Variance of body and organ weights in 28-day studies in mice

The OECD guideline 407 outlines the conduct of 28-day studies in rodents to detect systemic toxicity with focus on endocrine and immunotoxic effects. It was validated with the rat as preferred model species. Justification is required for other rodent species, as an increased variability is expected compared to the rat. We investigated the variability of organ weights in the mouse and compared this to data published for the rat in the validation report of test guideline 407. Furthermore, the influence of the immunotoxic model substance cyclophosphamide on spleen and thymus weights in the mouse in immunotoxicity studies (duration 28 days) is reported and discussed, an immunotoxic model substance was not included in the validation report. Historical control data were compiled for mouse studies performed according to OECD 407 and for immunotoxicity studies between 2008 and 2013 at BASF SE. For absolute weights, the coefficient of variation was determined for each study group and compared with the rat. Adrenal glands, ovaries and to lesser degree testes and prostate showed higher coefficients of variation in the mouse (most pronounced in adrenal glands in male animals: rat 5%-17%, CD1 mouse 20%-51%). Effects of cyclophosphamide were best detected measuring the thymus weight.