5) during irradiation with a high-pressure mercury lamp using a pyrex filter (>300 nm) purged with air. Analysing the results involved a combination of UV spectrophotometry, HPLC, GC/MS and NMR spectrometry. GC-analysis of UV-filter 3 in DMSO showed photodegradation after an irradiation time of 18 h, although the GC-column did not separate the diketo and enol form. In ethyl acetate only two degradation products were detected, whereas in cyclohexane nine products, in addition to UV-filter 3, were detected. HPLC analysis after 15–18 h of irradiation showed a decrease of 75% for the enol form of UV-filter 3 in DMSO, 56% in cyclohexane, 33% in ethyl acetate and only 7% in methanol. The comparison of the 1 H NMR spectra of UV-filter 3 before and after an irradiation time of 21 h showed an increase of the methylene proton-signal in DMSO, which indicated only photoisomerisation of the enol to the diketo form. The 1 H NMR revealed three major degradation products in cyclohexane after an irradiation time of 25 h, whereas in methanol no change between the unirradiated and the irradiated sample was detected. The absence of air had no effect on the photostability of UV-filter 3 in methanol, but in the other three solvents (cyclohexane, ethyl acetate and DMSO) the UV-filter was more stable in the absence of oxygen. While the difference was not significant in cyclohexane and ethyl acetate, in DMSO 97% of UV-filter 3 degraded after 40 min of irradiation in the presence of oxygen, while only 7% degraded in the absence of oxygen [64]. The instability in ethyl acetate was not consistent with the results of Huong et al. [63] (for details see above) (Table 5). Although Huong et al. only used UV spectrophotometry to analyse the photostability in different solvents [63], Mturi and Martincigh used a combination of analytical techniques [63,64]. A 1 H NMR showed the formation of 3.5% of the diketo form of UV-filter 3 in cyclohexane-d12 , but no diketo formation in isopropyl alcohol-d8 . These results show that the photodegradation of UV-filter 3 strongly depends on the formation of the 1,3-diketo form (see also Section 2.3) [36]. Photostability of the UV-filters 1, 5, 6, 9 and 15 was investigated in four solvents (equilibrated in air) of different polarities after using a Hg/Xe lamp ( > 290 nm) excluding infrared light. After 30 min of irradiation, photodegradation of UV-filtert 1 was observed in all four solvents: 90% in water, 40% in methanol, 45% in acetonitrile and 40% in hexane. The degradation in the least

polar solvent hexane was nearly complete (approximately 95%) after an irradiation time of 2 h. In all cases the degradation was attributed to a trans-cis isomerisation reaction (Scheme 1). Not only the degradation, but also the spectral band of absorbance was found to be solvent dependant. In water, the absorbance maximum was at 320 nm, whereas in hexane it was shifted to 289 nm. In addition, the 320 nm maximum in water shifted to about 280 nm after an irradiation time of 30 min, which has the ability to impact on its UV performance in a sunscreen. After 2 h of irradiation UV-filter 9 was more photostable than UV-filter 1. The degradation was about 5–10% in acetonitrile, 15% in hexane, 20% in water and around 95% in methanol, implying photostability in the first three solvents, but instability in methanol. In contrast to UV-filter 1, the shift of the absorbance maximum was relatively small, remaining in the range of 283–287 nm and 321–325 nm. After an irradiation time of 60 min, UV-filter 5 showed photodegradation in all four solvents with 87% in hexane, 65% in water, 60% in methanol and 45% in acetonitrile. The onset of the UV-absorption maximum was generally shifted from 325 nm in water to 305 nm in hexane, which has the ability to impact on the photoprotective character of UVfilter 5, due to an inability to provide adequate protection in the UVB range. UV-filter 6 showed a nearly complete photodegradation after 20 min of irradiation in hexane (97%) and acetonitrile (94%), while in water the degradation was 75% and in methanol it was the lowest at 15%. The absorption maximum of UV-filter 6 in all four solvents shifted during irradiation, in water from 311 nm to 277 nm after 30 min, in methanol from 311 nm to 292 nm after 80 min, in acetonitrile from 309 nm to 266 nm after 30 min and in hexane from 299 nm to 259 nm after 20 min of irradiation. These shifts thus impact negatively on the UV-protective effect for UVfilter 6. Because of the low solubility in hexane, the photostability of UV-filter 15 was only determined in the other three solvents. The photodegradation was rapid in water with 90% of UV-filter 15 degraded after only 10 min. After 20 min irradiation in acetonitrile 50% of 15 was degraded and after 2 h the degradation reached 70%. In contrast to the other tested UV-filters no shift in the absorption maximum or absorption range could be detected for UV-filter 15 [33]. The isomerisation of UV-filter 1 was investigated by HPLC analysis and calculation of the percentage of both isomers (trans- and cis-isomers) in heptane, dioxane, ethyl acetate, THF, acetonitrile, isopropyl alcohol and water was undertaken. The UV-filter was dissolved in each solvent and irradiated for 10 min at 250 W/m2 (= 150 kJ/m2 ) in a solar simulator. The recovery of the trans-isomer, which is the active form, was the highest in heptane and water with 53% and 55.4%, respectively, whereas in isopropyl alcohol and acetonitrile the recovery of the trans-isomer was the lowest with 34.2% and 33.6%, respectively. The analysis in the solvents dioxane, ethyl acetate and THF gave similar percentage recovery of the transisomer with 41.5%, 43.5% and 40.6%, achieved. The sum of the transand cis-isomer was in all solvents around 100%, only in water it was

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lower (71.7%) which indicates a structural degradation of UV-filter 1, with the mechanism for this degradation unknown [67]. In a photostability study of 16 commercial sunscreen products it was shown that the same UV-filter combinations are photostable in some formulations and unstable to light in others. The sunscreen products were spread on a quartz plate and irradiated in a solar simulator with different standard erythemal doses (5, 12.5, 25, 50) (SED), where 1 SED is equivalent to an erythemal effective radiant exposure of 100 J/m2 . The spectral absorbance before and after the different doses was measured for the UVA and UVB range. The mean difference of the absorbance before and after exposure A was ≤1% in the UVB range for all tested sunscreen products after UV exposure of 25 SED. In contrast, some sunscreen products showed a A of up to 48.4% in the UVA range, after an UV exposure of 25 SED. The sunscreen products with a A ≤ 1% were regarded as photostable. Out of three sunscreen products (A1, A2, A3), with the same UV-filter combination (TiO2 , 1, 3, 8, 13), two were regarded as photostable with A = 0.0% and 0.6%, respectively and one was regarded as unstable to light with A = 26.5% after UV exposure (Table 6). Another UV-filter combination (TiO2 , 1, 3, 13) was found in four tested sunscreen products (B1, B2, B3, B4), where three were unstable to light with A = 28.1%, 24.3% and 31.8%, respectively and one was photostable with A = 0.5% (Table 6). All these products were sun milks, except one of the photostable sunscreen products (A3), which was a lotion. Further details about the differences in the formulation or the reason for the different photostability were not provided [105]. These examples illustrate that not only the UV-filter combination has an influence on the photostability of a sunscreen product, but that the choice of the formulation components is also very important. There are different kinds of sunscreen formulations available on the market, for example creams, lotions, milks, sprays or gels. A sunscreen product should spread easily and uniformly on the skin for the entire time of the UV exposure. Achieving a uniform film is usually easier with emulsified systems such as creams and lotions in contrast to gels, whose sheer thinning capability is often much lower. However the oily component in creams and lotions, which tend to be greasy is not very popular with consumers. The advantage of O/W lotions and creams is that they contain less of the oily component, compared to W/O formulations, and are therefore less greasy. Alcohol/water-based sprays or gels are even less greasy and therefore highly favoured by consumers working outdoors. In addition, they have a cooling effect on the skin and are completely transparent after application. The disadvantage of these products with a high concentration of water is that they can degrade more readily, are more susceptible to microbial contamination and an uneven application is more likely to occur due to evaporation of the alcoholic/aqueous base [106,107]. 4.2. Inclusion of chemical UV-filters in cyclodextrins Cyclodextrins, which are often included in drug products because of their ability to improve the solubility of poorly water soluble drugs may affect the photostability of drugs in one of three ways: photostability may be increased, decreased or remains unchanged [108]. Glass and coworkers have reported on the improved photostability of triprolidine due to the presence of cyclodextrins, which is an important finding due to the potential of triprolidine to undergo a cis-trans isomerisation in the presence of light [109]. Thus, there is the potential to increase the photostability of UVfilters by addition of cyclodextrins to the formulations. UV-filter 6 showed an improved photostability when it was complexed with hydroxypropyl-␤-cyclodextrin (HP-␤-CD). In ethanol, the degradation of the UV-filter 6 was 54.6%, whereas for the complex with HP-␤-CD the degradation was 25.5% (measured by HPLC) after 4 h of

irradiation in a solar simulator with a 200 W xenon-mercury lamp. The same photostability test was performed in an O/W emulsion where the difference was not as pronounced as in ethanol. The degradation for the complex was 25.1%, whereas for the UV-filter 6 alone it was 33.4% [110]. The photostability of a complex between UV-filter 1 and cyclodextrins (␤-cyclodextrin (␤-CD) or HP-␤-CD) was studied by HPLC after incorporation in an O/W emulsion and irradiation for 4 h in a solar simulator with a 200 W xenon-mercury lamp. The complexation with HP-␤-CD showed no significant effect, but the degradation was reduced from 35.8% for the UV-filter 1 to 26.1% for the complex with ␤-CD [111]. Complexation of UV-filter 3 with HP-␤-CD in an ethanol-water solution increased the photostability of this UV-filter. The complex and UV-filter 3 alone were irradiated for 2 h in a solar simulator (290–410 nm) with an irradiance of about 0.4 mW/cm2 for UVB and 2.5 mW/cm2 for UVA. The degradation (measured by HPLC) was 63.1% for the UV-filter 3 and 27.6% for the complex of UV-filter 3 and HP-␤-CD [112]. In aqueous solution UV-filter 3 showed a recovery of 16.6% after an irradiation of 500 kJ/m2 in a solar simulator, whereas UV-filter 3 incorporated with 30% HP-␤-CD showed a recovery of 79.5%. To completely solubilise UV-filter 3 only 20% HP-␤-CD was required, but an excess of 10–30% HP-␤-CD shifted the theoretical equilibrium of the complexation reaction towards the complexed form and thus resulted in a greater photostabilisation of the UV-filter [113]. 5. Conclusions Protection against UV exposure and a reduction in photosensitivity effects requires that sunscreen formulations are photostable. Because of their ability to absorb UV-light, chemical UV-filters have the potential to degrade in the presence of light. Broad spectrum sunscreens often include more than one chemical UV-filter, a physical filter, an antioxidant and other formulation excipients. The photostability of chemical UV-filters alone and in combination has to some extent been investigated producing varying results. The effect on photostability of chemical UV-filters is shown when UV-filter 11 which showed 100% recovery after irradiation, in the presence of UV-filter 3 was reduced to 57%. Because broad spectrum sunscreens may contain in addition physical UV-filters such as titanium dioxide generating ROS, which may photocatalyse the degradation of the chemical UV-filters, photostability studies of these combinations are important. Even though this photocatalytic effect may be overcome by using coated oxides, this adds to the cost of the final product and may not be used by manufacturers since there is no regulatory requirement for photostability testing. It is therefore important that photostability testing of these products be undertaken to ensure not only the quality and safety but also the efficacy of these sunscreen products. Acknowledgements JK would like to thank the School of Pharmacy and Molecular Science at James Cook University for a scholarship. MO and BDG would like to express their appreciation to James Cook University for financial support (FAIG grants 2009 and 2011), for this project. References [1] L. Marrot, J.R. Meunier, J. Am. Acad. Dermatol. 58 (2008) S139–S148. [2] S. Madronich, R.L. McKenzie, L.O. Bjorn, M.M. Caldwell, J. Photochem. Photobiol. B: Biol. 46 (1998) 5–19. [3] M.V.R. Velasco, F.D. Sarruf, I.M.N. Salgado-Santos, C.A. Haroutiounian-Filho, T.M. Kaneko, A.R. Baby, Int. J. Pharm. 363 (2008) 50–57. [4] P. Kullavanijaya, H.W. Lim, J. Am. Acad. Dermatol. 52 (2005) 937–958.

J. Kockler et al. / Journal of Photochemistry and Photobiology C: Photochemistry Reviews 13 (2012) 91–110 [5] K. Hirakawa, H. Suzuki, S. Oikawa, S. Kawanishi, Arch. Biochem. Biophys. 410 (2003) 261–268. [6] R.B. Setlow, E. Grist, K. Thompson, A.D. Woodhead, Proc. Natl. Acad. Sci. U. S. A. 90 (1993) 6666–6670. [7] E.C. De Fabo, F.P. Noonan, T. Fears, G. Merlino, Cancer Res. 64 (2004) 6372–6376. [8] Cancer Council, Sun protection media kit, Summer 2010–11, 2010. Available: (accessed on: 23.11.10). [9] D.M. Parkin, F. Bray, J. Ferlay, P. Pisani, CA-Cancer J. Clin. 55 (2005) 74–108. [10] P.U. Giacomoni, L. Teta, L. Najdek, Photochem. Photobiol. Sci. 9 (2010) 524–529. [11] F. Urbach, J. Photochem. Photobiol. B: Biol. 64 (2001) 99–104. [12] J.F. Nash, Dermatol. Clin. 24 (2006) 35–50. [13] Australian/New Zealand Standard: Sunscreen products—Evaluation and classification, AS/NZS 2604:1998 Australian/New Zealand Standard. [14] G. Wakefield, M. Green, S. Lipscomb, B. Flutter, Mater. Sci. Technol. 20 (2004) 985–988. [15] D. Moyal, Photochem. Photobiol. Sci. 9 (2010) 516–523. [16] N.S. Agar, G.M. Halliday, R.S. Barnetson, H.N. Ananthaswamy, M. Wheeler, A.M. Jones, Proc. Natl. Acad. Sci. U. S. A. 101 (2004) 4954–4959. [17] A. Fourtanier, F. Bernerd, C. Bouillon, L. Marrot, D. Moyal, S. Seite, Photodermatol. Photoimmunol. Photomed. 22 (2006) 22–32. [18] European Commission, Commission Recommendation of 22 September 2006 on the efficacy of sunscreen products and claims made relating thereto. Available: , 2006 (accessed on: 09.03.11). [19] Therapeutic Goods Administration, Australian Regulatory guidelines for OTC medicines (ARGOM). Available: , 2003 (accessed on: 17.01.11). [20] Colipa, International sun protection factor (SPF) test method. Available: , 2006 (accessed on: 09.03.11). [21] J. Hojerová, A. Medovcíková, M. Mikula, Int. J. Pharm. 408 (2011) 27–38. [22] Sunscreen drug products for over-the-counter human use; propopsed amendment of Final Monograph; proposed rule. Federal Register/Vol. 72, No. 165 , 2007. [23] Sunscreen drug products for over-the counter human use; Final Monograph Federal Register/Vol. 64, No. 98 , 1999. [24] B.D. Glass, Aust. Pharm. 25 (2006) 148–153. [25] B.D. Glass, C. Novak, M.E. Brown, J. Therm. Anal. Calorim. 77 (2004) 1013–1036. [26] D.S. Maharaj, S. Anoopkumar-Dukie, B.D. Glass, E.M. Antunes, B. Lack, R.B. Walker, S. Daya, J. Pineal Res. 32 (2002) 257–261. [27] D.S. Maharaj, B.D. Glass, S. Daya, S. Afr. J. Pharm. 69 (2002) 29–31. [28] D.S. Maharaj, H. Molell, E.M. Antunes, H. Maharaj, D.M. Maree, T. Nyokong, B.D. Glass, S. Daya, J. Pineal Res. 38 (2005) 153–156. [29] P.M. Drummond, B.D. Glass, M.E. Brown, in: A. Albini, E. Fasani (Eds.), Drugs: Photochemistry and Photostability, Royal Society of Chemistry, 1998, pp. 134–150. [30] B.D. Glass, in: A. Griesbeck, F. Ghetti, M. Oelgemöller (Eds.), CRC Handbook of Organic Photochemistry and Photobiology, 3rd ed., CRC Press, Boca Raton, FL, 2012, pp. 1003–1028. [31] C.A. Bonda, in: N. Shaath (Ed.), Sunscreens Regulations and Commercial Development, 3rd ed., Taylor and Francis Group, New York, 2005, pp. 321–349. [32] B. Brewster, in: A.C. Kozlowski (Ed.), Sun Science: Formulating of Protection, Allured Books, Carol Stream, IL, 2009, pp. 3–14. [33] N. Serpone, A. Salinaro, A.V. Emeline, S. Horikoshi, H. Hidaka, J.C. Zhao, Photochem. Photobiol. Sci. 1 (2002) 970–981. [34] C. Couteau, M. Power, E. Paparis, L.J.M. Coiffard, Pharmazie 62 (2007) 449–452. [35] G. Vielhaber, S. Grether-Beck, O. Koch, W. Johncock, J. Krutmann, Photochem. Photobiol. Sci. 5 (2006) 275–282. [36] W. Schwack, T. Rudolph, J. Photochem. Photobiol. B: Biol. 28 (1995) 229–234. [37] The Merck Index [Online]. Medicines Complete Available: , (accessed on: 12.07.11). [38] N.A. Shaath, Photochem. Photobiol. Sci. 9 (2010) 464–469. [39] British Pharmacopoeia. Volume II. Monographs: Medicinal and Pharmaceutical Substances. Mexenone, British Pharmacopoeia Commision, London, 2011. [40] U. Osterwalder, H. Luther, B. Herzog, Bundesgesundheitsbl. Gesundheitsforsch. Gesundheitsschutz 44 (2001) 463–470. [41] European Commission, Regulation (EC) No 1223/2009 of the European Parliament and the Council of 30 November 2009 on cosmetic products. Available: , 2009 (accessed on: 09.03.11). [42] N. Tarras-Wahlberg, G. Stenhagen, O. Larko, A. Rosen, A.M. Wennberg, O. Wennerstrom, J. Invest. Dermatol. 113 (1999) 547–553. [43] C. Couteau, A. Faure, J. Fortin, E. Paparis, L.J.M. Coiffard, J. Pharm. Biomed. Anal. 44 (2007) 270–273. [44] C. Couteau, S. El-Boury, E. Paparis, V. Sebille-Rivain, L.J.M. Coiffard, Pharm. Dev. Technol. 14 (2009) 369–372. [45] E. Chatelain, B. Gabard, Photochem. Photobiol. 74 (2001) 401–406. [46] V. Lhiaubet-Vallet, M. Marin, O. Jimenez, O. Gorchs, C. Trullas, M.A. Miranda, Photochem. Photobiol. Sci. 9 (2010) 552–558. [47] L.R. Gaspar, P. Campos, Int. J. Pharm. 307 (2006) 123–128. [48] J.M. Allen, C.J. Gossett, S.K. Allen, J. Photochem. Photobiol. B: Biol. 32 (1996) 33–37. [49] J.M. Allen, C.J. Gossett, S.K. Allen, Chem. Res. Toxicol. 9 (1996) 605–609. [50] J.J. Inbaraj, P. Bilski, C.F. Chignell, Photochem. Photobiol. 75 (2002) 107–116. [51] K.M. Hanson, E. Gratton, C.J. Bardeen, Free Radic. Biol. Med. 41 (2006) 1205–1212.

109

[52] E. Damiani, W. Baschong, L. Greci, J. Photochem. Photobiol. B: Biol. 87 (2007) 95–104. [53] B. Herzog, M. Wehrle, K. Quass, Photochem. Photobiol. 85 (2009) 869–878. [54] A. Ricci, M.N. Chretien, L. Maretti, J.C. Scaiano, Photochem. Photobiol. Sci. 2 (2003) 487–492. [55] R. Rodil, M. Moeder, R. Altenburger, M. Schmitt-Jansen, Anal. Bioanal. Chem. 395 (2009) 1513–1524. [56] G. Bonda, Cosmet. Toiletries 123 (2008) 49–60. [57] A. Kikuchi, M. Yagi, Chem. Phys. Lett. 513 (2011) 63–66. [58] C. Bonda, A. Pavlovic, Cosmet. Toiletries 125 (2010) 40–48. [59] C. Bonda, J. Zhang, Cosmet. Toiletries 126 (2011) 40–48. [60] C. Bonda, J. Zhang, A. Pavlovic, Cosmet. Toiletries 126 (2011) 652–660. [61] N.M. Roscher, M.K.O. Lindemann, S.B. Kong, C.G. Cho, P. Jiang, J. Photochem. Photobiol. A: Chem. 80 (1994) 417–421. [62] European Commission, Health and Consumers, Cosing. Available: 2011 (accessed , on: 09.05.11). [63] S.P. Huong, E. Rocher, J.D. Fourneron, L. Charles, V. Monnier, H. Bun, V. Andrieu, J. Photochem. Photobiol. A: Chem. 196 (2008) 106–112. [64] G.J. Mturi, B.S. Martincigh, J. Photochem. Photobiol. A: Chem. 200 (2008) 410–420. [65] S. Pattanaargson, P. Limphong, Int. J. Cosmet. Sci. 23 (2001) 153–160. [66] S. Pattanaargson, T. Munhapol, N. Hirunsupachot, P. Luangthongaram, J. Photochem. Photobiol. A: Chem. 161 (2004) 269–274. [67] S.P. Huong, V. Andrieu, J.P. Reynier, E. Rocher, J.D. Fourneron, J. Photochem. Photobiol. A: Chem. 186 (2007) 65–70. [68] D. Dondi, A. Albini, N. Serpone, Photochem. Photobiol. Sci. 5 (2006) 835–843. [69] Merck, Merck Manual Online medical Library—Photosensitivity reactions. Available: , 2011 (accessed on: 12.04.11). [70] S. Lautenschlager, H.C. Wulf, M.R. Pittelkow, Lancet 370 (2007) 528–537. [71] L. Marrot, J.P. Belaidi, F. Lejeune, J.R. Meunier, D. Asselineau, F. Bernerd, Br. J. Dermatol. 151 (2004) 1234–1244. [72] S.T. Butt, T. Christensen, Radiat. Prot. Dosim. 91 (2000) 283–286. [73] I. Karlsson, L. Hillerstrom, A.L. Stenfeldt, J. Martensson, A. Borje, Chem. Res. Toxicol. 22 (2009) 1881–1892. [74] S. Wahie, J.J. Lloyd, P.M. Farr, Clin. Exp. Dermatol. 32 (2007) 359–364. [75] B.D. More, Indian J. Dermatol. Venereol. Leprol. 73 (2007) 80–85. [76] Australian Government (DHATGA), A Review of the scientific literature on the safety of nanoparticulate titanium dioxide or zinc oxide in sunscreens. Available: , 2009 (accessed on: 17.01.11). [77] H. Gonzalez, Photochem. Photobiol. Sci. 9 (2010) 482–488. [78] K. Schilling, B. Bradford, D. Castelli, E. Dufour, J.F. Nash, W. Pape, S. Schulte, I. Tooley, J. van den Bosch, F. Schellauf, Photochem. Photobiol. Sci. 9 (2010) 495–509. [79] M. Buchalska, G. Kras, M. Oszajca, W. Lasocha, W. Macyk, J. Photochem. Photobiol. A: Chem. 213 (2010) 158–163. [80] O.K. Dalrymple, D.H. Yeh, M.A. Trotz, J. Chem. Technol. Biotechnol. 82 (2007) 121–134. [81] L. Sun, J.A. Rippon, P.G. Cookson, O. Koulaeva, X.G. Wang, Chem. Eng. J. 147 (2009) 391–398. [82] J. Schwitzgebel, J.G. Ekerdt, H. Gerischer, A. Heller, J. Phys. Chem. 99 (1995) 5633–5638. [83] C.D. Jaeger, A.J. Bard, J. Phys. Chem. 83 (1979) 3146–3152. [84] R. Konaka, E. Kasahara, W.C. Dunlap, Y. Yamamoto, K.C. Chien, M. Inoue, Free Radic. Biol. Med. 27 (1999) 294–300. [85] G. Wakefield, S. Lipscomb, E. Holland, J. Knowland, Photochem. Photobiol. Sci. 3 (2004) 648–652. [86] A. Achilleos, E. Hapeshi, N.P. Xekoukoulotakis, D. Mantzavinos, D. FattaKassinos, Chem. Eng. J. 161 (2010) 53–59. [87] R. Dunford, A. Salinaro, L.Z. Cai, N. Serpone, S. Horikoshi, H. Hidaka, J. Knowland, FEBS Lett. 418 (1997) 87–90. [88] M.A. Mitchnick, D. Fairhurst, S.R. Pinnell, J. Am. Acad. Dermatol. 40 (1999) 85–90. [89] S. El-Boury, C. Couteau, L. Boulande, E. Paparis, L.J.M. Coiffard, Int. J. Pharm. 340 (2007) 1–5. [90] M.E. Carlotti, E. Ugazio, S. Sapino, I. Fenoglio, G. Greco, B. Fubini, Free Radic. Res. 43 (2009) 312–322. [91] J.Q. Albarelli, D.T. Santos, S. Murphy, M. Oelgemöller, Water Sci. Technol. 60 (2009) 1081–1087. [92] K. Rajeshwar, M.E. Osugi, W. Chanmanee, C.R. Chenthamarakshan, M.V.B. Zanoni, P. Kajitvichyanukul, R. Krishnan-Ayer, J. Photochem. Photobiol. C: Photochem. Rev. 9 (2008) 171–192. [93] A. Rampaul, I.P. Parkin, L.P. Cramer, J. Photochem. Photobiol. A: Chem. 191 (2007) 138–148. [94] B. Park, P.A. Martin, C. Harris, R. Guest, A. Whittingham, P. Jenkinson, Nanotoxicology 3 (2009) 73–90. [95] P.S. Casey, C.J. Rossouw, S. Boskovic, K.A. Lawrence, T.W. Turney, Superlattic. Microst. 39 (2006) 97–106. [96] G. Wakefield, J. Stott, J. Cosmet. Sci. 57 (2006) 385–395. [97] L. Tiano, T. Armeni, E. Venditti, G. Barucca, L. Mincarelli, E. Damiani, Free Radic. Biol. Med. 49 (2010) 408–415. [98] S.R. Pinnell, D. Fairhurst, R. Gillies, M.A. Mitchnick, N. Kollias, Dermatol. Surg. 26 (2000) 309–313. [99] E. Damiani, L. Rosati, R. Castagna, P. Carloni, L. Greci, J. Photochem. Photobiol. B: Biol. 82 (2006) 204–213.

110

J. Kockler et al. / Journal of Photochemistry and Photobiology C: Photochemistry Reviews 13 (2012) 91–110

[100] D. Darr, S. Combs, S. Dunston, T. Manning, S. Pinnell, Br. J. Dermatol. 127 (1992) 247–253. [101] R.M.W. Moison, R. Doerga, G. Van Henegouwen, Int. J. Radiat. Biol. 78 (2002) 1185–1193. [102] R.M.W. Moison, G.M.J. Beijersbergen van Henegouwen, Radiat. Res. 157 (2002) 402–409. [103] M.E. Carlotti, E. Ugazio, L. Gastaldi, S. Sapino, D. Vione, I. Fenoglio, B. Fubini, J. Photochem. Photobiol. B: Biol. 96 (2009) 130–135. [104] G. Marti-Mestres, C. Fernandez, N. Parsotam, F. Nielloud, J.P. Mestres, H. Maillols, Drug Dev. Ind. Pharm. 23 (1997) 647–655. [105] H. Maier, G. Schauberger, K. Brunnhofer, H. Honigsmann, J. Invest. Dermatol. 117 (2001) 256–262.

[106] H. Maier, A.W. Schmalwieser, Photochem. Photobiol. Sci. 9 (2010) 510–515. [107] G.A. Neudahl, J. Zhang, Household Personal Care Today 3 (2010) 18–20. [108] S.P. Jones, D.J.W. Grant, J. Hadgraft, G. Parr, Acta Pharm. Technol. 30 (1984) 263–277. [109] V.J. Ndlebe, M.E. Brown, B.D. Glass, J. Therm. Anal. Calorim. 77 (2004) 459–470. [110] S. Scalia, S. Villani, A. Casolari, J. Pharm. Pharmacol. 51 (1999) 1367–1374. [111] S. Scalia, A. Casolari, A. Iaconinoto, S. Simeoni, J. Pharm. Biomed. Anal. 30 (2002) 1181–1189. [112] S. Scalia, S. Simeoni, A. Barbieri, S. Sostero, J. Pharm. Pharmacol. 54 (2002) 1553–1558. [113] J. Yang, C.J. Wiley, D.A. Godwin, L.A. Felton, Eur. J. Pharm. Biopharm. 69 (2008) 605–612.