Strontium

Endpoint summary

Description of key information

Read-across:

Strontium metal is highly reactive and instantly oxidizes upon contact with water. It decomposes completely. During the redox-reaction with water, a strong evolution of hydrogen gas and an immediate precipitation of a white, crystalline solid (i.e. Sr(OH)2) is observed (Sr2+ + 2OH- + H2 (g). The amount of dissolved Sr cations is determined by the solubility of the Sr(OH)2 precipitate. According to OECD guideline 105 (1995) and EU method A.6 (2006), the water solubility of strontium was determined to be 6.74 ± 0.14 g/L under the conditions of the test (flask method under protective gas atmosphere; loading of 41 g Sr/L, at 20.0 ± 1.0 °C, pH >13).

Strontium ions are highly mobile, occur only in one valence state (2+), i.e. are not oxidized or reduced, and do not form strong complexes with most inorganic and organic ligands (Krupka et al. 1999. EPA 402-R-99-004B; Salminen et al. 2015; Carbonaro and Di Toro. 2007. Geochim Cosmochim Acta 71 3958–3968; Carbonaro et al. 2011. Geochim Cosmochim Acta 75: 2499-2511 and references therein). Thus, it may be assumed that systemic toxicological effects (not local) are related to the strontium ion. Therefore, the assessment of systemic toxicity of strontium is based on elemental strontium concentrations. Read-across of systemic toxicity data available for soluble strontium substances is applied since the strontium ions determine the toxicological potential of strontium. Read-across from strontium chloride hexahydrate to strontium is possible since as a first surrogate for bioavailability, the solubility of a test substance in water may be applied. Both substances (strontium chloride hexahydrate and strontium) are soluble (> 5 g/L). Hence, it can be concluded that adverse effects observed with strontium chloride hexahydrate are due to the presence of the strontium ion and are relevant for strontium metal. The NOAEL (males) of 300 ppm strontium chloride hexahydrate (equivalent to 30 mg/kg/day, nominal) was derived, based on the significant increase of the relative thyroid weights, found for the males at the 1200 ppm and 4800 ppm dose levels. The substance NOAEL corresponds to NOAEL (males) of 9.9 mg strontium/kg/day (nominal).

Key value for chemical safety assessment

Repeated dose toxicity: via oral route - systemic effects

Link to relevant study records

Reference

Endpoint:

sub-chronic toxicity: oral

Type of information:

read-across based on grouping of substances (category approach)

Adequacy of study:

key study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: Well-documented study. The study fulfils the requirements of the current test guideline for oral sub-chronic exposure to a great extent, but the study was not performed under GLP requirements.

Justification for type of information:

Strontium metal is highly reactive and instantly oxidizes upon contact with water. It decomposes completely. During the redox-reaction with water, a strong evolution of hydrogen gas and an immediate precipitation of a white, crystalline solid (i.e. Sr(OH)2) is observed (Sr2+ + 2OH- + H2 (g). The amount of dissolved Sr cations is determined by the solubility of the Sr(OH)2 precipitate. According to OECD guideline 105 (1995) and EU method A.6 (2006), the water solubility of strontium was determined to be 6.74 ± 0.14 g/L under the conditions of the test (flask method under protective gas atmosphere; loading of 41 g Sr/L, at 20.0 ± 1.0 °C, pH >13).
Strontium ions are highly mobile, occur only in one valence state (2+), i.e. are not oxidized or reduced, and do not form strong complexes with most inorganic and organic ligands (Krupka et al. 1999. EPA 402-R-99-004B; Salminen et al. 2015; Carbonaro and Di Toro. 2007. Geochim Cosmochim Acta 71 3958–3968; Carbonaro et al. 2011. Geochim Cosmochim Acta 75: 2499-2511 and references therein). Thus, it may be assumed that systemic toxicological effects (not local) are related to the strontium ion. Therefore, the assessment of the systemic toxicity of strontium is based on elemental strontium concentrations. Read-across of systemic toxicity data available for soluble strontium substances is applied since the strontium ions determine the toxicological potential of strontium.

Reason / purpose for cross-reference:

reference to same study

Qualifier:

no guideline followed

Principles of method if other than guideline:

Rats received SrCl2 hexahydrate in a diet at dose levels of 0, 75, 300, 1200, and 4800 ppm for 90 days. The diet contained adequate levels of Ca, Mg, P and vitamin D3. Growth, food intake, behaviour and mortality were measured, extensive haematology and clinical biochemistry carried out, organ weight determined, X-ray photographs of the bones taken and complete histopathological examination was performed. In addition, the Sr content of blood, bone and muscles was determined in additional groups of rats.

GLP compliance:

no

Limit test:

no

Species:

rat

Strain:

Wistar

Sex:

male/female

Details on test animals or test system and environmental conditions:

TEST ANIMALS- Source: SPF wistar rats from the breeding stock of the National Institute of Public Health- Weight at study initiation: 40-60 g bw- Housing: under conventional conditions, littermate-divided in wire cages, two in a cage according to sex- Diet (ad libitum): semipurified diet (Muracon SSp-tox, Trouw Ltd., Putten, The Netherlands); the diet contained 0.05 % Mg, 0.75 % P, 0.85 % Ca and 1.8 % I.U. Vit. D.3 per gram- Water (ad libitum): tap waterNo further information given.

Route of administration:

oral: feed

Vehicle:

unchanged (no vehicle)

Details on oral exposure:

DIET PREPARATION- SrCl2 in a semi-purified diet at dose levels of 0, 75, 300, 1200, and 4800 ppm- the diet contained adequate levels of Ca, Mg, P and vitamin D3 (see above, details on test animals and environmental conditions)No further information given.

Analytical verification of doses or concentrations:

not specified

Details on analytical verification of doses or concentrations:

no data

Duration of treatment / exposure:

90 days

Frequency of treatment:

ad libitum in diet (continuous)

Remarks:

Doses / Concentrations:75 ppm SrCl2*6H2OBasis:nominal in diet

Remarks:

Doses / Concentrations:300 ppm SrCl2*6H2OBasis:nominal in diet

Remarks:

Doses / Concentrations:1200 ppm SrCl2*6H2OBasis:nominal in diet

Remarks:

Doses / Concentrations:4800 ppm SrCl2*6H2OBasis:nominal in diet

No. of animals per sex per dose:

- 10 males and 10 females per group- additional 15 males in the control group for determination of Sr-levels in blood, bone and muscle- additional 10 males in the other groups for determination of Sr-levels in blood, bone and muscle

Control animals:

yes

Details on study design:

- Dose selection rationale: A range-finding experiment has been performed (for details see "Any other information on materials and methods incl. tables"; for results see "Remarks on results incl. tables and figures")No further information given.

Positive control:

no data

Observations and examinations performed and frequency:

CAGE SIDE OBSERVATIONS: Yes- Time schedule: daily- Cage side observations checked: behaviour, mortalityDETAILED CLINICAL OBSERVATIONS: No dataBODY WEIGHT: Yes - Time schedule for examinations: weeklyFOOD CONSUMPTION AND COMPOUND INTAKE (if feeding study): Yes, intake was measured in week 2, 5, 9, 12- Food consumption for each animal determined and mean daily diet consumption calculated as g food/kg body weight/day: No data - Compound intake calculated as time-weighted averages from the consumption and body weight gain data: No dataFOOD EFFICIENCY: Yes- Body weight gain in kg/food consumption in kg per unit time X 100 calculated as time-weighted averages from the consumption and body weight gain data: No dataWATER CONSUMPTION AND COMPOUND INTAKE (if drinking water study): No dataOPHTHALMOSCOPIC EXAMINATION: No dataHAEMATOLOGY: Yes- Time schedule for collection of blood: after 90 weeks- Anaesthetic used for blood collection: No data- Animals fasted: No data- How many animals: 10 female and 10 male rats of each group- Parameters checked: Hb, Ht and the number of erythrocytes and leucocytes; the MCV, MCHC and MCH were calculated; in addition blood picture was studiedCLINICAL CHEMISTRY: Yes (for method description, see below )- Time schedule for collection of blood: Ca, Mg and P in the beginning of the experiment and after 4, 8, and 12 weeks; SGPT, Alk. PAse and urea content after 6 and 12 weeks- Animals fasted: No data- How many animals: Ca, Mg and P of 5 control males (in the beginning of the experiment) and in 5 males per group after 4, 8, and 12 weeks; SGPT, Alk. PAse and urea conten of 5 males per group- Parameters checked: Ca, Mg and P analysis; activity of SGPT, Alk. PAse and urea content in the serumURINALYSIS: Yes- Time schedule for collection of urine: after 12 weeks- Metabolism cages used for collection of urine:No data- Animals fasted: No data- Parameters checked: urinalysis was carried out with Bililabstix (Ames Cy.) in 10 female and 10 male rats; presence of protein, blood bilirubin and ketones were studied, and pHNEUROBEHAVIOURAL EXAMINATION: No dataOTHER:- X-ray photographs of the bones were taken in the ninth and twelfth week of the experiment from 2 females and 2 males of group 1 and from 5 females and 5 males of group 5 (for method description, see below )- the Sr content of blood, bone and muscles was determined in additional groups of rats (10 males each and 15 control males) at weeks 0, 4, 8, and 12 (for method description, see below )- the activities of the microsomal liver enzymes AH, and APDM were determined in 5 males per group after 4 and 12 weeks- the glycogen content in liver was determined in 5 males after 8 week s and in 6 females and 6 males after 12 weeksNo further information given.

Sacrifice and pathology:

After 12 weeks the remaining animals were killed.GROSS PATHOLOGY: Yes; organ weight determined: brain, pituitary, heart, thyroid, liver, kidneys, spleen, adrenals, ovaries or testes, uterus or prostateHISTOPATHOLOGY: Yes, complete histopathological examination was carried out by preparing paraffin sections (5 µm) stained by haemalum and eosin: brain, pituitary, heart, thyroid, liver, kidneys, spleen, adrenals, ovaries or testes, uterus or prostate, lungs, thymus, pancreas, mesenterial lymph nodes, stomach, duodenum, ileum, jejunum, coecum, colon, rectum, urinary bladder, nervus ischiadicus, musculus quadriceps and femur

Other examinations:

no

Statistics:

For the purposes of objective quantification of observed differences between separate treatment groups and corresponding controls, Student's t-test was used. The results of these testings are indicated as follows: *=P<0.05, **=P<0.01 and ***=P<0.001. According to the general statistical theory it is advisable not to attach too much weight to a formal significance result (i.e. P<0.05). in such cases of marginal statistical significance it is important to consider carefully the test results obtained for corresponding comparisons regarding the other dose levels.

Clinical signs:

no effects observed

Mortality:

no mortality observed

Body weight and weight changes:

no effects observed

Food consumption and compound intake (if feeding study):

no effects observed

Food efficiency:

no effects observed

Water consumption and compound intake (if drinking water study):

not specified

Ophthalmological findings:

not specified

Haematological findings:

no effects observed

Clinical biochemistry findings:

effects observed, treatment-related

Urinalysis findings:

no effects observed

Behaviour (functional findings):

not specified

Organ weight findings including organ / body weight ratios:

effects observed, treatment-related

Gross pathological findings:

no effects observed

Histopathological findings: non-neoplastic:

effects observed, treatment-related

Histopathological findings: neoplastic:

no effects observed

Details on results:

CLINICAL SIGNS AND MORTALITY- no relevant effects- one control female died during bleeding procedure after more than 11 weeksBODY WEIGHT AND WEIGHT GAIN- no relevant effectsFOOD CONSUMPTION AND COMPOUND INTAKE (if feeding study)- food intake was not affected by exposure to strontium chloride hexahydrate- no data on daily food intake are available in order to calculate daily dose levels- according to the estimation given in the IUCLID Data Set Strontium carbonate, the NOAEL of 300 ppm corresponds to a dose of 22.5 mg SrCl2/kg bw/dFOOD EFFICIENCY- no relevant effectsHAEMATOLOGY- the slightly elevated erythrocyte count noticed in the range-finding experiment was not confirmed in the 90-day study- a lower leucocyte count noticed in the males of the 300 ppm group is not considered to be caused by the treatment, since at higher dose levels such an effect was not observedCLINICAL CHEMISTRY- Analysis of Alk. Pase, SGPT and urea in serum did not reveal any significant changes although in the highest dose group an indication of an increased activity of Alk. Pase was noticed. No clinical differences in clinical chemistry were noted except an indication of increased alkaline phosphatase activity in the highest dose group- the levels of Ca, Mg and P in blood were similar for all dose levels and the Ca/P ratio was constant- blood concentrations of Ca, Mg and P were higher at 8 weeks than 12 weeks, which seems a physiological condition URINALYSIS- urinalysis showed no differences in the groupsORGAN WEIGHTS- in males, relative thyroid weights were significantly increased in the 1200 and 4800 ppm groups (see Table III in attachment)- in females, relative pituitary weights were significantly decreased in the 300 and 4800 ppm group, but not in the 1200 ppm group (see Table III)- the relative prostate weights were significantely decreased at 75 and 1200 ppm (see Table III in attachment); this, however, must be considered with care, because proper preparation of the rat prostate is difficultHISTOPATHOLOGY: NON-NEOPLASTIC- there were no changes seen on histopathological examination except slight changes in the liver and thyroid after blind examination (see Table IV in attachment)(the changes consisted of a loss of glycogen in the liver at the highest dose level and a slightly increased activity in the thryroid of the males of the highest dose group)- glycogen depletion may be caused by several factors other than strontium, such as stress , starvation or dirunal rhythmOTHER FINDINGS- The glycogen concentration in the liver after 12 weeks showed a dose-related decrease, which was only significant in the female ath the highest doselevel (see Table II in attachment), which is may be caused by stress, starvation or diurnal rhythm- microsomal liver enzyme activities did not show any changes- detectable amounts of Sr in blood and muscle were only noticed at the dose of 4800 ppm (Table Vin attachment )- the Sr content in bone was increased at all dose levels having a constant level from 4 weeks onwards (steady-state level)(Table V in attachment)- there were no changes seen in the X-ray photographs - thus, up to the highest dose of 4800 ppm no rachitic changes occurredNo further information given.

Dose descriptor:

NOAEL

Remarks:

Sr

Effect level:

12.4 mg/kg bw/day (nominal)

Sex:

male

Basis for effect level:

other: See "Remarks"

Dose descriptor:

LOAEL

Remarks:

Sr

Effect level:

49.6 mg/kg bw/day (nominal)

Sex:

male

Basis for effect level:

other: If the increased concentrations of strontium in the bone can be considered a non-toxic effect, a LOAEL of 1200 ppm SrCl2 can be derived from this study which is based on the weight changes of thyroids at the doses of 1200 ppm and 4800 ppm.

Critical effects observed:

not specified

RANGE-FINDING EXPERIMENT

Behaviour, growth, food intake and food efficiency were not affected in the range-finding experiment. Haematological investigation revealed only a slight elevation of the total number of erythrocytes in males and females and a slight increase of the white cell count in the males at the highest dose level. No differences were found in liver and kidney weights and histopathological examination revealed no abnormalities. Sr in blood and muscle were only noted at the highest dose level whereas from 300 ppm onwards increased concentrations were found in bone (see Table I in attachment).

Conclusions:

If the increased concentrations of strontium in the bone can be considered a non-toxic effect, a NOAEL of 300 ppm SrCl2 can be derived form this study which is based on the weight changes of thyroids in males (but not females) at the doses of 1200 ppm (LOAEL) and 4800 ppm.

Endpoint conclusion

Endpoint conclusion:

adverse effect observed

Dose descriptor:

NOAEL

9.9 mg/kg bw/day

Study duration:

subchronic

Species:

rat

Quality of whole database:

The study is used as key study since it nearly meets the requirements of the current test guideline for oral sub-chronic exposure, however, the study was not performed under GLP (Klimisch score=2).

Organ:

other: glandular: thyroid

Repeated dose toxicity: inhalation - systemic effects

Endpoint conclusion

Endpoint conclusion:

no study available

Repeated dose toxicity: inhalation - local effects

Endpoint conclusion

Endpoint conclusion:

no study available

Repeated dose toxicity: dermal - systemic effects

Endpoint conclusion

Endpoint conclusion:

no study available

Repeated dose toxicity: dermal - local effects

Endpoint conclusion

Endpoint conclusion:

no study available

Additional information

Repeated-dose toxicity, oral:

A sub-chronic study was conducted with strontium chloride hexahydrate using groups of 10 male and 10 female Wistar rats. The rats were given the test substance via diet at dose levels of 0, 75, 300, 1200 or 4800 ppm (0, 7.5, 30, 120, or 480 mg/kg/day, respectively) for a duration of 90 days. The following parameters were investigated: clinical signs/mortality, body weight, food consumption/food efficiency, haematology, clinical chemistry, urinalysis, organ weights, and histopathology. Additional 10 males were included as satellite animals for the determination of the strontium levels in blood, bone and muscle, which were examined in weeks 0, 4, 8 and 12.

In a range-finding experiment, rats with an initial body weight of 130-170 g were divided into 5 groups each consisting of 3 females and 3 males, receiving 0, 3, 30, 300 and 3000 ppm strontium chloride hexahydrate in the diets respectively during two weeks. The animals were weighed at the beginning of the experiment and after 1 and 2 weeks. Food intake was recorded during the experimental period and was measured per cage (two rats) and expressed as the average intake per day per rat. Food conversion was calculated, blood samples were taken at the end of the experiment and haematological investigation was restricted to haemoglobin, haematocrit and the number of erythrocytes and leucocytes. The mean cell volume, mean cell haemoglobin concentration, and mean cell haemoglobin were calculated. At the end of the 2 weeks X-ray photographs were made of all animals. Strontium concentration was measured in blood, bone and muscle. Liver and kidneys were weighed and examined histopathologically by preparing paraffin sections (5 µm) stained with hematoxylin and eosin. Behaviour, growth, food intake and food efficiency were not affected in the range-finding experiment. Haematological investigation revealed only a slight elevation of the total number of erythrocytes in males and females and a slight increase of the white cell count in the males at the highest dose level. No differences were found in liver and kidney weights and histopathological examination revealed no abnormalities. Strontium in blood and muscle was only observed at the highest dose level whereas from 300 ppm onwards, increased concentrations were measured in bones.

In the main study, growth, behaviour, food intake and food efficiency were not affected by the treatment. Apart from a slight increase in the ALP activity in the highest dose group, no differences in clinical chemistry were noted. Levels of Ca, Mg and P in blood were similar for all animals and the Ca/P ratio was unaffected. Significant increase of the relative thyroid weights was found for the males at the 1200 ppm (control group: 0.0054 %; treatment group: 0.0072 %; P < 0.01) and 4800 ppm dose levels (control group: 0.0054 %; treatment group: 0.0068 %; P < 0.001). Although, no reasoning for this finding was be given, it was regarded as treatment-related. Relative pituitary weights of the females at 300 ppm dose level (control group: 0.0074 %; treatment group: 0.0062 %; P < 0.05) and at 4800 ppm dose level (control group: 0.0074 %; treatment group: 0.0056 %; P < 0.01) were significantly decreased, but not in the 1200 ppm group. Due to the lack of a dose-response relationship, this finding is not considered biologically relevant. Glycogen depletion of the liver was noted in the highest dose group. However, this was may be caused by stress, starvation or diurnal rhythm and not by treatment with the test substance. Increased strontium concentrations in blood and muscle were solely measured in animals at 4800 ppm. The strontium content in bone was increased at all dose levels with a plateau on week 4 and onwards (steady state). No treatment-related changes were observed in the X-ray photographs or during microscopical examination. Slight changes in the liver (glycogen depletion) and thyroid (activation). Thus, up to the highest dose of 4800 ppm no rachitic changes occurred.

The NOAEL (males) of 300 ppm strontium chloride hexahydrate (equivalent to 30 mg/kg/day, nominal) was derived, based on the significant increase of the relative thyroid weights, found for the males at the 1200 ppm and 4800 ppm dose levels. The substance NOAEL corresponds to NOAEL (males) of 9.9 mg strontium/kg/day (nominal).

 

Repeated dose toxicity, dermal:

According to Regulation (EC) 1907/2006 Annex IX section 8.6.2 and column 2, testing by the dermal route is not appropriate. Further, according to regulation (EC) 1907/2006 Annex XI (weight of evidence), testing for sub-chronic dermal toxicity is not considered to be required, for the following reasons:

- Repeated dose toxicity study via dermal route does not need to be performed since the physico-chemical and toxicological properties do not suggest potential for a significant rate of absorption through the skin.

- Strontioum is corrosive to skin. Thus, due to animal welfare, testing should be omitted. A qualitative approach was selected for the hazard assessment.

- In accordance with ECHA guidance on IR & CSA - chapter R.8: characterisation of dose [concentration] - response for human health (version 2.1, November, 2012), a DNEL for systemic effects could be derived by route-to-route extrapolation from a 90-day oral toxicity study in rats with strontium chloride hexahydrate.

Repeated dose toxicity, inhalation:

According to Regulation (EC) 1907/2006, Annex XI (weight of evidence), testing for sub-chronic inhalation toxicity is not considered to be required, for the following reason:

- Strontium is water soluble (> 5 g/L) at pH > 13. Any lung overload associated with inert particles can obviously be excluded.

- Strontium is corrosive. Therefore, the substance is assumed to be also corrosive to the respiratory tract. Testing by the inhalation route is further not appropriate since the substance releases in contact with water (air moisture) flammable gases, which may ignite spontaneously. Due to animal welfare, testing is not foreseen. 

 

In accordance with ECHA guidance on information requirements and chemical safety assessment-chapter R.8: characterisation of dose [concentration]-response for human health (Version 2.1; November 2012), a DNEL for systemic effects was derived by route-to-route extrapolation from a 90-day oral toxicity study in rats with strontium chloride hexahydrate.

Justification for classification or non-classification

Repeated dose toxicity, oral:

The study by Kroes (1977) is considered as the key study for repeated dose toxicity via oral application and is the basis for classification conclusions. Rats were dosed at 0, 75, 300, 1200, and 4800 ppm for 90 days. Since the increased concentrations of strontium in the bone can be considered a non-toxic effect, a NOAEL of 300 ppm SrCl2 is derived based on the weight changes of thyroids at the doses of 1200 ppm (LOAEL) and 4000 ppm. The NOAEL of 300 ppm strontium chloride corresponds to a dose of 9.9 mg Sr/kg bw/d (nominal).

 

The classification criteria according to regulation (EC) 1272/2008 as specific target organ toxicant (STOT) – repeated exposure, oral are not met, and thus no classification for specific target organ toxicant (STOT-RE) is required. However, some evidence of an effect of Sr on thyroid function is observed in the 90-day oral toxicity study in rats, but the incidence is very slight and seen only in males, but not in females at the highest dose that is above cut-off level for STOT-RE classification Cat2 (> 100 mg/kg bw/d rat oral, 90-day).

 

An indirect effect of Sr through activation of thyrocalcitonin production was regarded as unlikely by the authors of the study, since a parafollicular cell hyperplasia and effects on CA serum levels were not observed. Based on available animal data, Sr does not have the potential to produce significant toxicity, or to be harmful to humans, following repeated exposure at low or moderate exposure concentrations relevant for classification.

 

Repeated dose toxicity, inhalation:

Strontium is corrosive. Therefore, the substance is assumed to be also corrosive to the respiratory tract. Due to animal welfare, testing is not appropriate.