Drug metabolism is an important process in the action of a drug. The drug compound must be metabolised in order for it to be excreted. Cytochrome P450 isozymes are used for this purpose, with about 50 known in the human body. These oxidise the xenobiotic drug compound, inserting oxygens onto the target molecule, marking it for the second stage.
The second stage of drug metabolism is conjugation. This involves adding functional groups to the drug molecule, marking it for excretion.
This process can be exploited, administering a prodrug that is then acted upon by the P450 isozymes to produce a more active molecule. The speed at which the P450 isozymes metabolise the drug molecule is important – fast metabolism may require a higher or more frequent dosing regimen. Exceptionally slow metabolism may need accelerating to prevent toxic effects from occurring.
Some drug molecules may be converted to toxic metabolites. Paracetamol is known to undergo this, with high doses depleting the liver’s stores of glutathione. This can have fatal effects, and so doses must be carefully monitored and restricted.