ABC and SLC Transporters

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ATP-binding cassette (ABC) transporters and solute carrier (SLC) transporters are the two main classes that drug transporters can be categorized into. The ABC family of transporters requires ATP hydrolysis to transport substrates across membranes. Therefore, ABC transporters are primarily active transporters. Notable examples of ABC transporters include P-gp, multidrug resistance-associated protein (MRP), and breast cancer resistance protein (BCRP). In contrast to ABC transporters, SLC transporters do not have ATP-binding sites. Transport by SLC transporters use either an electrochemical potential difference in the substrate (i.e., facilitated transporters) or an ion gradient across membranes produced by primary active transporters and transport substrates against an electrochemical difference (i.e., secondary active transporters). Examples of SLC transporters include OATPs, organic anion transporters (OAT), organic cation transporters (OCT), etc. Most known drug transporters are SLC transporters.

The following are brief descriptions of select ABC transporters:

P-gp is involved in the ATP-dependent efflux of xenobiotics from cells and is probably the best characterized of all transporters. It was discovered originally as a result of its ability to confer resistance of tumors to anticancer drugs. P-gp is found to be expressed in the intestine, kidney, liver, and the brain. It plays a role in limiting the entry of certain drugs through the blood-brain barrier. It can also play a role in intestinal absorption and in biliary and urinary excretion. Digoxin is the best characterized of the P-gp substrates. Breast Cancer Resistance Protein (BCRP, MXR)

BCRP is a half ABC transporter that is expressed in the gastrointestinal tract, liver, kidney, brain, mammary tissues, testes, and the placenta. Similar to P-gp, BCRP was initially discovered due to its ability to confer resistance in cancer cell lines in vitro. BCRP plays a role in limiting oral bioavailability of certain drugs and limits entry of selected substrates through the blood-brain barrier, blood-testis barrier, and blood-placenta barrier.

The following are brief descriptions of select SLC transports: Organic Cation Transporter (OCT) and Organic Anion Transporter (OAT)

OCTs and OATs are found in kidney and play a role in the excretion of cations and anions (xenobiotics or endogenous), respectively, into the urine. They can also be found in other tissues such as hepatocytes where they act primarily as uptake transporters and intestinal epithelia (OCT1). Organic Anion Transporting Polypeptides (OATPs) OATPs are involved in the sodium-independent transport of a diverse range of amphiphilic organic compounds including bile acids, thyroid hormones and steroid conjugates, and many xenobiotics. OATPs can be found in liver, intestine, kidney, and blood-brain barrier. This transporter appears to be involved in clinically relevant transporter drug-drug interactions that are of the largest magnitude.

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