What Is Bioavailability?

Learn about bioavailability, or the rate at which a drug is absorbed

Circulatory system

Photographer's Choice/Getty Images

Bioavailability (BA) is a term used in pharmacology and nutritional and environmental sciences. In pharmacology, it refers to the degree and rate at which an administered drug is absorbed by the body's circulatory system, the systemic circulation.

Bioavailability is an essential measurement tool since it determines the correct dosage for non-intravenous administration of a drug. In clinical research trials, the bioavailability of a drug is a key factor to be measured in Phase 1 and Phase 2 trials.

Determining the absolute bioavailability of a drug is done through a pharmacokinetic study. The plasma drug concentration is plotted against time and measures concentration vs time after intravenous and non-intravenous administration. Non-intravenous applications include oral, rectal, topical, subcutaneous and sublingual.

The measurement of bioavailability is represented by the letter f or F if expressed in percent. Relative bioavailability is a measure used to assess bioequivalence (BE) between drug products.

In order to gain FDA approval for a generic drug, the drug sponsor must show a 90 percent confidence interval of its product to that of the branded drug.

Bioavailability is one of the essential tools in pharmaceutical drug development, since bioavailability must be considered when calculating dosages for non-intravenous routes of administration based on absorption.

The percentage of the drug absorbed is a measurement of the drug formulation's ability to deliver the drug to the target site.

“Amount absorbed” is conventionally measured by one of two criteria, either the area under the time-plasma concentration curve (AUC) or the total (cumulative) amount of drug excreted in the urine following drug administration,” according to the Boston University School of Medicine. “A linear relationship exists between “area under the curve” and dose when the fraction of drug absorbed is independent of dose, and elimination rate (half-life) and volume of distribution are independent of dose and dosage form. Alinearity of the relationship between area under the curve and dose may occur if, for example, the absorption process is a saturable one, or if drug fails to reach the systemic circulation because of, e.g., binding of drug in the intestine or biotransformation in the liver during the drug’s first transit through the portal system.”

Factors That Affect Bioavailability

When administered by a non-intravaneous route, the bioavailability of a drug varies from person to person. It may be affected by physiological and other factors including whether the drug is taken with other drugs or with or without food, and the presence of a disease that affects the gastrointestinal system or liver function.

Other factors affecting a drug's bioavailability are its physical properties, the drug formulation, such as extended release or immediate release, an individual's circadian rhythm, drug interactions, food interactions, rate of metabolism (the effect of enzyme induction or inhibition by other drugs and foods), health of the gastrointestinal tract, age of the patient, and the disease stage.

When a drug is administered intravenously, it has a bioavailability of 100 percent.

If a drug is taken orally, it quickly reaches the stomach, dissolves and some of it is absorbed by the small intestine. From the small intestine, it travels to the hepatic portal vein before it reaches the systemic circulation. Some of the factors that may prevent the drug from reaching the systemic circulation are the drug's properties and the patient's physiological state.

A drug such as benzyl penicillin cannot withstand the stomach's low pH, which destroys it. Digestive enzymes can destroy insulin and heparin.

Drugs which are highly hydrophobic may not be readily absorbed because they are insoluble in body fluids, while highly hydrophilic drugs – which have an attraction for fluids – cannot cross lipid-rich cell membranes.