Pharmacokinetics

Study of how drugs move through the body

Example of linear pharmacokinetics

Pharmacokinetics is a branch of pharmacology dedicated to determining the fate of substances administered to a living organism. The substances of interest include any chemical xenobiotic such as pharmaceutical drugs, pesticides, food additives, cosmetics, etc. It attempts to explain what the body does to a drug, through the processes of absorption, distribution, metabolism, and excretion (ADME).

Overview

Pharmacokinetics describes how the body affects a specific chemical after administration. It is concerned with the movement of drug into, through, and out of the body. The four main parameters generally examined in pharmacokinetics are absorption, distribution, metabolism, and excretion, collectively known as ADME.

Absorption

Oral administration of drugs is a common route of absorption.

Absorption is the process by which a drug enters the bloodstream. The rate and extent of drug absorption depend on the route of administration, the drug's formulation, and the presence of food or other substances in the gastrointestinal tract. Common routes of administration include oral, intravenous, intramuscular, and subcutaneous.

Distribution

Once absorbed, the drug is distributed throughout the body. Distribution is influenced by factors such as blood flow, tissue permeability, and the binding of the drug to plasma proteins. The volume of distribution is a key parameter that describes the extent to which a drug is distributed in body tissues.

Metabolism

Metabolism refers to the chemical alteration of the drug in the body, primarily by the liver. The liver's cytochrome P450 enzyme system plays a significant role in drug metabolism, converting lipophilic compounds into more water-soluble metabolites for excretion.

Excretion

Excretion is the process of eliminating the drug from the body, primarily through the kidneys in urine, but also through bile, sweat, saliva, and breast milk. The rate of excretion is a critical factor in determining the drug's half-life and dosing frequency.

Pharmacokinetic Models

Pharmacokinetic models are mathematical models that describe the time course of drug concentrations in the body. These models can be classified as either compartmental or non-compartmental.

Compartmental Models

Linear pharmacokinetics

Compartmental models assume that the body is divided into compartments where the drug is uniformly distributed. The simplest model is the one-compartment model, where the body is viewed as a single, homogeneous compartment. More complex models, such as the two-compartment model, consider the central and peripheral compartments.

Non-Compartmental Models

Non-compartmental analysis does not assume any specific compartmental model but uses statistical moment theory to estimate pharmacokinetic parameters. This approach is often used when the compartmental model is not clear or when a simpler analysis is desired.

Linear and Non-linear Pharmacokinetics

Non-linear pharmacokinetics

In linear pharmacokinetics, the drug concentration in the body is directly proportional to the dose administered. This means that doubling the dose will double the concentration. In contrast, non-linear pharmacokinetics occurs when the drug concentration does not change proportionally with the dose, often due to saturation of metabolic pathways or transport mechanisms.

Applications

Pharmacokinetics is crucial in the development of new drugs, determining appropriate dosing regimens, and understanding drug interactions. It helps in predicting the onset, duration, and intensity of a drug's effect, as well as its potential toxicity.

Related pages

• Pharmacodynamics
• Toxicokinetics
• Bioavailability
• Therapeutic drug monitoring