Diphenyl methylphosphonate

Basic toxicokinetics

Administrative data

Endpoint:

basic toxicokinetics in vivo

Type of information:

other: Expert Statement

Adequacy of study:

key study

Study period:

2012-2013

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: An extensive assessment of the toxicological behaviour of diphenyl methylphosphonate was performed, taking into account the chemical structure, the available physico-chemical-data and the available (eco-)toxicological data.

Data source

Reference

Reference Type:

other: Expert statement

Title:

Unnamed

Year:

2013

Report date:

2013

Materials and methods

Test guideline

Qualifier:

no guideline followed

Test material

Test material information

Constituent 1

Reference substance name:

Diphenyl methylphosphonate

EC Number:

231-388-1

EC Name:

Diphenyl methylphosphonate

Cas Number:

7526-26-3

Molecular formula:

C13H13O3P

IUPAC Name:

diphenyl methylphosphonate

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

Main ADME results

Main ADME results 1

Type:

absorption

Results:

absorption after oral exposure rather extensive

Main ADME results 2

Type:

absorption

Results:

intermediate dermal absorption

Main ADME results 3

Type:

absorption

Results:

diphenyl methylphosphonate is marginally available in the air for inhalation, due to its low vapour pressure and the fact that the substance is at room temperature a low melting, non particulate solid.

Main ADME results 4

Type:

distribution

Results:

expected to be wide, reaching also the intracellular compartment, may be more extensive in fat tissues than in other tissues, due to its better solubility in octanol than in water.

Main ADME results 5

Type:

excretion

Results:

parent substance will be excreted after extensive metabolism as metabolites and/or conjugates. A part might be excreted unchanged (low molecular weight / water solubility).

Main ADME results 6

Type:

excretion

Results:

Based on its chemical structure and its molecular weight, it is assumed to be excreted via the urine and to a minor extent via the bile as conjugates with glucuronic acid.

Toxicokinetic / pharmacokinetic studies

Details on absorption:

Diphenyl methylphosphonate: Considering its logPow above 0 and below 4, the absorption into the body is assumed to be favoured (logPow between > 0 and < 4 are favourable for absorption). Moreover, it is favourable for absorption, when taking its molecular weight (248.22 g/mol) and its water solubility (1.56 g/L) into account. Further enhancement of absorption is not expected, as diphenyl methylphosphonate is not irritating to skin or eyes.
The above mentioned properties determine the absorption of diphenyl methylphosphonate to be rather extensive, based on the absorption-easing properties (low molecular-weight, water solubility and intermediate logPow) and the observed effects in toxicological experiments.

Diphenyl methylphosphonate does not hydrolyse under acidic and neutral conditions and under basic conditions hydrolysis is only moderate (DT50 ca. 33 hrs). In accordance with the above mentioned principles, it is very unlikely for diphenyl methylphosphonate to be hydrolysed in the stomach.
The available data suggest that orally administered diphenyl methylphosphonate will be absorbed well, possibly also via micelles. However, the extent of absorption is dependent on the hydrophilicity / lipophilicity of a substance, which influences its uptake via the gastrointestinal fluids.

Concerning absorption in the respiratory tract, it can be expected that diphenyl methylphosphonate is marginally available in the air for inhalation, due to its low vapour pressure and the fact that the substance is at room temperature a low melting, non particulate solid. Due to its intermediate logPow, the amount available is assumed to be absorbed directly across the respiratory tract epithelium or through aqueous pores.

As the molecular weight of diphenyl methylphosphonate is above 100 and below 500, this indicates already a moderate potential to penetrate the skin. The amount of diphenyl methylphosphonate, which is absorbed following dermal exposure into the stratum corneum is however likely to be also transferred into the epidermis, due to its optimal logPow for dermal absorption. The systemic toxicity of diphenyl methylphosphonate via the skin has been shown to
be intermediate, as the results of the acute dermal toxicity study, give a LD50 value of 786 and 569 mg/kg bw for male and female rats, respectively.
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 an intermediate dermal absorption. In detail, despite it’s molecular weight, it’s physical state and the results of the irritation studies; the use of a factor of 100 % for the estimation of dermal uptake for diphenyl methylphosphonate is required, due to water solubility, optimal logPow and the results for acute toxicity (Schuhmacher –Wolz et al.,2003).

Details on distribution in tissues:

In case of diphenyl methylphosphonate, no data is available for distribution patterns. The distribution is expected to be wide, reaching also the intracellular compartment, may be more extensive in fat tissues than in other tissues, due to its better solubility in octanol than in water (logPow of 2.8).

Details on excretion:

For diphenyl methylphosphonate no data is available concerning its elimination. Concerning the above mentioned behaviour predicted for its metabolic fate, it is very likely that the parent substance will be excreted after extensive metabolism as metabolites and/or conjugates. A part might be excreted unchanged (low molecular weight / water solubility). Based on its chemical structure and its molecular weight, it is assumed to be excreted via the urine and to a minor extent via the bile as conjugates with glucuronic acid. However, it has to be kept in mind, that metabolites conjugated to glucuronic acid, can be subject to entero-hepatic recycling, and re-enter the system. Additionally, phenols can also be exhaled to a smaller percentage.

Metabolite characterisation studies

Metabolites identified:

yes

Details on metabolites:

Hydrolysis does not apply for diphenyl methylphosphonate. Its metabolism is likely to occur also via the Cytochrome P450 group of metabolising enzymes, as it has been predicted with the TOXTREE modelling tool (Chemservice S.A., 2011). There, the chemical has been identified to bear primary, secondary and tertiary sites and more than 4 sites for metabolism by the Cytochrome P450 group of metabolising enzymes. The primary, secondary and tertiary sites of metabolism are the carbon-atoms of the phenyl-rings which are predicted to be subject to aromatic hydroxylation. According to modelling results obtained using TOXTREE diphenyl methylphosphonate, contains the structural alerts: phosphonic acid derivative, phosphonic acid ester and aromatic compound.
To identify all possible sites for phase I and II reactions, the molecular structure of diphenyl methylphosphonate was investigated in detail. As specified above, the carbon atoms of the rings can be subject to aliphatic hydroxylation, possibly yielding multiple hydroxyl groups, which is facilitating the elimination.
Regarding other possible phase I reactions, it is stated that the ester-bond is most likely subject to hydrolytic cleavage via unspecific esterases, resulting in phenol and a phenylphosphonic acid ester. This, however, can be subject to further metabolism by esterases, resulting in another phenol and methylphosphonic acid. This enzymatic hydrolysis is likely to occur after absorption of DPP into the blood. However, the phenol can be subject to further phase I metabolism by Cytochrome P450s to hydroquinone.
Concerning the metabolite methyl phosphonic acid (CAS 993-13-5), its toxic effects have been studied (Munro et al., 1999, Watson et al., 2007). Its oral LD50 for rats and mice has been reported to be 5000 mg/kg bw (Munro et al., 1999) and 1888 mg/kg bw (Watson et al., 2007) . Moreover it is known to be a skin and eye irritant in humans. Moreover, it is possible for the methylphosphonic acid to be subject to oxidative desalkylation by cytochrome P450-mediated oxidation, by replacement of the methyl-group bound to the phosphorus with hydrogen, resulting in liberation of H-CHO.
The existing or newly introduced functional groups will react in phase II of the biotransformation with different molecules, leading to the formation of conjugations. For the hydroxyl-groups it is most likely that they will be conjugated to glucuronic acid, activated sulphate or activated methionine. This is true for phenol and phenolic compounds, as they are known to be conjugated to glucuronic acid and activated sulphate and subsequently excreted mainly via the urine and to a minor extent via the faeces. However, it has to be kept in mind, that metabolites conjugated to glucuronic acid, can be subject to entero-hepatic recycling, and therefore re-enter the system.
In conclusion, it is most likely that the substance of interest will be subject to metabolism by esterases and either before or after by cytochrome P450 enzymes and subsequent glucuronidation with occurring first-pass metabolism. Concerning the possibility of protein binding, this cannot be ruled out, without adequate experimental data.

Any other information on results incl. tables

Background:

There is data available on the physico-chemical properties of diphenyl methylphosphonate. The substance is a white solid (MW 248.22 g/mol), its melting point is 35.6 °C and it boils at 354 °C (Fox, 2012). The substance is soluble in water (1.56 g/L at 20 °C, Fox, 2012) and has a logPow of 2.8 (Fox, 2012). Hydrolysis (as a function of pH) has also been determined and was found to be negligible at pH 4 and 7 and at pH 9 only moderate hydrolysis (DT50 ca 33 hrs) was reported (Fox, 2012). The vapour pressure amounts to 0.0089 Pa at 25 °C (Atwal, 2012) hence can be considered to be very low.

The substance is acutely toxic, when administered to rats orally (LD50 233 mg/kg bw, Loeser 1980), or dermally (LD50 786 mg/kg bw for male and 569 mg/kg bw for female rats, Bomhard, 1991). It is not a skin (Thyssen, 1979) or eye irritant (Thyssen, 1979). Additionally, the substance is reported not to bear genotoxic potential in a mouse lymphoma assay (Wollny, 2012).

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.

Diphenyl methylphosphonateis water soluble (1.56 g/l), which eases absorption. In addition, the value of the logPow of 2.8 demonstrates that the substance has likely a better solubility in octanol than in water (positive logPow for lipophilic substances, negative logPow for hydrophilic substances). Considering its logPow above 0 and below 4, the absorption into the body is assumed to be favoured (logPow between > 0 and < 4 are favourable for absorption). Moreover, it is favourable for absorption, when taking its molecular weight (248.22 g/mol) into account. Further enhancement of absorption is not expected, as diphenyl methylphosphonateis not irritating to skin or eyes.

The above mentioned properties determine the absorption of diphenyl methylphosphonate to be rather extensive, based on the absorption-easing properties (low molecular-weight, water solubility and intermediate 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. Fordiphenyl methylphosphonatea study was undertaken to determine its potential to hydrolyse. However, the substance does not hydrolyse under acidic and neutral conditions and under basic conditions hydrolysis is only moderate (DT50 ca. 33 hrs). In accordance with the above mentioned principles, it is very unlikely fordiphenyl methylphosphonateto be hydrolysed in the stomach.

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≥4) 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 administereddiphenyl methylphosphonatewill be absorbed well, possibly also via micelles. However, the extent of absorption is dependent on the hydrophilicity / lipophilicity of a substance, which influences its uptake via the gastrointestinal fluids.

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.

Diphenyl methylphosphonatehas a very low vapour pressure, which indicates only marginal availability for inhalation. Due to its intermediate logPow, the amount available is assumed to be absorbed directly across the respiratory tract epithelium or through aqueous pores. Based on this data, it can be expected thatdiphenyl methylphosphonateis marginally available in the air for inhalation, due to its low vapour pressure and the fact that the substance is at room temperature a low melting, non particulate solid.

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 VI). 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 is most likely more than 10 % and less than 100 % 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 can be subject to biotransformation.

In case ofdiphenyl methylphosphonate, the molecular weight is above 100 and below 500, which indicates already a moderate potential to penetrate the skin. The amount ofdiphenyl methylphosphonate,which is absorbed following dermal exposure into the stratum corneum is however likely to be also transferred into the epidermis, due to its optimal logPow for dermal absorption. The systemic toxicity of diphenyl methylphosphonatevia the skin has been shown to be intermediate, as the results of the acute dermal toxicity study, give a LD50 value of 786 and 569 mg/kg bw for male and female rats, respectively.

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 an intermediate dermal absorption. In detail, despite it’s molecular weight, it’s physical state and the results of the irritation studies; the use of a factor of 100 % for the estimation of dermal uptake for diphenyl methylphosphonate is required, due to water solubility, optimal logPow and the results for acute toxicity (Schuhmacher –Wolz et al.,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. 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 ofdiphenyl methylphosphonate, no data is available for distribution patterns. The distribution is expected to be wide, reaching also the intracellular compartment, may be more extensive in fat tissues than in other tissues, due to its better solubility in octanol than in water (logPow of 2.8).

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 experimentally determined logPow, water solubility and predicted behaviour concerning absorption and metabolism ofdiphenyl methylphosphonate do not indicate a potential for accumulation in the body.

 

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 not apply fordiphenyl methylphosphonate. Its metabolism is likely to occur also via the Cytochrome P450 group of metabolising enzymes, as it has been predicted with the TOXTREE modelling tool (Chemservice S.A., 2011). There, the chemical has been identified to bear primary, secondary and tertiary sites and more than 4 sites for metabolism by the Cytochrome P450 group of metabolising enzymes.The primary, secondary and tertiary sites of metabolism are the carbon-atoms of the phenyl-rings which are predicted to be subject to aromatic hydroxylation. According to modelling results obtained using Toxtree diphenyl methylphosphonate, contains the structural alerts: phosphonic acid derivative, phosphonic acid ester and aromatic compound.

 

To identify all possible sites for phase I and II reactions, the molecular structure of diphenyl methylphosphonate was investigated in detail. As specified above, the carbon atoms of the rings can be subject to aliphatic hydroxylation, possibly yielding multiple hydroxyl groups, which is facilitating the elimination.

Regarding other possible phase I reactions, it is stated that the ester-bond is most likely subject to hydrolytic cleavage via unspecific esterases, resulting in phenol and a phenylphosphonic acid ester. This, however, can be subject to further metabolism by esterases, resulting in another phenol and methylphosphonic acid. The phenol can be subject to further phase I metabolism by Cytochrome P450s to hydroquinone.

Concerning the metabolite methyl phosphonic acid (CAS 993-13-5), its toxic effects have been studied (Munro et al., 1999, Watson et al., 2007). Its oral LD50 for rats and mice has been reported to be 5000 mg/kg bw (Munro et al., 1999) and 1888 mg/kg bw (Watson et al., 2007) . Moreover it is known to be a skin and eye irritant in humans.

Moreover, it is possible for the methylphosphonic acid to be subject to oxidative desalkylation by cytochrome P450-mediated oxidation, by replacement of the methyl-group bound to the phosphor with hydrogen, resulting in liberation of H-CHO.

The existing or newly introduced functional groups will react in phase II of the biotransformation with different molecules, leading to the formation of conjugations. For the hydroxyl-groups it is most likely that they will be conjugated to glucuronic acid, activated sulphate or activated methionine.

This is true for phenol and phenolic compounds, as they are known to be conjugated to glucuronic acid and activated sulphate and subsequently excreted mainly via the urine and to a minor extent via the faeces.However, it has to be kept in mind, that metabolites conjugated to glucuronic acid, can be subject to entero-hepatic recycling, and therefore re-enter to system.

In conclusion, it is most likely that the substance of interest will be subject to metabolism by esterases and either before or after by cytochrome P450 enzymes and subsequent glucuronidation with occurring first-pass metabolism.

Concerning the possibility of protein binding, this cannot be ruled out, without adequate experimental data.

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 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 or without metabolism with skin cells.

Fordiphenyl methylphosphonateno data is available concerning its elimination. Concerning the above mentioned behaviour predicted for its metabolic fate, it is very likely that the parent substance will be excreted after extensive metabolism as metabolites and/or conjugates. A part might be excreted unchanged (low molecular weight / water solubility).

Based on its chemical structure and its molecular weight, it is assumed to be excretedvia the urineand to a minor extent via the bile as conjugates with glucuronic acid. However, it has to be kept in mind, that metabolites conjugated to glucuronic acid, can be subject to entero-hepatic recycling, and re-enter the system. Additionally, phenols can also be exhaled to a smaller percentage.

Applicant's summary and conclusion

Conclusions:

An extensive assessment of the toxicological behaviour of diphenyl methylphosphonate 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 diphenyl methylphosphonate, the available physico-chemical and toxicological data have been evaluated. The substance is expected to be absorbed well after oral exposure, based on its low molecular weight, its water solubility and its logPow of 2.8. Concerning the absorption after exposure via inhalation, as the chemical has low vapour pressure, it is clear, that the substance is poorly available for inhalation. Given its lipophilicity (logPow 2.8 - if absorbed - it is expected to be absorbed directly across the respiratory tract epithelium or through aqueous pores.Diphenyl methylphosphonate is expected to be also absorbed following dermal exposure into the stratum corneum and into the epidermis, due to its molecular weight and its logPow. Concerning its distribution in the body diphenyl methylphosphonate is expected to be distributed also into the cells, due to its logPow. The substance does not indicate a significant potential for accumulation. Diphenyl methylphosphonate is expected to be metabolised mainly via unspecific esterases to firstly monophenyl methylphosphonate and later methylphosphonic acid and phenol and to a minor extent via Cytochrome P450s. The test item and its metabolites will be eliminated via urine or to a minor extent via the faeces i.e. as glucuronic acid conjugates.