In pharmacology, bioavailability is used to describe the fraction of an administered dose of unchanged drug that reaches the systemic circulation, one of the principal pharmacokinetic properties of drugs. By definition, when a medication is administered intravenously, its bioavailability is 100%. However, when a medication is administered via other routes (such as by mouth), its bioavailability decreases (due to incomplete absorption and first-pass metabolism). Bioavailability is one of the essential tools in pharmacokinetics, as bioavailability must be considered when calculating dosages for non-intravenous routes of administration. Pharmacology (in Greek: pharmacon (φάρμακον) meaning drug, and logos (λόγος) meaning science) is the study of how substances interact with living organisms to produce a change in function. ... Dose can refer to: Dose (album), an album by the jam band Govt Mule Dose (song), a song by the band Filter Dose (magazine), a free daily Canadian magazine In medicine: Effective dose, the smallest amount of a substance required to produce a measurable effect on a living organism... Systemic circulation is the portion of the cardiovascular system which carries oxygenated blood away from the heart, to the body, and returns deoxygenated blood back to the heart. ... Pharmacokinetics is a branch of pharmacology dedicated to the study of the time course of substances and their relationship with an organism or system. ... Oral medication A medication is any drug taken to cure or reduce the symptoms of an illness or ongoing medical condition. ... An intravenous drip in a hospital Intravenous therapy or IV therapy is the administration of liquid substances directly into a vein. ... In pharmacology and toxicology, a route of administration is the path by which a drug, fluid, poison or other substance is brought into contact with the body 1. ... The first pass effect (or first pass metabolism) is a phenomenon of drug metabolism. ...

Definition

Bioavailability is a measurement of the extent of a therapeutically active drug that reaches the systemic circulation and is available at the site of action.^[1]

It is expressed as the letter F.

Absolute bioavailability

Absolute bioavailability measures the availability of the active drug in systemic circulation after non-intravenous administration (i.e., after oral, rectal, transdermal, subcutaneous administration). An intravenous drip in a hospital Intravenous therapy or IV therapy is the administration of liquid substances directly into a vein. ... In pharmacology and toxicology, a route of administration is the path by which a drug, fluid, poison or other substance is brought into contact with the body 1. ...

In order to determine absolute bioavailability of a drug, a pharmacokinetic study must be done to obtain a plasma drug concentration vs time plot for the drug after both intravenous (IV) and non-intravenous administration. The absolute bioavailability is the dose-corrected area under curve (AUC) non-intravenous divided by AUC intravenous. For example, the formula for calculating F for a drug administered by the oral route (po) is given below. Pharmacokinetics is a branch of pharmacology dedicated to the study of the time course of substances and their relationship with an organism or system. ...

Therefore, a drug given by the intravenous route will have an absolute bioavailability of 1 (F=1) while drugs given by other routes usually have an absolute bioavailability of less than one.

Relative bioavailability

This measures the bioavailability of a certain drug when compared with another formulation of the same drug, usually an established standard, or through administration via a different route. When the standard consists of intravenously administered drug, this is known as absolute bioavailability.

Factors influencing bioavailability

The absolute bioavailability of a drug, when administered by an extravascular route, is usually less than one (i.e. F<1). Various physiological factors reduce the availability of drugs prior to their entry into the systemic circulation,

Such factors may include, but are not limited to:

• poor absorption from the gastrointestinal tract
• degradation or metabolism of the drug prior to absorption
• hepatic first pass effect

Each of these factors may vary from patient to patient, and indeed in the same patient over time. Whether a drug is taken with or without food will affect absorption, other drugs taken concurrently may alter absorption and first-pass metabolism, intestinal motility alters the dissolution of the drug and may affect the degree of chemical degradation of the drug by intestinal microflora. Disease states affecting liver metabolism or gastrointestinal function will also have an effect. The first pass effect (or first pass metabolism) is a phenomenon of drug metabolism. ...

See also

• ADME-Tox
• Lipinski's Rule of 5
• Biopharmaceutics Classification System
• Caco-2

ADME is an acronym for Absorption, Distribution, Metabolization and Excretion and describes the effectiveness of a pharmaceutical compound within an organism. ... In 1997 Christopher A. Lipinski published a seminal paper identifying a series of features commonly found in orally active drugs. ... This system is a guidance provided by the U.S. Food and Drug Administration [1]. According to the Biopharmaceutics Classification System (BCS_(disambiguation)), drug substances are classified as follows: Class I - High Permeability, High Solubility Class II - High Permeability, Low Solubility Class III - Low Permeability, High Solubility Class IV - Low... Caco-2 cell monolayer absorption model Cell-based functional assays, such as the Caco-2 drug transport model for assessing intestinal transport, are extremely valuable for screening lead compounds in drug discovery. ...

References

1. ^ Shargel, L.; Yu, A.B. (1999). Applied biopharmaceutics & pharmacokinetics (4th ed.). New York: McGraw-Hill. ISBN 0-8385-0278-4