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Drug metabolism is the body’s way of transforming drugs, so they can be excreted from the body. Many drugs are not active until they have been metabolized in the body by enzymes that transform them. Most drugs are lipophilic, meaning they pass through membranes to reach their target site. Generally, they cannot be excreted in urine by the kidneys until they have had functional groups introduced into them that make them soluble in water. Most of drug metabolism takes place in the liver, but all tissues are capable of carrying out these reactions to some degree.
Most drugs are treated by the body like foreign substances, also known as xenobiotics. Humans have evolved a complex system for xenobiotic metabolism. It is thought that exposure to the many toxic compounds in plants has facilitated the development of enzyme systems to detoxify foreign substances. Occasionally, though, the enzymes make an intermediate more toxic than the parent compound. This can happen with some of the compounds in cigarette smoke.
The metabolism of drugs varies based on many factors, including sex, age, and even the bacteria which live in the intestinal tract. Genetic variation can greatly affect the metabolism of some drugs. For instance, there are poor metabolizers of codeine, and people who metabolize it very quickly. This can affect the dosage of the drug. People who metabolize it poorly may be prone to overdose even when taking a low dose, while extensive metabolizers may need a higher dosage.
There are genetic tests called drug reaction testing that checks for specific liver enzymes that affect drug metabolism. This allows doctors to tailor the dosage of particular drugs to an individual, based on the person’s drug metabolism. For instance, this type of test can predict a person’s ability to metabolize codeine.
One of the problems with taking drugs orally is that some can be excreted from the body without even being metabolized. To avoid this, scientists have utilized drug design to develop prodrugs. These are drugs that are initially less active or inactive, but once in the body are metabolized into an active metabolite. Also, they can be targeted to certain types of cells, remaining inactive until they reach these cells. For example, by targeting specific types of cancer cells, patients can avoid some of the side effects of chemotherapy.
Drug metabolism usually consists of two phases. Phase 1 involves introducing a functional group onto the drug that makes it water-soluble. Frequently, this reaction is mediated by a cytochrome P450 enzyme that introduces an atom of oxygen onto the drug. This usually results in an OH group being added to the molecule. Humans have over 50 different cytochrome P450s, with many differing specificities.
Phase 2 metabolism consists of adding a compound that will allow the intermediate to be excreted by the kidneys. This step is called conjugation. Often, it involves adding glucuronic acid or sulfate to the molecule. This increases its solubility in water, so that it can be excreted from the body.
The metabolism of one drug can frequently cause an interaction with another. The presence of a drug can induce a greater concentration of particular cytochrome P450s that can then metabolize other drugs in the body. This leads to lowered concentrations of the other drugs. Another possibility is that the drug may directly inhibit the cytochrome P450-mediated metabolism of an alternate drug, leading to excessive levels in the body.
Natural compounds may also have such effects. A classic example is grapefruit, which contains a compound that inhibits the metabolism of many drugs. Many people who take prescription drugs, especially statins, avoid consuming grapefruit or its juice for this reason. The herbal remedy Saint John’s wort also inhibits a number of cytochrome P450s.