New and highly effective UV filters in sunscreen products
may protect against adverse effects including erythema, premature aging,
photosensitivity disorders and some forms of cancer. Recent studies have
shown that UVA radiation (320 - 400 nm) leads to erythemal damage, alters
human skin and reduces the immunosuppression reaction. The relative effectiveness
of UVB and UVA radiation is unclear in the case of melanoma which are
not only related to UV irradiation.
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
