The doses of N-methylolacrylamide used ranged from 25 to 400 mg/kg. All rats that received 400 mg/kg died within 4 days, and 3/5 male rats that received 200 mg/kg also died before the end of the studies. Compound-related clinical signs seen with 200 mg/kg included ataxia, muscle tremors, and hyperirritability. Ataxia after dosing was observed from day 7 to the end of the studies for rats that received 100 mg/kg. The final mean body weight of male rats that received 100 or 200 mg/kg was 10% or 27% lower than that of the vehicle controls. The final mean body weight of female rats that received 200 mg/kg was 20% lower than that of the vehicle controls. Compound-related lesions in rats included hyperplasia of the bronchiolar and tracheal epithelium, dysplasia of the nasal and tracheal epithelium, centrilobular hepatocellular necrosis, lymphoid depletion of the spleen, and myelin degeneration of the lumbar ventral spinal nerve.
All 5 male and 4/5 female mice that received 400 mg/kg N-methylolacrylamide died on the second day of the 16-day studies. The surviving female mouse in the 400 mg/kg group and the male and female mice in the 200 mg/kg groups were ataxic after they were dosed, starting on day 2. Weight changes were inconsistent among dose groups. Bronchial epithelial hyperplasia (mild) appeared to be dose related in males and females. Sinusoidal congestion of the liver and vacuolar degeneration of myocardial fibers were seen in males and females given 400 mg/kg.
The doses of N-methylolacrylamide used ranged from 12.5 to 200 mg/kg. All rats that received 100 or 200 mg/kg died before the end of the studies. Rats that received 100 or 200 mg/kg had hind limb ataxia, which progressed to hind limb paralysis. Rats that received 50 mg/kg had hind limb ataxia beginning at week 8, which progressed to hind limb paresis by week 11. The final mean body weight of rats that received 25 or 50 mg/kg was 8% or 16% lower than that of the vehicle controls for males and 6% or 10% lower for females. In neurobehavioral assessments, decreased forelimb and hind limb grip strength was seen at doses as low as 25 mg/kg for female rats and at doses as low as 12.5 mg/kg for male rats. A decreased startle response was seen for females at doses as low as 25 mg/kg. The landing foot spread was significantly increased for male and female rats that received 50 mg/kg.
Axon filament and myelin sheath degeneration of the brain stem, spinal cord, and/or peripheral nerves was seen in rats at increased incidences at 25 mg/kg and higher doses. Inflammation and/or hemorrhage and edema of the urinary bladder mucosa were seen with doses of 25 mg/kg or more in a few rats that had distended bladders at gross examination.
All mice that received 200 mg/kg N-methylolacrylamide died before the end of the studies. Final mean body weights of dosed and vehicle control mice were similar. A decreased relative testis weight was observed for mice that received 12.5 mg/kg or more. The relative kidney weights for male mice receiving 50 or 100 mg/kg were greater than that for vehicle controls. Neurobehavioral studies indicated decreased forelimb grip strength in male and female mice at doses as low as 25 mg/kg. An exaggerated startle response was seen for female mice given 100 mg/kg. A reduction in rotarod performance was seen for male and female mice receiving 100 mg/kg and for male mice receiving 25 mg/kg.
Hepatocellular necrosis and thymic lymphocytic necrosis were compound-related effects in mice given 200 mg/kg N-methylolacrylamide. Hemorrhage, necrosis, and mineralization of the zona reticularis of the adrenal gland were present in 3/10 female mice given 200 mg/kg, and cytoplasmic vacuolization of the adrenal cortex was seen with lower doses.
Based on the results of these short-term studies, 2-year studies were conducted by administering 0, 6, or 12 mg/kg N-methylolacrylamide in water by gavage, 5 days per week for 103 weeks, to groups of 50 rats of each sex. Groups of 50 mice of each sex were administered 0, 25, or 50 mg/kg on the same schedule.
Body Weight and Survival in the Two-Year Studies:
Mean body weights of dosed rats were within 6% of those of vehicle controls throughout most of the studies. Mean body weights of dosed mice were as much as 25% greater than those of vehicle controls for females and as much as 13% greater for males. The survival of female rats given 25 mg/kg per day was lower than that of vehicle controls after day 550, but survival of female rats given 50 mg/kg per day was not different from that of vehicle controls (vehicle control, 35/50; low dose, 22/50; high dose, 33/50). No differences in survival were observed between any other groups of rats or mice of either sex (male rats: 28/50; 22/50; 27/50; male mice: 30/50; 20/50; 21/50; female mice: 41/50; 35/50; 33/50).
Nonneoplastic and Neoplastic Effects in the Two-Year Studies:
In rats, no biologically important nonneoplastic or neoplastic lesions were attributed to administration of N-methylolacrylamide. Higher doses might have increased the sensitivity of the studies to determine the presence or absence of a carcinogenic response.
In mice, the incidences of adenomas of the Harderian gland were increased in males given either dose of N-methylolacrylamide and in females given the top dose (male: vehicle control, 1/48; low dose, 14/49; high dose, 29/50; female: 5/47; 8/45; 20/48). The incidences of carcinomas of the Harderian gland were not significantly increased by N-methylolacrylamide administration (male: 1/48; 0/49; 2/50; female: 0/47; 3/45; 2/48).
The incidences of hepatocellular adenomas were increased in male and female mice given 50 mg/kg N-methylolacrylamide (male: 8/50; 4/50; 19/50; female: 3/50; 4/50; 17/49). The incidences of hepatocellular carcinomas were also marginally increased in dosed male mice (male: 6/50; 13/50; 12/50; female: 3/50; 3/50; 2/49). Hepatocellular adenomas and carcinomas (combined) occurred with positive trends, and the incidences in male and female mice receiving 50 mg/kg were increased compared with those in the vehicle controls (male: 12/50; 17/50; 26/50; female: 6/50; 7/50; 17/49).
Chronic inflammation and alveolar epithelial hyperplasia of the lung were observed at increased incidences in mice given N-methylolacrylamide. Sentinel mice were seropositive for Sendai virus at 18 months. The incidences of alveolar/bronchiolar adenomas (3/49; 6/50; 11/50) and carcinomas (2/49; 4/50; 10/50) were increased in male mice given 50 mg/kg. Alveolar/bronchiolar adenomas or carcinomas (combined) occurred with a positive trend in male mice (5/49; 10/50; 18/50). The incidence of alveolar/bronchiolar adenomas or carcinomas (combined) was increased in female mice given the top dose of 50 mg/kg (6/50; 8/50; 13/49).
Ovarian atrophy was observed at increased incidences in female mice receiving N-methylolacrylamide (3/50; 39/45; 38/47). The incidences of benign granulosa cell tumors were also increased in the dosed groups (0/50; 5/45; 5/47).
The incidence of adenomas of the pars distalis in high dose female mice was significantly lower than that in vehicle controls (13/49; 5/14; 4/43).
N-Methylolacrylamide was not mutagenic in S. typhimurium strains TA97, TA98, TA100, or TA1535 when tested with or without exogenous metabolic activation. N-Methylolacrylamide induced both sister chromatid exchanges (SCEs) and chromosomal aberrations in CHO cells with and without metabolic activation. No increase in micronucleated polychromatic erythrocytes (PCEs) was observed in the bone marrow of B6C3F1 mice after intraperitoneal injection of N-methylolacrylamide.
Under the conditions of these 2-year studies, there was no evidence of carcinogenic activity of N-methylolacrylamide for male or female F344/N rats receiving doses of 6 or 12 mg/kg per day by aqueous gavage. There was clear evidence of carcinogenic activity of N-methylolacrylamide for male B6C3F1 mice, based on increased incidences of neoplasms of the Harderian gland, liver, and lung. There was clear evidence of carcinogenic activity of N-methylolacrylamide for female B6C3F1 mice, based on increased incidences of neoplasms of the Harderian gland, liver, lung, and ovary.
In rats, because no biologically important toxic effects were attributed to N-methylolacrylamide administration, somewhat higher doses could have been used to increase the sensitivity of these studies for determining the presence or absence of a carcinogenic response. In female mice, ovarian atrophy was compound related.