Photostability enhancement by encapsulation of α-tocopherol into lipid-based nanoparticles loaded with a UV filter

The aim of this study was to develop some sunscreen formulations able to maintain their photoprotection when exposed to UV radiation. In this context, the influence of the antioxidant α-tocopherol on the photostability of lipid-based nanoparticles, namely solid lipid nanoparticles (SLNs) and nanostructured lipid carriers (NLCs), encapsulated with a UVA filter, has been investigated. The nanoparticles co-encapsulated with both actives exhibited dimensions smaller than 200 nm and zeta potentials of –45 mV. The photoprotection of the creams based on lipid nanoparticles was evaluated in terms of two protection factors, SPF and erythemal UVA–PF. By exposing the creams to UV radiation, it was observed that tocopherol results in obtaining quite stable formulations, but it does not improve the overall photoprotection much. However, by adding the antioxidant to the formulation confers a double action: protection of the skin against reactive oxygen species and the photostabilization of the UVA filter into lipid nanoparticles.     

Time-dependent decrease in sunscreen protection against chronic photodamage in UVB-irradiated hairless mouse skin

To determine the time dependence of sunscreen protection against chronic photodamage in hairless mice, the time was varied (0-8 h) between topical sunscreen treatment and UVB radiation exposure. Sunscreen products with labeled sun protection factor (SPF) values of 2, 4 and 8 were evaluated; these values were verified in a guinea pig model for SPF determinations. When applied immediately prior to UVB radiation exposure, these sunscreen products were very effective in prevention of skin wrinkling and tumor formation. Onset of photodamage was delayed, the delay being greater with higher SPF values. However, the sunscreen actives were rapidly lost from the skin surface, and their protective effect diminished strikingly as the time between treatment and irradiation increased. For daily protection against chronic photodamage, this suggests a need for photoprotectants with greater substantivity to achieve a high level of protection throughout the day.     

A Review of Sunscreen Safety and Efficacy

The use of sunscreen products has been advocated by many health care practitioners as a means to reduce skin damage produced by ultraviolet radiation (UVR) from sunlight. There is a need to better understand the efficacy and safety of sunscreen products given this ongoing campaign encouraging their use. The approach used to establish sunscreen efficacy, sun protection factor (SPF), is a useful assessment of primarily UVB (290–320 nm) filters. The SPF test, however, does not adequately assess the complete photoprotective profile of sunscreens specifically against long wavelength UVAI (340–400 nm). Moreover, to date, there is no singular, agreed upon method for evaluating UVA efficacy despite the immediate and seemingly urgent consumer need to develop sunscreen products that provide broad-spectrum UVB and UVA photoprotection. With regard to the safety of UVB and UVA filters, the current list of commonly used organic and inorganic sunscreens has favorable toxico-logical profiles based on acute, subchronic and chronic animal or human studies. Further, in most studies, sunscreens have been shown to prevent the damaging effects of UVR exposure. Thus, based on this review of currently available data, it is concluded that sunscreen ingredients or products do not pose a human health concern. Further, the regular use of appropriate broad-spectrum sunscreen products could have a significant and favorable impact on public health as part of an overall strategy to reduce UVR exposure.     

Assessment of the photoprotection properties of sunscreens by chromatographic measurement of DNA damage in skin explants

Evaluation of the photoprotection provided by sunscreens is performed either through the induction of erythema and expressed as the sun protection factor (SPF), or by the UVA-mediated persistent pigment darkening (PPD). None of these two endpoints has a link with skin cancer, the most deleterious consequence of excess exposure to solar UV radiation. We thus set up a complementary approach to evaluate the protection provided by sunscreens to the genome of human skin. This is based on the quantification of the thymine cyclobutane dimer (TT-CPD), the main DNA lesion induced by both UVB and UVA radiations. Irradiations were performed ex vivo on human skin explants and the level of TT-CPD in DNA was determined by HPLC associated with tandem mass spectrometry. The technique was first optimized and validated with three standard sunscreens. The study was then extended to the evaluation of a commercial high SPF sunscreen exhibiting efficient UVA photoprotection. The DNA protecting factor was found to reflect the ratio between UVB and UVA photoprotection, although the absolute values of the genomic protection were, as a general trend, lower than either SPF or PPD. These data show the usefulness of the proposed approach for the evaluation of the genoprotection afforded by sunscreens.     

Photostability of UV Absorber Systems in Sunscreens†

Sunscreens are used to protect the human skin against harmful UV radiation. Today there is a trend toward higher sun protection factors (SPF) and better UVA protection. Methods for the assessment of SPF and UVA protection involve irradiation of the product, and the photostability properties of the sunscreen have an influence on its performance. Sunscreens often contain more than one UV filter. Thus it is important to understand the photostability properties of the complete system. The filter combinations used may exhibit destabilizing, stabilizing or inert interactions. For that reason, besides assessment of the properties of the single filters, photostabilities of binary filter combinations are investigated. Destabilization occurs when two UV absorbers undergo a chemical reaction after absorption of UV radiation. Stabilization may be achieved when the optical density of the system is very high, giving rise to a self‐protection effect of the sunscreen film. Photounstable UV absorbers may be additionally stabilized by employing triplet quenchers. Being aware of these mechanisms and applying them for specific UV filter combinations can help in designing efficient sunscreens.     

Materials Science Challenges in Skin UV Protection: A Review

UV radiation is one of the critical environmental stress factors for human skin, which can trigger various problems such as pruritus, burning, erythema, premature skin aging and skin cancer. Hence, UV protection has become an indispensable daily routine and the use of topical sunscreen products is rapidly increasing. However, there are emerging concerns over the efficiency and safety of existing chemical and physical UV filters used in consumer products. Furthermore, there is no universally approved method for assessing sun protection efficiency regardless of the immediate end user need to develop safer sunscreen products that afford broad-spectrum photoprotection. It is evident that the current organic and inorganic UV filters have significant unfavorable impacts on human, environmental, and marine safety. Therefore, effective alternative UV filters should be established. This article comprehensively reviews the properties, safety, health and ecological concerns of various UV filters including TiO₂ and ZnO nanoparticles as well as the limitations of the testing protocols and guidelines provided by major regulatory bodies. The photoreactivity of UV filters used in sunscreen remains a major challenge, and it is crucial to develop new sunscreen ingredients, which not only protect the consumer, but also the environment.     

Alternative methods to evaluate the protective ability of sunscreen against photo-genotoxicity

Numerous epidemiological investigations show that sunlight is carcinogenic to humans and that the use of sunscreen may be effective in decreasing the risk of skin cancer. The biological activity of a sunscreen is evaluated by its ability to protect human skin from erythema as represented by a Sun Protection Factor (SPF). We propose that the sunscreen's protective effect against sunlight-induced genotoxicity, including mutation, should also be taken into account. In this study we examined the protective ability of sunscreens against natural sunlight and UV-induced genotoxicity in Drosophila somatic cells. We prepared three kinds of sunscreen samples, each with an SPF value of 20, 40 or 60 and compared their protective activities with commercial sunscreens. When a sunscreen of SPF 20, 40 or 60 was pasted on the plastic cover of a petri dish in which Drosophila larvae were exposed to the sun or UV lamps, genotoxicity decreased as the SPF of the sunscreen increased, relative to levels of genotoxicity observed in samples without sunscreen. However, the protective abilities of sunscreens were unexpectedly not so different from each other. To reveal the relationship between the protective activity of sunscreen and the wavelength of light with which larvae were irradiated through the sunscreen, we measured the transmittance of light through the petri dish cover on which the sunscreen was pasted. Effective protection was demonstrated by removing components of light whose wavelengths were below 315 nm. We suggest, that the measurement of anti-genotoxic activity and the determination of the wavelengths of light transmitted through the sunscreen should be an alternative method for evaluating the effectiveness of a sunscreen.     

Relevance of sunscreen application method, visible light and sunlight intensity to free-radical protection: A study of ex vivo human skin

With the continued rise in skin cancers worldwide there is a need for effective skin protection against sunlight damage. It was shown previously that sunscreens, which claimed UVA protection (SPF 20+), provided limited protection against UV-induced ascorbate radicals in human skin. Here the results of an electron spin resonance (ESR) investigation to irradiate ex vivo human skin with solar-simulated light are reported. The ascorbate radical signal in the majority of skin samples was directly proportional to the irradiance over relevant sunlight intensities (0.9-2.9 mW cm(-2)). Radical production (substratum-corneum) by UV (wavelengths < 400 nm) and visible components (> 400 nm) was approximately 67% and 33% respectively. Ascorbate radicals were in steady state concentration at low irradiance (approximately 1 mW cm(-2) equivalent to UK sunlight), but at higher irradiance (approximately 3 mW cm(-2)) decreased with time, suggesting ascorbate depletion. Radical protection by a four star-rated sunscreen (with UVA protection) was optimal when applied as a thin film (40-60% at 2 mg cm(-2)) but less so when rubbed into the skin (37% at 4 mg cm(-2) and no significant protection at 2 mg cm(-2)), possibly due to cream filling crevices, which reduced film thickness. This study validates ESR determinations of the ascorbate radical for quantitative protection measurements. Visible light contribution to radical production, and loss of protection when sunscreen is rubbed into skin, has implications for sunscreen design and use for the prevention of free-radical damage.     

Sun exposure and sun protection habits among high-school adolescents in Porto Alegre, Brazil

Adolescents constitute an important audience for photoprotection programs. Sun exposure and sun protection habits acquired during adolescence have a significant impact on skin cancer incidence. We administered a questionnaire to 724 students about ultraviolet radiation effects, opinions about tanning, total time of sun exposure per day, photoprotection and activities in the sun. About 90% were aware of the association between sun exposure and skin cancer, and mass media was the main source of information. However, the great majority believed that tanning improved their appearance, and that it was worth taking the risk. The most prevalent outdoor activity among boys was sports; girls preferred walks and sunbathing. Sun exposure was significantly longer in summer, when 90% of the students went to the beach. About 47% reported sunscreen use in summer and only 3% reported using sunscreen during winter. These results emphasize the need for the promotion of photoprotective habits in our population and the importance of engaging physicians and school teachers in developing campaigns directed at this issue to achieve effective, long-lasting results. Adolescents are aware of the effects of ultraviolet radiation on the skin but campaigns have not successfully changed their sun exposure habits.     

Reapplication improves the amount of sunscreen, not its regularity, under real life conditions

The sun protection factor (SPF) of sunscreens is determined using samples applied with a thickness of 2 mg cm(-2). Sunscreen users, however, typically apply sunscreen nonuniformly and in smaller amounts. The objective of our study was to verify whether sunscreen reapplication increases the amount and regularity of the product on the skin. Volunteers were asked to apply an SPF 6 sunscreen on their forearms and reapply it 30 min later on one forearm. Tape-strips were used to collect five samples from two different sites on each forearm. The concentration of benzophenone-3 in the samples was measured and the total amount of sunscreen was estimated using high-performance liquid chromatography. The median amount of sunscreen film was 0.43 mg cm(-2) (0.17-1.07) after one application and 0.95 mg cm(-2) (0.18-1.91) after two applications (P = 0.002). No significant difference was found in the film uniformity. Though sunscreen reapplication increases the amount of product on the skin, levels are still lower than the recommended amount, confirming that the protection level is less than the product-stated SPF. Our results are the first in the literature to support the recommendation for reapplying sunscreens. Based on our results, we recommend that sunscreens be labeled using qualitative measures.     

Sunscreens Containing Cyclodextrin Inclusion Complexes for Enhanced Efficiency: A Strategy for Skin Cancer Prevention

Unprotected exposure of skin to solar ultraviolet radiation (UVR) may damage the DNA of skin cells and can lead to skin cancer. Sunscreens are topical formulations used to protect skin against UVR. The active ingredients of sunscreens are UV filters that absorb, scatter, and/or reflect UVR. Preventing the formation of free radicals and repairing DNA damages, natural antioxidants are also added to sunscreens as a second fold of protection against UVR. Antioxidants can help stabilise these formulations during the manufacturing process and upon application on skin. However, UV filters and antioxidants are both susceptible to degradation upon exposure to sunlight and oxygen. Additionally, due to their poor water solubility, natural antioxidants are challenging to formulate and exhibit limited penetration and bioavailability in the site of action (i.e., deeper skin layers). Cyclodextrins (CDs) are cyclic oligosaccharides that are capable of forming inclusion complexes with poorly soluble drugs, such as antioxidants. In this review, we discuss the use of CDs inclusion complexes to enhance the aqueous solubility of antioxidants and chemical UV filters and provide a protective shield against degradative factors. The role of CDs in providing a controlled drug release profile from sunscreens is also discussed. Finally, incorporating CDs inclusion complexes into sunscreens has the potential to increase their efficiency and hence improve their skin cancer prevention.     

Darkness at noon: sunscreens and vitamin D3

We are assured by responsible scientific and governmental organizations that sunscreens should be routinely worn to reduce skin cancer risk. We are also advised that wearing sunscreens will not hinder our ability to produce sufficient previtamin D3 (preD3) from casual sunlight exposure. We report the examination of a series of 166 solar spectra, obtained on different days throughout a year, evaluated for erythemic and preD3 effectiveness and the relative effects of recommended Sun protection factor (SPF) 15 sunscreen. The results show that the sunscreen is much more effective in blocking the formation of preD3, than its labeled SPF for preventing sunburn. In fact with sunscreen applied only miniscule amounts of preD3 are predicted to be made outdoors even with extensive exposure. This raises important questions regarding the safest way to use sunlight exposure to promote healthy vitamin D3 levels and suggests the need to modify the public safety "Safe Sun" messages.     

Measurements of the Protective Effect of Topically Applied Sunscreens using in vitro Three-dimensional Dermal and Skin Equivalents

Abstract— For preventing or minimizing acute and chronic skin damage caused by UV radiation, the use of sunscreens is probably the most important measure. To screen the protective efficacy of new sunscreen molecules or formulations against UV rays, we evaluated as in vitro testing methods the use of two three-dimensional models, a dermal equivalent (DE) and a skin equivalent (SE). The DE is composed of a porous collagen-glycosaminoglycans-chitosan matrix populated by normal human fibroblasts. The SE is comprised of a fully differentiated epidermis realized by seeding keratinocytes onto the DE. In this study, we demonstrated that the DE and SE models react to the deleterious effects of UVA and UVB. Then, we extended our research to the evaluation of their usefulness for photoprotection trials. Sunscreen agents (Euso-lex 8020 and 6300) and commercially available sunscreens (chemical and physical filter formulations) that protect the skin against either UVA or UVB were evaluated. The tested products were applied (n = 6) topically (10 μL) and incubated for 30 min prior to irradiation over a range of UVA (0-50 J/cm²) or UVB (0-5 J/cm²). The photoprotection provided by the tested sunscreen molecules and formulations was evaluated by measurement of residual cellular viability 24 h postirradiation using the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetra-zolium bromide (MTT) test and assessment of the inflammation response by interleukin-la release assay. When sunscreens were applied prior to UV exposure, a higher residual cellular viability versus control was obtained, demonstrating the photoprotective effects of the tested products. These in vitro models could be used for screening tests to evaluate the protective effects of sunscreen molecules and formulations, especially for UVA trials because there is a lack of consensus for an in vivo method.     

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.     

p53 tumor suppressor gene: a critical molecular target for UV induction and prevention of skin cancer

The relationship between exposure to UV radiation and development of skin cancer has been well established. Several studies have shown that UVB induces unique mutations (C-->T and CC-->TT transitions) in the p53 tumor suppressor gene that are not commonly induced by other carcinogens. Our studies have demonstrated that UV-induced mouse skin cancers contain p53 mutations at a high frequency and that these mutations can be detected in UV-irradiated mouse skin well before the appearance of skin tumors. This observation suggested that it might be possible to use p53 mutations as a biologic endpoint for testing the efficacy of sunscreens in photoprotection studies. Indeed, application of SPF 15 sunscreens to mouse skin before each UVB irradiation resulted in reduction in the number of p53 mutations. Because p53 mutations represent an early essential step in photocarcinogenesis, these results imply that inhibition of this event may protect against skin cancer development. This hypothesis was confirmed by our finding that sunscreens used in p53 mutation inhibition experiments also protected mice against UVB-induced skin cancer.     

Protection Against UV-Induced Reactive Intermediates in Human Cells and Mouse Skin by Glutathione Precursors: A Comparison of N-Acetylcysteine and Glutathione Ethylester

Because glutathione (GSH) plays a central part in the endogenous defense against UV radiation, an increase in GSH might provide photoprotection. Two agents that increase GSH levels were investigated. Cultured human cells and mouse skin were treated with N -acetylcysteine (NAC) and glutathione ethylester (GSH-Et). After 30 min, the GSH level was determined by HPLC. Photoprotection was assessed by testing the ability of the thiols to scavenge UV-induced reactive intermediates in the same models. As compared to control cells, NAC and GSH-Et increased intracellular GSH in vitro to maximally 144% and 174% respectively. In vitro protection (maximum 23% for NAC and 21% for GSH-Et) did not correlate to the intracellular GSH level but to the concentration of the thiols in the medium. In vivo , epidermal GSH was increased to maximally 163% of the control level by NAC and 1234% by GSH-Et. The maximum in vivo photoprotection provided by GSH-Et was 55%, similar to what was found previously for NAC. Again, the protection seemed more closely correlated to the thiol dose than to the GSH level. The study showed that the protection against UV-induced reactive intermediates depends on a general antioxidant action of these thiols, rather than only on their role as GSH precursors.     

Advantages of using hairless mice versus haired mice to test sunscreen efficacy against photoimmune suppressions

We tested the hypothesis that the strain of mice used in sunscreen protection experiments may influence immune protection. Ultraviolet (UV) dose-response curves were done in the presence or absence of a sun protection factor (SPF) 15 sunscreen using SKH1:hrBR or C3H/HeN mice. SKH1:hrBR mice showed a higher sensitivity to the suppressive effects of UV radiation (50% immune suppression equal to 5.2 kJ/m2 UVB in SKH1:hrBR mice versus 18.5 kJ/m2 in C3H mice). Immune protection factors (IPF) and an erythema protection factor (Ery-PF) for SKH1:hr mice were derived. The Ery-PF in hairless mice was 13.5, which was similar to the SPF of 15 measured in humans. When IPF were calculated as a ratio of minimal immune suppressive doses, the IPF for the SKH1:hrBR mice was 8.23 and the IFP for the C3H/HeN mice was 1.92. When IPF were estimated using the entire UV dose-response range, they were equal to 9.01 for SKH1:hrBR mice and 1.79 for the C3H/HeN mice. Because IPF and SPF can be measured directly in hairless mice, we suggest that the use of hairless mice may provide a better model to measure sunscreen efficacy, especially when the use of human volunteers is inappropriate, unethical or impossible.     

New Advances in Protection Against Solar Ultraviolet Radiation in Textiles for Summer Clothing

Clothing is considered one of the most important tools for photoprotection against harmful solar ultraviolet radiation (UVR). The standard for sun‐protective clothing is based on erythema despite other biological effects of UVR on the skin. We analyzed the potential protection against UVR in fabrics destined for summer clothing based on several action spectra. We examined 50 garments classified by type of fabric composition, structure of the fiber yarn and color. The ultraviolet protection factor was calculated based on fabric ultraviolet transmittance corrected for erythema according to the EU standard E‐13758 as well as the UVA transmittance of fabrics. UVR protection was also analyzed in base of different action spectra as for previtamin D3, nonmelanoma skin cancer, photoimmunosuppression and photoaging. Most knitted fabrics used for sports T‐shirts offered excellent ratings for ultraviolet protection while normal shirts showed very low ratings, particularly against photoaging. The cover is the most influential variable in fabric photoprotection, having an exponential relationship with the UPF. The relation between cover and UVA protection was linearly negative. Information about ultraviolet protection in textiles used for summer clothing should be included in labeling as some types of fabrics, especially those used for shirts, offer very low UVR protection.