Toxicology - Prof. RNDr. Jiří Patočka, DrSc - Toxicological profile of menthol

Toxicological profile of menthol

Jiří Patočka

Menthol (2-isopropyl-5-methylcyclohexan-1-ol) is a naturally occurring cyclic terpene alcohol of plant origin, obtained from cornmint, peppermint or other mintoils. Menthol has been used since antiquity for medicinal purposes (Patel et al., 2007). Its use in dermatology is ubiquitous, where it is frequently part of topical antipruritic, antiseptic, analgesic, and cooling formulations. It is a waxy, crystalline substance, clear or white in color, which is solid at room temperature and melts slightly above. The menthols category is comprised of the isomers L-menthol (CAS Number 2216-51-5), D-menthol (CAS Number 15356-60-2) and the racemate D/L-menthol (CAS Number 89-78-1). The menthols can be considered as a category because of their similarity in physico-chemical, toxicological, ecotoxicological and environmental fate properties. The main form of menthol occurring in nature is L-menthol, which is assigned the (1R,2S,5R) configuration. Because menthol is use in many applications where is in contact with humans, the pharmacology and toxicology of this compound received considerable attention (Ahijevych and Garrett, 2004).

     Menthol has local anesthetic (Leffler et al., 2011) and counterirritant qualities (Huffman et al., 2010), and it is widely used to relieve minor throat irritation (Willis et al., 2011). Menthol also acts as a weak kappa opioid receptor agonist (Galeotti et al., 2002). Menthol's ability to chemically trigger the cold-sensitive TRPM8 receptors in the skin is responsible for the well-known cooling sensation it provokes when inhaled, eaten, or applied to the skin (Eccles, 1994; Keh et al., 2011).
      All menthol isomers are of very low acute oral toxicity with LD₅₀ values normally greater than 2000 mg/kg (Jenner et al., 1964). It can be estimated from unreferenced citations in pharmaceutical texts, such as Gleason et al. (1969), that the lethal human dose of menthol is 50-500 mg/kg. Clinical signs of intoxication are unspecific, and included apathy and reduced activity. Based on old and limited studies for the racemate and the unspecified isomer, it can be assumed that the acute dermal toxicity of the mentol isomers is low (Morton et al., 1995). All studied isomers of menthol are moderately irritating to the skin and slightly irritating to the eye. The skin sensitization potency of menthol isomers in animals and humans is low.
      The NOEL in 13-week studies of toxicity with D/L-menthol in the diet was 560 mg/kg per day in mice and 750 mg/kg bw per day in rats on the basis of slightly increased incidences of interstitial nephritis at the next highest dose (US National Cancer Institute, 1979). The only effect seen in mice of both sexes was a reduction in body weight gain in the highest dose group. The NOAELs derived from these studies were 937 mg/kg/day for the male rat, 998 mg/kg/day for the female rat, 1956 mg/kg/day for the male and 2386 mg/kg/day for the female mouse. Even the highest dose of D/L-menthol tested in the long-term studies in mice and rats had no specific toxic effect. As the survival of mice was reduced at the high dose of 600 mg/kg per day, the Committee allocated an ADI in the range of 0 - 4 mg/kg (IPCS, 1999).
      In rats given 200 mg/kg/d of L-menthol in soybean oil by gavage for 28 days, increased liver weights and a non dose-related vacuolization of hepatocytes were reported. The relevance of these findings remains unclear and a NOAEL could not be derived from this study (Thorup et al., 1983). No toxicity was observed in rats receiving diets providing up to 200 mg/kg/day of either L- or D/L menthol for 5.5 weeks. Therefore for L-menthol and the racemate D/L-menthol a NOAEL of 200 mg/kg/day can be deduced from this study. Irritant effects on lungs and trachea, but no systemic effects were found in rats that were whole body exposed to L-menthol vapour for 71-79 days (Edwards et al., 1988).
      A bioassay of D/L-menthol for possible carcinogenicity was conducted by administering the test chemical in feed to Fischer 344 rats and B6C3F1 mice. Groups of 50 rats of each sex and 50 mice of each sex were administered dl-menthol at one of the following doses, either 3,750 or 7,500 ppm for the rats and either 2,000 or 4,000 ppm for the mice, for 103 weeks, then observed for 1 or 2 additional weeks. Matched controls consisted of 50 untreated rats of each sex and 50 untreated mice of each sex. All surviving rats were killed at 105 weeks and all surviving mice at 104 weeks. Mean body weights of dosed rats and mice were only slightly lower than those of corresponding controls. No other clinical signs related to administration of the dl-menthol were noted in the dosed groups of animals. A dose-related trend in mortality was observed only in the female mice. Survival at the end of the bioassay was at least 62% in all dosed and control groups of animals of each species, and sufficient numbers of animals were at risk for the development of late-appearing tumors. In male rats, no tumors occurred at incidences which were considered to be related to the administration of dl-menthol. In female rats, no tumors occurred at higher incidences in the dosed groups than in the control groups. Fibroadenomas of the mammary gland occurred at lower incidences in the low-dose (10/49) and high-dose (7/49) groups than in the control group (20/50), and alveolar/bronchiolar adenomas or carcinomas of the lung occurred only in the controls (3/50). In mice of either sex, no tumors occurred in dosed groups at incidences that were significantly different from those for corresponding control groups. It is concluded that under the conditions of this bioassay, D/L-menthol was not carcinogenic for either Fischer 344 rats or B6C3F1 mice (National Toxicology Program, 1979).

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