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Link to relevant study record(s)

Reference
Endpoint:
basic toxicokinetics, other
Remarks:
expert statement
Type of information:
other: Expert Statement
Adequacy of study:
key study
Study period:
2019-05-03
Reliability:
1 (reliable without restriction)
Rationale for reliability incl. deficiencies:
other: Expert statement, no other study available
Principles of method if other than guideline:
Expert Statement
GLP compliance:
no
Details on absorption:
Oral route
Generally, oral absorption is favoured for molecular weights below 500 g/mol. The low water solubility of ca. 1.67 mg/L will probably not allow ready dissolution of the test item in the gastrointestinal fluids. Thus, a direct uptake into the systemic circulation through aqueous pores or via carriage with the bulk passage of water is considered as less relevant. The logPow value of 4.44 is also already above the range of -1 to 4 which is usually considered to be favored for absorption by passive diffusion through biological membranes. With a logPow of greater than 4, the test item may be taken up by micellular solubilisation. Moreover, hydrolysis of the test item in the GIT is not expected based on QSAR calculation the hydrolytic half-life is greater than a year. Overall, physicochemical properties of the test item indicate that it might not be readily absorbed via the oral route. This is supported by oral acute as well as repeated dose toxicity data, where no or only minor signs of toxicity were observed after administration up to the limit dose (2000 mg/kg bw and 1000 mg/kg bw/d, respectively).

Inhalation route
Due to its low volatile properties availability of the test item via the inhalation route cannot be completely ruled out. Since the substance is a liquid at ambient temperature the generation of dust is excluded. However, in case of inhalation, absorption via the respiratory tract cannot completely be ruled out.

Dermal route
Dermal absorption is considered limited since the logPow value is above 4 and thus considered to limit transfer between stratum corneum and the epidermis. However, uptake into the stratum corneum will probably be high. Another parameter that affects transport of a substance from the stratum corneum into the epidermis is the water solubility of a substance. Below 1 mg/L dermal uptake is considered to be low.
However, based on in vitro test battery results the test item was shown to have a skin sensation potential. Thus, even small amounts of the substance becoming bioavailable through the dermal route could be important. Moreover, the substance is classified as skin irritant and thus absorption may be facilitated due to the damage of the upper skin layers.
Details on distribution in tissues:
Physicochemical properties of the test item as well as toxicity data indicate a rather low systemic bioavailability following oral, inhalative and dermal uptake.
However, after being absorbed into the body, the test item is likely distributed into cells and intracellular concentrations may be higher than extracellular concentrations due to rather lipophilic properties (log Pow 4.44). This applies especially for fatty tissues.
However, the test item is unlikely to have a bio-accumulative potential, even though it might be considered as lipophilic (logPow is greater than 4). The subacute repeated dose toxicity study in rats did not reveal any signs of target organ toxicity or other indications for an accumulation in any organ or tissue, there is also no other evidence for an accumulative property of this compound. Moreover, its metabolic transformation to more hydrophilic compounds is anticipated as described below.
Details on excretion:
As discussed below, the test item is probably substrate to phase I metabolizing enzymes and its hydrophilicity will likely be increased by this process. Taking into account its low molecular weight it can be assumed that the test item is rather excreted via the kidneys and the urine than via feces after it has been metabolised.
Details on metabolites:
It can be assumed that P450-enzymes are substantially involved in the metabolism of the test item, based on findings in the bacterial reverse mutation assay (Ames) as well as in the mammalian cell gene mutation assay (HPRT). In both assays cytotoxicity was observed in the absence of the metabolizing system (S9). In the presence of S9, however, the same substance concentration did not induce cytotoxicity anymore. Thus, a detoxification by metabolism of phase I enzymes could be anticipated. This metabolism usually aims to enhance hydrophilicity of the substance, for example by hydroxylation reactions, in order to facilitate excretion of the xenobiotic. This supports the assumption that the test item is not considered of concern in regards to bioaccumulation.
Conclusions:
Based on physicochemical characteristics, particularly molecular weight and octanol-water partition coefficient, absorption by the dermal, oral and inhalation route is expected to be low. This assumption is further supported by the results of the oral acute toxicity study as well as the subacute repeated dose study. There are no indications that the test item or its metabolites could have a bioaccumulating potential. Metabolic transformation by phase I enzymes is expected and excretion via urine can be anticipated.

Description of key information

Key value for chemical safety assessment

Bioaccumulation potential:
no bioaccumulation potential

Additional information

Toxicokinetic Assessment Dihydromyrcene

There are no experimental data available on toxicokinetic behavior of the test item. Therefore, the following considerations are based on physico chemical properties of the test item as well as on results of in vivo and in vitro toxicity studies available.

Phys-Chem properties
The test item is a colorless liquid at ambient conditions with a melting point <-30 °C and a boiling point of 163 °C. The substance is an UVCB consisting of different isomers with the same molecular formula. Therefore, molecular weight of every constituent is always 138.25 g/mol.

Vapor pressure of the test item was determined to be quite high with 2.14 hPa at 20°C and the test item is considered to be volatile. The test item is only poorly soluble in water based on QSAR calculations (WS = ca. 1.67 mg/L) and there are no indications that it undergoes hydrolysis in contact with water. The octanol/water partition coefficient of the test item was calculated to be 4.44.

Summary of toxicological profile

Acute toxicity: 
An acute oral toxicity study was carried out using the class method according to OECD guideline 423, applying 2000 mg/kg bw/d. No mortality and no test item related adverse effects regarding body weight development or clinical observations were revealed. Therefore, the test item is not classified according to the GHS classification and the median lethal dose derived was: LD50 cut off ≥ 5000 mg/kg bw according to the guideline.

Irritation and sensitisation: 

In vitro tests systems for skin and eye irritation revealed that the test item is irritating to the skin but not to the eye. Moreover, the test item is also considered to be a skin sensitizer based on in vitro test battery data.

 

Repeated dose toxicity:

The test item was assessed for its repeated dose toxicity in combined Repeated Dose Toxicity Study with the Reproduction/Developmental Toxicity Screening Test (OECD 422). No adverse effects were observed up to the highest dose tested i. e. 800 mg/kg bw/d. In male rats hyaline-like droplets were observed by histopathological examination. Further assessment confirmed alpha-u2-globulin positive. Thus, the agglomerates are a sex and species related finding and are not indicative for effects of human relevance.

Genotoxicity: 
The test item was shown to be not mutagenic or clastogenic in bacterial as well as in mammalian cell systems.

 

Absorption

Oral route
Generally, oral absorption is favoured for molecular weights below 500 g/mol. The low water solubility of ca. 1.67 mg/L will probably not allow ready dissolution of the test item in the gastrointestinal fluids. Thus, a direct uptake into the systemic circulation through aqueous pores or via carriage with the bulk passage of water is considered as less relevant. The logPow value of 4.44 is also already above the range of -1 to 4 which is usually considered to be favored for absorption by passive diffusion through biological membranes. With a logPow of greater than 4, the test item may be taken up by micellular solubilisation. Moreover, hydrolysis of the test item in the GIT is not expected based on QSAR calculation the hydrolytic half-life is greater than a year. Overall, physicochemical properties of the test item indicate that it might not be readily absorbed via the oral route. This is supported by oral acute as well as repeated dose toxicity data, where no or only minor signs of toxicity were observed after administration up to the limit dose (2000 mg/kg bw and 1000 mg/kg bw/d, respectively).

Inhalation route          
Due to its volatile properties availability of the test item via the inhalation route cannot be completely ruled out. Since the substance is a liquid at ambient temperature the generation of dust is excluded. However, in case of inhalation, absorption via the respiratory tract cannot be ruled out due to the relatively high vapor pressure.

Dermal route
Dermal absorption is considered limited since the logPow value is above 4 and thus considered to limit transfer between stratum corneum and the epidermis. However, uptake into the stratum corneum will probably be high. Another parameter that affects transport of a substance from the stratum corneum into the epidermis is the water solubility of a substance. Below 1 mg/L dermal uptake is considered to be low.
However, based on in vitro test battery results the test item was shown to have a skin sensation potential.
Moreover, the substance is classified as skin irritant and thus absorption may be facilitated due to the damage of the upper skin layers.

Distribution

Physicochemical properties of the test item as well as toxicity data indicate a rather low systemic bioavailability following oral, inhalative and dermal uptake.However, after being absorbed into the body, the test item is likely distributed into cells and intracellular concentrations may be higher than extracellular concentrations due to rather lipophilic properties (log Pow 4.44). This applies especially for fatty tissues. However, the test item is unlikely to have a bio-accumulative potential, even though it might be considered as lipophilic (logPow is greater than 4).The subacute repeated dose toxicity study in rats did not reveal any signs of target organ toxicity or other indications for an accumulation in any organ or tissue, there is also no other evidence for an accumulative property of this compound. Moreover, its metabolic transformation to more hydrophilic compounds is anticipated as described below.

 

Metabolism

It can be assumed that P450-enzymes are substantially involved in the metabolism of the test item, based on findings in the bacterial reverse mutation assay (Ames) as well as in the mammalian cell gene mutation assay (HPRT). In both assays cytotoxicity was observed in the absence of the metabolizing system (S9). In the presence of S9, however, the same substance concentration did not induce cytotoxicity anymore. Thus, a detoxification by metabolism of phase I enzymes could be anticipated. This metabolism usually aims to enhance hydrophilicity of the substance, for example by hydroxylation reactions, in order to facilitate excretion of the xenobiotic. This supports the assumption that the test item is not considered of concern in regards to bioaccumulation.

Excretion
As discussed above, the test item is probably substrate to phase I metabolizing enzymes and its hydrophilicity will likely be increased by this process. Taking into account its low molecular weight it can be assumed that the test item is rather excreted via the kidneys and the urine than via feces after it has been metabolised.

Summary

Based on physicochemical characteristics, particularly molecular weight and octanol-water partition coefficient, absorption by the dermal, oral and inhalation route is expected to be low. This assumption is further supported by the results of the oral acute toxicity study as well as the subacute repeated dose study. There are no indications that the test item or its metabolites could have a bioaccumulating potential. Metabolic transformation by phase I enzymes is expected and excretion via urine can be anticipated.