2,4,6-triisopropyl-m-phenylene diisocyanate

Basic toxicokinetics

Administrative data

Endpoint:

basic toxicokinetics

Type of information:

other: Expert statement

Adequacy of study:

key study

Study period:

2011

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: see 'Remark'

Remarks:

A read-across statement regarding the toxicological behaviour of the group of diisocyanates 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:

2020

Report date:

2020

Materials and methods

Test guideline

Qualifier:

no guideline required

GLP compliance:

no

Test material

Test material information

Constituent 1

Reference substance name:

2,4,6-triisopropyl-m-phenylene diisocyanate

EC Number:

218-485-4

EC Name:

2,4,6-triisopropyl-m-phenylene diisocyanate

Cas Number:

2162-73-4

Molecular formula:

C17H22N2O2

IUPAC Name:

2,4-diisocyanato-1,3,5-tris(propan-2-yl)benzene

Test material form:

liquid

Radiolabelling:

other: not applicable in this expert statement

Test animals

Species:

other: not applicable

Strain:

other: not applicable

Details on test animals or test system 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:

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, 2,4,6-triisopropyl-m-phenylene-diisocyanate and TDI would be favourable for absorption, when only taking into account their molecular weights. However, as 2,4,6-triisopropyl-m-phenylene-diisocyanate is practically insoluble in water, it is apparent that its absorption is hindered. This is also seen in the value calculated for the LogPow 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 P 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 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. 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 2,4,6-triisopropyl-m-phenylene-diisocyanate 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 2,4,6-triisopropyl-m-phenylene-diisocyanate has a relatively low vapour pressure (0.19 Pa) and a high boiling point (calculated 325.14°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 as 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 speculated that 2,4,6-triisopropyl-m-phenylene-diisocyanate 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 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 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 2,4,6-triisopropyl-m-phenylene-diisocyanate 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 2,4,6-triisopropyl-m-phenylene-diisocyanate is with 7.56 very high and this also indicates as stated above low absorption. Due to the vapour pressure of 0.19 Pa, 2,4,6-triisopropyl-m-phenylene-diisocyanate 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, 2,4,6-triisopropyl-m-phenylene-diisocyanate 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 2,4,6-triisopropyl-m-phenylene-diisocyanate 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.

Details on distribution in tissues:

In general, the following principle applies: the smaller the molecule, the wider the distribution. A lipophilic molecule (Log P >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 2,4,6-triisopropyl-m-phenylene-diisocyanate, 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 2,4,6-triisopropyl-m-phenylene-diisocyanate is expected to be more extensive in fat tissues than in other tissues.

Details on 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 2,4,6-triisopropyl-m-phenylene-diisocyanate 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).

Metabolite characterisation studies

Details on metabolites:

After oral exposure TDI, which is hydrolysed to TDA in the gastrointestinal tract and subsequently absorbed, it 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).
Similar to TDI, 2,4,6-triisopropyl-m-phenylene-diisocyanate is expected to form conjugates with glutathione (and other peptides) because the carbon atoms in the isocyanate group represent an electrophile centre susceptible to nucleophile attack by such strong nucleophiles as lysine, 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 2,4,6-triisopropyl-m-phenylene-diisocyanate hydrolyses 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.

Any other information on results incl. tables

Background:

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 physical-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 2,4,6-triisopropyl-m-phenylene-diisocyanate between -90°C and 50°C (Kintrup, 2012). Glass transition temperature (amorphous components) in the first heating run was determined to be -56°C. The calculated melting point was 82.9°C. The boiling point of 305.8°C was measured for 2,4,6-triisopropyl-m-phenylene-diisocyanate (Svobodova, 2012).

Hydrolysis as a function of pH has not been determined, but comparison to its structural analogue toluene diisocyanate revelaed a high likelihood for hydrolysis. The substance is not toxic when administered orally to ratsc (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 OECD471, 473 and 476.

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 log P values tend to have longer half-lives. On this basis, there is the potential for highly lipophilic substances (Log P >4) to accumulate in individuals that are frequently exposed. Highly lipophilic substances (Log P 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 2,4,6-triisopropyl-m-phenylene-diisocyanate. Similar to TDI, the substance is expected to form conjugates with glutathione (and other peptides) because the carbon atoms in the isocyanate group represent an electrophile centre susceptible to nucleophile attack by such strong nucleophiles as lysine, 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 2,4,6-triisopropyl-m-phenylene-diisocyanate 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.

Applicant's summary and conclusion

Conclusions:

A read-across statement regarding the toxicological behaviour of several diisocyanates, taking into account the chemical structure, the available physico-chemical-data and the available (eco-)toxicological data is available.

Executive summary:

2,4,6-triisopropyl-m-phenylene-diisocyanate and TDI are aromatic diisocyanates with different alkyl rests attached to benzene ring (three isopropyl groups in 2,4,6-triisopropyl-m-phenylene-diisocyanate and one methyl group in TDI). As a result of the structural differences, such physic-chemical properties as melting point, LogPow and water solubility differ significantly from each other.

2,4,6-triisopropyl-m-phenylene-diisocyanate is expected to be absorbed to a lesser extent than TDI into the organism after oral exposure. Absorption via 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, 2,4,6-triisopropyl-m-phenylene-diisocyanate 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 2,4,6-triisopropyl-m-phenylene-diisocyanate. Excretion of 2,4,6-triisopropyl-m-phenylene-diisocyanate is expected via urine.