IUR of imatinib in HEK293 and K562 cells. (A) OCT1 mRNA expression levels in arbitrary units (a.u.) in OCT1-transfected HEK293 cells (HEK293/hOCT1) compared with mock (pcDNA3)-transfected control cells (HEK293/Neo). (B) HEK293/hOCT1 cells display increased uptake of ASP⁺, a fluorescent substrate of OCT1, confirming that OCT1 is functional. ASP⁺-associated fluorescence (a.u.) was measured by FACScan flow cytometry as described previously.¹¹  (C) The IUR of imatinib was slightly (∼25%), but significantly (P < .05), increased in the HEK293/hOCT1 cells compared with the HEK293/Neo control cells. Cells were exposed to 30 µM of imatinib for 1 hour. Effect of amantadine (D,E,H) and prazosin (F,G,I) on the IUR of imatinib in HEK293/hOCT1 (D,F), HEK293/Neo control (E,G) cells and K562 cells (H-I). Cells were pretreated for 30 minutes with the indicated concentrations of inhibitor followed by an additional 30 minutes of exposure of the cells to 10 µM of imatinib. Uptake data are expressed as the mean ± standard deviation (derived from at least 3 replicates), and the Student t test was used to detect significant differences as indicated by an asterisk (*P < .05, **P < .01, ***P < .001) in IUR between HEK293/hOCT1 and HEK293/Neo cells (C) or between the IUR ± inhibitor (D-I). Amantadine showed a maximal inhibition of ∼65% at 1000 µM, whereas prazosin markedly inhibited the IUR of imatinib by ∼50% at 30 µM and up to ∼90% at 100 µM. It should be noted that the inhibitory effect of these compounds on the IUR of imatinib in HEK293 cells did not differ between OCT1-transfected and Neo control cells and was comparable with that seen in K562 cells having low OCT1 expression. Apparently, the inhibitory effects of these compounds do not depend on the level of OCT1 expression.