Workshop on risk assessment in reproductive and developmental toxicology: addressing the assumptions and identifying the research needs

Biomarkers of adult and developmental neurotoxicity

Neurotoxicity may be defined as any adverse effect on the structure or function of the central and/or peripheral nervous system by a biological, chemical, or physical agent. A multidisciplinary approach is necessary to assess adult and developmental neurotoxicity due to the complex and diverse functions of the nervous system. The overall strategy for understanding developmental neurotoxicity is based on two assumptions: (1) significant differences in the adult versus the developing nervous system susceptibility to neurotoxicity exist and they are often developmental stage dependent; (2) a multidisciplinary approach using neurobiological, including gene expression assays, neurophysiological, neuropathological, and behavioral function is necessary for a precise assessment of neurotoxicity. Application of genomic approaches to developmental studies must use the same criteria for evaluating microarray studies as those in adults including consideration of reproducibility, statistical analysis, homogenous cell populations, and confirmation with non-array methods. A study using amphetamine to induce neurotoxicity supports the following: (1) gene expression data can help define neurotoxic mechanism(s), (2) gene expression changes can be useful biomarkers of effect, and (3) the site-selective nature of gene expression in the nervous system may mandate assessment of selective cell populations.

Is there a no-effect dose for corticosteroid-induced cleft palate? The contribution of endogenous corticosterone to the incidence of cleft palate in mice

Teratology and genetic counselors are frequently asked whether very low exposures of drugs and chemicals can cause a child's congenital malformations. One critical factor on which the counseling is based is the dose. Because teratogenic effects follow a toxicologic dose-response curve with a no-effect dose, frequently counselors can refute a causal relationship because the dose was far below the no-observable-effect dose. Recently, some investigators have suggested that some teratogens which are present in physiologic levels such as cortisone, glucose, insulin, or sex steroids may contribute to the background incidence of congenital malformations and, therefore, there is no safe dose. Using corticosteroid-induced cleft palate in mice as the model, we conducted experiments to test this hypothesis. Adrenalectomy of A/J or CD-1 dams resulted in a reduction of endogenous corticosterone, but did not reduce the spontaneous incidence of cleft palate in the offspring. In A/J mice, the incidence of isolated cleft palate increased with adrenalectomy indicating that the spontaneous incidence of this defect is not due to endogenous corticosterone. Adrenalectomy did not affect the susceptibility of CD-1 mice to cortisone induced cleft palate demonstrating that endogenous corticosterone did not contribute significantly to the incidence of cleft palate induced by the exogenous corticosteroid. Finally, results in CD-1 mice clearly indicate that cortisone, like other teratogens, has a no-effect level for teratogenesis. These studies support the concept of a threshold in the dose-response relationship for corticosteroid-induced cleft palate in mice.

Risk assessment strategies for neuroprotective agents

Neurotoxicity may be defined as any adverse effect on the structure or function of the central and/or peripheral nervous system by a biological, chemical, or physical agent. Neurotoxic effects may be permanent or reversible, produced by neuropharmacological or neurodegenerative properties of a neurotoxicant, or the result of direct or indirect actions on the nervous system. A multidisciplinary approach is necessary to assess neurotoxicity because of the complexity and diverse functions of the nervous system. Many of the relevant effects can be measured directly by neurochemical, neurophysiological, and neuropathological techniques, whereas, others must be inferred from observed behavior. Some neurotoxicological data can be derived directly from humans. Neurotoxicity in humans is most commonly measured by relatively noninvasive neurophysiologic and neurobehavioral methods that assess cognitive, affective, sensory, and motor function. For most toxicological assessments, however, it is necessary to rely on information derived from animal models. There are many approaches that can be used to assess neurotoxicity, including whole animal (in vivo) and tissue/cell culture (in vitro) testing. Neurotoxicity can be described at multiple levels of organization, including neurochemical, anatomical, physiological, and behavioral. An important aspect of neurotoxic endpoint evaluation involves risk assessment procedures. Risk assessment may be defined as an empirically-based process used to determine the probability that adverse or abnormal effects are associated with exposure to a chemical, physical or biological agent. Risk management, on the other hand, is the process that applies information obtained through the risk assessment process to determine whether the assessed risk should be reduced and, if so, to what extent. For chemicals such as neuroprotective agents and other drugs designed to provide therapeutic benefits, information concerning these benefits is considered during the risk management phase. The risk assessment process usually involves four steps: hazard identification, dose-response assessment, exposure assessment, and risk characterization. Neurotoxicity risk assessment models of the future may well include biomarkers of both effect and exposure as well as biologically-based mechanistic and pharmacokinetic considerations derived from both epidemiologic and experimental data.