Avobenzone

Avobenzone is one of the best filters of UVA radiation, making it a good candidate for sun protection to prevent photoaging.

Effects


Grade Level of Evidence
A Multiple double-blind, controlled clinical trials.
B 1 double-blind, controlled clinical trial.
C At least 1 controlled or comparative clinical trial.
D Uncontrolled, observational, animal or in-vitro studies only.
Grade Effect Size of Effect Comments

D

Photoprotection

Strong

An excellent UVA absorber that absorbs in the entire UVA spectrum.

D

Wrinkle treatment

Mild

May help prevent skin wrinkling, since it inhibits the UVA-induced expression of collagen-degrading enzymes.

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Scientific Research


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Table of contents:

1. Sources

Avobenzone is an organic molecule used in topical sunscreen products.[1] It attenuates UV radiation by absorption.[2]

Avobenzone is consistently one of the most common UV filters contained within personal care products, including skin care, facial makeup and lip care products. A survey of sunscreen products in Denmark in 2002 found that avobenzone was the second most frequently used UV filter, present in 44% of 75 products.[3] More recent and more comprehensive surveys in the UK, Germany and Switzerland also agree that it is the most commonly included UV filter.[4][5][6]

2. Skin penetration

Minimal skin penetration of sunscreens is desirable for maximal photoprotection and to prevent any possible local or systemic toxicity.[7] Although a small amount (<1%) of topically applied avobenzone penetrated through baby mouse skin,[8] avobenzone is not able to pass through human epidermis.[9][10][11] It is however able to penetrate the stratum corneum and spread laterally to untreated areas of the skin.[12]

One study has indicated that of the up to 16.8% of free avobenzone that can penetrate human skin, the majority is localized in the stratum corneum and the remainder in the epidermis.[13] Another observed that avobenzone was located in the upper 30% of the stratum corneum 1 hour after application and did not reach the living cells.[14] The absorption of avobenzone is not increased in photoaged skin.[15]

Complexing avobenzone with cyclodextrins helps reduce its skin penetration flux, prolonging the absorption lag time.[16] This can also lower the amount of avobenzone that accumulates in the epidermis[13] as well as decrease transdermal penetration.[17]

The vehicle, too, influences the skin penetration of avobenzone.[11][18] Liposomes,[19] lipid microparticles,[20][21][22] and nanodelivery systems[23][24] can aid in retaining avobenzone at the superficial layers of the skin.

On the other hand, the use of silicone emulsifers in cosmetic emulsions did not significantly affect the skin permeation of avobenzone, despite influencing the permeation of octinoxate.[7]

3. Effects on the skin

3.1 Photoprotection

Avobenzone is a highly efficient absorber of UVA, capable of absorbing across the entire UVA spectrum from 320-400 nm.[2] Its absorption maximum is at 357 nm.[25]

In fact, avobenzone is among the best sunscreen agents for protecting against UVA radiation. An early study that evaluated 5 commercially available sunscreens in humans found that the product containing 3% avobenzone was the most effective.[26] Avobenzone is also more effective than titanium dioxide, which effectively attenuates UVA wavelengths up to only 360 nm,[2] and more effective than oxybenzone in both normal unsensitized and sensitized skin.[27]

In a controlled in vivo study involving 10 human volunteers, a SPF 10 sunscreen containing avobenzone, enzacamene and ensulizole was successful in protecting against UVB-induced DNA damage as measured by the induction of cyclobutane thymine dimers.[28]

Sunscreens containing avobenzone were also effective in photoprotecting patients with lupus erythematosus.[29][30] However, a SPF-16 sunscreen containing avobenzone was not as effective as a SPF 40 sunscreen containing benzophenone in protecting patients with actinic reticuloid, despite having a higher protection value in the UVA range.[31]

The addition of other sunscreen actives can improve the photoprotective efficacy of avobenzone. 2 separate studies on photosensitized subjects show that avobenzone + Padimate O provides greater protection than avobenzone alone, for instance.[32][33] The bioconvertible antioxidants vitamin E acetate and sodium ascorbyl phosphate, too, improve photoprotection by converting to vitamin E and vitamin C within the skin, thereby forming an antioxidant reservoir that deactivates reactive oxygen species (ROS) generated by UV photons not blocked by the UV filters avobenzone and octinoxate.[34]

The formulation influences the photoprotective effect of avobenzone as well. For example, avobenzone is retained better in the stratum corneum in an oil-in-water emulsion gel than in petrolatum,[11] which helps enhance its photoprotective efficacy. Lipid nanoparticles loaded with avobenzone also demonstrated an enhanced UVA blocking effect compared to conventional emulsions,[35] while other nanocarriers both enhanced and prolonged avobenzone's UV-protection efficacy.[36] Complexation with cyclodextrins and poly(methyl methacrylate) (PMMA)-encapsulation also improves avobenzone's photoprotective ability.[17][37]

However, incorporating avobenzone within lipid microparticles can adversely affect avobenzone's UV filter efficacy since it enhances the removal of avobenzone by watering.[38]

3.2 Prevention of photoaging

UVA radiation is implicated in photoaging[39][40] that avobenzone, as a UVA filter, may be able to prevent. In mice, topical avobenzone applied to the backs prior to UVA dosing reduced the induction of an elastin gene,[41] indicating that it may inhibit solar elastosis, the accumulation of massive amounts of abnormal elastic tissue that is a major histopathologic change in the dermis of photoaged skin.[42]

Avobenzone can also help prevent skin wrinkling caused by degradation of collagen fibres via inhibiting the UVA-induced expression of matrix metalloproteinase-1 (MMP-1). It was less efficacious than 2 experimental UVA filters as well as Uvinul A Plus in this respect, but more effective than Tinosorb S.[25]

A commercial broad-spectrum sunscreen (Avène 50+) containing avobenzone, Tinosorb S, Tinosorb M and titanium dioxide dramatically reduced the UVA-induced expression of matrix-metalloproteinase (MMP)-1, MMP-3 and MMP-9 by 83%, 80% and 65% respectively in human keratinocytes, compared with the unprotected irradiated controls.[43]

A full-UV spectrum absorbing cream containing avobenzone, octocrylene and Mexoryl SX applied to the UV-exposed buttock skin of human volunteers successfully prevented many biological changes associated with photoaging, including the darkening of the skin, the increase in skin dryness, the increased density and deepening of skin furrows, increases in stratum corneum and stratum granulosum thicknesses, the decrease in the expression of type I pro-collagen and the upregulation of matrix metalloproteinase-2 (MMP-2) expression.[44]

3.3 Antioxidant effect

Whether avobenzone has antioxidative properties is debatable. It has been suggested that avobenzone can behave as a photoantioxidant or photoactivated antioxidant under conditions where its enol isomers can be accumulated.[45] However, avobenzone failed to prevent the photooxidation of the fluorescent indicator 2,7-dichlorofluorescin (DCFH) when incorporated in liposomal membranes.[46] Moreover, in human keratinocytes it did not minimize UVA-induced increases in oxidative stress and lipid peroxidation; on the contrary, it actually enhanced the levels of lipid hydroperoxides.[47]

Avobenzone-induced lipid peroxidation can be reduced by the addition of antioxidants.[48]

3.4 Other effects and uses

Sunscreen products containing avobenzone have demonstrated effectiveness in preventing polymorphic light eruptions (PMLE) in independent trials.[49][50][51] One trial comparing the efficacy of a sunscreen cream containing avobenzone in combination with Mexoryl SX, octocrylene and titanium dioxide to a similar cream without avobenzone found that the former inhibted PMLE flares significantly better than the latter, proving that the inclusion of avobenzone provides clinical benefit.[51]

In addition, an open-label study of a sunscreen combining avobenzone and Padimate O found that it reduced the cutaneous signs and symptoms of discoid lupus erythematosus or subacute cutaneous lupus erythematosus such as hyperpigmentation, papules, scaling, and erythema.[29]

4. Stability

Upon sunlight irradiation, avobenzone tautomerises from the enol form to the keto form.[52] The keto form is generated in a triplet excited state, which can interact with and damage key building blocks of biomolecules such as thymidine, tryptophan and tyrosine.[53][54] Alternatively, the keto form can degrade, forming a photoproduct with strong absorption in the UVB region.[55]

Avobenzone's photo-instability compromises its protection against UVA radiation.[55] It has been reported that a cosmetic preparation containing avobenzone lost 85% of its UVA absorbance after 2 hours of sunlight exposure.[56] This is consistent with another study showing that avobenzone in a cosmetic emulsion degraded chemically by about 60% after 1 hour and 90% after 4 hours of UV exposure.[2]

The sensitivity of avobenzone to light is also dependent on the experimental conditions. Avobenzone is stable in polar solvents such as alcohol, but not in non-polar solvents.[57]

It is important to note that some sunscreen actives are chemically incompatible with avobenzone and should be avoided or tested before their addition to a avobenzone-containing formulation. An example is octinoxate, which reacts with avobenzone under irradiation, leading to the destruction of both compounds and a loss of protection efficacy.[58][59][60] The less frequently used UVB sunscreen active Padimate O has been put forward as another example.[2]

Once incompatible ingredients are avoided, avobenzone can be photostabilized in a few ways. The presence of the sunscreen actives octocrylene and oxybenzone, believed to be efficient singlet and triplet quenchers respectively,[61] can enhance the photostability of avobenzone.[62][63] One study showed that the addition of 10% octocrylene kept the chemical structure of avobenzone virtually intact, and that 6% oxybenzone was able to maintain about 70% of the original concentration of avobenzone after 4 hours of UV irradiation.[2] The UVB filters 4-methylbenzylidene camphor (MBC) and DOMBM, and the broad-spectrum UV absorber Tinosorb S are also effective.[64][65][66] The combination of 5% octisalate + 12% homosalate was less effective, but still provided modest benefit.[2]

Manganese-doped titanium dioxide can also stabilize avobenzone and the avobenzone + octinoxate combination.[67][68] Similarly, the antioxidant quercetin has been found to significantly reduce the photodegradation of avobenzone and octinoxate.[69] Quenching agents such as DEHN, DESM, polycrylene and diethylhexyl 2,6-naphthalate can serve as photostabilizers as well.[61][70][71][72][73]

Avobenzone's photostability can also be improved by encapsulation within poly(methyl methacrylate) (PMMA),[37] lipospheres[74][75] and nanocarriers,[23] or through complexation with cyclodextrins.[17]

5. Safety

Avobenzone is approved by the US FDA as an active ingredient in sunscreens up to a concentration of 3%.[76] It is also an approved UV filter under the EU Cosmetics Directive, which stipulates a maximum concentration of 5%.[77]

5.1 Possible skin toxicity

Avobenzone displayed a pronounced, UVA dose-dependent phototoxicity in the 3T3 Neutral Red Uptake Phototoxicity Test.[78] This agrees with the results of another in vitro experiment using cultured human keratinocytes, which found changes in cell morphology and proliferation as evidence of toxicity. The poor penetration of avobenzone into human skin means that its concentration in the viable epidermis after topical application is probably several times lower than that required to induce toxicity however.[10]

The triplet excited state of the keto form that is generated upon irradiation of avobenzone can interact with and damage key building blocks of biomolecules such as thymidine, tryptophan and tyrosine.[53] This helps explain the observations of earlier studies in which avobenzone was found to induce the production of free radicals and to cause DNA and protein damage. Fortunately, these adverse effects can be inhibted by adding triplet quenchers and free radical scavengers to formulations containing avobenzone[53][79][80] or by complexing it with hydroxypropyl-beta-cyclodextrin (HP-beta-CD).[81]

Avobenzone does not appear to be photomutagenic however, as its presence in irradiated Chinese hamster ovary cells did not cause a significant increase in UV-induced chromosomal aberrations.[82]

5.2 Photoallergic contact dermatitis

Even though photoallergic reactions to UV filters are considered rare in general,[83][84] there are numerous reports in the medical literature of photoallergic contact dermatitis caused by avobenzone.[85][86][87][88][89][90][91][92][93][94]

In fact, avobenzone is one of the most common UV filter photoallergens. A 1998 review of 7 years' data from Swedish dermatology clinics showed that of 34 allergic reactions originating from photocontact, 6 (18%) were caused by avobenzone.[95] Similarly, a more recent photopatch test study in Europe conducted on 1031 patients with suspected photoallergic contact dermatitis revealed that avobenzone, octocrylene and oxybenzone were the most frequent elicitors among the organic UV absorbers.[96]

It has been suggested that these photocontact allergies are due to the arylglyoxals that are formed upon photodegradation of avobenzone, since they are strong sensitizers in the murine local lymph node assay as well as highly reactive toward the nucleophile arginine, which indicates that the immunogenic hapten-protein complex could be formed via an electrophilic-nucleophilic pathway.[97]

Curiously, the addition of avobenzone to topical formulations of the NSAID ketoprofen appears to be effective in inhibiting ketoprofen contact dermatitis.[98]

5.3 No evidence of estrogenicity

Unlike oxybenzone, homosalate, octinoxate, Padimate O and 4-methyl-benzylidene camphor (4-MBC), avobenzone did not stimulate the proliferation of human breast cancer cells in vitro nor did it increase the uterine weights of rats in an uterotrophic assay, indicating that it is does not possess estrogenic activity.[99]

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