Final Report on the Safety Assessment of Urocanic Acid

Final Report On the Safety Assessment of Glycolic Acid, Ammonium, Calcium, Potassium, and Sodium Glycolates, Methyl, Ethyl, Propyl, and Butyl Glycolates, and Lactic Acid, Ammonium, Calcium, Potassium, Sodium, and Tea-Lactates, Methyl, Ethyl, Isopropyl, and Butyl Lactates, and Lauryl, Myristyl, and Cetyl Lactates

This report provides a review of the safety of Glycolic Acid, Ammonium, Calcium, Potassium, and Sodium Glycolates, Methyl, Ethyl, Propyl, and Butyl Glycolates, Lactic Acid, Ammonium, Calcium, Potassium, Sodium, and TEA-Lactates, and Lauryl, Myristyl, and Cetyl Lactates. These ingredients belong to a group known as alpha-hydroxy acids (AHAs). Products containing these ingredients may be for consumer use, salon use, or medical use. This report does not address the medical use. In consumer and salon use, AHAs can function as mild exfoliants, but are also used as pH adjusters and skin-conditioning agents. AHAs are absorbed by the skin; the lower the pH, the greater the absorption. Metabolism and distribution studies show expected pathways and distribution. Consistent with these data, acute oral animal studies show oxalate-induced renal calculi, an increase in renal oxalate, and nephrotoxic effects. No systemic effects in animals were seen with dermal application, but irritation at the sight of application was produced. While many animal studies were performed to evaluate AHA-induced skin irritation, it was common for either the AHA concentration or the pH of the formulation to be omitted, limiting the usefulness of the data. Clinical testing using AHA formulations of known concentration and pH was done to address the issue of skin irritation as a function of concentration and pH. Skin irritation increased with AHA concentration at a given pH. Skin irritation increased when the pH of a given AHA concentration was lowered. Repeat insult patch tests using lotions and creams containing up to 10% Glycolic or Lactic Acid were negative. Glycolic Acid at concentrations up to 10% was not comedogenic and Lactic Acid at the same concentrations did not cause immediate urticarial reactions. Glycolic Acid was found to be nonirritating to minimally irritating in animal ocular tests, while Lactic Acid was found to be nonirritating to moderately irritating. In vitro testing to predict ocular irritation suggested Glycolic Acid would be a minimal to moderate-severe ocular irritant, and that Lactic Acid would be a minimal to moderate ocular irritant. Developmental and maternal toxicity were reported in rats dosed by gavage at the highest dose level used in a study that exposed the animals on days 7-21 of gestation. No developmental toxicity was reported at levels that were not maternally toxic. AHAs were almost uniformly negative in genotoxicity tests and were not carcinogenic in rabbits or rats. Clinical reports suggested that AHAs would enhance the penetration of hydroquinone and lidocaine. Animal and clinical tests were done to further evaluate the potential ofAHAs to enhance the skin penetration of other chemical agents. Pretreatment of guinea pig skin with Glycolic Acid did not affect the absorption of hydroquinone or musk xylol. Clinical tests results indicated no increase in penetration of hydrocortisone or glycerin with Glycolic Acid pretreatment. Because AHAs can act to remove a portion of the stratum corneum, concern was expressed about the potential that pretreatment with AHAs could increase skin damage produced by UV radiation. Clinical testing was done to determine the number of sunburn cells (cells damaged by UV radiation that show distinct morphologic changes) produced by 1 MED of UV radiation in skin pretreated with AHAs. A statistically significant increase in the number of sunburn cells was seen in skin pretreated with AHAs compared to controls. These increases, however, were less than those seen when the UV dose was increased from 1 MED to 1.56 MED. The increase in UV radiation damage associated with AHA pretreatment, therefore, was of such a magnitude that it is easily conceivable that aspects of product formulation could eliminate the effect. Based on the available information included in this report, the CIR Expert Panel concluded that Glycolic and Lactic Acid, their common salts and their simple esters, are safe for use in cosmetic products at concentrations ≤10%, at final formulation pH≥3.5, when formulated to avoid increasing sun sensitivity or when directions for use include the daily use of sun protection. These ingredients are safe for use in salon products at concentrations ≤30%, at final formulation pH ≥3.0, in products designed for brief, discontinuous use followed by thorough rinsing from the skin, when applied by trained professionals, and when application is accompanied by directions for the daily use of sun protection.

Increased sensitivity of histidinemic mice to UVB radiation suggests a crucial role of endogenous urocanic acid in photoprotection

Urocanic acid (UCA) is produced by the enzyme histidase and accumulates in the stratum corneum of the epidermis. In this study, we investigated the photoprotective role of endogenous UCA in the murine skin using histidinemic mice, in which the gene encoding histidase is mutated. Histidase was detected by immunohistochemistry in the stratum granulosum and stratum corneum of the normal murine skin but not in the histidinemic skin. The UCA content of the stratum corneum and the UVB absorption capacity of aqueous extracts from the stratum corneum were significantly reduced in histidinemic mice as compared with wild-type mice. When the shaved back skin of adult mice was irradiated with 250 mJ cm(-2) UVB, histidinemic mice accumulated significantly more DNA damage in the form of cyclobutane pyrimidine dimers than did wild-type mice. Furthermore, UVB irradiation induced significantly higher levels of markers of apoptosis in the epidermis of histidinemic mice. Topical application of UCA reversed the UVB-photosensitive phenotype of histidinemic mice and increased UVB photoprotection of wild-type mice. Taken together, these results provide strong evidence for an important contribution of endogenous UCA to the protection of the epidermis against the damaging effects of UVB radiation.

Some photophysical studies of cis- and trans-urocanic acid

Urocanic acid, an important human skin chromophore, undergoes a variety of photochemical transformations when exposed to the near-UV portion of sunlight and natural daylight, the principal reaction being the transformation from the stable trans- or (E)-form of the chromophore (trans-UA) to the biologically active cis- or (Z)-form (cis-UA), which is claimed to induce immunosuppression linked to the onset of skin cancer. This study is concerned with the comparative photophysical behaviour of the two urocanic acid isomers in aqueous solution using both continuous irradiation and pulsed irradiation techniques. The UV absorption maximum for both isomers occurs in the region of 270 nm with the absorption shape varying characteristically with pH, the cis-isomer showing a lower overall molar absorptivity. Both isomers exhibit weak fluorescence (quantum yields estimated to be less than 10(-4)) with each isomer showing small differences in the way in which pH and excitation wavelength influence the fluorescence emission characteristics. Pulsed nanosecond laser irradiation at 266 nm of aqueous solutions at pH 7 shows that both isomers undergo photo-ionisation with a quantum yield of 0.02 for the hydrated electron production, a quantum yield value comparable with that for photoisomerisation at this wavelength. Laser flash studies also show that the photo-ionised species reacts efficiently with oxygen (quenching rate kQ = 1.3 x 10(9) M(-1) s(-1)), while some preliminary experiments indicate that both cis- and trans-urocanic acids react with the semiquinone radical of L-3,4-dihydroxyphenylalanine (L-DOPA) with a fast reaction rate constant of approximately 5 x 10(7) M(-1) s(-1). The photophysical characteristics of trans-UA and cis-UA reported here are discussed in the context of other recent pulsed irradiation studies on urocanic acid over nanosecond and picosecond time scales, in an attempt to clarify the complex photo-behaviour of this interesting biomolecule.

Histamine fish poisoning revisited

Histamine (or scombroid) fish poisoning (HFP) is reviewed in a risk-assessment framework in an attempt to arrive at an informed characterisation of risk. Histamine is the main toxin involved in HFP, but the disease is not uncomplicated histamine poisoning. Although it is generally associated with high levels of histamine (> or =50 mg/100 g) in bacterially contaminated fish of particular species, the pathogenesis of HFP has not been clearly elucidated. Various hypotheses have been put forward to explain why histamine consumed in spoiled fish is more toxic than pure histamine taken orally, but none has proved totally satisfactory. Urocanic acid, like histamine, an imidazole compound derived from histidine in spoiling fish, may be the "missing factor" in HFP. cis-Urocanic acid has recently been recognised as a mast cell degranulator, and endogenous histamine from mast cell degranulation may augment the exogenous histamine consumed in spoiled fish. HFP is a mild disease, but is important in relation to food safety and international trade. Consumers are becoming more demanding, and litigation following food poisoning incidents is becoming more common. Producers, distributors and restaurants are increasingly held liable for the quality of the products they handle and sell. Many countries have set guidelines for maximum permitted levels of histamine in fish. However, histamine concentrations within a spoiled fish are extremely variable, as is the threshold toxic dose. Until the identity, levels and potency of possible potentiators and/or mast-cell-degranulating factors are elucidated, it is difficult to establish regulatory limits for histamine in foods on the basis of potential health hazard. Histidine decarboxylating bacteria produce histamine from free histidine in spoiling fish. Although some are present in the normal microbial flora of live fish, most seem to be derived from post-catching contamination on board fishing vessels, at the processing plant or in the distribution system, or in restaurants or homes. The key to keeping bacterial numbers and histamine levels low is the rapid cooling of fish after catching and the maintenance of adequate refrigeration during handling and storage. Despite the huge expansion in trade in recent years, great progress has been made in ensuring the quality and safety of fish products. This is largely the result of the introduction of international standards of food hygiene and the application of risk analysis and hazard analysis and critical control point (HACCP) principles.

Epidermal trans-urocanic acid and the UV-A-induced photoaging of the skin

The premature photoaging of the skin is mediated by the sensitization of reactive oxygen species after absorption of ultraviolet radiation by endogenous chromophores. Yet identification of UV-A-absorbing chromophores in the skin that quantitatively account for the action spectra of the physiological responses of photoaging has remained elusive. This paper reports that the in vitro action spectrum for singlet oxygen generation after excitation of trans-urocanic acid mimics the in vivo UV-A action spectrum for the photosagging of mouse skin. The data presented provide evidence suggesting that the UV-A excitation of trans-urocanic acid initiates chemical processes that result in the photoaging of skin.