Drug Absorption Mechanisms

Drug absorption is the movement of a drug from its site of administration into the systemic circulation. For orally administered drugs - the most common route - this means crossing the gastrointestinal epithelium, a step controlled by the drug's dissolution and its ability to permeate cell membranes. Absorption is one of the four ADME processes and is a key determinant of how much of an administered dose becomes available to act.

Definition

Drug absorption is the process by which a drug passes from its site of administration across biological membranes into the systemic circulation, determined chiefly by dissolution and by membrane permeability through passive and carrier-mediated pathways.

Scope

This topic covers the routes by which drugs cross biological barriers, the molecular mechanisms of membrane permeation (passive diffusion, carrier-mediated transport, and paracellular passage), and the physicochemical properties that govern them. It treats absorption as a pharmacokinetic and physicochemical topic; it does not provide dosing or formulation prescriptions.

Core questions

• By what mechanisms do drug molecules cross epithelial membranes?
• Which physicochemical properties make a molecule well or poorly absorbed?
• How do solubility and permeability jointly limit oral absorption?

Key concepts

• Passive transcellular diffusion
• Carrier-mediated (active) transport
• Paracellular transport
• Dissolution and solubility
• Permeability
• Lipophilicity (logP / logD)
• Ionisation and the pH-partition hypothesis
• Efflux transporters (e.g., P-glycoprotein)

Key theories

Biopharmaceutics Classification System (BCS)

Amidon and colleagues proposed classifying drugs into four classes by their aqueous solubility and intestinal permeability, providing a framework that links in vitro dissolution to in vivo absorption and predicts which compounds are absorption-limited.

Rule of five

Lipinski's analysis associated poor oral absorption and permeability with molecules exceeding thresholds in molecular weight, lipophilicity (logP), and hydrogen-bond donors and acceptors, giving medicinal chemists structural guidelines for designing absorbable compounds.

Mechanisms

A drug must first dissolve in the gastrointestinal fluids and then permeate the intestinal epithelium. Most small lipophilic drugs cross by passive transcellular diffusion, driven by the concentration gradient and favoured by appropriate lipophilicity and low hydrogen-bonding capacity; the degree of ionisation, set by the drug's pKa and the local pH, modulates the fraction available to diffuse (the pH-partition hypothesis). Polar or charged molecules may instead use carrier-mediated transporters or pass between cells through tight junctions (paracellular route). Both passive and carrier-mediated processes commonly operate together for the same drug (Sugano, 2010). Efflux transporters such as P-glycoprotein can pump drug back into the lumen and reduce net absorption. Whether a compound is ultimately absorption-limited depends on the interplay of solubility and permeability captured by the BCS (Amidon, 1995).

Clinical relevance

Absorption mechanisms explain why some drugs are well suited to oral administration while others require alternative routes, and why formulation and food can influence the extent of absorption. This entry describes the determinants of absorption conceptually and is not a basis for individual dosing or administration decisions.

Evidence & guidelines

The Biopharmaceutics Classification System underpins regulatory frameworks for biowaivers, in which in vitro dissolution data can substitute for in vivo bioequivalence studies for certain high-solubility, high-permeability drugs (Amidon, 1995). Permeability and solubility screening assays are standard early ADME tools, interpreted in light of physicochemical design rules (Lipinski, 2001).

History

The pH-partition hypothesis of the mid-twentieth century framed absorption in terms of lipid solubility and ionisation. The Biopharmaceutics Classification System (1995) reframed it around the joint roles of solubility and permeability, and the rule of five (2001) translated absorption determinants into structural guidelines, embedding absorption thinking into early drug design. Recognition that carrier-mediated and passive routes coexist has since refined the simple passive-diffusion picture (Sugano, 2010).

Debates

How much of drug absorption is carrier-mediated versus passive?

A long-standing question is whether passive transcellular diffusion dominates intestinal absorption or whether transporters play a larger, sometimes underappreciated role; the consensus view is that passive and carrier-mediated processes coexist, with their relative contribution varying by compound.

Key figures

• Gordon Amidon
• Hans Lennernas
• Christopher Lipinski
• Kiyohiko Sugano
• Per Artursson

Seminal works

• amidon-1995
• lipinski-2001
• sugano-2010

Frequently asked questions

What is the difference between passive and active drug absorption?

Passive absorption (diffusion) moves drug down its concentration gradient without energy or a carrier, favouring lipophilic, uncharged molecules. Active or carrier-mediated absorption uses membrane transporter proteins and can move drug against a gradient; the two often operate together for the same compound.

Why are some drugs poorly absorbed when taken orally?

Poor oral absorption usually reflects low aqueous solubility, low membrane permeability, or both - the combination captured by the Biopharmaceutics Classification System - and can be compounded by efflux transporters and metabolism in the gut wall.