Registration Dossier

Administrative data

Link to relevant study record(s)

Description of key information

Short description of key information on bioaccumulation potential result: 
1). Read Across Statement, Chemservice S.A., 2011
2). Read across from TDI, basic toxikokinetics, Review Doe et al.,1995
3). Read across from TDI, basic toxicokinetics, Fischer 344 rats, oral administration or inhalation of vapour, Timchalk, 1994
4). Read across from TDI, basic toxicokinetics, Sprague Dawley rats, oral administration of TDI, Kennedy, 1994
5). Read across, basic toxicokinetics, Hartley guinea pigs, inhalation of vapours, Kennedy, 1989
6). Prediction using TOXTREE (v.2.1.0), Chemservice S.A. 2011
Short description of key information on absorption rate:
1). Read across from TDI, dermal absorption, Spraque Dawley rats, dermal application, semiocclusive, Yeh, 2008

Key value for chemical safety assessment

Bioaccumulation potential:
low bioaccumulation potential
Absorption rate - oral (%):
100
Absorption rate - dermal (%):
10
Absorption rate - inhalation (%):
100

Additional information

Conclusion on toxicokinetical behaviour, metabolism, excretion and dermal penetration of TRIDI

TRIDI and TDI are both aromatic diisocyanates with different alkyl rests attached to benzene ring (three isopropyl groups in TRIDI and one methyl group in TDI). As a result of the structural differences, such physico-chemical properties as melting point, LogPow and water solubility differ significantly from each other. As no information on the toxicokinetic behaviour of TRIDI is available, the similar substance TDI was assessed as a possible read-across substance. Data revealed TDI to be an adequate read-across substance.

Taking into account the available information on the toxicokinetics of TDI and the detailed assessment of toxicokinetics of TRIDI, TRIDI is expected to be absorbed into the organism by all routes of exposure. However, absorption via the oral route is assumed to be low. Absorption after inhalation is considered to be rather fast. Dermal absorption, however will be limited by its molecular weight, its high lipophilicity and its high logPow. Due to high logPow, TRIDI is not expected to penetrate easily through the skin but tends to migrate towards fat tissues but certain reactivity of isocyanate groups with peptides and proteins might hinder accumulation. Inhalation and dermal routes can represent sensitising and irritating hazard for respiratory system. Nucleophilic substitution by SN2 mechanism with electron rich nucleophile amino acids of peptides and proteins is considered to be primary detoxifying mechanism of TRIDI. Excretion of TRIDI is expected via urine.

 

Discussion on bioaccumulation potential result:

There is no data on toxicokinetical behaviour of TRIDI. The data on its structural analogue TDI were taken into account to assess toxicokinetics of TRIDI. A detailed assessment of adsorption, distribution, metabolism pathways and excretion of target chemical was performed and is also attached in the section 13 "Assessment reports".

Toxicokinetics of TDI:

The substance Toluene diisocyanate (TDI) was subject of a detailed review of the existing toxicological information by Doe and coworkers (1995). The absorption, distribution, and kinetics of TDI are qualitatively and quantitatively different following inhalation exposure when compared with oral dosing, with inhalation being the relevant route in humans. Following oral dosing of TDI, dose-dependent percentages of the compound are converted to TDA (a mutagen and rodent carcinogen) by hydrolysis (mainly at aqueous tissue surfaces), which is consistent with the carcinogenicity observed following TDI gavage studies. The lower pH levels in i.e. the stomach are leading to high protonation of biological NH2 groups and this facilitates hydrolysis of TDI to TDA and subsequent formation of polyureas. These observations are consistent with comparative toxicokinetic studies in rats, which demonstrate significant levels of TDA following oral dosing with TDI - due to the acidic environment in the stomach - but not after inhalation.

Upon inhalation exposure TDI is conjugated to protein preferentially before formation of oligoureas or hydrolysis to TDA takes place. After inhalation a rather large portion (of 34% recovered radioactivity) is found in the carcass 48 hours after termination of exposure, indicating a slow release of protein-bound material, as suggested by in vitro data. Overall these conclusions are consistent with the lack of carcinogenicity observed in inhalation two-year studies with TDI and the tumours observed in rodents after oral dosing of TDI in corn oil. Additionally there are human exposure data indicating that the metabolism and kinetics in humans might be similar to that in animals, therefore valid extrapolations can be made. The conventional hazard assessment and resulting risk evaluation comes to the conclusion that there is no risk for carcinogenicity associated with the inhalation of TDI. Considering additional data (gavage studies with TDI or TDA and mechanistic biochemical data), three approaches of risk characterisation arrive at the conclusion that the highest possible risk associated with the inhalation of TDI at workplaces with TLV level exposure is about 5 x 10E-6. TDI inhalation at workplaces therefore will not present an unacceptable risk of carcinogenicity to man.

The substance Toluene diisocyanate was investigated for its toxicokinetic properties in Fischer 344 rats by Timchalk and coworkers (1994). The study was conducted similar to OECD TG417 with only minor deviations, why it was considered to be of high reliability (Klimisch 2). The rats received either 14C-radiolabelled TDI orally (60 mg/kg) or via inhalation (4 h, 2 ppm). The metabolism and excretion were determined also via HPLC and GC-MS.

The data suggest that orally administered 2,4-[14C]TDI is not very well absorbed. The rats eliminated approximately 8% of the radioactivity in the urine and cage wash while 4% were recovered in the tissues/carcass. Thus the minimum estimate for absorption was 12%, which assumed that the radioactivity recovered in the faeces (~ 81 %) represented un-absorbed material. A more realistic estimate for absorption was obtained by assuming that some of the radioactivity in the faeces was absorbed. In addition, orally administered TDI was reported to undergo rapid hydrolysis under aqueous conditions to form TDA which reacted with available isocyanate groups (TDI) to form polyureas. Under appropriate conditions 2,4-TDI readily hydrolyses to form 2,4-TDA. The 2,4-TDA can react with free 2,4-TDI forming polyurea polymers, which appear to be poorly absorbed from the gastrointestinal tract. Absorbed 2,4-TDA can be excreted in the urine either unchanged or as acid-labile conjugates. Additionally, 2,4-TDA can be N-acetylated forming mono- and diacetylated TDA metabolites which are readily excreted in the urine. Based on the rapid reactivity it is doubtful that TDI was absorbed prior to its hydrolysis to TDA. Therefore, the 12-20% of the 2,4-[14C]TDI dose that was absorbed, most probably represented 2,4-[14C]TDA that had not reacted to form polyureas. Based on the above, it was assumed that the radioactivity that was absorbed and excreted in the urine following the 2,4-[14C]TDI oral dose was absorbed primarily as 2,4-[14C]TDA.

Rats which were exposed to 2,4-[14C]TDI vapours retained essentially all the radioactivity that they inhaled. Inhaled 2,4-[14C]TDI appeared to be retained by the rat, and the data suggest that a large percentage of the radioactivity was absorbed through the lungs into the blood. At 48 hr following inhalation exposure, the urine and cage wash accounted for 19% of the recovered radioactivity, the tissues/carcass accounted for 34%, and 47% was recovered in the faeces. However, assuming 100% retention of inhaled TDI and minimal excretion over the 4-hr exposure, the quantitation of radioactivity detected in the tissues/carcass immediately postexposure would suggest that as much as 90% of the radioactivity was absorbed. The remaining radioactivity may have been rapidly cleared from the respiratory tract and subsequently swallowed. The pulmonary clearance of radioactivity from the lung to the gastrointestinal tract was supported by the fact that approximately 10% of the recovered inhalation dose was detected in the gastrointestinal tract contents of rats immediately postexposure. These data suggest that between 61 and 90% of the inhaled TDI dose was absorbed and the remaining radioactivity was rapidly cleared from the respiratory tract, ingested, and then eliminated in the faeces. In conclusion, comparison of the total amount of radioactivity in the tissues/carcass of rats indicated that a larger fraction of the recovered radioactivity was in the tissues/carcass following the inhalation vs oral exposure to 2,4-TDI. These data suggest that following inhalation exposure, a large percentage of the 2,4-[14C]TDI was absorbed through the lungs and existed in a different form than what was absorbed following oral 2,4-TDI and/or 2,4-TDA doses. The urinary excretion of radioactivity by the kidneys was slower following inhalation exposure (t1\2 = 20 hr) when compared to the oral 2,4-[14C]TDI dose group (t1\2 = 7.5 hr), suggesting that inhaled 2,4-TDI was eliminated in the urine in a different form having a longer biological half-life than orally administered 2,4-TDI and/or 2,4-TDA. This suggests that in the rat, very little 2,4-TDA is formed following inhalation exposure to 2,4-[14C]TDI vapours. In addition 90% of the quantitated metabolites in the urine specimens following inhalation exposure to 2,4-[14C]TDI existed as acid-labile conjugates of TDI/TDA while only 10% existed as acetylated TDA. This indicated that following inhalation exposure, a larger percentage of the 2,4-[14C]TDI was excreted in the urine in a conjugated form and not as free or acetylated TDA.

Overall, these data suggest that the metabolic disposition and carcinogenic potential of 2,4-TDI are dependent upon the route of exposure. Oral administration enhances the hydrolysis of 2,4-TDI, forming 2,4-TDA which is readily absorbed, whereas inhalation exposure to 2,4-TDI primarily results in the formation of 2,4-TDI conjugates and only small amounts of acetylated 2,4-TDA are produced. These findings are consistent with the chronic bioassay data which indicated that 2,4-TDI was not carcinogenic following inhalation exposure, but did result in tumour formation following oral administration in corn oil. Considering that the primary route of occupational exposure to TDI is via the inhalation route, then the data would suggest that the carcinogenic potential of TDI is low.

The substance Toluene diisocyanate was investigated for its toxicokinetic properties by Kennedy and coworkers (1994) in Sprague Dawley rats. The study was conducted similar to OECD TG417 with only minor deviations, and is considered to be of high reliability (Klimisch 2). Rats were exposed to 14C-TDI vapours at concentrations ranging from 0.026 to 0.821 ppm for 4 h. The distribution was determined. All tissues examined showed detectable quantities of radioactivity, with the airways, gastrointestinal system and blood having the highest levels, which increased with exposure concentration. The concentration of radioactivity in the bloodstream after exposure was linear with respect to dose. The majority (74-87%) of the label associated with the blood was recovered in the plasma, and of this, 97-100% of the 14C existed in the form of biomolecular conjugates. Analysis of stomach contents shows that the majority of the label is also associated with high (>10 kDa) molecular weight species. While a larger percentage (28%) of the label is found in the low molecular weight fraction relative to blood, this low molecular weight labelled material represents at least eight different components. Thus, over the vapour exposure concentrations and time tested, it appears that conjugation is the predominant reaction and that free TDA is not a primary in vivo reaction product under the conditions tested.

The substance Toluene diisocyanate was investigated for its toxicokinetic properties by Kennedy and coworkers (1989) in Hartley guinea pigs. The study was conducted similar to OECD TG417 with only minor deviations, why it was considered to be of high reliability (Klimisch 2). The guinea pigs received 14C-radiolabelled TDI via inhalation (1 h, in certain cases also 4 or 5 hours, 0 to 0.146 ppm). Exposures to 14C TDI were performed over a range of relatively low concentrations, including levels at and below the TLV for TDI which has been established at 0.005 ppm (Amer. Conf. Gov. Ind. Hyg., 1988). The absorption, distribution and excretion were determined via scintillation analysis and also via the Marcali- and the PNBPA method. The Marcali, PNBPA, and radioactivity values yielded comparable exposure concentration results. This confirms that the exposure atmospheres contained reactive TDI at the desired levels, all animals received equivalent concentrations, and chamber atmospheres were maintained throughout the exposure. Analysis of the uptake and distribution of radioactivity in the TDI-exposed fluids and tissues showed that some form of the labelled compound whether TDI, a conjugate, metabolite, or hydrolysis product, entered and penetrated throughout the entire system, even at the 0.004 ppm level. The urine and bile profiles demonstrated the rapid penetration of some form of the radioactivity through the system since at all concentrations the highest level of radioactivity in the bile and urine was found immediately following exposure. The post exposure increase of blood radioactivity is presumably due to the processing or desorption of the compound from the sites of entry (i.e., nasal passage, conducting airways, and alveoli) into the bloodstream for clearance. This study shows that the rate of uptake into the blood is linear during exposure to concentrations ranging from 0.00005 to 0.146 ppm and that the uptake continues to increase slightly postexposure. It also demonstrates that the radioactivity clears from the bloodstream to a level corresponding to approximately a 100 nM concentration of tolyl group after 72 hr and persists at a nanomolar level even 2 weeks following the exposure. After 2 weeks of recovery the animals still had measurable levels of blood radioactivity at a nearly constant amount (8.3 X 10E-8 M), regardless of initial dose, which suggests the saturation of a particular target which as a reacted form does not have a rapid turnover rate. The initial rate of 14C uptake is also a linear function of the concentration of TDI when expressed either as concentration (ppm) or as concentration multiplied by duration of exposure (ppm * hr). This is discussed in comparison with the toxic responses as a function of both ppm and ppm * hr.

Prediction for TRIDI using TOXTREE

The chemical structure of 2,4,6 -triisopropyl-m-phenylene diisocyanate was assessed by Toxtree (v.2.1.0) modelling tool for possible metabolism. SMART Cyp is a prediction model, included in the tool, which identifies sites in a molecule that are labile for the metabolism by Cytochromes P450.

2,4,6 -triisopropyl-m-phenylene diisocyanate is expected to be well metabolized by the Cytochrome P450 group of metabolizing enzymes.The molecule possesses equal or more than three sites of metabolism. The primary and secondary sites of metabolism are the diisopropyl-groups, which are predicted to be subject to aliphatic hydroxylation. The tertiary sites of metabolism are the carbon-atoms of the aromatic ring, which are predicted to be subject to aromatic hydroxylations.

 

Prediction of the toxicokinetic behaviour of TRIDI (Read-Across statement):

There is little data available on physico-chemical properties of 2,4,6,-triisopropyl-m-phenylene diisocyanate. With the aid of the EPIWIN software some physico-chemical properties were calculated.

The substance is at room temperature a light yellowish liquid with a slight odour. The substance is insoluble in water (< 0.05 mg/L at 20°C) and has a logPow of 7.56. It has a low vapour pressure (0.19 Pa at 20°C). No exact value of melting point could be determined experimentally for TRIDI between -90°C and 50°C (Kintrup, 2012). Glass transition temperature (amorphous components) in the first heating run was determined to be at -56°C. The calculated melting point was 82.9°C. The boiling point of 305.8°C was measured for TRIDI (Svobodova, 2012). Hydrolysis as a function of pH has not been determined, but comparison to its structural analogue toluene diisocyanate revealed a high likelihood for hydrolysis. The substance is not toxic when administered orally to rats (LD50 > 2000 mg/kg bw). However, it has been determined to be toxic after inhalation (LC50 < 110mg/m³). It is not an eye, but a skin irritant, and skin sensitising properties have been predicted. Additionally, the substance was shown to be not mutagenic in studies according to OECD 471, 473 and 476.

Absorption:

In general, absorption of a chemical is possible, if the substance crosses biological membranes. The EU Technical Guidance Document on Risk Assessment (TGD, Part I, Appendix VI) gives a number of physico-chemical properties that normally determine oral, inhalation and dermal absorption (LINK to Guidance Document:http://ecb.jrc.ec.europa.eu/tgd/). 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). Firstly, TRIDI and TDI would be favourable for absorption, when only taking into account their molecular weights. However, as TRIDI is practically insoluble in water, it is apparent that its absorption is hindered. This is also seen in the value calculated for the LogPow (7.56) that shows the substance to be better soluble in octanol than in water. Considering its low water solubility and the value for LogPow calculated to be above 4, the absorption into the body will not be favoured (LogPow between 0 and 4 are favourable for absorption). In general, the absorption of chemicals, which are surfactants or irritants may be enhanced, because of damage to cell membranes. This is the case for both substances of interest.

 

Absorption from the gastrointestinal tract

Regarding oral absorption, in the stomach, a substance will most likely be hydrolysed, as this is a favoured reaction in the acidic environment of the stomach. In accordance with the above mentioned principles it has been reported for TDI to be hydrolysed in the stomach to toluene diamine (TDA). The lower pH levels in i.e. the stomach are leading to high protonation of biological NH2 groups and this facilitates hydrolysis of TDI to TDA and subsequent formation of polyureas

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 may 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.

The toxicological data available for both substances show, that the substances resemble each other in the endpoints: acute toxicity oral (LD50 > 2000 mg/kg bw for both substances) and skin irritation (both irritating). As the results for these endpoints are identical, it can be presumed that these substances have in principle the same mode of action. The available toxicokinetic data for TDI suggest that orally administered 2,4-TDI is not very well absorbed (Timchalk et al., 1994). The minimum estimate for absorption was 12%, which assumed that the radio-labelled substance was recovered in the faeces (~ 81 %), represented un-absorbed material. A more realistic estimate for absorption was obtained by assuming that some of the radioactivity in the faeces was absorbed. However, the hydrolysed TDI can react as 2.4-TDA with free 2,4-TDI forming polyurea polymers, which appear to be poorly absorbed from the gastrointestinal tract. Based on the rapid reactivity it is doubtful that TDI was absorbed prior to its hydrolysis to TDA. Therefore, the 12-20% of the 2,4-TDI that was absorbed, most probably represented 2,4-TDA that had not reacted to form polyureas.

Based on the abovementioned data for the closest analogue, it can be presumed that the absorption of TRIDI via the oral route will be slower than that of TDI, which will result in a lower toxicity of the substance (also by the long-term exposure).

 

Absorption from the respiratory tract

Regarding absorption in the respiratory tract, any gas or vapour has to be sufficiently lipophilic to cross the alveolar and capillary membranes (moderate Log P 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 may pass across the respiratory epithelium via aqueous membrane pores. Lipophilic substances (Log P >0) would have the potential to be absorbed directly across the respiratory tract epithelium. Very hydrophilic substances might be absorbed through aqueous pores (for substances with molecular weights below around 200) or be retained in the mucus.

Even though TRIDI has a relatively low vapour pressure (0.19 Pa) and a high boiling point (calculated 305.8°C), which would indicate a low availability for inhalation, it is known, that isocyanates bear a high potential for respiratory sensitisation and irritation. As isocyanates are highly reactive, irritating compounds it is clear, that contact to the epithelium will produce irritation and therefore enhance absorption. The toxicokinetic data available for TDI show that no relevant hydrolysis occurs in the respiratory tract, as TDA was barely detectable in the urine, following inhalation of TDI (Timchalk et al., 1994). Additionally, essentially all the radioactivity inhaled via 2.4-TDI vapours was retained (Timchalk et al., 1994). The data suggest that a large percentage of the radioactivity was absorbed through the lungs into the blood. The data suggest that between 61 and 90% of the inhaled 2,4-[14C]TDI dose was absorbed and the remaining radioactivity was rapidly cleared from the respiratory tract, ingested, and then eliminated in the faeces.

Kennedy and co-workers found in all tissues examined detectable quantities of radioactivity, with the airways, gastrointestinal system and blood having the highest levels which increased with exposure concentration (Kennedy et al., 1989 and 1994). The concentration of radioactivity in the bloodstream after exposure was linear with respect to dose. The results showed that greater than 95% of the plasma-associated radioactivity existed in the form of biomolecular conjugates (reaction of TDI with biological macromolecules successfully competes with hydrolysis to the diamine). Thus, over the vapour exposure concentrations and time tested, it appears that conjugation (mainly with serum-albumin) is the predominant reaction and that free TDA is not a primary in vivo reaction product under the conditions tested.

Based on this data, it can be assumed that TRIDI might act in the same way as TDI. Presumably it is expected to be bonded to proteins of cells in the respiratory epithelium and/or be hydrolyzed to an amine derivative as well, triggering respiratory sensitization and irritation reactions.

 

Absorption following dermal exposure

In order to cross the skin, a compound must first penetrate into the stratum corneum and may subsequently reach the viable epidermis, the dermis and the vascular network. The stratum corneum provides its greatest barrier function against hydrophilic compounds, whereas the viable 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 Log Pow values between 1 and 4 favour dermal absorption (values between 2 and 3 are optimal). Above 4, the rate of penetration may be limited by the rate of transfer between thestratum corneumand 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 may be more than 10% 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 may be subject to biotransformation.

In case of TRIDI and TDI, the molecular weight is above 100 and below 500, which would normally indicate low potential to penetrate the skin. Moreover, the logPow value of TRIDI is with 7.56 very high and this also indicates as stated above low absorption. Due to the vapour pressure of 0.19 Pa, TRIDI is once again represents rather an inhalation hazard than one by dermal route of exposure. One has to keep in mind, that absorption is influenced by the irritating potential of the two substances and might enhance penetration. It has been demonstrated that the absorption of 2,4- and 2,6-TDI through skin contact is possible, as toluene diamine was found in the urine (Yeh et al., 2008). A clear dose-dependent skin absorption for 2,4- and 2,6-TDI was demonstrated by the findings of AUC, Cmax and accumulative amounts (r = 0.968) (Yeh et al., 2008).

Based on these factors, TRIDI is expected to be partially absorbed following dermal exposure into the stratum corneum. The transfer of the substance into the epidermis will be limited, due to its molecular weight and high lipophilicity. Hence, the systemic toxicity of TRIDI via the skin is assumed to be low and 10% adsorption via dermal route is proposed based on the high logPow and low water solubility.

Distribution

In general, the following principle applies: the smaller the molecule, the wider the 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’s not possible to foresee protein binding, which can 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 of TRIDI, no data is available for distribution patterns. However, there are data available for TDI. It has been demonstrated, that the absorbed amount of TDI is distributed after oral, dermal and inhalatory exposure (Timchalk et al., 1994, Kennedy et al., 1989, and 1994). The gastrointestinal tract and contents accounted for a high amount of the recovered radioactivity in the tissues/carcass with the remaining radioactivity evenly distributed among the remaining tissues (Timchalk et al., 1994).

The distribution of TRIDI is expected to be more extensive in fat tissues than in other tissues.

Akkumulation

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 log P values tend to have longer half-lives. On this basis, there is the potential for highly lipophilic substances (Log Pow >4) to accumulate in individuals that are frequently exposed. Highly lipophilic substances (Log Pow 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

Following oral exposure little TDI is retained in the body (4 %) and following exposure via inhalation a larger amount of TDI (34%) is retained in the carcass, indicating a slow release of protein-bound material (Timchalk et al., 1994). However, to our knowledge the accumulation of TDI in the stratum corneum has not been investigated.

Metabolism

Route specific toxicity may result 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.

It has been shown, that after oral exposure TDI, which is hydrolysed to TDA in the gastrointestinal tract and subsequently absorbed, can be excreted in the urine either unchanged or as acid-labile conjugates. TDA, as a metabolite can be N-acetylated forming mono- and diacetylated TDA metabolites which are readily excreted in the urine (Timchalk, et al. 1994). After inhalatory exposure, very little 2,4-TDA is formed. In addition, 90% of the quantitated metabolites in the urine specimens following inhalation exposure to 2,4-TDI existed as acid-labile conjugates of TDI/TDA while only 10% existed as acetylated TDA. This indicated that following inhalation exposure, a larger percentage of the 2,4-TDI was excreted in the urine preferentially in a conjugated form (to proteins) and not as oligoureas or free or acetylated TDA (Timchalk et al., 1994).

There is no data on metabolism of TRIDI. Similar to TDI, the substance is expected to form conjugates with glutathione (and other peptides) because a carbon atom in the isocyanate group represents an electrophile centre susceptible to nucleophile attack by such strong nucleophiles as liysine, cysteine and histidine (Smith and Hotchkiss, 2001). The isopropyl groups on aromatic ring can however affect the reactivity of electrophile carbon due probably to sterical hindrance. If TRIDI hydrolises to its corresponding amine, the latter can be N-acetylated and then excreted in the urine. Aromatic and aliphatic hydroxylation can also occur leading to a hydrophyle which is easily to be excreted.

Excretion

The major routes of excretion for substances from the systemic circulation are in the urine and/or the faeces (via bile and directly from the gastrointestinal mucosa). For 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 skin cells.

For TRIDI no data is available concerning its elimination, but for TDI several studies have been undertaken. TDI is mainly eliminated via the faeces (80 %) after oral exposure; only 5 to 15 % are eliminated via the urine. However, this is in accordance with the above mentioned principles, as TDI reacts with hydrolysed TDA in the gastrointestinal tract to polyurea polymers, which have a high molecular weight and are subsequently not absorbed and therefore eliminated via the faeces. After inhalation 48 % is eliminated via the faeces and 15 % via the urine in 48 hours and no quantifiable elimination via exhalation occurred (Timchalk et al., 1994).

The urinary excretion by the kidneys was slower following inhalation exposure (t1\2 = 20 hr) when compared to the oral 2,4-TDI dose group (t1\2 = 7.5 hr), suggesting that inhaled 2,4-TDI was eliminated in the urine in a different form having a longer biological half-life than orally administered 2,4-TDI and/or 2,4-TDA (Timchalk et al., 1994).

 

Discussion on absorption rate:

There is no experimental data on dermal absorption of TRIDI. Therefore the available data on TDI , structurally similar analogue to TRIDI, was taken into account to assess dermal penetration of the target substance. A detailed assessment of possible dermal absorption is presented also in a separate file attached to the IUCLID file in section 13 "Assessment reports".

Toxicokinetic data on dermal absorption of TDI:

The toxicokinetics of the substance Toluene diisocyanate (TDI) were investigated in rats by Yeh et al. (2008) after dermal application in rats (dorsum, area approximately 3 * 5 cm). The exposure duration was 5 h, after which the substance was carefully washed of the skin, using a cleansing agent. It has been demonstrated that the absorption of 2,4- and 2,6-TDI through skin contact is possible. A clear dose-dependent skin absorption for 2,4- and 2,6-TDI was demonstrated by the findings of AUC, Cmax and accumulative amounts (r = 0.968). Excretory 2,4- and 2,6- TDA concentration profiles in 6-day consecutive urine samples were shown to fit in first-order kinetics, although higher order kinetics could not be excluded for high doses. The apparent half-lives for excretory urinary TDA were about 20 h at various skin exposures, similar to that from the inhalation exposure in the previous animal experiment. The overall yield ratios for 2,4- to 2,6-TDA in urine were found to be close to unity, apparently lower than the expectancy of 4:1, possibly due to the higher self-polymerization reactivity of 2,4- than 2,6-TDI.

It is concluded that skin absorption of TDI was confirmed in a rat model and a clear dose-dependent skin absorption relationship for 2,4- and 2,6-TDI was demonstrated. The findings in this study clearly demonstrate the skin absorption capability of topical TDI exposure based on the observation of the internal dose concentration profile of urinary TDA across 6 days.