Integrated Sun Protection Factor: A New Sun Protection Factor Based on Free Radicals Generated by UV Irradiation

Investigation of Model Sunscreen Formulations Comparing the Sun Protection Factor, the Universal Sun Protection Factor and the Radical Formation Ratio

In view of globally rising skin cancer rates and harmful effects exerted by sunlight throughout the ultraviolet, visible and infrared ranges, an objective, safe and comprehensive method for determining sunscreen efficacy is required in order to warrant safe sun exposure. In this study, the influence of characteristic active ingredients (chemical filters, physical filters and antioxidants) on different sunscreen indicators, including the universal sun protection factor and the radical formation ratio, was determined and compared to their influence on sun protection factor values. Spectroscopic universal sun protection factor measurements were conducted ex vivo by analyzing tape strips taken from human skin, and radical formation ratio determination was performed via electron paramagnetic resonance spectroscopy using porcine ear skin ex vivo. The sun protection factor determination was conducted according to ISO standards (ISO 24444:2010). It was shown that chemical filters provide a protective effect which was measurable by all methods examined (spectroscopy, electron paramagnetic resonance spectroscopy and erythema formation). Physical filters, when used as single active ingredients, increased protective values in universal sun protection factor and sun protection factor measurements but exhibited no significant effect on universal sun protection factor measurements when used in combination with chemical filters or antioxidants. Antioxidants were shown to increase sun protection factor values. Radical formation ratio values were shown to be influenced merely by chemical filters, leading to the conclusion that the universal sun protection factor is the most suitable efficacy indicator for the ultraviolet range.

Interactions of dietary carotenoids with activated (singlet) oxygen and free radicals: potential effects for human health

Molecular mechanisms associated with the anti-/pro-oxidative properties of carotenoids (CARs) are described in organic solvents, micro-heterogeneous environments and model lipid membranes and in cellular suspensions. Singlet oxygen is important in the skin and eye and CARs are efficient singlet oxygen (SO) quenchers with corresponding rate constants near diffusion controlled (typically app. 10¹⁰ M⁻¹ s⁻¹) with lycopene (LYC) exhibiting the most efficient quenching in organic solvents. However, in membrane environments there is little or no difference in the quenching efficiency between the dietary CARs. Furthermore, aggregation of CARs, particularly those in the macula (lutein and zeaxanthin), markedly reduces SO quenching efficiency. Free radical interactions with CARs leads to at least three processes, electron and hydrogen atom transfer and adduct formation. The most studied is electron transfer where the CAR loses an electron to become a radical cation. The reactivity/lifetime of such CAR radicals may lead to a switch from anti- to pro-oxidant behaviour of CARs. These reactions are related to CAR redox potentials with LYC being the lowest (most easily oxidised) allowing LYC to reduce/repair all other CAR radical cations and LYC 'sacrificed' where mixtures of CARs are present in oxidative environments. Such redox-controlled reactions may lead to deleterious as well as beneficial health effects.

Sunscreen use: controversies, challenges and regulatory aspects

Mismatches between skin pigmentation and modern lifestyle continue to challenge our naked skin. One of our responses to these challenges is the development and use of sunscreens. The management of sunscreens has to balance their protective effect against erythema, photocarcinogenesis and photoageing owing to the potential toxicity of the ultraviolet (UV) filters for humans and the environment. The protection against UV radiation offered by sunscreens was recently standardized in the European Union (EU) based on international harmonization of measurement techniques. Four different categories of sun protection have been implemented along with recommendations on how to use sunscreen products in order to obtain the labelled protection. The UV filters in sunscreens have long been authorized for use by the EU authority on the basis of data from studies on acute toxicity, subchronic and chronic toxicity, reproductive toxicity, genotoxicity, photogenotoxicity, carcinogenicity, irritation, sensitization, phototoxicity and photosensitization as well as on environmental aspects. New challenges with respect to the safety of UV filters have arisen from the banning of animal experiments for the development of cosmetics. Future debates on sunscreens are likely to focus on nanoparticles and environmental issues, along with motivation campaigns to persuade consumers to protect their skin. However, more efficient sunscreen use will also continue to raise questions on the benefit in preventing vitamin D synthesis in the skin induced by sunlight.

Sun protection factors: world wide confusion

The Sun Protection Factor (SPF) is a very popular instrument in the marketing of sunscreens. Unfortunately it is often not understood how sunscreens work and where the limitations of the SPF are. A lot of aspects of the SPF are confusing, e.g. the race for higher and higher numbers, the effect on SPF when less sunscreen is applied and if sunscreen should be used at all because they may block the Vitamin D synthesis. All this has a negative impact on compliance by the consumer or patient which is the most important influence factor in sun protection. This paper explains how sunscreens work, how the SPF is determined and where the limitations of the current methods exist. The dynamic view of 'UV radiation applied' and the 'UV dose transmitted' through the sunscreen onto the skin as well as onto a substrate in vitro help in the understanding and are also promising approaches in the in vitro assessment. A variation of the in vitro assessment of a sunscreen is the in silico calculation based on the absorption spectrum of the UV filters and an assumption about the irregular sunscreen film on the skin. The sunscreen simulator program can be used to determine how the SPF is affected by applying smaller amounts of sunscreen. Besides the SPF, UVA protection is also discussed. The degree of UVA protection determines the quality of the overall UV protection, whereas the SPF is an indication of the quantity of protection. Furthermore other protection factors such as IPF, iSPF, RSF and p53, and the inhibition of the Vitamin D3 synthesis by sunscreens are also discussed. In conclusion it is shown that the accuracy and robustness of the SPF and other Protection Factors will improve significantly with the availability of true broad-spectrum sunscreens rather than conventional UVB-biased sunscreens, because uniform protection profiles lead to protection independent of the action spectrum of the endpoint and the UV-radiation source.

Synthesis and application of a novel sunscreen-antioxidant

Background information on the inefficacy of sunscreens to provide free radical protection in skin, despite their usefulness in preventing sunburn/erythema, prompted us to synthesize a compound which would display in the same molecule both UV-absorbing and antioxidant capacities. For this purpose, the UVB absorber, 2-ethylhexyl-4-methoxycinnamate (OMC) was combined with the piperidine nitroxide TEMPOL, which has antioxidant properties. The spectral properties of the new nitroxide-based sunscreen (MC-NO) as well as its efficacy to prevent photo-oxidative damage to lipids induced by UVA, natural sunlight and 4-tert-butyl-4-methoxydibenzoylmethane (BMDBM), a photo-unstable sunscreen which generates free radicals upon UV radiation, was studied. The results obtained demonstrate that MC-NO: (a) absorbs in the UVB region even after UVA irradiation; (b) acts as free radical scavenger as demonstrated by EPR experiments; (c) strongly reduces both UVA-, sunlight- and BMDBM-induced lipid peroxidation in liposomes, measured as reduced TBARS levels; and (d) has comparable antioxidant activity to that of commonly used vitamin E and BHT in skin care formulations. These results suggest that the use of the novel sunscreen-antioxidant or of other nitroxide-based sunscreens in formulations aimed at reducing photoinduced skin damage may be envisaged.

[UV, visible and infrared light. Which wavelengths produce oxidative stress in human skin?]

Experimental evidence suggests that the creation of free radicals--mainly reactive oxygen species (ROS)--is the common photobiological answer to the skin-sunlight interaction. The free radical action spectrum (wavelength dependency) for ultraviolet and visible light (280-700 nm) has been determined by quantitative ESR spectroscopy. Visible light produces around 50% of the total oxidative stress caused by sunlight. Reactive species like *O(-)(2), *OH and *CHR are generated by visible light. The amount of ROS correlates with the visible light intensity (illuminance). We demonstrated the creation of excess free radicals by near-infrared light (NIR, 700-1600 nm). Free radical generation does not depend exclusively on the NIR irradiance, but also on the NIR initiated skin temperature increase. The temperature dependence follows the physiological fever curve. Our results indicate that the complex biological system skin creates the same type of free radicals over the entire active solar spectrum. This general response will make it possible to define the beneficial or deleterious action of sunlight on human skin by introduction of a free radical threshold value.

Comparison of human and porcine skin for characterization of sunscreens

The universal sun protection factor (USPF) characterizing sunscreen efficacy based on spectroscopically determined data, which were obtained using the tape stripping procedure. The USPF takes into account the complete ultraviolet (UV) spectral range in contrast to the classical sun protection factor (SPF). Until now, the USPF determination has been evaluated only in human skin. However, investigating new filters not yet licensed excludes in vivo investigation on human skin but requires the utilization of a suitable skin model. The penetration behavior and the protection efficacy of 10 commercial sunscreens characterized by USPF were investigated, comparing human and porcine skin. The penetration behavior found for typical UV filter substances is nearly identical for both skin types. The comparison of the USPF obtained for human and porcine skin results in a linear relation between both USPF values with a correlation factor R(2)=0.98. The results demonstrate the possibility for the use of porcine skin to determine the protection efficacy of sunscreens.

Ex-vivo spectroscopic quantification of sunscreen efficacy: proposal of a universal sun protection factor

The sun protection factor (SPF) describes the protective behavior of sunscreens insufficiently, because this factor takes into account only the UVB spectral range, and strains the volunteers during its determination by invasively invoking an erythema. A new noninvasive method is proposed that is based on the UV spectroscopic measurement of tape strips taken from a sunscreen-treated skin area. The resulting sum transmission spectra of the tape strips reflect the in-vivo distribution of the absorber on the skin and quantify the protective efficacy of the applied sunscreens over the complete UV spectral range. The spectroscopic data provide a basis for the calculation of a universal sun protection factor (USPF). The comparison of the concrete values of USPF and SPF results in the following statements. 1. An unique functional correlation is not to be expected because a different UVB / UVA dependence exists. 2. The size of the differences between both values is influenced clearly by the intensity relation of the average sum transmission in the UVB in comparison to the UVA range. 3. The USPF values objectively assess the efficacy of sunscreens considering a protection against all irradiation injuries.

Application of optical non-invasive methods in skin physiology: a comparison of laser scanning microscopy and optical coherent tomography with histological analysis

BACKGROUND/PURPOSE

Optical, non-invasive methods, such as fluorescence laser scanning microscopy (LSM) and optical coherent tomography (OCT), have become efficient tools for the characterization of the skin structure in vivo, as well as real-time investigation of distribution and penetration of topically applied substances.

METHODS

In the present paper, the results obtained with both non-invasive methods - OCT and LSM - were compared to conventional light microscopy of histological sections. Skin structure and the distribution of topically applied particulate and non-particulate substances on the skin surface and in the epidermis were analyzed.

RESULTS

None of the methods used are suitable for the realization of all diagnostic tasks, however, each method has advantages for particular applications. Fluorescence LSM is well suited for the investigation of the upper 150 microm of the skin as well as for the investigation of the kinetics of substances applied onto or into the epidermis. OCT can be applied for the investigation of vertical cross-sections of the skin up to a depth of 2 mm, albeit at lower resolution than achieved by LSM or conventional light microscopy. Conventional light microscopy of histological sections of biopsy specimens produces familiar high-resolution images of deeper tissue layers. However, the analysis of the kinetic processes is limited in this case.

CONCLUSIONS

LSM- and OCT-measurements are efficient non-invasive tools for the characterization of morphological structures of the skin. On the one hand, the optical methods have a clear advantage in the case of kinetic measurements. On the other hand, histological investigations are characterized by a high information density and a high resolution, also in deep tissue layers. The selection of the best method for the analysis of the skin morphology depends on the target and the task of the investigation.

Determination of Wavelength-Specific UV Protection Factors of Sunscreens in Intact Skin by EPR Measurement of UV-Induced Reactive Melanin Radical

There remains an unmet need for skin tissue-based assays for the measurement of the UVA protection and efficacy of sunscreens. Here we describe development of a novel electron paramagnetic resonance assay that uses the photogeneration of reactive melanin radical as a measure of UV light penetration to melanocytes in situ in skin. We have used areas of focal melanocytic hyperplasia in the skin of Monodelphis domestica to model the human nevus. We show that we are able to use this assay to determine the monochromatic protection factors (mPF) of research and commercial sunscreens at specific narrow wavebands of UVB, UVA and blue visible light. Both commercial sunscreens, a sun protection factor (SPF) 4 and an SPF 30 product, had mPFs in the UVB range that correlated well with their claimed SPF. However, their mPF in the UVA ranges were only about one-third of claimed SPF. This technique can be used to design and assay sunscreens with optimally balanced UVA and UVB protection.

Investigation of the homogeneity of the distribution of sunscreen formulations on the human skin: characterization and comparison of two different methods

The efficacy of sun protection, mostly realized by the application of sunscreen formulations, is commonly described by the sun protection factor (SPF). Previous investigations have shown that the efficacy of the sun protection inter alia depends on the homogeneity of the distribution of the topically applied sunscreen formulation on the human skin. Therefore, suitable methods are required to determine the homogeneity of topically applied substances on the skin surface. This study provides and compares two different methods, which enable this determination. Laser scanning microscopy allows the analysis of tape strips removed from skin treated with a sunscreen. These reflect the inhomogeneous distribution on the skin that can complementary be determined directly, utilizing a dermatological laser scanning microscope. For the second method, a chromatic confocal setup was utilized, which enables the study of the microtopography of skin replicas before and after the application of a sunscreen product. The two methods were applied for the evaluation of three different sunscreen formulations for each method. A correlation of the homogeneity of distribution with the in vivo SPF could be confirmed. Both methods are suitable to investigate the homogeneity of the tested sunscreen formulations, although they provide different advantages and disadvantages.

Anti-Inflammatory and Skin-Hydrating Properties of a Dietary Supplement and Topical Formulations Containing Oligomeric Proanthocyanidins

BACKGROUND

Anti-inflammatory and skin hydration properties of a dietary supplement and 2 topical formulations (Anthogenol) with oligomeric proanthocyanidins were investigated.

METHODS

Forty-two subjects were randomized into 2 groups: one taking the dietary supplement (100 mg/day) and the other without supplement. After 4 weeks, erythema was induced using UV radiation followed by treatment with topical cream or lotion. Erythema was measured for up to 72 h after irradiation. Skin hydration after 1 and 2 weeks of application of the cream and lotion was also measured in separate test fields.

RESULTS

Both topical formulations led to a significant suppression of erythema formation and the dietary supplement led to an additional slightly stronger suppression. Thus 72 h after UV exposure and compared to the control fields of patients that had not taken a dietary supplement, erythema was slightly (13.2%) lower in the subjects that had taken a dietary supplement. The cream resulted in a maximal reduction of erythema of 45.9% (p = 0.0015), while the lotion resulted in a maximal reduction of 53.1% (p = 0.0002). Both topical formulations also increased skin hydration (by nearly 20%; p < 0.002 for all combinations of dietary supplementation and topical treatment) and the hydration was higher in the group taking the dietary supplement.

CONCLUSION

The regular use of Anthogenol products may help to protect from free-radical-mediated skin inflammation and to increase skin hydration.

Changes in ultraviolet absorbance and hence in protective efficacy against lipid peroxidation of organic sunscreens after UVA irradiation

Owing to the spectral distribution of solar UV, the UVA component of sunlight is now believed to be the main cause of photoaging and photocarcinogenesis and is much more effective than UVB in inducing peroxidative damage. Consequently, most skin care cosmetic products now include UVA filters in their formulations along with UVB filters. These modern sunscreens should provide and maintain their initial absorbance, hence protection, throughout the entire period of exposure to sunlight. However, not all UVA and UVB filters are sufficiently photostable. In this study, we examine the correlation between the photochemical degradation of sunscreen agents under UVA irradiation, with particular reference to the UVA-absorber 4-tert-butyl-4'-methoxydibenzoylmethane, alone and in combination with other organic UV filters (2-ethylhexyl 4 methoxycinnamate and 2-ethylhexyl 2-cyano-3,3-diphenylacrylate) and their ability to prevent UVA-induced lipid peroxidation. Since antioxidants are also added to formulations to deactivate free radicals generated during UVA exposure, vitamin E and the synthetic antioxidant, bis(2,2,6,6-tetramethyl-1-oxyl-piperidine-4-yl)sebacate, a nitroxide derivative, were also included in this study. By using simple in vitro tests, the results show that a decrease in spectral absorbance of the UV filters correlates in most cases with increased UVA-induced lipid peroxidation; this depends on the specific UV absorber analysed and also on whether they are alone or in combination. Furthermore, the combined presence or absence of antioxidants has a profound effect on this oxidative event. In particular, the nitroxide appears to be a more efficient photo-antioxidant than vitamin E. Similar experiments were also performed under natural sunlight and the results obtained did not differ substantially from those performed under UVA. The results presented and discussed in this work may help in understanding the effects of UVA/UVB absorbers and antioxidants upon the level of UV-induced ROS generated under UVA exposure and in natural sunlight which could be relevant for improving the photoprotection and efficacy of skin care cosmetic formulations.