Absorption and Bioavailability
Absorption and bioavailability describe how a drug moves from its site of administration into the systemic circulation and what fraction of the administered dose reaches it intact. As the first phase of pharmacokinetics, this area sets the input that distribution, metabolism, and elimination then act upon, and it explains why the same dose given by different routes can produce very different exposures.
Definition
Absorption is the process by which an administered drug passes from its application site into the systemic circulation; bioavailability (F) is the fraction of the administered dose that reaches the systemic circulation in an unchanged form, taken as 1 for an intravenous dose.
Scope
This area orients the reader to drug absorption and the concept of bioavailability. It groups the routes by which drugs enter the body, the first-pass losses that occur before a drug reaches the systemic circulation, and the quantitative measure of bioavailability itself. It is a conceptual map of the absorption phase rather than a guide to dosing or product selection.
Sub-topics
Key concepts
- Absorption as the input phase of ADME
- Route of administration
- Extent versus rate of absorption
- First-pass (presystemic) elimination
- Absolute and relative bioavailability
- Dissolution, solubility, and permeability
Mechanisms
After administration, a drug must dissolve (if given as a solid) and then cross biological membranes — most often by passive diffusion of the un-ionised form, sometimes via transporters — to enter the blood. For orally administered drugs the absorbed fraction passes through the gut wall and the liver before reaching the systemic circulation, so presystemic (first-pass) metabolism can remove part of the dose. The net result of dissolution, membrane permeability, and first-pass loss determines bioavailability, which Amidon and colleagues linked to a substance's solubility and intestinal permeability in the Biopharmaceutics Classification System.
Clinical relevance
Because bioavailability governs how much of a dose actually reaches the circulation, it underlies why intravenous and oral exposures differ and why some drugs are given parenterally. This area describes the principles used to interpret exposure differences across routes and formulations; it is educational background for evidence appraisal and does not provide dosing or treatment recommendations.
Evidence & guidelines
The Biopharmaceutics Classification System of Amidon and colleagues is a widely used framework relating in vitro dissolution and intestinal permeability to in vivo bioavailability, and it informs regulatory thinking on oral drug products. General pharmacokinetic texts such as Rowland and Tozer codify the definitions of absorption and bioavailability used here.
History
The quantitative study of absorption and bioavailability matured through the second half of the twentieth century alongside biopharmaceutics and clinical pharmacokinetics. Pond and Tozer's 1984 synthesis clarified first-pass elimination, and Amidon and colleagues' 1995 Biopharmaceutics Classification System tied bioavailability to the measurable properties of solubility and permeability, shaping how absorption is now taught and regulated.
Key figures
- Gordon L. Amidon
- Malcolm Rowland
- Thomas N. Tozer
- Susan M. Pond
Related topics
Seminal works
- amidon-1995
- pond-tozer-1984
Frequently asked questions
- Why is intravenous bioavailability defined as 100%?
- An intravenous dose is placed directly into the systemic circulation, so by definition the entire dose reaches it; other routes are compared against this reference, giving an absolute bioavailability between 0 and 1.
- What is the difference between absorption and bioavailability?
- Absorption is the movement of a drug from its administration site into the blood; bioavailability is the fraction of the dose that actually reaches the systemic circulation intact, which can be lower than the absorbed fraction because of first-pass losses.