The effect of study type on body weight and tumor incidence in B6C3F1 mice fed the NTP-2000 diet

The B6C3F1 mouse is the standard mouse strain used in National Toxicology Program (NTP) carcinogenesis studies. Over time, increased liver tumorigenesis that was correlated with elevated body weights was noted in males and females. NTP therefore replaced the NIH-07 diet with the NTP-2000 diet and returned to group housing of females as lower body weights were noted in group housed mice. However, recent studies reported study-type differences in body weights at 3 months using the NTP-2000 diet with higher weights evident in drinking water and inhalation studies compared to feed studies. Therefore, body weight and tumor incidence data were collected for untreated control mice from all 2-year NTP feed (12), drinking water (8), water gavage (6) and inhalation (10) studies that used the NTP-2000 diet in order to assess the impact of study type on body weights and tumor incidences. Results show statistically significant elevated body weights and liver tumor incidences in males and females from drinking water, water gavage and inhalation studies compared to results from feed studies. Thus, the elevated body weights and liver tumorigenesis noted in mice using the NIH-07 diet were also evident using the NTP-2000 diet, which was introduced to address body weight elevations. Given the study-type dependent effects noted, these results emphasize the importance of carefully selecting historical control data for B6C3F1 mice. Moreover, because of the association between body weight and liver tumorigenesis, these results may have implications regarding dose-level selection for carcinogenicity studies involving B6C3F1 mice based on the maximum tolerated dose.