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Toxicodynamics
Toxicodynamics, termed pharmacodynamics in pharmacology, describes the dynamic interactions of a toxicant with a biological target and its biological effects.〔Boelsterli, 2003〕 A biological target, also known as the site of action, can be binding proteins, ion channels, DNA, or a variety of other receptors. When a toxicant enters an organism, it can interact with these receptors and produce structural or functional alterations. The mechanism of action of the toxicant, as determined by a toxicant’s chemical properties, will determine what receptors are targeted and the overall toxic effect at the cellular level and organismal level.
Toxicants have been grouped together according to their chemical properties by way of quantitative structure-activity relationships (QSARs), which allows prediction of toxic action based on these properties. endocrine disrupting chemicals (EDCs) and carcinogens are examples of classes of toxicants that can act as QSARs. EDCs mimic or block transcriptional activation normally caused by natural steroid hormones. These types of chemicals can act on androgen receptors, estrogen receptors and thyroid hormone receptors. This mechanism can include such toxicants as dichlorodiphenyltrichloroethane (DDE) and polychlorinated biphenyls (PCBs). Another class of chemicals, carcinogens, are substances that cause cancer and can be classified as genotoxic or nongenotoxic carcinogens. These categories include toxicants such as polycyclic aromatic hydrocarbon (PAHs) and carbon tetrachloride (CCl4).
The process of toxicodynamics can be useful for application in environmental risk assessment by implementing toxicokinetic-toxicodynamic (TKTD) models. TKTD models include phenomenas such as time-varying exposure, carry-over toxicity, organism recovery time, effects of mixtures, and extrapolation to untested chemicals and species. Due to their advantages, these types of models may be more applicable for risk assessment than traditional modeling approaches.
== Overview ==
While toxicokinetics describes the changes in the concentrations of a toxicant over time due to the uptake, biotransformation, distribution and elimination of toxicants, toxicodynamics involves the interactions of a toxicant with a biological target and the functional or structural alterations in a cell that can eventually lead to a toxic effect. Depending on the toxicant’s chemical reactivity and vicinity, the toxicant may be able to interact with the biological target. Interactions between a toxicant and the biological target may also be more specific, where high-affinity binding sites increase the selectivity of interactions. For this reason, toxicity may be expressed primarily in certain tissues or organs. The targets are often receptors on the cell surface or in the cytoplasm and nucleus. Toxicants can either induce an unnecessary response or inhibit a natural response, which can cause damage. If the biological target is critical and the damage is severe enough, irreversible injury can occur first at the molecular level, which will translate into effects at higher levels of organization.〔
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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