Contribution of equilibrative nucleoside transporters 1 and 2 to gemcitabine uptake in pancreatic cancer cells

Hepatic arterial infusion (HAI) chemotherapy is expected to be a more effective and safer method to treat the hepatic metastasis of pancreatic cancer than intravenous (iv) administration because of higher tumor exposure and lower systemic exposure. To clarify the uptake mechanism of nucleoside anticancer drugs, including gemcitabine (GEM), in pancreatic cancer, we investigated the uptakes of radiolabeled uridine (a general substrate of nucleoside transporters) and GEM in pancreatic cancer cell lines MIA‐PaCa2 and As‐PC1. Uridine uptake was inhibited by non‐labeled GEM and also by S‐(4‐nitrobenzyl)‐6‐thioinosine (NBMPR; an inhibitor of equilibrative nucleoside transporters, ENTs) in a concentration‐dependent manner, suggesting that ENTs contribute to uridine uptake in pancreatic cancer cells. As for GEM, saturable uptake was mediated by high‐ and low‐affinity components with K_m values of micromolar and millimolar orders, respectively. Uptake was inhibited in a concentration‐dependent manner by NBMPR and was sodium ion‐independent. Moreover, the concentration dependence of uptake in the presence of 0.1 μM NBMPR showed a single low‐affinity site. These results indicated that the high‐ and low‐affinity sites correspond to hENT1 and hENT2, respectively. The results indicated that at clinically relevant hepatic concentrations of GEM in GEM‐HAI therapy, the metastatic tumor exposure of GEM is predominantly determined by hENT2 under unsaturated conditions, suggesting that hENT2 expression in metastatic tumor would be a candidate biomarker for indicating anticancer therapy with GEM‐HAI.