- Pharmacokinetics is the study of how the body processes drugs over time, encompassing the complete journey of a drug from its administration to its elimination. This field is often summarized by the ADME principle: Absorption, Distribution, Metabolism, and Excretion.
- Absorption is the process by which a drug enters the bloodstream. The route of administration significantly affects absorption – intravenous delivery provides complete bioavailability, while oral administration subjects drugs to first-pass metabolism and varying absorption rates. Factors affecting absorption include drug solubility, particle size, blood flow to the absorption site, and the surface area available for absorption.
- Distribution describes how a drug moves throughout the body after entering the bloodstream. This process is influenced by factors such as blood flow, tissue affinity, protein binding, and the drug’s physical-chemical properties. Some drugs preferentially accumulate in specific tissues, while others distribute more uniformly. The volume of distribution (Vd) is a key parameter that relates the total amount of drug in the body to its plasma concentration.
- Metabolism, primarily occurring in the liver, is the process of converting drugs into more water-soluble compounds (metabolites) that can be more easily eliminated. Phase I reactions typically involve oxidation, reduction, or hydrolysis, while Phase II reactions involve conjugation with endogenous substances. Genetic variations in metabolic enzymes can significantly affect drug processing rates among individuals.
- Excretion is the final step, where drugs and their metabolites are eliminated from the body. The kidneys are the primary excretory organ for most drugs, though some are eliminated through bile, exhaled air, sweat, or other routes. The rate of elimination is often expressed as half-life, which helps determine dosing intervals.
- These processes are mathematically modeled using parameters like clearance, bioavailability, and area under the curve (AUC). Understanding pharmacokinetics is crucial for determining appropriate dosing regimens, predicting drug interactions, and adjusting doses for special populations like patients with kidney or liver dysfunction.
- Recent advances in pharmacokinetics include the development of physiologically-based Pharmacokinetics modeling and the integration of genetic factors (pharmacogenomics) to better predict individual drug responses.
