Protection Against Ultraviolet A (UVA) Radiation
Rating UVA Protection
The almost universal use of the sun protection factor (SPF) has lured many consumers into thinking that a higher SPF means a better sunscreen. Because SPF is mostly an indicator of UVB protection, it is difficult for consumers and physicians to compare the UVA protection afforded by sunscreens. For many years, some countries have been using UVA labeling systems that can provide guidance on both UVA and UVB protection that is offered by sunscreens. Other countries, like the US and Canada, have been slower to introduce guidelines for UVA protection labeling.
Modifications to Sunscreen Labeling
The US FDA recently proposed inclusion of a 4 star grading system in conjunction with a descriptor (i.e., low, medium, high, and highest) to rate UVA protection.2 This star rating system will depend on results of both in vitro and in vivo UVA testing. According to the agency, UVA ratings would be based on 2 tests: one would measure the sunscreen´s ability to reduce UVA penetration and the second would measure the product´s ability to prevent tanning. The test that yields the lowest level of UVA protection would determine the number of stars that the sunscreen would receive. This will help consumers and physicians identify the level of UVA protection provided by the different sunscreens. Other modifications include making minor changes to UVB testing procedures to improve accuracy, increasing the maximum sunburn protection factor from SPF 30+ to SPF 50+, and sanctioning the use of new combinations of active ingredients.
Avobenzone and Photostability
Protection against UVA radiation was revolutionized by the introduction of butyl methoxydibenzoylmethane (avobenzone) in the late 1980s and early 1990s. This was the first organic sunscreen agent that provided some protection for mid- to long-range UVA rays. However, the degradation of some UVA filters, such as avobenzone, by sunlight, prompted the need to stabilize the formulation in order to prevent the loss of efficacy. Upon absorption of UV radiation, the avobenzone molecule can transform into a molecule that no longer absorbs UVA radiation. In formulations where avobenzone is not photostable, UVA protection decreases with the time spent under the sun. This has been shown to occur after as little as 60-90 minutes of sun exposure.
A number of different companies have developed systems to stabilize avobenzone in the final formulations. For example, a combination of avobenzone and 2-ethylhexyl ester (octocrylene) has been shown to achieve a photostable product. The addition of diethylhexyl 2,6-naphthalate also makes avobenzone photostable. The combination of diethylhexyl 2,6-naphthalatate, avobenzone and oxybenzone is known under the commercial name of Helioplex™ and it is present in UltraSheer® and Age Shield® sunscreens (Neutrogena®/Johnson & Johnson). The addition ofTinosorb S® (Ciba Specialty Chemicals) has also been shown to photostabilize avobenzone. Confirmation of avobenzone´s photostability in a given formulation is difficult unless the sunscreen´s chemical stability has been studied and the results are published in a peer reviewed journal. In the absence of such studies, physicians can get indirect evidence of the photostability of a given formulation from UVA protection factor determination with methods such as persistent pigment darkening (PPD).
PPD evaluates pigmentation present at 2 hours after the end of exposure to different UVA fluences. Because UVA exposures for these methods are rather lengthy, a sunscreen formulation with unstable avobenzone will have a lower protection factor than a similar formulation with stabilized avobenzone.
Recently Introduced Organic UVA Sunscreen Agents
A number of UVA sunscreen agents have been introduced in the past few years. Unfortunately, their availability varies widely from country to country. For example, in the US and Canada, sunscreen agents are considered to be drugs. Sunscreen manufacturers must therefore submit a new drug application when they want to incorporate a new agent into a formulation. This explains why ecamsule (terephthalylidene dicamphor sulphonic acid [Mexoryl SX™, L´Oréal]) was only recently introduced in the US in 2006, whereas, this agent has been available in most other regions of the globe for more than 10 years. Mexoryl SX™ is a photostable chemical sunscreen agent that offers mid-range UVA protection. When combined with avobenzone, UVA protection is enhanced. Sunscreen products that contain Mexoryl SX™, and are available in the US, include Anthelios SX™ Daily Moisturizing Cream (SPF 15), Anthelios™ 15 Sunscreen Cream (SPF 15) and Anthelios SX™ 40 Sunscreen Cream (SPF 40, to be introduced in 2008).
Dometrizole trisiloxane (Mexoryl XL™) is another recently introduced organic sunscreen agent offering mid-range UVA protection. The addition of Mexoryl XL™ to Mexoryl SX™ has been shown to increase UVA protection in a synergistic manner, which may be attributable to its 2 phase component. Mexoryl XL™ was introduced in Canada in 2006. It has not yet been approved in the US, but has been available worldwide for many years in different sunscreens made by L´Oréal. In Canada, Mexoryl XL™ can be found in sunscreens sold under different brands including Anthelios™, Ombrelle™, Vichy™ and Biotherm™ (L'Oréal).
Bemotrizinol and bisoctrizole (Tinosorb S® and Tinosorb M® respectively, Ciba Specialty Chemicals) are organic compounds that also provide broad-spectrum UV protection. Tinosorb S® has been shown to increase photostability of avobenzone.6 Tinosorb S® and Tinosorb M® are mid-range photostable sunscreen agents that have been used in Europe for many years, but they are not yet approved in the US. These UV filters have recently been introduced in Canada and are formulated in Minesol® SPF 60 products (RoC®/Johnson & Johnson).
Skin Therapy Letter. 2008;13(6):5-7. © 2008 SkinCareGuide.com
Cite this: Update on Sunscreens - Medscape - Jul 01, 2008.