Pharmacokinetics and ADME Properties
Pharmacokinetics is the quantitative study of what the body does to a drug over time, summarised by the ADME processes: absorption into the systemic circulation, distribution to tissues, metabolism (biotransformation), and excretion. Within medicinal chemistry, ADME properties describe how a molecule's structure governs its fate in the body, and they are studied alongside potency because a compound must reach its target at an adequate concentration to be useful.
Definition
Pharmacokinetics is the description of the time course of drug absorption, distribution, metabolism, and excretion (ADME), typically expressed through parameters such as clearance, volume of distribution, bioavailability, and half-life that relate dose to the concentration of drug in plasma and tissues over time.
Scope
This area orients the reader to the four classical ADME processes and the parameters that summarise them - bioavailability, volume of distribution, clearance, and half-life - and to how these properties are designed into candidate molecules. It frames pharmacokinetics as a methodological and physicochemical topic in medicinal and pharmaceutical chemistry, not as clinical dosing guidance. Detailed treatment of each process is given in the child topics.
Sub-topics
Core questions
- How does a molecule's structure and physicochemical profile determine its absorption, distribution, metabolism, and excretion?
- Which pharmacokinetic parameters summarise drug disposition, and how are they related to one another?
- How can ADME liabilities be identified early and optimised during drug design?
Key concepts
- Absorption
- Distribution
- Metabolism
- Excretion
- Bioavailability
- Clearance
- Volume of distribution
- Half-life
- Plasma protein binding
- Lipophilicity and permeability
Mechanisms
After administration, a drug must dissolve and permeate biological membranes to be absorbed, then distribute between plasma and tissues according to its protein binding and lipophilicity. Enzymes - principally hepatic cytochrome P450s and conjugating enzymes - biotransform it, and the parent drug and its metabolites are eliminated mainly by the kidney and bile. These processes are governed by molecular properties such as size, solubility, lipophilicity, ionisation, and hydrogen-bonding capacity, which is why physicochemical rules of thumb such as the Lipinski rule of five emerged as guides for oral drug design (Lipinski, 2001). Across many marketed drugs, disposition parameters such as clearance and volume of distribution span wide but characterisable ranges (Obach, 2008).
Clinical relevance
ADME properties explain why two equally potent molecules can behave very differently in the body and why some candidates fail despite strong target activity. Understanding pharmacokinetics supports the interpretation of dose-exposure relationships and drug-interaction potential at a conceptual level. This entry describes how drug disposition is characterised and is not a source of dosing recommendations or individualised treatment advice.
Evidence & guidelines
ADME characterisation in drug development draws on a combination of in vitro assays (permeability, metabolic stability, protein binding), in vivo studies, and increasingly in silico prediction; regulatory agencies publish bioavailability, bioequivalence, and drug-interaction study guidance that operationalise these concepts. Compendia of human pharmacokinetic parameters provide reference ranges that inform expectations during candidate selection (Obach, 2008; Rowland & Tozer, 2011).
History
Pharmacokinetics emerged in the twentieth century as the mathematical description of drug concentrations in the body, formalised in monographs such as Gibaldi and Perrier (1982). From the 1990s, ADME thinking moved upstream into early drug discovery: high attrition attributed to poor pharmacokinetics motivated structure-based property optimisation, exemplified by Lipinski's rule of five (2001), and the integration of ADME screening into medicinal chemistry workflows.
Key figures
- Christopher Lipinski
- Malcolm Rowland
- Thomas Tozer
- Milo Gibaldi
- R. Scott Obach
Related topics
Seminal works
- lipinski-2001
- gibaldi-perrier-1982
- obach-2008
Frequently asked questions
- What does ADME stand for?
- ADME stands for Absorption, Distribution, Metabolism, and Excretion - the four processes that together describe a drug's pharmacokinetic fate in the body. ADMET adds Toxicity.
- How is pharmacokinetics different from pharmacodynamics?
- Pharmacokinetics describes what the body does to the drug (its concentration over time through ADME), whereas pharmacodynamics describes what the drug does to the body (the relationship between concentration and effect).