Uptake (SLC) Transporter Interaction Assays

In Vitro Cellular Uptake Assays for SLC Transporter Interaction Assessment

Drug transporters are routinely assessed in the drug discovery and development processes as potential sources of drug-drug interaction (DDI) or organ toxicity risks. They can also be targets for drug action or delivery. A test compound can be substrate and/or inhibitor to a transporter, and these interactions are mostly tested in vitro, using cell systems that overexpress a single transporter of interest. SOLVO, a Charles River Company, offers three such assay systems: the Vesicular Transport and the Transfected Monolayer Assay for assessing Efflux transporters, and a Cellular Uptake transporter assay. The choice from these setups is driven by client project needs and the test compound's PK properties.

Over the past few years various regulatory agencies have published new or updated guidelines on studying DDIs. Such recommendations were recently summarized in the internationally harmonized ICH M12 guidelines, which were adopted by the FDA, EMA, NMPA, PMDA, and other local agencies since its finalization in 2024. In addition to the transporter studies requested for drug approval (usually in the IND phase), there is also an expanding knowledge base on the general role of drug transporters in drug ADMET properties. Drug transporters are routinely screened in early development to assess compound permeability properties, as well as to detect potential toxic effects, such as hepatotoxicity, and their recognition as potential therapeutic targets or mediators of tissue-specific drug delivery is also growing. 

Why do SLC transporter interactions matter?

From a functional perspective, clinically relevant drug transporters are categorized as either efflux or uptake transporters depending on the relative direction of transport. Uptake transporters typically belong to Solute Carrier (SLC) protein families, and are usually facilitative or secondary-active, using either an electrochemical gradient or ion gradients to facilitate the movement of substrates across membranes. SLC transporters generally mediate active compound to active drug elimination via excretory organs, especially the liver and the kidneys, but can also mediate drug access to different other tissues. It is mostly uptake transporters specific to these excretory organs that are often involved in drug-drug interactions, therefore their assessment is required by regulatory agencies for IND applications. 

Multiple uptake transporters have a key role in mediating the traffic of endogenous substrates including signaling molecules and metabolites. Depending on their substrate range, this makes SLCs also popular pharmaceutical targets, for example, transport of signaling nucleosides via ENT transporters can be targeted in several CNS conditions such as epilepsy and seizures.

SLC Transporter Assays to Meet Your Needs

Compound interaction with uptake transporters can be twofold. A compound is substrate of an efflux transporter if it is relocated by it across the cell membrane in an active manner, and it can also be a transport inhibitor. Substrates are often inhibitors as well via a competitive mechanism, but non-competitive transporter inhibition is also often observed.

Assays using transporter-overexpressing cells are a popular and useful tool for assessing compounds as substrates to uptake transport. For substrate assessment, a test compound is co-incubated with the transporter-expressing cells, and its intracellular accumulation is measured, while inhibition assays measure the compound’s effect on the accumulation of a reference probe substrate. This assay is suitable for most types of test compounds regardless of their passive permeability or other PK properties.