7-hydroxycitronellal

Endpoint summary

Link to relevant study record(s)

Reference

Reference 1

Endpoint:

basic toxicokinetics in vivo

Type of information:

experimental study

Adequacy of study:

other information

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: Acceptable publication which meets basic scientific principles

Objective of study:

metabolism

Qualifier:

no guideline followed

Principles of method if other than guideline:

After test substance application, urine of rabbits was collected and analyzed to identify metabolites.

GLP compliance:

not specified

Radiolabelling:

no

Species:

rabbit

Strain:

not specified

Sex:

male

Details on test animals or test system and environmental conditions:

TEST ANIMALS
- Weight at study initiation: 2.5 - 3.0 kg
- Fasting period before study: 1 day
- Housing: in metabolism cages
- Diet and Water: ad libitum

Route of administration:

oral: unspecified

Vehicle:

other: Tween-80 in water (0.02 g/100 mL)

Details on exposure:

PREPARATION OF DOSING SOLUTIONS:
- samples were homogenized in water (100 ml) with Tween-80 (0.02 g) at 5°C

Duration and frequency of treatment / exposure:

once

Remarks:

Doses / Concentrations:
about 6 g test substance per animal

No. of animals per sex per dose / concentration:

6

Control animals:

not specified

Details on dosing and sampling:

METABOLITE CHARACTERISATION STUDIES
- Tissues and body fluids sampled: urine
- Time and frequency of sampling: daily for 3 days
- From how many animals: 6
- Method type(s) for identification: silica gel column chromatography, GC-FID/TCD

Metabolites identified:

yes

Details on metabolites:

(±)-7-Hydroxycitronellol and 7-Hydroxycitronellic acid were identified in urine samples

Reference 2

Endpoint:

basic toxicokinetics in vitro / ex vivo

Type of information:

experimental study

Adequacy of study:

other information

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: Acceptable publication/thesis which meets basic scientific principles

Objective of study:

metabolism

Qualifier:

no guideline available

Principles of method if other than guideline:

Investigation of the biotransformation of Hydroxycitronellal in human skin homogenate, in intact full-thickness and dermatomed rat skin using an in vitro flow-though diffusion cell system.

GLP compliance:

not specified

Radiolabelling:

yes

Remarks:

3,7-Dimethyl-7-hydroxy[7-14C]ocanal ([14C]-Hydroxycitronellal)

Species:

other: human and rat

Strain:

other: not applicable

Details on test animals or test system and environmental conditions:

TEST ANIMALS
- Source: Harlan Olac (Oxford, U.K.)
- Weight at study initiation: ca. 200 g
- Diet and Water: ad libitum

Route of administration:

dermal

Vehicle:

other: no vehicle or ethanol or ethanol:DEP (75:25)

Duration and frequency of treatment / exposure:

see "Any other information on materials and methods incl. tables"

Remarks:

Doses / Concentrations:
see "Any other information on materials and methods incl. tables"

No. of animals per sex per dose / concentration:

see "Any other information on materials and methods incl. tables"

Control animals:

not specified

Metabolites identified:

yes

Details on metabolites:

Hydroxycitronellol (HC-OH) and Hydroxycitronellic acid (HC-COOH)

HC skin metabolism in human skin homogenate:

- Identification of TMS-HC, mono- and di-TMS-HC-OH and di-TMS-HC-COOH in fraction A and B

- no HC related compounds were identified in the SFM extracts after ß-glucuronidase/sulfatase treatment

HC skin metabolism in intact full-thickness rat skin:

- Identification of TMS-HC, mono- and di-TMS-HC-OH and di-TMS-HC-COOH

- maximum amount of HC related compound between 40 and 60 h after dose application

- no HC was detected before 32 h (Up to 32 h, all available HC was metabilize to HC-OH and HC-COOH by the metabolic system by the skin. As soon as the maximal capacity of the enzymes exceeded, Hc was detected in the rezeptor fluid.)

- no HC related compounds were identified in the extracts after ß-glucuronidase/sulfatase treatment

HC skin metabolism in dermatomed rat skin:

- majority of absorbed HC related compounds was detected in the first receptor fluid sample (4-8 h after dose application)

- detection of HC, HC-COOH and HC-OH

- no HC related compounds were identified in the extracts after ß-glucuronidase/sulfatase treatment

Both the epidermis and the dermis are able to metabolise HC to HC-COOH and HC-OH.

Reference 3

Endpoint:

dermal absorption in vitro / ex vivo

Type of information:

experimental study

Adequacy of study:

key study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: Acceptable publication/thesis which meets basic scientific principles

Qualifier:

no guideline available

Principles of method if other than guideline:

Investigation of the absorption and distribution of Hydroxycitronellal through different layers of rat skin using an in vitro flow-though diffusion cell system.

GLP compliance:

not specified

Radiolabelling:

yes

Remarks:

[14C]HC (hydroxycitronellal)

Species:

rat

Strain:

Fischer 344

Sex:

male

Details on test animals or test system and environmental conditions:

TEST ANIMALS
- Source: Harlan Olac (Oxford, U.K.)
- Weight at study initiation: ca. 200 g
- Diet and Water: ad libitum

Type of coverage:

occlusive

Vehicle:

other: no vehicle or ethanol or ethanol:DEP (75:25)

Duration of exposure:

Absorption of HC through rat skin: up to 72 h
Distribution of HC between the different layers of rat skin: up to 6 h

Doses:

4 µL neat HC, 20 µL 20% HC in ethanol or 20 µL 20% HC in ethanol:DEP (mixture of unlabelled and [14C]-labelled HC, each dose contained 1 µCi of radioactivity).
HC neat: 12 mg/cm2
HC 20% in EtOH: 11.16 mg/cm2
HC 20% in EtOH/DEP: 12.59 mg/cm2

No. of animals per group:

see "Any other information on materials and methods incl. tables"

Details on study design:

DOSE PREPARATION
- Method for preparation of dose suspensions: the stock vial of [14C]HC was diluted appropriately with unlabelled HC and the dose solutions were prepared with or without vehicle.

Details on in vitro test system (if applicable):

SKIN PREPARATION
- Type of skin: dorsal region of male F344 rats
- Preparative technique: After shaving with animal clippers, the dorsal skin was cut out with dissecting scissors. The skin was placed uppermost on a plastic dissecting board and circles of skin 1.7 cm in diameter cut using a circular steel wad punch. Excess subcutaneous tissue was removed with scissors
- Storage conditions: The skin circles (0.32 cm2) were kept in a petridish on ice for a few minutes until required.

PRINCIPLES OF ASSAY
- Diffusion cell: 6 -14 teflon diffusion cell system with an extra large conical flask, a fraction collector and a peristaltic pump
- Receptor fluid: HEPES buffered Hank's balanced salt solution (HHBSS), pH 7.4
- Flow-through system: yes; Skin circles were placed epidermis side uppermost in the teflon cells and clamped into the cell with the threaded neck. peristaltic pump primed on maximum speed for approximately 3 - 5 min; after this time the pump was set to 1.5 mL/h for the duration of the experiment.
- Test temperature: The skin surface was maintained at 32°C

1) The absorption of neat HC through full-thickness rat skin and the effect of the vehicle on the absorption:

	Neat HC		20% HC in Ethanol		20% HC in Ethanol:DEP
	mean %	+/- SD	mean %	+/- SD	mean %	+/- SD
Mean culmulative % dose absorbed (receptor fluid)	30,34	7,18	28,81	5,36	33,38	4,05
Mean % dose skin (dermis/epidermis/stratum corneum)	33,89	9,47	31,58	5,04	27,67	8,37
Mean % dose swabbed off skin surface	23,2	10,09	15,65	4,91	19,39	9,70
Mean % dose tape-stripped off skin surface	3,65	1,09	3,08	1,22	1,73	0,57
Mean % dose on cell body and cell cap	15,15	5,71	11,78	5,89	7,69	1,42
Mean % dose total recovery	99,41	8,30	87,12	4,52	89,17	3,34
Sum of % dose receptor fluid + skin	64		60		61
Calculated dose bioavailable*	53		50		52

Values are from 3 individual experiments with skin from 3 different animals (n=12). skin from a single animal was used for each experiment.

*excluding estimated amount in stratum corneum acc. to ratio of experiment 4:

Mean % dose in receptor fluid + (mean % dose skin *2/3)

For all vehicles examined, no significant differences in the absorption of HC though rat skin were observed.

2) The effect of stratum corneum removal on the absorption of HC through rat skin:

	20% HC in Ethanol		20% HC in Ethanol
	Full thickness skin		Tape-stripped skin
	mean %	+/- SD	mean %	+/- SD
Mean culmulative % dose absorbed (receptor fluid)	22,4	12,44	64,34	10,17
Mean % dose skin (dermis/epidermis/stratum corneum)	10,69	4,72	10,39	2,06
Mean % dose swabbed off skin surface	42,3	10,21	21,34	9,98
Mean % dose tape-stripped off skin surface	1,09	0,21	0,45	0,12
Mean % dose on cell body and cell cap	11,05	7,00	4,29	2,37
Mean % dose total recovery	87,5	3,62	100,78	11,89
Sum of % dose receptor fluid + skin	33		75
Calculated dose bioavailable*	30		n.a.

Values are from a single experiment with skin from 1 animal (n=3).

*excluding estimated amount in stratum corneum acc. to ratio of experiment 4:

Mean % dose in receptor fluid + (mean % dose skin *2/3)

Compared to full-thickness skin, removal of the stratum corneum resulted in a 3-fold increase in the total amount of HC traversing the skin in 72 hours.

3) The effect of stratum corneum and epidermis removal on the absorption of HC through rat skin:

	20% HC in Ethanol		20% HC in Ethanol
	Full thickness skin		Dermatomed skin
	mean %	+/- SD	mean %	+/- SD
Mean culmulative % dose absorbed (receptor fluid)	21,56	8,21	80,48	9,79
Mean % dose skin (dermis/epidermis/stratum corneum)	15,46	6,60	9,51	5,96
Mean % dose swabbed off skin surface	41,01	5,90	0,96	0,21
Mean % dose tape-stripped off skin surface	1,5	0,33	0,06	0,02
Mean % dose on cell body and cell cap	7,57	2,75	1	0,57
Mean % dose total recovery	87,53	1,28	92,01	3,13
Sum of % dose receptor fluid + skin	37		90
Calculated dose bioavailable*	32		n.a.

Values are from a single experiment with skin from 1 animal (n=3).

*excluding estimated amount in stratum corneum acc. to ratio of experiment 4:

Mean % dose in receptor fluid + (mean % dose skin *2/3)

Compared to full-thickness skin, dermatomed skin (removal of the stratum corneum and epidermis) resulted in a statistically significant 4-fold increase in the total amount of HC traversing the skin in 72 hours.

4) Distribution of HC between different layers of rat skin:

HC in the stratum corneum rised to a max. level of 5.82 +/- 2.12% at 1h after application and remained approximately constant up to 5.63 +/-1.34% of applied dose at 6h (mean +/-SD, n=12).

HC in remaining skin (dermis/epidermis) rised to a max. level of 7.8 +/- 2.06% at 1h after application and remained approximately constant up to 10.78 +/-4.26% of applied dose at 6h (mean +/-SD, n=12).

No data were given for the amounts HC found in other compartments.

According to these data 1/3 of the amounts HC found in skin are estimated to be located in the SC and 2/3 in epidermis and dermis.

Description of key information

Bioavailable via oral (approx. 100%) and dermal route (approx. 50%). Bioavailability for inhalation route was assumed to be worst case, i.e. 100% absorption. Distribution across organisms was assumed. Two metabolites have been identified, namely 7 -hydroxycitronellol and 7 -hydroxycitronellic acid. Excretion was suggested to occur mainly via urine.

Key value for chemical safety assessment

Bioaccumulation potential:

no bioaccumulation potential

Absorption rate - oral (%):

100

Absorption rate - dermal (%):

50

Absorption rate - inhalation (%):

100

Additional information

The physicochemical properties of 7-hydroxycitronellal, i.e. small molecular weight, LogPow and high water solubility at room temperature (MW = 172.26, Log P_OW= 1.68, water solubility = 35 g/L), indicate a good bioavailability of the substance via the oral route.

Dermal absorption of 7-hydroxycitronellal was evaluated using [14C]-labeled hydroxycitronellal and full-thickness dorsal rat skin in an in vitro flow-through diffusion cell system (Tonge, 1995). Receptor fluid, dry lint swabs, tape-strips, teflon cell body and cap and remaining skin were assessed for the content of hydroxycitronellal in different experimental setups.

1) The absorption of neat HC through full-thickness rat skin and the effect of the vehicle (ethanol and ethanol:DEP) on the absorption was investigated.

2) Skin samples after stratum corneum removal were compared to intact skin samples on the absorption of HC.

3) ) Skin samples after stratum corneum and epidermis removal were compared to intact skin samples on the absorption of HC.

4) Distribution of HC between different layers of rat skin was investigated on intact skin samples.

HC distribution in different skin layers was reported to be approx. 5% of the applied dose in SC and approx. 10% of the applied dose in tape-stripped skin within the observation period of 6 hours. According to these data, 1/3 of the HC levels found in skin are estimated to be located in the SC and 2/3 in epidermis and dermis.

Application of neat HC and HC in different vehicles resulted in calculated sums (% dose of receptor fluid + intact skin) between 60% and 64%. For all vehicles examined, no significant differences in the absorption of HC though rat skin were observed. No determination of HC levels in the stratum corneum was performed in this experimental setup, however, considering the ratio of HC found in the skin layer distribution study, the bioavailable amount of HC, i.e. levels in receptor fluid, dermis and epidermis was estimated to be approx. 50 % of the applied dose or lower.

Compared to full-thickness skin, removal of the stratum corneum resulted in a 3-fold increase and removal of the stratum corneum and epidermis resulted in a 4-fold increase in the total amount of HC traversing the skin in 72 hours. These findings demonstrate the relevance of these skin layers as barrier.

Although human skin is described to result in lower skin penetration levels than rat skin, the estimated 50% bioavailability via the dermal route has been taken for the respective DNEL derivation as a worst case.

 

On the basis of the low vapour pressure at room temperature (vapour pressure = 0.54 Pa), it can be assumed that the exposure via inhalation of 7-hydroxycitronellal as a vapour is low.

Based on a pharmacokinetic study in rabbits, oral administration of hydroxycitronellal resulted in detectable amounts of hydroxycitronellol and hydroxycitronellic acid in ß-glucuronidase/arylsulfatase treated urine. Therefore a reduction to yield the corresponding alcohol and an oxidization to yield the corresponding carboxylic acid is evident and a subsequent conjugation with e.g. glucuronide is to be expected (Ishida et al., 1989). Furthermore, incubation of hydroxycitronellal in human skin homogenate or on intact full-thickness and dermatomed rat (excl. SC and epidermis) skin in a flow-through diffusion cell system resulted in the formation of hydroxycitronellol and hydroxycitronellic acid but no evidence for the presence of glucuronide or sulfate conjugates were given (Tonge, 1995).

Based on the information given above, there is no evidence for a bioaccumulative potential of 7-hydroxycitronellal.