Pharmacokinetics is the branch of pharmacology that studies how the body changes a drug rather than how a drug changes the body. It tracks a chemical substance from the moment it enters the organism as a xenobiotic until it is completely eliminated. This mathematical story of survival and elimination determines whether a treatment will heal or harm.
The ADME acronym stands for Liberation, Absorption, Distribution, and Excretion. Liberation separates the active ingredient from its pill form, while absorption moves the drug into the systemic circulation. Distribution disperses the drug through fluids and tissues, and excretion removes the substance or its metabolites from the body.
The half-life is the time required for the concentration of a drug to reach half of its original value. This metric dictates how often a patient must take a dose to maintain therapeutic levels without causing toxicity. Steady state is typically reached after three to five times the drug's half-life.
A one-compartment model assumes the organism is a single homogenous unit where blood plasma concentrations determine drug levels in other tissues. A two-compartment model distinguishes between a central compartment with rapid blood flow and a peripheral compartment with slower blood flow such as brain tissue or fat.
Mass spectrometry is the most common instrumentation used to measure drug concentrations in biological matrices like plasma or urine. It employs a triple quadrupole mass spectrometer to ensure high sensitivity and specificity for observing concentrations after a low dose and a long time period. This technology allows for the construction of concentration-time profiles that form the foundation of all pharmacokinetic analysis.
Population pharmacokinetics identifies sources of variability in drug concentrations among individuals based on factors like body weight and excretory functions. It allows for the individualization of patient doses by analyzing sparse data sets with only one concentration measurement per patient. This practice has facilitated organ transplants by ensuring drugs like ciclosporin remain within a narrow therapeutic range.