Effects of UV irradiation, which are invisible, may be classified as cellular and molecular effects. Urocanic acid (4-Imidazol acrylic acid), for instance, which is present in the stratum corneum in the trans form, undergoes isomerization to the cis form under UV irradiation. Cis urocanic acid is believed to be one of the initiators of the UV induced immunosuppression process at the origin of skin nonmelanoma skin cancer (NMSC). The p53 tumor suppressor gene is the “guardian of the genome” whose expression is a sensitive parameter for the detection of UV induced events which are considered as precursors of NMSC. Mutations in the p53 tumor suppressor gene have been found in 90 percent of squamous and 50 percent of basal cell carcinomas. The development of sunscreens which offer protection in the UVA range has been a major step in photoprotection. It is questionable, however, if a broad absorption or a full absorption sunscreen product should be applied to achieve a higher UVA protection.

To clarify the type of protection required, two identical SPF sunscreens with different absorption profiles were formulated by Serge Forestier and Romano Mascotto. One sunscreen was defined as “Broad Spectrum”, formulated with Octyl Methoxycinnamate and zink oxide and the other was defined as “Full Spectrum” (higher UVA protection), formulated with Octocrylene and Parsol 1789. As indicators of efficacy, the inhibition of UV-induced urocanic acid isomerization and p53 protein accumulation were chosen. The “Full Spectrum” sunscreen product offered a better continuous UV absorption than the “Broad Spectrum” product.

The two formulations were applied on twenty volunteers exposed to UVB and UVA irradiation or to UVA radiation only. The “Full Spectrum” product was significantly more effective than the “Broad Spectrum” one in preventing urocanic acid isomerization induced either by UVB and UVA radiation or by UVA radiation alone.

When the test products were applied on the buttocks of ten test subjects who were exposed daily to 5 MEDs of full spectrum UV source for eight times over two weeks, p53 positive nuclei were detected in the epidermis. Results of the tests show that the two products partially prevent the accumulation of p53 on the one hand, and that the “Full Spectrum” product is nearly twice more protective than the “Broad Spectrum” one.

These studies of Forestier and Mascotto showed that the SPF is not an indicator for the protection for other end points. The same observation has also been made with the photo immunosuppression where UVA plays a major role. As a consequence, the authors conclude that a sunscreen formulation must be proportionally protective against UVA radiation. Thus, a “Full Spectrum” sunscreen product offers a better reduction of the risk of nonmelanoma skin cancer.

Reference

Forestier, S.; Mascotto, R.; Sun and UVA; SÖFW-Journal, 8 (1999), 2-6