SLC transporters are attracting more attention in drug discovery, but many are quite difficult to work with. The biology may be compelling, yet the experimental tools are often the limiting factor. Cell-based uptake assays can support screening, but they do not always tell us whether a compound acts directly on the transporter. Conventional electrophysiology is also not well suited to every transporter, especially those with low turnover and small currents.

This new paper from Kathrin Lotz and colleagues at Sanofi offers a practical way forward.

The authors used an intact-cell ADE-MS assay to screen 8,000 compounds against SLC6A15, then applied SSM-based electrophysiology (SSME) on the SURFE²R 96SE to characterize selected hits.

The two platforms look at the same transporter activity from different experimental angles. ADE-MS provides the throughput needed for compound screening while keeping the transporter in a cellular environment. The SURFE²R assay measures transporter-generated currents in isolated membrane preparations. Combining both approaches revealed which screening hits translated into direct transporter modulation in the SSME assay and which appeared to be specific to the cellular assay, demonstrating the value of orthogonal validation.

For the compounds active in both assays, the paper reports potency shifts of up to 15-fold between the two formats. These potency shifts offer another example of what can be learned by looking at the same transporter in two different experimental settings. ADE-MS shows how compounds behave in intact cells, while SSME allows the sample conditions to be defined more selectively by removing cellular components at different levels. Depending on the question, measurements can be performed using native organelles, purified membrane preparations or proteoliposomes. Together, the two readouts provide a clearer picture than either could provide alone.

The broader value of this work is that the same strategy can be applied beyond SLC6A15. For transporters that are difficult to patch, poorly characterized or not yet supported by strong pharmacological tools, cell-based assays combined with direct electrophysiological validation can create a realistic path from screening hit to mechanism.

That could be useful for many members of the SLC family and for other challenging electrogenic membrane proteins where one assay alone does not tell the full story.

Congratulations to the authors and everyone involved in the RESOLUTE consortium.

https://doi.org/10.1021/acs.analchem.5c06828