(E)-1-(2,6,6-trimethyl-1,3-cyclohexadien-1-yl)-2-buten-1-one

Endpoint summary

Description of key information

The available evidence suggests that the substance is bioavailable via the oral route. Significant systemic dermal absortion is anticipated. Systemic absorption of this substance via inhalation is expected to occur but to a limited extent. The substance may cross cellular barriers or may be distributed into fatty tissues. The substance is expected to be mainly excreted in urine.

Key value for chemical safety assessment

Bioaccumulation potential:

low bioaccumulation potential

Additional information

In accordance with the section 8.1.1 of Annex VIII of Regulation (EC) No 1907/2006 (REACH), the toxicokinetic profile of the registered substance (i.e. absorption, distribution, metabolism and elimination) was derived from the relevant available information collated in the dossier. The physical chemical characteristics of the registered substance and supporting substances (see Section 13 for read-across justification), the results obtained from acute, repeated-dose, and reproductive toxicity studies, as well as information gained from genotoxicity assays were used to predict its toxicokinetic behaviour.

Moreover, the JECFA evaluated a group of 21 flavouring agents that includes alpha- and beta-ionone and structurally related substances. Each of these substances has a cyclohexane ring with an allyl or alkyl side-chain containing a ketone or secondary alcohol functional group. Each contains a 2,6,6-trimethylcylcohexyl carbon skeleton and an alkyl side chain of four to seven carbons located at the C-1 position. Each of these substances has at least one endocyclic double bond. In the ionones, the carbonyl or hydroxyl group is positioned gamma to the ring, while in the damascones the carbonyl group is positioned alpha to the ring. Data on ionones were therefore also taken into account in this toxicokinetic assessment.

Physical-chemical properties:

The registered substance is a mono-constituent, having a relatively low molecular weight of 190.28 g/mol. The registered substance is a slightly water soluble liquid (188 mg/L) and is lipophilic based on the octanol/water partition coefficient (log Kow = 3.4). The registered substance has low volatility according to its vapour pressure (3 Pa at 25°C).

Absorption:

The physical chemical characteristics described above suggest that the registered substance is of adequate molecular size to participate in endogenous absorption mechanisms within the mammalian gastrointestinal tract. Being lipophilic, the registered substance may be expected to cross gastrointestinal epithelial barriers even if the absorption may be limited by the inability of the substance to dissolve into gastro-intestinal fluids and hence make contact with the mucosal surface. Moreover, the absorption will be enhanced if the registered substance undergoes micellar solubilisation by bile salts. Substances absorbed as micelles will enter the circulation via the lymphatic system, bypassing the liver. In an acute oral gavage toxicity study conducted on the substance, there were no mortality and no clinical signs (LD50 < 2000). The 90-day repeated dose study conducted on the supporting substance using the oral route (gavage) gave a NOAEL of 30 mg/kg bw/day. Changes observed include increase in plasma creatinine, total protein, cholesterol and in plasma albumin in male and female rats, follicular cell hypertrophy of the thyroid and adipose infiltration on bone marrow indicative of marrow hyperplasia in male rats. The observation of systemic effects indicates the oral bioavailability of the substance and/or its metabolites.

In light of these data, and the lack of specific information on any the registered substance or its supporting substances, the registered substance was assumed to be 100% bioavailable by oral route for the purposes of human health risk assessment.

Regarding dermal absorption, the registered substance being lipophilic, the rate of uptake into the stratum corneum is expected to be high while the rate of penetration is likely to be limited by the rate of transfer between the stratum corneum and the epidermis. Moreover, it is assumed that the dermal uptake is also limited by the slight water solubility of the registered substance. However, the registered substance being a skin irritant, damage to the skin surface may enhance penetration and therefore enhance the dermal uptake. The presence of systemic effects following single-dose dermal application of the supporting substance, at doses above 2000 mg/kg bw would suggest a significant systemic absorption through cutaneous barriers.

In light of these data, and the lack of specific information on any the registered substance or its supporting substances, a dermal absorption of 100% was conservatively assumed for the purposes of human health risk assessment. The assumption of 100% dermal bioavailability is considered especially conservative given that in an in vitro dermal penetration / permeability study, only 0.7% or undetectable amounts of methyl ionone (mixture of isomers) were recovered in the fluid beneath the skin preparations of rats and pigs, respectively, 6 hours after application of a 3000 µg dose (600 µg/cm² over 5 cm² of the skin (Belsito et al., 2007). In this study, approximately 50% (rats) and 10% (pig) of methyl ionone 14C penetrated into, but not through the epidermis and dermis, while another 30% was lost to evaporation.

The potential for inhalation toxicity was not evaluated in vivo. However, the vapour pressure of the substance (3 at 25°C) indicated an absence of volatility and inhalability and therefore no exposure by inhalation is anticipated. Thus, at ambient temperature, no respiratory absorption is expected under normal use and handling of the substance.

Distribution:

Systemic distribution of the registered substance can be predicted from its physical chemical characteristics. Considering that the substance is lipophilic (log Pow >3) and slightly water soluble, it is suggested that, upon systemic absorption, the registered substance may be transported through the circulatory system in association with a carrier molecule such as a lipoprotein or other macromolecule. Afterwards, based on its lipophilic character, the substance may readily cross cellular barriers or may be distributed into fatty tissues with a low potential to accumulate. Data available to support this assessment is limited to a single study in mice reporting the presence of beta-ionone at trace levels (> 0.1 ng/mL) in the blood 30-90 minutes following a 1-hour inhalation exposure to 0.00001 ppm (Belsito et al., 2007).

Metabolism:

The ionones and damascones, because of their highly lipophilic nature would be expected to be extensively metabolized in vivo and eliminated as transformation products. This appears to be the case in several studies involving the administration of alpha- or beta-ionones to rabbits and dogs (Belsito et al., 2007). Little unchanged compound was recovered from the urine compared to relatively large amounts of transformation products that could be isolated. The available metabolic data are derived largely from studies on beta-ionone and indicate several possible detoxification pathways (Belsito et al., 2007; JECFA 1999):

1.Allylic hydroxylation of the ring at the 3 position, followed by oxidation of the hydroxyl group to the 3-oxo derivative,

2.Reduction of the ketone on the allyl side-chain to the corresponding secondary alcohol,

3.Oxidation of the angular methyl groups,

4.Reduction of the double bond in the exocyclic alkenyl side chain to form dihydro derivatives,

5.Conjugation of the hydroxylated metabolites with glucuronic acid,

6.Conjugation with glutathione

A combination of these reactions results in the formation of polar metabolites, which are excreted in the urine unchanged or conjugated with glucuronic acid.

The primary differences in the chemical structure between ionones and damascones that could affect metabolism, and potentially the toxicity of metabolites, are the position of the double bond in the allylic side chain and the potential for epoxidation depending upon the number and position of the double bonds in the cyclohexene ring. Since the allylic side chain of the damascones does not appear to have strong electrophilic activity, based on in vitro data, the damascone metabolites are unlikely to be of greater toxicity that those of the ionones.

The metabolism of ionones is expected to be similar in humans, i.e. oxidative and reductive transformation followed by conjugation. This is supported by human metabolism studies of retinoids and carotenoids such as cis-13-retinoic acid (i.e., isotretinoin) and beta-carotene, respectively, which possess ionone fragments. The primary blood and biliary metabolites following oral administration of isotretinoin to humans include the glucuronic acid conjugates of isotretinoin and the allylic oxidation product. Both metabolites were observed in the blood and bile of cynomologous monkeys provided in isotretinoin via the oral route. Allylic hydroxylation of the methyl ring substituent and subsequent conjugation with glucuronic acid also occurs in humans.

Excretion:

The registered substance, having a molecular weight lower than 300 g/mol, is expected to be mainly excreted in urine and no more than 5-10% may be excreted in bile. Any substance that is not absorbed from the gastro-intestinal tract, following oral ingestion, will be excreted in the faeces. Following dermal exposure, highly lipophilic substances, such as the registered substance, that have penetrated the stratum corneum but not penetrated the viable epidermis may be sloughed off with skin cells.

References:

-Belsito et al. (2007). A toxicologic and dermatologic assessment of ionones when used as fragrance ingredients. Food and Chemical Toxicology 45 (2007) S130-S167

- JECFA (1999). WHO Food Additives Series: 42. IONONES AND STRUCTURALLY RELATED SUBSTANCES. Prepared by the Fifty-first meeting of the Joint FAO/WHO Expert Committee on Food Additives