Bioavailability Study – Definition and Process

What Is a Bioavailability Study?

A bioavailability study is a scientific investigation that determines what proportion of an administered active substance reaches the systemic circulation in unchanged form and is thus available at the site of action. Bioavailability describes both the extent and the rate at which an active ingredient is absorbed by the body. These studies are fundamental to drug development, regulatory approval processes, and the optimization of therapeutic regimens.

Objectives and Importance

Bioavailability studies serve several key purposes:

• Quantifying the amount of an active substance absorbed after oral, intravenous, or other routes of administration
• Comparing different dosage forms (e.g., tablet vs. capsule vs. solution)
• Assessing the influence of food, age, sex, or disease states on drug absorption
• Providing the basis for determining appropriate dosage regimens
• Demonstrating the bioequivalence of generic medicines to their reference products

Types of Bioavailability Studies

Absolute Bioavailability

Absolute bioavailability is determined by comparing the systemic availability of a drug after non-intravenous administration (e.g., oral) with that following intravenous administration. Since intravenous delivery is by definition 100% bioavailable, this comparison reveals the true fraction of an orally administered dose that reaches the bloodstream.

Relative Bioavailability

Relative bioavailability compares two different formulations or dosage forms of the same active substance without using an intravenous reference. This approach is particularly relevant when developing new generic products or reformulating an existing drug.

Bioequivalence Studies

A specific type of bioavailability study is the bioequivalence study. Its purpose is to demonstrate that two drug formulations (e.g., a generic and the originator product) exhibit sufficiently similar bioavailability to be considered therapeutically interchangeable. Regulatory agencies such as the EMA (European Medicines Agency) and the FDA (U.S. Food and Drug Administration) require such studies for the approval of generic medicines.

How a Bioavailability Study Is Conducted

Bioavailability studies are typically conducted as Phase I clinical trials in healthy volunteers, or in certain cases in specific patient populations. The general procedure includes the following steps:

• Study design: A crossover design is commonly used, in which all participants receive each formulation in sequence to minimize individual variability.
• Drug administration: Subjects receive the test formulation (e.g., a tablet) and, where applicable, a reference formulation.
• Blood sampling: Blood samples are collected at predefined time points to measure plasma drug concentrations.
• Pharmacokinetic analysis: A concentration-time profile is generated from the measured values. Key parameters include the AUC (area under the concentration-time curve), Cmax (maximum plasma concentration), and Tmax (time to maximum concentration).
• Statistical evaluation: Results are analyzed statistically and compared against regulatory acceptance criteria.

Factors Affecting Bioavailability

The bioavailability of an active substance can be influenced by numerous factors:

• Chemical properties: Solubility, stability, and molecular size of the active substance
• Dosage form: Tablets, capsules, liquids, or patches may exhibit different absorption rates
• First-pass effect: Following oral administration, a portion of the drug may be metabolized in the intestinal wall or liver before reaching systemic circulation
• Food intake: Certain foods can accelerate, slow down, or inhibit drug absorption
• Individual factors: Age, sex, genetic differences in metabolism, and comorbidities
• Drug interactions: Concomitant medications may alter the bioavailability of the substance being studied

Regulatory Requirements

Bioavailability studies are a central component of drug authorization. Competent authorities, including the EMA in Europe and the FDA in the United States, have issued detailed guidelines for the design, conduct, and evaluation of such studies. Compliance with Good Clinical Practice (GCP) and Good Laboratory Practice (GLP) standards is mandatory throughout the process.

Relevance for Patients

For patients, bioavailability studies have significant practical implications. They provide scientific assurance that a generic medicine is as effective as the original product and help define optimal dosing schedules. They also generate evidence on whether a medication should be taken with or without food, and whether specific dietary components or other drugs may influence therapeutic outcomes.

References

1. European Medicines Agency (EMA): Guideline on the Investigation of Bioequivalence. CPMP/EWP/QWP/1401/98 Rev. 1, 2010. Available at: https://www.ema.europa.eu
2. U.S. Food and Drug Administration (FDA): Guidance for Industry - Bioavailability and Bioequivalence Studies for Orally Administered Drug Products. FDA, 2003. Available at: https://www.fda.gov
3. Rowland M., Tozer T.N.: Clinical Pharmacokinetics and Pharmacodynamics: Concepts and Applications. 4th edition. Lippincott Williams and Wilkins, 2010.