Why can an oral drug have low bioavailability even if it is well absorbed?

Drug absorbed from the gut passes through the liver before reaching the systemic circulation, where it may be extensively metabolised. This first-pass effect can sharply reduce the fraction that becomes systemically available even when intestinal absorption is good.

What does the Biopharmaceutics Classification System describe?

It classifies orally administered drugs by their aqueous solubility and intestinal permeability into four classes, providing a framework to anticipate how dissolution and permeability limit absorption.

Absorption, Distribution, and Bioavailability

Absorption is the process by which a drug moves from its site of administration into the systemic circulation; distribution is its subsequent partitioning between blood and tissues; and bioavailability is the fraction of an administered dose that reaches the circulation unchanged. Together these processes determine how much drug becomes available to act and how it spreads through the body.

Definition

Absorption is the transfer of a drug from its administration site into the systemic circulation; distribution is the reversible movement of a drug between the circulation and tissues; and bioavailability is the fraction (F) of an administered dose that reaches the systemic circulation in unchanged form.

Scope

This topic covers the first two steps of drug disposition — getting drug into the blood and spreading it through the body — and the parameter, bioavailability, that quantifies the completeness of absorption. It addresses oral absorption, the first-pass effect, factors governing tissue distribution and protein binding, and the Biopharmaceutics Classification System. It is conceptual and educational and gives no dosing guidance.

Core questions

What fraction of an oral dose survives absorption and first-pass metabolism to reach the systemic circulation?
Which physicochemical and physiological factors limit absorption rate and extent?
How do plasma protein binding and tissue affinity shape a drug's distribution?
How is absolute bioavailability measured against an intravenous reference?

Key concepts

Bioavailability (F)
First-pass (presystemic) metabolism
Biopharmaceutics Classification System (BCS)
Dissolution and solubility
Membrane permeability
Plasma protein binding
Tissue partitioning
Absolute versus relative bioavailability

Mechanisms

For an orally administered drug, absorption requires dissolution in gastrointestinal fluids and permeation across the intestinal epithelium; the Biopharmaceutics Classification System organises drugs by their solubility and permeability to predict absorption behaviour (Amidon et al., 1995). Drug absorbed from the gut passes through the portal circulation and liver before reaching the systemic blood, so presystemic (first-pass) metabolism can reduce the fraction that becomes available. Bioavailability is quantified by comparing systemic exposure after extravascular administration with that after an intravenous dose, for which availability is complete by definition (Toutain & Bousquet-Mélou, 2004). Once in the circulation, a drug distributes according to its binding to plasma proteins and its affinity for tissues, which together govern how widely it leaves the plasma.

Clinical relevance

Bioavailability and distribution explain why the same dose by different routes produces different exposures and why some drugs require formulation strategies to be absorbed reliably. This entry describes those principles as background to evidence appraisal and formulation science; it is not a basis for selecting routes or doses for an individual patient.

Evidence & guidelines

Regulatory bioequivalence assessment rests on bioavailability comparisons, and the Biopharmaceutics Classification System (Amidon et al., 1995) informs biowaiver frameworks used by drug regulators. The measurement principles and the definition of absolute and relative bioavailability are codified in standard pharmacokinetics texts (Rowland & Tozer, 2011) and review articles (Toutain & Bousquet-Mélou, 2004).

History

Quantitative treatment of absorption matured alongside the broader pharmacokinetic framework of the 1970s, but a defining advance came in 1995 with the Biopharmaceutics Classification System, which linked the in vitro dissolution and in vivo absorption of orally administered drugs through solubility and permeability and reshaped how absorption is predicted and regulated (Amidon et al., 1995).

Key figures

Gordon L. Amidon
Hans Lennernäs
Malcolm Rowland
Pierre-Louis Toutain

Seminal works

amidon-1995
toutain-bioavail-2004

Frequently asked questions

Why can an oral drug have low bioavailability even if it is well absorbed?: Drug absorbed from the gut passes through the liver before reaching the systemic circulation, where it may be extensively metabolised. This first-pass effect can sharply reduce the fraction that becomes systemically available even when intestinal absorption is good.
What does the Biopharmaceutics Classification System describe?: It classifies orally administered drugs by their aqueous solubility and intestinal permeability into four classes, providing a framework to anticipate how dissolution and permeability limit absorption.