Sunscreens: A Review of Formulation and Efficacy
Ms Elizabeth Tian, Senior Pharmacist
National Skin Centre


Introduction


Most modern sunscreens have highly efficient absorption or reflecting capabilities throughout ultraviolet B (UVB), partly ultraviolet A (UVA) and, in some instances, infrared wavelengths. Over the last several years, more efficient sunscreen ingredients (sunscreen actives) have been developed for improved skin protection. This article reviews the current status of sunscreen product development.

I. CHEMICAL SUNSCREENS


Sunscreens have been traditionally divided into chemical absorbers and physical blockers based on their mechanism of action. Chemical sunscreens are generally aromatic compounds conjugated with a carbonyl group. These chemicals absorb high intensity UV rays with excitation to a higher energy state. The energy lost results in conversion of the remaining energy into longer lower energy wavelengths with return to ground state.

A. UVB sunscreens


Octyl methoxycinnamate - The cinnamates have largely replaced PABA derivatives, as the next most potent UVB absorbers. Octyl methoxycinnamate (Parsol MCX) is the most frequently used sunscreen ingredient.


Octyl salicylate - Octyl salicylate or octisalate is used to augment the UVB protection in a sunscreen. Salicylates are weak UVB absorbers and are generally used in combination with other UVB filters. They have the benefit of being exceptionally stable, essentially nonsensitizing and water-insoluble, leading to a high substantivity. They are also useful as solubilizers of other poorly soluble sunscreen actives, such as the benzophenones.


Octocrylene - Octocrylene may also be used in combination with other UV absorbers to achieve higher SPF formulas. Octocrylene used in combination with other sunscreen ingredients, such as avobenzone, may add to the overall stability of these ingredients in a specific formula.


B. UVA sunscreens


Oxybenzone - Oxybenzone absorbs well through UVA II (320-340 nm). It significantly augments UVB protection when it is employed in a given formula. Oxybenzone is frequently implicated as the etiologic agent in photocontact allergy.

Avobenzone - Often referred to by its trade name, Parsol 1789, butyl methoxydibenzoylmethane or Avobenzone, provides superior protection through a large portion UVA range including UVA I (340-400 nm). Potentially a significant addition to sunscreen products for true broadspectrum UV protection, concerns have been raised regarding its photostability and potential to degrade other sunscreen ingredients in products where it is used.


Camphor derivatives - Mexoryl-SX (terephthalylidene dicamphor sulfonic acid) is a relatively new UVA sunscreen that is structurally based on existing benzylidene camphor UVB sunscreens such as 4-methylbenzylidene camphor. Mexoryl-SX has maximal absorption in the mid-UVA and also offers some UVB protection.

II. PHYSICAL SUNSCREENS


In the early 1990s, microfine (transparent) zinc oxide and titanium dioxide became available. Since micronized physical sunscreens reflect at wavelengths shorter than the visible spectrum, they are invisible and thus more cosmetically acceptable.


It was originally considered that, in contrast to organic (chemical) sunscreens, physical sunscreens acted as pure scatterers of UV light. Recent research indicates that the newer micro-sized forms of metal oxides attenuate light through a combination of scattering and absorption. Sometimes referred to as &/cgi-bin/WB_ContentGen.pl?id=147;non-chemical&&gid=83148; sunscreens, they may be more appropriately designated as inorganic particulate sunscreen actives. Titanium dioxide has been reported to photocatalyze a number of functional changes in the cell. To reduce the possibility of such activity on living tissues, titanium dioxide is often coated when used in cosmetic preparations.

A major difficulty in formulating micronized sunscreens is preventing agglomeration of the particles. If this occurs, the portion of the spectrum reflected will shift into the visible range and the product will have characteristics of traditional opaque physical blockers. Proper wetting and dispersion of metal oxides will prevent agglomeration and precipitation in the formula. Some advances have been made by treating or coating the particles to improve dispersion and stability. An example is the treatment of titanium dioxide with isopropyl titanium triisostearate.


Evaluation of UVB protection


The sun protection factor (SPF) is the universally accepted measure of UVB protection. Recently, however, the concepts of SPF usefulness, relevance, and methods of determination have been severely criticized. The main criticism is on the recommended quantities of sunscreen products applied to the skin for assessing SPF (2 ul/cm(2)). A number of studies have shown that consumers apply approximately one-quarter to one-half of the amounts used to measure SPF. Hence, manufacturers should consider testing sunscreens at an application thickness that more closely reflects consumer usage - for example 0.5 to 1 ul/cm(2).

What does SPF measure? - Erythema, the key measurement in the SPF assay, is a relatively crude biological endpoint. A decade ago, SPFs of 15 were touted as complete UVB blockers, a consequence of the idea that they would always prevent erythema and that erythema prevention equals prevention of all untoward effects of UV exposure. A omparison of a SPF 15 versus a SPF 30 sunscreen showed subclinical damage (sunburn cell formation) in the former without visible erythema. The SPF 30 product provided significantly greater protection. However, other forms of subclinical damage may occur through a SPF 15 formulation.


What SPF is considered adequate? - The current trend is to recommend higher factors. Arguably a SPF 15 sunscreen provides full UVB protection for normal individuals. A SPF 15 product filters out over 93% of the UVB radiation, a SPF 30 less than 97%. The difference of 4% would not seem significant to most individuals. However, there are reasons why high SPF products are of benefit. Firstly, sunscreens are usually not applied thickly enough to achieve their ideal SPF. Therefore sunscreen users are receiving a much lower (less than 50%) protection than the labeled SPF. Also, there is diminution of the labeled SPF by environmental factors (sweating, water immersion, rubbing, etc) as well as through photodegradation of the active ingredients.

Reapplication of sunscreens - is it necessary? - There are currently very few studies on the efficacy of sunscreen reapplication. While it is quite clear that one cannot gain additional (extra) SPF value from reapplication of a sunscreen, the reapplication might well ensure the presence of a given SPF for a longer period of time, particularly after swimming or profuse sweating, and so it should be recommended.

Sunscreen boosters - Because the majority of raw material costs for a sunscreen are for the sunscreen actives, the formulator must develop products with high SPF values and low concentrations of sunscreen actives. Recently, several companies have developed materials referred to as SPF boosters. Examples are 2-ethylhexyl salicylate and lauryl lactate. These raw materials attempt to increase the SPF of a formulation without increasing sunscreen actives. However, these SPF boosters are not universally effective; they may enhance the SPF of a particular type formulation, but not of another.

Evaluation of UVA protection


Currently, there is no consensus regarding an ideal in vivo assay to assess protection against UVA. The UVB spectrum readily causes erythema, and can be used for the SPF assay; UVA can cause erythema, but only after a much larger dose. However, the action spectra for other UVA-induced phenomena (e.g. photosensitivity dermatitis, photoaging, and development of melanoma and nonmelanoma skin cancer) are not specifically defined. A variety of different assays have been described for evaluation protection against UVA.


Phototoxic protection factor (PPF) - This assay involves sensitizing the subject’s skin with 8-methoxypsoralen either topically or orally, and then comparing the minimal erythema dose (MED), as for UVB sunscreen testing. Although a reliable assay, with clearly defined visual erythema, it can result in long-term pigmentation of skin sites.


UVA erythema protection factor (APF) - APF is defined analogously to SPF, and assesses the time to induce erythema in UVA-exposed skin. The reliability of such testing is of some question because of the inadequacy of current filters.

Pigment-darkening protection factor - Immediate or instant pigment darkening (IPD), a UVA and visible spectrum-mediated transient oxidation of pre-existing melanin, has also been used to evaluate UVA sunscreen efficacy. IPD is a measure of the darkening immediately after UVA rays exposure (between 0 to 15 minutes). This method has been criticized because it must be performed on individuals with darker skin, since pale-skinned individuals - who are the most in need of protection - do not demonstrate easily measurable IPD. The delayed or persistent pigment darkening (PPD) response evaluated 2 hours after UVA radiation may be more reproducible. This assay is more practical with a greater variety of skin prototypes than the IPD assay. In vitro transmission protection factor - The transmission protection factor is defined as the ratio of the photocurrent measured by a spectroradiometer through Transpore TM tape to the photocurrent measured through sunscreen-coated tape at a given wavelength. According to investigators, this in vitro method provides close agreement with in vivo recorded SPF data.

Photostability


Photostability refers to the ability of a molecule to remain intact with irradiation. Several chemical sunscreens undergo photolysis and thus lose their protective value when exposed to solar radiation. For example, avobenzone exhibits up to a 36% loss after 15 minutes of solar simulated light, while octyl methoxycinnamate loses only 4.5%, whereas the newer agent Mexoryl-SX is especially photostable and silicone-coated zinc oxide is completely photostable.


Higher SPF sunscreen products have led to the use of multiple individual sunscreen actives used in combinations at maximum concentrations. These sunscreen actives may interact. Some examples of photounstable sunscreen filters include combinations of avobenzone and octyl methoxycinnamate. Certain ingredients may have a stabilizing effect on others, as octocrylene and Mexoryl-SX have been shown to photostabilize avobenzone. The relevance of these observations to the in-vivo situation remains unclear. Much work remains to be done in this area. Solvents are needed to solubilize sunscreen actives. The solvents allow the sunscreen actives to emulsify and may have additional effects on the orientation of the sunscreen actives after the actives are applied to the skin. Some of these solvents, such as butyloctyl salicylate, may actually stabilize some sunscreen actives against photodegradation.

Toxicity


Organic sunscreens, especially PABA and its derivatives, have been the subject of extensive in-vitro photochemical and cytological studies suggesting that organic sunscreens like PABA interact with DNA following UV radiation and might potentiate photocarcinogenesis. Both acute and chronic in-vivo animal studies show sunscreens to be protective for both UV-induced DNA damage and skin tumor formation. The in-vivo data would seem to eliminate concerns related to photocarcinogenicity with the use of organic chemical sunscreens.

Directory of commercially available sunscreens

A directory of commercially available sunscreens in Singapore listing their SPFs, active ingredients, comedogenic potential, fragrance status, water resistance and other characteristics is given in Table 1. This directory will be a useful guide for doctors and pharmacists in recommending the appropriate sunscreens to patients.



References


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