Challenges in Making an Effective Sunscreen


Steven Q. Wang, MD

Director of DermatologicSurgery and DermatologyMemorial Sloan-Kettering Cancer Center At Basking Ridge, New Jersey

Judy Y. Hu, MD

Laser & Skin Institute Chatham, NJ

Ultraviolet (UV) radiation from the sun has long been associated with the development of skin cancer. The International Agency for Research on Cancer (IARC) has identified UV radiation as a human carcinogen,1 and nearly 90 percent of non-melanoma skin cancers and 65 percent of melanomas are associated with sun exposure.2,3 UV radiation also plays a role in accelerating skin aging, inducing immunosuppression, and aggravating photodermatoses. For these reasons, numerous campaigns to improve public attitudes towards UV protection and change behavior have been waged by health care communities, nonprofit organizations, and government agencies.

Comprehensive programs for photoprotection include avoiding excessive sun expo- sure, seeking shade, wearing sun-protective clothing (including long-sleeved shirts, long pants, UV-blocking sunglasses, and hats), and applying sunscreen. Among these measures, sunscreen remains the strategy most commonly adopted by people spending time outdoors.

Since the introduction of the first commercially available sunscreen in the US in 1928, tremendous advances have been made in sunscreen technology, improving efficacy and safety. The Final Rules on sunscreens, issued by the US Food and Drug Administration (FDA) on June 14, 2011,4 validated sunscreen manufacturers’ long-standing efforts at innovation, above all in improving broad-spectrum UVA/UVB protection. The FDA will now for the first time allow sunscreens with broad-spectrum SPF 15 or higher protection to display on their labels the claim, “if used as directed with other sun protection measures, (this sunscreen) decreases the risk of skin cancer and early skin aging caused by the sun.”

It has been a long journey to where sunscreens have arrived today—in a better place than they have ever been. Reviewing how they got here will help us understand just how significant the changes already made have been, and the considerable work that remains to be done to make sunscreens even better.

Sunscreen History

The first commercial sunscreen was probably developed by chemist Franz Greiter in 1938,5 but use was not common until 1944, when Benjamin Green6 sold red veterinarian petrolatum (“Red Vet Pet”) to the US Army to protect soldiers in the Pacific theater during World War II. The product had limited effectiveness and was disagreeable to use due to the red color and sticky texture.

In ensuing decades, sunscreens were designed to prevent sunburn and enhance tanning; many were called “tanning oils” or “tanning lotions.” Their UV filters blocked only UVB (280-320nm) radiation, often at very low SPFs (ranging from 2 or 3 to perhaps 8). The Skin Cancer Foundation, with its independent volunteer committee of photobiologists, beginning in 1979, was the first organization to clearly establish SPF 15 as the minimum standard for adequate SPF protection. The first UVA filter, a benzophenone,7 was introduced to manufacturers in the 1960s; however, the first purportedly broad-spectrum UVA/UVB sunscreen was not put on the market until 1980, and truly broad-spectrum sunscreens did not emerge until the 1990s. Even after that, no regulatory guidelines existed in the US for testing and labeling the degree of UVA protection, so many products claiming to have UVA or broad-spectrum protection actually offered inadequate protection or none.8,9

It was not until June 2011 when the FDA provided specific guidelines for testing and labeling UVA protection, naming the in vitro (lab-based) critical wavelength (CW) method4 as its test for UVA protection. Only sunscreens with a CW >370nm will be permitted to claim “broad spectrum” status. The critical wavelength test is a laboratory test method using a PMMA (polymethyl methacrylate) plate that measures UV transmission with and without sunscreen: the absorption spectrum of the sunscreen is measured against wavelength. The wavelength where 90 percent of absorption occurs is defined as the critical wavelength. The more potent the UVA protection, the longer the critical wavelength.

Technologies Used to Create an Effective Sunscreen

In addition to having an impeccable safety profile, sunscreens are expected to prevent both acute and long-term harmful effects of UV exposure. Multiple considerations are factored into the design and making of such products. The two most important objectives are improving the UV absorption profile and enhancing user compliance.

I. Improving the UV Absorption Profile

Sunscreens with superior UV absorption profiles combine high (magnitude) UV protection with broad-spectrum coverage extending to the long-wave UVAI range (340-400nm). To accomplish this, formulators need to consider both the UV filters and the inactive ingredients in the formulation. Most of the media attention has focused on UV filters, but inactive ingredients such as rheological additives, film-forming polymers, and light-scattering particles play an equal role in overall efficacy. In fact, many of the breakthroughs in modern-day sunscreens come from the addition of novel inactive ingredients; it is often easier to improve sunscreens in this way, since developing and marketing new UV filters are costly, and often hampered by long delays in the FDA approval process.

UV Filters

Currently only 17 UV filters, or active ingredients, are approved for use in the US— far fewer than in Europe and Australia. In addition, the allowable concentration of avobenzone, the most potent chemical UVA-I filter in the US, is only 3 percent, compared to 5 percent in Europe, and the FDA does not allow it to be combined with physical filters such as zinc oxide. These limitations pose a challenge for formulators attempting to design products with superior UVA protection.

A number of active filters (Table 1) are currently awaiting FDA approval via the Time and Extent Application (TEA) process. With TEA, the FDA can expedite approval of formulations and ingredients that have been available in foreign markets for five years. All of the new compounds fulfill this time requirement, having been marketed in Europe, Asia, and Australia over the last decade. The addition of these new filters would provide formulators with more options and, in theory, create superior products. Unfortunately, some of these products have been in the TEA pipeline for years without notable progress toward approval.

Efficacy Enhancers

Choosing the types and concentrations of UV filters is only the first component in the formulation process. As mentioned, inactive ingredients comprising the delivery vehicle play a major role in efficacy. Vehicles that dissolve and disperse the UV filters uniformly can enhance overall UV protection. Excess UV filters from a poorly formulated product tend to accumulate in the valleys of the skin, leaving poor coverage on the peaks of the skin. This uneven coverage translates to lower SPF protection and often sunburn. In contrast, UV filters from a well-formulated product deliver an even coating to the skin, providing equal coverage to both peaks and valleys and thus better protecting against sunburn. To deliver uniform and even coverage, additives and film-formers (agents such as acrylates, acrylamides, and copolymers that when applied to the skin, leave a pliable, cohesive, and continuous covering) have been developed.10,11

Light-scattering additives also boost the overall SPF of products. They scatter incident UV light and increase the distance it has to travel to reach the skin as it passes through the sunscreen layer. According to the Beer-Lambert law, the absorbance of any UV filter depends on the path length that light must travel. Hence, increasing the path length increases the filter’s UV absorbance. One example of this principle is the hollow sphere technology developed by Jones et al,12 composed of styrene/acrylates and copolymer-entrapping water molecules. These polymer spheres scatter light and increase path length.

Photostabilizers

A major weakness of some UV filters is their tendency to degrade after UV exposure; the original molecules convert to isomers, tautomers, or dimers, which are less effective or completely ineffective in providing UV protection. Avobenzone is the only long-range chemical UVA filter (340-400 nm) widely available to sunscreen manufacturers in the US. The molecule is inherently unstable, and by itself, loses nearly 50 percent of its screening capacity after just one hour of UV exposure.13 Furthermore, the frequent addition of octinoxate, a common UVB filter, to sunscreens containing avobenzone accelerates the degradation of both compounds. However, a number of photostabilizers have been incorporated in sunscreens to stabilize avobenzone. One, the UVB filter octocrylene, is widely used in the US.9 Another, comprised of octocrylene, oxybenzone, and diethylhexyl 2,6-naphthalate (DEHN), has been shown to provide over 80 percent photostabilization of avobenzone. This combination is patented and trademarked by Neutrogena as HelioplexTM. In Europe and Asia, other molecules and UV filters, such as bemotrizinol, 4-methylbenzylidene camphor, and polysilicone, are sanctioned to stabilize avobenzone.

II. Boosting User Compliance

The UV absorption profile is only one feature of an effective sunscreen. Perhaps every bit as important but less acknowledged are the qualities that inspire usage. These include fragrance, color, appearance, sensory pro- file, packaging, and last but not least, cost. Collectively, the appeal of these features often determines overall user compliance. It should be self-evident that consumers will not use a product that fails their demands in these areas, even if it has a superior UV absorption profile. At best, consumers will use the product in inadequate amounts and neglect to reapply as directed. The result is poor UV protection.

The sensory and tactile modifiers in sunscreens especially influence their appeal. Consumers often complain that most products are too greasy and oily. This poses a challenge to formulators, as most chemical UV filters are oil-soluble molecules. For example, octocrylene, a UVB filter and a photostabilizer for avobenzone, is extremely oily. To compound the problem, most recreational (sport) sunscreen products need to be water-resistant so that they won’t wash away with sweat. Formulators use water-resistant polymers such asBis-PEG-18methyl ether dimethylsilane, trimethylsiloxysilicate, and butylated PVP (polyvinylpyrrolidone) to form a physical film to help hold the sunscreen oil on the skin’s surface. These polymers can create a tacky and greasy feeling that makes sunscreens unpleasant to use.

A variety of ingredients, such as silicones, silicas, and other slipping agents, are added to decrease the tacky and sticky feeling and improve tactile and sensory profiles. Polymeric surfactants, such as acrylate cross polymers, can also serve as both water resistance agents and emulsifiers. These surfactants have rapid emulsion-breaking characteristics that allow consumers to spread the product on the skin easily and evenly, and improve the overall texture after the sunscreen dries.

Future Trends

A recent industry trend involves the addition of antioxidants to sunscreens. Many manufacturers have embraced the concept and now market products that include vitamin C, vitamin E, and other antioxidants. The scientific rationale is that antioxidants provide a second line of protection against the radical oxygen species (ROS) induced by UV, since conventional UV filters offer incomplete protection. A large reason for inadequate sunscreen protection, however, is simply that many individuals apply inadequate amounts. Furthermore, many products in the US still offer low or no UVA protection. ROS generated from UVA rays have the potential to react with various photosensitizers and generate damage to the DNA, proteins, and lipid membranes of skin tissue. Although the body has natural antioxidant defenses against the ROS, this endogenous system is quickly overwhelmed when faced with excessive oxidative stress. That is one reason it will be so important, now that the FDA has issued its final rules on sunscreen, for consumers to start looking for broad-spectrum UVA/UVB sunscreens that bear the aforementioned label, “if used as directed with other sun protection measures, (this sunscreen) decreases the risk of skin cancer and early skin aging caused by the sun.” Only broad-spectrum sunscreens with SPFs of 15 or higher will earn this label.

The concept of adding antioxidants to sunscreen is appealing. However, a recent study by Wang, et al14 showed that the protection against ROS in sunscreens containing antioxidants derives mainly from the UVA filters. The activity level of antioxidants in sunscreens is virtually nonexistent, for a number of reasons, including inadequate antioxidant concentration, inherently unstable formulations, and use of the wrong active forms. Thus, considerable progress is needed before any truly significant benefits can be achieved by adding antioxidants to sunscreens.

Conclusions

The functional role of sunscreen has progressed from merely preventing sunburn to reducing skin cancers and slowing skin aging. The sunscreen industry continues to develop novel UV filters and innovative vehicles to provide superior UV protection. With additional understanding of the potential dangers of UV radiation, the scientific communities and sunscreen industry will certainly continue to develop innovative, safer, and more effective sunscreens. Dermatologists and other physicians must educate consumers about new products with cutting edge technologies. More importantly, in addressing the public, we should always emphasize that sunscreen use is only one type of photoprotection. Avoiding excessive sun exposure, seeking shade, and wearing sun-protective clothing and sunglasses are also essential in reducing overall UV exposure.

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