Substrate-specific inhibition of organic cation transporter 1 revealed using a multisubstrate drug cocktail

Transporters of the SLC22 family, such as organic cation transporter 1 (OCT1), possess very broad substrate specificity. It is unclear to what extent the inhibitory potencies of OCT1 depend on the substrate used. Here, we describe a multisubstrate drug cocktail that allows for the simultaneous testing of drug-drug interactions using 8 different victim drugs: fenoterol, salbutamol, sumatriptan, zolmitriptan, ipratropium, trospium, methylnaltrexone, and metformin. There were no significant differences in Michaelis constant (K_M) and v_max of the OCT1-mediated uptake of the substrates alone or in the cocktail. Depending on the victim drug analyzed, we observed 6.7-fold differences in the inhibitory potency of fenoterol (IC₅₀ of 0.75 μM for metformin and 5.1 μM for sumatriptan). Similarly, the inhibitory potency of verapamil varied 6.7-fold (IC₅₀ of 1.3 μM for zolmitriptan and 8.7 μM for ipratropium). Two groups of inhibitors showed strong correlations in their victim-dependent inhibitory potencies. Group 1 comprised verapamil, quinidine, fenoterol, and ipratropium, and group 2 comprised metformin, sumatriptan, and trimethoprim. By comparing OCT1 paralogs and orthologs, the broadest substrate spectra were observed for OCT1 and multidrug and toxin extrusion 1, followed by OCT2, multidrug and toxin extrusion 2-K, and OCT3. In contrast, organic cation transporters novel 1 and organic cation transporters novel 2 exhibited very narrow substrate specificity, transporting only L-carnitine and L-ergothioneine, respectively. In conclusion, OCT1 demonstrates substantial differences in inhibitory potencies, depending on the victim drug used. We developed a cocktail approach that enables rapid screening for such differences, facilitating the identification of drug-drug interactions at the early stages of drug development. This approach can be extended to other transporters with broad substrate specificity. SIGNIFICANCE STATEMENT: Polyspecific transporters have a broad substrate-binding cavity with no defined single binding position. Consequently, inhibitors may exhibit different inhibitory potencies depending on the victim drug used for testing. Here, we demonstrate this for organic cation transporter 1 (OCT1, SLC22A1) and presents a drug cocktail designed to identify varying inhibitory potencies in vitro and prevent false-negative drug-drug interaction results during early drug development. This approach can be extended to other polyspecific drug transporters.