Tungsten

Neurotoxicity

Description of key information

The following information is taken into account for any hazard / risk assessment: No neurotoxicity data of sufficient quality are available for tungsten metal (target substance). However, neurotoxicity data are available for sodium tungstate (source substance), which are used for read-across. Due to lower water solubility and lower toxicity for the target substance compared to the source substance, the resulting read-across from the source substance to the target substance is appropriate as a conservative estimate of potential toxicity for this endpoint. In addition, read across is appropriate because the classification and labelling is more protective for the source substance than the target substance, the PBT/vPvB profile is the same, and the dose descriptors are, or are expected to be, lower for the source substance. For more details, refer to the read-across category approach in the Category section of this IUCLID submission or Annex 3 of the CSR.  The neurotoxicity potential of sodium tungstate is reported by two publications by Sachdeva et al, 2015 and Radcliffe et al, 2009. An inhalation study reported that sodium tungstate is not appreciably transported via the olfactory pathway to the brain following a single 90-min exposure in rats, although this pathway is known to transport a number of other metals (Radcliffe et al, 2009). Sodium tungstate exposure was reported in one study to produced oxidative stress in brains from rats exposed. However, the study did not elucidate and correlate these oxidative changes with behavioral and functional alterations (Sachdeva et al, 2015).

Key value for chemical safety assessment

Effect on neurotoxicity: via oral route

Link to relevant study records

Reference

Endpoint:

neurotoxicity: short-term oral

Type of information:

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

Adequacy of study:

weight of evidence

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

study well documented, meets generally accepted scientific principles, acceptable for assessment

Remarks:

Well documented scientfically sound study with sufficient information provided on materials and methods to evaluate results.

Justification for type of information:

1. HYPOTHESIS FOR THE CATEGORY APPROACH: The hypothesis is that properties are likely to be similar or follow a similar pattern because of the presence of a common metal ion, in this case tungstate.
2. SOURCE AND TARGET CHEMICAL(S) (INCLUDING INFORMATION ON PURITY AND IMPURITIES):
Source: Sodium tungstate
Target: Tungsten
3. CATEGORY APPROACH JUSTIFICATION: See Annex 3 in CSR
4. DATA MATRIX: See Annex 3 in CSR

Reason / purpose for cross-reference:

read-across: supporting information

Qualifier:

no guideline followed

Principles of method if other than guideline:

The effects of sodium tungstate on oxidative stress, few selected neurological variables like acetylcholinesterase, biogenic amines in rat brain regions (cerebral cortex, hippocampus and cerebellum)

GLP compliance:

not specified

Remarks:

Study has adequate and reliable coverage of the key parameters foreseen to be investigated in the corresponding test methods and adequate and reliable documentation of the study is provided

Limit test:

yes

Species:

rat

Strain:

Wistar

Sex:

male

Details on test animals or test system and environmental conditions:

Male Wistar rats, weighing approximately 100-120 g were used in the study. Prior to the dosing, they were acclimatized for 7 days. Briefly, animals were kept in polypropylene cages with clean dust free rice husk. The animals were housed in stainless steel cages in an air-conditioned room with temperature maintained at 25 ± 2 °C and relative humidity 60 ± 10%. Animals’ rooms were illuminated with 300 lux light, alternating with 12 h darkness light from 06:00 to 18:00 h and vice versa. Rats were allowed standard chow diet throughout the experiment and water ad libitum.

Route of administration:

oral: drinking water

Vehicle:

water

Details on exposure:

Rats were divided into eight groups of 10 rats each and were treated as below for 3 months (five days a week):
Group I: Normal animal, received normal water
Group II: Sodium tungstate, 100 ppm in drinking water, daily
Group III: Quercetin alone, orally, once, daily (0.30 mM)
Group IV: N-acetylcysteine (NAC) alone, orally, once, daily (0.30 mM)
Group V: Flavonoid (naringenin) alone, orally, once, daily (0.30 mM)
Group VI: Sodium tungstate + Quercetin as in group II and group III, respectively
Group VII: Sodium tungstate + NAC (as in group II + group IV, respectively)
Group VIII: Sodium tungstate + Flavonoid (naringenin) (as in group II + group V, respectively)

Analytical verification of doses or concentrations:

not specified

Duration of treatment / exposure:

90 days

Frequency of treatment:

Daily

Dose / conc.:

100 ppm

Remarks:

nominal in water

No. of animals per sex per dose:

10 rats per group

Control animals:

yes, concurrent vehicle

Specific biochemical examinations:

- Amount of reactive oxygen species in blood was measured using 2′, 7′-dichlrofluorescein diacetate (DCF-DA) that gets converted into highly fluorescent DCF by cellular peroxides (including hydrogen peroxide). The supernatant fractions obtained were used to determine the following parameters in three different brain regions:
- Measurement of lipid peroxidation was conducted by the thiobarbituric acid reactive substances (TBARS).
- Tissue reduced glutathione (GSH) and oxidized glutathione (GSSG)
- Tissue glutathione peroxidase (GPx)
- Tissue glutathione-S-transferase (GST
- Brain acetyl cholinesterase (AChE) activity
- Brain monoamine oxidase (MAO) activity
- Brain biogenic amine levels (dopamine and nor epinephrin)

Neurobehavioural examinations performed and frequency:

None

Sacrifice and (histo)pathology:

Brains were quickly dissected out, placed on a glass plate resting over crushed ice for the separation of various regions (cortex, cerebellum and hippocampus). The brain parts were homogenized in phosphate buffer (pH 7.4).

Statistics:

The results are expressed as the mean ± SEM of number of observations. Comparisons of means were carried out using ANOVA followed by Bonferroni multiple comparison test to compare means between the different treatment groups

Clinical signs:

not specified

Mortality:

no mortality observed

Body weight and weight changes:

not specified

Food consumption and compound intake (if feeding study):

not specified

Food efficiency:

not specified

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

not specified

Ophthalmological findings:

not specified

Haematological findings:

not specified

Clinical biochemistry findings:

effects observed, treatment-related

Description (incidence and severity):

Significant increase in oxidative stress variables.

Urinalysis findings:

not specified

Behaviour (functional findings):

not specified

Immunological findings:

not specified

Organ weight findings including organ / body weight ratios:

not specified

Gross pathological findings:

not specified

Neuropathological findings:

not specified

Histopathological findings: non-neoplastic:

not specified

Histopathological findings: neoplastic:

not specified

Other effects:

effects observed, treatment-related

Description (incidence and severity):

- Brain ROS increased significantly in the cortex and hippocampus; however, an increasing trend was noticed in the cerebellum of rats on sodium tungstate exposure compared to normal animals.
- increase in GPXactivity in cerebellum compared to cortex and hippocampus on sodium tungstate exposure. GST activity on the other hand, significantly decreased in cortex, cerebellum and hippocampus.
- Depletion of dopamine (DA) in cortex and cerebellum was observed in sodium tungstate exposed rats as compared to normal control rats.
- Following sodium tungstate exposure, 5-HT levels were depleted in the cortex and marginally in the cerebellum.
- Sodium tungstate exposure produced an increase in Nor Epinephrie (NE) levels in hippocampus while there was a significant reduction in cortex and marginal fall in cerebellum compared to the NE levels of control rats.
- Sodium tungstate exposed animals showed an alteration in AChE activity in cortex and cerebellum compared to hippocampus.
- Sodium tungstate exposed animals showed a more pronounced alteration in AChE activity in cortex and cerebellum compared to hippocampus
- Co-administration of NAC and flavonoids with tungstate provided moderately effective in reducing brain ROS, moderately prevent effects in restoring GPX and GST activities in the different brain regions, recover the altered DA levels in cortex and cerebellum; and provided a significant recovery in the 5-HT level of the cortex and hippocampus, restored the altered NE levels in all the brain regions, and increase in the activity of acetylcholinesterase.

Key result

Dose descriptor:

conc. level:

Effect level:

100 ppm (nominal)

Based on:

test mat.

Sex:

male

Basis for effect level:

other: Tungstate exposure produced an increase in biochemical variables indicative of oxidative stress but no correlation was conducted with behavioral and functional alteration.

Remarks on result:

not determinable

Conclusions:

Tungstate exposure produced an increase in biochemical variables indicative of oxidative stress while, neurological alterations were more pronounced in the cerebral cortex compared to other regions. Co-administration of NAC and flavonoids with sodium tungstate significantly restored glutathione, prevented changes in the brain biogenic amines, reactive oxygen species (ROS) and TBARS levels in the different brain regions. The protection was more prominent in the animals co-administered with NAC. Further studies are required for elucidating and correlating the behavioral and functional alterations.

Executive summary:

No neurotoxicity data of sufficient quality are available for tungsten (target substance). However, neurotoxicity data are available for sodium tungstate (source substance), which are used for read-across. Due to lower water solubility and lower toxicity for the target substance compared to the source substance, the resulting read-across from the source substance to the target substance is appropriate as a conservative estimate of potential toxicity for this endpoint. In addition, read-across is appropriate because the classification and labelling is more protective for the source substance than the target substance, the PBT/vPvB profile is the same, and the dose descriptors are, or are expected to be, lower for the source substance. For more details, refer to the read-across category approach in the Category section of this IUCLID submission or Annex 3 in the CSR. 

Endpoint conclusion

Endpoint conclusion:

no study available

Effect on neurotoxicity: via inhalation route

Link to relevant study records

Reference

Endpoint:

neurotoxicity: short-term inhalation

Type of information:

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

Adequacy of study:

weight of evidence

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

study well documented, meets generally accepted scientific principles, acceptable for assessment

Remarks:

Well documented scientfically sound study with sufficient information provided on materials and methods to evaluate results.

Justification for type of information:

1. HYPOTHESIS FOR THE CATEGORY APPROACH: The hypothesis is that properties are likely to be similar or follow a similar pattern because of the presence of a common metal ion, in this case tungstate.
2. SOURCE AND TARGET CHEMICAL(S) (INCLUDING INFORMATION ON PURITY AND IMPURITIES):
Source: Sodium tungstate
Target: Tungsten
3. CATEGORY APPROACH JUSTIFICATION: See Annex 3 in CSR
4. DATA MATRIX: See Annex 3 in CSR

Qualifier:

no guideline followed

Principles of method if other than guideline:

Olfactory transport of represents an important mechanism for direct delivery of certain metals to the central nervous system (CNS). The objective of this study was to determine whether inhaled tungsten (W) undergoes olfactory uptake and transport to the rat brain. To test this hypothesis, we used the unilateral nasal occlusion model which allows the test substance to be administered to the open (unoccluded) nostril while the animal is breathing freely. Since it is believed that the test substance will not cross the nasal septum during inspiration, limited deposition will occur in the nasal cavity on the side ipsilateral to the occluded nostril.

GLP compliance:

not specified

Remarks:

Study has adequate and reliable coverage of the key parameters foreseen to be investigated in the corresponding test methods and adequate and reliable documentation of the study is provided

Limit test:

yes

Specific details on test material used for the study:

Radiolabeled sodium tungstate (188W) was obtained in a nominally 250 mCi batch, with a radiopurity of >99% and specific activity of 1.47 mCi/ml

Species:

rat

Strain:

Sprague-Dawley

Sex:

male

Details on test animals or test system and environmental conditions:

Thirty-five 16-week-old male Sprague–Dawley rats were allowed a 2-week acclimation period and identified. Animals were weighed weekly and randomized to the experimental groups according to body weight. Animal weights were also recorded at the beginning of each exposure and at each post-exposure time point. The rats were housed individually in suspended-stainless steel wire cages in biologically clean rooms with HEPA-filtered air and a 12 h light/dark photoperiod cycle. The cage rooms were maintained at 22 4 8C and 50 20% relative humidity. All rats received pelleted diet and reverse osmosis water ad libitum.

Route of administration:

inhalation: aerosol

Vehicle:

water

Mass median aerodynamic diameter (MMAD):

1.5 µm

Geometric standard deviation (GSD):

1.33

Remarks on MMAD:

The Mass median aerodynamic diameter (MMAD) was 1.50 um while the geometric standard deviation was 1.33 mm. The average mass concentration for both exposures was 256 mg188 W/m3

Details on exposure:

Sodium tungstate aerosol was produced using a nebulizer (3-jet Collison Nebulizer). The aerosol passed through a diffusion dryer and 85 Kr bipolar discharger into a noseonly exposure system (52-port). A continuous flow of the exposure atmosphere was drawn through an optical particle sizing spectrometer. Aerosol mass concentration was measured using gravimetric filtration.

Analytical verification of doses or concentrations:

yes

Details on analytical verification of doses or concentrations:

Exposure solution specific activity was measured.

Duration of treatment / exposure:

90 minutes

Frequency of treatment:

One treatment. The right nostril of the rats was occluded (left side unoccluded) using a nasal plug 2.5 0.5 h prior to the start of the inhalation exposure

Dose / conc.:

223 mg/m³ air (analytical)

Dose / conc.:

288 mg/m³ air (analytical)

Dose / conc.:

256 mg/m³ air (analytical)

No. of animals per sex per dose:

- 7/group/time point
- The rats were exposed in two separate sessions (21 rats for exposure 1; 14 rats for exposure 2) in order to accommodate the necropsy and nasal plugging
procedures.

Control animals:

other: The right nostril of the rats was occluded (left side unoccluded) using a nasal plug

Details on study design:

The amount of radioactivity (188 W) in a representative preweighed tissue sample was determined using Gamma Counting System. The amount of 188 W was reported as nCi/g tissue wet weight and was corrected for radioactive decay of this isotope

Observations and clinical examinations performed and frequency:

The animals were euthanized immediately following the end of the exposure (0 hr time point), or 1 day, 3 days, 7 days and 21 days thereafter.

Sacrifice and (histo)pathology:

The animals were euthanized immediately following the end of the exposure (0 hr time point), or 1 day, 3 days, 7 days, and 21 days thereafter. The following tissues were taken for gamma spectrometry: (right and left sides) respiratory epithelium (RE), olfactory epithelium (OE), olfactory bulb (OB), olfactory tract/tubercle (OTT), striatum (ST), cerebellum (CE), trigeminal nerve (TN), pituitary gland, and the rest of brain (ROB).

Statistics:

Tissue concentration of 188 W at each time point was evaluated statistically with a one-way analysis of variance (ANOVA) followed by Student’s t-test.

Clinical signs:

no effects observed

Mortality:

no mortality observed

Body weight and weight changes:

not specified

Food consumption and compound intake (if feeding study):

not specified

Food efficiency:

not specified

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

not specified

Ophthalmological findings:

not specified

Haematological findings:

not specified

Clinical biochemistry findings:

not specified

Urinalysis findings:

not specified

Behaviour (functional findings):

not specified

Immunological findings:

not specified

Organ weight findings including organ / body weight ratios:

not specified

Gross pathological findings:

not specified

Neuropathological findings:

no effects observed

Histopathological findings: non-neoplastic:

not specified

Histopathological findings: neoplastic:

not specified

Details on results:

- Immediately following a 90-min nose-only inhalation exposure to 188 W (0 day), gamma spectrometry analyses demonstrated that the olfactory epithelium and the respiratory epithelium had high levels of188 W in the nasal passage without an occluded nostril.
- In the olfactory bulb there was a statistically significant difference in the concentration of 188W between unoccluded and occluded side (unoccluded side being higher) at 3 days post-exposure.
- The concentration of188 W in the olfactory tract/tubercle and the stratium remained low throughout the experiment, ie approximately 1-3% of the amount of tungsten seen in the olfactory epithelium.
- The 188 W concentrations in the trigeminal nerve, cerebellum, and rest of the brain did not differ significantly between the unoccluded and occluded sides immediately following exposure.
- Mean end of exposure pituitary 188 W concentrations were higher than those observed in other brain regions.
- Since 188W concentrations were not significantly different between the unoccluded side and the occluded side in the other brain regions (olfactory tract/tubercle, trigeminal nerve, cerebellum, and rest of the brain) the concentrations at each time point were combined for the pharmacokinetic analysis. As with the olfactory epithelium, respiratory epithelium, and olfactory bulb, there was rapid elimination of 188W between day 0 and day 1. The elimination of188 W in the other brain regions.
- The data revealed that the amount of188 W found in the right olfactory and respiratory epitheliums were significantly lower than that found in the left side, which is consistent with the unilateral nasal occlusion model

Key result

Dose descriptor:

conc. level:

Effect level:

288 mg/m³ air (analytical)

Based on:

test mat.

Sex:

male

Basis for effect level:

other: see 'Remark'

Remarks on result:

not determinable

Conclusions:

The present study has shown that sodium tungstate is not appreciably transported via the olfactory pathway to the brain following a single 90-min exposure in rats, although this pathway is known to transport a number of other metals

Executive summary:

No neurotoxicity data of sufficient quality are available for tungsten (target substance). However, neurotoxicity data are available for sodium tungstate (source substance), which are used for read-across. Due to lower water solubility and lower toxicity for the target substance compared to the source substance, the resulting read-across from the source substance to the target substance is appropriate as a conservative estimate of potential toxicity for this endpoint. In addition, read-across is appropriate because the classification and labelling is more protective for the source substance than the target substance, the PBT/vPvB profile is the same, and the dose descriptors are, or are expected to be, lower for the source substance. For more details, refer to the read-across category approach in the Category section of this IUCLID submission or Annex 3 in the CSR. 

Endpoint conclusion

Endpoint conclusion:

no study available

Effect on neurotoxicity: via dermal route

Endpoint conclusion

Endpoint conclusion:

no study available

Justification for classification or non-classification

No neurotoxicity studies are available for tungsten. However, data were available on sodium tungstate, which are used for read-across. The sodium tungstate neurotoxicity studies are more investigative reports conducted under no standard testing guidelines which limit their usability for regulatory purposes. Based on this, none of the data from these publications warrant any classification for sodium tungstate and subsequently tungsten as neurotoxicants per CLP as more information is needed.