Biliary Excretion

Biliary excretion is the elimination of drugs and metabolites by transport from the hepatocyte into bile and onward into the intestine. It is driven by efflux transporters on the canalicular membrane and tends to handle larger, more polar, and conjugated molecules. Because bile empties into the gut, excreted compounds may be reabsorbed, setting up enterohepatic recycling.

Definition

Biliary excretion is the elimination of drug from the body by carrier-mediated secretion across the hepatocyte canalicular membrane into bile, from which it passes into the intestine and is either eliminated in faeces or reabsorbed (enterohepatic recycling).

Scope

This entry covers the canalicular efflux transporters that secrete drugs into bile, the molecular and physicochemical features that favour biliary over renal elimination, and the phenomenon of enterohepatic circulation. It treats biliary excretion as a pharmacokinetic process for reference; it is not clinical guidance.

Core questions

• Which canalicular transporters secrete drugs and metabolites into bile?
• What molecular and physicochemical properties favour biliary over renal excretion?
• How does enterohepatic recirculation prolong a drug's residence in the body?
• How do hepatic uptake and canalicular efflux together set the rate of biliary clearance?

Key concepts

• Canalicular efflux transporters
• P-glycoprotein and BCRP
• Multidrug resistance-associated protein 2
• Conjugated metabolites
• Molecular-weight and polarity thresholds
• Enterohepatic circulation
• Biliary clearance

Mechanisms

After a drug is taken up into the hepatocyte and often conjugated, ATP-dependent efflux pumps on the canalicular (bile-facing) membrane move it against a concentration gradient into bile. P-glycoprotein, the breast cancer resistance protein, and multidrug resistance-associated protein 2 are principal exporters, with substrate preference shaped by size, charge, and conjugation; larger and more polar or conjugated compounds tend to favour the biliary route. Bile drains into the small intestine, where some excreted drug is eliminated in faeces while conjugated drug may be hydrolysed by gut bacteria back to the parent compound and reabsorbed, returning to the liver. This enterohepatic circulation can prolong residence in the body and produce secondary peaks in the concentration-time profile. Overall biliary clearance reflects the combined action of sinusoidal uptake and canalicular efflux.

Clinical relevance

Biliary excretion explains why some drugs are cleared mainly into faeces rather than urine, why genetic or pharmacological changes in canalicular transporters alter elimination, and why enterohepatic recycling can extend a drug's effect. The topic supports interpretation of disposition and drug-interaction studies; it describes mechanisms for reference and is not a basis for individual dosing.

Evidence & guidelines

The canalicular efflux transporters responsible for biliary secretion are catalogued in consensus reviews from the International Transporter Consortium, which also frame their role in drug development. The coordination of hepatic uptake with canalicular efflux is discussed in transporter reviews such as Niemi and colleagues, and the integration of these processes into clearance concepts is covered by standard pharmacokinetics texts.

History

Biliary excretion was long recognized physiologically, but its molecular basis was clarified from the 1990s onward with the cloning of canalicular ATP-binding cassette transporters such as P-glycoprotein, MRP2, and BCRP. Integration of these efflux systems with hepatic uptake transporters into a coherent hepatobiliary disposition framework followed in the 2000s and is summarized in International Transporter Consortium reviews.

Key figures

• Kathleen Giacomini
• Dietrich Keppler
• Yuichi Sugiyama

Seminal works

• giacomini-2010
• niemi-2011
• rowland-tozer-2011

Frequently asked questions

What kinds of drugs are mainly excreted in bile rather than urine?

Larger, more polar, and conjugated molecules tend to favour biliary excretion, because the canalicular efflux transporters that secrete drugs into bile handle these substrates well, whereas small water-soluble compounds are usually cleared renally.

What is enterohepatic circulation?

It is the recycling of a drug that is excreted into bile, delivered to the intestine, and then reabsorbed back into the blood, which can prolong how long the drug stays in the body and produce secondary concentration peaks.