Metabolism is the biotransformation process that drugs undergo in the body, usually as part of the strategy of elimination. A wide group of drug metabolizing enzymes are responsible for the biotransformation of drugs, mainly into more hydrophilic structures, ready for the excretion in urine or bile. Several organs can be involved in this process, the liver being the main one.

For the assessment of metabolic stability, new compounds are incubated in vitro in presence of liver subcellular fractions such as microsomes or in presence of hepatocytes. The results can be extrapolated to estimate the in vivo hepatic clearance.

Depending on the chemical structure, in the case of groups susceptible of hydrolysis, metabolic stability in plasma should also be checked.

Furthermore, the specific enzymes responsible for the metabolism of investigational drugs should be identified, as well as the chemical structure of the metabolites formed as a result of metabolizing enzymes action.