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Bio Available -- Definition

The term "bioavailable" is often used and seldom explained. It sounds good, but typically hides the truth behind that usual emotional reaction to that phrase.

Let's take a look, first, at some standard definitions and then some commercial usages of that term.

From Wikipedia, the free encyclopedia

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Bioavailability
From Wikipedia, the free encyclopedia

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 orally), 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.

Contents
1 Definition
2 Absolute bioavailability
3 Relative bioavailability
4 Factors influencing bioavailability

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).

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.

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.

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Bioavailability

Source

Bioavailability refers to the difference between the amount of a substance, such as a drug, herb, or chemical, to which a person is exposed and the actual dose of the substance the body receives. Bioavailability accounts for the difference between exposure and dose. A drug's therapeutic action or a chemical's toxicity is determined by the dose received at the target site in the body. The dose at the target site is determined by the amount of the substance absorbed by the body, which depends on its bioavailability. If a substance is ingested, for example, its bioavailability is determined by the amount that is absorbed by the intestinal tract. If a substance is inhaled, its bioavailability is determined by the amount that is absorbed by the lungs. Understanding bioavailability is critical to determining the amount of a drug to administer or the level of chemical exposure that is likely to produce toxicity.

The bioavailability of drugs depends on their formulation, which determines the rate at which they dissolve in the gastrointestinal tract. Although not legally considered to be drugs, the bioavailability of vitamin, mineral, and herbal supplements obey the same principles. For example, calcium (calcium bound to an organic acid such as citrate) is more easily absorbed by the gastrointestinal tract than calcium carbonate. Similarly, the bioavailability of chemical contaminants in the environment depends on the nature of the medium in which they are found. For example, the soil at locations of former manufactured gas plants can be very contaminated with chemicals (such as polycyclic aromatic hydrocarbons) that were produced by burning fuels, although very little of those chemicals is bioavailable because they are bound very tightly to the soil itself. The toxicity level of the chemicals in the soil, if measured in the laboratory, would be much greater than the toxicity level that would be experienced by someone exposed to the soil itself.

Questions of bioavailability are sometimes at the root of disagreements about what are the appropriate actions to take to protect public health and the environment from environmental contaminants. For example, sediment at the bottom of the Hudson River is contaminated with polychlorinated biphenyls (PCBs) due to past industrial disposal practices. Some argue that the PCBs in the sediment pose an unacceptable risk to the health of humans, fish, and other wildlife, and should be removed. Others argue that the PCBs are not a health hazard because of their low bioavailability in the sediment, and thus should be left in place because disturbing the sediment might make them more bioavailable. The bioavailability of chemical contaminants is often poorly understood, so it is sometimes not taken into account when the health risks from chemical exposures are assessed.

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