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What is Needed for a Sunscreen to Provide Complete Protection

P. Schroeder, PhD and J. Krutmann, MD
Institut für Umweltmedizinische Forschung (IUF) at the Heinrich-Heine-Universität Düsseldorf, Düsseldorf, Germany

ABSTRACT

Human skin is increasingly exposed to sunlight. In order to achieve complete protection against the cumulative detrimental effects from sun exposure, topical strategies must shield against the range of solar wavelengths that can damage the skin. Importantly, the harm sustained by the skin is not limited to that caused by the ultraviolet (UV) portion of the light spectrum, but also includes the adverse effects inflicted by near infrared energy. Consequently, in an attempt to provide the necessary broad spectrum coverage, innovative research continues through the exploration of new compounds and novel combinations of chemical and physical UV filters with molecules that are capable of interfering with and/or preventing the deleterious effects of infrared A (IRA) radiation. Existing examples of infrared-protective active agents include mitochondrially targeted antioxidants of synthetic or natural origin.

Key Words: infrared, IRA, photoaging, sunscreens, skin protection, UVA, UVB, ultraviolet

Adverse Skin Effects from Solar Radiation

Despite some positive and health promoting effects from sunlight, it is apparent that high acute, chronic low dose, and/or unprotected exposure have several detrimental effects, including premature skin aging and the development and progression of cancer.1 For many years the focus of research, and therefore for protective strategies, has been centered on the ultraviolet (UV) part of sunlight, i.e., ultraviolet B (UVB) (290-320 nm) and ultraviolet A (UVA) (320-400 nm), because their relatively high photon energy causes macroscopic skin changes that are visible even after a short duration of exposure. However, UV radiation only accounts for approximately 7% of the sun’s energy,2 which underlines the necessity to consider the detrimental effects from other parts of the sunlight spectrum. Accordingly, we and others identified infrared A (IRA) (760-1440 nm) as a damaging environmental factor to skin through its ability to engender alterations in gene expression of skin cells at multiple points,3 resulting in accelerated skin aging4,5 and contributing to the development of cancer.6

It is well known that the most effective protection against UV radiation is sun avoidance, e.g., by limiting exposure, or at least direct exposure during peak times, and by wearing appropriate clothing. However, in Western civilizations the level of sun exposure continues to rise, e.g., for recreational reasons and due to increased life expectancy.

Complete Photoprotection Considers All Relevant Parts of the Solar Spectrum

Taking into account recent findings, it is evident that effective photoprotection must provide more than UV coverage, but rather it should protect against IRA as well. It is estimated that about one-third of solar energy is comprised of IRA, which is capable of deep skin penetration.2

Multipronged Approach to Complete Photoprotection

Modern topical photoprotection integrates both primary protective factors (e.g., organic or inorganic light filtering agents) that absorb or reflect UV radiation and secondary factors (e.g., antioxidants, osmolytes, and DNA repair enzymes) that can disrupt the photochemical cascade triggered by UV-penetration, thereby limiting skin damage.

Primary Photoprotection

Primary photoprotection is achieved by using physical and/ or chemical UV filtering agents, which have been key active components in commercially available sunscreens for more than 60 years. The most frequently used physical UV filters are the inorganic micropigments, zinc oxide and titanium dioxide.

Most chemical filters absorb UV energy across a relatively narrow or specific wavelength range, converting UV radiation to longer wavelength photons. Due to the limited absorption spectrum of any single ingredient, a combination of sunscreen actives is required to yield both UVA and UVB protection, but the degradation of some UVA filters by sunlight presents formulary challenges. However, in recent years, tremendous progress has been made in developing more photostable UV filters, such as ecamsule (Mexoryl™ SX) and drometrizole trisiloxane (Mexoryl™ XL) and by formulating efficient combinations, such as avobenzone combined with diethylhexyl 2,6-naphthalate and oxybenzone (Helioplex™). Concerning UV filters used in commercially available products, it should be noted that there are differences between approved agents in the European Union when compared with the US, as the US FDA has been more conservative in sanctioning new chemical sunscreens. As for protection against other parts of the light spectrum (other than UV), these chemical compounds do not provide any benefit beyond their UV specificity.

Secondary Photoprotection

Secondary photoprotection involves the use of active agents to interfere with or counteract the inherent photochemical processes that can induce DNA damage in skin cells. Secondary photoprotection may be achieved by an extremely heterogeneous and constantly growing group of molecules that are termed “actives”. Examples of such actives include antioxidants, osmolytes, and DNA repair enzymes7,8 (e.g., photolyase and T4 endonuclease V).

Antioxidants that are typically used in sunscreens and other cosmetic products are comprised of vitamins and polyphenols. Prime examples of vitamins formulated in sunscreens are water soluble vitamin C and lipophilic vitamin E. The term “polyphenols” refers to compounds that possess at least 2 adjacent hydroxyl groups on a benzene ring. Natural polyphenols (e.g., flavonoids and procyanidins) are present in numerous foods and have been demonstrated to provide protective properties through topical application.9,10 In addition, antioxidants have also been shown to protect against IRA. Accordingly, significant importance resides with molecules that are targeted toward mitochondria, because of their central role in IRA-induced adverse effects.3,5,11,12 However, it should be noted that the precise mechanism of action of topically applied actives remain to be elucidated; there is a need to fully understand their effects at both cellular and molecular levels prior to supporting their therapeutic benefits as photoprotective agents.

Osmolytes are small molecules that control and stabilize the cellular environment by regulating hydration and responses to stress conditions. Osmolytes (compatible organic solutes) are not only utilized by cells to control cell volumes, but they have been identified as integral parts of the cellular defence against environmental noxae. The osmolytes taurine13 and ectoine14 have been demonstrated to protect against detrimental UV effects and are amalgamated into several commercially available sunscreens.

Conclusion

Complete topical photoprotection can only be obtained if a sunscreen formula defends against UVB, UVA, and IRA. Whether additional wavelengths contribute to skin damage is currently not known. In order to achieve as near complete broad spectrum protection as is possible, a sunscreen must combine multiple therapeutic approaches that incorporate both essential elements of primary and secondary photoprotection.

References

  1. Krutmann J, Gilchrest BA. Photoaging of skin. In: Gilchrest BA, Krutmann J (eds). Skin aging. New York: Springer, p33-44 (2006).
  2. Kochevar IE, Taylor CR, Krutmann J. Fundamentals of cutaneous photobiology and photoimmunology. In: Wolff K, Goldsmith LA, Katz S, et al. (eds). Fitzpatrick's dermatology in general medicine, 7th ed. New York: McGraw-Hill, p797-808 (2008).
  3. Calles C, Schneider M, Macaluso F, et al. Infrared A radiation influences the skin fibroblast transcriptome: mechanisms and consequences. J Invest Dermatol (In press 2010).
  4. Schroeder P, Pohl C, Calles C, et al. Cellular response to infrared radiation involves retrograde mitochondrial signaling. Free Radic Biol Med 43(1):128-35 (2007 Jul 1).
  5. Schroeder P, Lademann J, Darvin ME, et al. Infrared radiationinduced matrix metalloproteinase in human skin: implications for protection. J Invest Dermatol 128(10):2491-7 (2008 Oct).
  6. Jantschitsch C, Majewski S, Maeda A, et al. Infrared radiation confers resistance to UV-induced apoptosis via reduction of DNA damage and upregulation of antiapoptotic proteins. J Invest Dermatol 129(5):1271-9 (2009 May).
  7. Dong KK, Damaghi N, Picart SD, et al. UV-induced DNA damage initiates release of MMP-1 in human skin. Exp Dermatol 17(12):1037-44 (2008 Dec).
  8. Yarosh DB, O'Connor A, Alas L, et al. Photoprotection by topical DNA repair enzymes: molecular correlates of clinical studies. Photochem Photobiol 69(2):136-40 (1999 Feb).
  9. Allemann IB, Baumann L. Botanicals in skin care products. Int J Dermatol 48(9):923-34 (2009 Sep).
  10. Krutmann J, Yarosh D. Modern photoprotection of human skin. In: Gilchrest BA, Krutmann J (eds). Skin aging. New York: Springer, p103-12 (2006).
  11. Krutmann J, Schroeder P. Role of mitochondria in photoaging of human skin: the defective powerhouse model. J Investig Dermatol Symp Proc 14(1):44-9 (2009 Aug).
  12. Schroeder EK, Kelsey NA, Doyle J, et al. Green tea epigallocatechin 3-gallate accumulates in mitochondria and displays a selective antiapoptotic effect against inducers of mitochondrial oxidative stress in neurons. Antioxid Redox Signal 11(3):469-80 (2009 Mar).
  13. Rockel N, Esser C, Grether-Beck S, et al. The osmolyte taurine protects against ultraviolet B radiation-induced immunosuppression. J Immunol 15;179(6):3604-12 (2007 Sep).
  14. Buenger J, Driller H. Ectoin: an effective natural substance to prevent UVA-induced premature photoaging. Skin Pharmacol Physiol 17(5):232-7 (2004 Sep-Oct).

In this issue:

  1. Alefacept Treatment for Chronic Plaque Psoriasis
  2. What is Needed for a Sunscreen to Provide Complete Protection
  3. Update on Drugs and Drug News - April 2010