J. Cosmet. Sci., 71, 199–206 (July/August 2020) 199 Formulation of Sunscreens for Optimal Effi cacy DOMINIQUE MOYAL, THIERRY PASSERON, MARTIN JOSSO, STÉPHANE DOUEZAN, VÉRONIQUE DELVIGNE, and SOPHIE SEITÉ, La Roche-Posay Dermatological Laboratories, 92300 Levallois-Perret, France (D .M., V.D., S.S.), Department of Dermatology, Université Côte d’Azur, CHU Nice, 06000 Nice, France and Université Côte d’Azur, INSERM, U1065, C3M, 06000 Nice, France (T.P.) , L’Oreal Research and Innovation, 94550 Chevil ly Larue, France (M.J., S.D.) Synops i s Skin ex posure to solar radiation can cause many adverse effects. In addition to the sun protection factor (SPF), a parameter associated with Ultraviolet B (UVB) protection, signifi cant evidence emphasized the crucial importance of a well-balanced protection against ultraviolet A (UVA) and for some indications, against high-energy visible light. Synergy between UV fi lters and fi lter photostability together with fi lm-forming ingredients such as polymers that ensure the homogeneous distribution of UV fi lters on the skin are key factors to avoid UVA– and UVB– provoked detrimental effects of solar radiation. Clinical studies mimicking real conditions of use have been performed. The results show that a well-balanced sunscreen with at least an SPF-to-UVA protection factor ratio 3 provides the most effective protection against DNA damage, skin photoimmunosuppression, photodermatoses, and pigmentation disorders. In addition, cosmetically pleasant sunscreens allow a suffi cient amount to be applied and re-applied by consumers, ensuring continuous and even coverage of the exposed skin. INTRODU CTION Ultravi olet B (UVB) (290–320 nm) radiation accounts for 5% of total solar Ultraviolet (UV) radiation. It causes adverse effects, such as sunburn and long-term damage includ- ing skin cancer, principally melanoma and squamous cell carcinoma (1,2), by directly impacting cellular DNA (3). Irrespe ctive of the meteorological conditions, ultraviolet A (UVA) (320–400 nm) irradi- ance is at least 17 times higher than UVB irradiance (4). UVA penetrates deeper into the skin than UVB. Via photosensitization processes, it produces reactive oxygen species that damage DNA, cells, vessels, and elastic fi bers in connective tissues, leading to photoag- ing (5–8). Within the UVA band, long UVA or UVA1 rays cause the most damage (9). UVA and B are implicated in immune system depression (10,11). Photosensitivity and pho- todermatoses are mainly UVA-induced (12), and UVA is a key factor in the aggravation of pigmentation through melanin oxidation and distribution (13,14). The sun protection Address all correspondence to Sophie Seité at sophie.seite@loreal.com.

JOURNAL OF COSMETIC SCIENCE 200 factor (SPF) is still considered by many consumers and some physicians as the main crite- rion to choose a sunscreen. In addition to UVB protection, not only the protection against long-wave UVA (and in some conditions against visible light and short infrared) but also the stability of the fi lters, the esthetic of the formula, and practicability and ability of the fi nal product to homogeneously cover the entire skin are key factors that must be taken into consideration for an optimal formulation of a sunscreen. DEVELOP MENT OF MORE EFFICIENT SUNSCREENS CHOOSIN G THE APPROPRIATE FILTERS The rec ognition of UV-induced skin damage prompted the development of products of- fering greater protection against UVB and UVA radiation. Well-balanced protection against UVB and UVA radiation was defi ned in the 2006 EC recommendation (15) as an SPF-to-UVA protection factor (UVAPF) ratio of d3 (e.g., UVAPF at least 20 for SPF50+). This requirement has therefore been adopted in other countries worldwide. The UVAPF is determined according to International Organization for Standardization (ISO) (16,17) using the persistent pigment darkening (PPD) endpoint representative of UVA damage. Increased UV protection requires higher concentrations of UV absorbers, but their un- pleasant, greasy feeling may induce low cosmetic acceptance. Therefore, formulators must create elegant yet effective sunscreens with less UV fi lters but the same level of UVB/ UVA protection. Balanced protection requires a combination of lipophilic and hydro- philic absorbers in the UVB, short UVA, and long UVA (18). An examp le of synergy of organic UV fi lters is offered by mixing terephthalylidene dic- amphor sulfonic acid (Mexoryl® SX, Noveal, Le Thillay, France), a “short UVA” fi lter, with drometrizole trisiloxane (Mexoryl® XL, Noveal) and bis ethylhexyloxyphenol me- thoxyphenyl triazine (Tinosorb® S, BASF, Ludvigshafen, Germany), two broad UVB/ UVA fi lters. All three are photostable and are part of Mexoplex® technology (18). An- other example is offered by nanosized titanium dioxide (an inorganic UV fi lter) that, combined with organic UV fi lters, produces a high SPF through synergetic effects. Formulations should be photostable and not be degraded during exposure to sunlight (19). Formulators should ensure persistence of protection during exposure by using a photostable fi ltering system. The UV fi lte r avobenzone (butylmethoxydibenzoylmethane or BMDBM) has a very high molar extinction coeffi cient in the UVA1 wavelengths, but it is degraded signifi cantly on exposure to UV (20). This photo-instability is enhanced by the UVB fi lter ethylhexyl methoxycinnamate (21), so this combination should be avoided. Potent avobenzone photo- stabilizers include octocrylene, a UVB fi lter, Tinosorb® S (22–24), and the biodegradable oil Eldew® SL-205 (Ajinomoto, Tokyo, Japan), a sarcosine derivative, are used in the Mexoplex® technology (18). IMPORTANCE OF M AXIMIZING EFFICIENCY AND COMPLIANCE Increasing effi cacy while improving consumer observance irrespective of age and skin types requires products with good spreadability, pleasant feel, and transparency on the skin. Homogeneous distribution of UV fi lters is essential for maximizing effi cacy.