Toxicology and carcinogenesis studies of erythromycin stearate (USP grade, greater than 96% pure) were conducted by administering the antibiotic in feed to groups of F344/N rats and B6C3F1 mice of each sex for 14 days, 13 weeks, or 2 years. Erythromycin stearate was studied because of its widespread use in humans as a broad-spectrum macrolide antibiotic and because of the lack of adequate long-term studies for carcinogenicity.
Fourteen-day and thirteen-week studies
In the 14-day studies, none of the rats (at dietary concentrations up to 50,000 ppm) and 2/5 female mice that received 50,000 ppm died before the end of the studies. Final mean body weights of male rats that received 12,500, 25,000, or 50,000 ppm were 10%, 30%, or 36% lower, respectively, than that of controls; final mean body weights of female rats were 10%, 12%, or 32% lower. None of the dosed mouse groups gained weight. The final mean body weight of male mice that received 50,000 ppm was 10% lower than that of controls.
In the 13-week studies, none of the rats or mice (at dietary concentrations up to 20,000 ppm) died before the end of the studies. Final mean body weights of the 20,000-ppm groups of rats were more than 12% lower than that of the controls for males and 7% lower for females. Final mean body weights of mice that received 10,000 or 20,000 ppm were 15% or 19% lower than that of controls for males and 5% or 14% lower for females.
Multinucleated syncytial hepatocytes were observed in 10/10 male rats that received 20,000 ppm but in 0/10 male rats that received 10,000 ppm. No compound-related gross or microscopic pathologic effects were observed in mice.
Based on these results, 2-year studies of erythromycin stearate were conducted by feeding diets containing 0, 5,000, or 10,000 ppm erythromycin stearate to groups of 50 rats of each sex for 103 weeks. Diets containing 0, 2,500, or 5,000 ppm were fed to groups of 50 mice of each sex for 103 weeks.
Two-year studies
Body weight and survival
Mean body weights of high dose male rats were comparable to those of controls throughout the studies. Mean body weights of high dose female rats were 5%-10% lower than those of controls. Mean body weights of dosed and control mice were comparable. The average daily feed consumption was similar for dosed and control male and female rats. For mice, estimated daily feed consumption by low and high dose males was similar to that of the controls and by low and high dose females was 92% that of the controls. The average amount of erythromycin stearate consumed per day was approximately 180 or 370 mg/kg for male rats and 210 or 435 mg/kg for female rats; for mice, the average amounts were 270 or 545 mg/kg for males and 250 or 500 mg/kg for females.
No significant differences in survival were observed between any groups of rats or mice of either sex (final survival-- male rats: control, 28/50; low dose, 23/50; high dose, 27/50; female rats: 29/50; 30/50; 38/50; male mice: 34/50; 33/50; 40/50; female mice: 38/50; 34/50; 40/50).
Nonneoplastic and neoplastic effects
Granulomas of the liver were observed at increased incidences in high dose rats (male: 1/50; 2/50; 10/50; female: 18/50; 27/50; 43/50). Granulomatous inflammation or granulomas of the spleen were observed in dosed female rats (0/48; 1/49; 3/50). Reticulum cell hyperplasia in the bone marrow occurred at increased incidences in high dose female rats (10/50; 14/50; 25/50).
Squamous cell papillomas of the oral mucosa were observed in 1/50 control, 2/50 low dose, and 3/50 high dose female rats. These tumors were considered to be marginal and not related to exposure. Hyperplasia of the oral mucosa was not observed.
Pheochromocytomas of the adrenal gland in female rats occurred with a positive trend (1/50; 4/49; 5/50). The incidences in the dosed groups are similar to the average historical incidence (9%) of this tumor in untreated control female F344/N rats at the study laboratory. This marginal tumor increase is not considered to be biologically important. No increases in incidences of neoplasms were observed at any site in dosed male rats.
Inflammation in the glandular stomach was observed at increased incidences in dosed male mice (1/49; 4/50; 6/50). Lymphoid hyperplasia in the urinary bladder was observed at increased incidences in dosed female mice (1/50; 9/47; 7/48).
No increases in incidences of neoplasms were observed at any site in dosed male or female mice.
Genetic toxicology
Erythromycin stearate was not mutagenic in Salmonella typhimurium strains TA98, TA100, TA1535, or TA1537 when tested both with or without exogenous metabolic activation. Erythromycin stearate demonstrated equivocal mutagenicity in the mouse L5178Y lymphoma cell assay in the absence of exogenous metabolic activation (S9); erythromycin stearate was not mutagenic in the presence of S9. Treatment of cultured Chinese hamster ovary cells with erythromycin stearate did not produce an increase in the frequency of sister chromatid exchanges or chromosomal aberrations in either the presence or absence of metabolic activation.
Data audit
The data, documents, and pathology materials from the 2-year studies of erythromycin stearate have been audited. The audit findings show that the conduct of the studies is documented adequately and support the data and results given in this Technical Report.
Conclusions
Under the conditions of these 2-year studies, there was no evidence of carcinogenic activity of erythromycin stearate for male or female F344/N rats administered erythromycin stearate in the diet at 5,000 or 10,000 ppm. There was no evidence of carcinogenic activity of erythromycin stearate for male or female B6C3F1 mice administered erythromycin stearate in the diet at 2,500 or 5,000 ppm. Dose-related increases in the incidences of granulomas of the liver were observed in male and female rats. The absence of any biologically important chemical-associated effects in mice suggests that higher doses could have been given to male and female mice.