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.     

The effect of UV absorbing sunscreens on the reflectance and the consequent protection of skin

The in vivo reflectance spectra of Caucasian skin, coated with preparations containing sunscreen vehicle, vehicle with olive oil and vehicle with the UVB and UVA absorbers 2-ethylhexyl-4-methoxycinnamate and 4-t-butyl-4'-methoxydibenzoylmethane were determined. All preparations reduced the reflectance of skin throughout the UVA spectral range (320 to 400 nm), with the sunscreen preparations containing the UVB and UVB plus UVA absorbers reducing the reflectance more than the sunscreen vehicle alone. This phenomenon, which facilitates the penetration of UV radiation to the lower epidermis and dermal layers of skin and therefore lessens sunscreen efficacy, is attributed to optical coupling mediated by refractive index matching of the sunscreen to the upper epidermis. The greater reduction in skin diffuse reflectance caused by sunscreens containing methoxycinnamate is associated with this compound's high refractive index. Also, by determining the excitation spectra of the autofluorescence originating from the dermal layer of skin, the transmission spectra of the various components of sunscreen on skin were established, and these were in good general agreement with previously published spectra.     

Coencapsulation of Butyl‐Methoxydibenzoylmethane and Octocrylene into Lipid Nanocarriers: UV Performance,Photostability and in vitro Release

The coencapsulation of two UV filters, butyl‐methoxydibenzoylmethane (BMDBM) and octocrylene (OCT), into lipid nanocarriers was explored to develop stable cosmetic formulations with broad‐spectrum photoprotection and slow release properties. Different types of nanocarriers in various concentrations of the two UV filters were tested to find the combination with the best absorption and release properties. Solid lipid nanoparticles (SLNs) and nanostructured lipid carriers (NLCs) have been the two types of lipid nanocarriers used. The NLCs were based on either medium chain triglycerides (MCT) or squalene (Sq). The following physicochemical properties of the nanocarriers have been evaluated: particle size, morphology, zeta potential (ZP), entrapment efficiency, loading capacity, and thermal behavior. The nanocarriers have been formulated into creams containing low amounts of UV filters (2.5% BMDBM and 1% OCT). The best photoprotection results were obtained with the cream based on NLCs prepared with MCT, having a sun protection factor (SPF) of 17.2 and an erythemal UVA protection factor (EUVA–PF) of 50.8. The photostability of the encapsulated BMDBM filter was confirmed by subjecting the nanocarriers‐based creams to in vitro irradiation. The prolonged UV‐protection efficacy was coupled with a slow in vitro release of the synthetic UV filters, which followed the Higuchi release model.     

Development of different temoporfin-loaded invasomes—novel nanocarriers of temoporfin: Characterization, stability and in vitro skin penetration studies

A previous study revealed that the invasome dispersion containing 3.3% (w/v) ethanol and 1% (w/v) of the terpene mixture (cineole:citral:d-limonene = 45:45:10, v/v = standard mixture) could significantly enhance skin penetration of the highly hydrophobic photosensitizer temoporfin (mTHPC). Invasomes enhanced mTHPC-deposition in stratum corneum (SC) compared to liposomes without terpenes and conventional liposomes, and they were efficient in delivering mTHPC to deeper skin layers [J. Control. Release 127 (2008) 271–280]. The aim of this study was to develop new mTHPC-loaded invasomes in order to further enhance the drug penetration. The ratio between d-limonene, citral and cineole was varied in the standard terpene mixture and also single terpenes were used. As a result new mTHPC-loaded invasome dispersions were prepared, characterized and investigated for stability and in vitro penetration of mTHPC into abdominal human skin using Franz diffusion cells.Invasomes were of a small particle size (<150 nm), high homogeneity (<0.3), mostly unilamellar and spherical, but also deformed vesicles were detected. Invasomes containing 1% (w/v) cineole provided the highest skin penetration enhancement of mTHPC, i.e. they provided high amounts of mTHPC in the SC and deeper skin layers, indicating that also incorporation of a single terpene into invasomes could provide efficient nanocarriers of mTHPC. These invasomes could be considered as a promising tool for delivering the photosensitizer mTHPC to the skin.However, in contrast to most invasomes, being effective nanocarriers of mTHPC, there were also formulations less effective than liposomes containing 3.3% (w/v) ethanol and one formulation was less efficient than conventional liposomes.     

Development of liposomes containing ethanol for skin delivery of temoporfin: Characterization and in vitro penetration studies

The aim of this study was to develop ethanol-containing (3.3–20%, w/v) liposomes loaded with temoporfin (mTHPC), which presents a highly hydrophobic photosensitizer with low percutaneous penetration, and to investigate their skin penetration enhancing effect. Characterization parameters of liposomes were measured by photon correlation spectroscopy, lamellarity was analyzed by cryo-electron microscopy and mTHPC-content in formulations was determined spectrofotometrically. In order to assess the stability of mTHPC–liposomes at 4 and 23 °C, at predetermined time intervals characterization parameters and mTHPC-content were measured. The in vitro skin penetration of mTHPC was investigated using human abdominal skin mounted in Franz cells. The results indicated that mTHPC–liposomes were of a small particle size, small polydispersity index, negative surface charge, unilamellar or oligolamellar, and of a spherical or oval shape. All liposomes were stable during 12 months’ storage at 4 °C. Increasing the amount of ethanol in mTHPC–liposomes the skin deposition of mTHPC increased also. Liposomes without ethanol delivered the lowest amount of mTHPC into the skin, while liposomes containing 20% ethanol showed the highest penetration enhancement. In conclusion, mTHPC–liposomes containing 20% ethanol could be a promising tool for delivering temoporfin to the skin, which would be beneficial for the photodynamic therapy of cutaneous malignant or non-malignant diseases.     

Photostability of sunscreens

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.     

Development,Characterization, and Evaluation of Liposomes and Niosomes of Bacitracin Zinc

The skin permeation of bacitracin zinc in liposomes and niosomes after topical application were elucidated in the present study with the to increase its penetration capacity and, hence, efficiency. The formulations of bacitracin zinc were prepared by film hydration method and characterized for vesicle shape, size, entrapment efficiency, and drug permeation across rat skin and also evaluated for their stability. Formulation with niosomes demonstrated a better skin permeation potential, sustained release characteristic, and higher stability as compared to liposomes. The ability of liposomes and niosomes to modulate drug delivery makes the two vesicles useful to formulate topical bacitracin zinc.     

Recent developments in chemistry of sunscreens & their photostabilization

Excessive exposure of UV radiation from the sunlight to human skin is dangerous and causes various skin disorders including skin cancer. Dermatologists worldwide recommend use of skin creams/lotions containing UV-filters/sunscreens for protection from such harmful effects. There is continuous evolution and development of new organic, inorganic and hybrid sunscreens which are being used in such skin care preparations. In the recent past it has been seen that some of these materials are not stable to UV light and the sun protection offered by them decreases rapidly with time. Therefore, most of the formulators now prefer photostable sunscreens or the formulations are designed in such a way that they provide inherent photostability.In this review, we have covered the chemistry of synthesis of different organic sunscreens, issues related to photostability of some of them, and different approaches used to enhance their photostability.     

Photodegradation of Anti-Inflammatory Drugs: Stability Tests and Lipid Nanocarriers for Their Photoprotection

The present paper provides an updated overview of the methodologies applied in photodegradation studies of non-steroidal anti-inflammatory drugs. Photostability tests, performed according to international standards, have clearly demonstrated the photolability of many drugs belonging to this class, observed during the preparation of commercial forms, administration or when dispersed in the environment. The photodegradation profile of these drugs is usually monitored by spectrophotometric or chromatographic techniques and in many studies the analytical data are processed by chemometric procedures. The application of multivariate analysis in the resolution of often-complex data sets makes it possible to estimate the pure spectra of the species involved in the degradation process and their concentration profiles. Given the wide use of these drugs, several pharmaceutical formulations have been investigated to improve their photostability in solution or gel, as well as the pharmacokinetic profile. The use of lipid nanocarriers as liposomes, niosomes or solid lipid nanoparticles has demonstrated to both minimize photodegradation and improve the controlled release of the entrapped drugs.     

Photochemical and antioxidant properties of gamma-oryzanol in beta-cyclodextrin-based nanosponges

Gamma-oryzanol (GO), a mixture of ferulic acid esters, has recently attracted a great interest as natural antioxidant extracted from rice-bran oil, usually employed to stabilize food and pharmaceutical raw materials, moreover as sunscreen in cosmetic formulations. Its usefulness, however, is limited by its fast degradation. A recently proposed approach to increase the stability and effectiveness of antioxidants is based on the inclusion in supramolecular structures (nanoparticles, cyclodextrins, liposomes, etc.). In this work we studied the inclusion of GO in β-cyclodextrin-based nanosponges which in the last few years have been chosen for their ability to encapsulate a great variety of substances to decrease their side-effects and to protect them from degradation. The inclusion complex was prepared in 1:1 w/w ratio and characterized by DSC, XRPD and membrane diffusion runs. The photodegradation of GO upon either UVA or UVB irradiation was found to be slowed down by inclusion in nanosponges. The antioxidant effectiveness of the inclusion complex was also assessed and in vitro experiments on porcine ear skin revealed a certain accumulation of GO also when entrapped in the host structure.     

Photostability of Terbinafine Under UVA Irradiation: The Effect of UV Absorbers

The photostability of drugs administered topically on unprotected skin is a complex phenomenon that could be connected with the loss of activity or, rather rarely, the occurrence of toxic degradation products. In this study, an in‐depth investigation of the photostability of terbinafine, in both solutions and formulations, was conducted, taking into account the presence of UV absorbers such as TiO₂, ZnO, avobenzone, 3‐(4‐methylbenzylidene)camphor, octocrylene, benzophenone‐1 and benzophenone‐2. The clear photocatalytic degradation of terbinafine in ethanol solution was observed in the presence of TiO₂ and/or ZnO. In other cases, terbinafine was stable, with the exception of, in the presence of octocrylene. The presumed degradation products of terbinafine were identified for the first time using LC/MS/MS, and transformation pathways were proposed. In the case of a cream formulation, the percentage of initial terbinafine content was almost unchanged in the presence of the UV absorbers benzophenone‐1, benzophenone‐2 and 3‐(4‐methylbenzylidene)camphor. In vitro cytotoxicity risk assessment of terbinafine based on photostability under UVA irradiation was evaluated using the human skin fibroblast BJ (ATCC^® CRL‐2522?), and this showed no statistically significant difference in cell viability for all samples analyzed.     

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.     

Photostabilization of butyl methoxydibenzoylmethane (Avobenzone) and ethylhexyl methoxycinnamate by bis-ethylhexyloxyphenol methoxyphenyl triazine (Tinosorb S), a new UV broadband filter

It is now well documented that chronic UVA exposure induces damage to human skin. Therefore, modern sunscreens should not only provide protection from both UVB and UVA radiation but also maintain this protection during the entire period of exposure to the sun. UVA filters, however, are rare and not sufficiently photostable. We investigated the effect of the introduction of a new UV filter, bis-ethylhexyloxyphenol methoxyphenyl triazine (Tinosorb S), in oil in water sunscreen formulations on the photostability of butyl methoxydibenzoylmethane (Avobenzone [AVB]) after irradiation with an optically filtered Xenon arc source (UV irradiance adjusted at 1 mean effective dose [MED]/min). With spectrophotometrical methods to assess the sun protection factor (SPF) and UVA ratio and chromatographical methods to determine the amount of UV filters recovered after irradiation we showed that Tinosorb S prevented the photodegradation of AVB in a concentration-dependent way, leading to a sustained SPF and UVA ratio even after irradiation with doses of up to 30 MED. Since AVB was shown to destabilize ethylhexyl methoxycinnamate (EHM) we tested the effect of Tinosorb S in sunscreens containing this UV filter combination. Here too Tinosorb S showed photoprotective properties toward both UV filters. Thus, Tinosorb S can be used successfully to improve the photostability and efficiency of sunscreens containing AVB and EHM.     

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.     

FORWARD SCATTERING PROPERTIES OF HUMAN EPIDERMAL LAYERS

Abstract From an optical point of view the outermost skin layers contain numerous structures by which penetrating radiation may be scattered as well as absorbed. The nature and strength of this scattering may strongly influence the extent of penetration. We illuminated samples of stratum corneum and full-thickness epidermis with collimated radiation and measured the angular intensity distribution of the transmitted radiation; we did this in the ultraviolet for several angles of incidence, and in the visible for perpendicular incidence only. Skin samples were obtained from the skin of the lower back and upper leg of Caucasian volunteers. Epidermis and subsequently stratum corneum were separated by chemical methods. In the case of stratum corneum, the angular intensity distribution of the transmitted radiation peaks strongly at all wavelengths, in approximately the direction of the incident radiation, that has been refracted at the surface of the sample. With full-thickness epidermis, the distribution of the transmitted radiation also peaks, though less strongly than with stratum corneum. These features suggest a forward oriented scattering mechanism. Both in the case of stratum corneum and full-thickness epidermis, the angular distribution flattens towards the shorter wavelengths and with increasing thickness. The wavelength dependence suggests that both scattering and absorption increase towards the shorter wavelengths. The existence of a thickness dependence indicates that volume scattering occurs. Hydration of stratum corneum is found to influence its scattering properties. Dry samples scatter less than hydrated samples. The consequences of our findings for modelling skin optics are briefly discussed.     

Influence of the photostabilizer in the photoprotective effects of a formulation containing UV-filters and vitamin A

Vitamin A palmitate has been used in cosmetics; however, studies report that this substance shows photoreactivity that can lead to loss of safety and efficacy. On the other hand, photostabilizers have been used to increase sunscreen photostability and consequently their safety and effectiveness. Thus, this study aimed to evaluate the influence of photostabilizers on the photoprotective effects of a cosmetic formulation containing UV-filters and vitamin A palmitate. The formulation containing UV-filters was supplemented with vitamin A palmitate and the photostabilizers diethylhexyl 2,6-naphthalate (DEHN), bumetrizole and benzotriazolyl dodecyl p-cresol (BTDC). Hairless mice were treated daily by topical applications and irradiated (UVA/B). Erythema index, transepidermal water loss, histological/histometric analysis and number of sunburn cells (SBC) were evaluated. The results showed that all formulations protected from UV-induced enhancement of erythema and SBC but there was no difference among them. The formulation with no stabilizers reduced viable epidermis thickness due to atrophy induced by UV radiation. Thus, it can be concluded that the presence of photostabilizers influenced the effects of formulations containing UV-filters and vitamin A palmitate, which could be seen by histological and histometric analysis. Furthermore, the formulations containing the stabilizers DEHN and BTDC showed better protective effects on hairless mice skin.     

LACK OF CORRELATION BETWEEN SUPPRESSION OF CONTACT HYPERSENSITIVITY BY UV RADIATION AND PHOTOISOMERIZATION OF EPIDERMAL UROCANIC ACID IN THE HAIRLESS MOUSE

Abstract The immunological consequences of exposure to UVA (320–400 nm) radiation are unclear. This study describes the relationship between the generation of epidermal cis -urocanic acid and the ability to respond to a contact-sensitizing agent, in hairless mice exposed to different UV radiation sources, which incorporate successively greater short-wavelength cutoff by filtration of the radiation from fluorescent UV tubes. Mice were exposed to these radiation sources at doses systematically varying in UVB radiation content but supplying increasing proportions of UVA radiation. All radiation sources were found to generate approximately 35% cis -urocanic acid in the epidermis, thus normalizing the sources for cis -urocanic acid production. However, only those sources richest in short-wavelength UVB resulted in suppression of the systemic contact hypersensitivity response. These sources also induced the greatest erythema reaction, measured as its edema component, in the exposed skin. A strong correlation was thus demonstrated between the induction of edema and the suppression of contact hypersensitivity, but there appeared to be no correlation between the generation of epidermal cis-urocanic acid and suppression of contact hypersensitivity. The sources richest in UVA content did not result in suppression of contact hypersensitivity: furthermore mice previously irradiated with such UVA-rich sources were refractory to the immunosuppressive action of exogenous cis-urocanic acid. A protective effect of the increased UVA content thus appeared to be inhibiting immunosuppression by the available endogenously generated or exogenously applied cⁱs-urocanic acid.     

LONGWAVE ULTRAVIOLET RADIATION (UVA)-INDUCED ALTERATION OF EPIDERMAL DNA SYNTHESIS

Abstract— Longwave ultraviolet radiation(UVA–320–400nm) is known to induce inflammation, pigmentation and tumor production in mammalian skin. The mechanisms by which such radiation induces these biologic phenomena are poorly defined. In an effort to broaden our knowledge in this area, we examined the effect of UVA on DNA biosynthesis in Hartley strain albino guinea pig skin. The animals were irradiated with selected doses of solar simulated UVA, and DNA was assayed by [³H]thymidine incorporation into epidermal DNA by autoradiography. These studies revealed that UVA inhibited DNA synthesis in a dose dependent manner between 40 and 80 J cm^-2 at 3 h post-irradiation. This inhibition was followed by a stimulation of synthesis at 5 h and a second inhibition/ stimulation at 8 and 24 h, respectively. Although the mechanism of alteration is undefined, our data suggest that UVA has profound effects on DNA biosynthesis in mammalian epidermis.     

Chlorogenic Acid UVA–UVB Photostability

Chlorogenic acid is a natural potent antioxidant. It can be used in cosmetics formulations, but for this purpose its photochemical stability should be determined to ensure that the compound will not be degraded after UV radiation exposure. To evaluate this possibility, the concentration of a chlorogenic acid solution was determined by HPLC before and after UVA and UVB irradiation. The results indicate that chlorogenic acid is not degraded under UVA or UVB irradiation.     

Lipid-Based Nanostructures for the Delivery of Natural Antimicrobials

Encapsulation can be a suitable strategy to protect natural antimicrobial substances against some harsh conditions of processing and storage and to provide efficient formulations for antimicrobial delivery. Lipid-based nanostructures, including liposomes, solid lipid nanoparticles (SLNs), and nanostructured lipid nanocarriers (NLCs), are valuable systems for the delivery and controlled release of natural antimicrobial substances. These nanostructures have been used as carriers for bacteriocins and other antimicrobial peptides, antimicrobial enzymes, essential oils, and antimicrobial phytochemicals. Most studies are conducted with liposomes, although the potential of SLNs and NLCs as antimicrobial nanocarriers is not yet fully established. Some studies reveal that lipid-based formulations can be used for co-encapsulation of natural antimicrobials, improving their potential to control microbial pathogens.