ABC (Efflux) Transporters

Overview

P-gp (MDR1/ABCB1), BCRP (ABCG2) and BSEP (sPgp/ABCB11) are members of the ATP-binding cassette superfamily, are expressed in the luminal membrane of enterocytes, endothelial cells in the brain, brush border membrane of renal proximal tubules and canalicular membrane of hepatocytes where they limit the intestinal absorption, blood-brain barrier penetration and facilitate excretion into the bile and urine. BSEP is mainly expressed in the canalicular membrane of hepatocytes where it facilitates excretion into the bile. To evaluate if a test compound is a substrate of P-gp or BCRP, the bidirectional permeability of the test compound across a monolayer of cells (MDCKII-MDR1 or MDCKII-BCRP) in a transwell plate is measured. Inhibition is evaluated by measuring the ability of the test compound to reduce the bidirectional permeability of a probe substrate across caco-2 (P-gp) or MDCKII-BCRP cells. Accumulation of a test compound in BSEP expressing vesicles is measured to evaluate if the compound is a substrate of BSEP and inhibition is evaluated by measuring the effect of the test compound on the accumulation of a probe substrate.
View schematics for liver, kidney, intestine and brain transporters

P-gp(MDR1)

MDR1 (Multidrug Resistance protein, P-gp, ABCB1) is involved in the transport and excretion of mainly hydrophobic drugs out of cells. Drugs are excreted by MDR1 from cells to the lumen of the small intestine, therefore, it is an important factor in absorption and pharmacokinetics of many drugs. Typical substrates are digoxin and paclitaxel. MDR1 is highly expressed in tumor cells, and may cause multidrug resistance of anticancer drugs. MDR1 is expressed in the bile canaliculus of hepatocytes and in proximal tubules, and excretes compounds into the bile and urine. MDR1 is also expressed in the blood brain barrier (BBB) and prevents many compounds from entering the brain.

MRP2

MRP2 (cMOAT / ABCC2) is expressed in many tissues and transports relatively hydrophilic drugs. Drugs are excreted by MRP2 into bile from the liver and into the lumen of the small intestine or into urine from the kidney. MRP2 may cause multidrug resistance and drug-drug interactions along with MDR1 and BCRP. Typical substrates are cisplatin and indinavir.

BCRP

BCRP (MXR, ABCG2) is expressed in many tumor cells and like MDR1 and MRP2, it too may cause multidrug resistance against anticancer drugs. BCRP is also expressed in the small intestine, liver and brain and is involved in the excretion of drugs. Typical substrates are mitoxantrone and statins. Some substrates are known to overlap with MDR1 or MRP1.

BSEP

The bile salt export pump (BSEP, sPgp, ABCB11) is expressed on the canalicular (apical) membrane of hepatocytes and effluxes compounds into the bile. Inhibition of BSEP can cause cholestasis.

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Transwell assay design
Vesicle assay design
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