1,2-dichlorobenzene

Repeated dose toxicity: oral

Administrative data

Endpoint:

sub-chronic toxicity: oral

Type of information:

experimental study

Adequacy of study:

key study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: see 'Remark'

Remarks:

Comparable to guideline study with acceptable restrictions (food consumption not recorded; some serum parameters (sodium, urea, creatinine) not analysed; some organs (testes, epididymes) not weighted; some organs/tissues (spinal cord, aorta, peripheral nerves) not histopatholically examined)

Cross-reference

Reason / purpose for cross-reference:

reference to other study

Data source

Reference

Reference Type:

publication

Title:

Toxicology and carcinogenesis studies of 1,2-dichlorobenzene (o-dichlorobenzene) (CAS no. 95-50-1) in F344/N rats and B6C3F1 mice (gavage studies)

Author:

National Toxicology Program (NTP)

Year:

1985

Bibliographic source:

Techn. Rep. No. 255, US Dept. of Health and Human Services, Research Triangle Park, N.C. 27709, USA, 1985

Materials and methods

Test guideline

Qualifier:

equivalent or similar to guideline

Guideline:

OECD Guideline 408 (Repeated Dose 90-Day Oral Toxicity Study in Rodents)

Deviations:

yes

Remarks:

; food consumption not recorded; some serum parameters (sodium, urea, creatinine) not analysed; some organs (testes, epididymes) not weighted; some organs/tissues (spinal cord, aorta, peripheral nerves) not histopatholically examined

GLP compliance:

not specified

Limit test:

no

Test material

Test material information

Constituent 1

Reference substance name:

1,2-dichlorobenzene

EC Number:

202-425-9

EC Name:

1,2-dichlorobenzene

Cas Number:

95-50-1

Molecular formula:

C6H4Cl2

IUPAC Name:

1,2-dichlorobenzene

Details on test material:

- Name of test material (as cited in study report): 1,2-dichlorobenzene
- Analytical purity: >99%
- Impurities (identity and concentrations): 10.8 ppm acidic components (assumed to be hydrochloric acid); GC system: seven further impurities totalling less than 0.07%; two other GC-MS system: 0.7-0.84% 1,4-dichlorobenzene
- Lot/batch No.: SC61377 (ICC Solvent Chemical Company, New York, NY)
- Stability under test conditions: no notable change in the test material; storage conditions were satisfactory over the course of the study
- Storage condition of test material: 22°C in the dark

Test animals

Species:

rat

Strain:

Fischer 344

Sex:

male/female

Details on test animals or test system and environmental conditions:

TEST ANIMALS
- Source: Charles River Breeding Laboratories, Portage, MI
- Age at study initiation: 4 week old plus 17 days of acclimation period
- Housing: five animals/polycarbonate cage
- Diet (e.g. ad libitum): Purina Lab Chow 3/79 to 10/79; Zeigler Bros. N1N07 10/79 to 3/81 (ad libitum)
- Water (e.g. ad libitum): Edstrom automatic watering system, Waterford, WI (ad libitum)
- Acclimation period: 17 days

ENVIRONMENTAL CONDITIONS
- Temperature (°C): 21-23°C
- Humidity (%): 40-60%
- Air changes (per hr): 15 room air changes/hour
- Photoperiod (hrs dark / hrs light): 12/12

Administration / exposure

Route of administration:

oral: gavage

Vehicle:

corn oil

Details on oral exposure:

Amount of dosage (if gavage): 5 mL/kg bw

Analytical verification of doses or concentrations:

not specified

Duration of treatment / exposure:

13 w

Frequency of treatment:

5 d/w

Doses / concentrations

Remarks:

Doses / Concentrations:
30, 60, 125, 250 or 500 mg/kg bw/d
Basis:
actual ingested

No. of animals per sex per dose:

10 animals/sex/dose

Control animals:

yes, concurrent vehicle

Details on study design:

Post-exposure period: no

Examinations

Observations and examinations performed and frequency:

CAGE SIDE OBSERVATIONS: Yes
- Time schedule: observed twice daily for clinical signs of toxicity

BODY WEIGHT: Yes
- Time schedule for examinations: individual weights measured weekly

FOOD EFFICIENCY:
- 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
- food consumption measured weekly

OPHTHALMOSCOPIC EXAMINATION: No

HAEMATOLOGY: Yes, blood withdrawn from orbital venous plexus
- Time schedule for collection of blood: one day before death
- Anaesthetic used for blood collection: No data
- Animals fasted: No data
- How many animals: all animals
- Following parameters were examined: hematological profile (hemoglobin, RBC, WBC, hematocrit, MCV, platelet count, and reticulocyte count)


CLINICAL CHEMISTRY: Yes, blood samples were withdrawn by cardiac puncture after animals were anesthetized with sodium pentobarbital
- Time schedule for collection of blood: on the day of sacrifice
- Animals fasted: No data
- How many animals: all animals
- Following parameters were examined: alkaline phosphatase, serum glutamic pyruvic transaminase, gamma-glutamyltranspeptidase, bilirubin, cholesterol, triglycerides, blood urea nitrogen, glucose, total protein, and total globulin fractions


URINALYSIS: Yes
- Time schedule for collection of urine: one week before sacrifice; only animals of the control and the 500 mg/kg dose group
- Metabolism cages used for collection of urine: Yes (collection time 24h)
- Animals fasted: No data
- Following parameters were examined: uroporphyrins and coproporphyrins


NEUROBEHAVIOURAL EXAMINATION: No


OTHER:
- Total liver porphyrins from all animals
- Organ to body weight ratios:
Following organs weighed from all animals at necropsy: Lung, heart, liver, spleen, thymus, right kidney, brain, right testicle or right ovary, and uterus

Sacrifice and pathology:

GROSS PATHOLOGY: Yes, on all animals not completely autolyzed or cannibalized.

HISTOPATHOLOGY: Yes.
- The following specimens were examined histologically for control and high dose groups: gross lesions, tissue masses, abnormal lymph nodes, skin, thigh muscle, mandibular or mesenteric lymph nodes, mammary gland, salivary gland, bone marrow, sternebrae, femur, or vertebrae, thymus, trachea, lungs and bronchi, heart, thyroid, parathyroid, esophagus, stomach, small intestine, colon, liver, pancreas, spleen, kidneys, adrenals, urinary bladder, prostate/testes or ovaries/uterus, brain, and pituitary.
- Additional histopathologic examinations were limited to the kidneys, thymus, and liver for rats administered 125 or 250 mg/kg.

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 efficiency:

not specified

Ophthalmological findings:

not examined

Haematological findings:

effects observed, treatment-related

Clinical biochemistry findings:

effects observed, treatment-related

Urinalysis findings:

effects observed, treatment-related

Behaviour (functional findings):

not examined

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:

not specified

Details on results:

CLINICAL SIGNS AND MORTALITY
Two female rats receiving 500 mg/kg of 1,2-dichlorobenzene died, one at week 6 and one at week 9. One male each from the control, 30 mg/kg, and 125 mg/kg groups also died, presumably because of a gavage error (Table 1).

BODY WEIGHT AND WEIGHT GAIN
A dose-related decrease in weight gain was observed in male rats; however, except for the 500 mg/kg male rats, final body weights were within 7% of controls (refer to Table 1).

HAEMATOLOGY
Only minimal changes were observed in hematology. Minimal decreases in the hematocrit, the amount of hemoglobin, the number of red blood cells (significant only in males), and mean corpuscular volume were observed in male and female rats at the 500 mg/kg dose (Tables 3 and 4). The number of reticulocytes was increased slightly in high dose females. A minimal decrease in the number of lymphocytes and a small increase in the percentage of segmented neutrophils were observed in high dose males. These hematologic changes presumably reflect toxicity at the high dose. The number of platelets was increased at 60, 125, and 500 mg/kg in female but not in male rats. The reason for the increase in platelets is not apparent. This change is not considered to be biologically significant. 1,2-Dichlorobenzene did not affect the number of white cells, eosinophils, basophils, or monocytes.

CLINICAL CHEMISTRY
Only minimal changes were observed in clinical chemistry parameters. 1,2-Dichlorobenzene did not produce statistically significant increases in serum concentrations of SGPT (serum glutamic pyruvic transaminase), GGTP (gamma-glutamyl transpeptidase) or alkaline phosphatase (Tables 5 and 6). However, 1,2-dichlorobenzene produced slight dose-related increases in serum cholesterol at doses of 30, 125, 250, and 500 mg/kg (males) and 125-500 mg/kg (females), decreases in serum triglycerides at 500 mg/kg (males) and 250 mg/kg (females), and dose-related increases in total serum protein at 250-500 mg/kg (males) and at 30-500 mg/kg (females). All of these changes were relatively small; however, they may reflect hepatic effects of the test substance at these doses. Minimal increases in serum glucose levels in female rats were observed at 30 mg/kg and 125 to 500 mg/kg. Blood urea nitrogen was not affected in male or female rats receiving 500 mg/kg of 1,2-dichlorobenzene (Tables 5 and 6). However, 24-hour urine volume was increased 57% over controls in male rats receiving 500 mg/kg of 1,2-dichlorobenzene (Table 7).

ORGAN WEIGHTS
1,2-Dichlorobenzene increased liver weights in male and female rats in a dose-related manner (Table 2). Significant increases in liver weight/ body weight ratios were observed at the 125, 250, and 500 mg/kg doses in males and females. Decreases in the absolute organ weight and the organ weight/ body weight ratio for thymus in male rats at the 500 mg/kg dose may reflect general toxicity or organ-specific toxicity. However, slight increases in organ weight/ body weight ratios for lungs, kidney, and brain in male rats receiving 500 mg/kg without a corresponding effect on organ weight probably reflect the effect of the test substance on body weight.

GROSS PATHOLOGY
At necropsy, no consistent lesions were noted. The liver and kidneys from all rats (except for a few that died early in the study) were examined under long-range ultraviolet light. None had the positive reddish-brown pigmentation indicative of porphyria.

HISTOPATHOLOGY: NON-NEOPLASTIC
Microscopically, a number of lesions that appeared to be compound or dose related were observed. The two 500 mg/kg dose rats that died early had a moderate degree of centrolobular hepatocellular necrosis. Most of the surviving high dose rats (7/8 surviving females and 8/10 males) had liver lesions, either centrolobular degeneration or necrosis of individual hepatocytes. The necrosis of individual hepatocytes was characterized by randomly scattered hepatocytes that were pyknotic or karyolytic and had shrunken dark red cytoplasms. In addition, renal tubular degeneration was found in 6/10 high dose male rats and thymic lymphoid depletion was found in 4/10 high dose male rats.
The renal and thymic lesions were not present at the 250 mg/kg or 125 mg/kg doses. The individual hepatocellular necrosis persisted at the 250 mg/kg dose (4/9 males and 5/10 females). Individual hepatocellular necrosis was seen in one female rat at the 125 mg/kg dose. In addition, some focal hepatic necrosis was observed in one male rat at the 125 mg/kg dose that died early due to a ruptured esophagus and in two female rats at the 125 mg/kg dose; this was less pronounced than that observed at higher doses.
Yellow-green to gold pigment was observed in some of the livers at the 250 and 500 mg/kg doses. This pigment was periodic acid-Schiff (PAS) and Pens positive and was believed to be hemosiderin. Because the liver lesions found in rats receiving 250 mg/kg were considered to be potentially life-shortening by the original pathologist, doses of 60 and 120 mg/kg of 1,2-dichlorobenzene in corn oil were selected for rats in the 2-year study.

URINALYSIS
The urinary concentration of uroporphyrin and coproporphyrin was three to five times higher in male and female rats receiving 500 mg/kg of 1,2-dichlorobenzene than in controls (Table 7). However, this increase is not considered to be indicative of porphyria. The concentration of total porphyrins in the liver was not altered by 1,2-dichlorobenzene at any dose level (Table 7).

Effect levels

1

Key result

Dose descriptor:

NOAEL

Effect level:

60 mg/kg bw/day (nominal)

Sex:

male/female

Basis for effect level:

other: no adverse effects observed

2

Key result

Dose descriptor:

LOAEL

Effect level:

125 mg/kg bw/day (nominal)

Sex:

male/female

Basis for effect level:

other: minimal hepatocellular necrosis

Target system / organ toxicity

Critical effects observed:

yes

Lowest effective dose / conc.:

125 mg/kg bw/day (nominal)

System:

hepatobiliary

Organ:

liver

Treatment related:

yes

Dose response relationship:

yes

Any other information on results incl. tables

Table 1: Survival and mean body weights of male rats administered 1,2-dichlorobenzene by gavage for 13 weeks

		Mean Body Weights [g]
Dose [mg/kg]	Survival *	Initial	Final	Change **	Final Body Weights Relative to Controls [%] ***
0	9/10	93.3 ± 4.4	293.1 ± 7.8	+199.8 ± 4.9	--
30	9/10	104.1 ± 2.9	305.4 ± 3.6	+201.3 ± 3.9	+ 4
60	10/10	97.7 ± 5.9	292.9 ± 6.3	+195.2 ± 6.1	0
125	9/10	99.7 ± 3.4	281.4 ± 5.9	+181.7 ± 5.0	- 4
250	9/9 ****	98.9 ± 4.9	275.8 ± 8.3	+176.9 ± 7.2	- 6
500	10/10	101.8 ± 5.4	236.1 ± 12.1	+134.3 ± 11.4	- 19

* : Number surviving / number initially in the group

** : Mean weight change of the survivors of the group

*** : Weight of the dosed group relative to that of the controls

**** : One animal in this group was missexed.

Table 2: Organ weights of rats administered 1,2-dichlorobenzene by gavage for 13 weeks (a)

		Male						Female
Dose [mg/kg]		Body weight [g]	Lung	Liver	Thymus	Right kidney	Brain	Body weight [g]	Liver
0
	Mean [g]	274 ± 22	1.34 ± 0.18	8.75 ± 1.15	0.29 ± 0.09	0.97 ± 0.09	1.78 ± 0.08	169 ± 13	4.87 ± 0.24
	OW/BW (b)	--	0.49 ± 0.07	3.18 ± 0.20	0.11 ± 0.03	0.35 ± 0.01	0.65 ± 0.05	--	2.90 ± 0.20
30
	Mean [g]	275 ± 2	1.44 ± 0.17	9.00 ± 0.63	0.27 ± 0.07	0.94 ± 0.07	1.77 ± 0.07	166 ± 13	4.95 ± 0.53
	OW/BW	--	0.52 ± 0.06	3.28 ± 0.22	0.10 ± 0.02	0.34 ± 0.02	0.64 ± 0.02	--	2.98 ± 0.15
60
	Mean [g]	269 ± 17	1.46 ± 0.20	8.34 ± 0.73	0.28 ± 0.06	0.95 ± 0.10	1.80 ± 0.06	166 ± 16	4.84 ± 0.44
	OW/BW	--	0.54 ± 0.08	3.10 ± 0.15	0.10 ± 0.02	0.35 ± 0.03	0.67 ± 0.04	--	2.92 ± 0.16
125
	Mean [g]	261 ± 15	1.32 ± 0.14	8.98 ± 0.95	0.25 ± 0.02	1.02 ± 0.15	1.82 ± 0.07	164 ± 10	5.13 ± 0.53
	OW/BW	--	0.51 ± 0.07	3.43 ± 0.22 (c)	0.10 ± 0.01	0.39 ± 0.06	0.70 ± 0.05	--	3.13 ± 0.20 (c)
250
	Mean [g]	258 ± 25	1.36 ± 0.15	9.62 ± 1.28	0.23 ± 0.08	0.95 ± 0.06	1.77 ± 0.09	160 ± 13	5.33 ± 0.51
	OW/BW	--	0.53 ± 0.05	3.72 ± 0.29 (c)	0.09 ± 0.04	0.37 ± 0.03	0.69 ± 0.05	--	3.33 ± 0.18 (c)
500
	Mean [g]	224 ± 27 (c,e)	1.46 ± 0.69	10.27 ± 1.04 (c,d)	0.17 ± 0.11 (c,e)	0.91 ± 0.10	1.73 ± 0.08	165 ± 14	6.03 ± 0.54 (c,d)
	OW/BW	--	0.66 ± 0.34 (c,d)	4.61 ± 0.47 (c,d)	0.07 ± 0.05 (c,e)	0.40 ± 0.02 (c,d)	0.78 ± 0.07 (c,d)	--	3.78 ± 0.30 (c,d)

(a) : Groups contained 9 or 10 animals

(b) : Organ weight / body weight * 100; [%]

(c) : Dosed group significantly different from controls; P<0.05

(d) : Positive dose-response trend; P<0.05

(e) : Negative dose-response trend; P<0.05

Table 3: Results of blood analysis in male rats administered 1,2-dichlorobenzene by gavage for 13 weeks (a)

	Dose [mg(kg]
	0	30	60	125	250	500
Hemoglobin [g/L]	16.5 ± 0.9	16.9 ± 0.6	16.8 ± 0.9	16.6 ± 0.5	16.5 ± 1.1	15.0 ± 0.5 (b,c)
Hematocrit [%]	51 ± 3	53 ± 1	52 ± 1	49 ± 2	50 ± 3	46 ± 1 (b,c)
Red  blood cells [10^6 cu mm]	9.42 ± 0.53	9.70 ± 0.32	9.49 ± 0.48	9.46 ± 0.30	9.54 ± 0.63	8.57 ± 0.25 (b,c)
Mean corpuscular volume [µ^3]	52 ± 1	53 ± 1	52 ± 1	50 ± 1 (b)	50 ± 1 (b)	51 ± 1 (c)
Segs [%]	15 ± 4	19 ± 7	15 ± 3	16 ± 6	11 ± 4	22 ± 7 (b)
Lymphocytes [%]	84 ± 4	80 ± 8	83 ± 4	83 ± 7	88 ± 5	78 ± 7 (b)

(a) : Groups contained 9 or 10 animals. Values presented represent the mean ± standard deviation.

(b) : Dosed group significantly different from controls; P<0.05

(c) : Negative dose-response trend; P<0.05

Table 4: Results of blood analysis in female rats administered 1,2-dichlorobenzene by gavage for 13 weeks (a)

	Dose [mg/kg]
	0	30	60	125	250	500
Mean corpuscular volume  [µ^3]	54 ± 2	54 ± 1	53 ± 1	54 ± 1	53 ± 0.4 (b)	53 ± 1 (b,d)
Platelets x 10^5	2.96 ± 0.39	4.09 ± 1.32	4.81 ± 0.63 (b)	5.85 ± 2.32 (b)	3.66 ± 0.63	5.24 ± 0.76 (b,c)
Reticulocytes [%]	5.7 ± 2.2	4.7 ± 1.2	5.4 ± 0.7	5.8 ± 1.0	6.2 ± 1.2	8.2 ± 0.9 (b,c)

(a) : Groups contained 7 to 10 animals. Values presented represent the mean ± standard deviation.

(b) : Dosed group significantly different from controls; P<0.05

(c) : Positive dose-response trend; P<0.05

(d) : Negative dose-response trend; P<0.05

Table 5: Results of clinical chemistry in male rats administered 1,2-dichlorobenzene by gavage for 13 weeks (a)

	Dose [mg/kg]
	0	30	60	125	250	500
Cholesterol [mg/dL]	34 ± 4	51 ± 9 (b)	40 ± 7	43 ± 5 (b)	58 ± 6 (b)	71 ± 8 (b,c)
Triglycerides [mg/dL]	242 ± 28	285 ± 50	264 ± 60	228 ± 56	236 ± 70	142 ± 75 (b,d)
Total protein [g/dL]	7.0 ± 0.2	6.9 ± 0.4	7.1 ± 0.4	7.0 ± 0.2	7.5 ± 0.5 (b)	7.5 ± 0.3 (b,c)

(a) : Groups contained 9 or 10 animals. Values presented represent the mean ± standard deviation.

(b) : Dosed group significantly different from controls; P<0.05

(c) : Positive dose-response trend; P<0.05

(d) : Negative dose-response trend; P<0.05

Table 6: Results of clinical chemistry in female rats administered 1,2-dichlorobenzene by gavage for 13 weeks (a)

	Dose [mg/kg]
	0	30	60	125	250	500
Cholesterol [mg/dL]	49 ± 4	55 ± 7	55 ± 6	65 ± 4 (b)	62 ± 7 (b)	74 ± 7 (b,c)
Triglycerides [mg/dL]	180 ± 30	145 ± 33	172 ± 43	167 ± 32	141 ± 36 (b)	167 ± 17
Glucose [mg/dL]	176 ± 16	204 ± 12 (b)	195 ± 15	212 ± 16 (b)	206 ± 21 (b)	209 ± 17 (b,c)
Total protein [g/dL]	6.4 ± 0.3	6.9 ± 0.1 (b)	6.7 ± 0.2 (b)	6.8 ± 0.2 (b)	6.8 ± 0.3 (b)	7.5 ± 0.3 (b,c)

(a) : Groups contained 9 or 10 animals. Values presented represent the mean ± standard deviation.

(b) : Dosed group significantly different from controls; P<0.05

(c) : Positive dose-response trend; P<0.05

Table 7: Results of porphyrin analyses of urine in rats administered 1,2-dichlorobenzene by gavage for 13 weeks (a)

Dose [mg/kg]	Coproporphyrin [µg/24 hours]	Uroporphyrin [mg/24 hours]	Volume [mL]
Males
0	0.51 ± 0.23	1.38 ± 0.42	5.6 ± 1.8
500	4.99 ± 2.54 (b)	5.85 ± 2.78 (b)	8.8 ± 2.8 (b)
Females
0	0.46 ± 0.34	0.59 ± 0.14	3.1 ± 1.7
500	2.53 ± 1.16 (b)	2.02 ± 0.88 (b)	5.3 ± 3.8

(a) : Groups contained 6 or 10 animals. Values presented represent the mean ± standard deviation.

(b) : Dosed group significantly different from controls; P<0.05

Applicant's summary and conclusion

Conclusions:

Only minimal hepatocellular necrosis was observed at 125 mg/kg/day in a few rats. Therefore the NOAEL is 60 mg/kg/day.

Executive summary:

NTP report, 1985

A repeated dose toxicity study was conducted according to OECD Guideline 408. For 90 days, 1,2-dichlorobenzene was adminsitered to rats by gavage. The study was considered as reliable with restrictions (food consumption not recorded; some serum parameters not analysed; some organs not weighted; some organs/tissues not histopatholically examined).

For 13 weeks, male and female F344/N rats were dosed 5 days per week with 30, 60, 125, 250 or 500 mg/kg bw/day. According to guideline, 10 animals/sex/dose were used.

Two female rats receiving 500 mg/kg of 1,2-dichlorobenzene died, one at week 6 and one at week 9. One male each from the control, 30 mg/kg, and 125 mg/kg groups also died, presumably because of a gavage error. A dose-related decrease in weight gain was observed in male rats; however, except for the 500 mg/kg male rats, final body weights were within 7% of controls.

Only minimal changes were observed in hematology. Minimal decreases in the hematocrit, the amount of hemoglobin, the number of red blood cells (significant only in males), and mean corpuscular volume were observed in male and female rats at the 500 mg/kg dose. The number of reticulocytes was increased slightly in high dose females. A minimal decrease in the number of lymphocytes and a small increase in the percentage of segmented neutrophils were observed in high dose males. These hematologic changes presumably reflect toxicity at the high dose.

Only minimal changes were observed in clinical chemistry parameters. 1,2-Dichlorobenzene produced slight dose-related increases in serum cholesterol at doses of 30, 125, 250, and 500 mg/kg (males) and 125-500 mg/kg (females), decreases in serum triglycerides at 500 mg/kg (males) and 250 mg/kg (females), and dose-related increases in total serum protein at 250-500 mg/kg (males) and at 30-500 mg/kg (females). All of these changes were relatively small; however, they may reflect hepatic effects of the test substance at these doses. Minimal increases in serum glucose levels in female rats were observed at 30 mg/kg and 125 to 500 mg/kg.

Blood urea nitrogen was not affected, however, 24-hour urine volume was increased 57% over controls in male rats receiving the highest dose. The urinary concentration of uroporphyrin and coproporphyrin was three to five times higher in male and female rats receiving 500 mg/kg of 1,2-dichlorobenzene than in controls. However, this increase is not considered to be indicative of porphyria.

1,2-Dichlorobenzene increased liver weights in male and female rats in a dose-related manner. Significant increases in liver weight/ body weight ratios were observed at the 125, 250, and 500 mg/kg doses in males and females. Decreases in the absolute organ weight and the organ weight/ body weight ratio for thymus in male rats at the 500 mg/kg dose may reflect general toxicity or organ-specific toxicity.

At necropsy, no consistent lesions were noted. Microscopically, a number of lesions that appeared to be compound or dose related were observed. The two 500 mg/kg dose rats that died early had a moderate degree of centrolobular hepatocellular necrosis. Most of the surviving high dose rats had liver lesions, either centrolobular degeneration or necrosis of individual hepatocytes. In addition, renal tubular degeneration and thymic lymphoid depletion was found in high dose male rats. Individual hepatocellular necrosis was seen in one female rat at the 125 mg/kg dose. In addition, some focal hepatic necrosis was observed in one male rat at the 125 mg/kg dose that died early due to a ruptured esophagus and in two female rats at the 125 mg/kg dose; this was less pronounced than that observed at higher doses. Yellow-green to gold pigment was observed in some of the livers at the 250 and 500 mg/kg doses. This pigment was periodic acid-Schiff (PAS) and Pens positive and was believed to be hemosiderin. Liver lesions found in rats receiving 250 mg/kg were considered to be potentially life-shortening.

Because only minimal hepatocellular necrosis was observed at 125 mg/kg/day in a few rats. Therefore a NOAEL of 60 mg/kg/day is justified.