One-Compartment Open Model for Extravascular Administration: Zero-Order Absorption Model

Extravascular drug administration occurs via oral, intramuscular, or rectal routes.

It relies on absorption for the drug's therapeutic activity.

Here, the rate of absorption may be of two types: zero-order and first-order.

In zero-order absorption, the absorption rate remains constant over time independent of the amount remaining to be absorbed, unlike first-order absorption.

Now, the given equation expresses the rate of change in the amount of drug in the body post-administration.

For drugs following one-compartment kinetics, the plasma drug concentration-time profile is defined by absorption, post-absorption, and elimination phases.

During the absorption phase, the absorption rate exceeds the elimination rate. At peak plasma concentration, the rate of absorption equals its elimination rate.

During the post-absorption phase, the elimination rate exceeds the absorption rate.

Upon complete drug absorption, its rate becomes zero, defining the elimination phase in the plasma drug concentration-time curve.