2-ethylhexyl 10-ethyl-4,4-dimethyl-7-oxo-8-oxa-3,5-dithia-4-stannatetradecanoate

Endpoint summary

Link to relevant study record(s)

Reference

Reference 1

Endpoint:

basic toxicokinetics in vitro / ex vivo

Type of information:

experimental study

Adequacy of study:

key study

Study period:

15 June 2015 to 15 July 2015

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

other: Study conducted in accordance with generally accepted scientific principles, possibly with incomplete reporting or methodological deficiencies, which do not affect the quality of the relevant results.

Objective of study:

other: hydrolysis

Qualifier:

no guideline followed

Principles of method if other than guideline:

The purpose of the study was to verify the chemical pathways of the in-vitro metabolism of DMTE in order to determine if DMTE is metabolised via a hydrolytic mechanism at mammalian gastric pH and, if so, to identify the product of the metabolism.
DMTE was exposed at 37 °C for 30 seconds, 30 minutes and 1, 2, 4 and 72 hours to 0.1 M HCl ( pH 1.2) solution. The reaction products were extracted with hexane and analysed using 119Sn-NMR-spectroscopy.

GLP compliance:

no

Radiolabelling:

no

Species:

other: not applicable

Route of administration:

other: in vitro

Vehicle:

unchanged (no vehicle)

Remarks:

The test material was used without a co-solvent or a detergent.

Details on exposure:

PREPARATION OF DOSING SOLUTIONS
1 g (1.8 mmol) test material was added to 100 mL of a 0.1 M aqueous hydrochloric acid solution (pH 1.2). The solution was preheated to 37 °C in a 250 mL Erlenmeyer flask with ground in stopper.

Duration and frequency of treatment / exposure:

30 seconds, 30 minutes and 1, 2, 4 and 72 hours

No. of animals per sex per dose / concentration:

The experiments were run in duplicate

Control animals:

other: untreated test material

Details on dosing and sampling:

For the initial time of the experiment (30 seconds), the reaction products were extracted with hexane immediately according to the procedure described below.
For longer exposure/hydrolysis times, the flask was closed with a stopper and heated in a heating cabinet for 0.5, 1, 2, 4, and 72 hours at 37 °C. The mixture was stirred by a magnetic stirrer using a 40 x 7 mm stir bar at approximately 100 rpm.
After the pre-determined exposure time, the solution was allowed to cool down to room temperature. The solution was extracted 2 times with 25 mL hexane and the phases separated using a separatory funnel. The organic phase was transferred into a pre- weighed flask and the solvent was removed in a rotary evaporator (<40 °C, 10 mbar). The weight difference was recorded for the mass balance, and the samples were analysed by 119Sn-NMR Spectroscopy.

ANALYTICAL METHOD
119Sn-NMR Spectroscopy
The 119Sn-NMR was used to analyse the test material as well as the breakdown products, since it combines several unique aspects of analysing organotin substances:
- 119Sn-NMR detects all tin-containing substances qualitatively and quantitatively at the same time.
- 119Sn-NMR is a direct and non-destructive method. It does not require any sample digestion or derivatisation. Thus, it avoids errors associated with a) the sample derivatisation and b) misinterpretation of the results associated with analysing and quantifying the derivatives.
- The 119Sn spectra signals (peaks) are highly selective. They uniquely represent the corresponding tin compounds. Chemical shifts of differently substituted tin atoms are highly characteristic of the specific atom coordination.
- The 119Sn-NMR spectroscopy is very sensitive and reliable. Its detection limit was established to be 0.5 %.
-The 119Sn-NMR method has been used for decades by the industry as a standard analytical method on tin compounds for the purpose of quality control, process development and research.

Experimental:
- Apparatus: Bruker Advance 200
- Temperature: Ambient temperature
- Sample preparation: 370 µL/330 µL toluene-d8 (10 mg/mL CrAcAc)

Type:

other: simulated gastric hydrolysis

Results:

DMTE is relatively stable and only partially hydrolysed to DMTEC, its mono-chloro ester, after 72 hours under simulated gastric conditions. DMTEC is the only metabolite of DMTE formed in the simulated mammalian gastric environment, and no DMTC was formed.

Metabolites identified:

yes

Details on metabolites:

Results of in-vitro metabolism under the simulated gastric conditions over the specified periods of time are included in Table 1. The reported concentrations are average values of two repetitions.
The results show that the only product that is formed as a result of the in-vitro metabolism is DMTEC, a mono-chloro ester of DMTE.
The hydrolysis starts instantly when the test material is brought in contact with the HCl solution. After 30 seconds of contact with the acidic solution (followed by the "instant" work-up) the DMTEC concentration raised from 3.5 % in the untreated test material to 13.7 %.
After 30 minutes of exposure to the acidic solution, the concentration of DMTEC in the reaction mass reaches 25 %. The subsequent increase in the concentration of DMTEC is very moderate, and it reaches 29 % after 72 hours.

Mass balance

Amounts of the test material recovered upon completion of the respective hydrolysis periods followed by the hexane extraction of the aqueous phases are in Table 2.

The Recovered Weight was converted to the Adjusted Recovery per cent taking into consideration amounts of the formed DMTEC. Table 2 shows that except for the first two short experiments, the adjusted recovery per cent was high, ranging from 88.0 to 98.4 %.

 

Discussion

In-vitro metabolism of DMTE can be monitored using 119Sn-NMR Spectroscopy by the decrease in the relative intensity of the respective 119Sn-NMR signal at ~74 ppm attributable to DMTE and the increase of the signal at ~ 41 ppm characteristic to DMTEC

Under the simulated gastric conditions (0.1 M HCl/pH 1.2 /37 °C), the in-vitro metabolism of DMTE equilibrated within a short period and resulted in the formation of DMTEC (a mono-chloro-ester of DMTE) at about 30 %.

DMTC was not detected in any of the metabolised DMTE samples at δ(DMTC) = 131 ppm using the 119Sn-NMR spectroscopy.

Table 1: Kinetics of DMTE in-vitro metabolism

Time (hours)	DMTE (% w/w)	DMTEC (%)
0	92.9	3.5
0.001	82.3	13.7
0.5	68.7	25.0
1	69.5	23.0
2	68.3	25.0
4	69.2	24.0
72	61.8	29.0

No signal typical for DMTC was detected.

Table 2: Recovery of Test Material

Time of Hydrolysis (Hours)	Recovered Weight (g)	Concentration of Formed DMTEC (%)	Adjusted Recovery (%)
0.001	0.61	10.4	63.0
	0.62	10.0	63.9
0.5	0.92	21.8	98.4
	0.91	21.3	97.2
1	0.81	16.5	85.2
	0.9	22.7	96.6
2	0.89	18.5	94.2
	0.85	24.7	91.8
4	0.86	19.1	91.2
	0.84	22.0	89.9
72	0.81	28.4	88.5
	0.82	22.8	88.0

Conclusions:

Interpretation of results: DMTE is relatively stable and only partially hydrolysed to DMTEC, its mono-chloro ester, after 72 hours under simulated gastric conditions. DMTEC is the only metabolite of DMTE formed in the simulated mammalian gastric environment, and no DMTC was formed.
Under the conditions of this study, hydrolysis begins immediately on contact with the HCl solution. After 30 seconds of exposure followed by the "instant" work-up, the DMTEC concentration rose from 3.5 % in control samples to 13.7 %. By 30 minutes of exposure, the concentration of DMTEC reached 25 % and the reaction moderates with DMTE at 29 % after 72 hours.

Executive summary:

The purpose of the study was to verify the chemical pathways of the in-vitro metabolism of DMTE in order to determine if DMTE is metabolised via a hydrolytic mechanism at mammalian gastric pH and, if so, to identify the product of the metabolism.

DMTE was exposed at 37 °C for 30 seconds, 30 minutes and 1, 2, 4 and 72 hours to 0.1 M HCl ( pH 1.2) solution. The reaction products were extracted with hexane and analysed using 119Sn-NMR-spectroscopy.

The results show that the only product that is formed as a result of the in-vitro metabolism is DMTEC, a mono-chloro ester of DMTE. The hydrolysis starts instantly when the test material is brought in contact with the HCl solution. After 30 seconds of contact with the acidic solution (followed by the "instant" work-up) the DMTEC concentration raised from 3.5 % in the untreated test material to 13.7 %. After 30 minutes of exposure to the acidic solution, the concentration of DMTEC in the reaction mass reaches 25 %. The subsequent increase in the concentration of DMTEC is very moderate, and it reaches 29 % after 72 hours.

The Recovered Weight was converted to the Adjusted Recovery per cent taking into consideration amounts of the formed DMTEC. Except for the first two short experiments, the adjusted recovery per cent was high, ranging from 88.0 to 98.4 %.

The 119Sn-NMR Spectroscopy method replaces earlier analytical methods as the NMR method (a) does not require a derivatisation step, and (b) measures Sn components formed, not the EHTG ligand released. The 119-Sn NMR directly and correctly identifies and quantifies all organotin components which are formed as a result of the in-vitro metabolism of the DMTE.

Under the conditions of this study, hydrolysis begins immediately on contact with the HCl solution. After 30 seconds of exposure followed by the "instant" work-up, the DMTEC concentration rose from 3.5 % in control samples to 13.7 %. By 30 minutes of exposure, the concentration of DMTEC reached 25 % and the reaction moderates with DMTE at 29 % after 72 hours.

Under the conditions of this study (simulated gastric conditions; 0.1 M HCl, pH 1.2, 37 °C), DMTE is relatively stable and only partially hydrolysed to DMTEC, its mono-chloro ester by 72 hours. DMTEC is the only metabolite of DMTE formed in the simulated mammalian gastric environment, and no DMTC was formed under the conditions of this study.

Reference 2

Endpoint:

dermal absorption in vitro / ex vivo

Type of information:

experimental study

Adequacy of study:

key study

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

other: GLP, guideline study

Qualifier:

according to guideline

Guideline:

OECD Guideline 428 (Skin Absorption: In Vitro Method)

Version / remarks:

guideline was in draft form at the time the study was conducted

Deviations:

not specified

GLP compliance:

yes

Radiolabelling:

no

Species:

other: human and rat

Strain:

Wistar

Sex:

male

Details on test animals or test system and environmental conditions:

TEST ANIMALS
- Source: rat: Charles River UK Ltd., Margate, Kent, UK
- Age at study initiation: 28 +/- 2 days




IN-LIFE DATES: From: 1998-06-08 To: 1999-01-15

Type of coverage:

other: occluded and open

Vehicle:

ethanol

Duration of exposure:

24 h

Doses:

- Dose volume: 100 uL/cm2
- Rationale for dose selection: The first phase of the study was to identify the highest dose that could practically be applied to human epidermis, which was also determined to be non-damaging to human epidermis by measuring the alteration in barrier function of the epidermis, following exposures to various applications.
During the second phase of the study the absorption of tin was determined from applications of this non-damaging dose to both human and rat epidermis.

No. of animals per group:

Skin barrier damage assessment: 3 cells- human epidermis
Absorption test: 6 cells/species

Control animals:

yes

Remarks:

untreated in the skin barrier damage test

Details on study design:

DOSE PREPARATION
- Method for preparation of dose suspensions: receptor chambers filled with receptor fluid: water or 50% ethanol in water. Samples were diluted with a diluent prepared containing a known amount of indium internal reference.


APPLICATION OF DOSE: applied undiluted to human epidermis in the skin barrier damage assessment.


VEHICLE
- Justification for use and choice of vehicle (if other than water): 50% ethanol in water; the solubility in water alone in minimal
- Amount(s) applied (volume or weight with unit): 100 uL/cm2 (24300 ug/cm2 nominal)


TEST SITE
- Preparation of test site: Extraneous tissue was removed from human whole skin samples. The skin samples were immersed in water at 60 deg C for 40-45 seconds and te epidermis teased off the dermis. Each epidermal membrane was given an identifying number and stored frozen on aluminum foil until used. Skin from rats was carefully shaved using animal clippers in the dorsal and flank region, ensuring the skin was not damaged. The clipped area was excised and any subcutaneous fat removed. The skins were soaked for approximately 20 hours in 1.5 M sodium bromide then rinsed in distilled water. The epidermis was carefully peeled from the dermis and stored frozen on aluminum foil until use.
Epidermis samples were digested in nitric acid and diluted with deionised water. Donor / wash samples were concentrated to approximately 2 mL prior to digestion in nitric acid and dilution.
- Area of exposure: cells placed in receptor chambers
- Type of cover / wrap if used: no data
- Time intervals for shavings or clipplings: one time


SITE PROTECTION / USE OF RESTRAINERS FOR PREVENTING INGESTION: no; skin tissue was removed


REMOVAL OF TEST SUBSTANCE
- Washing procedures and type of cleansing agent: Afterthe final sample of receptor fluid had been taken at the end of the exposure period, the remianing fluid in the receptor chamber was discarded and the chamber rinsed with fresh receptor fluid (5 mL) which was also discarded. The surface of the epidermis was rinsed with 50% ethanol in water (2x5 mL) and the rinsings combined.
The donor chambers were carefully removed and washed with ethanol (10 mL) and the washings retained for analysis.
The surface of the epidermis was rinsed with further 50% ethanol in water (3x5 mL) and the rinsings combined with the first rinsings (i.e. total rinse volume = 25 mL).
The epidermis was carefully removed from the receptor chamber ad placed in a glass scintillation vial.
- Time after start of exposure: at the end of the exposure period (24-hr)



SAMPLE PREPARATION
- Storage procedure: stored frozen on aluminum foil
- Preparation details: Extraneous tissue was removed from human whole skin samples. The skin samples were immersed in water at 60 deg C for 40-45 seconds and te epidermis teased off the dermis. Each epidermal membrane was given an identifying number and stored frozen on aluminum foil until used. Skin from rats was carefully shaved using animal clippers in the dorsal and flank region, ensuring the skin was not damaged. The clipped area was excised and any subcutaneous fat removed. The skins were soaked for approximately 20 hours in 1.5 M sodium bromide then rinsed in distilled water. The epidermis was carefully peeled from the dermis and stored frozen on aluminum foil until use.
Epidermis samples were digested in nitric acid and diluted with deionised water. Donor / wash samples were concentrated to approximately 2 mL prior to digestion in nitric acid and dilution.


ANALYSIS
- Method type(s) for identification: The recedptor fluid diluted samples were analysed using ICP/MS; The epidermis diluted samples were anlaysed using ICP/AES
- Validation of analytical procedure: no data
- Limits of detection and quantification: Receptor fluid:0.2 - 2 ng tin/mL for samples for human experiments and 0.4 -20 ng tin/mL for samples from rat experiments, however in calculations the LOQ was set at a value of 1 ng/mL for both species.
Epidermis samples and donor/wash samples: the limit of quantitation using the above procedure was set at 12 ug tin for both human and rat epidermis and 6 ug tin/mL for the donor / wash samples.

Details on in vitro test system (if applicable):

SKIN PREPARATION
- Source of skin: extraneous human whole skin samples and skin from male rats of the Wistar-derived strain
- Type of skin: human and rat (dorsal and flank region)
- Preparative technique: Extraneous tissue was removed from human whole skin samples. The skin samples were immersed in water at 60 deg C for 40-45 seconds and te epidermis teased off the dermis. Each epidermal membrane was given an identifying number and stored frozen on aluminum foil until used. Skin from rats was carefully shaved using animal clippers in the dorsal and flank region, ensuring the skin was not damaged. The clipped area was excised and any subcutaneous fat removed. The skins were soaked for approximately 20 hours in 1.5 M sodium bromide then rinsed in distilled water. The epidermis was carefully peeled from the dermis and stored frozen on aluminum foil until use.
Epidermis samples were digested in nitric acid and diluted with deionised water. Donor / wash samples were concentrated to approximately 2 mL prior to digestion in nitric acid and dilution.
- Thickness of skin (in mm): no data
- Membrane integrity check: yes; Samples of epidermis were mounted in glass diffusion cells with an exposed area of 2.54 cm2. The cells were placed in a water bath maintained at 32 +/- 1 deg C.
The integrity of the membranes was determined by measurement of their electrical resistance across the skin membrane. Membranes with a measured resistance <10kΩ (human) or <2.5 kΩ (rat) were regarded as having a lower integrity than normal and not used for exosure to the test material.
- Storage conditions: stored frozen on aluminum foil until use
- Justification of species, anatomical site and preparative technique: In vitro techniques employing glass diffusion cells have been shown to predict percutaneous absorption of chemicals in vivo.


PRINCIPLES OF ASSAY
- Diffusion cell: donor chamber-skin-support grid-receptor chamber- sampling arm
- Receptor fluid: diluent containing a known amount of indium internal reference
- Solubility of test substance in receptor fluid: 50% ethanol in water as the solubility of the test material in water alone is minimal.
- Flow-through system: at recorde intervals, samples of receptor fluid were taken for analysis. The volume of fluid in the receptor chamber was maintained by the addition of (0.5 mL) of fresh receptor fluid to the chamber immediately after the removal of each sample.
- Test temperature: 32 +/- 1 deg C
- Occlusion: occluded and unoccluded
- Reference substance(s): no

Signs and symptoms of toxicity:

not examined

Dermal irritation:

no effects

Absorption in different matrices:

Human epidermis: Occluded applications: tin absorption was 0.005 ug/cm2/h during the first 10h of exposure, increased to a maximum of 0.018 µg/cm2/h during the 10-24 hour period. Unoccluded application: absorption rates during the 0-10 h and 10-24 h periods were 0.002 µg/cm2/h and 0.007 µg/cm2/h respectively. In terms of percent of applied tin, 134 x 10E-5% (0.327 µg tin/cm2) was absorbed from the occluded dose, and 44 x 10E-5 % (0.107 µg tin/cm2) was absorbed from the unoccluded dose after 24-h exposure.

Rat epidermis: Absorption was 133 times faster than human for the occluded application and 213 times faster for the unoccluded application. The occluded trial absorption rate during the first 3h was 0.069 µg/cm2/h, increasing to 2.39 µg/cm2/h between 3-24h.
From the unoccluded application the rate in the first 3h was 0.489 µg/cm2/h, increasing to 1.49 µg/cm2/h between 3-24h. In terms of percent of applied tin, 0.206% (50 µg tin/cm2) was absorbed from the occluded dose, while 0.137% (33 µg tin/cm2) was absorbed from the unoccluded dose after 24h exposure.

The amounts of tin absorbed were <0.001% (unoccluded) and 0.001% (occluded) through human epidermis and 0.14% (unoccluded) and 0.21% (occluded) through rat epidermis.

Total recovery:

- Total recovery: >80%- human; >70%- rat was obtained from the surface of the epidermis and donor chamber
- Recovery of applied dose acceptable: yes; between 80-88% from both the occluded and unoccluded applications. Of the applied tin, approximately 1.5-2% for human and 6.5-7.2% for rat was recovered from the epidermis.
- Results adjusted for incomplete recovery of the applied dose: no
- Limit of detection (LOD): no data
- Quantification of values below LOD or LOQ: where individual values were below the LOQ, the LOQ values were included in calculations to obtain mean absorption data. In such cases, ti absorption is overestimated.

Dose:

100 uL/cm2

Parameter:

percentage

Absorption:

0 %

Remarks on result:

other: 24h

Remarks:

135 x 10e-5; human; occluded

Dose:

100 uL/cm2

Parameter:

percentage

Absorption:

0 %

Remarks on result:

other: 24h

Remarks:

44 x 10e-5; human; unoccluded

Dose:

100 uL/cm2

Parameter:

percentage

Absorption:

0.2 %

Remarks on result:

other: 24h

Remarks:

rat; occluded

Dose:

100 uL/cm2

Parameter:

percentage

Absorption:

0.1 %

Remarks on result:

other: 24h

Remarks:

rat; unoccluded

Conversion factor human vs. animal skin:

not determined

The test substance did not damage the epidermis when applied to  human epidermis at a rate of 100 µl/cm2 and exposed under occlusion for  24 hours.

Conclusions:

Following 24-h dermal contact, the amount of the test substance required to alter the barrier function of human epidermis was >100 uL/cm2.
At this 100 uL/cm2 dose level, the absorption of tin from the test substance through human epidermis is extremely slow, when compared with the absorption rates of other penetrants measured using this in vitro technique.
The absorption of tin from the test substance through rat epidermis significantly overestimated absorption through human epidermis.
Absorption of tin through human epidermis after 24h exposure under occlusion was 135 x 10e-5% of the applied tin dose, but was only 44 x 10e-5% when left unoccluded. Throguh rat epidermis, 0.208% of the applied tin was absorbed by 24 h from the occluded application and 0.138% from the unoccluded applications.

Executive summary:

The absorption of the test substance has been measured in vitro through human and rat epidermis. The tin species absorbed were not individually identified; absorption determinations were based on measurements of total tin in the samples analysed. The first phase of the study was to idenitify the highest dose that could practically be applied, which was also determined to be non-damaging to human epidermis. During the second phase of the study, the absorption of tin was determined from both occluded and unoccluded applications of this non-damaging dose of the test substance (100 uL.cm2) human and rat epidermis.

The test substance did not damage the skin barrier (damage ratio = 1.3), when applied to human epidermis at a rate of 100 uL/cm2 and exposed under occlusion for 24h. This dose level is regarded as an excess application and increasing the dose would not further damage the membrane.

Fromt he occluded applications to human epidermis, tin absorption was 0.005 ug/cm2/h during the first 10 h of exposure, which increased to a maximum absorption rate of 0.018 ug/cm2/h during the 10 -24 h period. From the unoccluded application, absorption rates were slower during the 0 -10 h and 10 -24 h periods (0.002 ug/cm2/h and 0.007 ug/cm2/h, respectively). In terms of percent of applied tin, 134 x 10e-5% (0.327 ug tin/cm2) was absorbed from the occluded dose, while only 44 x 10e-5% (0.107 ug tin/cm2) was absorbed from the unoccluded dose after 24 h exposure.

When compared with human epidermis, absorption of tin from the test substance through rat epidermis was 133 times faster for the occluded application and 213 times faster for the unoccluded application.

The absorption rate for tin during the first 3h of exposure was 0.069 ug/cm2/h, increasing to 2.39 ug/cm2/h between 3 -24 h. Tin absorption form the unoccluded application during the first 3 h of exposure was 0.489 ug/cm2/h, increasing to 1.49 ug/cm2/h between 3 -24 h. In erms of percent of applied tin, 0.208% (50 ug tin/cm2) was absorbed from the occluded dose, while 0.138% (33 ug tin/cm2) was absorbed from the unoccluded dose after 24 h exposure.

The overall recovery of tin from the test system after 24 h exposure was between 81 -89% from both the occulded and unoccluded applications to human and rat epidermis.

Of the recovered tin, 82 -87% (human) and 74 -78% (rat) was washed off the epidermis and donor chamber at the end of the 24 h exposure period. The amounts of tin actuallly penetrated the epidermis into the recpetor solution were relatively insignificant (<0.001% and 0.001% through human epidermis adn 0.14% and 0.21% through rat epidermis). Of the applied tin, approximately 1.5 -2% (human) and 6.5 -7.3% (rat) was recovered from the epidermis.

Description of key information

Under simulated gastric conditions (0.1 M HCl, pH 1.2, 37 °C), DMTE is relatively stable and only partially hydrolysed to DMTEC, its mono-chloro ester by 72 hours. DMTEC is the only metabolite of DMTE

formed in the simulated mammalian gastric environment.

Key value for chemical safety assessment

Bioaccumulation potential:

high bioaccumulation potential

Absorption rate - oral (%):

100

Absorption rate - dermal (%):

10

Absorption rate - inhalation (%):

100

Additional information

TOXICOKINETIC ASSESSMENT: DMTE

BACKGROUND
New hydrolysis data utilising an advanced analytical methodology, ¹¹⁹Sn NMR Spectroscopy has allowed intermediate hydrolysis products to be identified and quantified directly (Naβhan, 2017). The purpose of the new study (Naβhan, 2017) was to verify the chemical pathways of the in vitro metabolism of DMTE in order to determine if DMTE is metabolised via a hydrolytic mechanism at mammalian gastric pH and, if so, to identify the product of the metabolism. DMTE was exposed at 37 °C for 30 seconds, 30 minutes and 1, 2, 4 and 72 hours to 0.1 M HCl (pH 1.2) solution. The reaction products were extracted with hexane and analysed using ¹¹⁹Sn-NMR spectroscopy. The results show that the only product that is formed as a result of the in vitro metabolism is dimethyltin chloro-2-ethylhexyl thioglycolate (DMTEC), a mono-chloro ester of DMTE. The hydrolysis starts instantly when the test material is brought in contact with the HCl solution. After 30 seconds of contact with the acidic solution (followed by the "instant" work-up), the DMTEC concentration raised from 3.5 % in the untreated test material to 13.7 %. After 30 minutes of exposure to the acidic solution, the concentration of DMTEC in the reaction mass reaches 25 %. The subsequent increase in the concentration of DMTEC is very moderate, and it reaches 29 % after 72 hours. The Recovered Weight was converted to the Adjusted Recovery per cent taking into consideration amounts of the formed DMTEC. Except for the first two short experiments, the adjusted recovery per cent was considered to be high, ranging from 88.0 to 98.4 %.

The ¹¹⁹Sn-NMR directly and correctly identifies and quantifies all organotin components which are formed as a result of the in vitro metabolism of the DMTE. Under the conditions of this study (simulated gastric conditions; 0.1 M HCl, pH 1.2, 37°C), DMTE is relatively stable and only partially hydrolysed to DMTEC by 72 hours. DMTEC is the only metabolite of DMTE formed in the simulated mammalian gastric environment.

INTRODUCTION
In accordance with the Section 8.8.1 of Annex VIII in Regulation (EC) No 1272/2008, the toxicokinetic profile of DMTE was derived from all available substance-specific information collated in the dossier. The assessment is based on the Guidance on information requirements and chemical safety assessment Chapter R.7c: Endpoint specific guidance (ECHA, 2017).
The registered substance is hydrolytically stable under the physiological conditions. Under the simulated gastric conditions (0.1 M HCl/pH 1.2 /37 °C; OECD 111; Naβhan, 2017), DMTE is relatively stable and only 30 % of DMTE was hydrolysed to DMTEC by 72 hours. DMTEC is the only metabolite of DMTE formed in the simulated mammalian gastric environment. The assay showed a high hydrolytic stability of the test material at higher pH values (4, 7, 9). Therefore, the physico-chemical properties of the registered substance are considered relevant to supplement toxicokinetic behaviour of the registered substance.

PHYSICOCHEMICAL PROPERTIES
DMTE is a mono-constituent organometallic substance with a high molecular weight of 555.42 g/mol. It is a colourless solid that decomposed prior to boiling at approximately 290 °C and has a low vapour pressure of 0.81 Pa at 25 °C. The registered substance has a low water solubility of 4.51 mg/L at 20 °C (0.964 mg/L Sn at 20 °C) with a relatively high log Pow value of > 4.74 (calculated value from the water solubility and n-octanol solubility). Due to its high molecular weight with highly lipophilic characteristics, oral and dermal absorption is considered to be limited. Bioaccessibility of the substance into GI fluids is expected to be low and its contact with the mucosal surface is expected to be limited. Its low vapour pressure and the fact it decomposes prior to boiling suggest that the substance is poorly available as a vapour for inhalation exposure.
Since its molecular weight is above 500 and its log Pow is outside the range of dermal absorption [-1, 4], a default dermal absorption rate of 10 % is appropriate for risk assessment of DMTE for workers.

ABSORPTION
Signs of oral absorption are seen in the results of the 14-day range-finding study (Kumar, 2018), 28-day neurotoxicity study (Kumar, 2019), 90-day combined repeated dose and neurotoxicity study (Kumar, 2020) and prenatal developmental toxicity study (de Groot, 2022), all of which were feeding studies conducted in the rat.
No signs of dermal absorption are observed in the acute dermal toxicity study in rabbits following topical application of DMTE under semiocclusive conditions (OECD 402; Schindler-Horvat and Baldwin, 1993). In the optimisation test in the guinea pig (Sachsse and Ullmann, 1975), the registered substance showed positive skin sensitisation response. In this test, intracutaneous sensitisation procedure was employed followed by intracutaneous challenge applications. The positive response in the optimisation test does not directly reflect a sign of dermal absorption since DMTE was administered directly into the dermis.
Tin content of DMTE was poorly absorbed via the dermal route, with tin content in humans being negligible, in the in vitro comparative dermal absorption test using the human and rat epidermal membrane (OECD 428; Ward, 1999). Following dermal applications of non-radiolabelled DMTE, the percentage of applied tin content was 0.0134 % (occluded conditions) and 0.0044 % (unoccluded conditions) in the human epidermis, and 0.206 % (occluded condition) and 0.137 % (unoccluded conditions) in the rat epidermis after 24 hours. Dermal absorption of tin content in rats was 133 times faster than human for the occluded application and 213 times faster for the unoccluded application, indicating inter-species differences.
There are no in vivo mammalian studies available via the inhalation route for the registered substance. However, due to its physico-chemical properties, inhalation is not considered to be the most significant route of exposure.

DISTRIBUTION
The high molecular weight and low water solubility of the registered substance are considered unlikely to allow it to disperse into the water compartment of blood for systemic distribution. Due to its high log Pow value, DMTE is likely to be accumulated in body fat.
In the 28-day neurotoxicity study in rats (Kumar, 2019) and 90-day combined repeated dose and neurotoxicity study in rats (Kumar, 2020), neurohistopathological changes were observed in various sites in the brain. Although neurobehavioural parameters were unaffected in these studies, the observed histopathological changes indicate distribution of DMTE and/or its metabolites to the central nervous system.
The positive skin sensitisation study in guinea pig (Sachsse and Ullmann, 1975) suggests that the registered substance binds to carrier proteins in the blood once molecules enter the circulatory system.

METABOLISM
The results of the repeat oral toxicity studies in the rat did not show any evidence of enhanced metabolism.
DMTE produced negative results in both the presence and absence of metabolic activation in the bacterial mutation test (Stankowski, 1996) and in vitro chromosome aberration test (Rashmi, 2019a), whereas it was shown to be weakly positive in the gene mutation assay in Chinese hamster ovarian cells in both the presence or absence of metabolic activation (Rashmi, 2019b). The results of available in vitro genotoxicity studies suggest that metabolism did not influence genotoxicity of DMTE.

EXCRETION
There is no evidence to indicate the route of excretion. Given its high molecular weight, biliary excretion may well be the primary route of excretion for the registered substance. Any test material that is not absorbed will be excreted in the faeces.
Due to its high lipophilicity, penetration of DMTE into the viable epidermal layer of the skin is limited and the substance may be sloughed off with the skin cells. In the in vitro dermal absorption test (Ward, 1999), approximately 1.5 -2 % (human) and 6.5 -7.3 % (rat) of the applied tin was recovered from the epidermis.