Permeability and Transport Services

Permeability and Transport Services

XenoGesis offers a complete panel of transporter assays including, but not limited to, all the transporters mentioned in the EMA and FDA guidelines.

Over the past decade drug-transporter interactions have gained significant importance in drug discovery and development. Not only can drug-transporter interactions affect all aspects of ADMET, they can also be a cause of clinically relevant drug-drug interactions.

Both the American Food and Drug Administration (FDA) and the European Medicines Agency (EMA) recently introduced recommendations for the assessment of drug-transporter interactions and their implementation in the planning of clinical DDI studies.

Knowing about potential interactions early in drug discovery can help to make the right decisions and mitigate against any potential issue.

Please see the International Transporter Consortium publications 2010, and 2013The FDA Guidance (2012) and the EMA Guideline on the investigation of drug interactions (2012) for reference.

A number of valuable tools have become available to assess drug-transporter interactions In vitro including cell-free, vesicle based assays, cell lines expressing individual transporters in monolayer or suspension cultures, polarised cells expressing combinations of uptake and efflux transporters (Caco‑2, MDCKII, LLC-PK1) and primary hepatocytes in sandwich cultures.

Drug transporter assays currently available from XenoGesis

SLC transporters

Transporter Recommended by


ABC transporters

Transporter Recommended by
MDR1 (P-gp) EMA | FDA


Drug transporters: An overview
Drug transporters belong to three categories of transport proteins: ATP-binding cassette (ABC) transporters, solute carriers (SLC) and P-type ATPases.

ABC transporters
Most eukaryotic ABC transporters are efflux pumps. Exceptions are the cystic fibrosis transmembrane conductance regulator (CFTR) and the sulfonylurea receptors (SUR), which are ion channels. ABC transporters are primary active transporters, i.e. they use ATP as an energy source in order to translocate their substrates across biological membranes and can generate huge concentration gradients. The ATP binding motifes have been highly preserved during evolution and can, therefore, be used to identify ABC superfamily members. ABC transporters of particular pharmacological interest include P-glycoprotein (P-gp), also known as multi-drug resistance protein 1 (MDR1), which is the first member of the ABCB family (gene symbol ABCB1). Other ABC transporters, for which an involvement in drug ADMET has been shown, are the Multidrug Resistance-Associated Proteins 1 (MRP1, ABCC1) and MRP2 (ABCC2), also known as canalicular Multi-specific Organic Anion Transporter (cMOAT), the Breast Cancer Resistance Protein (BCRP, ABCG2) and the Bile Salt Export Pump (BSEP, ABCB11).

SLC transporters
Slide3Most SLC transporters are secondary active transporters, i.e. they use the electrochemical potential stored in ion gradients such as sodium, protons or bicarbonate. Like primary active transporters, secondary active transporters can pump their substrates against a concentration gradient. Exceptions are the equilibrative nucleoside transporters (ENTs, SLC29).

SLC transporters that are of particular interest in drug R&D include the organic anion transporting polypeptides (OATPs), organic anion transporters (OATs), organic cation transporters (OCTs), the bile salt uptake transporters NTCP and ASBT as well as the multidrug and toxin extrusion transporters (MATEs).

P-type ATPases
P-type ATPases are an evolutionary very old class of proteins that translocate ions and lipids across biological membranes. Like ABC transporters, the P-typ ATPases are primary active transporters.

Two members, namely ATP7A and ATP7B are involved in the detoxification of copper. Both these proteins have been shown to transport the platinum containing cancer drugs cisplatin, carboplatin and oxaliplatin and might, therefore, play a role in drug resistance.

FDA guidelines

“Development of a drug should include identification of the principal routes of elimination, quantitation of the contribution by enzymes and transporters to drug disposition, and characterization of the mechanism of drug-drug interactions.”

FDA Guidance for Industry, Drug Interaction Studies —Study Design, Data Analysis, Implications for Dosing, and Labeling Recommendations.

Investigational drug as a transporter substrate

The current American Food and Drug Administration (FDA) draft guidelines suggest that “all investigational drugs should be evaluated in vitro to determine whether they are a potential substrate of P-gp and BCRP”.

When hepatic metabolism and/or biliary excretion constitute ≥25% of total drug clearance, then it should be investigated if this drug is a substrate of OATP1B1 and/or OATP1B3.”

Similarly, drugs that are ≥25% eliminated via active renal secretion should be investigated as potential substrates of OAT1, OAT3 and OCT2.

Other transporters should be investigated as and when their involvement appears likely based on experience from related compounds or the scientific literature.

Investigational drug as a transporter inhibitor

The current FDA draft guidelines also recommend to clarify whether an investigational drug is an inhibitor of all the above transporters.

EMA guidelines

“In vitro inhibition studies are recommended to investigate whether the investigational drug inhibits any of the transporters known to be involved in clinically relevant in vivo drug interaction studies.”

European Medicines Agency: Guideline on the investigation of drug interactions (2012)


Investigational drug as a transporter substrate

In agreement with the FDA guidance, the EMA guidance recommends to investigate whether an investigational drug is a substrate of OATP1B1 and OATP1B3, if it is ≥25% eliminated hepatically.

If an investigational drug is or may be ≥25% eliminated through renal, biliary or intestinal secretion, then the candidate main transporter(s) involved should be identified through in vitro studies.

Investigational drug as a transporter inhibitor

Like the FDA guideline, the EMA Guideline on the investigation of drug interactions suggests to explore whether an investigational drug is an inhibitor of P-gp, BCRP, OATP1B1, OATP1B3, OAT1, OAT3 and OCT2. In addition, inhibition studies with OCT1, MATE1 and MATE2 and BSEP should be considered.