3-cyclohexylaminopropane-1-sulphonic acid

Basic toxicokinetics

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:

2017

Report date:

2017

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 3-(Cyclohexylamino)-propane sulfonic acid (CAPS) was performed, taking into account the chemical structure, the available physico-chemical and toxicological data.

GLP compliance:

no

Test material

Test material information

Constituent 1

Reference substance name:

3-cyclohexylaminopropane-1-sulphonic acid

EC Number:

214-492-1

EC Name:

3-cyclohexylaminopropane-1-sulphonic acid

Cas Number:

1135-40-6

Molecular formula:

C9H19NO3S

IUPAC Name:

3-cyclohexylaminopropane-1-sulphonic acid

Radiolabelling:

other: not applicable

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 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 reference chemical:

not applicable

Details on study design:

not applicable

Details on dosing and sampling:

not applicable

Statistics:

not applicable

Results and discussion

Main ADME results

Main ADME results 1

Type:

absorption

Results:

The relevant absorption rates were estimated to: Oral absorption: approx. 100% Dermal absorption: approx. 10% Inhalative absorption: approx. 100%

Main ADME results 2

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.

Main ADME results 3

Type:

metabolism

Results:

N-dealkylation, Amine hydroxylation and Aliphatic hydroxylation were identified as the mode of action during Phase-I-metabolism, and subsequent conjugation is expected.

Main ADME results 4

Type:

excretion

Results:

CAPS 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. CAPS 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.
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 CAPS 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, a LLNA sensitization study, and an in vitro BCOP eye irritation study, and is supported by the determination of the pH of CAPS (4.2 – 5.7 (pure compound), 6.2 (10% aqueous solution)).
Due to the lack of experimental absorption data, the following physico-chemical parameters of 3-(Cyclohexylamino)-propane sulfonic acid will be taken into account when discussing its absorption into the body:
- Molecular weight = 221.317 g/mol
- Water solubility = 184 g/L @ 20°C
- Partition Coefficient log Pow calculated as -0.778 ± 0.020, stated as < 0.3
- Vapour pressure = 0.000000169 Pa at 25 °C (estimated)
- Melting / Boiling point: The substance decomposes (at ca. 327 °C) before melting occurs
- Particle size: Average particle size of the test substance = 4.848 µm (D10 = 1.15 µm, D50 = 4.10 µm, D90 = 9.64 µm); ~ 100% <100 µm (inhalable fraction), ~ 90% <10 µm (thoracic fraction), ~50% < 4 µm (respirable fraction)

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 221.317 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. With a LogPow of -0.778 / < 0.3 at 20°C, a passive diffusion in the gastrointestinal fluids is indicated, although by being only slightly above the boundary value, a beginning slightly hindered absorption e.g. by diffusion might be indicated. Nevertheless, based on the Log Pow, absorption must be taken into account.
Nevertheless, a substance with such a log P value can be slightly less soluble in lipids and hence not readily absorbed when its water solubility is very low [ECHA 2008]; only water-soluble substances will readily dissolve into the gastrointestinal fluids and hence be available for absorption. With a water solubility of 184 g/L at 20°C, an ample water solubility is given. Nevertheless, 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.
The substance is not readily biodegradable and presumably hydrolytically stable, so it is less likely that degradation products will occur and hence, those do not need to be regarded, only CAPS as such is relevant for absorption. A possible metabolism as described in chapter 3.4 Metabolism does only have to be regarded after absorption already has occurred.
Taking into account the acute and short oral toxicity data, no further information can be gathered from the acute oral toxicity study and the OECD 421 study due to the lack of effects noted. No treatment-related were noted up to 800 mg/kg bw/day, the highest dose tested. The only information on absorption can be drawn from the OECD 407 study. It is the only study in which effects were noted in the tested concentration range. So it is evident that 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.
In the OECD 407 study, a decrease in the number leukocytes and increased concentration of chlorides in males from groups 2 and 3 were observed. Those effects are strongly indicative for a wide distribution of the test items throughout the body. Hence, a preliminary occurrence of oral absorption of the test item after gavage can reasonably deducted.
So in summary, taking into account the available physico-chemical data of 3-(Cyclohexylamino)-propane sulfonic acid, especially its small molecular weight and high water solubility, its absorption via the GI tract can considered to be very high. This conclusion is supported by minor effects after oral application in an OECD 407 study 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.

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].
CAPS has a very low vapour pressure, 0.000000169 Pa at 25 °C (estimated), and decomposes without melting or boiling at 327°C, clearly showing that the inhalative absorption as a gas does not have to be regarded.
According to its particle size distribution, the average particle size of the test substance is 4.848 µm (D10 = 1.15 µm, D50 = 4.10 µm, D90 = 9.64 µm). Hence, the following distribution is biologically relevant: ~ 100% <100 µm (inhalable fraction), ~ 90% <10 µm (thoracic fraction), ~50% < 4 µm (respirable fraction). Those values are taken from the recent version of ECHA’s Guidance document [ECHA, 2015], the former version gave the following information: In humans, particles with aerodynamic diameters below 100 μm have the potential to be inhaled. Particles with aerodynamic diameters below 50 μm may reach the thoracic region and those below 15 μm the alveolar region of the respiratory tract [ECHA, 2008].
As a consequence, there is a rather large fraction potentially reaching the alveolar region of the respiratory tract beyond the bronchi, where no mechanical excretion mechanism as the ciliary movements is available. Although handling of the substance does not lead to a significant exposure due to implemented Risk Management Measures, the potential absorption will be regarded theoretically.
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 -0.778 / < 0.3 at 20°C and a water solubility of 184 g/L at 20 °C, the absorption of this fraction which may not be subjected to ciliary clearance can be considered as rather high.
In summary, taking into account the fraction of CAPS being able to reach the lower respiratory tract, 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.

Absorption after 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 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 CAPS, 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.
In general, it can be assumed that the substance will be predominantly present in the dissociated and hence charged anion will be present when getting into contact with the moistened skin. The molecular weight of the anion is with 220.32 g/mol rather low, which in general indicates a certain potential to penetrate the skin. However, due to the negative logPow of -0.778 ± 0.020 (stated as < 0.3), the high water solubility (184 g/L) and hence hydrophilicity, it is nearly impossible for 3-(Cyclohexylamino)-propane sulfonic acid to penetrate the stratum corneum and to be absorbed via the skin.
Therefore, a diminished dermal absorption rate can be assumed, most reasonably a resulting maximum dermal penetration of 3-(Cyclohexylamino)-propane sulfonic acid 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 CAPS however, ToxTree modelling [Ideaconsult Ltd, 2004-2013] gave no protein binding alerts. 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 of CAPS, no quantitative data is available for distribution patterns. Taking into account its rather low molecular weight of 221.317 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.
Possible metabolites and degradation products are expected to be of the same or smaller size and at least same or higher hydrophilicity as the parent compound. Hence, similar distribution patterns can be expected and no differentiation between the parent compound and metabolites has to be made regarding distribution. 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 complete, 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 CAPS, and which are also applicable for its metabolites, are furthermore supported by the results of repeated dose OECD 407 study. There was an increased concentration of chlorides in males during blood serum biochemistry examinations observed, which might be related to the test item. Hyperchloremia can lead to metabolic acidosis. Further, a statistically significant decrease in the number of leucocytes in males and females in groups 2 and 3, an increase in the number of neutrocytes and a decrease in the number of lymphocytes in females from group 3 were stated. As so hematological parameters of blood were affected, it is clearly indicated that the substance was distributed in the aqueous compartments of the whole body. As the substance did not show any other toxicologically relevant effects, no further conclusions on its distribution can be drawn.
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 CAPS 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. The exemption will be made for CO2 as it is expected to be excreted via exhalation. CAPS and its 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, Amine hydroxylation and Aliphatic hydroxylation.

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 3-(Cyclohexylamino)-propane sulfonic acid 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 3-(Cyclohexylamino)-propane sulfonic acid, the available toxicological, ecotoxicological and physico-chemical data were evaluated, as toxicokinetic test data is lacking.

With the molecular weight of 221.317 g/mol, a LogPow of -0.778 ± 0.020 (stated as < 0.3), and a water solubility of 184 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 repeated dose OECD 407 oral toxicity study, a certain absorption of the test compounds is evident, as changes in blood serum biochemistry and haematology can be seen. Hence, a preliminary occurrence of oral absorption of the test item after gavage can reasonably deducted.

CAPS has a very low vapour pressure and decomposes without boiling, so the inhalative absorption as a gas does not have to be regarded. Based on the distribution of the particle sizes, a relevant fraction reaches the alveolar region of the respiratory tract beyond the bronchi. So either a prolonged exposure due to deposition and subsequent absorption or immediate absorption by micellular solubilisation has to be assumed, and 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 -0.778 ± 0.020 (stated as < 0.3), the high water solubility (184 g/L) and hence hydrophilicity, it is nearly impossible for CAPS 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 the substance 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 CAPS via the dermal route, a minor peak exposure has to be considered. The sites of the observed effects in the repeated dose toxicity study support a wide distribution of the test item throughout the body.

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

Regarding its metabolic fate, N-dealkylation, Amine hydroxylation and Aliphatic hydroxylation 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 the substance. 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 CAPS will be excreted unchanged. the substance 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, 3-(Cyclohexylamino)-propane sulfonic acid has a minor potential for bioaccumulation, and will be excreted rapidly after metabolism.