Drug Elimination and Excretion

Drug elimination is the irreversible loss of drug from the body. It occurs by metabolism and by excretion — the removal of unchanged drug or its metabolites into urine, bile, or other routes. The kidney is the dominant excretory organ, handling drugs through glomerular filtration, active tubular secretion, and reabsorption.

Definition

Drug elimination is the irreversible removal of a drug from the body by metabolism and excretion; excretion is the transfer of unchanged drug or metabolites out of the body, predominantly by the kidney into urine and to a lesser extent by the liver into bile.

Scope

The entry covers routes and mechanisms of excretion, the renal processes of filtration, secretion, and reabsorption, the role of transporters, biliary and enterohepatic recycling, and how excretion contributes to total clearance. It is a conceptual reference and does not provide dosing guidance.

Core questions

• By what routes is drug removed from the body, and which predominates?
• How do glomerular filtration, tubular secretion, and reabsorption together determine renal excretion?
• What roles do membrane transporters play in renal and biliary excretion?
• How does excretion combine with metabolism to give total clearance?

Key concepts

• Glomerular filtration
• Active tubular secretion
• Tubular reabsorption
• Renal clearance
• Biliary excretion and enterohepatic recirculation
• Renal drug transporters (OATs, OCTs, MATEs, P-glycoprotein)
• Influence of urine pH and flow on reabsorption

Key theories

Three-process model of renal drug handling

Renal excretion of a drug is the net result of glomerular filtration of unbound drug, active tubular secretion via uptake and efflux transporters, and passive or active tubular reabsorption, so renal clearance can be lower or much higher than the filtration rate depending on which processes dominate.

Mechanisms

Most drugs leave the body either after metabolic conversion or by direct excretion of the unchanged molecule. Renal excretion combines three processes: unbound drug is filtered at the glomerulus; some drugs are actively secreted into the tubular lumen by uptake and efflux transporters (such as the organic anion and cation transporters and the MATE proteins); and lipophilic, un-ionised drug can be passively reabsorbed, a process sensitive to urine pH and flow. The net of these processes is renal clearance. The liver also excretes drugs and conjugates into bile, from which some may be reabsorbed in the intestine (enterohepatic recirculation), prolonging their presence in the body. Total elimination, expressed as total clearance, is the sum of renal, hepatic, and any other clearances.

Clinical relevance

Elimination determines how quickly drug and metabolites are removed and therefore how exposure declines over time; impaired excretory organ function reduces clearance and raises exposure. This entry describes those mechanisms as a reference and is not a basis for individualised dose adjustment.

Evidence & guidelines

The role of renal transporters in excretion and in transporter-mediated interactions is set out in the International Transporter Consortium's recommendations, which inform regulatory guidance on evaluating drug excretion and interactions during development; the renal handling model is standard in clinical-pharmacokinetics texts.

History

Classical renal physiology established that the kidney handles solutes by filtration, secretion, and reabsorption, and this framework was applied to drugs to explain why renal clearance can exceed the filtration rate. The molecular identification of organic anion and cation transporters, and later the MATE proteins, clarified the mechanisms of active secretion and the basis of transporter-mediated excretory interactions, consolidated by the International Transporter Consortium.

Key figures

• Kathleen M. Giacomini
• Malcolm Rowland
• Thomas N. Tozer
• Grant R. Wilkinson

Seminal works

• morrissey-2013
• iticc-2010

Frequently asked questions

What is the main route of drug excretion?

The kidney is the dominant route, removing unchanged drug and metabolites into urine through filtration, secretion, and reabsorption; the liver contributes by excreting drugs and conjugates into bile.

How can renal clearance be higher than the glomerular filtration rate?

When a drug is actively secreted into the tubule by transporters in addition to being filtered, its renal clearance can exceed the glomerular filtration rate; conversely, extensive reabsorption can make renal clearance much lower.