Role of the human concentrative nucleoside transporter (hCNT1) in the cytotoxic action of 5[prime]-deoxy-5-fluorouridine, an active intermediate metabolite of capecitabine, a novel oral anticancer drug

We attempt to identify the plasma membrane transporter involved in the upta ke of 5'-deoxy-5-fluorouridine (5'-DFUR), an intermediate metabolite of cap ecitabine. This novel oral fluoropyrimidine is used in cancer treatments an d is a direct precursor of the cytostatic agent 5'-fluorouracil. We also ex amine the role of the transporter in 5'-DFUR cytotoxicity. The human concen trative nucleoside transporter (hCNT1) was cloned from human fetal liver an d expressed in Xenopus laevis oocytes, The two-electrode voltage-clamp tech nique was used to demonstrate that 5'-DFUR, but not capecitabine or 5'-FU, is an hCNT1 substrate. Then, hCNT1 was heterologously expressed in the mamm alian cell line Chinese hamster ovary-K1. Functional expression was demonst rated by monitoring transport of radiolabeled substrates and by using a mon ospecific polyclonal antibody generated against the transporter. hCNT1-expr essing cells were more sensitive to 5'-DFUR than vector-transfected or wild -type cells. The sensitivity of the three cell types to other agents such a s cisplatin or 5'-FU was identical. In conclusion, this study shows that 1) the pharmacological profile of a nucleoside transporter can be determined by an electrophysiological approach; 2) the hCNT1 transporter is involved i n 5'-DFUR uptake; and 3) hCNT1 expression may increase cell sensitivity to 5'-DFUR treatment. This study also reports for the first time the generatio n of an antibody against hCNT1, which may be useful in the elucidation of t he relationship between hCNT1 expression and tumor response to capecitabine treatment.