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and even such common nutrients as glucose or cholesterol, can also be toxic at high levels, ourattention will be focussed mainly on xenobiotics.We use the terms target organismand non-target organismto discuss the deliberate use of toxicsubstances. For example, we use antibiotics (antibiotic = "against life") in human and veterinarymedicine to kill the pathogenic bacteria which caused illness. Such drugs must be carefully testedto ensure that they will be effective (toxic) against the bacteria but not harm the human patient.The pathogenic bacterium is called the target organism, and the human patient is the non-targetorganism. Other examples:•Ivermectin (antihelminthic veterinary drug used to treat sheep for worms): the worms arethe target organism; the sheep and its rumen microflora are non-target organisms.•2,4-D (herbicide used to control broad-leafed weeds in cereal crops and (formerly) in thesuburban lawn): the weeds are the target organism, the grasses (and the human applicator)are examples of non-target organisms.•Deltamethrin (insecticide used to spray dairy barns for flies): the flies are the targetorganisms; the cows and the farm worker are non-target organism. In the case ofinsecticides used, for example, to spray apple trees for aphids (target organism), the agentshould have low toxicity towards the honey bees in the orchard, to the human applicator ofthe insecticide, and towards wildlife, especially aquatic life such as fish, if residues of thepesticide should be washed into streams.Knowledge of the biochemical mechanism of action of toxicants is very important in developing acompounds that are toxic to the target organism but not to non-target organisms, at least under theconditions under which it is supposed to be used. For example, herbicides that are designed tointerfere with photosynthesis often have low mammalian toxicity, since photosynthesis is abiochemical process that is absent in animals.All antibacterial, antiparasitic, and chemotherapeutic (anti-cancer) drugs are toxic agents; weexploit the selective or differential toxicity of the drug against the cells we wish to eliminate fromthe body. The therapeutic index is the ratio of the toxic dose to the therapeutic dose.
1-3Therapeutic Index =Lethal dose to the patientEffective doseThe larger the therapeutic index, the greater the margin of safety to the patient. One could toleratea small therapeutic index for a drug that treat life-threatening diseases such as cancer (incidentally,what are the target and non-target life forms in this example?), but not for an "over-the-counter"medication for relieving headache or allergy.Toxicological scrutiny of prospective therapeutic agents and chemicals used in commerce hasincreased considerably over the years, as is shown by the examples below.