Registration Dossier

Administrative data

Endpoint:
basic toxicokinetics, other
Remarks:
Expert statement
Type of information:
other: Expert statement
Adequacy of study:
key study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: No GLP-conform guideline study, but scientifically valid expert statement based i.a. on studies assessed with Klimisch 1 or 2

Data source

Reference
Reference Type:
other: Expert Statement
Title:
Unnamed
Year:
2018
Report Date:
2018

Materials and methods

Objective of study:
absorption
distribution
excretion
metabolism
toxicokinetics
Test guideline
Qualifier:
no guideline required
Principles of method if other than guideline:
An extensive assessment of the toxicokinetic behaviour of 1-(2-hydroxy-3-sulphonatopropyl)pyridinium, inner salt (PPSOH) was performed, taking into account the chemical structure, the available physico-chemical and toxicological data.
GLP compliance:
no

Test material

Reference
Name:
Unnamed
Type:
Constituent
Details on test material:
SMILES Code: OC(C[n+]1ccccc1)CS(=O)(=O)[O-]
Radiolabelling:
other: not applicable

Test animals

Species:
other: not applicable
Strain:
other: not applicable

Administration / exposure

Route of administration:
other: All relevant routes of administration are discussed in the expert statement.
Vehicle:
other: not applicable
Details on exposure:
not applicable
Doses / concentrations
Remarks:
not applicable
Control animals:
other: not applicable
Positive control:
not applicable
Details on study design:
not applicable
Details on dosing and sampling:
not applicable
Statistics:
not applicable

Results and discussion

Main ADME resultsopen allclose all
Type:
absorption
Results:
The relevant absorption rates were estimated to: Oral absorption: approx. 100% Dermal absorption: approx. 10% Inhalative absorption: approx. 100%
Type:
distribution
Results:
Systemic bioavailability of the substance is very high. A high peak exposure can be expected, a very relevant AUC is not to be expected.
Type:
metabolism
Results:
N-dealkylation, aliphatic hydroxylation, and N-oxidation were identified as the mode of action during Phase-I-metabolism, and subsequent conjugation is expected.
Type:
excretion
Results:
PPSOH and its estimated metabolites are small, charged, hydrophilic and very soluble in water. A very fast excretion of the compounds via the kidneys and urine can be expected. PPSOH has a minor potential for bioaccumulation, and will be excreted rapidly.

Toxicokinetic / pharmacokinetic studies

Details on absorption:
Absorption
In this chapter, the physico-chemical properties of the substance are used to draw general conclusions for its behaviour and how these properties will influence its oral, inhalatory and dermal absorption. Furthermore, these conclusions will be supported by the available literature data and studies, both on PPSOH and a read-across substance, PPS.
In general, absorption of a chemical is possible, if the substance crosses biological membranes. In case where no transport mechanisms are involved, 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 g/mol are favourable for absorption). Generally, the absorption of chemicals which are surfactants or irritants may be enhanced, because of damage to cell membranes. However, since PPSOH resp. PPS was found to be non-irritating to the skin and eye, the possibility of an enhanced absorption due to damaged cell membranes can be excluded. This was shown in each a dermal toxicity study on PPS, a guinea pig maximisation test on PPS, an OECD 404 skin irritation study on PPS, an OECD 405 eye irritation study on PPS, and a in vitro skin irritation test on PPSOH.
Due to the lack of experimental absorption data, the following physico-chemical parameters of PPSOH will be taken into account when discussing its absorption into the body:
- Molecular weight = 217.24 g/mol
- Water solubility = 1280 g/L @ 23°C
- Partition Coefficient log Pow = <-2.0
- Vapour pressure = 7.12E-08 Pa at 25°C (estimated)
- Melting / Boiling point: The test material melted at 245.4 °C under degradation (OECD 102, capillary method).
- Particle size: Not available, the substance is only manufactured, distributed and used in an aqueous solution.

Absorption from the gastrointestinal tract
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 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. Consequently, immediate Cytochrome P450 metabolism is less important here as for substances which directly enter the hepatic system via the portal vein.
According to ECHA’s guidance R.7c [ECHA 2008], it is stated that the smaller the molecule the more easily it may be taken up. Molecular weights below 500 g/mol are favourable for absorption. With a molecular weight of 217.24 g/mol, absorption in general can be considered as possible. Also, substances with moderate log P values (between -1 and 4) are favourable for absorption by passive diffusion. The negative log Pow of <-2.0 indicates that PPSOH is more soluble in water than in octanol, i.e. hydrophilic. Water-soluble substances will readily dissolve into the GI fluids. However, absorption of very hydrophilic substances by passive diffusion may be limited by the rate at which the substance partitions out of the GI fluid. The high water solubility of PPSOH (1280 g/L) and the log Pow value of <-2.0 indicates that absorption from the GI tract may be limited. Absorption of rather hydrophilic substances by passive diffusion may be limited by the rate at which the substance partitions out of the gastrointestinal fluid. However, if the molecular weight is low (less than 200) the substance may pass through aqueous pores or be carried through the epithelial barrier by the bulk passage of water. With a molecular weight of 217.24 g/mol, absorption via the bulk passage of water may be not hindered, if, only to a minor extent.
The substance is not readily biodegradable, no degradation observed (DOC) and presumably hydrolytically stable, as hydrolysis can be excluded due to the lack of functional groups that are hydrolytically reactive. So it is less likely that degradation products will occur and hence, those do not need to be regarded, only PPSOH as such is relevant for absorption. A possible metabolism does only have to be regarded after absorption already has occurred.
Taking into account the acute and short term oral toxicity data, no relevant further information can be gathered from the acute oral toxicity study and the OECD 422 study on PPS due to the lack of relevant effects noted. The determined oral LD50 values were LD50 > 5000 mg/kg (PPSOH, rat, OECD 423), no mortality or signs of toxicity occurred, and LD50 > 5000 mg/kg (PPS, rat, OECD 401), the animals showed slightly ruffled fur, which is either indicative for a non-adsorption or very low toxicity of the test item. No treatment-related adverse / toxicologically relevant effects were noted up to 1000 mg/kg bw/day, the highest dose tested in the available OECD 422 study on the read-across substance PPS. So it is evident from the physico-chemical parameters and the OECD 401 on PPS, that in general absorption via the GI-tract has occurred. The lack of toxic effects in the other studies is hence less indicative that no absorption has occurred but rather that the substance less hazardous.
So in summary, taking into account the available physico-chemical data of PPSOH, especially its small molecular weight and high water solubility, its absorption via the GI tract can be considered to be given. This conclusion is supported by minor effects after oral application in an OECD 401 study on PPS indicating a distribution of the compounds throughout the body. Due to the lack of further data, an absorption of 100% should be taken into account when performing the subsequent risk assessment, although the actual absorption is possibly less.

Absorption from the respiratory tract
Concerning absorption in the respiratory tract, any gas, vapour or other substances inhaled as respirable dust (i.e. particle size ≤ 15 µm) has to be sufficiently lipophilic to cross the alveolar and capillary membranes (moderate LogPow values between 0-4 are 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. Any lipophilic compound may be taken up by micellular solubilisation but this mechanism may be of particular importance for highly lipophilic compounds (log P >4), particularly those that are poorly soluble in water (1 mg/l or less) that would otherwise be poorly absorbed [ECHA, 2008].
PPSOH has a very low vapour pressure, 7.12E-08 Pa at 25°C (estimated), test material melted at 245.4 °C under degradation, clearly showing that the inhalative absorption as a gas does not have to be regarded.
The substance is only manufactured, distributed and used in an aqueous solution, so no particles of inhalable size are present. Further, there is only a very limited potential for inhalation during use, e.g. the aerosol formation of solutions of PPSOH is very low. Nevertheless, in order to estimate the potential of the substance to be absorbed via the inhalatory route for this rare cases, its physico-chemical properties will be taken into account, although handling of the substance does not lead to a significant exposure due to implemented Risk Management Measures, and the potential absorption can only be regarded theoretically.
For absorption of deposited material similar criteria as for GI absorption can be applied. In general, either a prolonged exposure due to deposition and subsequent absorption or immediate absorption by micellular solubilisation has to be assumed. The latter mechanism may be of particular importance for highly lipophilic compounds (LogPow >4), particularly those that are poorly soluble in water (1 mg/l or less) and is hence not relevant here. To be readily soluble in blood, a gas, vapour or dust must be soluble in water and increasing water solubility would increase the amount absorbed per breath. However, the gas, vapour or dust must also be sufficiently lipophilic to cross the alveolar and capillary membranes. Therefore, a moderate log P value (between -1 and 4) would be favourable for absorption. Generally, liquids, solids in solution and water-soluble dusts would readily diffuse/dissolve into the mucus lining the respiratory tract. Hence, with a LogPow of <-2.0 and a water solubility of 1280 g/L at 23°C, the absorption of this fraction which may not be subjected to ciliary clearance can be considered as present but may be may be limited by the rate at which the substance partitions out of the fluid in the lung surface.
In summary, assuming that PPSOH may be in in theory able to reach the lower respiratory tract, and taking into account its potential to be absorbed and a certain precaution due to the lack of toxicokinetic test data, the inhalative absorption can be estimated to be 100% as a worst case assumption, although the actual absorption is likely to be less.

Absorption after dermal exposure
In general, dry particles will have to dissolve into the surface moisture of the skin before uptake can begin. 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 through the skin and substances above 500 g/mol 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.
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 of PPSOH, an evaporation after skin contact does not need to be regarded due to the high decomposition temperature and low vapour pressure, and hence it can be assumed that the substance will remain on the skin until mechanical removal. Furthermore, since the substance is neither corrosive nor a skin irritant, additional absorption-enhancing effects can be disregarded, too.
The molecular weight of the substance is with 217.24 g/mol rather low, which in general indicates a certain potential to penetrate the skin. However, due to the negative logPow of <-2.0, the high water solubility (1280 g/L) and hence hydrophilicity, it is nearly impossible for PPSOH to penetrate the stratum corneum and to be absorbed via the skin. PPSOH is not sufficiently lipophilic to cross the lipid rich environment of the stratum corneum and thus dermal uptake will be low [ECHA, 2015]. This is in accordance with the result of the acute dermal toxicity study on PPS, in which no systemic toxicity or local irritation was found up to 2000 mg/kg bw in male and female rats. Similarly, in the skin sensitisation study (GPMT on PPS) a negative result was obtained, also not indicating that some dermal absorption takes place.
Therefore, a diminished dermal absorption rate can be assumed, most reasonably a resulting maximum dermal penetration of PPSOH of 10% as a worst case, which is compliant with i.a. the European ECHAs Guidance documents [ECHA, 2008] and scientifically reasonable when e.g. performing route-to-route extrapolations during risk assessment.
Details on distribution in tissues:
Distribution
In general, it can be stated that the smaller the molecule, the wider is its 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 is not possible to absolutely foresee protein binding, which can limit the amount of a substance available for distribution. In the case of PPSOH however, ToxTree modelling [Ideaconsult Ltd, 2004-2013] gave one protein binding alert, as the substance was identified to undergo arylic nucleophilic substitutions. The reactivity of pyridine ring can however not be predicted. Toxtree modelling however gave no DNA-binding alert, was is consistent with the negative genotoxicity tests. Further, consistent with the negative GPMT, no skin sensitizing effects were noted. One of the key events in skin sensitization is protein binding of the hapten to form an antigen. So as the substance is not a skin sensitizer, it is likely that no protein binding at all occurs. So it can be assumed that, if any, only a very minor portion of the applied and absorbed amount is not distributed through the entire body but will be retained on the site of impact, which can hence be neglected.
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. ToxTree modelling of the CYP450 metabolites indicated that various modifications of the molecule occur. Not only general structure of PPSOH remains intact and is only modified, also the pyridine ring could be separated from the side chain. The addition of a hydroxyl group could only enhance water solubility, but taking into account the already very high water solubility of 1280 g/l and the low logPow of <-2.0, a relevant enhancement of these parameters is not expected. Hence, distribution of the metabolites is expected to follow a similar pattern than the one of the parent compound and no differentiation between the parent compound and metabolites has to be made regarding distribution.
Small water-soluble molecule and ions will diffuse through aqueous channels and pores. The rate at which very hydrophilic molecules diffuse across membranes could limit their distribution. In case of PPSOH, no quantitative data is available for distribution patterns. Taking into account its rather low molecular weight of 217.24 g/mol, its hydrophilicity and high water solubility, the absolute systemic bioavailability is very high, certainly rather in the aqueous compartment and to a lesser extent in fatty tissues. After oral exposure, the first target will be the gastrointestinal tract, where the substance and possibly bacterial metabolites will be absorbed in high quantities, most likely close to 100% of the ingested amount, and transferred via the blood stream to the liver. After reaching the liver via the portal vein, the substance will be further distributed via the bloodstream. Here, especially the kidneys due to their filter function and the heart due to its enormous need for nutrients and consequently large blood flow through coronary arteries will be exposed.
Due to the hydrophilicity, high water solubility and small size, a possible accumulation can be neglected. Since the solubility and hence absorption via the GI tract of the parent compound is rather high, a high peak exposure to the compound(s) and hence high systemic bioavailability can be expected. However, due to their tendency to be excreted rather fast, a very relevant AUC is not to be expected. After the poorer absorption of the parent compound via dermal route, a minor peak exposure has to be considered. The affection of the lymphatic system via micellar uptake however is only of minor importance.
These conclusions are based on the physico-chemical properties of PPSOH, and which are also applicable for its metabolites. From the repeated dose OECD 422 study, no further conclusions on its distribution can be drawn, as the substance did not show any other toxicologically relevant effects.
As a consequence, a wide distribution of the test items throughout the body can be reasonably assumed. Also, due to the nature of the observed effects, secondary effects due to an localized phenomenon, e.g. inflammation of the GI tract without substance uptake, can be excluded.
Details on excretion:
Excretion
In general, 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 GI tract 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.
For PPSOH no test data is available regarding its elimination. Concerning the above mentioned behaviour predicted for its metabolic fate, it is unlikely that the parent substance will be excreted unchanged. Nevertheless a similar behaviour regarding excretion can be assumed for the parent compound and most of its degradation products. PPSOH and its known and estimated degradation products are small, charged and hence hydrophilic and very soluble in water. So a very fast excretion of the compounds via the kidneys and so urine can be expected. Excretion via the GI tract (unabsorbed material) and via the bile and consequent subjection to enterohepatic recycling can be neglected.

Metabolite characterisation studies

Metabolites identified:
yes
Details on metabolites:
For details, see attached file. Metabolites of the substance are estimated to be formed via N-dealkylation, aliphatic hydroxylation, and N-oxidation

Any other information on results incl. tables

See attached expert statement.

Applicant's summary and conclusion

Conclusions:
The present expert statement covers all relevant toxicokinetic parameters to assess the behaviour of 1-(2-hydroxy-3-sulphonatopropyl)pyridinium, inner salt (PPSOH) in the body, the available information is sufficient to enable one to perform a proper risk assessment. Hence, no further information needs to be gathered and further studies can be omitted due to animal welfare. In conclusion, the substance has no potential for bioaccumulation in its non-metabolized or metabolized form.
Executive summary:

In order to assess the toxicokinetic behaviour of 1-(2-hydroxy-3-sulphonatopropyl)pyridinium, inner salt (PPSOH), the available toxicological, ecotoxicological and physico-chemical data were evaluated, also derived from a suitable read-across substance, PPS, as toxicokinetic test data is lacking.

The molecular weight of 217.24 g/mol, a LogPow of <-2.0, and a water solubility of 1280 g/L, a high potential for oral absorption is given. The substance is not ready biodegradable and hydrolytically stable, so degradation products do not need to be considered. Taking into account the OECD 401 oral study on PPS, a certain absorption of the test compounds is evident, as signs of toxicity were seen. Hence, a preliminary occurrence of oral absorption of the test item after gavage can reasonably deducted.

PPSOH has a very low vapour pressure and decomposes during melting, so the inhalative absorption as a gas does not have to be regarded. As the substance is only manufactured, distributed and used in an aqueous solution, no particles of inhalable size are present, and the aerosol formation of solutions of PPSOH is very low, so inhalative absorption is only regarded in theory. With a very low logPow and a high water solubility, the absorption of the fraction which may not be subjected to ciliary clearance can be considered as rather high.

The substance is neither corrosive nor a skin irritant, additional absorption-enhancing effects can be disregarded. The low molecular weight indicates a certain potential to penetrate the skin, but due to the negative logPow of <-2.0, the high water solubility (1280 g/L) and hence hydrophilicity, it is nearly impossible for PPSOH to penetrate the stratum corneum and to be absorbed via the skin.

So in summary, the absorption rates may be estimated to:

-      Absorption via oral route: 100%

-      Absorption via inhalative route: 100%

-      Absorption via dermal route: 10%

The absolute systemic bioavailability of PPSOH is very high. Similar distribution patterns can be expected for its metabolites and a possible accumulation can be neglected. A high peak exposure can be expected, but due to their tendency to be excreted rather fast, a very relevant AUC (Area under the curve, i.e. measure for systemic bioavailability) is not to be expected. After the poorer absorption of PPSOH via the dermal route, a minor peak exposure has to be considered.

The negative logPow and the very high water solubility are clearly indicating that for PPSOH, a rather fast excretion can be expected and the potential of PPSOH for bioaccumulation in its classic sense is virtually not existing.

Regarding its metabolic fate, N-dealkylation, aliphatic hydroxylation, and N-oxidation were identified as the mode of action during Phase-I-metabolism. The metabolites formed are not expected to modify essentially the molecular weights, physico-chemical properties and hence ADME behaviour of PPSOH. It is most likely that the substance of interest will be subject to metabolism by cytochrome P450 enzymes and subsequent conjugation. It is unlikely that PPSOH will be excreted unchanged. PPSOH and its estimated metabolites are small, charged and hence hydrophilic and very soluble in water. So a very fast excretion of the compounds via the kidneys and so urine can be expected.

In conclusion, PPSOH has a minor potential for bioaccumulation, and will be excreted rapidly after metabolism.