Research Projects

Sunscreen Research – Prof Bice Martincigh

Sunscreen Research – Prof Bice Martincigh

Prof Bice Martincigh

Prof Bice Martincigh

School of Chemistry, University of KwaZulu-Natal, Westville Campus

Title of the project

Photostability of Sunscreen Absorbers

Highlights of the project

A suitable method to quantify the amounts of active ingredients in commercial sunscreen products has been devised. This includes the extraction of the ingredients, their separation and quantitation, and determining the limit of detection and limit of quantitation. Both chemical and physical absorbers have been adequately quantified and most products were found to conform to the limits set by the European Cosmetic Toiletry and Perfumery Association (COLIPA).

The photostability of a number of commercially available sunscreen products on the Australian and South African markets has been assessed by measuring their transmission characteristics on exposure to measured doses of solar ultraviolet radiation. Solar light intensities were measured by valerophenone chemical actinometry. Those products that exhibited photoinstability were then investigated further by high performance liquid chromatography. Attempts have been made to identify the breakdown products from the photoinstability exhibited by sunscreen absorbers by means of high performance liquid chromatography-mass spectrometry. We have identified some of the causes of the observed photoinstabilities. These include photoisomerisation, phototautomerisation and photodegradation of some of the absorber molecules. In addition, in some cases photosensitisation causes the degradation of the absorbers.

One of the main causes of product photoinstability was found to be the inclusion of the most widely used UVA absorber in broad-spectrum sunscreens, namely 4-tert-butyl-4′-methoxydibenzoylmethane (avobenzone). Hence, this absorber was investigated in detail. The photostability of avobenzone was found to be solvent-dependent. In particular, the photostability of avobenzone depended on both the polarity and proticity of the solvent. Four solvents were employed, namely, cyclohexane, ethyl acetate, dimethylsulfoxide and methanol. The cause of the instability of avobenzone in these solvents was determined by means of ultraviolet spectroscopy, high performance liquid chromatography, gas chromatography–mass spectrometry and nuclear magnetic resonance spectroscopy. The effect of oxygen on the photo-instability was also determined. Avobenzone was found to lose absorption efficacy as a result of photoisomerisation from the enol to the keto form and/or photodegradation to form photoproducts that absorb principally in the UVC region, depending on the solvent. It was found to be essentially photostable in the polar protic solvent methanol but photoisomerised in the polar aprotic solvent dimethylsulfoxide. In the nonpolar solvent cyclohexane, it photodegraded appreciably. Both photoisomerisation and photodegradation occurred to a similar extent in the moderately polar aprotic solvent ethyl acetate. Photoisomerisation occurred only in the presence of oxygen whereas photodegradation occurred irrespective of oxygen. This knowledge is important in order to achieve the correct formulation for sunscreens incorporating avobenzone.

The possible use of plant polyphenols as potential stabilisers of photo-unstable sunscreen absorbers, and in particular, avobenzone, was investigated by ultraviolet spectroscopy. Polyphenols can act as free radical scavengers and appear to be able to quench some of the degradation reactions. The polyphenol extracts offered some degree of photostabilisation for the photo-unstable avobenzone in various solvents. A photostabilisation mechanism was proposed based on the photochemical pathway followed by avobenzone that results in its photo-instability.

The polyphenol extracts have been assessed for possible use as sunscreen-active ingredients. Promising results were obtained. The isolation of the active flavonoid compounds in these extracts was pursued. Although these have not yet been conclusively identified, we have been able to identify the parent aglycones. The antioxidant properties of the polyphenol extracts have also been measured.

Investigations have been carried out to determine whether the antioxidants vitamin C and vitamin E can prevent the well-known photosensitiser and once widely used sunscreen absorber, para-aminobenzoic acid (PABA), from photosensitising the formation of thymine dimer and singlet oxygen, both of which are damaging to DNA and may induce skin cancer. Both vitamins are found extensively in sunscreen formulations because of their photoprotective properties. Both synergistic and antagonistic properties of the two vitamins were observed in the PABA-photosensitised thymine dimer formation. In the case of singlet oxygen generation, the introduction of the antioxidants either individually or together decreased the yield of singlet oxygen.

A range of sunscreen absorbers have been screened for their ability to photosensitise the reactive oxygen species, singlet oxygen. The knowledge generated is important because singlet oxygen is toxic.

Peer-reviewed publications

How was this project of value in the struggle against cancer?

South Africa has a high incidence of skin cancer. The knowledge gained from this research will assist in choosing the correct strategies for protection against skin cancer. A knowledge of the photochemical behaviour of the active ingredients in sunscreen preparations is essential in order to know which actives are most effective and do not cause any potential harm, as well as knowing which are stable under typical usage conditions and do not give the consumer a false sense of protection. This work has shown that a number of the commercial sunscreen preparations available on the market in South Africa are not sufficiently photoprotective, particularly in the longwave UVA region, to afford the consumer adequate protection. This is a cause of concern since it is now known that UVA radiation can induce skin cancer. This information needs to be taken cognisance of in public health campaigns. Consequently, it is imperative to understand how to stabilise these products. Our work has shown that a South African plant extract can be used to improve the photostability of commonly used sunscreen absorbers and can act as a sunscreen-active agent itself. Once this extract is completely characterised and has been assessed for possible undesirable effects it can be used to develop a new sunscreen product which hopefully will prove effective in protecting the consumer from the undesirable effects of the sun.

Future plans with the project

The photostability of a range of sunscreen products available on the South African and Australian market was investigated as were means of stabilising sunscreen preparations. This knowledge will assist in the formulation of products which will afford the consumer the required degree of protection. The research methodology devised in this work will also enable CANSA to determine which experimental procedures should be used to test the products to which it awards its seal of recognition.

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