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

286 DEVELOPMENTAL REPRODUCTIVE TOXICOLOGY: A PRACTICAL APPROACH, SECOND EDITIONThe availability of sufficient litters to produce the number of animals required for the study maybe a limiting factor, and overselection from a small number of litters to yield the desired number ofanimals per group may compromise the study (a discussion of litter-based selection will follow). Inmore focused study designs, smaller numbers of animals may be adequate to accurately assess thetoxic potential of the test article, as demonstrated in the ramipril study described previously. 92 Inreproductive and developmental toxicity studies adapted to address specific concerns in the juvenile,the availability of an adequate sample size is of less concern because these studies are typicallyconducted in the rodent, with approximately 20 to 30 litters per group available for analysis. Selectionof a subset of animals from these litters for juvenile testing is quite feasible in the adapted adult studydesigns, and the researcher is still capable of fulfilling the basic requirements of the standard study.A critical issue that must be addressed when determining the appropriate number of animalsfor use in a study is the effect of the litter on the response to test article exposure. It was demonstratedin the Collaborative Behavioral Teratology Study (CBTS) that offspring from the same litter respondmore similarly to a test article than do nonlittermates. 105 This response is known as the litter effect.Therefore, guidelines for developmental toxicity studies require that the litter be the experimentalunit for statistical and biological analysis. 54,106–110 Unfortunately, many researchers fail to carry thisrequirement into the postweaning period. Instead, littermates are treated as though they are notsiblings. Determination of the appropriate number of animals for use for a study should take intoaccount the number of litters available for selection. Attempts must be made to select animals fromas many litters as possible in order to create the required sample size for each group. This can beaccomplished by conducting within-litter selections, fostering, or selecting only one animal per sexper litter (between-litter selection) for analysis. Between-litter designs should only be consideredwhen culling is planned, when data from siblings are used to obtain a litter mean, or when siblingsare selected for different endpoints of analysis. The issue of litter-based selection is addressed inmore detail later in the chapter.C. ExposureWhen designing a juvenile toxicity study, the proper exposure paradigm must be determined. Thisinvolves selection of an acceptable route of administration, consideration of the frequency andduration of exposure and the timing of dose administration, and appropriate dose level selection.When feasible, the route of exposure should match the expected exposure scenario for the pediatricpopulation.1. Route of AdministrationChoosing an acceptable route of administration may seem an easy task for most test articles, inthat the most likely route of clinical administration or exposure to the human population wouldalready have been determined. However, there are many more challenges when the route ofadministration is selected for a juvenile toxicity study. The first issue that must be considered isthe feasibility by which the test article may be administered via a particular route. Although thecommon route of administration in preweaning animals is by gavage, oral intubation can betechnically challenging at some ages (Figure 8.10 and Figure 8.11). For example, in a PND 4mouse pup, the technical challenges involved in administering the test article can be extreme. Whenthe test article must be administered as a viscous suspension through a small gauge cannula, thesechallenges may be impossible to overcome. While a rat pup is several times larger than a mousepup of the same age (at PND 1, rats weigh approximately 6 g whereas mice weigh approximately2 g), the size of the animal and the susceptibility of the esophagus to perforation early in postnatallife cannot be ignored. The staff involved in administering doses to these young animals must bethoroughly trained; never assume that an individual who is capable of dosing an adult animal willbe able to dose a neonate of the same species. At the authors’ laboratory, staff members are required© 2006 by Taylor & Francis Group, LLC