In a two-compartment pharmacokinetic model, what does the alpha (distribution) phase describe?

• A. Rapid distribution between central and peripheral compartments
• B. Slower distribution with elimination culminating in terminal elimination
• C. Absorption into the systemic circulation
• D. Equilibration of drug concentrations without distribution

Answer: (A)

The alpha distribution phase is the rapid exchange of the drug between the central (blood/plasma) and peripheral (tissue) compartments after an intravenous dose. This quick distribution causes an early, steep drop in plasma concentration as the drug moves from the blood into tissues. The alpha phase ends when the concentrations in the two compartments approach a quasi-equilibrium, after which the slower elimination from the body (the beta or terminal phase) dominates. The other possibilities don’t fit because absorption isn’t involved with IV bolus, the slower distribution with elimination describes the later beta phase, and equilibration without distribution contradicts the idea of drug movement between compartments.