NTP has developed a range of techniques and testing regimes to evaluate the potential of environmental and occupational substances to affect development and damage reproductive systems. Testing is focused on the following areas of study.

Prenatal Developmental Toxicity Studies

The prenatal developmental toxicity study (also known as embryo-fetal developmental study, teratology study, or Segment II study) is undertaken to identify substances that may pose a risk to the developing fetus if pregnant women are exposed. Regulatory agencies use the results of well-conducted animal studies to help set human exposure guidelines. The experimental protocols, in general, are outlined by EPA Health Effects Test Guidelines OPPTS 870.3700 and ICH S5(R2).

Prenatal developmental toxicity studies are typically carried out using rodent models. Substances are tested in pregnant animals for their potential to cause birth defects and other signs of toxicity during embryo-fetal development. If a substance induces an adverse effect in the offspring, a second species may be used to confirm this response. Pregnant females are usually exposed to the substance from implantation, that is, the attachment of the embryo in the uterus (day 6 of the embryo development in the rodent) to the day before birth. However, they may be exposed to the substance throughout pregnancy. This testing paradigm can also be used within a larger study to assess multigenerational responses. See Modified One-Generation study design for details.

Reproductive Studies

NTP has developed a range of techniques and testing regimes for evaluating the potential toxic effects of exposure to environmental and occupational substances on the reproductive system. These studies are carried out primarily using rodent models. Characterization of these potential effects on reproduction are now typically assessed using the Modified One-Generation study design.

Modified One-Generation Studies

Historically, NTP evaluated the potential for a substance to cause reproductive harm via a multigenerational reproduction experimental design. NTP has modified this study design to use fewer animals, while generating information on the effects of substances on prenatal development, postnatal development, and reproduction.

NTP's Modified One-Generation Reproductive study design employs pregnant animals with dosing beginning at implantation and continuing throughout gestation and lactation. At weaning, the offspring are administered the test substance at the same dose level as their respective mother and are subsequently assigned to a number of different cohorts including:

• a subchronic toxicity cohort – to evaluate target organ toxicity, pathology, clinical pathology on subchronic toxicity, that is, of moderate duration and not long-term.
• a teratology cohort – to evaluate prenatal development, that is, development before birth. Teratology specifically refers to the scientific study of congenital abnormalities and abnormal development of organs or tissues that impact function and vitality.
• a littering cohort – to evaluate breeding and littering to assess potential effects in the subsequent generation.
• a developmental neurotoxicity (DNT) cohort – to evaluate neurobehavioral end-points and neurohistopathology. Neurotoxicity refers to a form of toxicity when a substance effects the central or peripheral nervous system, and neurohistopathology is the study of changes caused by disease at the cellular level in neural tissues.

Studies Using Zebrafish Embryos

The small size and rapid development of the zebrafish make it a useful vertebrate model for assessing potential effects of substances on development in a mid- to high-throughput manner. However, deficits in several key areas hinder the broader adoption of the zebrafish model for toxicological screening, including consistency of experimental protocol elements; understanding of mechanisms of substance absorption, distribution, metabolism, and excretion in zebrafish; and consistency of informatics approaches used for classification of outcomes. NTP established the Systematic Evaluation of the Application of Zebrafish in Toxicology (SEAZIT) program to enable the broader adoption of zebrafish for toxicological screening.

• MOG Study Design Overview Figure
• Toxicology Specifications
• EPA Health Effects Test Guidelines OPPTS 870.3700