The concept of a spare receptor

The higher the therapeutic index, the safer the drug the lower the therapeutic index, the greater the possibility of toxicity. The therapeutic index for barbiturate as a class is 10, whereas the therapeutic index for cardiac glycoside as a class is 3. Because the usual therapeutic dose of cardiac glycoside is 1 mg, death may result if only 3 mg has been administered. 

The therapeutic index deals with the ratio of lethal doses to 50% of the population (LDjq) over the median minimum effective dose (ED50). 

By far the major portion of biotransformation is carried out in the liver by cytochrome P-450 (P 50), which is a collective term for a group of related enzymes or isoenzymes that are responsible for the oxidation of numerous drugs endogenous substances such as fatty acids, prostaglandins, steroids, and ketones and carcinogens such as polycychc aromatic hydrocarbons, nitrosamines, hydrazines, and arylamines. 

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