Sodium methanethiolate

Repeated dose toxicity: inhalation

Administrative data

Endpoint:

sub-chronic toxicity: inhalation

Type of information:

migrated information: read-across from supporting substance (structural analogue or surrogate)

Adequacy of study:

supporting study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: Study well documented, meets generally accepted scientific principles, acceptable for assessment

Data source

Reference

Reference Type:

publication

Title:

Unnamed

Year:

1981

Materials and methods

Test guideline

Qualifier:

equivalent or similar to guideline

Guideline:

OECD Guideline 413 (Subchronic Inhalation Toxicity: 90-Day Study)

Deviations:

yes

Remarks:

No hematological and limited histopathological examinations

GLP compliance:

no

Limit test:

no

Test material

Test material information

Constituent 1

Reference substance name:

Methanethiol

EC Number:

200-822-1

EC Name:

Methanethiol

Cas Number:

74-93-1

IUPAC Name:

methanethiol

Details on test material:

Source: Matheson, Inc., Bridgeport, NJ, USA
Purity: no data

Test animals

Species:

rat

Strain:

Sprague-Dawley

Sex:

male

Details on test animals or test system and environmental conditions:

TEST ANIMALS
- Source: Charles River
- Age at study initiation: no data
- Weight : 90-100 g on day of receipt
- Housing: 6/cage
- Diet (ad libitum): Purina Laboratory Chow
- Water (ad libitum): tap water
- Acclimation period: 7 days

ENVIRONMENTAL CONDITIONS
- Temperature (°C): 22
- Humidity (%): no data
- Air changes (per hr): no data
- Photoperiod (hrs dark / hrs light): 12/12

Administration / exposure

Route of administration:

inhalation

Type of inhalation exposure:

whole body

Vehicle:

other: unchanged (no vehicle)

Remarks on MMAD:

MMAD / GSD: Only vapour

Details on inhalation exposure:

Short-term chronic and sham exposures were conducted in two 11.4-ft3 stainless steel commercial chambers (Young and Bertke Co., Cincinnati, Ohio). These chambers permitted continuous observation during exposure and were designed to ensure spatial distribution of metered gas mixtures introduced through the inlet port. Flow rates were calculated to yield the desired gas concentrations and were verified by analysis of gas samples.

Analytical verification of doses or concentrations:

yes

Details on analytical verification of doses or concentrations:

Samples were collected at 1.0 I/min in a midget impinger containing 15 ml mercuric acetate-acetic acid solution. The collected mercaptan was then determined spectrophotometrically by the method of Katz (1977).

Duration of treatment / exposure:

3 months

Frequency of treatment:

7 h/day ; 5 d/week

Doses / concentrations

Remarks:

Doses / Concentrations:
2, 17, 57 ppm
Basis:
analytical conc.

No. of animals per sex per dose:

31

Control animals:

yes, concurrent no treatment

Details on study design:

Post-exposure period: none

Examinations

Observations and examinations performed and frequency:

CAGE SIDE OBSERVATIONS: Yes

DETAILED CLINICAL OBSERVATIONS: No

BODY WEIGHT: Yes
- Time schedule for examinations: terminal body weight

FOOD CONSUMPTION: Yes

FOOD EFFICIENCY: No data

WATER CONSUMPTION: Yes

OPHTHALMOSCOPIC EXAMINATION: No

HAEMATOLOGY: No

CLINICAL CHEMISTRY: Yes
- Time schedule for collection of blood: terminal sacrifice
- Animals fasted: No data
- How many animals: all rats
- Parameters checked: total protein, albumine, Ca2+, Pi, cholesterol BUN, uric acid, total bilirubin, alkaline phosphatase, LDH, SGPT, SGPT, SGOT, glucose

URINALYSIS: No

NEUROBEHAVIOURAL EXAMINATION: No

Sacrifice and pathology:

GROSS PATHOLOGY: No data

ORGAN WEIGHTS: Yes
Brain, lung, liver, spleen, heart, kidneys, and adrenals in all rats

HISTOPATHOLOGY: Yes
Heart, lungs, small intestine, and kidneys in 5 rats/groups
Complete histopathology of livers in all rats

Other examinations:

A subset of 10 animals from each group was designated for special metabolic performance studies by an independent true random process.  To minimize possible differences in feeding behavior during exposure periods, the sham and experimental groups were deprived of food during the exposure periods.  Tap water was provided ad libitum. At the end of the exposure day the metabolic subsets were placed overnight in metabolism cages and the appropriate measurements were made. Metabolic performance measurements were made for 17 h periods on 5 consecutive days.
At the end of the 3-mo experimental period the metabolic subsets served as the subjects for the following tests: intestinal transit time, systolic blood pressure effects. The observations were made at least 24 h later than the end of the last exposure day. 

Statistics:

AII recorded values from the metabolic performance studies were normalized to 100 g body weight to reduce the variations that would be caused by differences in weights of the rats. The daily mean of individual normalized values for each variable was plotted for the sham control and experimental groups and least-squares linear regression lines were fitted through the points. Then a pooled variances t-test of the difference between regression coefficients was performed. This type of test only allows a determination of the significance of differences between average rates of change of metabolic performance, but if the difference between the rates is significant, it can be assumed that the metabolic performances of the two groups with respect to the variable being studied are not the same. An F-test to check for homogeneity of variance between the two least-squares lines to be compared was performed. If the F-test showed that the ratios of the mean square deviations from regression were not satisfactorily homogeneous, a modified Welch test was used to test the significance of the difference between slopes. Propulsive motility, systolic blood pressure, 02 consumption, SMA 12/60 blood analysis, and organ weights for each sham control and experimental group were tabulated. Mean and standard deviations were computed and comparisons were made by Student's t-test under the null hypothesis that the means were equal. The alternate hypothesis was twotailed: that the means were different. An overall 5% confidence level was set in advance.

Results and discussion

Results of examinations

Clinical signs:

no effects observed

Mortality:

no mortality observed

Body weight and weight changes:

effects observed, treatment-related

Food consumption and compound intake (if feeding study):

no effects observed

Food efficiency:

not specified

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

no effects observed

Ophthalmological findings:

not examined

Haematological findings:

not examined

Clinical biochemistry findings:

effects observed, treatment-related

Urinalysis findings:

not examined

Behaviour (functional findings):

not examined

Organ weight findings including organ / body weight ratios:

no effects observed

Gross pathological findings:

not specified

Histopathological findings: non-neoplastic:

no effects observed

Histopathological findings: neoplastic:

no effects observed

Details on results:

CLINICAL SIGNS AND MORTALITY
No mortality response was observed in any sham or exposed population of rats during the 3-mo period.  However, during actual exposures the rats tended to huddle in groups of 5 or 6 toward the periphery of the chamber with noses pointed outward from the chamber's vertical axis.  This behavior was not observed in the sham group but was markedly obvious at 57 ppm.

BODY WEIGHT AND WEIGHT GAIN
Average terminal body weights were lower than those of sham controls for all rats in the exposed groups.  This difference was statistically significant in the 57 ppm group and showed a statistically significant dose-related trend (Table 1).

FOOD CONSUMPTION
Average rates of change of food intake and wet and dry fecal weights were not significantly different from those of the sham controls. Fecal pellet production rate increases (data not shown) were significantly lower for the 2 and 17 ppm subsets and nonsignificantly greater for the 57 ppm subset.

WATER CONSUMPTION
Rates of water intake increase were less for all exposed subsets, although this was not significant for the 57 ppm subset. Rates of water excretion (total urine and fecal water) increase were slightly higher for all exposed subsets, although the rate of increase for the 57 ppm subset was not significant.

OPHTHALMOSCOPIC EXAMINATION
Not examined

HAEMATOLOGY
Not examined

CLINICAL CHEMISTRY
Statistically significant changes were observed in serum components of terminal blood samples from animals of all exposed groups subjected to SMA 12/60 analysis (Table 2).
Average total serum proteins were significantly higher for all exposed groups. Average albumin concentrations were significantly lower for all exposed groups. Significant reductions in inorganic phosphate occurred in the 2 and 17 ppm groups. Cholesterol was significantly elevated in the 2 ppm group and total bilirubin was significantly higher in the 2 and 17 ppm groups. Blood urea N was significantly lower in the 57 ppm group and lactate dehydrogenase was significantly lower in all three exposed groups. None of these trends were dose-related at the 95% confidence level.

URINALYSIS
Not examined

NEUROBEHAVIOUR
Not examined

ORGAN WEIGHTS
Although some average organ weights were significantly different from corresponding sham values (Table 1), there was no obvious dose-related trend such as was apparent with whole-body weights, these significant differences could be due to chance alone.

GROSS PATHOLOGY
No data

HISTOPATHOLOGY: NON-NEOPLASTIC
Routine histopathological examination was conducted for five rats per dose group and was negative for the heart, small bowel, and kidneys. Lungs exhibited the pneumonia, emphysematic changes, and occasional fibrosis that are characteristic of rat colonies. These pictures did not appear to be different in samples from exposed rats. Some evidence of pathological changes was noted in liver sections from 31 rats each in the 2, 17, and 57 ppm groups. In all cases there was evidence of inflammatory cells and possibly enlarged bile ductules. Hyperplastic nodules were observed in one liver section from the 2 ppm and in three liver sections from the 57 ppm group. 

HISTOPATHOLOGY: NEOPLASTIC (if applicable)
One hepatic carcinoma was visually observed (and sampled for histopathological examination) on the ventral surface of the liver of a rat in the 17 ppm group. In the case of the sham control group, 31 livers were hand-sectioned (2-3 µm) and examined under a dissecting microscope. Two small nodular lesions were detected in two livers; under light microscopy they were observed to be a hyperplastic region similar to that observed in the 2 and 57 ppm groups. Therefore, the treatment relation-ship of the hyperplastic nodules observed in the treated animals can be ruled out.

OTHER FINDINGS
No significant differences in intestinal transit performance parameters were observed in any of the metabolic performance subsets.  No consistent patterns were observed in average values of systolic blood pressure in the metabolic subset rats. After week 1, average values of O2 consumption measured in special subsets tended to be lower for exposed than for sham rats, but the differences were not consistently significant. All of these average values tended to decrease with time during the limited course of the observations. However, for both the sham and exposed groups, the mean 02 consumption rates were higher than those expected for normal rats. These studies could not be conducted beyond 3 week because the rats grew too large to fit into the apparatus used for measuring O2 consumption. 

Effect levels

1

Dose descriptor:

NOAEL

Effect level:

17 ppm

Sex:

male

2

Dose descriptor:

LOAEL

Effect level:

57 ppm

Sex:

male

Target system / organ toxicity

Critical effects observed:

not specified

Any other information on results incl. tables

  TABLE 1. Changes in Body and Normalized Tissue Wet Weights (g) Resulting from 3 mo Exposure to Methyl Mercaptan Vapora^a

Tissue	Sham control group	2 ppm	17 ppm	57 ppm +
Whole body	458.58 ±53.53	446.48 ± 48.57	443.45 ± 45.63	391.71 ± 45.02^b,c
Brain	0.44 ± 0.06	0.45 ± 0.05	0.44 ± 0.09	0.49 ± 0.09^b
Lung	0.35 ± 0.07	0.32 ± 0.05	0.35 ± 0.07	0.34t0.05
Liver	2.78 ±0.41	2.81 ± 0.46	2.84 ±0.46	2.75±0.43
Spleen	0.16 ± 0.02	0.15 t 0.03	0.15 ± 0.02^b	0.18 ±0.02^b
Heart	0.29± 0.03	0.30 ± 0.03	0.29 ± 0.04	0.28 ±0.03
Kidneys	0.66 ± 0.05	0.64 ± 0.06	0.65 ± 0.05	0,64 ±0.05
Adrenals	0.012 ± 0.004	0.011 ± 0.002	0.010 ± 0.002^b	0.017 ±0.007^b

aMean ± SD represents 31 animals in each group.

^bStatistically significant difference compared with the mean values for sham control rats (p<0.015 for each pairing; p<0.05 overall).

^cDose-related change statistically significant at 95% confidence level.
The same was true when average rates of body weight increases were determined by regression analyses for the metabolic subsets.
 

TABLE 2. SMA 12/60 Blood Serum Analyses after 3 mo Exposure to Methyl Mercaptan Vapor^a

Parameters	Sham control group	2 ppm	17 ppm	57 ppm
Total protein (g%)	6.69 ± 0.49	7.23 ± 0.53^c	7.47 ± 0.54^c	7.14 ± 0.85^c
Albumin (g%)	3.44 ± 0.25	3.00 ± 0.22^c	2.99 ± 0.20^c	2.92 ± 0.25^c
Ca^++(mg%)	4.92 ± 0.32	5.01 ± 0.28	5.03 ± 0.30	4.90 ± 0.55
Pi (mg%P)	8.25 ± 1.10	7.40 ± 0.79^c	7.49 ± 0.63^c	7.73 ± 0.58
Cholesterol (mg%)	65.84 ± 15.21	75.68 ± 13.79^c	69.10±9.30	66.39 ± 16.38
BUN(mg%)	22.48 ± 3.61	23.68 ± 5.37	22.27 ± 3.63	20.00 ± 3.54^c
Uric acid (mg%)	1.86 ± 0.75	1.68 ± 0.41	1.70 ± 0.41	1.44 ± 0.59
Total bilirubin (mg%)	0.11 ± 0.08	0.41 ± 0.26^c	0.43 ± 0.19^c	0.09 ± 0.14
Alkaline phosphatase (mU/ml)	241.64 ± 98.97	209.29 ± 92.22	210.07 ± 81.28	248.94 ± 112.93
LDH (mU/ml)	597.32 ± 130.45	465.94 ± 146.46^c	468.67± 116.92^c	522.35 ± 100.73^c
SGPT (mU/ml)	86.90 ± 28.42	77.10 ± 19.97	77.00 ± 19.6487.	71 ± 26.91
SGOT (mU/ml)	301.55 ± 74.46	276.84 ± 46.32	273.00 ± 49.73	277.64 ± 55.27
Glucose (mg%)	122.82 ± 18.33	125.16 ± 16.75	123.53 ± 18.05	132.97 ± 20.81

^aA total of 31 rats were used at each dose level. Values are expressed as mean ± SD.

^bAbbreviations: SMA, sequential multiple analyzer; Pi, inorganic phosphate; BUN, blood urea N; LDH, lactic dehydrogenase; SGPT, serum glutamic-pyruvic transaminase; and SGOT, serum glutamic-oxaloacetic transaminase.

^cStatistically significant difference compared with mean values for sham control rats (p<0.015 for each pairing; p<C 0.05 overall).

Applicant's summary and conclusion

Conclusions:

The authors of the study report concluded that the NOAEC and LOAEC were considered to be 17 (0.033 mg/L) and 57 (0.118
mg/L) ppm, respectively.

Executive summary:

In a 3-month inhalation toxicity study, Sprague-Dawley rats (31 males/concentration) were exposed (whole body) to methanethiol at 0, 2, 17 or 57 ppm (0, 0.0039, 0.033 or 0.118 mg/L, respectively) for seven hours per day, five days per week (Tansy et al., 1981). There were no deaths, clinical signs of toxicity, food intake or fecal weight effects or organ weight changes observed. During actual exposures the rats tended to huddle in groups of 5 or 6 toward the periphery of the chamber with noses pointed outward from the chamber's vertical axis. This behavior was not observed in the sham group but was markedly obvious at 57 ppm. Average terminal body weights were lower in all treated groups and statistically significant (p<0.05) in the 57 ppm group indicating a statistically significant dose-related trend. Significant changes in organ weights (spleen and adrenals) were not considered relevant by the study authors. Average total serum proteins were significantly higher for all exposed groups but no dose-response relationship was evident. Average albumin concentration was significantly lower for all exposed groups. The authors noted that these changes in blood chemistry could indicate liver damage but also might indicate dehydration. In addition, statistically-significant reductions in inorganic phosphate (2 and 17 ppm groups), slightly elevated cholesterol (2 ppm group) and elevated total bilirubin (2 and 17 ppm groups) were observed but these were not considered treatment-related because of the lack of effect at 57 ppm (i.e. no dose response). Blood urea nitrogen was significantly lowered in the 57 ppm group and lactate dehydrogenase was significantly lower in all exposed groups (only increases in these parameters are generally considered of toxicological relevance). It can be concluded with 95% confidence that there were no dose-related trends in any clinical chemistry parameters. There were no exposure-related histopathological effects (including kidneys) observed in this study. All animals at all concentrations had inflammatory cells in their livers and possibly enlarged bile ducts; these effects were not seen in the controls. One hepatic carcinoma was seen in a rat in the 17 ppm exposure group. The authors of the study report concluded that the NOAEC and LOAEC were considered to be 17 (0.033 mg/L) and 57 (0.118 mg/L) ppm, respectively.