Reaction products of diphosphorus pentaoxide and alcohol C7-9-iso, C8 rich, salted with 2-ethylhexylamine

Endpoint summary

Description of key information

The absorption of the target UVCB substance into the body is expected to be low via inhalation route of exposure. Therefore, distribution, metabolism and excretion of the target substance in its salted form are not key toxicokinetic considerations for this exposure route. However, according to the criteria outlined in ECHA Toxicokinetics guidance (Chapter R.7C – Endpoint specific guidance) the target substance meets criteria for 100 % absorption by inhalation due to its MW, logPow  and water solubility values.  These values are in the range suggestive that absorption via the lung cannot be ruled out. If administered orally or in contact with skin, the target substance is expected to dissociate releasing its main constituents: mono- and di-alkyl phosphates (predominantly C8 compounds) and 2-ethylhexyl amine. These components are expected to be well absorbed and distributed throughout the body. Phosphoric acid, C8 alcohols (predominantly octanol, 6-methyl-1-heptanol and 2,5-dimethyl-3-hexanol), and their corresponding carboxylic acids will be primary metabolites of mono-, and di-alkyl phosphates. 2-Ethylhexylamine will be oxidised by drug metabolising enzymes to hydroxylated derivatives with subsequent oxidation to aldehydes and carboxylic acids. All predicted metabolites are expected to be excreted via the urine. Based on the logPow values of < 4.5 (0.84, 0.28, and 4.28, respectively) for the amine, mono-alkyl phosphate, and di-alkyl phosphate analytes of the substance and evidence that alkylamines and alkyl phosphates/C8 alcohols are readily metabolised and eliminated, the substance is not expected to bioaccumulate.

Key value for chemical safety assessment

Bioaccumulation potential:

no bioaccumulation potential

Absorption rate - oral (%):

100

Absorption rate - dermal (%):

50

Absorption rate - inhalation (%):

100

Additional information

General

The toxicokinetic profile of the Reaction products of diphosphorus pentaoxide and alcohol C7-9-iso, C8 rich, salted with 2-ethylhexylamine was not determined by actual absorption, distribution, metabolism or excretion measurements. Rather, the physico-chemical properties of the target substance were integrated with the available toxicological data for the target substance to create a prediction of the toxicokinetic behavior. Because the target substance is composed of an anionic alkylated phosphate and cationic alkylamine moieties, salted alkylated phosphate and a salted amine, both components are considered in detail.

The EU Technical Guidance Document on Risk Assessment (TGD, Part I, Appendix IV, 2003) provides guidance on the physico-chemical properties that commonly determine oral, inhalatory and dermal absorption, distribution, metabolism and elimination of substances (Access to Guidance Document: http://ec.europa.eu/environment/chemicals/exist_subst/pdf/tgdpart1_2ed.pdf). ECHA REACH Guidance R.7c – Endpoint Specific Guidance (Version 1.1, November, 2012) offers further information regarding toxicokinetics. ECHA REACH Guidance R.11 provides information on trigger values for bioaccumulation which can be taken into account for toxicokinetics assessment.

Toxicological profile of the Reaction products of diphosphorus pentaoxide and alcohol C7-9-iso, C8 rich, salted with 2-ethylhexylamine

The UVCB substance consists predominantly of C8 octyl and isooctyl esters of phosphoric acid salted with 2-ethylhexylamine. The main representative constituents among alkyl phosphate ester species are n-octyl phosphate, 6-methyl-1-heptyl phosphate and 2,5-dimethyl-3-hexyl phosphate. The substance has an average molecular weight of 400 g/mol. The di- and mono-octyl representative structures shown above have molecular weights of 451 g/mol and 339 g/mol, respectively. The substance has a pour point of -12 ± 3 °C and decomposes at approximately 145 °C at 101.4 kPa (Fox, 2013). Due to this decomposition, no boiling point value could be determined (Fox, 2013). Moreover, it has low vapour pressure of 0.0004 Pa at 25 °C (Tremain, 2013b). The substance is expected to be absorbed based on its solubility in water, which is loading rate dependent (0.0569 g/L, 0.386 g/L and 1.71 g/L at 0.1, 1.0 and 10 g/L loading rate). The logPow of amine, mono-alkyl phosphate, and di-alkyl phosphate analytes are 0.84, 0.28, and 4.28, respectively (Fox, 2013). Furthermore, the substance is expected to be hydrolytically stable under environmental conditions. The main functional groups within the test item are a salted alkylated phosphate and a salted amine. Neither of these groups would readily hydrolyse at relevant temperatures and pH’s (Fox, 2013). In contrast, hydrolysis for the component alkyl phosphate esters is reported. According to Svara et al. (2012), the esters of phosphoric acid hydrolyse in the presence of water. This is also mentioned by Morrison and Boyd (1987) who state that di-alkyl phosphate esters hydrolyse in an acidic environment. The rate of hydrolysis varies widely and depends on the degree of esterification, the pH value and the nature of the substituents. This will be further discussed in section “Absorption from gastrointestinal tract”.

The substance has been evaluated for mutagenic potential and reproductive/developmental toxicity. It was not mutagenic in Mouse Lymphoma Test with or without metabolic activation (OECD 476, Flanders, 2013). Cytotoxicity observed at high doses indicated that it was absorbed. In the oral gavage reproduction/developmental toxicity screening test in rats, the substance produced systemic effects: post-dose salivation and findings in kidneys of male rats (Thorsrud, 2013). Microscopic changes in the males at 500 mg/kg/day (α2μ-globulin nephropathy) was limited to the male rat and is not relevant for human risk assessment. The NOAEL for reproductive/development toxicity was determined to be 500 mg/kg bw/day, the highest dose level tested.

 

Toxicokinetic analysis of the Reaction products of diphosphorus pentaoxide and alcohol C7-9-iso, C8 rich, salted with 2-ethylhexylamine

Absorption

The relatively high molecular weight of the substance (400 g/mol (451 g/mol and 339 g/mol for the representative structures) and the logPow values < 4.5 (0.84, 0.28, and 4.28, respectively) for the amine, mono-alkyl phosphate, and di-alkyl phosphate analytes indicates that absorption will be limited. However, the water solubility data of the substance indicates that it is moderately soluble (100 -1000 mg/L) and likely to dissolve and be absorbed in the gastrointestinal tract. The target substance possesses surface activity (Fox, 2013) which may enhance absorption to some extent. Furthermore, because the substance has low vapour pressure (0.0004 Pa at 25 °C), absorption by inhalation is not relevant.

Absorption from the gastrointestinal tract

Regarding oral absorption, a substance will most likely be hydrolysed in the stomach, because this is a favoured reaction in an acidic environment. The substance includes two main functional groups: a salted alkylated phosphate and a salted amine. According to Fox (2013), neither of these groups would readily hydrolyse at environmentally relevant temperatures and pH values. However, the substance is expected to dissociate in the stomach releasing its main constituents mono-, and di-alkyl (C8, octyl and isooctyl) phosphates and 2-ethylhexylamine. Therefore, the absorption of the substance will be confined to the absorption of these dissociation products.

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 4.5 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 with the bile acids. Substances absorbed as micelles enter the circulation via the lymphatic system, bypassing the liver.

Oral absorption of alkyl phosphates is described by Daunderer (1991). The author states that alkyl phosphates are rapidly absorbed by oral route of exposure. The substance’s amine, mono-alkyl phosphate, and di-alkyl phosphate analytes have logPow values less than 4.5 (0.84, 0.28, and 4.28, respectively) indicating favoured absorption via gastrointestinal tract. Some uptake of the substance may occur via solubilisation with the bile acid and absorption in small intestines.

With regard to acidic solutions, alkyl phosphates are known to hydrolyse gradually to phosphoric acid and an appropriate alkyl alcohol (Morrison and Boyd, 1987) and hydrolysis is considered as likely bond-breaking process. Therefore, the hydrolysis of the dissociation products of the target substance applies too. This will be based on a nucleophilic substitution reaction where the hydroxide ion attacks the electrophilic phosphorus, which leads to the formation of C8 alcohols i.e. octanol, isooctanol (6-methyl-1-heptanol) and 2,5-dimethyl-3-hexanol and phosphoric acid. Dealkylation of phosphoric acid esters as primary metabolism pathway was also experimentally confirmed in metabolism studies with members of “Phosphoric Acid Derivatives Category” (US EPA, 2009; PAD, 2005). The products of hydrolysis, however, are all ionisable structures - as the parent compound - and ionisation would hinder absorption. On the other hand, a C8 alcohol 2-ethylhexanol, which is contained only at negligible amounts in the UVCB target substance, was demonstrated to be efficiently absorbed following oral administration to rats (Albro, 1975). Generally, aliphatic alcohols are absorbed by all common routes of exposure, widely distributed within the body and efficiently eliminated (Veenstra et al., 2009; SIDS, 2006). Hydrolysis for the second dissociation product 2-ethylhexylamine will not appear because the substance does not possess any hydrolysable groups.

In conclusion, based on the physico-chemical properties of the target substance and its dissociation and hydrolysis products, significant absorption of the target substance by oral route of exposure is expected. This prediction is confirmed in the Oral (gavage) Reproduction/developmental Toxicity Screening test in the rat (Thorsrud, 2013) where the substance produced test-article related systemic effects. Therefore, 100 % oral absorption is considered appropriate for the purposes of the hazard assessment: DNEL derivation.

Absorption from the respiratory tract

Because the substance is a liquid with a low vapour pressure (0.0004 Pa at 25 °C), the potential for inhalation is limited to mists that can be generated during spraying. However, according to the criteria outlined in ECHA Toxicokinetics guidance (Chapter R.7C – Endpoint specific guidance) the target substance meets criteria for 100 % absorption by inhalation due to its MW, logPow and water solubility values. These values are in the range suggestive that absorption via the lung cannot be ruled out: logPow < 4.5 (0.84, 0.28, and 4.28, respectively for the amine, mono-alkyl phosphate, and di-alkyl phosphate analytes), moderate solubility in water (0.0569 g/L, 0.386 g/L and 1.71 g/L at 0.1, 1.0 and 10 g/L loading rate), and a molecular weight < 500 g/mol (451 g/mol and 339 g/mol for the representative structures). The substance may also be removed from the lung via muco-cilliary transport and then ingested. Based on these data, 100 % absorption by inhalation is considered appropriate for the purposes of the hazard assessment: DNEL derivation).

Absorption following dermal exposure

The substance has a molecular weight range above 100 and below 500 (451 g/mol and 339 g/mol for the representative structures) which is not favourable for skin penetration. While the substance is soluble in water above 1 mg/L where dermal uptake is anticipated to be low to moderate (0.0569 g/L, 0.386 g/L and 1.71 g/L at 0.1, 1.0 and 10 g/L loading rate) the logPow values for the analytes (0.84, 0.28, and 4.28, respectively for the amine, monoalkyl phosphate, and dialkyl phosphate analytes) are below or above the optimal logPow value range (logPow values between 1 and 4 favour dermal absorption (values between 2 and 3 are optimal) for dermal absorption (TGD, Part I, Appendix IV, 2003). Moreover, the dissociation of the substance with subsequent hydrolysis of alkyl phosphate moieties in contact with skin cannot be ruled out as natural skin surface pH is below 5 (Lambers et al., 2006) and the alkyl phosphates moieties are known to dissociate at acidic conditions (Morrison and Boyd, 1987). Based on these data, dermal absorption is expected to be limited and 50 % dermal absorption is considered appropriate for the risk assessment: DNEL derivation.

Distribution

Although no ADME studies have been conducted on this substance, the reproductive/developmental toxicity study in rats shows that it is distributed to the kidneys. Based on the substance’s molecular size (MW < 500) and lipophilicity (log Pow > 0), it is likely to distribute to other tissues.

The substance and its dissociation products mono-, and di-alkyl phosphates and 2-ethylhexylamine do not bind to proteins (confirmed by the OECD QSAR Toolbox, v1.3, 2013). Therefore, protein binding is not expected to affect distribution. According to the general mechanistic profiling methods included in the software, the dissociation products and their product of hydrolysis C8 alcohols are discrete chemicals and do not possess functional groups which could bind to proteins (no alerts found). Further distribution of the hydrolysis products (octanol, isooctanol (6-methyl-1-heptanol) and 2,5-dimethyl-3-hexanol, phosphate and 2-ethylhexylamine) and metabolites of the substance is expected similar to other aliphatic alcohols and amines (WHO, 2007; SIDS, 2006; Cavender et al., 2000; Yu and Dong-Mei, 1993; Albro, 1975).

Accumulation

Based on the logPow values of < 4.5 (0.84, 0.28, and 4.28, respectively) for the amine, mono-alkyl phosphate, and di-alkyl phosphate analytes of the substance, it is not expected to bioaccumulate. Potential alcohol and amine metabolites also are not known to bioaccumulate. In general, aliphatic amines are rapidly absorbed in the gastrointestinal tract and transformed to polar metabolites that are readily eliminated in the urine (Rechenberger, 1940 cited in Yu and Dong-Mei, 1993; WHO, 2007).

Metabolism

The target substance is expected to dissociate in body compartments where pH is acidic. In the stomach, mono- and di-alkyl (octyl and isooctyl) phosphates and 2-ethylhexylamine will be the main dissociation products. The dissociation products, if absorbed, can be subject for further transformation reactions. In a study, a structurally similar alkyl ester of phosphoric acid bis(2-ethylhexyl)phosphate was administered to rats by diet (Lundgren and DePierre, 1987). It induced liver epoxide hydrolase activity and increased microsomal Cytochrome P450 content, the facts pointing to a certain metabolic activity. Generally, alkyl esters of phosphoric acid are known to hydrolyse gradually to phosphoric acid and corresponding alcohols (Morrison and Boyd, 1987). Dealkylation as primary route of metabolism was described also for structurally similar alkyl esters of phosphoric acid (PAD, 2005). Tributyl phosphate was metabolised in rats by stepwise debutylation through hydroxylated intermediates (SIDS, 2002). As confirmed by the OECD QSAR Toolbox, hydrolysis does apply for the main dissociation products of the target substance octyl-, 6-methyl-1-heptyl phosphate and 2,5-dimethyl-3-hexyl phosphate in the acidic environment. Therefore, the expected main metabolites for the target substance are aliphatic alcohols: octanol, 6-methyl-1-heptanol and 2,5-dimethyl-3-hexanol. Metabolism of primary aliphatic alcohols is described for C6-C22 long-chain alcohols (Veenstra et al., 2009; SIDS, 2006). They are oxidised to the corresponding carboxylic acids, followed by a stepwise elimination of C2 units in β-oxidation process (Veenstra et al., 2009; SIDS, 2006). Additionally, Albro (1975) determined metabolites for a structurally similar C8 alcohol 2-ethylhexanol which was administered to rats in diet. 2-Ethylhexanoic acid was the main metabolite found. Thus it is very likely that the released C8 alcohols will undergo carboxylation reactions in a similar way. Phosphoric acid will be involved into the intermediary metabolism.

2-Ethylhexylamine, the other dissociation products of the target salted substance, will be metabolized by Cytochrome P450 group of metabolising enzymes. Aliphatic amines are metabolized primarily by flavin-containing monooxygenases, monoamino oxidases or amino oxidases by a process known as oxidative deamination (Beard and Noe, 1981; Yu and Dong-Mei, 1993; Cavender et al., 2000; WHO, 2007). The second metabolic pathway known for primary amines is N-oxidation leading to formation of nitroso-compounds (WHO, 2006). It can be assumed that 2-ethylhexylamine as primary aliphatic amine will be substrates of mitochondrial monoamine oxidase (MAO) and will undergo ready metabolism in the mammalian organism. Its primary metabolites are expected to be hydroxylated derivatives (C-oxidation step), which can subsequently be hydrolysed to their corresponding aldehydes. These compounds can be further oxidised to carboxylic acids. All the metabolites are supposed to be excreted mainly via the urine (Bourke et al., 1972; WHO, 2006).

Excretion

No studies have been conducted to determine the elimination of the substance. However, the elimination of the substance can be predicted based on their chemical nature. The expected hydrolysis products ((mono-, and di-alkyl (octyl and isooctyl) phosphates and alkylamine) and metabolites (C8 alcohols and carboxylic acids) are polar molecules that are expected to be eliminated mainly via the urine. According to Albro (1975), the metabolites of similar C8 isooctyl alcohol 2-ethylhexanol were rapidly excreted in respiratory CO2 (6 – 7 %), faeces (8 – 9 %) and urine (80 – 82 %), with essentially complete elimination by 28 h after administration. Only 3 % of 2-ethylhexanol was excreted unchanged. 2-Ethylhexylamine and its hydroxylated products are supposed to be excreted mainly via the urine (Bourke et al., 1972; WHO, 2006).

Conclusion

The toxicokinetics of the substance (Reaction products of diphosphorus pentaoxide and alcohol C7-9-iso, C8 rich, salted with 2-ethylhexylamine) was assessed based on its physical chemical properties and toxicity data, QSAR modelling, and published data on structurally similar substances. Because the substance is a salt which dissociates and subsequently its alkyl phosphate moieties undergo hydrolysis under acid conditions in the gut and even the skin (pH 4 to 7), the toxicokinetics of the alkylamine and alkylphosphate hydrolysis products as well as the parent amine-salted alkylated phosphate were assessed.

The substance is expected to be readily absorbed via oral and inhalation (mists) routes of exposure. Because the substance is a liquid with a low vapour pressure (0.0004 Pa at 25 °C), inhalation of vapours is not expected. Dermal absorption is expected to be limited because it has a molecular weight > 100 and <500 g/mol and because the logPow values for the analytes are below or above the optimal logPow value range (between 2 and 3) for dermal absorption. 

The results of the reproductive/developmental toxicity study in rats shows that it is distributed to the kidneys. The substance of relatively small molecular size (MW < 500) and lipophilicity also indicates that it is likely to be distributed to other tissues, but it does not allow predictions of specific targets or concentrations. In contrast, once dissociated both alkylamine and alkyl phosphate species are expected to be subject to first pass metabolism. Alkyl phosphates will undergo hydrolysis to phosphoric acid and C8 alcohols which are expected to be further metabolised to their corresponding carboxylic acids and excreted predominantly via the urine. The other dissociation product 2-ethylhexylamine will not undergo hydrolysis due to the absence of hydrolysable groups. In analogy to other primary alkyl amines, it is expected to be readily absorbed and metabolised by oxidative deamination or N-oxidation. The expected metabolites are hydroxylated derivatives, aldehydes and carboxylic acids. All the metabolites are supposed to be excreted mainly via the urine.

Based on the logPow values of <4.5 (0.84, 0.28, and 4.28, respectively) for the amine, mono-alkyl phosphate, and di-alkyl phosphate analytes of the substance and evidence that alkylamines and alkyl phoshphates/C8 alcohols are readily metabolised and eliminated, the substance is not expected to bioaccumulate.