Welcome to an introduction to pharmacokinetics. Pharmacokinetics is the study of how drugs move through the body over time. It describes what the body does to a drug, from administration to elimination. The key processes in pharmacokinetics are often abbreviated as ADME: Absorption, Distribution, Metabolism, and Excretion.
Let's explore the first two processes of pharmacokinetics: absorption and distribution. Absorption is the process by which a drug moves from the site of administration into the bloodstream. This can occur through various routes such as oral, intravenous, or transdermal. Distribution is the process by which a drug disperses throughout the body's fluids and tissues after entering the bloodstream. The drug's chemical properties, such as solubility and protein binding, affect how widely it distributes in the body.
Now let's examine the final two processes of pharmacokinetics: metabolism and excretion. Metabolism is the process by which the body chemically alters a drug, usually making it less active and easier to eliminate. This primarily occurs in the liver through various enzymatic reactions. The resulting compounds are called metabolites. Excretion is the process by which drugs and their metabolites are eliminated from the body. This occurs primarily through the kidneys, which filter the blood and excrete waste products in urine. The liver also plays a role in excretion by removing drugs from the blood and secreting them into bile.
Pharmacokinetic parameters are quantitative measurements that describe how the body handles a drug. Half-life is the time required for the drug concentration to decrease by 50%. As shown in the graph, after one half-life, the concentration drops from 100% to 50%, and after two half-lives, it drops to 25%. Clearance measures how quickly the body removes a drug, expressed as volume of blood cleared of drug per unit time. Volume of distribution is a theoretical volume that relates the amount of drug in the body to the concentration in the blood. Finally, bioavailability represents the fraction of the administered drug that reaches systemic circulation unchanged, which is especially important for orally administered medications.
Pharmacokinetics has numerous clinical applications. First, it's essential for drug dosing, helping clinicians determine the appropriate dose and dosing intervals for different patients. The graph shows how drug concentrations change over time in patients with normal versus impaired kidney function. Patients with impaired function eliminate drugs more slowly, requiring dose adjustments to stay within the therapeutic range. Pharmacokinetics also guides drug development by helping researchers optimize new medications. For special populations like elderly patients, children, or those with kidney or liver disease, pharmacokinetic principles allow for customized therapy. Finally, understanding pharmacokinetics helps predict and manage drug interactions, where one medication affects the metabolism or elimination of another.