Metal elements are present in all living organisms and play different roles in them. Metal intoxication may occur due to essential metal overload or exposure to heavy metals. It is affected by factors, such as dose or concentration of the substance, time and route of exposure, chemical and physical form of the metal. Common method of treatment is chelation therapy, in which various chelating agents are used. Chelating agents are ligands that form a complex with a metal cation, called chelate (chelate complex). The formation or presence of two or more coordinate bonds of the same ligand with one central atom is called chelation. This reaction happens when the chemical affinity of the chelating agent for a metal ion is higher than that of similar nonchelating (monodentate) ligands for the same metal ion. Chelation is an equilibrium reaction which is affected by temperature, pH, ionic strength and the presence of other ions. The equilibrium constant is in this case called the stability constant K and is a measure of affinity of the chelating agent for a particular metal. Chelating agents, used in chelating therapy, are (among others) dimercaprol, DMSA, DMPS, D-penicillamine, EDTA, CaNa2EDTA, DTPA, deferoxamine, deferiprone, deferasirox and nitrilotriacetic acid.
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