Sodium cyanate

Repeated dose toxicity: oral

Administrative data

Endpoint:

short-term repeated dose toxicity: oral

Type of information:

experimental study

Adequacy of study:

weight of evidence

Study period:

1992-04-07 to 1992-07-16

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

other: Read across to sodium cyanate. For justification of read across see endpoint summary.

Data source

Reference

Reference Type:

study report

Title:

Unnamed

Year:

1993

Report date:

1993

Materials and methods

Test guideline

Test guideline 1

Qualifier:

according to guideline

Guideline:

OECD Guideline 407 (Repeated Dose 28-Day Oral Toxicity Study in Rodents)

Deviations:

no

Test guideline 2

Qualifier:

according to guideline

Guideline:

EU Method B.7 (Repeated Dose (28 Days) Toxicity (Oral))

Version / remarks:

84/449/EEC

Deviations:

no

Principles of method if other than guideline:

NA

GLP compliance:

yes

Limit test:

no

Test material

Test material information

Constituent 1

Reference substance name:

Sodium cyanate

EC Number:

213-030-6

EC Name:

Sodium cyanate

Cas Number:

917-61-3

Molecular formula:

CNO.Na

IUPAC Name:

sodium cyanate

Details on test material:

- Chemical name: sodium cyanate
- Analytical purity of the test material: 90 %

Test animals

Species:

rat

Strain:

Wistar

Sex:

male/female

Details on test animals or test system and environmental conditions:

- age at the beginning: 7 weeks (males); 6 weeks (females)
- body weight: 145 - 185 g (males); 128 - 160 g (females)

Administration / exposure

Route of administration:

oral: gavage

Vehicle:

water

Details on oral exposure:

NA

Analytical verification of doses or concentrations:

yes

Details on analytical verification of doses or concentrations:

The method was based on the precipitation with semicarbazide hydrochloride. Subsequently the dried precipitate was weighed.

Duration of treatment / exposure:

Duration of the study was 4 weeks treatment period (substance administration up to the day before autopsy) and 6 weeks treatment-free recovery period.

Frequency of treatment:

Frequency of treatment was once daily a.m. (7 days per week).

Doses / concentrations

Doses / concentrations 1

Remarks:

Doses / Concentrations:
0 mg/mL
Basis:
nominal in water

Doses / concentrations 2

Remarks:

Doses / Concentrations:
14.7 mg/mL
Basis:
nominal in water

Doses / concentrations 3

Remarks:

Doses / Concentrations:
31.6 mg/mL
Basis:
nominal in water

Doses / concentrations 4

Remarks:

Doses / Concentrations:
68.1 mg/mL
Basis:
nominal in water

No. of animals per sex per dose:

5 male and 5 female animals in groups 2 and 3 each. 10 male and 10 female animals in groups 1 and 4 each.
The last 5 animals of each sex of group 1 were used as recovery animals.

Control animals:

yes, concurrent vehicle

Details on study design:

After death of 2 male and 1 female animals in group 4 only the last 3 respectively 4 animals were used as recovery animals.

Positive control:

NA

Examinations

Observations and examinations performed and frequency:

Clinical investigations:
- Mortality: Mortality was checked twice daily (a.m. and p.m., on Saturdays, Sundays, and on national and business holidays only once daily (a.m.).
- Behaviour and general condition of the animals (clinical symptoms): The animals were observed daily for the occurrence of toxicity symptoms as well as their severity and duration.
- Food consumption: Once a week, starting with pre-test period.
- Body weight: Once a week, starting with pre-est period.
- Reflexes: Pain, pinna, and corneal reflexes were tested once week, starting with pre-test period.
- Examinations of eyes, hearing, and teeth: Prior to first substance administration and in week 4 as well as in week 10 from recovery animals. The hearing was checked with a simple noise test. The teeth were inspected. Eye examination using a focussed visual light beam.
Clinical pathology:
- Blood collection: In weeks 1 and 4 a.m. as well as in week 10 from recovery animals. Blood was collected from the retro-orbital venous plexus of 1 eye under CO2 anaesthesia from all animals.
- Anticoagulants: ETDA was used as an anticoagulant for haematological parameters. The clinical chemistry was done with serum. Only for the determination of methaemoglobin and oxygenated haemoglobin heparinized capillary tubes were used.
- Urine collection: Urine was collected in metabolism cages in week 4 a.m. from all animals. Pre-treatment before sampling with tap water (10 mL/kg bw).
- Haematology: In weeks 1 and 4 as well as in week 10 from recovery animals: Erythrocytes (RBC), haematocrit (Hct), haemoglobin (Hb), leucocytes (WBC), mean corpuscular haemoglobin concentration (MCHC), mean corpuscular volume (MCV) thrombocytes (Platelets).
- Clinical chemistry: In weeks 1 and 4 as well as in week 10 from recovery animals: Alanine aminotransferase (ALAT), albumin, alkaline phosphatase (AKP), asparatate aminotransferase (ASAT), blood urea, calcium (Ca), chloride (Cl), cholinesterase (CHE), creatine kinase (CK), creatinine, iron (Fe), gamma-glutamyltransferase (gamma-GT), glucose, glutamate dehydrogenase (GLDH), inorganic phosphate (P=, methaemoglobin (MetHb), oxygenated haemoglobin (O2ct, HbO2), potassium (K), serum electrophoresis, sodium (Na), total bilirubin (tot. Bili). total cholesterol (Chol), total protin (tot. Prot), triglycerides (triglyc).
- Urinalysis: in week 4: Bilirubin, glucose, haemoglobin/erythrocytes, ketones, leucocytes, nitrite, osmolality, pH-value, protein, urobilinogen.
Additionally the volume of the collected urine was determined. Microscopical examinations of the urine sediment were done in animals whose urine state showed pathological changes in haemoglobin/erythrocytes, leucocytes, or protein.

Sacrifice and pathology:

Pathology:
Sacrifice: At the end of the treatment or recovery period the surviving animals were anesthetized with CO2 and sacrificed by exsanguination.

Other examinations:

Gross Necropsy:
- Autopsy: All animals were subjected to full gross necropsy which included examination of the external surface of the body, all orifices and abdominal cavities and their contents. The following organs or tissues or a respresentative sample were preserved: all gross lesions, adrenal glands (r/l), bone marrow, bone marrow smear, brain, cecum, colon, duodenum, epididymides (r/l), heart, ileum,jejunum, kidneys (r/l), liver, lungs, ovaries (r/l), rectum, spleen, stomach, testes (r/l).
- Organs: Adrenals (r/l), brain, heart, kidneys (r/l), liver, ovaries (r/l), spleen, testes (r/l).
The organ weights were expresssed as absolute values and relative to body weight after exsanguination. No organ weights were taken from the intercurrently dead animals.

Histopathology:
- Fixation, Staining: All above mentioned organs and tissues except for bone marrow smears were fixed in a 4 % neutral buffered formaldehyde solution. The bone marrow smears were air dried. All fixed organs and tissues of group 1 animals and group 4 animals as well as of the intercurrently dead animals were trimmed, embedded in paraffin wax, sectioned at approximately 4 µm, and stained with Hematoxylin and Eosin. Additionally sections of the following organs of group 2 and 3 animals as well as of the recovery animals of group 1 and group 4 were prepared: adrenals, bone marrow smears, brain, cecum, colon, duodenum, epididymides, heart, ileum, jejunum, kidneys, liver, lungs, ovaries, rectum, spleen, stomach, and testes. Additional sections of the stomach were stained with the periodic acid Schiff reagent, with the silver impregnation technique according to Grimelius and according to the staining produre of Schiebler and Behr.
- Microscopical examination: All slides prepared were examined microscopically, except for the sections of the following organs of group 2 and 3 animals: brain, cecum, colon, duodenum, heart, ileum, jejunum, liver, lungs, ovaries, rectum.

Statistics:

Mean values of all blood parameters and - where indicated - standard deviations were calculated separately for each group and sex.
For statistical evaluation of food consumption, body weights, and organ weights the Dunnet-Test was used. For values of haematological and clinical chemistry examinations the Dunnet- Test was used in case of normal distribution, otherwise the STEEL-Test was employed. Significant differences between mean values of control group 1 and dose groups 2, 3, and 4 were marked with * or ** , respectively.

Results and discussion

Results of examinations

Clinical signs:

effects observed, treatment-related

Mortality:

mortality observed, treatment-related

Body weight and weight changes:

effects observed, treatment-related

Food consumption and compound intake (if feeding study):

effects observed, treatment-related

Food efficiency:

not specified

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

not specified

Ophthalmological findings:

not examined

Haematological findings:

effects observed, treatment-related

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:

effects observed, treatment-related

Histopathological findings: non-neoplastic:

effects observed, treatment-related

Histopathological findings: neoplastic:

effects observed, treatment-related

Details on results:

Clinical Investigations:
- Clinical signs: In the low dose group (2, 68.1 mg/kg) the behaviour and general condition of the animals was not affected in 1 rat, was recorded at the same incidence in the other dose and control groups and is therefore considered as incidental.
Of the animals of the mid dose group (3, 147 mg/kg) only 1 rat showed salivation, 1 kept her head in a tilted position, and 2 exhibited alopecia. Salivation was detected between days 10 and 21 of the treatment. Tilted head and alopecia were observed firstly in weeks 1 or 3, respectively, and lasted until the end of the treatment period.
The animals of group 4 (316/237 mg/kg) exhibited a whole number of symptoms starting on days 7 to 14. Signs were slight to moderate hypokinesia, coordination disturbances, restrained or stilted gait, salivation, piloerection, and sunken sides. Two rats exhibited reddish salivation for 1 day each. One rat showed tilted head. During the recovery period all clinical findings faded within 2 weeks. Nine days after the end of the administration in 2 males the reduced size of testes was noted. This finding remained until termination of the study. Deaths occurred only in the high dose group (316 mg/kg). After 11 to 14 days 2 males and female were found dead. To prevent further fatalities the dose was reduced to 237 mg/kg on day 15.
- Food consumption: In animals of the low and mid dose groups (2,3) a slight, respectively moderate retardation of the increase in food consumption during the treatment period was recorded. Both sexes were equally affected. The maximum differences of mean values in group 2 were about 9 % in female and 4 % in males. In group 3 the maximum mean differences were about 15 % in males and 13 % in females. In the high dose group there was a very steep reduction of food intake in week 2. The mean difference relative to the control animals reached about 45 % in males and 28 % in females. After the reduction of the dose at the end of week 2 food consumption improved slightly and the difference was about 35 % in males and 28 % in females in week 4. After termination of the treatment the food uptake reached control values within 3 weeks.
-Body weights: The body weight gain was affected by the test substance only in the mid and high dose groups. In group 3 only males showed a very slight effect. In males as well as in females of group 4 body weight remained almost constant during the treatment period. The maximum difference of means relative to control, reached in week 4, was about 33 % in males and 16 % in females. Only during the first 4 weeks of the recovery period the body weight increased steeply. Afterwards the increase of body weight slowed down in males and stopped in females. Body weight reached about 90 % of control values.
- Examinations of reflexes, eyes, hearing, and teeth: No defects were detected during examinations of eyes, teeth, and ears in week -1 and 4. In weeks 4 or 10 1 control female each exhibited mydriasis of the left eye. No changes were recorded during the weekly examination of pinna, corneal, and pain reflexes.

Clinical Pathology
- Haematology:
After 2 days of administration the examination of cellular blood constituents did not reveal any toxicologically significant differences in treated animals. Although there is a statistical significant increase of the absolute numbers of lymphocytes in group 4 males, the value is still within the normal range. Therefore the finding is considered as incidental. During the second investigation in week 4 changes were detected predominantly in red blood cells. The number of red cells (erythrocytes), haemoglobin content, hematocrit, mean corpuscular volume (females only), and the number of reticulocytes (females only) was significantly increased in males and females of groups 3 and 4. The changes (hematocrit, mean corpuscular volume, mean corpuscular hemoglobin) were dose dependent and could be detected also in group 2, although they mostly were not significant in this group. The high value of mean corpuscular hemoglobin in group 2 is within the normal range, does not show a correlation to the dose, and is therefore considered as incidental. Among the white blood cells the numbers of lymphocytes, absolute as well as relative, were reduced reaching significant levels with significant differences only in group 4. At the end of the recovery period (week 10) all changed parameters had returned to normal values. The only exceptions were mean corpuscular volume, which had been increased in females and now was above control in males, too, and mean corpuscular hemoglobin, which also was high in males and females.
- Clinical chemistry: The clinical chemistry examination performed in parallel to the haematology in weeks 1, 4, and 10 showed changes in a whole number of parameters. In week 1 males of group 4 exhibited increases in oxygenated hemoglobin concentration, blood urea, and cholesterol. Slight decreases were noticed in the activities of cholinesterase and glutamate dehydrogenase. While in group 3 animals levels of oxygenated hemoglobin, cholinesterase, and glutamate dehydrogenase still were altered, animals of group 2 did not show significant changes. The low potassium concentration in males of group 3 is not dose dependent and therefore considered as incidental. In females the values for creatine kinase and alanine aminotransferase activities, total protein, alpha1- und beta-globulins, and potassium were low. Only the concentration of iron was increased. Changes were mostly observed in groups 3 and/or 4, but group 2 also showed slight, insignificant effects (iron, creatine kinase): At the end of the treatment period (week 4) animals of both sexes showed considerable increases in the concentrations of oxygenated hemoglobin and iron in groups 3 and 4. In group 2 slight increases of oxygenated hemoglobin were noted, although these were not statistically significant. The concentration of iron also was increased. Also in rats of both sexes (groups 3 and/or 4) the concentrations or activities of glucose, total protein, cholineesterase, glutamate dehydrogenase, albumin (minimally), potassium (minimally), alpha1-globulins /absolute and relative), and alpha2-globulins (absolute) were significantly reduced. Additionally alanine aminotransferase (only in group 3), alkaline phosphatase, and triglycerides (only in group 4) appeared decreased in females of these groups but were still in the control range and are considered as incidental findings. Males of groups 3 and 4 showed reductions in gamma-glutamyltransferase (only group 4) and alpha2-globulins (relative) and at the same time minimal increases in cholesterol, and beta- globulins (relative). The small alterations in chloride, creatinine, calcium, albumin (relative) are all within the normal range, not dose dependent and therefore are considered as incidental findings. At the end of the recovery period all parameters had returned to normal except for a remaining, slightly high iron concentration in female rats. There are some slight statistically significant changes in methaemoglobin, aspartate aminotransferase, creatinine, total protein, albumin (absolute), alpha1-globulins (absolute, in females) and blood urea and alpha2-globulins, but they are well within the normal range. They possess no toxicological importance and are judged as incidental findings.
- Urinalysis: No test substance related findings were detected during the examination of urine.

Pathology:
- Gross Necropsy: The macroscopical inspection of the rats during section revealed changes in stomachs and male genital organs. The mucous membranes of the glandular stomachs were thickened in male and female animals of the high dose group (group 4). The organs of the male genital tract were small.
- Organ Weights: At necropsy absolute organ weights were significantly altered only in male rats of group 4. Relative organ weights were changed in females of groups 3 and 4 and males of group 4. The organs affected were brain, heart, liver, kidneys, spleen, and testes with respect to absolute weights and brain, heart, kidneys, and adrenals with respect to relative weights. In females absolute organ weights appeared normal, but relative weights of brain, liver, kidneys, and spleen were increased. The alterations probably were due to direct effects of the test substance only spleen, adrenals, and testes. Absolute weights were decreased for spleens in males of group 4, while the relative weights were unchanged. In group 4 females the absolute weights seemed to be normal (although body weights were decreased) and therefore the relative weights appeared high. Adrenals only showed slightly increased relative weights in males of group 4. Testes were grossly reduced in size and their absolute weights were reduced by 40 to 50 %. The relative weight was only very slightly lowered. All findings are considered as secondary effects due to the severely reduced body weight.

Histopathology:
Treatment related changes were seen in stomach, spleen, adrenals, testes, epididymides, prostate, and seminal vesicles.
-Stomach: Treatment related changes of the stomach consisted of a diffuse hyperplasia of the mucosa and of diffuse mixed inflammatory cell infiltrates. The hyperplasia of the mucosa was restricted to the glandular stomach and was more pronounced in the fundic than in the pyloric area. In sections stained with the PAS reagent it was possible to identify chief cells, parietal cells, and mucous neck cells in the hyperplastic mucosa. The neck region of the gastric glands, characterized by the occurrence of mucous neck cells, seemed to be enlarged. The same results were obtained from sections stained according to the procedure of Shiebler and Behr. No hyperplasia of endocrine cells was observed in sections stained with the silver impregnation technique according to Grimelius. Diffuse mucosal hyperplasia of the glandular stomach was observed in all treated animals. There was no obvious tendency towards a dose related increase in the severity of this change. The finding was graded slight to moderate with the exception of each 1/5 female of the low dose and high dose groups.Incidence and severity of mucosal hyperplasia showed a tendency towards regression during the recovery period. The finding was graded slight to moderate with the exception of each 1/5 female of low dose and high dose groups. Incidence and severity of mucosal hyperplasia showed a tendency towards regression during the recovery period. The finding was observed in 1/3 males and 2/4 females of the high dose group, graded as minimal. The microscopic finding of diffuse mucosal hyperplasia correlated with the macroscopic observation of a thickened mucosa in each 1/5 male and 1/5 female of the high dose group. Diffuse mixed inflammatory cell infiltrates occurred in the lamina propria mucosa of the glandular stomach, predominantly in the fundic area. It was always associated with diffuse hyperplasia of the mucosa. Mixed inflammatory cell infiltrates were observed in 3/5, 4/5, and 5/5 males and in 4/5, 5/5, and 5/5 females of groups 2, 3, and 4, respectively. It was graded minimal to slight in the low dose group and slight to moderate in the mid and high dose groups. The inflammatory changes showed nearly complete regression during the recovery period. Only 1/4 females had minimal diffuse mixed inflammatory cell infiltrates. A focal submucosal edema, graded as moderate, was observed in 1/5 high dose group female. This finding is presumably treatment related, although the same change was found in 1/5 female control rat of the recovery group.
-Spleen: The spleen was changed by a treatment related reduction of the marginal zone and a treatment related increase in the extramedullary haematopoiesis. A reduced marginal zone was observed only in males. One/5 animals of the mid and 3/5 of the high dose groups had this finding. It was graded minimal in the case of the group 3 animal and minimal to moderate in the group 4 rats. The finding showed complete regression during the recovery period. An increased extramedullary haematopoiesis was observed in animals of all groups. In both males and females the finding showed a minimal dose related increase in incidence and/or severity. In males, 1/5, 2/5, 4/5, and 3/5 animals of groups 1, 2, 3, and 4, respectively, had this finding. It was graded minimal to moderate without a clear tendency towards a dose related increase of severity. Three/5, 2/5, 4/5, and 5/5 females of groups 1, 2, 3, and 4, respectively, had an increased extramedullary haematopoiesis, graded as minimal to moderate. The mean severity of this finding increased dose related from 1.3 in the control group to 2.0, 2.0, and 2.2 in groups 2, 3, and 4, respectively (explanation of codes for the grading: 1=minimal, 2=slight, 3=moderate, 4=marked, 5=massive). The finding showed strong tendency towards regression during the recovery period. 4/5 group 1 and 3/3 group 4 recovery males had an increased extramedullary haematopoiesis, with a mean severity of 1.0 in group 1 and 1.3 in group 4. In females the change was observed in 3/5 controls and 2/4 high dose group rats, with a mean severity of 2.0 in group 1 and 1.5 in group 4.
- Treatment related changes of the adrenals consisted of an increased cortical vacuolization. The vacuolization was most pronounced in the zona glomerulosa and fasciculata. The zona reticularis was less intensely affected. In both males and females the finding showed a dose related increase in incidence and severity. Two/5, 2/5, and 5/5 males of groups 2, 3, and 4, respectively, had an increased cortical vacuolization. The mean severity of the finding increased from 1.0 in group 2 to 1.5 in group 3, and 2.0 in group 4. In females, the finding was observed in 3/5 group 2 and each 5/5 group 3 and 4 animals. The mean severity was 1.0 in group 2, 1.6 in group 3, and 1.8 in group 4. In the recovery groups the finding occurred in group 4 animals as well as in control rats. Three/5 group 1 and 2/3 group 1 and 2/3 group 4 males had this finding. In females the change occurred in 3/5 controls and 3/4 high dose group animals. The mean severity was 2.0 and 2.5 in group 1 and group 4 males, respectively, and 2.3 in each controls and high dose group females.

Male genital system:
- Testes: The testes of treated animals were changed by a degeneration of the germinal epithelium and a hyperplasia of Leydig cells. The degeneration of the germinal epithelium was in most cases diffuse. The severity of the change differed between the tubules of one testis and between both testes of the same animal. Tubules with marked degeneration were lined only by Sertoli cells, while less intensely affected tubules had spermatogonia and reduced numbers of spermatocytes and spermatides. In these tubules also multinucleated giant cells or necrotic spermatocytes, spermatides and spermatozoa occurred. Degeneration of the germinal epithelium was observed in all animals of the high dose group. Diffuse degeneration occurred in 1/5 group 2 rat, graded as minimal, and in 4/5 group 4 animals, graded as moderate to massive. One/5 group 4 male had slight focal degeneration of the germinal epithelium. No group 3 animal had degeneration of the germinal epithelium. The degeneration of the germinal epithelium showed a minimal tendency towards regression during the recovery period. In affected organs, intact tubules were observed next to tubules lined only by Sertoli Cells, suggesting that only less intensely changed tubules showed regeneration. Two/3 high dose group rats had marked diffuse degeneration of the germinal epithelium, 1/3 had moderate focal degeneration. Diffuse hyperplasia of Leydig cells occurred in 2/5 high dose group males, graded minimal and slight. This finding showed tendency towards progression during the recovery period. Two/3 group 4 recovery males were affected, graded slight to moderate.
- Epididymides: As a result of the testicular degeneration of the germinal epithelium, the epididymides had a treatment related reduction of spermatosoma. The finding was diagnosed in 5/5 high dose group animals, graded slight to marked. During the recovery period the finding showed minimal tendency towards regression. Two/3 high dose group rats had severe reduction of spermatosoma. The animal with focal testicular degeneration of the germinal epithelium had no absence of spermatozoa in the epididymides.
- Prostate and seminal vesicles: Prostate and seminal vesicles of the one examined high dose group male had diffuse atrophy. This microscopical finding correlated to the macroscopical observation of a small prostate and seminal vesicles. All other findings noted are regarded as incidental (e.g. mononuclear infiltrates liver, kidneys, heart, brain, lung) or caused by the sacrifice procedure and not to be treatment related. Two/7 high dose group males and 1/6 high dose group females died intercurrently. In these animals, autolytic changes occurred, predominantly in the digestive tract. In organs without dominant autolysis, the same treatment related findings occurred as in animals terminally sacrificed. Additional findings which occurred in the three animals with intercurrent death were regarded as agonal changes.

Effect levels

1

Key result

Dose descriptor:

LOAEL

Effect level:

147 mg/kg bw/day (nominal)

Based on:

test mat.

Sex:

male/female

Basis for effect level:

food consumption and compound intake

haematology

2

Key result

Dose descriptor:

NOEL

Effect level:

68.1 mg/kg bw/day (nominal)

Based on:

test mat.

Sex:

male/female

Basis for effect level:

other: overall effects

Target system / organ toxicity

1

Key result

Critical effects observed:

yes

Lowest effective dose / conc.:

147 mg/kg bw/day (nominal)

System:

haematopoietic

Organ:

blood

spleen

Treatment related:

yes

Dose response relationship:

not specified

Relevant for humans:

not specified

2

Key result

Critical effects observed:

yes

Lowest effective dose / conc.:

147 mg/kg bw/day (nominal)

System:

male reproductive system

Organ:

adrenal glands

other: testes

Treatment related:

yes

Dose response relationship:

not specified

Relevant for humans:

not specified

3

Key result

Critical effects observed:

yes

Lowest effective dose / conc.:

147 mg/kg bw/day (nominal)

System:

hepatobiliary

Organ:

liver

Treatment related:

yes

Dose response relationship:

not specified

Relevant for humans:

not specified

Applicant's summary and conclusion

Conclusions:

The data of the present study show that sodium cyanate, if ingested, is able to damage a number of organs. The main target organs are the red blood cells, spleen, testes and male genital system, liver and adrenals. Local effects can occur in the stomach.
The study also shows that all effects are in principle reversible, although complete recovery can take rather long time. The no observed effect level cannot be determined exactly from these data. The NOEL is expected slightly below 68.1 mg/kg, because there were only slight, mostly statistically insignificant effects in this dose group (hyperplasia and inflammatory infiltrates in the stomach, cortical vacuolization in the adrenals, diffuse degeneration of the germinal epithelium in the testes, extramedullary haematopoiesis in the spleen, increase of oxygenated haemoglobin iron, reduced food intake).

Executive summary:

In this subacute toxicity study sodium cyanate (90 %) was administered to 5 Wistar rats/sex/dose in water by gavage at dose levels of 0, 68.1, 147, 316 (reduced to 237 mg/kg/day after 2 weeks) mg/kg bw/day.

The results of this 4-week oral toxicity study in Wistar rats indicate that sodium cyanate can affect the general condition and behaviour of the animals. Clinically the main findings were a considerable retardation of body weight gain, accompanied by reduced locomotive activity, impaired coordination, and clonic seizures at 147 and 316 mg/kg bw/day. Additionally there were signs of bad general condition, e. g. piloerection and sunken sides. These findings were recorded predominantly at the high dose (316/237 mg/kg). Changes in blood parameters indicate a negative influence of sodium cyanate on the function of the red blood cells and hence a decrease of the availability of oxygen to the tissue. The increase in red blood cell count, haematocrit, mean corpuscular volume, and reticulocytes can be understood as an attempt to increase the oxygen pressure in the tissue. This also correlates to the increased extramedullary haematopoiesis in the spleen and the elevated iron concentration in plasma. The reduction of the activities of almost all liver enzymes in serum indicates an effect of the test substance on the liver, which is not clearly a damage to the liver cells (no histological correlate), but seems to reduce protein synthesis and/or secretion of proteins into the blood. This is emphasized also by the decrease in albumin and globulin proteins. Similar effects of sodium cyanate on protein synthesis and secretion are described in the literature. The macroscopical changes in the size of the male genital tract are correlated with a microscopically observed degeneration of germinal epithelium in the testes. Whether this degeneration is the consequence of a direct action of the test substance on the germinal epithelium or due to hypoxia within the tissue caused by impaired oxygen delivery from the blood, cannot be conclusively determined from the data available but is likely. Nevertheless, the other changes in the male genital system (size of organs, proliferation of Leydig cells) are most likely secondary to the damage of the testes. The slight increased cortical vacuolization in the adrenals and the decrease of the number of lymphocytes has to be considered as a substance related effect, although the mechanisms involved are not clear. The diffuse hyperplasia of the mucosa in the glandular part of the stomach accompanied by an also diffuse mixed inflammatory cell infiltration appears to be due to a slight local irritation of the relatively high concentration of the test substance. The finding correlates to the macroscopically detected thickening of the mucous membrane. Except for the effects on the male genital system and the adrenals all changes were almost fully reversible within the 6-week recovery period. The vacuolization of the adrenals remained unchanged. The germinal epithelium also showed a tendency to recovery, but recovery is expected to take quite long periods of time to be complete in this tissue.

The data of the present study show that sodium cyanate, if ingested, is able to damage a number of organs. The main target organs are the red blood cells, which then might cause secondary toxicity to spleen, testes and male genital system, liver and adrenals. Local effects can occur in the stomach. The study also shows that all effects are in principle reversible, although complete recovery can take rather long time. The no observed effect level cannot be determined exactly from these data. The NOEL is expected slightly below 68.1 mg/kg, because there were only slight, mostly statistically insignificant effects in this dose group (hyperplasia and inflammatory infiltrates in the stomach, cortical vacuolization in the adrenals, diffuse degeneration of the germinal epithelium in the testes, extramedullary haematopoiesis in the spleen, increase of oxygenated hemoglobin iron, reduced food intake).

This subacute toxicity study in the rat is acceptable and satisfies the guidelines requirement for a subacute oral study (EU B.7 and OECD 407) in rats.