Effects of corn oil, time-related changes, and inter-laboratory variability on tumor occurrence in control Fischer 344 (F344/N) rats

Survival, body weight, and site-specific tumor rates in untreated, corn oil gavage, and water gavage control Fischer 344 (F344/N) rats from 88 National Toxicology Program (NTP) long term carcinogenicity studies were evaluated to determine which factors were primarily responsible for inter-study variability. For male rats, previously-reported decreases in leukemia and increases in body weight, survival, and pancreatic acinar cell tumors attributable to corn oil gavage were confirmed. Corn oil did not appear to affect tumor rates in female rats. The gavage technique per se did not appear to influence tumor rates in rats of either sex. Previously reported time-related increases in certain site-specific neoplasia in control rats appeared to have stabilized in recent years, but control tumor rates are still much greater than those seen a decade ago. More recent studies continue to show increasing rates of leukemia and mammary gland tumors and decreasing survival. Female rats also continue to show time-related increases in maximum mean body weight. Inter-laboratory variability in body weight and in the rates of a number of site-specific neoplasms were also significant. High mean body weights in control groups were found to be associated with increased rates of mammary and pituitary tumors. Our evaluation supports the view that if historical control data are to be utilized in the interpretation of experimental results, primary emphasis should be given to lab and route of administration-specific tumor rates for studies that are contemporary to the study under evaluation. It also suggests that certain experimental design changes (e.g., dietary modifications) may be needed to reduce tumor rates and to increase survival.