A Practical Approach, Second Edition=Ronald D. Ho.pdf

PERSPECTIVES ON THE DEVELOPMENTAL AND REPRODUCTIVE TOXICITY GUIDELINES 771For drugs where alterations in plasma kinetics are seen following repeated administration, thepotential for adverse effects on embryo-fetal development may not be fully evaluated in studiesaccording to 4.1.3. In such cases it may be desirable to extend the period of drug administrationto females in a 4.1.1 study to day 17. With sacrifice at term, both fertility and embryo-fetaldevelopment can be assessed.Note 12 (4.1.1) Premating treatmentThe design of the fertility study, especially the reduction in the premating period for males, is basedon evidence accumulated and re-appraisal of the basic research on the process of spermatogenesisthat originally prompted the demand for a prolonged premating treatment period. Compoundsinducing selective effects on male reproduction are rare; mating with females is an insensitivemeans of detecting effects on spermatogenesis; good pathological and histopathological examination(e.g., by employing Bouin’s fixation, paraffine embedding, transverse sections of 2-4 micronsfor testes, longitudinal sections for epididymides, PAS and haematoxylin staining) of the malereproductive organs provides a more sensitive and quicker means of detecting effects on spermatogenesis;compounds affecting spermatogenesis almost invariably affect post meiotic stages; thereis no conclusive example of a male reproductive toxicant the effects of which could be detectedonly by dosing males for 9-10 weeks and mating them with females.Information on potential effects on spermatogenesis can be derived from repeated dose toxicitystudies. This allows the investigations in the fertility study to be concentrated on other, moreimmediate, causes of effect. It is noted that the full sequence of spermatogenesis (including spermmaturation) in rats lasts 63 days. When the available evidence, or lack of it, suggests that the scopeof investigations in the fertility study should be increased, or extended from detection to characterization,appropriate studies should be designed to further characterise the effects.Note 13 (4.1.1, 4.1.2, 4.1.3) Number of animalsThere is very little scientific basis underlying specified group sizes in past and existing guidelinesnor in this one. The numbers specified are educated guesses governed by the maximum study sizethat can be managed without undue loss of overall study control. This is indicated by the fact thatthe more expensive the animal is to obtain or keep, the smaller the group size proposed. Ideally,at least the same group size should be required for all species and there is a case for using largergroup sizes for less frequently used species such as primates. It should also be made clear that thenumbers required depend on whether or not the group is expected to demonstrate an effect. For ahigh frequency effect few animals are required, to presume the absence of an effect the numberrequired varies according to the variable (endpoint) being considered, its prevalence in controlpopulations (rare or categorical events) or dispersion around the central tendency (continuous orsemi-continuous variables). See also Note 23.For all but the rarest events (such as malformations, abortions, total litter loss), evaluation ofbetween 16 to 20 litters for rodents and rabbits tends to provide a degree of consistency betweenstudies. Below 16 litters per evaluation, between study results become inconsistent, above 20–24litters per group consistency and precision is not greatly enhanced. These numbers relate toevaluation. If groups are subdivided for different evaluations the number of animals starting thestudy should be doubled. Similarly, in studies with 2 breeding generations, 16-20 litters would berequired for the final evaluation of the litters of the F1 generation. To allow for natural wastage,the starting group size of the F0 generation must be larger.Note 14 (4.1.1) MatingMating ratios: When both sexes are being dosed or are of equal consideration in separate male andfemale studies, the preferred mating ratio is 1:1 since this is the safest option in respect of obtaininggood pregnancy rates and avoiding incorrect analysis and interpretation of results.© 2006 by Taylor & Francis Group, LLC