Abstract

Cellular uptake of many chemotherapeutic nucleoside analogs is dependent on the activity of a family of nucleoside transport proteins located in the cell plasma membrane. In the present study, we examined the role of these transporters in the accumulation of gemcitabine by a human head and neck squamous carcinoma cell line. The uptake of [³H]gemcitibine was compared with that of [³H]uridine and [³H]formycin B in the parent cell line (HN-5a) and in a gemcitabine-resistant variant (GEM-8e). The HN-5a and GEM-8e cells were similar in their transport characteristics and expressed predominantly the es(equilibrative, inhibitor-sensitive) transporter subtype; less than 10% of the influx of [³H]formycin B or [³H]uridine was mediated by the ei(equilibrative inhibitor-resistant) system, and there was no evidence for Na⁺-dependent nucleoside transporters. [³H]Gemcitabine (10 μM) entered these cells via both the es and ei transporters with an initial rate of uptake similar to that seen with the use of [³H]formycin B or [³H]uridine. In addition, ATP-replete cells accumulated significantly less [³H]gemcitabine than did ATP-depleted cells, which is indicative of an active efflux mechanism for gemcitabine. These results show that gemcitabine is a substrate for both the es andei nucleoside transporters of HN-5a and GEM-8e cells and that gemcitabine resistance of the GEM-8e cells cannot be attributed to changes in transporter activity. Further studies to define the characteristics of the putative efflux mechanism are clearly warranted because this system has the potential to significantly affect the clinical efficacy of gemcitabine.