|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|
Absorbs in the range 290-315 nm, which covers nearly the whole UVB spectrum. Alone, its SPF reaches a maximum of about 4 at a concentration of 6%.
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Homosalate, also known as homomenthyl salicylate, is a clear, colourless to pale yellow liquid with a slight mint odour. It has a long history of use as a UV filter in sunscreens, but a recent analysis of 100 sunscreen cosmetics found that it was used in only 4% of the products.
2. Skin penetration
Homosalate can penetrate through the skin, with an early study showing that 1.1% of the applied dose from a sunscreen containing 10% homosalate was absorbed through fresh dermatomed human abdominal skin. The proportion absorbed was substantially higher in rat skin, which had a mean total absorption of 8.7%. A more recent study found that 4-6% of the applied dose in a gel became bioavailable in rats.
However, one study in which 5% homosalate was applied to full-thickness human breast or abdominal skin showed that homosalate was absorbed only in the epidermis, and did not penetrate through the skin. Another study demonstrated that penetration of homosalate from a sunscreen lotion containing 8% homosalate was minimal, with the vast majority retained in the stratum corneum.
The choice of vehicle can influence the extent and amount of homosalate absorption. Homosalate in a gel accumulated in the stratum corneum and viable skin of rats between 3 to 10-fold more than homosalate in petrolatum, in a lotion or in an oily solution. Similarly, an oil-in-water emulsion gel formulation of homosalate showed approximately 3-fold higher distribution into the stratum corneum of human skin than homosalate in petrolatum.
3. Effects on the skin
Homosalate's absorbance range is between 290–315 nm. It has 2 absorbance peaks at 238 and 306 nm. The US FDA's standard sunscreen preparation, which contains 8% homosalate, has a sun protection factor (SPF) of 4.47 ± 1.279. In a comparison of 18 sun filters all at their highest concentrations allowed by European legislation, homosalate ranked 9th in terms of efficacy. The authors also noted that the trend curve of SPF against homosalate concentration is asymptotic, with the SPF reaching a maximum value of about 4 as soon as concentration is at 6%. Homosalate has a significant anti-inflammatory effect, which is thought to influence its in vivo SPF value.
Homosalate is photostable as an emulsion and in various solvents. Its shelf life has been estimated to be at least 2-3 years.
Homosalate is an approved sunscreen active ingredient in the US, where it can be included up to a concentration of 15%. In Europe, the Scientific Committee on Consumer Products established in 2007 that homosalate at a maximum use concentration of 10% in cosmetic sunscreens does not pose a risk to the health of consumers. This has formed the basis for its inclusion in the EU Cosmetics Directive.
5.1 Skin irritation
Homosalate administered dermally to rabbits at a single dose of 5 g/kg body weight led to signs of skin irritation in the form of slight or moderate redness and edema. Investigations using mice and guinea pigs revealed no photoallergic potential. These results are in line with a quantitative structure-toxicity relationship model, which predicts that homosalate has weak sensitizing potential.
Homosalate has also been tested for potential sensitization on human skin in a maximization test involving 25 volunteers. No signs of skin irritation or sensitization were observed. Numerous human repeated insult patch tests of cosmetic products containing 10-15% homosalate also found no clinically relevant potential for dermal irritation or sensitization.
Photopatch tests on 118 patients with suspected photoallergic contact dermatitis revealed that none reacted to homosalate as a 5% preparation in petrolatum. Moreover, there are only isolated cases reported concerning induction of skin sensitization or photoallergic reactions to homosalate, indicating that it has a negligible potential to induce adverse skin reactions in humans.
5.2 Potential endocrine disruptor
Data on the estrogenicity of homosalate is conflicting. The earliest investigation found that homosalate induced proliferation in human breast cancer cells, but important shortcomings have been identified in this study. Other studies confirm that homosalate has estrogenic activity toward estrogen receptors in vitro, but caution should be taken to avoid over-interpreting these data, since homosalate was not estrogenic in 2 in vivo experiments.
Homosalate appears to be a potent antagonist of the human androgen receptor. It has been demonstrated to suppress the dihydrotestosterone-induced activation of the androgen receptor in vitro, though these preliminary results may not be relevant for the in vivo situation.
5.3 Enhanced absorption of a herbicide
A 5% homosalate has been shown to significantly increase the total penetration of 2,4-dichlorophenoxyacetic acid, a moderately lipophilic herbicide, in vitro, which may make it a risk to agricultural workers who use sunscreens as protection against UV-induced skin cancer.
5.5 No genotoxic potential
Homosalate was not mutagenic in Salmonella assays with and without metabolic activation. It did not induce an increase structural chromosomal aberrations in Chinese hamster ovary cells either, indicating no clastogenic potential. As a result, the European Scientific Committee on Consumer Products concluded in 2007 that homosalate poses no genotoxic or mutagenic risk to humans.
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