Uptake Transporter Assay

The cellular uptake mechanism of transporters plays a key role in the pharmacokinetic profile of many structurally unrelated drugs. Present at all important pharmacological barrier tissues, these proteins enable the translocation of medium to low permeability drug substances across plasma membranes, and thereby determine absorption, distribution and elimination routing. Testing for interactions with uptake transporters has the potential to predict the main factors in the system level movement of drugs. Assessing the inhibitory potential of test articles on uptake transporters may indicate drug-drug interaction implications, such as in the case of cyclosporin A and statins1, 2.

Today, combination therapies are widely used for drugs of many different therapeutic classes. Statins are often co-medicated with other drugs like immuno-suppressants, antidiabetic drugs or fibrates to treat patients in clinics. This increases the risk for significant drug-drug interactions amongst these drugs3, 4, 5, 6.

Therefore, it is important to address the possible drug-drug interaction risks relevant for the respective patient population by using appropriate in vitro tools to guide tailor-made clinical interaction studies in drug development. In addition to the well established cytochrome P450 interaction studies, drug-drug interactions can also occur at the level of active drug transport, involving OATP and OAT carriers and/or different ABC transporters. In the context of drug-drug interactions involving statins, members of the OATP solute carrier family were found to be involved in addition to cytochrome P450 enzymes.

Recombinant cell lines expressing human OATP carriers are often used to study the in vitro drug-drug interaction potential at the level of these carriers7. The indirect and direct uptake transporter assays are performed with stable transfectants expressing selected human or rat uptake transporters. Cells adhere to treated surfaces, which allow the easy separation of transported and excess low permeability substrate molecules.

Inhibitor assessment studies

In these assays the inhibitory potential of the test article (TA) on the uptake transporter-mediated transport of a labeled probe substrate is measured. The assay is performed with cold TA. The probe substrate is transported into the transfected cells by the selected human uptake transporter. Values are presented on a relative scale with 100% defined as transport in the presence of the solvent and without TA (no inhibition), and 0% defined as transport detected without transporter activity. IC50 is defined as the concentration required to inhibit the transport of the reporter substrate by 50%. This assay provides information on any interaction (substrate or inhibitor) between selected uptake transporters and the investigated TA that would affect the transport of the probe substrate. This assay does not give information on the nature of the interaction (transported substrate or inhibitor).

Substrate assessment studies

Substrate assessment is usually performed as a follow-up on an inhibition study, but can be performed in separate modules. Each module is planned and initiated based on the result of the previous module.

Module 1 – Pilot experiments to detect active transport:

The expected outcome is the determination of whether or not the uptake transporter-mediated uptake of the cold (or isotope-labeled) compound into the cells can be measured.

Module 2 – Detailed kinetic characterization of the transport:

The aim of Module 2 is the determination of KM and Vmax values. Inhibition of transport will be studied with a known substrate and inhibitor (used as probe substrate and reference inhibitor in the inhibition study). The effect of known inhibitors/substrates on the transport of the TA will be investigated at the optimized concentration of the TA and seven concentrations of the inhibitor/substrate molecule.

Module 3 – Characterization of potential drug-drug interactions:

The IC50 of selected known or unknown perpetrator drugs is determined on the transport of the test article. Learn more about our uptake transporter assay services. SOLVO Biotechnology has developed and acquired a wide range of stably transfected cell lines for studying the interaction of test articles with uptake transporters. Our uptake transporter portfolio is continuously expanding. Custom development of other transfected cell lines is also available from SOLVO. Licensing of uptake transporter overexpressing cell lines is also available.


1. Shitara Y, Itoh T, Sato H, Li AP and Sugiyama Y (2003) J Pharm Exp Ther 304:610-616; 2. Shitara Y, Horie T and Sugiyama Y (2006) Eur J Pharm Sci 27:425-446; 3. Backman JT, Kyrklund C, Neuvonen M and Neuvonen PJ (2002) Clin Pharmacol Ther 72:685-691.; 4. Hodel C (2002) Toxicol Lett 128:159-168.; 5. Williams D and Feely J (2002) Clin Pharmacokinet 41:343-370.; 6. Campbell SD, de Morais SM and Xu JJ (2004) Chem Biol Interact 150:179-187.; 7. Hirano M, Maeda K, Shitara Y and Sugiyama Y (2006) Drug Metab Dispos 34:1229-1236.