Pharmacology: Distribution and Elimination Phase

Distribution Phase

• During the distribution phase, the fraction of drug in the tissue compartment increases up to a maximum in a given tissue, which may be greater or less than the plasma drug concentration.
• At maximum tissue concentrations, the rate of drug entry into the tissue equals the rate of drug exit from the tissue.
• This marks the achievement of distribution equilibrium, where the fraction of drug in the tissue compartment is in equilibrium with the fraction of drug in the central compartment.

Distribution Equilibrium

• During distribution equilibrium, drug concentrations in both the central and tissue compartments decline in parallel and more slowly compared to the distribution phase.
• The drug concentrations in both compartments decline at a slower rate during this phase.

Elimination Phase

• In the elimination phase, drug kinetics appear to follow a one-compartment model.
• Drug elimination is a first-order process described by β.
• The drug concentration in the tissue compartment represents the average drug concentration in a group of tissues, rather than a real anatomic tissue drug concentration.

Tissue Drug Concentrations

• In reality, drug concentrations may vary among different tissues and possibly within an individual tissue due to differences in the partitioning of drug into the tissues.