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Metabolism: The Third Stage of Pharmacokinetics

Updated: Oct 24, 2022

Metabolism is the third stage of pharmacokinetics and the process where drug particles in the circulatory system can experience chemical changes within the liver or various tissues. Biotransformation is the chemical adjustments of a compound that happens inside the body, I.e., metabolism. The liver is utilized to clean toxins from the body and is the site of drug metabolism, with drugs undergoing two forms of metabolic processes inside the liver.

drugs metabolic processes inside the body

In the first form of metabolism, drugs are created more polar via hydrolysis or oxidation-reduction reactions. Metabolic enzymes, mainly those from the cytochrome P450 enzyme system, are utilized to catalyze the biotransformation in the reactions. For enzyme-catalyzed reactions, the rate of the chemical reaction is increased via enzymes. A small number of enzymes are nearby at any point within the liver.

The rate of enzyme-catalyzed drug metabolism is restricted by the number of accessible enzymes. Metabolism in these instances is called a saturable process. This represents the rate of conversion will remain at a steady pace until the remaining quantity of enzymes is used. In this instance, metabolism is reduced until the enzymes are available again. Normal doses of drugs cannot arrive at the saturation point of the enzymes for their reactions. There are some drugs where doses achieve the saturation point of enzymes. When enzymes are saturated, blood levels greatly increase in toxicity.

drug metabolism phases

The second form of metabolism includes conjugation reactions. For these reactions, the drug experiencing change is affiliated with a different substance. Conjugation yields a more water-soluble compound that is effortless for the kidneys to eliminate. These formed metabolites are usually therapeutically inactive. Certain agents are conducted as inactive compounds or prodrugs to improve availability or lessen adverse effects. Metabolism changes

the prodrug into its active drug.

Influences on Metabolism

Variation in the metabolism of drugs exists amongst population groups. The inadequacy of certain drug metabolizing enzymes is genetic and yields deficient tolerance of certain drugs. Moreso, this populace can have detrimental drug reactions at a greater rate than the normal population.

Age is another factor that influences metabolism. Organ performance progressively reduces with age, and the elderly may defectively tolerate drugs that necessitate metabolism. Those near infancy need unique attention to drug dosing due to the immaturity of their organ systems.

decoration picture of drug processing by the body

Drug interactions might happen amongst drugs metabolized by similar enzyme systems in the liver. Due to the limit on accessible enzymes for metabolism, surplus drugs will stay active and free to influence other body areas. It is common for two drugs to be metabolized by the same enzyme system. One has a greater affinity for the enzyme, and levels of the other drug increase. In certain instances, the drug being metabolized will cause the formation of more enzymes. Enzyme induction sets up another type of drug interaction due to the elevated production of metabolizing enzymes may yield greater rates of removal and the requirement for a greater dose of the second drug.


Metabolism in pharmacokinetics is the chemical changes of a drug in the body.

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