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Toxicological information

Basic toxicokinetics

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Administrative data

Endpoint:
basic toxicokinetics
Type of information:
other: Expert statement
Adequacy of study:
key study
Study period:
2013
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: see 'Remark'
Remarks:
An extensive assessment of the toxicological behaviour of 1,3,4-Thiadiazolidine-2,5-dithione, reaction products with hydrogen peroxide and tert-nonanethiol was performed, taking into account the chemical structure, the available physico-chemical-data and the available (eco-)toxicological data.

Data source

Reference
Reference Type:
study report
Title:
Unnamed
Year:
2013
Report Date:
2013

Materials and methods

Test guideline
Qualifier:
no guideline required

Test material

Reference
Name:
Unnamed
Type:
Constituent
Details on test material:
not applicable
Radiolabelling:
other: not applicable in this expert statement

Test animals

Species:
other: not applicable
Strain:
other: not applicable
Details on test animals and environmental conditions:
not applicable

Administration / exposure

Route of administration:
other: all routes of administration are discussed in the expert statement
Vehicle:
other: not applicable
Details on exposure:
all routes of administration are discussed in the expert statement
Details on study design:
not applicable
Details on dosing and sampling:
not applicable

Results and discussion

Toxicokinetic / pharmacokinetic studies

Details on absorption:
Based on physico-chemical properties which are not suggestive for a significant absorption, absorption of the substance into the body is expected to be low via all exposure routes.
Details on distribution in tissues:
For the target substance, even though the very high LogPow would indicate the possibility to reach the intracellular compartment, this seems to be unlikely as the molecular weight of the unmetabolised substance is rather high and the absorption of unchanged substance is considered to be very limited. Therefore, the distribution is also expected to be limited.
Details on excretion:
For the target substance no data is available concerning its elimination. Concerning the fate of the metabolites formed, the metabolites should be eliminated mainly via the urine.

Metabolite characterisation studies

Details on metabolites:
If absorbed, the substance is expected to be metabolised by Phase I drugs metabolising enzymes with followed conjugation by Phase II reactions. Thioester derivatives and hydroxylated derivatives at C atoms of the hydrocarbon chain are primary metabolites expected.

Any other information on results incl. tables

Background

There is data available on the physico-chemical properties of 1,3,4-Thiadiazolidine-2,5-dithione, reaction products with hydrogen peroxide and tert-nonanethiol. The substance is a viscous, liquid with a pungent odour (MW of representative structure = 466.84 g/mol, Afton, 2012). It has been observed that the substance has its pour point at -3 +/- 3°C (Fox, J.M., 2012). The substance was found to decompose at approximately 223°C. Due to this decomposition, no boiling point value could be determined (Fox, J.M., 2012) Moreover, its vapour pressure was determined to be 0.0021 Pa at 25°C (Atwal, 2012) and its density was reported to be 1.12 (Fox, J.M., 2012). The substance is insoluble in water (< 1.0 x 10-4g/L) and the LogPow was determined as > 9.4 (Fox, 2012). Moreover, the substance was reported to hydrolyse at pH 7, but not at pH 4 or 9 (Rocchio, 1990). For the substance itself data on toxicity is available. 1,3,4-Thiadiazolidine-2,5-dithione, reaction products with hydrogen peroxide and tert-nonanethiol was shown not to be acutely toxic, the oral LD50 being above 10,000 mg/kg bw, the dermal LD50 being greater than 2000 mg/kg bw and the inhalation LC50 being above 2.75 mg/L air (Reckers, 1981a,b, Findlay, 1981). It has been shown to be neither a skin nor an eye irritant (Reckers, 1982, Reckers, 1981c). 1,3,4-Thiadiazolidine-2,5-dithione, reaction products with hydrogen peroxide and tert-nonanethiol was shown not to bear a potential to cause allergic reactions (Rodabaugh, 2006; Smedley, 2003). Repeated oral exposures (28-day study) revealed a NOAEL of 200 mg/kg bw (discounting all non-adverse effects at this dose level as well as kidney effects in male rats which are species-specific and are not relevant to humans, Ullman, 1989). Similar NOAEL (250 mg/kg bw) was established for systemic effects in the Oral (gavage) reproduction/developmental toxicity screening test in the rat (OECD 421, Harlan Laboratories, 2013). Concerning gene mutation, it was shown not to bear mutagenic potential (Timm, 1989) and not to be clastogenic (Heidemann, 1989).

Absorption

In general, absorption of a chemical is possible, if the substance crosses biological membranes. 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.

The molecular weight of the representative structure of 1,3,4-Thiadiazolidine-2,5-dithione, reaction products with hydrogen peroxide and tert-nonanethiolis at the upper frontier and considered not to be favourable for absorption (molecular weight = 466.86 g/mol), and due to its water insolubility (< 1.0 x 10-4g/L) and its logPow (> 9.4). Such lipophilic water insoluble substances are hindered from being absorbed because their dissolution in the gastrointestinal fluids is impaired. On the other hand, any lipophilic compound may be taken up by micellular solubilisation with the help of bile salts. The substance does not bear a significant surface activity; therefore an enhancement of absorption is not likely. Studies which show that it is not irritating to skin or eyes help confirm that no further enhancement of absorption is applicable. This hypothesis is supported by the reported LD50 and LC50 values, which confirm a poor absorption:

The above mentioned properties determine the absorption of 1,3,4-Thiadiazolidine-2,5-dithione, reaction products with hydrogen peroxide and tert-nonanethiol to be rather limited, based on the absorption-hindering properties (water insolubility and high LogPow) and the observed effects in toxicological experiments.

Absorption from the gastrointestinal tract

Regarding oral absorption, in the stomach, a substance will most likely be hydrolysed, because this is a favoured reaction in the acidic environment of the stomach.1,3,4-Thiadiazolidine-2,5-dithione, reaction products with hydrogen peroxide and tert-nonanethiolis expected to hydrolyse only at pH7. Accordingly, it is not very likely for1,3,4-Thiadiazolidine-2,5-dithione, reaction products with hydrogen peroxide and tert-nonanethiolto be hydrolysed in the acidic environment of the stomach, where a pH value of approximately 2 – 5 is expected. However, in the small intestine (with pH values of 5-8), hydrolysis might occur. This will probably be based on a hydrolysis of the disulphide-bridges, which leads to the formation of thio-alkanes and the heterocyclic remainder. These, however, are all ionisable structures (the sulphur or the nitrogen atoms) - as the parent compound - and ionisation would hinder absorption.

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 (LogPow 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.

The available data suggest that orally administered1,3,4-Thiadiazolidine-2,5-dithione, reaction products with hydrogen peroxide and tert-nonanethiolwill be absorbed unchanged only to a limited extent, even though micellular solubilisation, pinocytosis and persorption can not be ruled out. This thesis is supported by the LD50 value > 10,000 mg/kg bw available, which shows that the substance is not acutely toxic after oral exposure (also see above). The metabolites of hydrolysis will most likely not be absorbed to a greater extent, as they are ionisable, which hinders absorption.

Absorption from the respiratory tract

Concerning absorption in the respiratory tract, any gas or vapour has to be sufficiently lipophilic to cross the alveolar and capillary membranes (moderate LogPow values between 0-4 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 (LogPow >0) have the potential to be absorbed directly across the respiratory tract epithelium. Very hydrophilic substances can be absorbed through aqueous pores (for substances with molecular weights below and around 200) or be retained in the mucus.

The substance of interest is a non-volatile substance and hasa rather low vapour pressure (0.0021 Pa at 25°C), which indicates only low availability for inhalation. The molecular weight of 466.86 g/mol and the very high LogPow also indicate no possibility for absorption through aqueous pores. Based on this data and even though the LogPow value above 0 indicates the potential for absorption directly across the respiratory tract epithelium (which is unlikely as the substance is ionisable), it can be expected that1,3,4-Thiadiazolidine-2,5-dithione, reaction products with hydrogen peroxide and tert-nonanethiolis marginally available in the air for inhalation and any inhaled substance is expected not to be absorbed.

Absorption following 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 of 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, LogPow values between 1 and 4 favour dermal absorption (values between 2 and 3 are optimal; TGD, Part I, Appendix IV). 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 too. 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% of the amount that would be absorbed by inhalation (TGD, Part I, Appendix IV, 2003). 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 1,3,4-Thiadiazolidine-2,5-dithione, reaction products with hydrogen peroxide and tert-nonanethiol, the molecular weight is around 500, which indicates already a low potential to penetrate the skin. This is accompanied by a very low hydrophilicity of the substance and even though the stratum corneum is open for lipophilic substances, the epidermis is very resistant against penetration by highly lipophilic substances. In addition, the amount of1,3,4-Thiadiazolidine-2,5-dithione, reaction products with hydrogen peroxide and tert-nonanethiol,which is absorbed following dermal exposure into the stratum corneum is unlikely to be transferred into the epidermis. 1,3,4-Thiadiazolidine-2,5-dithione, reaction products with hydrogen peroxide and tert-nonanethiol is not irritating to skin and eyes, and therefore this does not enhance dermal absorption. The vapour pressure of the substance is very low (0.0021 Pa) and therefore it will not partition into the air. Thus, no skin exposure is expected by airborne concentrations of the target substance.

In support of this hypothesis (the very low dermal absorption), the systemic toxicity via the skin is low (acute dermal toxicity, LD50 value of > 2000).

In conclusion, the evaluation of all the available indicators and the results of toxicity studies allow the allocation of the chemical in question into the group of chemicals with a low dermal absorption. In detail, due to it’s molecular weight and the results for acute toxicity, the use of a factor of 10 % for the estimation of dermal uptake for 1,3,4-Thiadiazolidine-2,5-dithione, reaction products with hydrogen peroxide and tert-nonanethiol is justified(Schuhmacher –Wolz et al.,2003; TGD, Part I, 2003).

Distribution

In general, the following principle applies: the smaller the molecule, the wider the distribution. A lipophilic molecule (LogPow >0) is likely to distribute into cells and the intracellular concentration may be higher than extracellular concentration particularly in fatty tissues. If a substance binds to proteins, it will limit the amount of a substance available for distribution. 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 of1,3,4-Thiadiazolidine-2,5-dithione, reaction products with hydrogen peroxide and tert-nonanethiol, no data is available for distribution patterns. The substance might bind to proteins due to the presence of disulphide moieties in the molecule (see section 2.4.3). It can limit the distribution.Even though the very high LogPow would indicate the possibility to reach the intracellular compartment, this seems to be unlikely as the molecular weight of the un-metabolised substance is at the upper limit which is favourable for absorption(500 g/mol versus 466.86 g/mol, respectively)and therefore absorption of unchanged substance is considered to be very limited. Therefore, the distribution is also expected to be limited.

Accumulation

It is also important to consider the potential for a substance to accumulate or to be retained within the body. Lipophilic substances have the potential to accumulate within the body (mainly in the adipose tissue), if the dosing interval is shorter than 4 times the whole body half-life. Although there is no direct correlation between the lipophilicity of a substance and its biological half-life, substances with high LogPow values tend to have longer half-lives. On this basis, there is the potential for highly lipophilic substances (LogPow >4) to accumulate in biota which are frequently exposed. Highly lipophilic substances (LogPow between 4 and 6) that come into contact with the skin can readily penetrate the lipid rich stratum corneum but are not well absorbed systemically. Although they may persist in the stratum corneum, they will eventually be cleared as the stratum corneum is sloughed off. A turnover time of 12 days has been quoted for skin epithelial cells.

Accordingly, the high LogPow and the predicted behaviour concerning absorption and metabolism of1,3,4-Thiadiazolidine-2,5-dithione, reaction products with hydrogen peroxide and tert-nonanethiolindicate a potential for accumulation in the body. This, however, is limited as the absorption is expected to be low via all route of exposure and because metabolism of 1,3,4-Thiadiazolidine-2,5-dithione, reaction products with hydrogen peroxide and tert-nonanethiol is expected to influence this initial prediction.

Metabolism

Route specific toxicity results from several phenomena, such as hydrolysis within the gastrointestinal or respiratory tracts, also metabolism by gastrointestinal flora or within the gastrointestinal tract epithelia (mainly in the small intestine), respiratory tract epithelia (sites include the nasal cavity, tracheo-bronchial mucosa [Clara cells] and alveoli [type 2 cells]) and skin. As specified above,hydrolysis does apply for 1,3,4-Thiadiazolidine-2,5-dithione, reaction products with hydrogen peroxide and tert-nonanethiol. The substance will partly hydrolyse most likely in the small intestines and the resulting thioalkanes and the heterocyclic ring will be subject to further metabolism, as described below. However, its metabolism is very likely to occur via the Cytochrome P450 group of metabolising enzymes, as it has been predicted with the TOXTREE modelling tool (Chemservice, S.A., 2013a) and with the OECD QSAR Toolbox (v3.0).

According to the TOXTREE modelling results, 1,3,4-Thiadiazolidine-2,5-dithione, reaction products with hydrogen peroxide and tert-nonanethiol, containing the structural alerts: disulphide, sulfenic acid derivative and heterocyclic compound (Class 1: At least one functional group), is expected to be well metabolized by the Cytochrome P450 group of metabolizing enzymes. The primary, secondary and tertiary sites of metabolism are the sulphur atoms, which are predicted to be subject to S-oxidation, which can take place by hydrolytic cleavage. Moreover, sites of metabolism are the pre-terminal carbon-atoms of the chain, which are predicted to be subject to aliphatic hydroxylation. In detail, the S-oxidation of the disulphide-bond will most likely lead firstly to a formation of thioalkanes, which are in a second step oxidised to sulphenic acid groups and as this is expected to be unstable, will break down in the following to thiol- and sulphinic acid groups. The chief end-product of other disulphide-containing agents (like cysteine) is sulphate (via oxidation of sulphite) and this is also expected to apply for the target substance. In addition, it is possible (before or after the S-oxidation of the disulphide-bonds), that the long carbon chains are subject to initial omega- and then successive beta-oxidation. The heterocyclic ring will probably react via metabolic reduction of the nitrogen-bridge, which leads to 2 amino-groups. This is most likely followed by desamination-, desulfonation-processes and S-oxidation. However, it is also possible that the newly build amino-groups are subject to N-acetylation.

The expected metabolic pathways deduced from the TOXTREE prediction were confirmed by the OECD QSAR Toolbox (v.3.0). Using the metabolite simulators (microbial, hydrolysis, rat liver S9 and skin metabolism simulator) included in the software, metabolites were predicted for the target substance.The substance is expected to be metabolised predominantly by microorganisms (128 metabolites). Entering the body, it is likely to undergo hydrolysis at diazolidine moiety releasing thioester derivatives and hydrazine. In small intestines, newly formed thioester derivatives can undergo enzymatic cleavage by Phase I and Phase II drugs metabolising enzyme reactions, leading to increasing of hydrophilicity facilitating the excretion. Reduction of disulphide bridges is considered to be primary reactions of Phase I reactions leading to forming either thiol derivatives or sulfonic acid derivatives. Concomitantly, oxidation of sulphur atom in diazolidyne moiety is likely to occur. The predicted metabolites by skin metabolism simulator are products of hydroxylation reactions of the tail chains at the terminal position and the oxidation of the sulphur atom in the diazolidyne moiety. The metabolites can already be excreted via the urine or can react in phase 2 of the biotransformation with different molecules, leading to the formation of conjugations. This might be necessary for the parent compound, as its water solubility is fairly low and it cannot be eliminated via the urine without further metabolism.

The possibility of protein binding of the metabolites (i.e. the thiol-group) of the target substance cannot be ruled out. It was suggested by the OECD QSAR Tollbox. According to the general mechanistic profiling methods included in the software, the target substance is a discrete chemical and possesses two functional groups: disulphide and thiadiazol which determine the toxicological behaviour of the chemical. According to the profiling rules, chemicals which have disulphide functional group in their structures can undergo adduct formation with proteins by SN2 mechanism. The protein conjugation can take place via protein thiol-disulfide interchange. The target substance can interact with thiol groups of cysteine via the protein thiol-disulfide exchange interactions, causing cysteine depletion. However, interaction between lysine residues of proteins with the sulphur atom in the disulphide bond of the chemical of interest is unfavourable and can hardly occur. In these respect low degree of reactivity towards Lysine peptide could be expected.According to the endpoint specific profiling, disulphide moieties in the molecule are identified as the structural alerts for protein binding which can cause skin sensitization. Namely, the target substance is expected to react with protein amino or thiol groups via addition-elimination process or via SN2 mechanism (protein thiol-disulfide interchange) leading to hapten formation and therewith inducing allergic contact dermatitis (explanation of profiling results is given in the table below). However, the skin sensitization potential of the target substance has not been proven in two reliable skin sensitization studies that were both Buehler tests (OECD 406, Rodabaugh, 2006, Smedley, 2003). The rules of the general mechanistic profilers are based on the structural similarity only, while the rules of the endpoint specific profilers are based on the statistical frequency of occurrence of an effect in reality (= on experimental data). The both profiling methods provide only a direction of expected property (toxicity) and cannot predict an outcome in an assay/test. They rather overpredict an effect. There are cascades of cellular events and pathways which are responsible for the sensitization response of a chemical. Therefore, the only structural similarity can be used for a prediction on a limited basis. In conclusion, it is likely that the substance of interest will be subject to metabolism by cytochrome P450 enzymes, by omega- and beta-oxidation, via S-oxidation, metabolic reduction, desulfonation and possibly deamination andN-acetylation. The parent substance or metabolites without strecally hindrance of disulphide bridges can bind to proteins leading to adduct formation.

Excretion

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 gastrointestinal tract 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.

For1,3,4-Thiadiazolidine-2,5-dithione, reaction products with hydrogen peroxide and tert-nonanethiolno data is available concerning its elimination. Concerning the fate of the metabolites formed of1,3,4-Thiadiazolidine-2,5-dithione, reaction products with hydrogen peroxide and tert-nonanethiol, the metabolitesshould be eliminated mainly via the urine.

Applicant's summary and conclusion

Conclusions:
Interpretation of results (migrated information): low bioaccumulation potential based on study results
An extensive assessment of the toxicological behaviour of 1,3,4-Thiadiazolidine-2,5-dithione, reaction products with hydrogen peroxide and tert-nonanethiol was performed (expert statement), taking into account the chemical structure, the available physico-chemical-data and the available (eco-)toxicological data.
Executive summary:

In order to assess the toxicological behaviour of 1,3,4-Thiadiazolidine-2,5-dithione, reaction products with hydrogen peroxide and tert-nonanethiol,the available physico-chemical and toxicological data have been evaluated. The substance is expected to be absorbed to a limited extent after oral exposure, based on its rather high molecular weight (466.86 g/mol, which is at the upper limit favourable for absorption), its low water solubility and its high LogPow. Concerning the absorption after exposure via inhalation, as the chemical is considered to have a fairly low vapour pressure, is highly lipophilic, has a high LogPow, and has a rather high molecular weight, it is clear, that the substance is poorly available for inhalation and will not be absorbed significantly. The substance is also not expected to be absorbed following dermal exposure into the epidermis, due to its low water solubility and its fairly high molecular weight. Concerning its distribution throughout the body, the target substanceis not expected to be distributed extensively due to the low absorption. If absorbed, it is likely to distribute into the both intravasal and intracellular compartments.

The substance does indicate a significant potential for accumulation based on the high logPow (9.4), this, however, is limited as the absorption is expected to be lowvia all routes of exposure. Moreover,metabolism of the substance is expected to influence this initial prediction. The substance is expected to hydrolyse under neutral conditions (i.e. small intestines) and be extensively metabolisedby cytochrome P450 enzymes, by omega- and beta-oxidation, via S-oxidation, metabolic reduction, desulfonation and possibly deamination andN-acetylation andto be eliminated mainly via the urine.