and even such common nutrients as glucose or cholesterol can also be toxic at

And even such common nutrients as glucose or

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and even such common nutrients as glucose or cholesterol, can also be toxic at high levels, our attention will be focussed mainly on xenobiotics. We use the terms target organism and non-target organism to discuss the deliberate use of toxic substances. For example, we use antibiotics (antibiotic = "against life") in human and veterinary medicine to kill the pathogenic bacteria which caused illness. Such drugs must be carefully tested to 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-target organism. Other examples : Ivermectin (antihelminthic veterinary drug used to treat sheep for worms): the worms are the 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 the suburban 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 target organisms; the cows and the farm worker are non-target organism. In the case of insecticides used, for example, to spray apple trees for aphids (target organism), the agent should have low toxicity towards the honey bees in the orchard, to the human applicator of the insecticide, and towards wildlife, especially aquatic life such as fish, if residues of the pesticide should be washed into streams. Knowledge of the biochemical mechanism of action of toxicants is very important in developing a compounds that are toxic to the target organism but not to non-target organisms, at least under the conditions under which it is supposed to be used. For example, herbicides that are designed to interfere with photosynthesis often have low mammalian toxicity, since photosynthesis is a biochemical process that is absent in animals. All antibacterial, antiparasitic, and chemotherapeutic (anti-cancer) drugs are toxic agents; we exploit the selective or differential toxicity of the drug against the cells we wish to eliminate from the body. The therapeutic index is the ratio of the toxic dose to the therapeutic dose.
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1-3 Therapeutic Index = Lethal dose to the patient Effective dose The larger the therapeutic index, the greater the margin of safety to the patient. One could tolerate a 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 has increased considerably over the years, as is shown by the examples below.
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  • Spring '14
  • Toxicology, adverse effects, toxic substances, Toxicity

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