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

Zero-order absorption is a constant-rate drug absorption process where the absorption rate remains steady over time, independent of the amount of drug remaining to be absorbed. This differs from first-order absorption, where the rate declines as drug amount decreases. Zero-order kinetics commonly applies to controlled-release systems like sustained-release tablets or transdermal patches, where absorption continues steadily until the drug at the absorption site is depleted.

During the absorption phase, the absorption rate exceeds the elimination rate, causing plasma concentration to rise. At peak plasma concentration, absorption and elimination rates are equal. During the post-absorption phase, elimination rate exceeds absorption rate, causing concentration to decline. Once drug absorption completes, the elimination phase begins where absorption rate becomes zero and only elimination continues.

Extravascular administration via oral, intramuscular, or rectal routes is non-invasive and often preferred for ease of use and improved patient compliance. These routes rely on absorption to deliver the drug's therapeutic activity. However, absorption rate—whether zero-order or first-order—becomes a critical factor determining how quickly and effectively the drug enters the bloodstream from the administration site.

Key pharmacokinetic parameters estimated include the elimination rate constant (k), absorption rate constant (ka), maximum plasma concentration (Cmax), and time to reach maximum concentration (tmax). These parameters provide critical insights into drug behavior and effectiveness. Mathematical equations governing the one-compartment open model determine how absorption and elimination rates balance to produce the observed plasma concentration-time profile.

Zero-order absorption maintains a steady absorption rate similar to constant-rate intravenous infusion, where drug enters the body at a constant rate. The mathematical equations and principles governing plasma concentration-time profiles in constant-rate infusion also apply to zero-order absorption. Both processes result in linear drug input until the source is depleted, distinguishing them from first-order processes where input rate declines over time.

Extravascular administration includes oral, intramuscular, and rectal routes. These non-invasive delivery methods rely on absorption for the drug to enter the bloodstream and exert therapeutic activity. The rate of absorption—either zero-order or first-order—determines how quickly the drug becomes available systemically, making absorption kinetics a fundamental consideration in pharmacokinetic modeling for these routes.

Controlled-release systems such as sustained-release tablets and transdermal patches typically exhibit zero-order absorption kinetics. These formulations are designed to deliver drug at a constant rate over an extended period. Absorption continues steadily until the drug at the absorption site, such as the gastrointestinal tract, is completely depleted, providing predictable and consistent plasma concentration profiles.