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Key value for chemical safety assessment

Effects on fertility

Description of key information

Background

 

A read-across category-approach is used for the assessment of the toxicological properties of selenium and selenium compounds. The following Se-substance are included in the category:

  • Se-metal (massive, powder)
  • Disodium selenate
  • Disodium selenite
  • Selenium dioxide / selenious acid
  • Zinc selenite
  • Barium selenite

A detailed rationale for the read-across hypothesis has been outlined in the read-across report that was generated according to the principles laid out in the Read-Across Assessment Framework (RAAF). In summary, the physico-chemical behavior of elemental selenium (once it has formed an ion-from its metal state), disodium selenite, disodium selenate and selenium dioxide/selenious acid is the same with regard to their metabolic fate. All selenium compounds (organic and inorganic, including elemental selenium), do share the very same metabolic fate in that after their resorption, reduction to the selenide moiety [Se2-], which is the single common precursor for its further metabolic conversion, takes place.

Therefore, there seems to be good evidence that different selenium moieties will behave very similar also for their ability to form reactive species which may play a decisive role in the generation of cytotoxicity followed likewise by unspecific and secondary clastogenicity and read-across can be made from the available data for disodium selenite. It is concluded that additional testing for each individual member of the proposed Se-category is not necessary and scientifically not meaningful.

In the case of inorganic salts like barium selenite and zinc selenite, uptake is always associated with a dissolution of the substance, i.e. dissociation into the metal cation (Zn2+, Ba2+) and the selenite anion (SeO32-). It can safely be assumed that the selenium/selenite moiety of barium/zinc selenite is generally of higher toxicological relevance than the zinc/barium cations. Therefore, the subsequent assessment of the toxicity of barium/zinc selenite focuses on the selenium moiety. As noin vivotoxicokinetic data orin vitrobioaccessibility data are available for a comparative assessment of relative bioavailability of various selenite substances, water solubility is adopted as a surrogate for bioavailability. Disodium selenite is readily soluble, with a water solubility of 800-900 g/L at 20°C. Barium selenite and zinc selenite, on the other hand, are poorly soluble salts (water solubility at 20°C of 66.7 mg/L and 16 mg/L, respectively, i.e. a difference of four/five orders of magnitude). Based on that, an intrinsically very conservative read-across from highly soluble forms to the poorly soluble barium/zinc selenite is proposed as the latter are assumed to have a lower solubility. It should also be noted that selenite anions in the tests with disodium selenite are formed under most physiological relevant conditions (i.e. neutral pH), thus facilitating unrestricted read-across between the various substances. In slightly acid conditions (pKa:8.32) the hydrogen selenite ion (HSeO3-) is formed whereas in more acidic conditions (pKa:2.62) the formation of selenious acid is observed (H2SeO3). Based on such existing equilibrium conditions, read-across between selenites, hydrogen selenites and selenious acid (solubility of 1670 g/L at 20°C) is justified.

 

Read-across from sodium selenite and selenious acid to barium/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/barium ion itself with the zinc/barium levels that are associated with the DNELs for barium/zinc selenite (based on selenite-data) indicated significantly higher values (in the range of factor 10 to 20) for the DNELs derived for the barium/zinc ion itself. Therefore, the subsequent assessment of the toxicity of barium/zinc selenite focuses on the selenium moiety.

Key information

  • Abdo (1994): NOAEL mice, female: 0.5 mg Se/kg bw/d (sub-chronic tests with sodium selenite), NOAEL ≥2.6 mg Se/kg bw/d (sub-chronic test with sodium selenate
  • Abdo (1994): NOAEL rats, female: 0.2 mg Se/kg bw/d (sub-chronic tests with sodium selenite), LOAEL 0.1 mg Se/kg bw/d (sub-chronic test with sodium selenate)
  • Abdo (1994): NOAEL rats male: ≥0.8 mg Se/kg bw/d (sub-chronic tests with sodium selenite), NOAEL 0.4 mg Se/kg bw/d (sub-chronic test with sodium selenate)
  • Abdo (1994): NOAEL mice: ≥1.6 mg Se/kg bw/d (sub-chronic tests with sodium selenite), NOAEL ≥2.6 mg Se/kg bw/d (sub-chronic test with sodium selenate)
  • Nobunaga et al. (1979): NOAEL mice: 0.18 mg Se/kg bw/d (test with sodium selenite pentahydrate)

 

It has to be emphasized, that the NOAEL, which is used as starting point for DNEL derivation is based on human data (see chapter “repeated dose toxicity”). The existing studies on humans are considering a wealth of toxicological endpoints, but effects on reproduction were neither observed on workers nor on humans with an increased uptake of selenium above the DNEL. From this follows, that the substance is of no concern with respect to reproduction toxicity.

Additional information

General

The ATSDR Toxicological Profile on Selenium (2003), which is currently the most comprehensive review, was used as key source of relevant data on selenium compounds. It already contains a detailed evaluation of toxicity data, performed by a renowned scientific body. More recent reviews, e. g. the work conducted for the Canadian Soil Quality Guidelines / Sudbury Soil Study, were also been screened for additional data.

 

All key references identified by ATSDR were re-evaluated and re-assessed for use in the REACH dossiers of the Se-compounds that are covered by the Se-category. Assessment was conducted according to Klimisch, and with respect to the requirements for risk assessment. Studies which were assessed as not adequate, not relevant or unreliable by expert judgement during the screening procedure were assigned to "disregarded study", and rated as "not reliable" (RL=3), with the rationale being included in the endpoint study record.

 

Evaluation of references

 

According to the evaluation criteria used by the experts of ATSDR (2003), a set of studies on reproductive toxicity of selenites providing reliable, quantitative estimates of No-Observed-Adverse-Effect Levels (NOAELs) or Lowest-Observed-Adverse-Effect Levels (LOAELs) are available. All these references were obtained and re-evaluated according to the criteria of REACH. Two key studies performed with sodium selenite were identified in which effects on female fertility and in one of those also effects on male fertility are reported: Abdo, 1994 and Nobunaga, et al. 1979 (data entry in IUCLID section 7.8.2).

 

In the NTP studies (Abdo, 1994), treatment of male and female mice via drinking water with concentrations of 2, 4, 8, 16 and 32 ppm sodium selenite (corresponding to 0.14, 0.3, 0.5, 0.9 or 1.6 mg Se/kg bw/d) caused significant increases in oestrus cycle lengths in female mice of the 32 ppm group (1.6 mg Se/kg bw/d). Because the next lower group treated with 16 ppm (0.9 mg Se/kg bw/d) was not evaluated for fertility parameters, the dose level of 8 ppm Na2SeO3 or 0.5 mg Se/kg bw/d can be regarded as NOAEL for female fertility. Treatment of male and female rats via drinking water with concentrations of 2, 4, 8, 16 and 32 ppm Na2SeO3 (corresponding to 0.08, 0.13, 0.2, 0.4 or 0.8/0.9 mg Se/kg bw/d) caused increases in oestrus cycle lengths in female rats of the 16 ppm group (0.4 mg Se/kg bw/d) (32 ppm not evaluate), which was regarded as treatment related. All other findings at high exposure concentrations were regarded as confounded by dehydrated and unthrifty conditions of the animals. Thus, the dose level of 8 ppm Na2SeO3 or 0.2 mg Se/kg bw/d can be regarded as lowest NOAEL for female fertility resulting from this study. The effect on oestrus cycle length was regarded as treatment related, because it was seen in rats and mice at dose levels that caused less than 10% body weight depression.

 

In the same NTP studies (Abdo, 1994), there is also some information on male fertility. No treatment- related effects were observed on spermatid and epididymal spermatozoal measurements in treated groups of male rats and mice. In male rats, epididymal sperm concentrations were slightly, but significantly decreased in all groups (4, 8, 16, 32 ppm Na2SeO3) without dose-relation, and this effect was therefore regarded as not treatment related. Thus, no treatment related effect on male fertility could be observed up to a dose of 32 ppm Na2SeO3.

In addition, results from a developmental toxicity study (Nobunaga, et al. 1979) confirmed effects on oestrus cycle length at 0.6 mg Na2SeO3 x 5H2O/kg bw/d (corresponding to 0.18 mg Se/kg bw/d, but no maternal toxicity and effects on conception rates and the average time until conception as well as fertility in general were observed.

Thus, based on a weight of evidence approach of the studies presented above there is some evidence of treatment-related effects on female oestrus cycle length in rats and mice with an NOAEL of 0.18 mg Se/kg bw/d, but no clear evidence for impaired fertility are available. In comparison, the NOAEL of 0.12 mg Se/ kg bw/d is somewhat lower and is used for risk assessment purposes.

 

The three supportive studies investigating male fertility (Kaur, 1994; Kaur, 2005; and Shalini, 2008) are considered inadequate for the evaluation of this endpoint (rated as not reliable RL=3), because of their limitations with respect to study animals and dose selection. In the study reported by Kaur (1994), the ingestion of 2 and 4 ppm sodium selenite via diet was investigated in house rats (Rattus rattus) captured from poultry houses with no information on age, body weight and health status of animals. Only 6 animals per group were used and the results showed a very high variability between animals. In two studies conducted in mice (Kaur 2005, Shalini 2008), male mice were fed diets either with an adequate Se status (0.2 ppm), deficient (0.02 ppm) or excess Se (1 ppm) amount of Se over up to 8 weeks. However, these studies were not designed to evaluate a dose–response relationship of sodium selenite on the fertility of male mice, because only one dose level with selenium in access of the adequate group was given, and these two groups were compared to each other.

 

The data requirement for a screening study as laid down in regulation (EC) 1907/2006, Annex VIII, Column 1, Section 8.7.1 is nearly fulfilled by applying a weight-of-evidence approach to the two key studies Abdo (1994) and Nobunaga (1979).

 

The outcome of a 3-generation study with sodium selenate (Schroeder and Mitchener 1971b) and a 2-generation study with potassium selenate were also assessed in the ATSDR Toxicological Profile on Selenium (2003), and are also included in the REACH registration dossier.

As can be seen in Schroeder and Mitchener (1971b), a three-generation reproduction study in which selenium was administered to breeding mice via drinking water as sodium selenate (0.57 mg selenium/kg/day) resulted in adverse effects on reproduction. The most notable observed effects included the failure of about half of the F3 generation pairs to breed successfully.

In the two-generation study using rats, selenium administered as potassium selenate (Rosenfeld and Beath 1954) no effect on reproduction were noted at a dose of 0.21 mg selenium/kg/day for 1 year. Decreased fertility and pup survival were noted at 1.05 mg Se/kg/day. At 0.35 mg selenium/kg/day for 1 year, the number of young successfully reared by the females was reduced by 50%, and the body weight of the females was approximately 20% less than that of the control.

 

 

Furthermore, a short term reproductive study with sodium selenate is presented in ATSDR (2003), which should be included as supportive information in the evaluation of the effects on fertility: “A short-term reproductive study of the effects of sodium selenate in drinking water on rats reported some female reproductive toxicity (reduced corpora lutea, reduced implants per litter, shorter estrous cycle), but only at doses (0.418 mg Se/kg/day) that produced signs of severe maternal toxicity, including a large reduction in water consumption (NTP 1996). ”

 

Summarising the results of these studies, the following NOAELs – expressed as Se – can be put forward; a similar conclusion was also made by ATSDR:

·     3-generation study: no NOAEL derived; LOAEL: 0.57 mg Se/kg/day

·     2- generation study: NOAEL: 0.21 mg Se/kg/day

·     30-day study: no NOAEL derived; LOAELreproduction: 0.5 mg selenium/kg/day for female rats

 

 

Since no NOAEL could be derived from the 3-generation study, the lowest available NOAEL (rat) resulting from the 2-generation study is 0.21 mg selenium/kg bw/d. This no-adverse effect level is in the same range as the NOAELs derived from the two studies (Nobunaga, 1979; Abdo, 1994) conducted with sodium selenite (0.18 mg Se/kg bw/d).

 

Based on the above, the conduct of a new study is not considered necessary,since reliable data fulfilling the requirements of the REACH regulation are available in the ATSDR Toxicological Profile on Selenium (2003).

Effects on developmental toxicity

Description of key information

Nobunga (1979): Oral exposure of mice via drinking water. NOAEL = 3 ppm: 0.6 mg/kg bw/d Na2SeO3x 5H2O, which is equivalent to 0.18 mg Se/kg bw*d

It has to be emphasized, that the NOAEL, which is used as starting point for DNEL derivation is based on human data. The existing studies on humans are considering a wealth of toxicological endpoints, but effects on reproduction were neither observed on workers nor on humans with an increased uptake of selenium above the DNEL. From this follows that the substance is of no concern with respect to developmental toxicity. 

Additional information

General

 The ATSDR Toxicological Profile on Selenium (2003), which is currently the most comprehensive review, was used as key source of relevant data on selenium compounds. It already contains a detailed evaluation of toxicity data, performed by a renowned scientific body. More recent reviews, e. g. the work conducted for the Canadian Soil Quality Guidelines / Sudbury Soil Study, were also been screened for additional data.

 

All key references identified by ATSDR were re-evaluated and re-assessed for use in the REACH dossiers of the Se-compounds that are covered by the Se-category. Assessment was conducted according to Klimisch, and with respect to the requirements for risk assessment. Studies which were assessed as not adequate, not relevant or unreliable by expert judgement during the screening procedure were assigned to "disregarded study", and rated as "not reliable" (RL=3), with the rationale being included in the endpoint study record.

 

 

Evaluation of references

Several studies on developmental toxicity with selenites were assessed in the ATSDR and reported quantitative estimates of No-Observed-Adverse-Effect Levels (NOAELs) or Lowest-Observed-Adverse-Effect Levels (LOAELs). All these references were obtained and re-evaluated according to the criteria of REACH.

 

One key study was identified for developmental toxicity and teratogenicity: Nobunaga (1979). Treatment of female mice with sodium selenite pentahydrate in drinking water (3 and 6 ppm Se) for 30 days before gestation and following mating until day 18 of gestation did not cause maternal toxicity, but oestrus cycle length was increased in comparison to control in the high dose group (6 ppm). Beside a statistically significant decrease in foetal body weights in high dose animals (6 ppm), no further signs of embryo-/foetotoxicity or teratogenicity were observed. Thus, the lower dose level of 3 ppm represents a NOAEL. This corresponds to 0.6 mg/kg bw/d Na2SeO3 x 5H2O, or 0.18 mg Se/kg bw/d.

 

This study has some restrictions because it was conducted at a time when OECD guidelines were not available and reporting of results is not comparable to information coming from a recently conducted OECD and GLP toxicity study. There are, however, a number of additional studies that provide reliable, supportive information. Developmental studies using the oral route of administration indicate that excessive sodium selenate or sodium selenite intake can result in fetal toxicity and reduced growth in experimental mammals (Dinkel et al. 1963; Ferm et al. 1990; NTP 1996; Rosenfeld and Beath 1964; Wahlstrom and Olson 1959a), but generally only at doses that produce maternal toxicity. Developmental effects were not observed in macaque fetuses from mothers given toxic oral doses of L-selenomethionine during gestation (Tarantal et al. 1991). Intravenous injection of sodium selenite in mice did not indicate that the compound is teratogenic in rodents (Yonemoto et al. 1984). Intravenous injections of sodium selenate, D, L-selenomethionine, and D, L-selenocystine into neonatal rats indicated that some selenium compounds can contribute to the formation of one type of cataracts (Ostadalova and Babicky 1980). Cataracts were not observed in the offspring of macaques treated orally with L-selenomethionine during gestation (Tarantal et al. 1991). Additional developmental toxicity studies of selenium compounds in mammals do not seem to be necessary at this time.

Since no further relevant developmental effects (at concentrations without maternal toxicity) are described in ATSDR (2003), the lowest NOAEL obtained for developmental effects was0.18 mg Se/kg bw/d.

 

 

Since there are no indications of effects of treatment with sodium selenite on development of mice (Nobunaga), and taking into account the study results cited above,the conduct of a new OECD developmental toxicity study is not considered necessary.

Mode of Action Analysis / Human Relevance Framework

In the current version of Regulation (EC) No 1272/2008, neither “selenium” or “selenium compounds with the exception of cadmium sulphoselenide and those specified elsewhere in this Annex”have a harmonised classification for the reproductive toxicity endpoint.

In the classification system of Regulation (EC) No 1272/2008, reproductive toxicity includes adverse effects on sexual function and fertility in adult males and females, as well as developmental toxicity in the offspring. Classification as a reproductive toxicant should be made on the basis of an assessment of the total weight of evidence. Evaluation of substances chemically related to the substance under study may also be included, particularly when information on the substance is scarce. If, in some reproductive toxicity studies in experimental animals the only effects recorded are considered to be of low or minimal toxicological significance, classification may not necessarily be the outcome.

 

Since the available data on selenate and selenite are not considered as sufficient to allow a conclusive decision for classification or non-classification, study results from related selenium compounds as cited in ATSDR (2003) were included in the assessment of the total weight of evidence. ATSDR summarises the reproductive effects of various selenium compounds as follows: “Very high amounts of selenium have caused decreased sperm counts, increased abnormal sperm, changes in the female reproductive cycle in rats, and changes in the menstrual cycle in monkeys. The relevance of the reproductive effects of selenium exposure in animals studied to potential reproductive effects in humans is not known. Selenium compounds have not been shown to cause birth defects in humans or in other mammals.

 

For sodium selenate and sodium selenite, effects on oestrus cycle lengths of females were observed in standard toxicity studies in rats and mice and in a developmental toxicity in mice. However, no evidence for an impairment of fertility was seen in the developmental study. The available reliable developmental study showed no significant adverse effects on foetal development and no indications of a teratogenic potential of sodium selenite were observed.

 

In conclusion, no classification for reproductive toxicity is considered necessary for the Se-compounds that are included in the assessed Se-category.

Justification for classification or non-classification

Additional information

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