Inhibition of sodium-independent and sodium-dependent nucleobase transport activities by tyrosine kinase inhibitors

Purpose Effects of tyrosine kinase inhibitors (TKIs) on equilibrative nucleobase transport (ENBT) and sodium-dependent nucleobase transport (SNBT) activities were investigated in normal human renal proximal tubule epithelial cells (hRPTECs) and in pig kidney cell line (LLC-PK1). Methods Uptake assays were performed by assessing accumulation of radiolabeled nucleobases over time into hRPTECs or LLC-PK1 cell lines which express ENBT and SNBT activities, respectively. Dose–response curves for inhibition of 1 µM [ 3 H]adenine or 1 µM [ 3 H]hypoxanthine were examined in hRPTECs and in LLC-PK1 cells with varying TKI concentrations (0–100 µM) to calculate the IC 50 values (mean ± S.E) for inhibition. Results Gefitinib inhibited ENBT activity with an IC 50 value of 0.7 µM, thus indicating strong interactions of ENBT with gefitinib in hRPTECs. Erlotinib > sorafenib > imatinib > sunitinib inhibited ENBT with IC 50 values of 15, 40, 60, 78 µM, respectively, whereas dasatinib, lapatinib, and vandetanib were not inhibitory at concentrations >100 µM. Similar studies in LLC-PK1 cells which exhibit SNBT activity showed that vandetanib was the most potent inhibitor followed by sorafenib > erlotinib > gefitinib > sunitinib > imatinib with IC 50 values of 14, 25, 28, 40, 47, 94 µM, respectively, whereas dasatinib and lapatinib were not inhibitory at concentrations >100 µM. Conclusions These results suggest for the first time inhibition of both ENBT and SNBT transport activities by TKIs. These results suggest that it is important to consider potential effects on combination regimens using TKIs with nucleobase drugs such as 5-FU in cancer treatment.