Phosphoric acid, 2-ethylhexyl ester, ammonium salt

Endpoint summary

Description of key information

Toxicological Data

Acute toxicity:  In an acute oral toxicity study conducted in accordance with OECD TG 423, mortality was observed when tested up to 2,000 mg/kg bw. No overt toxicity, clinical signs, death or abnormality at necropsy were observed amongst animals treated at 300 mg/kg.  Therefore it is concluded that the test item is Acute Tox. Cat. 4 (H302: Harmful if swallowed) with a reported LD50 as  300 - 2000 mg/kg bw/day.

The substance was not acutely toxic via the dermal route, although there was well defined to moderate erythema noted at the application sites of all treated animals on Days 2 and 3, with very slight to well-defined erythema persisting up to Day 14.  Other adverse dermal reactions noted were scabbing and patchy hair growth which developed from Day 3 and persisted to the end of the observation period. There were no mortalities observed, the reported LD50 > 2000 mg/kg.

Local Toxicity:  Although not corrosive the substance is considered a Skin Irrit. 2 (H315 – Causes skin irritation) and Eye Irrit. 2 (H319 – Causes serious eye irritation) following in vitro studies conducted in accordance with OECD 431, OECD 439, OECD 437 and OECD 492. The test item was also non-sensitising following conduct of an in chemico and in vitro studies (OECD 442C and OECD 442D, respectively).

There are no repeated dose toxicity studies or reproductive/developmental studies conducted on the test item as data on the read-across source substance reaction mass of bis(2-ethylhexyl) hydrogen phosphate and 2-ethylhexyl dihydrogen phosphate (CAS Number 12645-31-7, EC Number 235-741-0) was used to meet these data requirements.

Repeated dose toxicity: A sub-acute repeated dose toxicity study with combined reproductive and developmental screening (OECD 422) conducted on the source substance is available.  The substance was administered twice daily to 10 male and 10 female Wistar Han™:RccHan™: rats by oral gavage at dose levels of 60, 125 and 250 mg/kg bw/day for up to eight weeks (including a two week maturation phase, pairing, gestation and early lactation for females).  Although there was no recovery phase, this is not a guideline requirement.

The key findings in this study were episodes of increased salivation which were evident in animals of either sex treated with 250 mg/kg bw/day and in males treated with 125 mg/kg bw/day during the treatment period. An isolated incident of noisy respiration was also evident in one female treated with 250 mg/kg bw/day on Day 6. No such effects were detected in females treated with 125 mg/kg bw/day or animals of either sex treated with 60 mg/kg bw/day. Males treated with 250 mg/kg bw/day showed a reduction in body weight gain during Weeks 1, 2 and during the final week of treatment. No such effects were detected in females treated with 250 mg/kg bw/day or animals of either sex treated with 125 or 60 mg/kg bw/day.  Males treated with 250 mg/kg bw/day showed a slight reduction in overall food consumption when compared to controls. No such effects were detected in females treated with 250 mg/kg bw/day or in animals of either sex treated with 125 or 60 mg/kg bw/day. Sloughing on the non-glandular and/or glandular regions of the stomach were evident in a number of animals treated with 250 and 125 mg/kg bw/day and in one male treated with 60 mg/kg bw/day. No such effects were detected in females treated with 60 mg/kg bw/day. 

Whilst microscopic changes identified in the stomach may be considered to be adverse, they are also considered to be a result of local irritation of the test item rather than a true effect of systemic toxicity. The ‘No Observed Adverse Effect Level’ (NOAEL) for systemic toxicity was therefore considered to be 250 mg/kg bw/day for females and 125 mg/kg bw/day for males.

There were no effects reported for reproductive and developmental parameters, so the NOAEL for reproduction/development was => 250 mg/kg bw/day.

Absorption

The molecular weight of the substance is low to moderate i.e. 227 - 339 g/mol, it’s n-octanol/water partition coefficient is -1.6, its water solubility is 9.536 g/L, and vapour pressure of  0.00018 Pa with surface tension of 34.0mN/m.  These physiochemical properties are suggestive of absorption via oral and dermal routes. Oral uptake is more favourable, this is based on the very hydrophilic nature of the test item which encourages weight passive diffusion into portal circulation to the liver where the amount reaching systemic circulation is reduced.  Furthermore being surface active and an irritant, gastrointestinal membrane penetration will be enhanced and therefore uptake will be favourable following oral exposure. This is supported by the microscopic changes in the liver and forestomach, which included focal to multifocal squamous hyperplasia with hyperkeratosis, submucosal inflammation, and erosion in the forestomach following sub-acute exposure in all dose groups.  

The substance is of low molecular weight and surface active, and is therefore likely to cause adverse reactions to the skin, as supported by the classification as Skin Irrit. 2. Dermal irritation would cause damage that would enhance penetration through the skin, however, transfer between the stratum corneum and the epidermis is low and therefore overall systemic bioavailibility of the substance compared to oral exposure.

Based on the very low vapour pressure of the test item, uptake via the inhalation route is considered negligible.

Distribution

The substance has physico-chemical properties that mean systemic bioavailability is possible, based on the molecular weight, n-octanol/water partition coefficient and the solubility and as such, a widely systemic distribution is expected.  However, since passive diffusion of the substance into portal circulation to the liver is expected, plasma concentration and plasma half-life of the parent compound in blood plasma is expected to be low. This is because hydrolysis of the substance in the liver mediated by hepatic enzymes is expected to be rapid. Based on the n-octanol/water partition coefficient (log Pow = -1.6), bioaccumulation of the test substance is not expected.

Metabolism

The substance is expected to undergo rapid transformation through the liver via phase I and II enzymes, i.e. hydrolysis of the parent compound into ammonia, phosphate and 2-ethylhexanol. The alcohol derivative can undergo conjugation by phase II enzymes into  metabolites which would undergo phase II metabolism and eliminated via urine. This is supported by the increased relative liver and kidney weight.       

Excretion

Based on the molecular structure and solubility, excretion into urine as conjugated metabolites is assumed to be a preferred route of elimination.  The physio-chemical properties of the substance means  uptake following exposure is enhanced and with potential passive diffusion into portal circulation to the liver metabolism is expected to be rapid.  As a result, elimination is assumed to be rapid, therefore potential for bioaccumulation is to be expected to be low.

Conclusion

The substance has physico-chemical properties which lead to favourable absorption, distribution, metabolism and elimination (ADME). Oral uptake is the most favourable route of uptake, this is based on the very hydrophilic nature of the test item which encourages weight passive diffusion into portal circulation to the liver where rapid metabolism will occur ensuring low plasma concentration and  plasma half-life of the parent compound in the blood. Although dermal exposure is expected, systemic bioavailibility is expected to be low.  Whilst microscopic changes identified in the stomach during repeat dose toxicity studies may be considered to be adverse clinical signs, in this instance they are considered to be a result of local irritation of the test item rather than a true effect of systemic toxicity.  The clinical signs observed following oral sub-acute exposure support oral absorption of the tested item and clinical signs reported in liver are demonstrative of distribution, biotransformation and elimination of the test item. It can be concluded that the toxicokinetics of the test item does not pose significant toxicological concern and bioaccumulation is not expected.

Key value for chemical safety assessment

Bioaccumulation potential:

no bioaccumulation potential

Absorption rate - oral (%):

100

Absorption rate - dermal (%):

100

Absorption rate - inhalation (%):

100