Registration Dossier

Administrative data

Key value for chemical safety assessment

Additional information

Background

A comprehensive literature search has been conducted by the registrant in factual data bases and secondary literature sources. During this search the most comprehensive and recent review document " Toxicological Profile for Selenium”, ATSDR - U.S. Department of Health and Human Services, Public Health Service (2003)" has been identified and will be used as primary database for the hazard assessment of zinc selenite.

Studies performed with organic selenium compounds were excluded from evaluation as the bioavailability is much higher than for inorganic selenium compounds.

The ATSDR Toxicological Profile on Selenium (2003), was used as the key source of relevant data on selenium and selenium compounds, because it contains an evaluation of the toxicity information performed by a renowned scientific body. More recent reviews, e.g. the work conducted for the Canadian Soil Quality Guidelines / Sudbury Soil Study, have also been screened for additional data. The underlying assumption is that the key literature considered by renowned international organisations such as ATSDR has usually already been subjected to a reliability assessment. All key references identified by ATSDR have been screened for use in the REACH dossiers according to Klimisch and with respect to the requirements for risk assessment.

In addition, after the evaluation of the ATSDR review and further other secondary publications (as documented in the IUCLID section 12) it was suggested that a follow-up endpoint specific literature search should be conducted in bibliographic databases, restricting the search to references published since the beginning of 2000. The results of this search have also been screened for use in the REACH dossiers.

Literature cited in the ATSDR

The ATSDR document identified a set of 27 references on genetic toxicity which can be subdivided in (i) 17 references on in vitro test systems, of which 5 references are on bacterial tests systems and 12 on mammalian cell test systems (ii) 10 references on in vivo tests all on clastogenic effects. All references cited in the ATSDR were acquired in full text and evaluated with regard to relevance and reliability. A small subset of references was added which were considered relevant during the literature search, mainly based on the test substance used. All studies indentified by ATSDR as key references concerning tests with selenites or selenous acid were included in the IUCLID as study records. Following the quality and reliability screening, those references which were assessed as not adequate, relevant or reliable by expert judgement during screening procedure were assigned to "disregarded study", and rated as "not reliable" (RL=3 or 4), with the rationale being included in the endpoint study record.

Evaluation of relevant references

In vitro data:

One well described study (Slapsyte, 2003; not cited in ATSDR) on the induction of chromosome aberrations on isolated human lymphocytes following treatment with disodium selenite (Na2SeO3), reports no biological relevant increase in chromosome aberrations following 24h exposure. The maximum dose was limited by cytotoxicity. Clastogenic effects were reported at top concentrations for which a significant cytotoxicity was observed.

A data gap for the in vitro gene mutation endpoint has been identified. Therefore a state of the art, guideline and GLP compliant in vitro gene mutation test was conducted (Lloyd, 2010). It is concluded that zinc selenite did not induce increases in mutation at the hprt locus of L5178Y mouse lymphoma cells when tested up to precipitating and toxic concentrations in two independent experiments, with and without rat liver metabolic activation system (S9 mix).

The lack of an in vitro gene mutation test in bacteria constitutes a formal data gap. However, tests on the mutagenic potential of selenite in bacteria is considered dispensable for principal considerations, since inorganic metal compounds are frequently negative in this assay due to limited capacity for uptake of metal ions (Guidance on information requirements and chemical safety assessment, Chapter R.7a, p. 387; HERAG facts sheet mutagenicity, Chapter 2.1).

One reference on in-vivo micronucleus induction:

Itoh, (1996) described the induction of micronuclei in the bone marrow of mice following i.p. exposure towards selenous acid (H2SeO3). A significant increase in micronucleated cells was observed, with a clear positive finding in the maximum dose of 5 mg/kg. All lower dose levels were in the range of the control (0.0-0.3% MNPCE). However, the dose selection was not clearly justified and a description of the toxic effects is lacking. As stated below one might speculate that the maximum dose already showed signs of near-lethal toxicity, making this positive finding of questionable biological significance. Due to reporting deficiencies this reference will only be used as supportive information.

Results

A state of the art and guideline and GLP compliant in vitro gene mutation test (according to OECD 476) did not show a positive response. It is concluded that zinc selenite does not induce gene mutations in an in vitro mammalian test system when tested up to precipitating and toxic concentrations.

The findings in the in vivo test for selenites (Itoh, 1996) indicate that positive findings are only obtained at very high, near lethal doses. Furthermore, an overwhelming database of positive findings in in vitro tests was identified during the literature search and in the ATSDR. However, especially for metal compounds false positive findings have repeatedly been published which, can be attributed to osmolality or pH instead of genotoxic effects exerted by the metal ion itself and are therefore of limited biological relevance.

Furthermore, the risk assessment document published by the European Food Safety Authority (EFSA, 2006) summarise the genotoxic effects of selenium compounds as follows: “In vivo, only toxic amounts were shown to be active, keeping in mind the central role of hydrogen selenide in the metabolism of most selenium compound it is likely that overproduction of this and other auto-oxidisable selenium metabolites could promote the formation of DNA reactive oxygen radicals. [...] Genotoxicity has been seen in a number of in vitro systems and also in vivo at toxic doses. It is likely, however, that these effects may be related to the generation of reactive oxygen radicals, being dose dependent and showing a threshold in vivo and not occurring at nutritionally adequate intakes.

Based on the above given information it can be concluded that in vivo clastogenic effects may occur at high, nearly lethal doses which are not attributable to exposure conditions for workers, consumers and humans exposed via the environment. Thus, this effect is considered of no biological relevance for humans.


References

Selenium, In: European Food Safety Authority (EFSA) (2006) Tolerable upper intake levels for vitamins and minerals, page 65f.

Read-across from sodium selenite and selenous acid to zinc selenite

Based on a comparison between toxicity reference values of zinc compounds and selenium compounds, it can safely be assumed that the selenium/selenite moiety of zinc selenite is generally of higher toxicological relevance than the zinc cations. Comparing the DNELs for the zinc ion itself with the DNELs for the zinc ion in zinc selenite indicated significantly higher values (in the range of factor 10 to 20) for the DNELs derived for zinc ion itself (see endpoint summary, IUCLID section 7 “Toxicological information” for a detailed comparison of the toxicity of the zinc and selenite components in zinc selenite). Therefore, the subsequent assessment of the toxicity of zinc selenite focuses on the selenium moiety.

Very little toxicological test data exist with the test substance zinc selenite itself. However, for any kind of systemic toxicity a substance needs to be taken up systemically. In the case of an inorganic salt like zinc selenite, such an uptake is always associated with a dissolution of the substance, i.e. dissociation into ions. Zinc selenite initially dissociates into zinc cations (Zn2+) and selenite anions (SeO32-).

Since toxicological test data are available for sodium selenite (Na2SeO3) and selenous acid (H2SeO3), read-across is applied from these to substances to zinc selenite.

Justification:

a) In vivo toxicokinetic data or in vitro bioaccessibility data for a comparative assessment of relative bioavailability of various selenite substances are not available. Thus, water solubility is adopted as a surrogate of bioavailability:

substance

water solubility

sodium selenite

800-900 g/L

selenous acid

1670 g/L

zinc selenite

16 mg/L

Sodium selenite and selenous acid are readily soluble, with water solubilities of 800 – 900 g/L and 1670 g/L at 20 °C, respectively. Zinc selenite is a salt which is only slightly soluble in water at a concentration of 16 mg/L at 20 °C. Based on that, conservative read-across from the highly soluble substances to the poorly soluble zinc selenite is possible for the purposes of this dossier since zinc selenite is assumed to have a lower bioavailability than sodium selenite and selenous acid.

b) Sodium selenite, selenous acid and zinc selenite all liberate the selenite anion upon dissolution. Assuming that (i) sodium selenite, and zinc selenite both dissociate in water to SeO32-and the cationic counter-ions, and (ii) the potential effects are caused by SeO32-and not by the cations, the results from the available studies with sodium selenite can be used for read across to zinc selenite. The selenite anions are formed under most physiologically relevant conditions (i.e., neutral pH), thus facilitating unrestricted read-across between these species. In slightly acid condition, the hydrogenselenite ion, HSeO3-, is formed; in more acidic conditions selenous acid, H2SeO3, exists.

H2SeO3<=> H++ HSeO3- (pKa= 2.62)

HSeO3- <=> H++ SeO32-   (pKa= 8.32)

Based on these equilibrium conditions, read-across between the groups of selenites, hydrogenselenites and selenous acid is possible.


Short description of key information:
Zinc selenite did not show a significant or dose-dependent increase in mutations in cultured mouse lymphoma cells (L5178Y) up to the maximum dose of 120 µg/mL. In a weight of evidence it has to be concluded that in vivo clastogenicity is only observed at extremely high, nearly lethal doses. Thus, this effect is considered of no biological relevance. Zinc selenite is considered non-genotoxic.

Endpoint Conclusion: No adverse effect observed (negative)

Justification for classification or non-classification

Zinc selenite did not show a significant or dose-dependent increase in mutations in cultured mouse lymphoma cells (L5178Y) up to the maximum dose of 120 µg/mL.

In a weight of evidence it has to be concluded that in vivo clastogenicity is only observed at extremely high, nearly lethal doses. Thus, this effect is considered of no biological relevance.

The classification criteria according to regulation (EC) 1272/2008 as germ cell mutagen are not met, no classification is required.

Further testing of in vivo genetic toxicity tests is not considered necessary.