J. Cosmet. Sci., 71, 149–165 (May/June 2020) 149 Effect of Solvents on the In Vitro Sun Protection Factor and Broad-Spectrum Protection of Three Organic UV Filters MARIAM ABOU-DAHECH, ALLISON SCHAEFER, LAURA LAM-PHAURE, AN HUYNH, MARK CHANDLER, and GABRIELLA BAKI , Department of Pharmacy Practice, College of Pharmacy and Pharmaceutical Sciences, University of Toledo, Toledo, OH 43614 (M.A., A.S., L.L.-P., A.H.,G.B.), ACT Solutions Corp, Newark, DE 19713 (M.C.) Accepted for publication February 28, 2020. Synopsis Solvents play an essential role in the performance of ultraviolet (UV) fi lters. The goal of this study was to understand how the in vitro sun protection factor (SPF) and broad-spectrum protection of three organic UV fi lters (homosalate, ethylhexyl salicylate, and butyl methoxydibenzoylmethane) and a combination of these are infl uenced by solvents. Twenty-four solvents were selected based on the ingredient active gap for testing. Mixtures of UV fi lters and solvents were formulated, and in vitro SPF, wavelength of maximum absorbance, broad-spectrum protection, and spreadability were evaluated. Results indicate that in vitro SPF of organic sunscreens can be signifi cantly enhanced by solvents. Relying on solubility data only was not found to be a good approach in this study. The most effi cient solvents shared multiple similar structural characteristics, including ester bonds, conjugated structure, aromatic rings, and –CN groups however, the absence of some of these structural elements did not necessarily prevent a solvent from being a booster. The wavelength of maximum absorbance was signifi cantly shifted in the UVA range by most solvents, whereas minimal or no shift was observed in the UVB range. Results of this study provide practical information that can guide sunscreen formulators in selecting solvents for UV fi lters and making more effective sunscreens. INTRODUCTION Ultraviolet (UV) radiation has a variety of biological and clinical effects on the skin rang- ing from erythema to pigmentation (1) to premature aging (2) and skin cancer. A com- mon approach to prevent the damaging effects of UV radiation on the skin is to reduce intentional exposure to UV radiation and use sunscreens. Recent legislations in Hawaii Address all correspondence to Gabriella Baki at Gabriella.Baki@utoledo.edu. Part of the work was presented as poster presentations at the following meetings: Michigan Society of Cos- metic Chemists Fall Symposium, Grand Rapids, MI, on September 6, 2018 New York Society of Cosmetic Chemists Meeting, Totowa, NJ, on March 13, 2019 American Chemical Society Central Regional Meeting in Midland, MI, on June 4–8, 2019 and Midwest Society of Cosmetic Chemists Fall Technical Symposium, Chicago, IL, on October 8, 2019.
JOURNAL OF COSMETIC SCIENCE 150 (3) Key West, FL (4) and the Virgin Islands (5) in the United States have banned two monographed organic UV fi lters, namely, oxybenzone [International Nomenclature of Cosmetic Ingredients (INCI) benzophenone-3] and octinoxate (INCI: ethylhexyl methoxycin- namate) because of their harmful environmental effects (6). The United States has a low num- ber of UV fi lters to start with and the over-the-counter (OTC) monograph restricts certain combinations. Now, with the bans, sunscreen formulators in the United States have a very limited selection of organic UV fi lters they can incorporate into sunscreen products, and it is challenging to achieve a reasonably high SPF and broad-spectrum protection. The need for SPF and broad-spectrum protection boosting technologies is greater than ever. For an organic UV fi lter to be an effective sunscreen, it must be soluble in at least a por- tion of the sunscreen formulation (7). Therefore, solvents play an essential role in the performance of sunscreens. Traditionally, octanol–water partition coeffi cient (logP) was used as an indication of lipophilicity and solubility. A newer approach is to use Hansen solubility parameters (HSPs), which are more versatile and look at solubility from a more complex approach (8). Each ingredient has three HSPs, δD for dispersion, δP for polarity, and δH for hydrogen bonding, which defi ne its location in a three-dimensional space, the HSP space. HSP is based on the theory of “like dissolves like.” Ingredients with similar HSPs dissolve well in each other, whereas dissimilar HSP values indicate that two ingre- dients will not mix/dissolve in each other. Formulating for Effi cacy™ (FFE, ACT Solutions Corp., Newark, DE) is a computer program that matches active and inactive ingredients based on their HSPs (9). In addition to matching ingredients, FFE also calculates the ingredient active gap (IAG). IAG refers to the similarity of the ingredient and active ingredient (i.e., solvent and UV fi lter in this study). In general, the smaller the IAG, the more alike the active and solvent, and as “like dissolves like,” the solubility will be higher. HSPs and IAG are applied in many sectors, including pharmaceuticals (10,11), personal care products (12), polymers (13–15), and paints and coatings (16,17). Using HSPs and IAG allows for a logical and streamlined selection process, which can save time and money for formulators. Solve nts often have different labels on supplier datasheets, including emollient, photosta- bilizer, and SPF booster. The current trend is to achieve a high SPF with only small amounts of UV fi lters because of economical, ecological, sensorial, and health-related reasons (7). A way to achieve this goal is to incorporate ingredients in the formulation that can attain better UV performance with less UV fi lters. Solvents have an important role in dissolving UV fi lters and contributing to the aesthetics of the product. In addition, certain emol- lients have been found to contain the properties of UV fi lters and, therefore, impart an SPF of their own (18). Solvent polarity was also found to affect the wavelength of maxi- mum absorbance (λmax) and critical wavelength in sunscreen formulations (19). In this project, our goal was to evaluate the effect of 24 solvents on the in vitro SPF, λmax, and broad-spectrum protection (i.e., critical wavelength) of three organic UV fi lters and a blend of these three UV fi lters. Solvent selection was based on a modern approach, i.e., IAG calculated by FFE. MATERIA LS UV fi lt ers included avobenzone [INCI: butyl methoxydibenzoylmethane (BMDM), United States Adopted Name (USAN): avobenzone] and homosalate (INCI/USAN: homosalate) purchased from Making Cosmetics (Redmond, WA), and Neo Heliopan OS® (INCI: ethyl- hexyl salicylate, USAN: octisalate) provided by Symrise (Elyria, OH). Today, the most widely
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