Bioavailability Refers to the fraction of an administered drug that reaches the systemic circulation.

It is expressed as the fraction of administered drug that gains access to the systemic circulation in a chemically unchanged form.

It is determined by comparing plasma levels of a drug after a particular route of administration with plasma drug levels achieved by IV injection in which all of the agent rapidly enters the circulation.

By plotting plasma concentrations of the drug versus time, the area under the curve (AUC) can be measured, and this curve reflects the extent of absorption of the drug.

The bioavailability of a drug administered orally is the ratio of the area calculated for oral administration compared with the area calculated for IV injection.

Bioavailability factors include first-pass hepatic metabolism, which refers to the drug absorption across the GI tract.

Oral drugs enter the portal circulation before entering the systemic circulation, and if the drug is rapidly metabolized by the liver, the amount of unchanged drug that gains access to the systemic circulation is decreased.

Many drugs undergo major biotransformation during a single passage through the liver.

Highly hydrophilic drugs are poorly absorbed because they are unable to cross the lipid-rich cell membranes.

Extremely hydrophobic are also poorly absorbed, as they are insoluble in aqueous body fluids and cannot access cell surfaces.

Drug absorption requires drugs to be largely hydrophobic, yet have some solubility in aqueous solutions.

Some drugs are highly lipid-soluble, and are transported in the aqueous solutions of the body on carrier proteins such as albumin.

Some drugs are unstable in the pH of the gastric contents and include agents such as penicillin G. Some drugs are destroyed in the GI tract by degradative enzymes and includes insulin.

Drug absorption is altered by factors such as particle size, it’s salt form, crystal polymorphisms, enteric coatings and excipients such as binders and dispersing agents.

The above factors can influence the rate of drug dissolution and alter the rate of absorption.

Bioequivalent drugs have comparable bioavailability and similar times to achieve peak blood concentrations.

Drugs with a significant difference in bioavailability are said to be bioinequivalent.

Therapeutic equivalence refers to drugs that have comparable efficacy and safety.

Two drugs that are bioequivalent may not be therapeutically equivalent.

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