How do transporter interactions differ from CYP enzyme interactions?

Enzyme interactions change how fast a drug is chemically metabolized, whereas transporter interactions change where the drug goes — its absorption, entry into tissues, or excretion — by altering carrier-mediated movement across membranes; the two can occur together because transporters and enzymes often act at the same sites.

Why is P-glycoprotein important in drug interactions?

P-glycoprotein pumps drugs out of intestinal, brain, and kidney cells, so inhibiting it can increase a substrate's oral absorption, its access to the brain, or its retention in the body, changing exposure even when metabolism is unaffected.

Transporter-Mediated Drug Interactions

Membrane transporters move drugs into and out of cells across the gut, liver, kidney, and blood-brain barrier, and they are a second major source of pharmacokinetic drug interactions alongside metabolizing enzymes. When one drug inhibits or competes with a transporter, it changes the absorption, distribution, or elimination of another, altering its exposure. This topic covers the main transporter families and how their interactions arise and are classified.

Definition

A transporter-mediated drug interaction is a pharmacokinetic interaction in which one substance inhibits or competes for a membrane transporter, changing the absorption, tissue distribution, or excretion of a co-administered drug that is a substrate of that transporter.

Scope

The topic covers efflux transporters such as P-glycoprotein and uptake transporters such as the organic anion transporting polypeptides (OATPs) and organic cation/anion transporters, the tissues where they govern drug disposition, and how transporter inhibition produces interactions. It is framed as mechanistic reference knowledge, not as prescribing guidance.

Core questions

Which uptake and efflux transporters most often govern drug disposition?
How does transporter inhibition alter absorption, hepatic uptake, and renal or biliary excretion?
How do transporter and enzyme interactions overlap, and how are they distinguished?
How is the clinical significance of a transporter interaction predicted and graded?

Key concepts

Efflux transporters (e.g., P-glycoprotein/ABCB1)
Uptake transporters (e.g., OATP1B1)
Organic cation and anion transporters
Substrate and inhibitor
Blood-brain barrier and intestinal efflux
Hepatic uptake and biliary excretion
Renal tubular secretion

Mechanisms

Transporters are membrane proteins that carry drugs across cell membranes either into cells (uptake transporters such as OATP1B1 in hepatocytes) or out of them (efflux transporters such as P-glycoprotein in the intestine, blood-brain barrier, and renal tubule). Inhibiting an efflux transporter can increase the absorption of a substrate or its penetration into protected tissues, while inhibiting an uptake transporter can reduce a drug's clearance by limiting its delivery to metabolizing organs, raising plasma concentrations. Because transporters often act at the same sites as drug-metabolizing enzymes and may share substrates with them, transporter and CYP interactions can occur together and must be disentangled when interpreting exposure changes. The International Transporter Consortium identified a priority set of transporters whose interactions are most likely to be clinically important, providing a framework for prediction.

Clinical relevance

Transporter interactions explain exposure changes that cannot be attributed to metabolism alone, and they inform the transporter-related warnings in product information and decision support. This entry describes the mechanism and classification of such interactions for reference and does not provide dosing or individualized management advice.

Evidence & guidelines

Mechanistic and pharmacokinetic studies, together with the International Transporter Consortium's framework and regulatory transporter-interaction recommendations, form the evidence base for identifying clinically relevant transporter interactions. Here that evidence is summarized to explain mechanism rather than to direct therapy.

History

Although active drug transport was recognized for decades, the systematic role of named transporters in drug interactions came into focus in the 2000s, when P-glycoprotein and the OATPs were shown to govern oral absorption and hepatic uptake of widely used drugs. The 2010 International Transporter Consortium synthesis consolidated this knowledge into a prioritized framework for drug development and interaction prediction.

Key figures

Mikko Niemi
Pertti J. Neuvonen
Grant R. Wilkinson

Seminal works

itc-2010
niemi-2011

Frequently asked questions

How do transporter interactions differ from CYP enzyme interactions?: Enzyme interactions change how fast a drug is chemically metabolized, whereas transporter interactions change where the drug goes — its absorption, entry into tissues, or excretion — by altering carrier-mediated movement across membranes; the two can occur together because transporters and enzymes often act at the same sites.
Why is P-glycoprotein important in drug interactions?: P-glycoprotein pumps drugs out of intestinal, brain, and kidney cells, so inhibiting it can increase a substrate's oral absorption, its access to the brain, or its retention in the body, changing exposure even when metabolism is unaffected.