Bioavailability

AUC IVPO

Bioavailability refers to the proportion of a drug or other substance which enters the circulation when introduced into the body and so is able to have an active effect. It is a critical concept in pharmacology, nutrition, and pharmaceutical sciences, as it helps determine the correct dosage for non-intravenous administration of a drug. In simpler terms, bioavailability measures how much and how quickly a drug reaches its site of action in the body.

Definition

Bioavailability is defined as 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 generally decreases (due to incomplete absorption and first-pass metabolism) or may vary from patient to patient. Bioavailability is one of the essential tools in pharmacokinetics, as it must be considered when calculating dosages for non-intravenous routes of administration.

Factors Influencing Bioavailability

Several factors can influence the bioavailability of a substance. These include:

- Drug formulation and physical properties: The physical characteristics of a drug and its formulation play a significant role. For example, certain excipients used in drug formulations can enhance or inhibit drug absorption.

- Route of administration: The route of administration (oral, intravenous, intramuscular, inhalation, etc.) significantly affects bioavailability. Oral administration often results in lower bioavailability compared to intravenous administration due to the effects of digestive enzymes and first-pass metabolism.

- First-pass metabolism: Drugs administered orally are first exposed to the liver and may be extensively metabolized before reaching the systemic circulation. This first-pass effect can significantly reduce the bioavailability of the drug.

- Gastrointestinal tract factors: Factors such as gastric pH, motility, and the presence of food in the gut can influence the dissolution, absorption, and ultimately the bioavailability of a drug.

- Genetic factors: Genetic polymorphisms in drug-metabolizing enzymes can lead to variations in bioavailability among individuals.

Measurement of Bioavailability

Bioavailability is typically assessed by determining the area under the plasma concentration-time curve (AUC) from zero to infinity after the administration of a dose of the drug. The AUC is directly proportional to the total amount of unchanged drug that reaches systemic circulation. Bioavailability is usually expressed as a percentage of the AUC for the administered dose compared to the AUC for the same dose given intravenously.

Importance of Bioavailability

Understanding and optimizing the bioavailability of drugs is crucial for drug development and the safe and effective use of medications. It helps in determining the correct dosage for oral administration of drugs, ensuring that the drug reaches the required concentration in the systemic circulation to elicit the desired therapeutic effect without causing toxicity.

See Also

• Pharmacokinetics
• Drug metabolism
• Pharmacodynamics
• Drug delivery