Pharmacokinetics

Pharmacokinetics is the quantitative study of what the body does to a drug over time: how a drug is absorbed, distributed to tissues, metabolised, and eliminated. It describes the time course of drug concentrations using a small set of parameters — bioavailability, volume of distribution, clearance, and half-life — that together explain why a given dose produces a particular concentration profile.

Definition

Pharmacokinetics is the branch of pharmacology that characterises the rates and extent of drug absorption, distribution, metabolism, and excretion, expressing drug disposition through parameters such as bioavailability, volume of distribution, clearance, and half-life.

Scope

This area orients the reader to the four classical processes summarised by the acronym ADME (absorption, distribution, metabolism, excretion) and to the parameters that quantify them. It frames pharmacokinetics as the disposition half of the dose-concentration-effect chain, leaving the concentration-effect relationship to pharmacodynamics. The treatment is conceptual and educational; it does not give dosing or individualised therapeutic recommendations.

Sub-topics

• Absorption, Distribution, and Bioavailability
• Drug Elimination and Clearance
• Half-Life and Steady-State Kinetics
• Metabolism and Biotransformation
• Volume of Distribution

Core questions

• How much of an administered dose reaches the systemic circulation, and how fast?
• How widely does a drug distribute relative to plasma, and what does that imply about a loading dose?
• By what routes and at what rate is a drug eliminated from the body?
• Which independent parameters (clearance, volume) govern the derived parameter half-life?
• How do these parameters together determine the concentration-time profile after single and repeated dosing?

Key concepts

• ADME (absorption, distribution, metabolism, excretion)
• Bioavailability
• Volume of distribution
• Clearance
• Elimination half-life
• First-order (linear) versus zero-order kinetics
• Compartmental modelling
• Steady state and accumulation

Mechanisms

Pharmacokinetics treats the body as a system through which drug mass moves and from which it is removed. Clearance and volume of distribution are the two primary, independent parameters: clearance expresses the body's overall capacity to eliminate drug and is the parameter that controls total exposure, while volume of distribution relates the amount of drug in the body to the measured plasma concentration. Half-life is a derived quantity that depends on both — longer when volume is large or clearance is low. The order of a process (first-order, where rate is proportional to concentration, versus zero-order, where rate is constant) determines whether the kinetics are linear and predictable across doses.

Clinical relevance

Pharmacokinetic parameters underlie how clinicians and pharmacologists reason about drug exposure, the rationale for therapeutic drug monitoring, and why renal or hepatic impairment alters drug handling. This entry explains the framework that such reasoning rests on; it describes how disposition is conceptualised and is not a source of dosing or treatment instructions for any individual.

Evidence & guidelines

The conceptual framework summarised here is codified in standard clinical pharmacokinetics textbooks (Rowland & Tozer) and in foundational papers that defined clearance and half-life as the organising parameters of drug disposition (Rowland and colleagues, 1973; Toutain & Bousquet-Mélou, 2004). Regulatory bioequivalence and drug-interaction frameworks build on these same parameters, though specific guidance documents are treated at the topic level.

History

The conceptual vocabulary of modern pharmacokinetics was consolidated in the early 1970s, when clearance was articulated as the central parameter relating dose to steady-state concentration (Rowland, Benet & Graham, 1973), and when the physiological determinants of hepatic clearance — organ blood flow and intrinsic clearance — were formalised (Wilkinson & Shand, 1975). Later expository work re-grounded the discipline by distinguishing primary parameters (clearance, volume) from the derived half-life.

Key figures

• Malcolm Rowland
• Leslie Z. Benet
• Grant R. Wilkinson
• Pierre-Louis Toutain

Seminal works

• rowland-1973
• wilkinson-shand-1975
• toutain-clearance-2004

Frequently asked questions

What is the difference between pharmacokinetics and pharmacodynamics?

Pharmacokinetics describes what the body does to the drug (absorption, distribution, metabolism, excretion), while pharmacodynamics describes what the drug does to the body (the concentration-effect relationship). Together they link dose to effect.

Which two parameters are considered the most fundamental in pharmacokinetics?

Clearance and volume of distribution are the two primary, independent parameters. Clearance controls overall drug exposure and volume of distribution relates total drug in the body to plasma concentration; half-life is derived from both.