Pharmacokinetics asks what the body does to the drug
While pharmacodynamics tracks a drug's effects on the organism, pharmacokinetics (PK) maps the chemical's journey from dosing to total elimination—usually summarized as ADME: absorption, distribution, metabolism, and excretion. Steady state on regular dosing is often reached after three to five half-lives.
Pharmacokinetics (from Greek pharmakon, drug, and kinetikos, putting in motion) describes how the body handles xenobiotics—medicines, pesticides, additives, cosmetics—using math that links plasma concentration to time since administration. Together with pharmacodynamics it shapes dosing, benefit, and harm in PK/PD models. Phases after contact are Liberation, Absorption, Distribution, Metabolism (biotransformation by enzymes such as cytochrome P450), and Excretion; LADME adds liberation from the dosage form, while some texts fold distribution–metabolism–excretion into disposition or group metabolism and excretion as elimination. Rarely, drugs accumulate irreversibly in tissue. Understanding kinetics also needs excipient properties, membrane crossing, and enzyme induction or inhibition.
Modelers choose noncompartmental or compartmental approaches. Noncompartmental analysis reads concentration–time tables without assuming a structure, often estimating exposure via area under the curve (AUC) with the trapezoidal rule—results that depend heavily on sampling density across absorption, distribution, and elimination. Metrics include Cmax, Tmax, clearance, and volume of distribution. Compartmental models treat the body as linked tanks described by differential equations; one- and two-compartment schemes are common because more parameters complicate fitting. A one-compartment model assumes plasma levels scale other tissues by constants and often uses first-order (linear) elimination proportional to concentration. Two-compartment thinking separates a well-perfused central compartment from a slower peripheral one; the brain may sit either side depending on lipophilicity and efflux at the blood–brain barrier. After an IV bolus, plasma often falls in an alpha distribution phase then a beta elimination phase.
Industry uses these outputs for bioequivalence of generics; clinicians use them for safer regimens. Multi-compartment and physiologically based (PBPK) models approach reality yet always simplify. Educators note students struggle with core PK concepts and math transfer—so team-based cases and simulations aim to bridge the gap between curves on a page and decisions at the bedside.
Source: Pharmacokinetics
