Shown are five compounds that potently inhibited MATE2K in the screening studies at or below therapeutically relevant unbound concentrations. Increasing concentrations of (a) moxifloxacin, (b) norfloxacin, (c) ondansetron, (d) ranitidine and (e) nizatidine were analyzed for their ability to inhibit metformin uptake in cells stably overexpressing OCT1, OCT2, OCT3, MATE1, MATE2K and PMAT. IC50 values were determined for drugs/cell lines that display ≥50% inhibition of metformin uptake (horizontal red line). The vertical grey and black lines are drawn at 1x and 10x the inhibitor’s Cmax,u, respectively. Values are presented as mean ± SEM (n = 3).