Compartmental pharmacokinetics

Several models have been devised to facilitate understanding and prediction of drug pharmacokinetic behaviour. Of these, compartmental models are the most widely used. These models are comprised typically of one, two or three inter-connected compartments representing combinations of body fluids, tissues and organs that share characteristics relating to the rate (and sometimes the extent) to which drug equilibrates with the compartment and is lost from the compartment. The calculated volumes of the compartments, and the concentration-time profiles of drug present in the compartments, provide information about pharmacokinetic behaviour of that drug that is required in order to determine appropriate dosing regimens. While calculated compartmental volumes sometimes appear to equate to physiological volumes (for example, the plasma volume, or total body water volume), this is usually coincidental, and such comparisons are best avoided.

It is important to understand that compartments are not fixed entities. Whether one or more compartments should be used to describe a drug’s behaviour, and the identities of the tissues and organs that are included in each compartment, are dependent entirely upon the drug of interest. The concept of compartments simply provides a model framework to facilitate understanding and interpretation of a drug’s concentration-time profile after administration.