Registration Dossier

Administrative data

Endpoint:
basic toxicokinetics
Type of information:
other: Assessment based on available information
Adequacy of study:
key study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: see 'Remark'
Remarks:
A full toxicokinetic assessment of the substance is not deemed necessary. The toxicokinetic profile of this test substance has been addressed in a variety of studies since manufacture of the substance commenced in the 1950's, with detailed assessments throughout publically available literature being available. As such, it is deemed appropriate on animal welfare grounds to provide this as a written synopsis assessment only, rather than a definitive study. This is detailed below in "overall remarks".

Data source

Reference
Reference Type:
other: Assessment based on existing data
Title:
Unnamed
Year:
2010
Report Date:
2010

Materials and methods

Objective of study:
toxicokinetics
Test guideline
Qualifier:
no guideline followed
Principles of method if other than guideline:
Assessment of available data
GLP compliance:
no

Test material

Reference
Name:
Unnamed
Type:
Constituent
Test material form:
liquid: viscous
Radiolabelling:
no

Test animals

Details on test animals and environmental conditions:
Assessment of available data

Administration / exposure

Details on exposure:
Assessment of available data
Details on study design:
Assessment of available data

Results and discussion

Toxicokinetic / pharmacokinetic studies

Details on absorption:
Assessment of available data
Details on distribution in tissues:
Assessment of available data
Details on excretion:
Assessment of available data

Metabolite characterisation studies

Details on metabolites:
Assessment of available data

Bioaccessibility

Bioaccessibility testing results:
Assessment of available data

Any other information on results incl. tables

Organophosphorus compounds are usually esters, amides or thiol derivatives of phosphonic acid. They form a large family of ca. 50 000 chemical agents with biological properties that have important and sometimes unique implications for man.

Toxicokinetics of these types of compounds are fairly well documented and understood from works on associated pesticides and industrial chemicals, and are widely available within the public domain literature. As such, further investigation of these types of effects via further experimental animal studies are not considered appropriate. 

Most of the ill-health following exposure to organophosphorus compounds has been attributed to the inhibition of cholinesterases. However, current literature has justifiably challenged this view, as the inhibition of cholinesterases by itself cannot account for the wide range of disorders that have been reported following organophosphorus poisoning. It is becoming apparent that, although inhibition of cholinesterases plays a key role in the toxicology of organophosphates, individual susceptibility, the inhibition of other enzyme systems and the direct effects of organophosphates on tissues are also important. The absorption, distribution, metabolism, and elimination of organophosphates are therefore critical to the toxicological effects of these compounds.

Mechanism Of Toxic Effects

 As discussed above, one mode of action of organophosphate compounds is the phosphorylation and inactivation of acetylcholinesterases. This causes an increase and accumulation of acetylcholine at nerve endings, stimulating neuro-effector junctions, skeletal neuro-muscular junctions, autonomic ganglia and in the brain. Overstimulation causes a depolarising block of neuromuscular junction receptors. This gives rise to a large number of clinical effects in the central nervous system, autonomic nervous system and leads eventually to paralysis. 

After the initial organophosphate acetylcholinesterase bonds are formed a conformational change in the molecular structure of the organophosphate occurs which increases the binding and subsequently makes the organophosphate-acetylcholinesterase complex irreversibly bound. This process is called “ageing”and ishighly dependent upon the type of organophosphatesuch that significant aging varies between, 2-36 hours after initial binding.

 In addition to acetylcholinesterase inactivation and subsequent acetylcholine accumulation there is also central nervous system antagonism ofγ-aminobutyric acid (GABA) and dopaminergic neurons. Neurocognitive effects and late onset peripheral neuropathy is well described 

Mechanism of action

Most organophosphates are highly lipid-soluble agents and are well absorbed from the skin, oral mucous membranes, conjunctiva and gastrointestinal and respiratory routes. The onset, severity and duration of toxicity is determined by the dose, route of exposure, physicochemical properties of the organophosphate (e.g. lipid solubility), rate of metabolism (whether transformation in the liver is required before the compound becomes toxic) and whether the organophosphorylated cholinesterase “ages” rapidly.

When inhibition of cholinesterases does occur, assays of plasma butyryl cholinesterase and red blood cell acetylcholinesterase (AChE) are widely used for confirming and assessing exposure.

Exposure to organophosphorus agents causes sequential toxic effects in man. In most instances the earliest cholinergic phase may only be observed. This cholinergic phase progresses to the intermediate syndrome in ~20% of subjects. Both the acute cholinergic phase and the intermediate syndrome are associated with a high risk of mortality. The final phase, organophosphate-induced delayed polyneuropathy, which does not carry the risk of death, sets in 7–21 days after exposure to an organophosphorus agent and may not be preceded by either the cholinergic phase or the intermediate syndrome.

The inactivation of the cholinesterases occurs in the blood and in a wide range of nerve, neuromuscular (skeletal, smooth and cardiac) and glandular tissues where these enzymes have a role in cell-to-cell communication and the hydrolysis of xenobiotics. These enzymes have possible (but as yet unidentified) roles such as cell development and growth. The inhibition of AChE leads to the accumulation of acetylcholine, the neurotransmitter at all ganglia in the autonomic nervous system and at many synapses in the brain, skeletal neuromuscular junctions, at some postganglionic nerve endings of the sympathetic nervous system and adrenal medulla. The role of butyryl cholinesterase in the body is yet to be fully identified, but it is known to be involved in the hydrolysis of many therapeutic agents (e.g. suxamethonium, esmolol, procaine and cocaine). There are many other roles speculated for butyryl cholinesterase and these include cellular differentiation and growth, as a scavenger in xenobiotic exposure and as a modulator in lipid metabolism.

The consequences of inhibition of other enzyme systems by organophosphorus compounds are as yet uncertain. A variety of tissue carboxylesterases exist in the serum, liver, intestine and other tissues. Although inhibition of one specific carboxylesterase (neuropathy target esterase) has toxic effects, no direct detrimental effects of inhibition of other carboxylesterases have been demonstrated. However, carboxylesterases may contribute markedly to the metabolic degradation of

organophosphates and inhibition of these enzymes may increase the toxicity of organophosphorus compounds. The search for effects of inactivation or changes in other physiological systems is still currently under investigation. The following effects of organophosphorus agents have been demonstrated in animals and are theoretically possible effects in man:

1. Inactivation by phosphorylation of other beta esterases.

2. Altering the release of neurotransmitters, e.g.γ-aminobutyric acid (GABA) and glutamate.

3. Increasing the number of GABA and dopamine receptors.

4. Acting as agonists at M2/M4 muscarinic receptors.

5. Inhibition of mitochondrial enzymes, respiration and ATP generation.

6. Induction of mast cell degranulation, probably causing the release of histamine or histamine-like compounds.

With regards to study specific information on toxicokinetics, the following data on organophosphates is available from review of the available literature:

Absorption

All organophosphates are known to be absorbed from the small intestine or dermal exposure. Peak concentrations may occur within a few hours, although rate of absorption is known to be dependant on the chemical structure of the organophosphate in question. The following routes are considered in studies on organophosphates available within the literature, and are provided here as an indication: 

Dermal

No specific studies were identified that investigated the dermal absorption of the organophosphates in humans.

It has been suggested that similarities with regard to structure and physical properties among the isomeric tricreysl phosphates (one form of organophosphate) make it likely that the isomers of this type of organophosphate could also be readily absorbed through the skin (NTP 1994). In the cat, 73% of the radioactivity from a 50-mg/kg dose of14C-tricreysl phosphate was no longer present at the application site (intrascapular region) after 12 hr. Maximum concentrations of radioactivity were reached in the examined tissues within 24 hr. By day 10, at least 48% of the dose was absorbed as indicated by urinary and fecal excretion data (Nomeir and Abou-Donia 1986, as reviewed by NTP 1994).

Hodge and Sterner (1943), described by IPCS (1990), found that32P- tricreysl phosphate (200 mg/kg) was poorly absorbed through dog abdominal skin. The absorption of 2 to 4 mg/kg tricreysl phosphate by human palm skin was approximately 100 times faster than through the dog abdominal skin based on urinary excretion and surface-area data. Additional details were not provided.

Inhalation

No definitive studies in support of the registration have been identified that have investigated the absorption of organophospates in humans or laboratory animals following inhalation exposure.

Oral

At least 41% of a single gavage dose of 7.8 mg/kg14C-labeled tricresyl phosphate in rats was excreted in the urine over 7 d following administration (Kurebayashi et al. 1985). About 12% of a single gavage dose of 89.6 mg/kg in rats was excreted in the urine. Most of the urinary excretion occurred within the 24 hr after administration.

Distribution

Distribution of the metabolites of an organophosphate substance is known to occur to a wide variety of tissues, although evidence suggests that these do not bioaccumulate on the basis of the excretion data. An organophosphate will undergo significant alteration following adsorption in the body to form a diverse group of compounds with a wide range of lipid/water solubility characteristics and variable volume of distribution. Dermal

Data available indicates that the distribution of radioactivity in the dog following a single 200-mg/kg application of32P-tricresyl phosphate to the abdominal skin was highest in the liver followed by the blood, kidney, lung, muscle and spinal cord, brain and sciatic nerve at 24 hr post-exposure (Hodge and Sterner 1943, as reviewed by IPCS 1990). In cats, the highest levels of radioactivity occurred in the bile, gall bladder, urinary bladder, kidney, and liver at 1–10 d after application of 50 mg/kg of14C- tricresyl phosphate (Nomeir and Abou-Donia 1986, as reviewed by IPCS 1990). In addition, low levels of radioactivity were found in the spinal cord and brain. Analysis showed that the parent compound was found primarily in the brain, spinal cord, and sciatic nerve, while metabolites were primarily found in the liver, kidney, and lung. It is not known if the patterns of distribution for tricresyl phosphate and metabolites can be generalized to other organophosphates; however given the likely mode of action within biological systems, this cannot be precluded.

Inhalation

No definitive studies in support of the registration have been identified that have investigated the absorption of organophospates in humans or laboratory animals following inhalation exposure.

Oral

Twenty-four hr after 89.6 mg/kg of14C- tricresyl phosphate was administered by gavage to rats, the highest concentrations of radioactivity were found in the intestine (including contents), followed by the stomach, adipose tissue, liver, and kidneys (4–13-fold higher than blood concentrations). The lowest concentrations were found in heart, muscle, and brain (lower than blood concentrations) (Kurebayashi et al. 1985).

In rats,14C-organophosphates were rapidly distributed to muscle and liver following intravenous administration. This was followed by a redistribution of radioactivity to adipose tissue and skin. The parent compounds were rapidly cleared rapidly from the tissues and did not bioaccumulate. Details of the study were not reported.

Metabolism

It is understood that some organophosphates are metabolised in the liver to much more active metabolites (-oxons). These poisons are also usually highly lipid soluble. Thus the slow conversion of these substances, which are widely distributed into fat, may lead to delayed and/or prolonged cholinesterase inhibition and toxic effects. This slow redistribution and/or activation may have implications for treatment: longer treatment and late commencement may be of benefit in these patients In rats, metabolism of tricresyl phosphate following oral gavage of 7.8 or 89.6 mg/kg was found to involve successive oxidations and hydrolysis resulting in the production ofp-hydroxybenzoic acid (Kurebayashi et al. 1985). The major urinary metabolites identified werep-hydroxybenzoic acid, di-p-cresyl phosphate, andp-cresylp-carboxyphenyl phosphate. The main biliary metabolites were di-p-cresyl phosphate,p-cresylp-carboxyphenyl phosphate, and the oxidized triesters, di-p-cresylp-carboxyphenyl phosphate, andp-cresylp-carboxyphenyl phosphate. Fecal metabolites were similar to the biliary metabolites.14CO2was found in expired air following administration and appeared to be formed probably through decarboxylation ofp-hydroxybenzoic acid by intestinal microbes.

Elimination

Elimination of the substance via excreted fluids is known to happen with the majority of the metabolites excreted within a short period of time. The following routes are considered in studies on organophosphates available within the literature, and are provided here as an indication: 

Dermal

About 48% of a single dermal application of a 50 mg/kg dose of organophosphate was excreted by d 10 post-exposure with 28% of the dose excreted in the urine while 20% of the dose was excreted in the feces (Nomeir and Abou-Donia 1986, as reviewed by NTP 1994).

Approximately 40–60% of an intravenous injection of 2 or 20 mg/kg of a radiolabelled organophosphate underwent biliary excretion within 6 hr of administration (NTP 1994). It was determined that biliary excretion increased with increasing dose from 2–20 mg/kg resulting in a doubling of biliary excretion. For a number tricresyl phosphates, the percentage of administered radioactivity excreted in the feces was less than the percentage excreted in bile suggesting that the isomers underwent enterohepatic recirculation.

Inhalation

No definitive studies in support of the registration have been identified that have investigated the absorption of organophospates in humans or laboratory animals following inhalation exposure.

Oral

Excretion of radioactivity following oral administration of14C- tricresyl phosphate in rats at doses of 0.5 2, 20, and 200 mg/kg was investigated by NTP (1994). Radioactivity from tricresyl phosphate was excreted primarily in the feces at all dose levels. Radioactivity from TPCP was excreted primarily in the urine at 0.5 and 2 mg/kg and primarily in the feces at 20 and 200 mg/kg. Radioactivity from tricresyl phosphate was excreted primarily (70%) in the urine at all doses tested.

Rats that received14C- tricresyl phosphate as a single gavage dose of 7.8 mg/kg excreted 41% of the dose of radioactivity in the urine, 44% in the feces, and 18% in the expired air within 7 d (Kurebayashi et al. 1985). A majority of the excretion occurred within 24 hr. Rats with cannulated bile ducts excreted 28% of the administered radioactivity in the bile during the first 24 hr. Rats treated in a similar manner with 89.6 mg/kg of14C- tricresyl phosphate excreted 12% of the administered radioactivity in the urine, 77% in the feces, and 6% in the expired air. The radiolabeled material excreted in urine and bile was identified as metabolites of tricresyl phosphate in high dose rats (see Metabolism section for details). Parent compound was the dominant isomer excreted in the feces with some lesser amounts of metabolites present.

Measurement of urinary metabolites

An alternative or complementary approach to biological monitoring for organophosphates (by measurement of reduction in blood cholinesterases) is based on the analysis of metabolites in urine. These methods can either use metabolites specific to the organophosphate under study or the dialkyl phosphate metabolites that are common to a large number of different organophosphates [24]. In an informative review of such analyses over 10 years, Cockeret al. found that, in non-occupationally

exposed people, 95% of urinary alkyl phosphates do not exceed 72 μmol/mol creatinine. In occupationally exposed people, the corresponding 95th percentile of total urinary alkyl phosphates is 122 μmol/mol creatinine.

In volunteer studies with 1 mg oral doses of the organophosphate pesticides, chlorpyrifos, diazinon and propetamphos, the mean peak values were 160, 750 and 404 μmol/mol creatinine, respectively,

and were not associated with any reduction in blood cholinesterase activity. They concluded that the levels of organophosphate metabolites in the urine from workers potentially exposed to organophosphates are generally low and unlikely to cause a significant reduction in blood cholinesterase activity. This is the probable explanation for the lack of correlation in many instances when both urinary metabolites and red blood cell AChE activity have been measured, and, in most cases, no inhibition of red blood cell AChE was found. Thus, no correlation could be made between urinary metabolites and organophosphate levels associated with cholinesterase inhibition

Conclusions.

Depending on the compound, metabolism and absorption route, the peak excretion might be reached at different times after exposure. Absorption after dermal exposure is generally slower than after ingestion or presumably inhalation.

Toxicological effects are very much dependant on the type of organophosphate ingested; the mode of that ingestion and the type and amount of the dose. It is not possible to determine exactly the toxicokinetics of the substance subject to the registration specifically; however given the overall data available in the literature, it is proposed that the modes of action within this assessment are appropriate for the assessment of the potential toxicokinetic actions of the substance.

Applicant's summary and conclusion

Conclusions:
Interpretation of results (migrated information): low bioaccumulation potential based on study results
Depending on the compound, metabolism and absorption route, the peak excretion might be reached at different times after exposure. Absorption after dermal exposure is generally slower than after ingestion or presumably inhalation.
Executive summary:

Toxicological effects are very much dependant on the type of organophosphate ingested; the mode of that ingestion and the type and amount of the dose. It is not possible to determine exactly the toxicokinetics of the substance subject to the registration specifically; however given the overall data available in the literature, it is proposed that the modes of action within this assessment are appropriate for the assessment of the potential toxicokinetic actions of the substance.