Drug transporters !

Membrane proteins that pump drugs into or out of cells in the gut, liver, kidney and brain, and a frequent cause of drug interactions.

Updated 2026-11-01

Many drugs do not simply diffuse across membranes: transporter proteins carry them. Efflux pumps push drugs out of cells (P-glycoprotein, BCRP); uptake carriers bring them in (OATP, OAT, OCT). Transporters shape absorption, entry into the brain, uptake into the liver and secretion into urine and bile, and blocking them changes drug levels just as blocking an enzyme does.

Two families

ABC transporters use ATP to pump substrates out of cells: P-glycoprotein (ABCB1), BCRP (ABCG2), MRPs
SLC transporters move substrates with or against gradients, mostly into cells: OATP1B1/1B3 (liver uptake), OAT1/OAT3 and OCT2 (kidney uptake), MATE (efflux into urine)

Where they matter

Gut: P-gp in the intestinal wall limits absorption of some drugs
Blood–brain barrier: P-gp keeps many drugs out of the brain
Liver: OATP carries drugs in; P-gp, BCRP and MRP2 push them into bile
Kidney: OAT and OCT take drugs up from blood; P-gp and MATE push them into urine
Grapefruit juice can also lower absorption of some drugs (such as fexofenadine) through gut uptake transporters

Classic interactions

Digoxin is a P-gp substrate with a narrow margin. The digoxin label lists amiodarone, verapamil, quinidine and propafenone among drugs that raise digoxin levels, and the FDA lists amiodarone and verapamil as P-gp inhibitors. Statins depend on OATP1B1 to reach the liver, where they act and are cleared; blocking OATP1B1 keeps them in the blood. Probenecid blocks renal OAT1/OAT3 and slows the secretion of anionic drugs; cimetidine blocks OCT2/MATE and can raise metformin levels.

Transporters and the clinic

When a label or interaction checker mentions P-gp, BCRP, OATP, OAT or OCT, it means the same logic as with CYP enzymes: an inhibitor raises the level of the substrate and an inducer lowers it. Genetic variants in transporter genes can also change response.

Test yourself

Think of the answer, then tap the question to see it.

How can a P-glycoprotein inhibitor raise digoxin levels?

Less digoxin is pumped back into the gut and into urine and bile, so more is absorbed and less is eliminated.

Why does inhibiting OATP1B1 increase the risk of statin muscle toxicity?

OATP1B1 carries statins into liver cells; when it is blocked, statins stay in the blood at higher levels and reach muscle.

Which renal transporters carry metformin into urine?

OCT2 takes it into kidney tubule cells and MATE transporters push it into the urine; inhibitors such as cimetidine can raise its level.

What role does P-glycoprotein play at the blood–brain barrier?

It pumps many drugs out of the brain capillary cells back into the blood, limiting their entry into the central nervous system.

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Example drugs

← All of pharmacology

Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.

Sources: For Healthcare Professionals | FDA’s Examples of Drugs that Interact with CYP Enzymes and Transporter Systems (U.S. Food and Drug Administration, 2026); DailyMed - DIGOXIN tablet (DailyMed (U.S. National Library of Medicine), 2026); Grapefruit Juice and Some Drugs Don't Mix (U.S. Food and Drug Administration, 2026).

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Representative model, for educational purposes only.