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Key value for chemical safety assessment

Additional information

Justification of read-across from other arsenic compounds to diarsenic trioxide

Diarsenic trioxide is soluble in water (17.8 g/L at 20°C) and thus readily bioavailable. It has been shown that the predominant dissolved species in water is As(III). Whereas a number of studies have noted differences in the relative toxicity of arsenic compounds, with trivalent arsenites tending to be somewhat more toxic than pentavalent arsenates, these distinctions are generally not emphasised in the assessments of systemic effects of arsenic compounds, for several reasons: (1) in most cases, the differences in the relative potency are reasonably small (about 2– 3-fold), often within the bounds of uncertainty regarding effect levels; (2) different forms of arsenic may be interconverted, both in the environment and the body; and (3) in many cases of human exposure the precise chemical speciation is not known (adopted from ATSDR, 2007).

Read-across from data on other (bioavailable) arsenic compounds, specifically from arsenites, to diarsenic trioxide is thus considered justified without restriction for systemic toxicological effects, including genetix toxicity / mutagenicity.

Data on individual endpoints discussed under genetic toxicity/mutagenicity

 (i) bacterial assays

Readily bioavailable arsenicals such as “arsenites” are not mutagenic in the Ames test, as has been shown in Salmonella typhimurium (Löfroth and Ames 1978) and Escherichia coli (Rossman et al. 1980). A detailed summary has recently been published by ATSDR (2007).

 (ii) in vitro mammalian systems, gene mutation:

“Arsenite” does not induce point mutation in mammalian cell systems, as summarised in detailby ATSDR (2007; for details, please refer to Table 3 -8 of the ATSDR review): the overwhelming majority of assays conducted in mouse lymphoma cells, Chinese hamster V79 cells, Chinese hamster ovary cells and Syrian hamster embryo cells yielded negative results. Because of the large number of such results, a detailed presentation of data is not considered to be required.

(iii) in vitro mammalian systems, cytogenicity:

A vast amount of positive results obtained in in vitro cytogenicity assays with human fibroblasts, lymphocytes and leukocytes, mouse lymphoma cells, Chinese hamster ovary cells, and Syrian hamster embryo cells demonstrate that trivalent inorganic arsenic substances can induce chromosomal aberrations and sister chromatid exchanges; likewise, in vitrostudies in human, mouse, and hamster cells have also been positive for DNA damage and repair, as well as enhancement or inhibition of DNA synthesis (ATSDR, 2007;for details, please refer to Table 3-8 of the ATSDR review).Because of the large number of such results, a detailed presentation of data is not considered to be required.

(iv) in vivo data:

Numerous published data are available on in vivo studies in animals via the oral route, including rat and mouse bone marrow assays, as well as human (oral exposure) data investigating chromosomal aberrations or micronuclei formation and sister chromatid exchanges in peripheral lymphocytes. The overall positive result of these investigations demonstrates the clastogenicity of trivalent inorganic arsenic species (ATSDR, 2007; for details, please refer to Table 3-8 of the ATSDR review). Because of the large number of such results, a detailed presentation of data is not considered to be required. Human and animal data are similarly available indicating that inorganic arsenic is clastogenic also via inhalation exposure. Most noteworthy, workers exposed to unspecified concentrations of arsenic trioxide at the Ronnskar copper smelter in Sweden were found to have a significant increase in the frequency of chromosomal aberrations in peripheral lymphocytes (Beckman et al. 1977; Nordenson et al. 1978; cited in ATSDR, 2007).

 

 

Overall summary and conclusion on genetic toxicity / mutagenicity:

Trivalent inorganic arsenical have been shown to void of mutagenic activity both in bacterial as well as mammalian test systems. In contrast, readily bioavailable “arsenite” is a powerful inducer of chromosomal aberrations and sister chromatid exchanges in vitro and in vivo.

Mechanistic aspects: whereas inorganic trivalent arsenicals such as „arsenite“ do not react directly with DNA, treated cells nevertheless show increased oxidative DNA damage. Cytotoxic concentrations of trivalent arsenicals also cause DNA strand breaks and/or alkali-labile sites. Statistically significant increases in chromosomal aberrations apparently occur only at toxic doses. An analysis of micronuclei induced by As(III) in human fibroblasts shows that at lower (relatively non-toxic) doses, As(III) acts as an aneugen by interfering with spindle function and causing micronuclei with centromeres, but at high (toxic) doses, it acts as a clastogen, inducing micronuclei without centromers. Humans exposed to high concentrations of inorganic arsenic in drinking-water also show increased micronuclei in lymphocytes, exfoliated bladder epithelial cells and buccal mucosa cells, and sometimes chromosomal aberrations and sister chromatid exchange in whole blood lymphocyte cultures (ATSDR, 2007).

The induction of oxidative stress, inhibition of DNA repair and changes in DNA methylation patterns are discussed as underlying mechanisms (Beyersmann, D.; Hartwig, A. (2008).

Overall, particularly in view of the primarily secondary genotoxic effects, it is assumed that a threshold exists for these effects (Hassauer & Kalberlah, 2010). This is supported by a meta analysis of available genotoxicity data by Rudel et al. (1996), who concluded thatthe dose-response relationships for the majority of the observed genotoxic effects are seemingly sublinear.


Short description of key information:
Arsenic in the form of biologically available trivalent As(III) (“arsenite”) ions is void of primary genotoxicity as manifested in the absence of gene mutation effects in prokaryotic or mammalian cell systems. Instead, arsenic is postulated to exert its genotoxicity by secondary mechanisms such as induction of oxidative stress, inhibition of DNA repair and changes in DNA methylation patterns.

Endpoint Conclusion:

Justification for classification or non-classification

Diarsenic trioxide is currently classified as “Cat 1A” human carcinogen. In the derivation of this classification (as also supported by reviewing organisations such as IARC and ATSDR, for example), it has been recognised that arsenic in the form of biologically available trivalent As(III) (“arsenite”) ions is void of primary genotoxicity as manifested in the absence of gene mutation effects in prokaryotic or mammalian cell systems. Instead, arsenic is postulated to exert its genotoxicity by secondary mechanisms such as induction of oxidative stress, inhibition of DNA repair and changes in DNA methylation patterns. In conclusion, a separate classification for genotoxicity is not warranted.