Role of equilibrative nucleoside transporters 1 and 2 in the transport and disposition of gemcitabine and its metabolites in cervical carcinoma.

Abstract

Gemcitabine is a nucleoside analog used as a radiosensitizer for the treatment of locally advanced cervical carcinoma. Yet, despite its efficacy when administered concomitantly with radiation, gemcitabine therapy is not without side effects. The utility of delivering gemcitabine directly to the cervix was explored through the use of a novel drug delivery device, CerviPrep(TM). Local administration to the cervix led to clinically relevant concentrations of gemcitabine in cervical tissue and plasma, while no gemcitabine was detected in the systemic circulation and no side effects were reported. Our data suggest that targeting gemcitabine delivery to the cervix can limit systemic exposure and toxicity while achieving cytotoxic concentrations of drug at the target site.

Despite its widespread use in cervical carcinoma, little is known about the disposition of gemcitabine in this tissue. As a nucleoside analog, gemcitabine is a substrate for the equilibrative nucleoside transporters (hENT), and patient response to gemcitabine therapy has been associated with the expression of these proteins. A characterization of hENT1 and hENT2 in both malignant and normal cervical tissue was undertaken, and while no effect of malignancy was observed on hENT1 protein expression, hENT2 protein was nearly three-fold higher in malignant cervical tissue when compared to normal tissue. Expression of hENT mRNA was highly variable and not associated with malignancy.

We also examined the effect of dFdU on gemcitabine disposition, as this relatively inactive metabolite is present at much higher concentrations in the plasma than gemcitabine following intravenous administration of the parent compound. We report a novel interaction between dFdU and gemcitabine whereby dFdU competes with gemcitabine for transport via hENT1 and hENT2. The presence of dFdU appears to enhance the retention of gemcitabine intracellularly leading to an increase in the amount of active gemcitabine triphosphate. As more gemcitabine is phosphorylated in the presence of dFdU, a "metabolic sink" is created, further increasing gemcitabine uptake into the cell via hENT1 and hENT2. These data suggest that both transport and intracellular metabolism are equally important components of gemcitabine disposition and cytotoxic potential, and that the presence of dFdU increases intracellular exposure to this nucleoside analog.