Unnecessary use of additional animals for determination of sexual maturation in the EOGRTS

The Extended-One-Generation Study [EOGRTS, OECD 443] is a study in which the toxic effects of test substances on reproduction (Cohort 1), neurodevelopment (Cohort 2), and development of the immune system (Cohort 3) in rats are evaluated. The latter two Cohorts are not always required according to the European Chemicals Agency (ECHA) based on data from previously performed toxicity studies. Although the Cohorts for developmental neurotoxicity (DNT) and developmental immunotoxicity (DIT) are often omitted, the F₁-animals normally required for these Cohorts are still maintained for evaluation of sexual maturation since three F₁-animals/sex/litter/group are required according to OECD Guidance Document (GD) No. 151. This review investigates whether two F₁-animals/sex/litter/group would suffice for this endpoint by investigating the rationale provided by the GD and by comparing results of eighteen EOGRTSs in which three versus two F₁-animals/sex/litter/group were evaluated. After a comprehensive literature research, we concluded that the rationale in the GD does not substantiate the decision to use three F₁-animals/sex/litter/group. The scientific papers provided as rationale focused on male observations and the observations discussed do not match the observations for sexual maturation mentioned by the guidelines. The investigation using data from eighteen EOGRTSs showed that the toxicological conclusions, whether the test substance affected sexual maturation or not, matched when comparing data of two F₁-animals/sex/litter/group to three F₁-animals/sex/litter/group. To conclude, two F₁-animals/sex/litter/group would suffice for the evaluation of sexual maturation, which negates the requirement for a so called "Cohort 1 C" (i.e. 160 animals (80 males and 80 females)) per EOGRTS, as well as the number of regulated procedures that need to be performed.