Basics of Graph Construction
To perform an accurate analysis of pharmacokinetic curves, one must clearly understand the structure of the graph. The plasma drug kinetics are visualized using a two-dimensional coordinate system.
- Vertical axis (Y): Represents the concentration of the drug in blood plasma, most commonly expressed as a percentage.
- Horizontal axis (X): Reflects the time elapsed since drug administration, typically graduated in hours.
Comparing these two parameters allows clinicians and researchers to observe the dynamic behavior of a drug based on its route of administration.
Characteristics of the Intravenous Administration Curve
The graph describing drug kinetics following intravenous administration has several distinct and easily recognizable features:
- Graph Origin: The curve does not start at zero, but immediately at the maximum value on the Y-axis. This is due to the instantaneous entry of the entire drug dose directly into the systemic circulation.
- Curve Shape: The graph is a descending exponential curve. Since the absorption phase is absent, the curve exclusively reflects drug distribution and elimination processes.
- Area Under the Curve: The $AUC_{IV}$ (Area Under the Curve) parameter demonstrates the maximum possible area in this case. In pharmacokinetics, this area is considered the absolute standard against which the efficacy of other administration routes is compared.
Characteristics of the Oral Administration Curve
Drug kinetics following oral administration appear entirely different because the substance requires time to enter the systemic circulation.
- Starting Point: Unlike the intravenous route, the curve begins strictly at zero.
- Ascending Phase: A clear upward slope is visible on the graph. During this period, the process of absorption predominates over elimination.
- Peak Concentration: The curve reaches its highest point, which corresponds to the maximum drug concentration in the blood plasma.
- Descending Phase: Following the peak, a decline begins as elimination overtakes absorption.
A critical parameter is $AUC_{oral}$. The area under this curve is always smaller than that of intravenous administration. This reduction is driven by two main factors: incomplete gastrointestinal absorption and the first-pass hepatic effect (where a portion of the drug is metabolized before reaching the systemic circulation).
Key Pharmacokinetic Processes on the Graph
By analyzing the shape of the curves, one can identify the primary processes acting on the drug:
- Absorption: Forms the ascending phase on the graph during oral administration.
- Distribution and Elimination: Form the descending exponential curve during intravenous administration and the post-peak decline following oral administration.
- Metabolism (First-Pass Effect): Not directly visible on the graph, but it (along with incomplete absorption) accounts for why $AUC_{oral}$ is smaller than the standard $AUC_{IV}$.