Drug Absorption and Permeability

Drug absorption is the process by which a drug moves from its site of administration into the systemic circulation. For orally administered drugs, the rate and extent of absorption depend on the drug dissolving in gastrointestinal fluids and then permeating the intestinal epithelium, so solubility and membrane permeability are the two pivotal properties.

Definition

Drug absorption is the transfer of a drug from its administration site into the systemic circulation; permeability is the propensity of a drug to cross a biological membrane, a principal determinant of the rate and extent of absorption.

Scope

The entry covers the routes and mechanisms of absorption, the determinants of intestinal permeability, the role of membrane transporters and efflux, and the classification of drugs by solubility and permeability. It is a conceptual reference on how the absorption step is characterised, not a guide to formulation or dosing decisions.

Core questions

• By what routes and mechanisms does a drug cross biological membranes after administration?
• How do a drug's solubility and permeability determine the extent of its absorption?
• What roles do uptake and efflux transporters play in intestinal absorption?
• Why does the same dose produce different exposure across formulations and individuals?

Key concepts

• Passive diffusion
• Carrier-mediated (active and facilitated) transport
• Paracellular vs transcellular pathways
• Solubility and dissolution
• Lipophilicity and ionisation (pH-partition)
• Uptake and efflux transporters (e.g. P-glycoprotein)
• First-pass metabolism limiting bioavailability

Key theories

Biopharmaceutics Classification System (BCS)

Classifies drugs by aqueous solubility and intestinal permeability into four classes, correlating in vitro dissolution with in vivo absorption and providing a rational basis for predicting and regulating oral bioavailability.

Rule of five (drug-likeness for absorption)

An empirical set of physicochemical limits — on molecular weight, lipophilicity, and hydrogen-bond donors and acceptors — beyond which passive oral absorption becomes less likely, used to flag poor permeability early in development.

Mechanisms

Most small-molecule drugs cross the intestinal epithelium by passive transcellular diffusion, driven by the concentration gradient and governed by lipophilicity, degree of ionisation, and molecular size; small polar molecules may also pass paracellularly. Carrier-mediated transport adds selectivity: uptake transporters can carry drugs into enterocytes, while efflux transporters such as P-glycoprotein pump them back into the lumen, limiting absorption. For oral drugs the rate-limiting step is often dissolution or permeability, which is why the BCS pairs these two properties; molecules that violate empirical drug-likeness limits tend to permeate poorly. Drug that is absorbed may still be lost to metabolism in the gut wall and liver before reaching the systemic circulation, reducing bioavailability.

Clinical relevance

Absorption and permeability determine how much of an administered dose becomes systemically available, which is central to comparing formulations and understanding why oral exposure varies. This entry describes the science of the absorption step as a reference; it does not provide formulation or dosing recommendations.

Evidence & guidelines

The Biopharmaceutics Classification System underlies international regulatory guidance on dissolution testing and biowaivers, and the International Transporter Consortium's recommendations frame how transporter-mediated effects on absorption are assessed in drug development.

History

Early understanding of absorption rested on the pH-partition hypothesis, which related membrane crossing to lipophilicity and ionisation. Recognition that carrier proteins mediate uptake and efflux complicated this passive picture, and by the 1990s the Biopharmaceutics Classification System combined solubility and permeability into a predictive scheme. Lipinski's rule of five then gave medicinal chemistry a rapid filter for oral absorptive potential, and the transporter consortium consolidated how transporters are evaluated.

Key figures

• Gordon L. Amidon
• Hans Lennernäs
• Christopher A. Lipinski
• Kathleen M. Giacomini

Seminal works

• amidon-1995
• lipinski-2012
• iticc-2010

Frequently asked questions

What two properties most determine oral drug absorption?

Aqueous solubility (whether the drug dissolves in gastrointestinal fluids) and membrane permeability (whether it can cross the intestinal epithelium) — the two axes of the Biopharmaceutics Classification System.

How can a drug be absorbed yet still have low bioavailability?

Drug absorbed across the gut wall can be metabolised in the intestinal wall and liver before reaching the systemic circulation; this first-pass effect can substantially reduce the fraction that becomes systemically available.