3.24:
Kinetics of Drug Elimination
Every drug that enters the body gets removed over time. The drug is either excreted intact or metabolized first and then eliminated from the body.
The rate of drug elimination is related to the drug's plasma concentration through a parameter called clearance. It estimates the volume of plasma being cleared of the drug in unit time.
Usually, drug clearance follows first-order kinetics, where a constant fraction of the drug is eliminated per unit of time. As a result, the plot of drug concentration versus time is exponential, indicating that an increase in drug concentration increases its elimination rate.
In contrast, some drug eliminations, such as for phenytoin and aspirin, follow zero-order kinetics. High therapeutic doses of these drugs saturate the metabolic capacity of enzymes, resulting in only a fixed amount of drugs being eliminated at a constant rate.
The plot of drug concentration versus time becomes linear, implying that increasing the drug concentration does not affect the elimination rate.
3.24:
Kinetics of Drug Elimination
Eliminating drugs from the body is a vital process that occurs through excretion or metabolism. Understanding the kinetics of drug elimination is crucial for drug development, dosage determination, and optimizing patient outcomes.
Drug clearance depends on the rate of drug elimination and its plasma concentration. Another important parameter is the half-life of a drug, which is the time required for its concentration to decrease by half. In most cases, drug clearance follows first-order kinetics, where a constant fraction of the drug is eliminated per unit time. This means that as the drug concentration increases, its elimination rate also increases. The relationship between drug concentration and time is exponential.
However, certain drugs, like phenytoin and aspirin, exhibit zero-order kinetics. When administered at high therapeutic doses, these drugs saturate the enzymes responsible for their metabolism. As a result, only a fixed amount of the drug is eliminated at a constant rate, regardless of its concentration. The plot of drug concentration versus time in zero-order kinetics is linear, indicating that increasing the drug concentration does not affect its elimination rate.