Zinc EDDHA

Basic toxicokinetics

Administrative data

Endpoint:

basic toxicokinetics in vivo

Type of information:

other: expert statement

Adequacy of study:

key study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: expert statement based on the ECHA Guidance R.7c

Cross-reference

Reason / purpose for cross-reference:

read-across: supporting information

Data source

Reference

Reference Type:

review article or handbook

Title:

Guidance on Information Requirements and Chemical Safety Assessment, Chapter R.7c: Endpoint specific guidance, Version 3.0 – June 2017

Author:

ECHA

Year:

2017

Bibliographic source:

ISBN: 978-92-9495-838-9, DOI: 10.2823/43472

Materials and methods

Objective of study:

absorption

distribution

enzyme clearance

excretion

Test guideline

Qualifier:

no guideline required

Principles of method if other than guideline:

expert assessment of ADME based on the ECHA Guidance R.7c

GLP compliance:

no

Test material

Test material information

Constituent 1

Reference substance name:

Acetic acid, oxo-, sodium salt, reaction products with ethylenediamine and phenol, zinc sodium salts

Molecular formula:

non specified (UVCB substance)

IUPAC Name:

Acetic acid, oxo-, sodium salt, reaction products with ethylenediamine and phenol, zinc sodium salts

Test material form:

solid: particulate/powder

Test animals

Sex:

male/female

Results and discussion

Toxicokinetic / pharmacokinetic studies

Details on absorption:

Oral absorption
The registered substance is not expected to be readily absorbed after oral exposure, based on its rather high molecular weight, very high water solubility and negative LogP value. This agrees with the LD50 > 2000 mg/kg bw, determined in rats after oral exposure to the structural analogue Fe(Na)EDDHA. In case of absorption through the GI epithelium, it is expected to be by passive diffusion through aqueous pores or by carriage across membranes with the bulk passage of water. The systemic toxicity observed in long-term studies show that the EDDHA chelate is absorbed when applied orally at an amount which is sufficient to produce adverse effects. The absorption will consist of the intact chelator-iron complexes or released iron ions and chelator moieties. The same behaviour is assumed for the positional isomers and the condensation products of these chelates. Although the stability data indicate that Fe-EDDHA is stable up to the pH of 3, the observation of anaemia effects in repeated-dose studies shows that Fe-EDDHA and Fe-EDDHMA complexes or their positional isomers and condensation products release iron either before or after absorption. The amount of the ligands is sufficient to sequester body own iron leading to anaemia effects. As the EDDHA complex with zinc was shown to be rather less stable than the iron complex (see section 5.1. -Stability), for Zn(Na2)EDDHA it is also expected that the chelated metal manganese is released leading to free EDDHA chelates in the blood.

Zinc is an essential trace element which is regulated and maintained in the various tissues mainly by the gastrointestinal absorption and secretion during high and low dietary zinc intake and because of the limited exchange of zinc between tissues, a constant supply of zinc is required to sustain the physiological requirements. Zinc absorption in the gastrointestinal tract occurs throughout the entire small intestine with the highest rate in the jejunum and the rate of total absorption appears to be concentration-dependent. The zinc absorption process in the intestines includes both passive diffusion and a carrier-mediated process. The absorption can be influenced by several factors such as ligands in the diet and the zinc status. Humans with adequate nutritional levels absorb 20 – 30 % and animals absorb 40 – 50 %. Humans that are zinc deficient absorb more, while persons with excessive zinc intake absorb less.

Inhalation absorption
As the registered substance has a low vapour pressure, respiratory exposure of vapours is very unlikely. The target substance may, however be available for inhalation as particles. 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. 34.3 % of the particles of the registered substance Zn(Na2)EDDHA have a size between 1.4 and 2 mm. 41.2 % are between 0.6 and 1.4 mm. 3.0 % of the particles range between 0.25 and 0.6mm, while 1.2 % are below 0.25mm. Those particles have a d10 of 9.141 µm, a d50 of 75.555 µm and a d90 of 251.82 µm. Thus, a low percentage of the particles of the registered substance will be able to be inhaled and reach the thoracic region. Further, it cannot be excluded that a minor percentage of the particles is smaller than 15 µm and will thus be able to also reach also the alveolar region. As the substance has a high water solubility (200 g/L), it is expected to readily dissolve in the mucus lining the respiratory tract. Moderate log P values (between -1 and 4) are favourable for absorption directly across the respiratory tract epithelium by passive diffusion. The log P was not determined for the target substance itself. However, it is expected to be comparable to the source substance FeNaEDDHA. For this substance a log P of -4.2 was determined. It is expected that the log P of the substance is in a comparable range. So, the substance may be too hydrophilic be absorbed directly across the respiratory tract epithelium. Based on the high molecular weight (469 g/mol), also absorption through aqueous pores is not likely. This is favoured for hydrophilic substances with a molecular weight up to 200 g/mol. So, the particles of the target substance will most likely be retained in the mucus and transported out of the respiratory tract.

Dermal absorption
The registered substance is not expected to be absorbed significantly following dermal exposure due to its rather high molecular weight, negative LogP value and high water solubility. Accordingly, the systemic toxicity via the skin has been proven to be low for the structural analogue Fe(Na)EDDHA. No mortality of rats was observed after dermal application of 2000 mg/kg bw. With regard to zinc the available information from in vivo as well as the in vitro studies suggests the dermal absorption of soluble zinc compounds through intact skin to be less than 2% (solutions or suspensions).

Details on distribution in tissues:

If absorbed, the registered substance is not expected to bear an accumulative potential based on its high water-solubility and the low logPow value. The EDDHA constituents, its positional isomers and condensation products are all very soluble in water and, if absorbed, are expected to be distributed to vascular system 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. Based on their very low BCF values the substances are very unlikely to bioaccumulate in the human body. Also, no enhanced risk for accumulation will be associated with the substance, due to its negative LogPow.
With regard to zinc, after absorption zinc ions are primarily bound to albumin in plasma. Then they are transported to the liver before they distribute throughout the body. The normal plasma zinc concentration is ca. 1 mg/L, the total zinc content of the human body (70 kg) is in the range of 1.5-2 g. Zinc is found in all tissues and tissue fluids and it is a co-factor in over 300 enzyme systems. In humans, the major part of total body zinc is found in muscle and bone, approximately 60% and 30%, respectively. Under normal conditions, the highest zinc concentration per tissue weight is found in bone, hair and in the prostate.

Details on excretion:

Metabolism and excretion
Free of metal EDDHA and EDDHMA moiety as well as their positional isomers are not expected to be metabolised in the body but are rather excreted as a chelate. No data on metabolites of the organic moiety of the target and the source substances is available. In case chelates are de-chelated, based on the structure of the molecules and their nature, metabolism in the human body will mainly consist on phase-II metabolising steps, leading to an even better water solubility for excretion. Glucuronidation is one of such pathways leading to a better water solubility for excretion and it is most probable to occur also for EDDHA and its derivatives. This is in compliance with the results obtained in the genotoxic tests with the source substances showing no effects with and without metabolising system. Metabolic activation leading to more toxic metabolites is thus not very likely. In case of formation of glucuronid derivates, there is a possibility of entero-hepatic recycling, and the risk of a re-entering into system, but it does not bear any risk for the organism.
Additionally, extensive experimental data exist on the metabolism of EDTA compounds. Neither the iron nor the EDTA moiety of EDTA-FeNa undergoes biotransformation. Evidence for this conclusion is based on a number of absorption and metabolism studies in animals and humans which indicated that both EDTA and iron are excreted unchanged following ingestion of NaFeEDTA.
Based on the water solubility and the logPow value, excretion via the urine is likely. Urine parameters (acidic, reddish urine) were induced in the repeated dose toxicity studies with these chelates indicating an extensive urinary excretion of the chelate complexes or their metabolites. Further, due to the high stability constant of the iron chelate complex Fe-EDDHA (logK = 36.89, as mixture of meso and rac isomers), it is clear that if chelated, it exerts a low reactivity in the organism. Therefore, it is assumed that most of this very water-soluble iron fraction will be excreted unchanged in the chelated form mainly in the faeces and in the urine. As the substances have molecular weights above 300 g/mol the excretion of a considerable amount via the bile is also possible, especially if phase-II conjugation takes place e. g. with formation of glucuronid derivates.
With regard to zinc ion, within certain limits, mammals can maintain the total body zinc and the physiologically required levels of zinc in the various tissues, constant, both at low and high dietary zinc intakes. In humans, the faecal zinc consists of un-absorbed dietary zinc and endogenous zinc from bile, pancreatic juice and other secretions. About 70 – 80 % of the ingested amount of zinc is excreted via faeces (5 to 10 mg/day depending upon the dietary zinc concentration) and about 10 % of consumed zinc is excreted in urine.

Applicant's summary and conclusion

Conclusions:

The registered substance can be orally absorbed to a certain extent. They are not expected to be absorbed significantly following dermal and inhalative exposure. If absorbed the chelates are expected to be distributed to vascular system and will not be distributed into the cells. In case of the dissociated fraction, i.e. zinc for the target substance, absorbed zinc ions will be widely distributed throughout the body. The source and target substances are not expected to bear an accumulative potential. In case chelates are de-chelated, based on the structure of the molecules and their nature, metabolism in the human body will mainly consist on phase-II metabolising steps, leading to an even better water solubility for excretion. Excretion via the urine is likely. With regard to the target substance any absorbed zinc 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.

Executive summary:

The ADME parameters were assessed based on stability data, available toxicological studies and physico-chemical properties in accordance with ECHA Guidance R.7c.

The registered substance can be orally absorbed to a certain extent. They are not expected to be absorbed significantly following dermal and inhalative exposure. If absorbed the chelates are expected to be distributed to vascular system and will not be distributed into the cells. In case of the dissociated fraction, i.e. zinc for the target substance, absorbed zinc ions will be widely distributed throughout the body. The source and target substances are not expected to bear an accumulative potential. In case chelates are de-chelated, based on the structure of the molecules and their nature, metabolism in the human body will mainly consist on phase-II metabolising steps, leading to an even better water solubility for excretion. Excretion via the urine is likely. With regard to the target substance any absorbed zinc 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.