The body reacts to the drugs by transforming them in the liver to eliminate them, through the process of biotransformation.
The term "biotransformation" refers to the various chemical changes that drugs undergo in the body to give rise to metabolites. The biotransformations are mainly carried out by enzymatic reaction. A drug can undergo several biotransformations resulting in the formation of several metabolites.
The main function of biotransformations is to make water-soluble lipophilic molecules to promote the elimination of the body: indeed, the lipophilic molecules pass the membranes during the absorption and distribution phases, but conversely their liposolubility does not allow their renal elimination in unchanged form. They will either be excreted directly via the bile ducts or biotransformed before renal or biliary excretion. Some drugs do not undergo biotransformations: they are said to be eliminated from the body in unchanged form.
In general, biotransformations are defense reactions of the body that lead to less toxic and less active molecules than the parent molecule. Nevertheless, the metabolites may also be more active or more toxic than the drug administered. When the active ingredient is inactive and its metabolite is active, the drug is called "prodrug". For example, valganciclovir is a prodrug of ganciclovir.
The biotransformations of the drugs are essentially hepatic and intestinal even if there is also a pulmonary, renal or plasma metabolism. There are two types of biotransformations, classified in Phase I and Phase II (functionalization reaction and conjugation reaction).