Pharmacokinetics part 1: Overview, Absorption and Bioavailability, Animation

Pharmacokinetics studies the events that happen to a drug from its administration to the time it is excreted from the body. Pharmacokinetics is one of the 2 major branches of pharmacology, the other being pharmacodynamics, which describes the effects a drug has on the body.
Pharmacokinetics involves 4 processes: absorption, distribution, metabolism, and excretion.
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Voice by : Marty Henne
Absorption is when the drug moves from the site of administration to the bloodstream,
Distribution is how the drug is distributed from the bloodstream to body’s tissues,
Metabolism describes how the body chemically modifies the drug,
and excretion is how the drug is eliminated from the body.
Because a drug typically has to be metabolized before it can be excreted, drug metabolism and excretion are sometimes collectively referred to as clearance, or elimination.
Pharmacokinetics of a drug depends on its properties as well as patient-related factors. For example, older patients or those with kidney or liver problems may have slower clearance rates, causing some drugs to stay longer in their blood.
Drug absorption varies depending on the route of administration, the drug’s chemical properties, formulation, and patient factors.
When a drug is given intravenously, it absorbs immediately, reaching maximum concentration in the blood plasma almost instantaneously. This is not the case with other routes of administration, in which the drug needs some time to get into the bloodstream, and drug concentration in blood plasma increases gradually. The maximum, or peak, plasma concentration is reached when drug absorption rate equals clearance rate. Peak time, when peak concentration occurs, is an indicator of absorption rate - the faster the absorption, the sooner the peak time.
Oral administration is the most common route because of its convenience, but it may not be suitable for some drugs.
Absorption of oral drugs involves transport across membranes of epithelial cells in the digestive tract, primarily the small intestine. Cell membrane is a barrier that all drugs must overcome to get into the bloodstream, unless they are injected intravenously.
Drugs in solid forms must first be dissolved. The rate of dissolution can be the limiting factor if it is slower than absorption. For example, sustained-release forms are specifically designed to dissolve slowly, releasing small amounts of the drug as it passes through the digestive tract. The goal is to produce a steady level of plasma drug concentration, to have a stable therapeutic effect with minimum toxicity.
A measure of the extent and rate of absorption is bioavailability. Bioavailability is usually referred to as the fraction of the administered drug that is absorbed into the bloodstream and is therefore available to the target tissue. A drug given intravenously immediately enters the systemic circulation and therefore has a bioavailability of 100%.
Bioavailability is usually calculated as the area under the “plasma concentration versus time” curve, AUC. Because bioavailability of a drug is considered 100% when delivered intravenously, bioavailability of a drug given other ways can be calculated by dividing its AUC by the AUC of the same drug given intravenously.
The use of nitroglycerin in angina management is an example of how different routes of administration result in different bioavailabilities and therefore producing different therapeutic effects. Sublingual nitroglycerin diffuses immediately into the bloodstream, producing a fast, strong, but short-lived action, and is typically used for immediate angina relief. On the other hand, oral nitroglycerin absorbs slowly, resulting in weaker but more sustained effects, and is usually prescribed for preventive purposes.
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