Reaction mass of disodium [2,2'-(imino-kappaN)dibutanedioato-kappa2O1,O4(4-)]manganese(2-) and sodium sulphate

Endpoint summary

Description of key information

Mn(2Na)IDHA is expected to be moderately absorbed after oral exposure, based on its high water solubility and negative logPow as well as absorption rates known for free IDHA and manganese. The substance is poorly available for inhalation since it is in a micro granulated form with particles above 100 µm, has a low vapour pressure, and is highly hydrophilic. Mn(2Na)IDHA is also not expected to be absorbed following dermal exposure into the stratum corneum and into the epidermis, due to its high water solubility. Concerning its distribution in the body Mn(2Na)IDHA, if absorbed, is expected to be distributed mainly in the intravasal compartment, due to its high water solubility. The substance does not indicate a significant potential for accumulation and is not expected to be significantly metabolised but to be eliminated unchanged via the bile and, to a lesser extent, via the urine. 

Key value for chemical safety assessment

Bioaccumulation potential:

no bioaccumulation potential

Absorption rate - oral (%):

50

Absorption rate - dermal (%):

10

Absorption rate - inhalation (%):

50

Additional information

General

There are no ADME studies available for Mn(2Na)IDHA. The toxicokinetic profile of the test substance was not determined by actual absorption, distribution, metabolism or excretion measurements. Rather, the physico-chemical properties of this substance were integrated with its toxicological data and the data on its structural analogues IDHA (iminodisuccinic acid) and Mn(2Na)EDTA to create a prediction of toxicokinetic behaviour. The results of toxicokinetic studies available for another aminopolycarboxylate chelate like Ethylenediaminetetraacetic acid calcium disodium salt (Ca(2Na)EDTA; CAS 62-33-9) and not chelated Ethylenediaminetetraacetic acid disodium salt (Na2EDTA; CAS 139-33-3) as well as manganese metal ion (in form of salts) are also taken into account to assess the toxicokinetic behaviour of Mn(2Na)IDHA.

There are acute (oral and dermal), irritation (skin and eye), and sensitization toxicity studies available for Mn(2Na)IDHA. The results of repeated dose toxicity studies and genetic toxicity studies conducted with the read-across substances chelating agent IDHA (iminodisuccinic acid) and Mn(2Na)EDTA have been used to cover the endpoints where the data are lacking for the target substance (please refer to read-across statement attached to the IUCLID file under section 13). The target substance and the read-across substances show very similar physical/chemical properties (high water solubility, negative logPow, no hydrolysis in water at environmentally pH values, low vapour pressure) and are thus believed to behave very similar in aqueous solutions and in living organisms. The complexing metal ion manganese, which is expected to be released under strong acidic conditions of stomach, is believed to contribute significantly to the toxicity observed in the long-term studies. Generally, toxicity of Mn(2Na)IDHA is expected to be mediated by excess exposure to elemental manganese. Therefore, ADME data on manganese have been considered to predict toxicokinetics behaviour of Mn(2Na)IDHA. Regarding toxicokinetic behaviour of Mn(2Na)IDHA in the chelated form, the substance is expected to exhibit similar biological activities with those of Mn(2Na)EDTA. In addition, similar patterns have been observed for the toxicological effects (e.g., available data showed similar level of acute oral and dermal toxicity, not irritating and not sensitisation properties and similar ecotoxicity potential). These common behaviours suggest a common mechanism and mode of action thereby providing evidence for the read-across between manganese chelates of different aminopolycarboxylates (please refer to read-across statement).

Toxicological profile of Mn(2Na)IDHA

The target substance Mn(2Na)IDHA is not toxic by oral route of exposure (LD50 is greater than 2000 mg/kg bw; Gruszka, 2007, Report No. OS 45/06) and not toxic by dermal route of exposure in rats (LD50 is greater than 2000 mg/kg bw; Kropidło, 2010, Report No. DER -11/13). The substance is not a skin or eye irritant (Sornat, 2013, Report No. DDR 15/13; Sornat 2013, Report No. ODR 11/13) and do not possess skin sensitisation potential (Kropidło, 2013, Report No. AI -9/13).

Effects in animal studies conducted with read-across substances which are relevant in the assessment of toxicokinetic behaviour of Mn(2Na)IDHA.

Long-term studies:

NOAEL of 1000 mg/kg bw, the highest dose tested, was established for females for the nearest analogue chelating agent IDHA in a sub-acute study in rats (Stropp and Popp, 1997; Report No. PH 26446). No toxicologically relevant clinical signs were noted in the animals treated with the substance. All blood chemistry parameters were unaffected and no toxicologically relevant findings were observed at necropsy. NOAEL of 200 mg/kg bw was established for males based on decreased motor activity.

The read-across substance Mn(2Na)EDTA produced adverse effects in animals treated by 1500 mg/kg bw (the highest dose group) in the Combined oral repeated dose toxicity study with reproduction / developmental toxicity screening test (OECD 422; Wolterbeek, 2010, Report No. V8650). The target organ was kidney. NOAEL was 500 mg/kg bw.

The chelating agent IDHA and Mn(2Na)EDTA were no reproductive or developmental toxicants in animal studies. This is proved by an one-generation reproduction toxicity study (Eiben and Rinke, 2002; Report No. PH-32294), developmental study conducted with IDHA (Klaus, 2002, Report No. PH-32141) and the Combined oral repeated dose toxicity study with reproduction / developmental toxicity screening test conducted with Mn(2Na)EDTA (Wolterbeek, 2010; Report No. V8650).

Toxicokinetic study conducted with the chelating agent IDHA sodium salt:

In a toxicokinetic study, the pharmacokinetic behaviour (absorption, distribution, excretion) and metabolism of the complexing agent IDHA sodium salt for metal ions were investigated (OECD 417; Koester, 2007; Report No. M81819180). The test material was labelled with 14C and 13C in the 2 and 3 as well as 2' and 3' - positions of imminodisuccinic acid. The radiolabelled and unlabelled test material was administered to two groups of 5 male Wistar rats, respectively, at dose level of 1000 mg/kg bw. The rats received the test item by oral gavage as an aqueous solution. They were sacrificed 72 h after dosing. The total radioactivity that included the test item and possible metabolites was determined in plasma samples, the excreta (urine and faeces) as well as in organs and tissues. The metabolism was investigated by radio-HPLC and spectroscopic methods in selected urine samples and faeces extract. Further, osmolality in urine as well as pH-values and metals in urine and plasma samples collected from rats which received the unlabelled test item were investigated.
The kinetic and metabolic behaviour of IDHA sodium salt in male rats can be characterised by the following observations:
A first portion of the administered radioactivity was rapidly absorbed, widely distributed into organs and tissues and rapidly eliminated from the body via urine and faeces. For a second portion, the absorption and elimination periods were significant longer than for the first one which indicated a higher systemic exposure to the test item related radioactivity during this phase. An exact value for the absorption rate could not be determined from these observations. But taking the urinary excretion behaviour and the remaining radioactivity in the body at sacrifice into account it can be concluded that at least 37% of the administered dose becomes systemically available.
The distribution of the radioactivity within the central compartments of the body (i.e. blood, liver, and kidney) showed a distinctive preference towards the liver and - to a lower extent - to the kidney which had the highest TRR-values at sacrifice.
IDHA sodium salt was metabolically stable as no metabolites were detected in the excreta in significant amounts.
The excretion of radioactivity via urine and faeces was fast and almost completed at 24 h after administration. The by far major part of the dose (68.7%) was excreted with faeces and 34.7% with urine. Relating to the administered dose, IDHA sodium salt amounted to 34.5% in urine and 61.2% in faeces. No single unknown metabolite was higher than 0.2% in urine and 2% in faeces. From the renal and faecal excretion rate as well as the low accumulation factor it was concluded that the residual radioactivity in the organs and tissues of the rat are subject to further elimination.
The administration of IDHA sodium salt led to transient changes of the urinary osmolality-and pH-values within the test period of 3 days. Based on a high variability of urine pH- and osmolality-values in animal species used in toxicological studies, these effects however were regarded as insignificant. Differences were obtained in the amount of metals in urine and plasma samples between treated and untreated rats.

Summary of toxicity effects of manganese and its inorganic compounds:

Regarding manganese toxicity, the most commonly reported adverse health effect in humans are neurologic effects occurring after physiologically excessive amounts of manganese (ATSDR, 2012; SCOEL, 2011). The effects appear to increase in severity as the exposure level or duration of exposure increases. Chronic exposure to manganese at very high levels results in permanent neurological damage, as is seen in former manganese miners and smelters. Chronic exposure to much lower levels of manganese (as with occupational exposures) has been linked to deficits in the ability to perform rapid hand movements and some loss of coordination and balance, along with an increase in reporting mild symptoms such as forgetfulness, anxiety, or insomnia. Results from animal studies indicate that the solubility of inorganic manganese compounds can influence the bioavailability of manganese and subsequent delivery of manganese to critical toxicity targets such as the brain; however, the influence of manganese oxidation state on manganese toxicity is not currently well understood.

In humans, inhalation of particulate manganese compounds such as manganese dioxide or manganese tetroxide can also lead to an inflammatory response in the lung. Symptoms and signs of lung irritation and injury may include cough, bronchitis, pneumonitis, and minor reductions in lung function as well as pneumonia. It is assumed that the increased susceptibility to pneumonia is mainly secondary to the lung irritation and inflammation is a consequence of inhaled particulate matter at all and not the manganese per se that causes the response. Inflammatory responses in the lungs of animals were observed in animal studies as well. Further effects described for workers exposed to manganese were cardiovascular effects such as lower mean systolic and diastolic blood pressure, abnormal electrocardiograms and sudden death mortality.

In case of oral exposure, most studies in animals indicate that manganese compounds have low acute oral toxicity when provided in feed. Nephropathy and renal failure were common effects observed in treated animals. No systemic toxic effects in humans who have ingested manganese are described. This is likely due to the strong homeostatic control the body exerts on the amount of manganese absorbed following oral exposure; this control protects the body from the toxic effects of excess manganese. Studies in humans and animals provide limited data regarding the effects of manganese ingestion on systemic target tissues.

For healthy adults, estimated acceptable or adequate dietary intakes range from 1–12.2 mg manganese/day (SCOEL, 2011). A guidance value of 0.16 mg manganese/kg/day, based on the Tolerable Upper Intake Level for 70 kg adults of 11 mg manganese/day is recommended for public health risk assessment (ATSDR, 2012). For inhalation exposure, respirable Indicative Occupational Exposure Limit Value (IOELV) of 0.05 mg/m³ and an inhalable IOELV of 0.2 mg/m³ are recommended and neither respiratory nor cardiovascular toxicity would be expected at inhalable exposures of 1 mg/m³ or less (SCOEL, 2011).

Toxicokinetic analysis of Mn(2Na)IDHA

The substance Mn(2Na)IDHA is an odourless, dark white solid in a microgranulated form (MW is 346.08 g/mol) at 20 °C. The substance is soluble in water (491 g/L at 20 °C, Klimas, 2014) and has a negative partition coefficient (logPow =-2.69; Stegient-Nowicka, 2013, Report No. 2013/294). It has a very low vapour pressure (3.36 x 10^-5Pa; Petryka, 2013, Report No. BC-34/13) and it decomposes at 258 °C under atmospheric conditions (Sobera-Madej, 2013, Report No. 384900030). Hydrolysis as a function of pH does not apply as the substance forms very stable complexes (log of stability constant is 7.26, Hyvönen et al., 2003). Therefore, chelate stability is more applicable instead. Mn(2Na)IDHA is stable over alkaline pH range (7-12) but it is not stable under acidic conditions (Lucena et al., 2003; Hyvönen et al., 2003). The following situation applies by pH below 7:

4H⁺+ MnIDHANa₂+ H₂O⇔H₄IDHA + 2Na⁺+ Mn²⁺

In the following, ADME parameters have been predicted based on data for manganese, free IDHA, Mn(2Na)IDHA and Mn(2Na)EDTA.

Absorption

Oral absorption:

Oral absorption is favoured for small (with MW below 500 g/mol) water soluble molecules. Molecular weight of Mn(2Na)IDHA is 346.08 g/mol pointing to a possible absorption in the gastrointestinal (GI) tract while high water solubility (491 g/L) and the very low logPow value (-2.69) suggest that Mn(2Na)IDHA may be too hydrophilic to be readily absorbed via GI tract. The substance may be taken up by passive diffusion. As the substance’s molecular weight is lower than 500, it is likely to pass through aqueous pores or be carried through the gastrointestinal epithelial barrier by the bulk passage of water. However, the substance is instable under acidic condition (by pH under 7; Hyvönen et al., 2003). Therefore, the complex Mn(2Na)IDHA is expected to be in de-chelated form in the stomach: sodium ions, released Mn2+ ions from the complex and free IDHA. In small intestines, free IDHA can chelate metal ions again because of increased pH. Thus, oral absorption of chelated Mn(2Na)IDHA will result from absorption of manganese ions, sodium ions and free IDHA. Additionally, toxicity data on Mn(2Na)IDHA and Mn(2Na)EDTA can elucidate the pattern and rate of absorption.

With regard to the toxicity data on Mn(2Na)IDHA and Mn(2Na)EDTA:

Mn(2Na)IDHA did not produce death or toxic effects up to 2000 mg/kg bw in an acute oral toxicity study in rats (Gruszka, 2007, Report No. OS 45/06). Similarly, the read-across substance Mn(2Na) EDTA was not acutely toxic by oral (gavage) route in rats (LD50 greater than 2000 mg/kg bw; Beerens, 2010, Report No. 494026). In the Combined oral repeated dose toxicity study with reproduction / developmental toxicity screening test, Mn(2Na)EDTA produced clinical signs and histopathological findings in kidneys at the highest dose of 1500 mg/kg bw (NOAEL is 500 mg/kg bw; Wolterbeek, 2010, Report No. V8650). This confirms the assumed absorption via the gastrointestinal (GI) tract.

With regard to the absorption data on free EDTA and other EDTA metal complexes:

In the toxicokinetic studies with other EDTA derivatives: EDTA-CaNa2 and Na salts of EDTA are poorly absorbed from the gastrointestinal tract (2 -18% in rats; less than 5% in humans; RAR, 2004). Poor absorption was observed also for sodium iron EDTA administered orally to animals and humans: “Only a very small fraction of the NaFeEDTA complex (less than 1 %) is absorbed intact and this is completely excreted in the urine. An additional small fraction (less than 5 %) of the EDTA moiety is absorbed, presumably bound to other metals in the gastrointestinal tract, and is also completely eliminated in the urine” (IPCS, 2014). In absorption studies with sodium iron EDTA, only a very small fraction of the sodium iron EDTA complex (less than 1–2%) is absorbed intact and is rapidly and completely excreted via the kidneys in the urine (WHO, 2008)

With regard to the absorption data on free metal cations released from EDTA or IDHA metal complexes:

Regarding absorption of released manganese from both IDHA and EDTA, the rates of their absorption are assumed to be dependent on the pH optimum at which chelated agents form efficiently complexes with metal cations. For instance, in absorption studies with sodium iron EDTA the fate of different complex species is investigated (WHO, 2008; IPCS, 2014). Administered orally, “iron (primarily Fe3+) remains complexed with EDTA under the acidic conditions prevailing in the stomach because pH of 1 is optimal for efficient complex formation of EDTA with iron. The chelate holds the iron in solution as the pH rises in the upper small intestine, but the strength of the complex is progressively reduced allowing at least partial exchange with other metals and the release of some of the iron for absorption. Further results indicate that iron dissociates from the chelate and is released into the common non-haem iron pool before absorption” (IPCS, 2014).

With regard to the toxicity and ADME data on free IDHA chelating agent:

In the acute oral and sub-acute oral studies, conducted with IDHA chelating agent (in form of its sodium salt), only minimal effects were noted in treated animals. LD50 of greater than 2000 mg/kg bw in the acute study and NOAEL of 1000 mg/kg bw and 200 mg/kg bw were established for females and males in the sub-acute study, respectively (Stropp, 1996, Report No. T4060981; Stropp and Popp, 1997; Report No. PH 26446). The effects in males demonstrate certain absorption of IDHA but it also shows its relatively low intrinsic toxicity. In the toxicokinetic study, the chelating agent IDHA was extensively absorbed and at least 37% of the administered dose became systemically available in rats (Koester, 2007; Report No. M81819180).

With regard to manganese (based on inorganic manganese compounds):

Manganese is required by the body and is found in virtually all diets (ATSDR, 2012). Adult humans generally maintain stable tissue levels of manganese through the regulation of gastrointestinal absorption and hepatobiliary excretion. The amount of manganese absorbed across the gastrointestinal tract in humans is variable, but typically averages about 3–5 % (SCOEL, 2011). The absorption is expected to be higher for soluble forms of manganese compared with relatively insoluble forms of manganese. The absorption of manganese from the gut is dependent on several factors, including the amount ingested, iron status and other dietary components. There is very tight biological regulation of the gastro-intestinal absorption of manganese which is not the case for inhalation exposure (ATSDR, 2012).

Based on these data, it can be concluded that a moderate direct absorption across the gastrointestinal tract epithelium will occur when Mn(2Na)IDHA is applied orally. The complex Mn(2Na)IDHA is expected to dissociate at high acidic conditions of stomach. In the upper intestines, manganese cation will tend to be complexed again with free ligand IDHA because pH reaches the optimum (7) for complex formation. Therefore, the absorbed fraction will result from intact Mn(2Na)IDHA and its released components: Mn2+ ions and free IDHA. Intestinal uptake of released Mn2+ ions is low, 3 to 5%, while intestinal uptake of sodium IDHA is at least 37 % comparing to 5 % of sodium EDTA Therefore, oral absorption of intact manganese IDHA chelates is expected to be somewhat higher than oral absorption of manganese of EDTA complexes or free EDTA.

In conclusion, taking into account low absorption potential based on physico-chemical properties of chelate Mn(2Na)IDHA, its instability at low pH values, absorption of 5 % for manganese and 37% for free IDHA, 50 % oral absorption (as worst-case) for Mn(2Na)IDHA is considered appropriate in case of hazard assessment (DNEL derivation).

Absorption by inhalation:

Based on the low vapour pressure of Mn(2Na)IDHA, inhalation exposure is not likely. There are no particles with aerodynamic diameter less than 100 µm (Stefaniak, 2013, Report No. 2013/292). Moreover, final product has a microgranulated form. Thus, it is very unlikely, that big amounts of the substance reach the lung. In case of dust formation, it is expected that 100% of the inhaled substance will be deposited in the upper respiratory tract, where the particles may be moved by mucociliary transport to the throat and where the substance is swallowed and, conclusively, enters the stomach. The particles are not expected to reach alveolar region. If the substance reaches the lung, it is not very likely that the substance is taken up rapidly (based on physical-chemical properties). The substance is expected to be predominantly in chelated form since pH of healthy lungs is between 7.38 and 7.43 (Effros and Chinard, 1969). In case of a negligible fraction of released manganese ions, the respirable manganese will be readily taken up (ATSDR, 2012). It is mainly absorbed into blood and lymph fluids, while manganese from larger particles or nanosized particles deposited in the nasal mucosa may be directly transported to the brain via olfactory or trigeminal nerves. There is experimental evidence of olfactory uptake of manganese to the brain. The toxicological significance of this olfactory uptake to humans remains uncertain (ATSDR, 2012).

In an acute inhalation study, the read-across substance Mn (2Na) EDTA was absorbed by lungs of rats as confirmed by clinical signs and findings at necropsy (Jonker and van Triel, 2012). The substance was inhaled in form of aerosol (particles of 3.2 – 3.3 µm). The 4-h LC50 value exceeded 5.16 g/m³, therefore it does not trigger C&L. Thus, this indicates low systemic availability after inhalation and if bioavailable, low toxicity effects via this route of administration.

Based on this information, the absorption by inhalation is expected to confine to the amount of Mn(2Na)IDHA deposited in upper airways which can be swallowed. Therefore, as worst case, 50 % absorption by inhalation (similar with oral absorption) is considered appropriate for the purposes of hazard assessment (DNEL derivation). This absorption rate covers sufficiently also oral absorption of free IDHA chelating agent which is at least 37 %.

Dermal absorption:

Similarly, based on physico-chemical properties of Mn(2Na)IDHA, the substance is not likely to penetrate skin to a large extent due to negative logPow: -2.69 and a very high water solubility: 491 g/L. Water solubility above 10.000 mg/L combined with a log P value below 0 indicate that the substance may be too hydrophilic to cross the lipid rich environment of the stratum corneum. Dermal uptake for these substances will be low. The molecular weight of 346.08 g/mol indicates theoretically a certain potential to penetrate the skin (< 500) but in case of such a hydrophilic substance it is rather unlikely. This is supported by the findings of acute dermal toxicity studies of the target substance Mn(2Na)IDHA and free IDHA where no systemic toxicity after exposure via the skin was noted (LD50 > 2000 mg/kg bw; Kropidło, 2010, Report No. DER -11/13; Stropp, 1997, Report No. T3061600). Moreover, an acute dermal irritation / corrosion study in the rabbit (according to OECD 404) for Mn(2Na)IDHA did not demonstrate any irritation after 14 days (Sornat, 2013, Report No. DDR 15/13). This information indicates that Mn(2Na)IDHA is unlikely to penetrate the skin. In a human study, EDTA-CaNa2 did not penetrate the skin, only 0.001% was absorbed within 24 hours of administration (RAR, 2004). In case of dissociated complexes, manganese ions uptake across intact skin would be expected to be extremely limited (ATSDR, 2012). Low absorption potential through the skin would also apply to free IDHA chelating agent due to its high hydrophilicity (water solubility 564 g/L; data for Baypure CX 100 (Lanxess, 2014)).

Based on very low logPow values, high water solubility and absence of toxicity effects in animal studies conducted with different aminopolycarboxylate chelates: free IDHA, Mn(2Na)IDHA, Mn (2Na) EDTA and Ca (2Na) EDTA, a similar behaviour regarding absorption through the skin is expected. Dermal absorption is considered to be negligible and equal to 10 % (worst-case) as established for substances which meet criteria of low dermal absorption potential mentioned in ECHA guidance R7c. (the value used for hazard assessment: DNEL derivation).

Distribution and accumulative potential

When reaching the body, only a limited amount of Mn(2Na)IDHA is expected to be available for distribution. The minor amount absorbed into the body, will most likely exist only in the intravascular compartment (due to its low logPow and high water solubility) and will not be distributed into the cells, as the cell membranes require a substance to be soluble also in lipids to be taken up. In a human study with Ca(2Na)EDTA, intravenously injected EDTA was excreted within 24 hours in the urine, 50% of the substance in the first hour and 90% within 7 hours (RAR, 2004). No test substance was detected in blood. On the other hand, Ca(2Na)EDTA was also excreted in the expired air in treated animals. This indicates a wide distribution potential and no accumulative potential.

In case of dissociated fraction of Mn(2Na)IDHA, absorbed manganese ions will be widely distributed throughout the body (ATSDR, 2012). Manganese is a normal component of human and animal tissues and fluids. Adult humans normally maintain stable tissue levels of manganese regardless of intake; this homeostasis is maintained by regulation absorption and excretion (ATSDR, 2012). In humans, following inhalation exposure, manganese can be transported into olfactory or trigeminal presynaptic nerve endings in the nasal mucosa with subsequent delivery to the brain, across pulmonary epithelial linings into blood or lymph fluids, or across gastrointestinal epithelial linings into blood after mucociliary elevator clearance from the respiratory tract (ATSDR, 2012). Manganese is found in the brain and all other mammalian tissues, with some tissues showing higher accumulations of manganese than others. For example, liver, pancreas, and kidney usually have higher manganese concentrations than other tissues. The lowest levels were in bone and fat. Following oral exposure, manganese preferentially accumulates in brain but to a lesser extent than after inhalation exposure (ATSDR, 2012).

Distribution of the absorbed free IDHA is expected to have similar pattern observed in the toxicokinetic study with free IDHA, sodium salt (Koester, 2007; Report No. M81819180). Liver and kidney were the target organs. As evidence for systemic distribution the results of the sub-acute toxicity study in rats can serve also where males had clinical signs of toxicity indicating absorption and distribution of the substance into the organism (Stropp and Popp, 1997; Report No. PH 26446).

Based on these data and taking into account the fact, that “substances with logPow values of 3 or less would be unlikely to accumulate with the repeated intermittent exposure patterns normally encountered in the workplace” (TGD, Part 1; ECHA guidance R.7C, 2012), no enhanced risk for accumulation will be associated with the target substance Mn(2Na)IDHA.

Metabolism and excretion

No studies are available for free IDHA, Mn(2Na)IDHA, Mn(2Na)EDTA and Ca(2Na)EDTA in which metabolites would be detected and studied in details. Due to the chelated nature of the molecule, metabolism in the human body will unlikely to occur. This is confirmed by the metabolism studies in animals and in humans with Ca (2Na) EDTA where the substance was rapidly excreted predominantly in faeces (more than 90 %). No metabolites were reported (RAR, 2004). In Ames test and in in vitro Micronucleus test with Human Lymphocytes, Mn (2Na) EDTA showed no effects with and without metabolising system (BASF, 1992; Report No. 40M0401/914239; Usta, 2013, Report No. V20217/03). Metabolic activation leading to more toxic metabolites is thus not very likely. Further, due to the high stability constant of the manganese chelate IDHA complex (K = 10^-7.26) (Hyvönen et al., 2003), it is clear that it exerts a low reactivity in the organism. Therefore, it is assumed that most of this very water soluble manganese fraction will be excreted unchanged in the chelated form mainly in the faeces in analogy with Ca(2Na)EDTA or other salts of EDTA. Small amount of absorbed fraction will be also excreted unchanged via the urine.

In case of dissociation of Mn(2Na)IDHA complexes in stomach under very acidic conditions, metabolism of free IDHA is expected to be similar to that described in the toxicokinetic study in rats (Koester, 2007). The substance was metabolically stable as no metabolites were detected in the excreta in significant amounts. Thus, biotransformation of iminodisuccinate is possible only to a minor extent. With the aid of skin metabolism simulator*, a tool of the OECD QSAR Toolbox (v3.2, 2013), several metabolites were predicted for IDHA. They are all intermediates in mammals: oxaloacetate, aspartic acid, ß-alanine and pyruvate. The excretion pattern of IDHA were similar to those observed by Ca (2Na) EDTA. The excretion via urine and faeces was fast and almost completed at 24 h after administration. The major part of the administered dose (61.2 %) was excreted with faeces and 34.5% with urine (Koester, 2007).

Regarding manganese ions, limited data suggest that it may undergo changes in oxidation state within the body (ATSDR, 2012). Probably, it is converted from Mn (II) to Mn (III), but the formation of complexes between Mn (II) and biomolecules is also possible (bile salts, proteins, necleotids, etc., ATSDR, 2012). Absorbed manganese is removed from the blood by the liver where it conjugates with bile and is excreted into the intestine with following excretion predominantly via the faeces. However, some of the manganese in the intestine is reabsorbed through enterohepatic circulation. Small amounts of manganese can also be found in urine, sweat, and milk. Absorbed manganese is eliminated with a half-life of 10 to 30 days (SCOEL, 2011) whereby it is dependent on route of exposure. In humans who inhaled manganese chloride or manganese tetroxide, about 60% of the material originally deposited in the lung was excreted in the faeces within 4 days, while humans who ingested tracer levels of radioactive manganese (usually as manganese chloride) excreted the manganese with whole-body retention half-times of 13–37 days (ATSDR, 2012). Manganese that is delivered to the brain is eliminated over time with reported half-life of 50 to 220 days (SCOEL, 2011). It is important to recognise that accumulation and clearance of manganese from the brain might have important implications for neurofunctional effects which are reported in a number of occupational studies (SCOEL, 2011).

*SMILES for assessed structure of IDHA: OC(=O)CC(C(O)=O)NC(CC(O)=O)C(O)=O