1-(5,5-dimethyl-1-cyclohexen-1-yl)pent-4-en-1-one

Endpoint summary

Description of key information

The available evidence suggests that the substance is bioavailable via the oral route and by inhalation if exposure occurs. Systemic absorption of this substance via dermal route is expected but to a limited extent. The substance is expected to be mainly excreted in urine. The substance has a potential to bioaccumulate.

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 substance (i.e. absorption, distribution, metabolism and elimination) was derived from the relevant available information collated in the dossier. The physical chemical characteristics, the results obtained from acute, repeated-dose, and reproductive toxicity studies on both the source and the target substances, as well as information gained from genotoxicity assays were used to predict its toxicokinetic behaviour.

Physical-chemical properties:

The target substance is a mono-constituent, having a relatively low molecular weight of 192.3 g/mol. The substance is a slightly water-soluble liquid (56.2 mg/L) and is lipophilic based on the octanol/water partition coefficient (Log Kow = 4.14). The substance has low volatility according to its vapour pressure (1.34 Pa at 25°C).

The source substance is a multi-constituent composed of the target substance (60-75 % w/w) and an isomer (25-35 % w/w). The source substance is a slightly water-soluble liquid (26.6 mg/L), is lipophilic (Log Kow 4.4-4.5)) and has a low volatility (VP = 0.67 Pa at 25°C).

Absorption:

Oral/GI absorption:

The physical chemical characteristics described above suggest that the target substance is of adequate molecular size to participate in endogenous absorption mechanisms within the mammalian gastrointestinal tract. Being lipophilic, the target substance may be expected to cross gastrointestinal epithelial barriers with the absorption may be potentiated by the ability of the substance to dissolve into gastro-intestinal fluids and hence make contact with the mucosal surface.

These hypotheses are supported by oral systemic effects observed in the combined toxicity study with the reproduction/developmental toxicity screening test performed on the source substance in rats by dietary administration. Hypertrophy of the urothelium of the urinary bladder was observed in females at 2000 ppm and renal changes were noted in males at 2000 ppm (hyaline droplet accumulation, accompanied by renal tubular basophilia and granular casts and associated higher renal weight). The NOAEL was set to 700 ppm (corresponded to mean daily test item intake levels of 51 mg/kg bw/day in males, and 59 (pre-mating period), 87 (post-coitum period) and 129 (lactation period) mg/kg bw/day in females. The observation of systemic effects indicates the oral bioavailability of both the target and the source substances and/or its metabolites.

In light of these data, and the lack of specific information, the target substance was assumed to be 100% bioavailable by oral route for the purpose of human health risk assessment.

Dermal absorption:

Regarding dermal absorption, the target substance being lipophilic (log Kow = 4.14), 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 target substance. The absence of systemic effects following a single-dose dermal application of the source substance up to 1000 mg/kg bw would suggest a limited systemic absorption through cutaneous barriers.

In light of these data, and the lack of specific information on the target substance, a dermal absorption of 100% was conservatively assumed for the purposes of human health risk assessment.

Respiratory absorption:

The potential for inhalation toxicity was not evaluated in vivo.

The vapour pressure of the target substance (VP = 0.34 Pa 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.

However, when used as a vapour or as mist (droplet aerosol), the substance is expected to be directly absorbed across the respiratory tract epithelium by passive diffusion.

In light of these data, and the lack of specific information on respiratory absorption, the substance was conservatively assumed to be 100% bioavailable by inhalation for the purposes of human health risk assessment.

Distribution:

Systemic distribution of the target substance can be predicted from its physical chemical characteristics. Considering that the substance is lipophilic (log Pow >4) and slightly water soluble, it is suggested that, upon systemic absorption by oral or by inhalation, the target 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 target substance may readily cross cellular barriers or may be distributed into fatty tissues with a low potential to accumulate. Due to its lipophilic property, the target substance could readily penetrate the stratum corneum following dermal exposure but then it is not expected to be systemically absorbed to a major extent.

Metabolism:

Specific data on metabolism of the registered substance is not available. However, the family of alicyclic ketones (ACK) may include one or more of the following biotransformation pathway (Belsito et al., 2013):

Primary Metabolism:

- Reduction of the ketone group to a secondary alcohol

- Hydroxylation/oxygenation of the cyclohexene ring

- Oxidation of the angular methyl groups

- Reduction of the double bond in the exocyclic alkenyl side chain or cyclic portion of the molecule to form dihydroderivatives

- Epoxidation of isolated (non-conjugated) double bonds of exocyclic alkyl side chains and subsequent reaction with epoxide hydrolases or glutathione transferase.

Secondary Metabolism:

- Conjugation of the hydroxylated metabolites with glucuronic acid

- Conjugation of epoxide metabolites with glutathione.

Carbonyl reduction of the ketone (oxo) functionality by carbonyl reducing (reductase) enzymes has been well-characterized and should be the major and predominant primary route for ACK metabolism unless the ketone is sterically hindered. The biotransformation of the ketone is mediated by alcohol dehydrogenase and by NADH/NADPH-dependent cytosolic carbonyl reductases. The secondary alcohol metabolite may be either converted back to the parent ketone (and excreted unchanged) or conjugated with glucuronic acid and excreted. On a case-by-case basis, differences in the alkyl and cyclic hydrocarbon group functionality and steric hindrance may affect the rate of metabolism of the specific ACK fragrance material by either inhibiting or decreasing the activity of carbonyl reductases or other enzymes involved in primary or secondary metabolism.

Primary secondary alcohol (hydroxyl) metabolites are less lipophilic than the parent ketones and are generally not retained. Secondary metabolism, conjugated with glucuronic acid, occurs readily and is followed by elimination of the glucuronide in the urine. Carbonyl reducing enzymes are found mainly in the liver and kidney, but are also reported to be present in the brain, lung, spleen and adrenal glands. The difference in tissue and intracellular distribution substantiate the possibility that several enzymes could be involved in the reduction of the ACK fragrance materials. These ACK reducing enzymes may include a short-chain dehydrogenase/reductase (SDR), such as NADPH secondary alcohol oxidoreductases; pluripotent hydroxysteroid dehydrogenases (HSD); and/or a pyridine nucleotide-dependent oxidoreductase that may also catalyze carbonyl reduction of nonsteroidal ketone, which mediate the inter-conversion of the alkyl cyclic alcohol metabolite back to the parent ACK.

It is also possible that the double bonds of ACK fragrance ingredients may undergo other biotransformations that may precede or occur in tandem with carbonyl reductases. The available literature for unsaturated compounds with one or more non-conjugated or conjugated double bonds in alkyl or cyclic hydrocarbon groups generally supports metabolism that results in a possible complex mixture of oxidative metabolites that are primary alkyl, secondary alkyl, and ring hydroxyl metabolites. Depending on the structural properties of the particular ACK in which steric hindrance may shield the double bond or ketone, these types of oxidative biotransformation would either precede or occur in tandem with carbonyl reduction.

In vivo reduction of the double bond in α,β-unsaturated ketones by hepatic cytosolic reductase may be an initial primary route of metabolism to generate the saturated ketone, which may then be acted upon by carbonyl reductase to generate the secondary alcohol metabolite.

Isolated non-conjugated double bonds such as in the terminal double bond of the target substance may undergo initial primary cytochrome P450 oxidation to form an epoxide. The epoxide should have a transient existence and be rapidly detoxified by epoxide hydrolase. There is also the possibility that the epoxy intermediate could bind to epidermal proteins. The resultant diol may be conjugated and excreted by various mechanisms. However, if carbonyl reduction occurs prior to epoxidation, then the ACK alcohol may be excreted as the glucuronide with the double bond intact.

Excretion:

The target 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 target substance, that have penetrated the stratum corneum but not penetrated the viable epidermis may be sloughed off with skin cells.

Reference:

(Belsito et al., 2013). Toxicological and Dermatological Assessments for One Group of Fragrance Ingredients: Alkyl Cyclic KetonesD. Belsito et al. / Food and Chemical Toxicology 62 (2013) S1–S44