Graph Basics
To properly analyze pharmacokinetic curves, one must clearly understand the structure of the graph itself. The visualization of differences in drug kinetics in blood plasma is built in a two-dimensional coordinate system.
- Vertical axis (Y) demonstrates the concentration of the drug in blood plasma. Most often, this indicator is expressed as a percentage.
- Horizontal axis (X) reflects the time elapsed since drug administration. As a rule, the time scale is graded in hours.
It is the comparison of these two parameters that allows us to see the dynamics of drug behavior depending on the route by which it was administered into the patient's body.
Features of the IV curve
The graph describing drug kinetics during intravenous administration has a number of specific and easily recognizable features.
- Start of the graph: The curve does not start from zero, but immediately from the maximum value on the Y-axis. This is due to the instant delivery of the entire dose of the drug directly into the systemic circulation.
- Curve shape: The graph represents a falling exponent. Since the absorption phase is absent, the curve exclusively reflects the processes of elimination and distribution of the drug in the body.
- Area under the curve: The $AUC_{iv}$ (Area Under the Curve) indicator in this case demonstrates the maximum area. In pharmacokinetics, this particular area is taken as the absolute standard against which the effectiveness of other routes of administration is compared.
Features of the oral curve
Drug kinetics upon oral administration looks completely different, as the substance takes time to enter the systemic circulation.
- Starting point: Unlike the intravenous route, the curve starts strictly from zero.
- Ascent phase: A rise is clearly visible on the graph. During this period, the process of absorption predominates over elimination processes.
- Peak concentration: The curve reaches its highest point, which corresponds to the maximum concentration of the drug in blood plasma.
- Descent phase: After passing the peak, a decline begins when elimination starts to predominate.
A crucial characteristic is the $AUC_{oral}$ indicator. The area under this curve is always smaller than with intravenous administration. This reduction is due to two main factors: incomplete drug absorption in the gastrointestinal tract and the first-pass hepatic effect (where part of the substance undergoes metabolism even before entering the systemic circulation).
Key pharmacokinetic processes on the graph
By analyzing the shape of the curves, one can identify the main processes occurring with the drug:
- Absorption: Forms the ascent phase on the graph upon oral administration.
- Distribution and elimination: Form the falling exponent during intravenous administration and the descent phase after the peak during oral administration.
- Metabolism (first-pass effect): Not directly visible on the graph, but it is precisely this (along with incomplete absorption) that causes $AUC_{oral}$ to be smaller than the standard $AUC_{iv}$.
Briefly: answers to topical questions
By what formula is the absolute bioavailability of a drug calculated using the AUC indicator?
The absolute bioavailability of a drug is calculated as the ratio of the area under the curve for oral administration to the area under the curve for intravenous administration. To quantify what fraction of the administered dose reached the systemic circulation, the following formula is used:
$$F = \frac{AUC_{oral}}{AUC_{iv}} \times 100\%$$
In this calculation, bioavailability upon intravenous administration is taken as the standard (100%).
What exact pharmacokinetic parameters ($C_{max}$, $T_{max}$) can be determined from the peak of the curve upon oral administration?
The maximum drug concentration and the time of its achievement are determined from the peak of the pharmacokinetic curve during extravascular (oral) administration.
- Maximum concentration ($C_{max}$) is the highest point of the curve reached after the growth phase (when absorption prevails over elimination).
- Time to maximum concentration ($T_{max}$) is an absorption parameter that can vary among different drugs even with identical bioavailability.
To remember the difference in starting points: "Intravenous — straight into the vein (starts at maximum), oral — takes time (starts at zero)."
Class and exam questions
Why does the intravenous curve start at the maximum?
This is due to the instant delivery of the entire dose of the drug into the systemic circulation, bypassing the absorption phase.
What does the ascent phase mean on the oral administration graph?
The ascent phase means that the process of drug absorption from the gastrointestinal tract prevails over its elimination processes.
Why is AUC lower for oral administration than for intravenous administration?
The area under the curve is smaller due to incomplete drug absorption and the first-pass hepatic effect, where part of the substance is degraded.
What else to study on this topic
- Mechanisms of drug absorption
- First-pass hepatic effect (presystemic metabolism)
- Processes of drug distribution in the body
- Routes of drug elimination
- Clinical significance of the AUC indicator
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