Photoprotection advances stress standardization, stability

"UVA is an important component of sunlight and sun exposure. However, at this time in the United States, there's no specific guideline or method that has been approved" by the Food and Drug Administration (FDA) for measuring UVA protection, says Henry W. Lim, M.D., chairman and C.S. Livingood chair, department of dermatology, Henry Ford Hospital, Detroit.

Exploring UVA protection

Meanwhile, persistent pigment darkening (PPD) represents the most widely used means of measuring UVA protection worldwide, Dr. Lim says (J Am Acad Dermatol. 2001 Mar; 44[3]:505-508). PPD is a response that is detected two to 24 hours after exposure to UVA, Dr. Lim, who adds that it is secondary to photo-oxidation of melanin, tells Dermatology Times.

Additional in vivo methods of measuring UVA protection include immediate pigment darkening (IPD) and UVA protection factor (PFA, APF), Dr. Lim says. Conversely, he adds that critical wavelength represents an in vitro method, while UVA balance expresses the ratio of in vitro PPD protection factor and in vivo UVB protection (i.e., label SPF).

Dr. Lim says research has shown that exposure to a single instance of suberythemal solar simulating radiation is immunosuppressive (J Invest Dermatol. 2003;121:ix-x), and that immune protection factor (IPF) correlates more closely with UVA protection, not SPF (J Invest Dermatol. 2005;125:403-409).

Categorizing sunscreens

The FDA categorizes sunscreen agents as organic filters and inorganic filters.

Organic filters include UVA sunscreens such as avobenzone/Parsol 1789 and oxybenzone/benzophenone-3, along with UVB sunscreens such as aminobenzoic acid and cinoxate.

Inorganic filters include zinc oxide and titanium dioxide, Dr. Lim says.

Among newer sunscreen agents, he says, "The only one that is approved (July 2006) by the FDA is the UVA filter ecamsule (mexoryl SX, an ingredient in Anthelios SX, LaRoche-Posay)."

However, he says two other UVB/UVA filters - Tinosorb M and Tinosorb S (bisoctrizole and bemotrizinol, both made by Ciba Specialty Chemicals) - were undergoing the FDA approval process at press time.

Also under FDA review is T4N5 liposome lotion, which, in one study, provided DNA repair after daily application for a year (Lancet. 2001 Mar 24;357[9260]:926-929), Dr. Lim says.

"In this study," he says, "It decreased the development of actinic keratoses and basal cell carcinomas."

Manufacturers stepping up

Dr. Lim says the sunscreen industry is very effectively addressing the fact that some of the most effective UV filters are photounstable.

For example, "Parsol 1789 by itself is a very good UVA filter - it effectively covers any wavelength of UVA," he notes.

However, he says that because of its photoinstability, manufacturers have begun mixing it (and other products lacking photostability) with other agents such as oxybenzone, titanium dioxide and zinc oxide to increase photostability.

On the other hand, Dr. Lim says that, in small studies, the OTC plant extract Polypodium leucotomos, available in some sunscreens in Europe, has been shown to decrease redness and tanning caused by UV radiation.