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

DEVELOPMENTAL TOXICITY TESTING — METHODOLOGY 227continuous for the entire dosing period ad libitum. For cutaneous application and inhalationexposures, the daily duration is usually 6 to 8 h (corresponding to a human workday), althoughsome studies have employed exposures of 22 h/d, with 2 h/d for housekeeping and allowing theanimals to eat and drink unencumbered.The termination of exposures was previously specified as the end of major organogenesis. It issignaled by the closure of the secondary palate and the change in designation of the conceptusfrom embryo to fetus. The cessation of exposure prior to term allowed for a postexposure recoveryperiod for both the dam and the fetuses, and an assessment could be made regarding whether theobserved maternal effects (body weights, clinical observations) are transient or permanent. However,fetal evaluations take place at term, and there is no commonly employed way to detect early adverseeffects on the conceptus that resolve (are repaired, compensated for, or result in in utero demise)earlier in gestation. What was observed at term was the net result of the original insult and anyrepair or compensation that occurred subsequently in the conceptus, as well as any observableeffects on the dam after a postexposure period. Thus, no detailed information on the dam wasobtained during the exposure period, and there was no way to distinguish effects during exposurefrom those occurring afterward. The new developmental toxicity testing guidelines require exposureuntil term, which includes the fetogenesis period.The consequences to exposures continuing until term are as follows:• There is no postdosing recovery period so the outcome at term is not confounded by insult duringdosing and compensation in the postdosing period. Example: There is no increase in maternal feedconsumption or weight gain in postdosing period and no obscuring of effects on fetal body weight(since most fetal body weight gain is in the “last trimester”). There may be increased in utero fetaldeath due to continued direct or indirect toxic insult.• The NOAEL may allow a lower dose (than if the dosing ended on GD 15), which may be a moreappropriate value.• The incidence of fetal malformation may be underestimated because more malformed fetuses maydie with prolonged maternal dosing.• Dams will require more test material as they gain weight in the “last trimester” (to maintainconstant dose in milligrams per kilogram per day), so maternal and/or fetal toxicity may be greaterat a given dose (i.e., the maternal liver will have to metabolize more test material).• Fetal malformation rates will probably not increase because they are usually induced during theperiod of major organogenesis (GD 6 through 15), common to both dosing regimens.• Maternal assessments at scheduled necropsy on GD 20 (e.g., toxicokinetic, biochemical, physiologicalendpoints) reflect effects of continued dosing (not after a postdosing recovery period). Asatellite group of dams to be sacrificed at the end of dosing under the shorter dosing period forsuch assessments is, therefore, not necessary (this saves animals, time, and money).• More test material is necessary (more dosing days and dams are heavier in the “last trimester,” somore test material is administered to maintain a constant dose in milligrams per kilogram per day).• The costs to perform a Phase II study with prolonged dosing are increased because more testchemical is required and there is increased labor to dose the dams on the additional days.The rationale for continuing exposure until term includes:• Maternal exposure until term is a better model for human exposure than exposure only during aportion of gestation, with abrupt cessation at the end of embryogenesis.• Continued maternal responses at term (e.g., changes in organ weights, hematology, clinical chemistry,histopathology) can be better interpreted in terms of causality; there is no maternal postexposureperiod for compensatory changes to occur.• Many systems continue to develop in the fetal period, both in terms of increases in cell size andthe number and differentiation of specialized cells, tissues, and organs (e.g., central nervous,pulmonary, renal, gastrointestinal systems, etc.). The effects on these processes occurring in thepresence of continual maternal exposure will be manifested at term (for most of the systems) andwill not be confounded by compensatory processes that may occur in a postexposure period.© 2006 by Taylor & Francis Group, LLC