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Please be aware that this old REACH registration data factsheet is no longer maintained; it remains frozen as of 19th May 2023.

The new ECHA CHEM database has been released by ECHA, and it now contains all REACH registration data. There are more details on the transition of ECHA's published data to ECHA CHEM here.

Diss Factsheets

Administrative data

basic toxicokinetics, other
Expert statement
Type of information:
other: Expert statement
Adequacy of study:
key study
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: No GLP-conform guideline study, but scientifically valid expert statement based i.a. on studies assessed with Klimisch 1 or 2

Data source

Reference Type:
other: Expert Statement
Report date:

Materials and methods

Objective of study:
Test guideline
no guideline required
Principles of method if other than guideline:
An extensive assessment of the toxicokinetic behaviour of 2,4-diethyl-9H-thioxanthen-9-one (DETX) was performed, taking into account the chemical structure, the available physico-chemical and toxicological data.
GLP compliance:

Test material

Constituent 1
Chemical structure
Reference substance name:
EC Number:
EC Name:
Cas Number:
Molecular formula:
other: not applicable

Test animals

other: not applicable
other: not applicable
Details on test animals or test system and environmental conditions:
not applicable

Administration / exposure

Route of administration:
other: All relevant routes of administration are discussed in the expert statement.
other: not applicable
Details on exposure:
not applicable
Duration and frequency of treatment / exposure:
not applicable
Doses / concentrations
not applicable
No. of animals per sex per dose / concentration:
not applicable
Control animals:
other: not applicable
Details on study design:
not applicable
Details on dosing and sampling:
not applicable
not applicable

Results and discussion

Main ADME resultsopen allclose all
The relevant absorption rates were estimated to: Oral absorption: approx. 50% Dermal absorption: approx. 10% Inhalative absorption: approx. 10%
The absolute systemic bioavailability is rather low and expected to be more extensive in fat tissues than in other tissues.
S-oxidation, aliphatic hydroxylation and aromatic hydroxylation were identified as the mode of action during Phase-I-metabolism, and subsequent conjugation is expected.
The excretion, if any, of DETX will occur via the gastrointestinal tract and the bile, for the metabolites of 2,4-diethyl-9H-thioxanthen-9-one, a elimination of the Phase 2 conjugated metabolites, via the bile is expected.

Toxicokinetic / pharmacokinetic studies

Details on absorption:
In general, absorption of a chemical is possible, if the substance crosses biological membranes. In case where no transport mechanisms are involved, this process requires a substance to be soluble, both in lipid and in water, and is also dependent on its molecular weight (substances with molecular weights below 500 are favourable for absorption). Generally, the absorption of chemicals which are surfactants or irritants may be enhanced, because of damage to cell membranes. However, since 2,4-diethyl-9H-thioxanthen-9-one was found to be non-irritating to the skin or eyes, the possibility of an enhanced absorption due to damaged cell membranes can be excluded.
Due to the lack of experimental absorption data, the following physico-chemical parameters of 2,4-diethyl-9H-thioxanthen-9-one will be taken into account when discussing its absorption into the body:
- Molecular weight = 268.37 g/mol
- Water solubility = < 0.66 mg/L
- Partition Coefficient Log Pow = > 6.5
- Vapour pressure = 0.000103 Pa (Modified Grain Method, EpiSuite estimation)
- Boiling point: 423.7°C
- Particle size: 22.3 %10-100 µm (inhalable), 2.7% are 4-10 µm (thoracic), and 0.13% <4 µm (respirable)

Absorption from the gastrointestinal tract
In the small intestine absorption occurs mainly via passive diffusion or lipophilic compounds may form micelles and be taken into the lymphatic system. Additionally, metabolism can occur by gut microflora or by enzymes in the gastrointestinal mucosa. However, the absorption of highly lipophilic substances (Log Pow of 4 or above) may be limited by the inability of such substances to dissolve into gastrointestinal fluids and hence make contact with the mucosal surface. The absorption of such substances will be enhanced if they undergo micellular solubilisation by bile salts. Substances absorbed as micelles enter the circulation via the lymphatic system, bypassing the liver. Consequently, immediate Cytochrome P450 metabolism is less important here as for substances which directly enter the hepatic system via the portal vein.
According to ECHA’s guidance R.7c [ECHA 2008], it is stated that the smaller the molecule the more easily it may be taken up. Molecular weights below 500 are favourable for absorption. With a molecular weight of 268.37 g/mol, absorption in general can be considered as possible. However, substances with high Log Pow values, i.e. >6.5 in this case, are less favourable for absorption by passive diffusion, which diminishes the ability of 2,4-diethyl-9H-thioxanthen-9-one to be absorbed. Only water-soluble substances will readily dissolve into the gastrointestinal fluids and hence be available for absorption. The substance of interest though has a very low water solubility, i.e. < 0.66 mg/L. This data indicates that the substances is practically insoluble in water, and so it can be assumed that the solved amount of the substance in e.g. the upper GI tract will be very limited, clearly indicative for a poor absorption.
Data on the read-across substance ITX show that it is potentially susceptible to hydrolysis. However, half-lives increase with decreasing logPow values, i.e. 88.3 h at pH7 and 20°C, and 126.5 h at pH4 and 20°C. As the pH in the stomach can be assumed to be 1-2, an even prolonged half-life can be assumed. Hence, it is considered that no very high amounts of the degradation product are formed during the passage time of the substance, approx. 12-24h in the human body. Further, the substance is not readily biodegradable. So in general a degradation via gut bacteria can be assumed not to occur. Neither bacterial metabolites nor hydrolysis products need to be regarded when assessing the absorption of DETX, and it is sufficient to focus on the substance itself only. Also, a possible metabolism does only have to be regarded after absorption already has occurred. 2,4-diethyl-9H-thioxanthen-9-one is non-toxic, i.e. the available acute 50% lethal doses are above the classification limit of 2000 mg/kg bw, i.e. > 2000 mg/kg bw in rats, there was no death and no overt clinical signs following treatment, necropsy on Day 8 revealed no macroscopic changes, so the actually determined values are LD0 values of ≥ 2000 mg/kg. Taking into account the subchronic oral toxicity data, at least some information can be taken to regard that absorption occurs; However, the NOAEL of 1000 mg/kg bw/d furthermore indicates either a relative non-toxicity of the substance or its poor absorption.
Based on the available data, an absorption rate of 50% or less can be estimated, predominantly due to the insolubility of the substance in water and the high logPow.

Absorption from the respiratory tract
Concerning absorption in the respiratory tract, any gas, vapour or other substances inhaled as respirable dust (i.e. particle size ≤ 15µm) has to be sufficiently lipophilic to cross the alveolar and capillary membranes (moderate Log Pow values between 0-4 are favourable for absorption). The rate of systemic uptake of very hydrophilic gases or vapours may be limited by the rate at which they partition out of the aqueous fluids (mucus) lining the respiratory tract and into the blood. Such substances may be transported out of the lungs with the mucus and swallowed or pass across the respiratory epithelium via aqueous membrane pores. Lipophilic substances (Log Pow >0) have the potential to be absorbed directly across the respiratory tract epithelium. Any lipophilic compound may be taken up by micellular solubilisation but this mechanism may be of particular importance for highly lipophilic compounds (Log Pow >4), particularly those that are poorly soluble in water (1 mg/L or less) that would otherwise be poorly absorbed [ECHA, 2008].
2,4-diethyl-9H-thioxanthen-9-one has a very low vapour pressure (0.000103 Pa) and boils at 423.7°C, clearly showing that the inhalative absorption as a gas does not have to be regarded.
According to its particle size distribution, 25% of the particles are <100µm, of which the mayor part is only inhalable, but does not reach the thoracic or alveolar region: 22.3% are 10-100 µm (inhalable), 2.7% are 4-10 µm (thoracic), and 0.13% <4 µm (respirable). Those boundary values are taken from the recent version of ECHA’s Guidance document [ECHA, 2015], the former version gave the following information: In humans, particles with aerodynamic diameters below 100 μm have the potential to be inhaled. Particles with aerodynamic diameters below 50 μm may reach the thoracic region and those below 15 μm the alveolar region of the respiratory tract [ECHA, 2008]. So, based on the particle size distribution, max. 25% of the applied dose would reach the body via inhalation, all other particles are too large to be bioavailable. Further, most of the amount of particles are subject to nasal clearance, and only a minor part to tracheobronchial clearance, and further only a negligible amount (0.13%) of the particles are able to reach the alveoli. As a consequence, the fraction potentially reaching the alveolar region of the respiratory tract beyond the bronchi, where no mechanical excretion mechanism as the ciliary movements is available, is negligible. So, the potential absorption needs mostly be regarded theoretically.
For absorption of deposited material similar criteria as for GI absorption can be applied. In general, either a prolonged exposure due to deposition and subsequent absorption or immediate absorption by micellular solubilisation has to be assumed. The latter mechanism may be of particular importance for highly lipophilic compounds (LogPow >4), particularly those that are poorly soluble in water (1 mg/l or less) and is hence the only relevant here. To be readily soluble in blood, a gas, vapour or dust must be soluble in water and increasing water solubility would increase the amount absorbed per breath. However, the gas, vapour or dust must also be sufficiently lipophilic to cross the alveolar and capillary membranes. Therefore, a moderate log P value (between -1 and 4) would be favourable for absorption. Generally, liquids, solids in solution and water-soluble dusts would readily diffuse/dissolve into the mucus lining the respiratory tract. Hence, with a logPow of >6.5 and a virtually non-existing water solubility, the absorption of this fraction which may not be subjected to ciliary clearance can be considered as very limited by the rate at which the substance partitions out of the fluid in the lung surface.
Hence, as only, a very minor, if any, portion of the particles may reach the part of the lungs where no clearance mechanisms such a ciliary movements exist and a prolonged high exposure can be excluded, and further, micellular solubilisation is the only reasonable uptake mechanism, the inhalative absorption of the substance can be considered as minor. In theory, if absorption was not limited by the particle size, a similar absorption rate as via the oral route, i.e. 50%, could be assumed. Due to the physical limitations however, an inhalative absorption of approx. 10% can be assumed as a worst case.
Due to ciliary movements in the upper respiratory tract, the secretion of possibly unsolved, deposed particles with a subsequent swallowing and hence oral exposure after inhalation has to be additionally regarded.

Absorption after dermal exposure
In order to cross the skin, a compound must first penetrate into the stratum corneum and may subsequently reach the epidermis, the dermis and the vascular network. The stratum corneum provides its greatest barrier function against hydrophilic compounds, whereas the epidermis is most resistant to penetration by highly lipophilic compounds. Substances with a molecular weight below 100 are favourable for penetration through the skin and substances above 500 are normally not able to penetrate. The substance must be sufficiently soluble in water to partition from the stratum corneum into the epidermis. Therefore, if the water solubility is below 1 mg/L, dermal uptake is likely to be low. Additionally, Log Pow values between 1 and 4 favour dermal absorption.
Above 4, the rate of penetration may be limited by the rate of transfer between the stratum corneum and the epidermis, but uptake into the stratum corneum will be high. Above 6, the rate of transfer between the stratum corneum and the epidermis will be slow and will limit absorption across the skin. Uptake into the stratum corneum itself may be slow. Moreover vapours of substances with vapour pressures below 100 Pa are likely to be well absorbed and the amount absorbed dermally is most likely more than 10% and less than 100 % of the amount that would be absorbed by inhalation. If the substance is a skin irritant or corrosive, damage to the skin surface may enhance penetration. During the whole absorption process into the skin, the compound can be subject to biotransformation.
In case of 2,4-diethyl-9H-thioxanthen-9-one, an evaporation after skin contact does not need to be regarded due to the high boiling point and low vapour pressure, and hence it can be assumed that the substance will remain on the skin until mechanical removal. Furthermore, since the substance is not a skin irritant, additional absorption-enhancing effects can be disregarded, too.
With a molecular weight of 268.37 g/mol in theory a high absorption via the skin and hence a default dermal absorption rate of 100% [ECHA, 2008] could be assumed.
However, a log Pow of >6.5 and the fact that the substance is virtually insoluble in water indicate the opposite and are considered to be more important and so, dermal uptake is likely to be low. In addition, dry particulates will have to dissolve into the surface moisture of the skin before uptake can begin [ECHA, 2008]. Therefore, a limited dermal absorption rate can be assumed, most likely about 10%.
Details on distribution in tissues:
In general, it can be stated that the smaller the molecule, the wider is its distribution. A lipophilic molecule (Log Pow >0) is likely to distribute into cells and the intracellular concentration may be higher than extracellular concentration particularly in fatty tissues. It is not possible to foresee protein binding, which can limit the amount of a substance available for distribution. ToxTree modelling revealed protein binding alerts, both a Michael acceptor and for nucleophilic aliphatic substitution, but the extent however is unknown. Furthermore, if a substance undergoes extensive first-pass metabolism, predictions made on the basis of the physico-chemical characteristics of the parent substance may not be applicable.
In case of 2,4-diethyl-9H-thioxanthen-9-one, no quantitative data is available for distribution patterns. Taking into account its medium molecular weight of 268.37 g/mol, its lipophilicity and poor water solubility, the absolute systemic bioavailability is rather low and expected to be more extensive in fat tissues than in other tissues, especially when absorption occurred while bypassing the liver.
After oral exposure, the first target will be the gastrointestinal tract, where the substance and possibly, but here less likely bacterial metabolites will be absorbed in small quantities and transferred via the blood stream to the liver, which may be however limited due to the insolubility in water. However, adaptive affection of the liver as shown in the available OECD 407 study indicate a certain absorption and distribution especially into the liver. After first pass metabolism (see 3.4), the substance will be further distributed via the bloodstream. Here, especially the kidneys due to their filter function and the heart due to its enormous need for nutrients and consequently large blood flow through coronary arteries will be affected. In general, the estimated metabolites are max. slightly more hydrophilic due to the hydroxyl groups, which is however expected to be of minor importance given the very high logPow of the parent compound, influences are considered to be only minor. Since the solubility and hence absorption via the GI tract of the parent compound is rather limited, an excessive formation of metabolites is unlikely and so is a peak exposure to the metabolites. After absorption of the parent compound via other routes, only a subsequent metabolism has to be taken into account, also leading to no relevant peak exposure.
Additionally, accumulation of the parent compound in all directly and indirectly (general supply via bloodstream) involved organs has to be considered. Furthermore, the affection of the lymphatic system via micellar uptake must also be taken into consideration.
Details on excretion:
In general, the major routes of excretion for substances from the systemic circulation are the urine and/or the faeces (via bile and directly from the gastrointestinal mucosa). For non-polar volatile substances and metabolites exhaled air is an important route of excretion. Substances that are excreted favourable in the urine tend to be water-soluble and of low molecular weight (below 300 in the rat) and be ionized at the pH of urine. Most will have been filtered out of the blood by the kidneys, though a small amount may enter the urine directly by passive diffusion and there is the potential for reabsorption into the systemic circulation across the tubular epithelium. Substances that are excreted in the bile tend to be amphipathic (containing both polar and nonpolar regions), hydrophobic/strongly polar and have higher molecular weights and pass through the intestines before they are excreted in the faeces and as a result may undergo enterohepatic recycling which will prolong their biological half-life. This is particularly a problem for conjugated molecules that are hydrolysed by gastrointestinal bacteria to form smaller more lipid soluble molecules that can then be reabsorbed from the GI tract. Those substances less likely to recirculate are substances having strong polarity and high molecular weight of their own accord. Other substances excreted in the faeces are those that have diffused out of the systemic circulation into the GIT directly, substances which have been removed from the gastrointestinal mucosa by efflux mechanisms and non-absorbed substances that have been ingested or inhaled and subsequently swallowed. Non-ionized and lipid soluble molecules may be excreted in the saliva (where they may be swallowed again) or in the sweat. Highly lipophilic substances that have penetrated the stratum corneum but not penetrated the viable epidermis may be sloughed off with or without metabolism with skin cells.
For 2,4-diethyl-9H-thioxanthen-9-one no data is available regarding its elimination. Concerning the above mentioned behaviour predicted for its metabolic fate, it is unlikely that the parent substance will be excreted unchanged. However, if unchanged excretion is assumed, based on the chemical structure of 2,4-diethyl-9H-thioxanthen-9-one, its molecular weight (close to 300) and its non-existent water solubility, it is less likely to be excreted via the urine. The excretion, if any, of the parent compound will occur via the gastrointestinal tract (unabsorbed material) and the bile (small amounts of unchanged compound), and it could be subject to enterohepatic recycling. The same considerations apply to
Regarding the metabolites of the substance, a elimination of the Phase 2 conjugated metabolites, via the bile is expected.

Metabolite characterisation studies

Metabolites identified:
Details on metabolites:
For details, see attached file. Metabolites of the substance are estimated to be formed via S-oxidation, aliphatic hydroxylation and aromatic hydroxylation.

Any other information on results incl. tables

See attached expert statement.

Applicant's summary and conclusion

The present expert statement covers all relevant toxicokinetic parameters to assess the behaviour of DETX in the body, the available information is sufficient to enable one to perform a proper risk assessment. Hence, no further information needs to be gathered and further studies can be omitted due to animal welfare. In conclusion, the substance has no potential for bioaccumulation in its non-metabolized or metabolized form.
Executive summary:

In order to assess the toxicokinetic behaviour of 2,4-diethyl-9H-thioxanthen-9-one, the available toxicological and physico-chemical data were evaluated.

The substance is expected to be moderately absorbed via the oral route. An oral absorption rate of 50% or less was estimated due to its very low water solubility and high logPow. Only 25% of the particles could be inhaled, only 0.13% reach the alveoli where no mechanical excretion mechanisms take place, so the inhalation of particles practically does not have to be regarded, however in theory it has a potential for absorption via respiratory tract epithelium by micellular solubilisation. As a consequence, due to physical limitations, an inhalative absorption rate of 10% can be estimated. Taking into account its poor water solubility, a diminished dermal absorption rate can be assumed, approx. 10%.

So in summary, the absorption rates may be estimated to:

-      Absorption via oral route: 50%

-      Absorption via inhalative route: 10%

-      Absorption via dermal route: 10%

The absolute systemic bioavailability is rather low and expected to be more extensive in fat tissues than in other tissues. The estimated Cytochrome P450 metabolites of the substance are formed via S-oxidation, aliphatic hydroxylation and aromatic hydroxylation, and need to be regarded, too, as either a first-pass metabolism or a metabolism by other tissues is expected to occur. The introduction of single hydroxyl groups into the otherwise unchanged molecule, is not expected to alter its ADME behaviour substantially given the magnitude of its logPow.

Due to the high logPow and the insolubility in water of 2,4-diethyl-9H-thioxanthen-9-one, a certain potential for accumulation in the body is indicated. Nevertheless, a prolonged, extensive accumulation is not expected.

It is unlikely that the parent substance will be excreted unchanged. However, if unchanged excretion is assumed, it is unlikely to be excreted via the urine. The excretion, if any, of the parent compound will occur via the gastrointestinal tract (unabsorbed material) and the bile (small amounts of unchanged compound), and it could be subject to enterohepatic recycling. Regarding the metabolites of 2,4-diethyl-9H-thioxanthen-9-one, a elimination of the Phase 2 conjugated metabolites, via the bile is expected.

In conclusion, 2,4-diethyl-9H-thioxanthen-9-one has a not very high potential for bioaccumulation, and will be excreted after metabolism.