Successful stem cell transplant with antibody-based conditioning for XIAP deficiency with refractory hemophagocytic lymphohistiocytosis.

not rely on OCT1 transporter activity. Functional OCT1 activity was determined by subtracting the uptake in the presence of amantadine, a specific OCT1 inhibitor according to the authors, from the uptake observed in unmanipulated cells. Examination of the data presented in their Figure 2 suggest that the IUR of imatinib in the absence of 500 mM of amantadine was increased by;50% in OCT1-transfected KCL22 cells compared with mock-transfected cells. It was assumed that this difference in imatinib uptake could be attributed to OCT1 activity. Remarkably, although mock-transfected KCL22 cells did not express detectable OCT1 mRNA and protein (Figure 4, Giannoudis et al), a substantial IUR of imatinib was observed, which could be partially inhibited by amantadine. According to the authors (Figure 4, Giannoudis et al), this decrease in IUR of imatinib is attributable to OCT1 activity. On the basis of these observations and our experimental data (see below), we argue that these inhibitor-based assays, designed to quantify functional OCT1 transporter activity, are in fact measuring additional, as yet unidentified, processes involved in imatinib IUR. Therefore, we have serious concerns about the use of “specific”OCT1 inhibitors to demonstrate OCT1-mediated uptake of imatinib. Using HEK293 (human embryonal kidney) cells, we show that overexpression of OCT1 resulted in a limited (;25%) increase in imatinib IUR (Figure 1A-C). These results coincide with other imatinib uptake studies using OCT1-transfected leukemic cells or oocytes. Furthermore, we show a dose-dependent decrease of imatinib uptake by amantadine in HEK293/OCT1 cells, but importantly, amantadine also decreased the imatinib uptake in HEK293/Neo control cells having an almost 4-log lower OCT1 expression (Figure 1D-E). Similar, even more profound results were obtained with prazosin (Figure 1F-G). Analogous inhibitory effects were also seen in the leukemic K562 cell line (Figure 1H-I). Importantly, both amantadine and prazosin decreased the IUR of imatinib independent of OCT1 expression levels. In conclusion, we demonstrated that these inhibitors are not exclusively specific for OCT1 and, therefore, we would like to emphasize to be very cautious in the interpretation of inhibitor-based OCT1 data. In fact, our data indicate that prazosin and amantadine may also interfere with other imatinib uptake processes evidently distinct from that of OCT1-mediated IUR.