Assessment of male rodent fertility in general toxicology 6-month studies.

An outcome and statistical review of male reproductive performance assessed by including a mating phase within 6-month general toxicity studies in the Han Wistar rat was undertaken. The basic study design was 1620 animals per group dosed for approximately 9 weeks before pairing the male rats with undosed females. This design provides opportunity for remating and automatically includes general toxicity parameters. The dose levels used in the 1- and 6-month studies show that male reproduction was assessed at generally similar doses. The majority of males (compound-dosed and controls) mated within 7 days. All vehicle-dosed males mated and 98.5% of these females were pregnant. Modeling shows that a pregnancy rate of less than 14 out of 16 pregnant animals is very unlikely to occur due to biological variability. Power calculations based on vehicle control data show that group sizes of >10 males have a >80% power of detecting a decrease in median of three embryos per group compared with the control group. Even if the number of pregnancies decreased by a third, a group size of =12 would still detect a decrement in the median of three embryos with >80% power. Based on the statistical modeling and inherent strengths of the study design, this review indicates that decrements in male reproductive function can be successfully detected by incorporating a mating phase into a 6-month rat study and that a group size of 1216 is generally adequate rather than the 1620 group size indicated as a generic default within ICHS5(R2).