The Process of Drug Metabolism

The term drug metabolism describes the process by which a drug is broken down in the body by the metabolism and then used around the body for various functions and eventually excreted. This is a term used in bioanalysis – which is the process of detecting substances in solutions and samples as well as measuring their quantities. Bioanalysis is used in order to observe drug metabolism by testing for the presence of the drug and using biomarkers to observe its progress. In vitro in vivo drug metabolism means 'in glass in body drug metabolism' which means the process is tested in the test tube and in the blood of a subject.


Drug metabolism is linked closely to pharmacokinetics and often they are both described as DM/PK (drug metabolism/pharmacokinetics). Pharmacokinetics is the process of measuring the effect that the body has on a drug, rather than vice versa, following its progress through the body as it is broken down, absorbed, distributed and eventually expelled one way or other (in most cases).


Drug metabolism will generally take place in the liver. This is the organ most responsible for the breaking down and utilization of various substances and plays an important role in digestion, detoxification and the elimination of substances in the body – including toxins. Enzymes in the liver serve as catalysts for chemical reactions which change the drugs into other substances which are called 'metabolites'. These are then bound to other materials and excreted through the lungs, the pores or elsewhere as sweat, urine, saliva etc; or are reabsorbed via the intestines.


The family of liver isoenzymes known as cytochrome P-450 are critical for drug metabolism also. These CYP-450 enzymes (CYP1A2, CYP2C9 etc) have catabolic reactions to substances thereby breaking them down into metabolites. As a result of this they also act to lower concentrations of medications in the bloodstream. As such CYP-450 inhibition CYP-450 induction processes are both in bionalysis as ways to control the amount of the substance in the blood stream. Drug interactions can occur when one drug inhibits or induces a CYP-450 enzyme. Individuals with liver disease may have low levels of CYP-450 enzymes and thus the CYP-450 inhibition CYP-450 induction processes are an important area of study for these conditions. At the same time certain foods can also play a role in the CYP-450 inhibition CYP-450 induction actions. For instance grapefruit juice has been shown to have an effect. Drug dosages must account for individual levels of isoenzymes and of diet and lifestyle.


Metabolic rates vary greatly from person to person, and while one drug might be rapid acting in one individual, it may not work well, or may not work at all for others. Age, nutrition, environment and more all affect a person's metabolism. Infants and the elderly for example have a reduced ability to metabolize various drugs and so require different dosages. It is important that patients be entirely open and honest with their physicians regarding their diet, lifestyle and any other courses of medication.


In vitro in vivo drug metabolism looks at the break down and use of drugs in the test tube and in the bottle. For more on DM/PK, click the links.