Skin surface lipid fractions represent the first target of ultraviolet (UV) irradiation of the skin. Following irradiation, squalene (SQ), a peculiar polyunsaturated sebum fraction, undergoes a rapid and time dependent oxidation and decomposition with generation of reactive radical by-products, possible mediators of the biological effects of UV-R on skin. In this connection, SQ decomposition can be viewed as a hallmark of epidermal oxidative stress. Our work aimed at verifying if a relationship does exist between UV induced SQ oxidation and protection afforded by commercial UV-B and UVA sunscreen agents (Octyldimethyl PABA, UV-A filter; Parsol MCX, UV-B filter; Benzophenone-3, UV-B+UVA filter; UV-B; Parsol 1789, UV-A filter). Moreover, we evaluated if the sunscreens were able to exert a antioxidant action when subjected to oxidative sources other than UV irradiation.
We have investigated by GC-MS methods, following different periods of UV exposure (UV-B and UV-A ranges): the degree of inhibition of peroxidation of SQ and of a tri-unsaturated fatty acid (a-linolenic acid, LIN), when subjected to different oxidative sources, in three in vitro experimental models: photo-oxidation, auto-oxidation, chemical oxidation with an initiator of lipid peroxidation (in the absence of light).
We have measured: a) the rate of SQ and/or LIN decomposition in the presence or in the absence of sunscreens tested at concentrations and in combinations similar to those present in cosmetic products; b) sunscreen stability under the same experimental conditions; c) the possible inhibition exerted by filters on the formation of a stable byproduct of LIN oxidation (i.e. azelaic acid, AZ).
Depending on concentration and time of irradiation, all sunscreens displayed the capacity of inhibiting, to different degrees, SQ oxidation. Our results show that sunscreen efficacy correlates well with the inhibition of SQ oxidation. The protective effect observed on LIN oxidation was significantly correlated with the inhibition of the production of AZ (p<0.01). As for AMVN oxidation, all filters tested exerted a weak but significant protection on LIN, indicating that antioxidant power of sunscreens relies mainly on their capability of absorbing UV-R, but may in sone cases due to a true antioxidant action.

Independently of the mechanism of action, the biological effects of irradiation by-products of sunscreens such as Octyldimethyl PABA and Parsol 1789, should be carefully investigated, considering the wide use of these molecules.

Dr. Chiara De Luca

IDI - Centro Invecchiamento Cellulare

Via Dei Monti Di Creta 104

I-00167 Rome, Italy

Telefono: ++39 6 66464787 (86)

Fax: ++39 6 66464705