Benzenesulfonic acid, 4-(branched alkyl derivs.) and benzenesulfonic acid, 4-(linear alkyl dervis.), calcium salts

Endpoint summary

Description of key information

The calcium sulfonate target substance (C15-C36) is expected to be absorbed to a limited extent after oral exposure, based on its high molecular weight, its low water solubility and its high LogPow. Concerning the absorption after exposure via inhalation, as the chemical is considered to have a low vapour pressure, is highly lipophilic, has a high LogPow, and has a rather high molecular weight, it is clear, that the substance is poorly available for inhalation and will not be absorbed significantly. The calcium sulfonate target substance (C15-C36) is also not expected to be absorbed following dermal exposure into the epidermis, due to its low water solubility and its fairly high molecular weight and its high LogPow. Concerning its distribution in the body the calcium sulfonate target substance (C15-C36) is expected to be distributed into the intravasal compartment and possibly also into the intracellular compartment. The substance does not indicate a significant potential for accumulation, when taking into account the predicted behaviour concerning absorption and metabolism. The calcium sulfonate target substance (C15-C36) is expected to be extensively metabolised (metabolism by cytochrome P450 enzymes, followed by omega- and beta-oxidation and cleavage of the aromatic ring and desulfonation) and to be eliminated mainly via the urine and also via the bile.

Key value for chemical safety assessment

Bioaccumulation potential:

low bioaccumulation potential

Absorption rate - oral (%):

50

Absorption rate - dermal (%):

10

Absorption rate - inhalation (%):

100

Additional information

Toxicokinetic behaviour of the calcium sulfonate target substance (C15 -C36)

The toxicokinetic profile of the test substance was not determined by actual absorption, distribution, metabolism or excretion measurements. Rather, the physical chemical properties of the calcium sulfonate target substance (C15 -C36) and acute and repeated-dose toxicity data on the calcium sulfonate target substance (C15-C36) and the calcium read across substances were used to predict toxicokinetic behaviour of the calcium sulfonate target substance (C15-C36).

Toxicological profile of the calcium sulfonate target substance (C15-C36)

The calcium sulfonate target substance (C15-C36) is a viscous, dark brown coloured liquid with a relatively high molecular weight (MW >= 817.31 and <= 1111.87 g/mol, Anon, 2013). It has a pour point of -9 °C (Anon., 2013). Via TGA, the thermal decomposition and relative weight per cent value was observed. Overall weight loss onset occurs at 183 ˚C for the oil portion of the substance whereas for the soap fraction, overall weight loss onset occurs at 417 ˚C (Anon., 2013). In addition, the calcium sulfonate target substance (C15 -C36) has a low vapour pressure, 0.01 Pa at 25°C (Tremain, 2014 and Tremain, 2013). Its relative density was reported to be 0.9432 (Anon., 2013). The calcium sulfonate read across substances (CAS 70024-69-0 and CAS 61789-86-4) are slightly soluble in water (1.69 mg/L at 20°C (Fox and White, 2011a) and 0.113 mg/L at 20°C, (Fox and White, 2011b)). The partition coefficient LogPow of the calcium read across substances has been estimated to be > 5.47 (CAS 70024-69-0; Fox and White, 2011a) or 6.65 (Fox and White, 2011b).

The calcium sulfonate read across substances were shown not to be skin or eye irritating (Kern, T.G., 1999a/b; Swan, 1972, Hoff 2002a/b, Buehler, 1990a/b, 1991 a/b, Costello, 1986b, Ohees, 1968 c/d, Gabriel, 1981b/c, Kern, 1999).

Systemic toxicity is expected to be low; as the calcium sulfonate read across substances were non-toxic by ingestion (Swan, 1972 - LD50: > 10,000 - < 20,000 mg/kg bw; Sanitised, F., 1989 - LD50 < 5,000 mg/kg bw; Sanitised, A., 1981 - LD50 < 5,000 mg/kg bw, Sanitised, C., 1984 - LD50 >5.000 mg/kg bw; Sanitised, E., 1985 - LD50 >5.000 mg/kg bw; Ohees, P. 1968a - LD50 > 20,000 mg/kg bw; Regel, L., 1970 - LD50 > 10,000 mg/kg bw; Ohees, P., 1968b - LD50 > 20,000 mg/kg bw, Gabriel, K. L:, 1981a - LD50 > 16,000 mg/kg bw). In addition no toxicity was found after percutaneous absorption (Sanitised, G., 1989, LD50 < 2,000 mg/kg bw; Sanitised, B., 1981 - LD50 > 5,000 mg/kg bw, Sanitised, J., 1993 - LD50 > 2000 mg/kg bw, Costello, B. A., 1986a - LD50 > 4000 mg/kg bw). The calcium sulfonate read across substance (CAS 75975-85-8) was shown to bear a potential to cause allergic reactions (Shults, 1993, Bonnette, 1993, Lees, 1996). However, the other calcium sulfonate read across substance (CAS 61789-86-4) was shown not to be skin sensitising (Kiplinger, 1992a/b/c, Blaszcak, 1992, Reagan, 1988). This was analysed in a weight-of-evidence approach (in conjunction with various human patch test data) and the conclusion was drawn, that, a low TBN calcium sulfonate target substance (C15-C36) (TBN < 300) is a skin sensitiser (Cat. 1B, H317) with a specific concentration limit (SCL) of 10%, in accordance with CLP 1272/2008. In contrast, a high TBN calcium sulfonate target substance (C15-C36) (TBN ≥ 300) is not a skin sensitiser. Repeated oral exposures to the calcium sulfonate read across substance CAS 70024-69-0 (28-day study) revealed a NOAEL of 500 mg/kg bw (based on a decrease in serum cholesterol and a LOAEL of 1000 mg/kg bw, the highest dose tested (Wong, Z., 1989). Additionally, the calcium sulfonate read across substance CAS 115733-09-0 showed a systemic NOAEL of 1000 mg/kg bw for rats (Rush, R.E., 2003). The calcium sulfonate read across substance CAS 61789-86-4 was shown to have a dermal NOAEL of 1000 mg/kg bw (Laveglia, 1988). Concerning gene mutation, the calcium sulfonate read across substance CAS 70024-69-0 was shown not to bear genotoxic potential (Sanitised, C., 1989, and Sanitised, L., 1995) and the calcium sulfonate read across substance CAS 68783-96-0 not to be mutagenic (Sanitised, D. 1984, Sanitised, N., 1995). Moreover, the calcium sulfonate read across substance (Analogue of 70024-69-0) was shown not to induce micronuclei (Sanitised,I., 1989). In addition, the calcium sulfonate read across substances (CAS 68783-96-0 and CAS 61789-86-4) were also shown not induce micronuclei (Sanitised, N., 1995, Loveday, 1988b).

The calcium sulfonate read across substance (CAS 115733-09-0) was reported not to induce adverse effects in a 1-generation reproduction toxicity study (Bjorn, 2004).

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.

The calcium sulfonate target substance (C15-C36) is not favourable for absorption, due to its molecular weight (MW >= 817.31 and <= 1111.87 g/mol), limited water solubility (1.69 mg/L) and a logPow (>6.65). Such lipophilic low water soluble substances are hindered to be absorbed because the dissolving in the gastrointestinal fluids is impaired. On the other hand, any lipophilic compound may be taken up by micellular solubilisation and this mechanism may be of particular importance for the calcium sulfonate target substance (C15 -C36) since it is poorly soluble in water. The substance does bear, however, surface activity; therefore an enhancement of absorption is theoretically possible, but it is not irritating to skin or eyes confirming that no further enhancement of absorption seems to be applicable.

The above mentioned properties determine the absorption of the calcium sulfonate target substance (C15 -C36) to be, rather limited, based on the absorption-hindering properties (molecular weight, slight water solubility and high LogPow) and the observed effects in toxicological experiments.

Oral route

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. The calcium sulfonate target substance (C15-C36) is not expected to hydrolyse (due to its low water solubility). Accordingly, it is very unlikely for the calcium sulfonate target substance (C15-36) to 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 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.

The available data suggest that orally administered calcium sulfonate target substance (C15-C36) will be absorbed to a limited extent, even though micellular solubilisation, pinocytosis and persorption cannot be ruled out. This thesis is supported by the LD50 value of 10000 mg/kg bw available for the calcium sulfonate read across substance CAS 70024-69-0, which shows that the substance is not acutely toxic after oral exposure.

Inhalation route

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.

The calcium sulfonate target substance (C15-C36) has a low vapour pressure (0.01 Pa at 25°C), which indicates only low availability for inhalation. The high molecular weight and the high LogPow also indicate no possibility for absorption through aqueous pores. Based on this data and even though the LogPow value above 0 indicates the potential for absorption directly across the respiratory tract epithelium (which is unlikely as the substance is ionisable), it can be expected that the calcium sulfonate target substance (C15-C36) is marginally available in the air for inhalation and any inhaled substance is expected not to be absorbed.

Dermal route

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 IV). 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. Vapours of substances with vapour pressures below 100 Pa are likely to have enough contact time to be 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 the case of the calcium sulfonate target substance (C15-36), the molecular weight is above 500, which indicates already a marginal potential to penetrate the skin. This is accompanied by a low hydrophilicity of the substance and even though the stratum corneum is open for lipophilic substances, the epidermis is very resistant against penetration by highly lipophilic substances. However, the amount of the calciumsulfonate target substance C15-36), which is absorbed following dermal exposure into the stratum corneum is unlikely to be transferred into the epidermis. Although the substance does show characteristics of a surfactant, the calcium sulfonate target substance (C15-C36) is not irritating to skin and eyes, and therefore this does not enhance dermal absorption.

In support of this hypothesis (the low dermal absorption), the systemic toxicity of the calcium sulfonate read across substance CAS 70024-69-0 and of 115733-09-0 via the skin is low (acute dermal toxicity, LD50 value of > 2000 and > 5000 mg/kg bw for 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 a low dermal absorption. In detail, due to its molecular weight and the results for acute toxicity, the use of a factor of 10 % for the estimation of dermal uptake for hasa low vapour pressure (0.01 Pa at 25°C), which indicates only lowdermal absorption. In detail, due to its molecular weight and the results for acute toxicity, the use of a factor of 10% for estimation of dermal uptake for the calcium sulfonate target substance (C15 -36) is justified (Schuhmacher –Wolz et al.,2003; TGD, Part I, 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 of the calcium sulfonate target substance (15-36), no data is available for distribution patterns. Even though the high LogPow would indicate the possibility to reach the intracellular compartment, this seems to be unlikely as the molecular weight of the un-metabolised substance is so high. Therefore, the distribution is expected to be limited.

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 high LogPow and the predicted behaviour concerning absorption and metabolism of the calcium sulfonate target substance (C15-C36) might indicate a potential for accumulation in the body. This, however, is limited as the absorption is expected to be low via all routes of exposure and because metabolism of the calcium sulfonate target substance (C15-C36) is expected to influence this initial prediction.

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 for the calcium sulfonate target substance (C15-C36). However, its metabolism is very likely to occur via the Cytochrome P450 group of metabolising enzymes, as it has been predicted with the TOXTREE modelling tool (Chemservice, S.A., 2013). According to the modelling results, the calcium sulfonate target substance (C15-C36), containing the structural alerts: cation, anion, sulfonic acid derivative and aromatic compound (Class 1: At least one functional group), is expected to be well metabolized by the Cytochrome P450 group of metabolizing enzymes. The primary and secondary sites of metabolism are the carbon atoms of the chain, next to aromatic ring, which are predicted to be subject to aliphatic hydroxylation. The tertiary sites of metabolism are the terminal carbon-atoms of the chain, which is predicted to be also subject to aliphatic hydroxylation.

Moreover, it is possible that the long carbon chains are subject to initial omega- and then successive beta-oxidation, possibly followed by oxidative scission of the aromatic ring and desulfonation. The above mentioned functional groups can react in phase 2 of the biotransformation with different molecules, leading to the formation of conjugations. This might be necessary for the parent compound, as its water solubility is fairly low and it cannot be eliminated via the urine without further metabolism. Further metabolism is most likely the conjugation of the hydroxyl-groups with glucuronic acid, activated sulphate or activated methionine.

In conclusion, it is likely that the substance of interest will be subject to metabolism by cytochrome P450 enzymes, followed by omega- and beta-oxidation and cleavage of the aromatic ring and desulfonation.

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 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 the calcium sulfonate target substance (C15-36) no data is available concerning its elimination. Concerning the fate of the metabolites formed of the calcium sulfonate target substance (C15-36), the metabolites should be eliminated mainly via the urine and to a smaller extent via the bile.

Prediction using TOXTREE

A representative chemical structure of the calcium sulfonate target substance (C15-36) was assessed by Toxtree (v.2.5.0) modelling tool for possible metabolism. SMART Cyp is a prediction model, included in the tool, which identifies sites in a molecule that are labile for the metabolism by Cytochromes P450.

The calcium sulfonate target substance (C15-36), containing the structural alerts: cation, anion, sulfonic acid derivative and aromatic compound (Class 1: At least one functional group), is expected to be well metabolized by the Cytochrome P450 group of metabolizing enzymes. The primary and secondary sites of metabolism are the carbon atoms of the chain, next to the aromatic ring, which are predicted to be subject to aliphatic hydroxylation. The tertiary sites of metabolism are the terminal carbon-atoms of the chain, which are predicted to be subject to aliphatic hydroxylation.

ADME Studies on Related Alkyl/aryl Sulfonates

Linear alkylbenzene sulfonates (LAS), alpha-olefin sulfonates (AOS) and alkyl sulfates (AS)

The World Health Organization (WHO assessment of alkyl/aryl sulfates and sulfonates (ISBN 92 4 157169 1, 1996), shows that linear alkylbenzene sulfonates, alpha-olefin sulfonates and alkyl sulfonates are readily absorbed by the gastrointestinal tract, widely distributed throughout the body and extensively metabolised. They can undergo omega- and beta-oxidation, which is followed by oxidative scission of the aromatic ring and desulfonation. For LAS, the parent compounds and the metabolites are excreted mainly through the kidneys and via the bile. AOS and AS are mainly excreted via the urine. Only minimal amounts of LAS and AS are absorbed through intact skin.

Dodecyl benzenesulfonate (DOBC)

The percutaneous absorption of DOBC surfactant was studied in in vitro studies in rats and humans (Howes, 1975). Two test solutions of the [14C]DOBC were used, the first a 3 mM solution in 25 % v/v Polyethylene Glycol 400 in water and a second 3 mM suspension in water. 0.25 mL of the test solution was applied to the rat skin (4.9 cm²) and 0.1 mL to the human epidermal samples (0.78 cm²). The concentration of the test material in the solutions was 1.2 mg/mL. After 2, 6 and 24 hours (rats) or after 0.5, 1, 2, 3, 4, 6, 7, 8, 24 and 48 hours (human) of exposure, the skin samples were washed with excess of water and the radioactivity was measured in the rinsings and in the tested skin samples to determine the penetration rates. The results show no measurable penetration (<0.1 µg/cm²) of DOBS through the rat skin up to 24 hours. DOBC did not penetrate human epidermis as well (<0.1 µg/cm²). 30% [14C] DOBS was recovered in the rinsings and 70% remained associated with the skin of rats. 30-50% of [14C] DOBC retained in the human epidermis after rinsing.

To calculate percutaneous absorption rates, penetration of 0.1 µg/cm², the lower limit of detection, was taken as worst-case. Based on the applied amount of 0.25 mL to the rat skin (4.9 cm²) and 0.1 mL to the human epidermis (0.78 cm²) and ´the concentration of DOBC in vehicle of 1.2 mg/mL, 0.16% and 0.065% absorption was calculated for the rat skin and human epidermis, respectively. Monitoring at 2, 6, and 24 hours after exposure to rat and human skin showed no measurable percutaneous penetration of 14C (< 0.1 µg/cm²) for DOBS.

Howes (1975) also studied the in vivo dermal absorption of DOBC in rats (Howes, 1975). The [14C]-labelled DOBS was applied (0.2 ml) as a 3 mM aqueous suspension over 7.5 cm² of skin for 15 min. The applied dose resulted in 250 µg per test site. Then the test solution was rinsed off with distilled water and the animals were fitted with either restraining collars or non-occlusive protection patches and placed in the metabolism cages for collection of excreta. The expired CO2, urine, faeces and the carcasses of the animals, after excision of the treated skin, was monitored for 14C at 24 h after treatment. The excised skin was monitored for 14C and examined by autoradiography. Autoradiography of the skins showed heavy deposition of the surfactant on the skin surface and in the upper regions of the hair follicles. 11± 4 µg/cm² was recovered in the excised skin of rats, 135 µg in the rinsing and < 2 µg in the protection patches, so the total recovery was complete. All the tissue and excreta samples examined did not contain quantifiable amounts of 14C. The penetration of the DOBC was below the limit of detection (<0.1 µg/cm²). Based on the amount applied (250 µg) and the penetration of 0.1 µg/cm², rat skin absorption would result in 0.3%.

The turnover (elimination) of [14C]-labelled DOBC was also measured by injecting three animals intraperitoneally and three animals subcutaneously with 0.1 or 0.5 mL of surfactant solution (Howes, 1975). Two test solutions of the [14C] DOBS were used, the first a 3 mM solution in 25% v/v Polyethylene Glycol 400 in water and a second a 3 mM suspension in water. The administered dose resulted in 1.02 mg/kg bw. The animals were placed in sealed metabolism cages where urine, faeces and expired air were collected and monitored for 14C. After 6 or 24 h the animals were killed and the radioactivity in their carcasses was measured. The rate and route of excretion of 14C from intraperitoneally administered [14C] surfactant solutions were the same as that from subcutaneously administered solutions. Most of the administered 14C was recovered in the urine at 24 h after dosing (78%). 22% and 1.5% of applied dose were recovered in carcasses and in faeces, respectively. Less than 0.1% radioactivity was measured in the expired air.

Conclusion:

The toxicology profile for the calcium sulfonate target substance (C15 -C36) and the calcium sulfonate read across substances indicate that oral and dermal absorption is expected to be limited. The in vitro and in vivo studies with DOBC and other related alkyl/aryl sulfates and sulfonates provide additional evidence that the skin absorption is expected to be low.

No measurable skin penetration could be determined in the in vitro and in vivo studies in rats and/or humans.