Gemcitabine and
capecitabine are
nucleoside analogues used in
chemotherapy strategies for the treatment of
breast cancer. We previously demonstrated that deficiency in hENT1, the most abundant and widely distributed plasma membrane
nucleoside transporter in human cells, confers high-level resistance to
gemcitabine toxicity in vitro, whereas the relationship between hENT1 activity and
capecitabine toxicity is unknown. To determine the relationship between
capecitabine cytotoxicity and hENT1 abundance, cultured MDA-MB-435s
human mammary carcinoma cells were exposed to graded concentrations of the
capecitabine metabolites,
5'-deoxy-5-fluorouridine or
5-fluorouracil, in the presence and absence of nitrobenzylmercaptopurine ribonucleoside (
NBMPR), a tight-binding inhibitor of hENT1. The presence of
NBMPR reduced the cytotoxic effects of
5'-deoxy-5-fluorouridine, indicating that hENT1 also enabled cellular uptake of this
capecitabine metabolite by
breast cancer cells. We report here the development of an immunohistochemical method to assess the hENT1 abundance of malignant cells in solid
tumors. Frozen sections of 33 primary breast
cancers were stained with
monoclonal antibodies raised against a synthetic
peptide derived from the large intracellular loop of hENT1, and staining intensity was scored on a 0-4+ scale. hENT1 staining intensity varied markedly among breast samples (4 with score 0, 5 with score 1+, 7 with score 2+, 14 with score 3+, 3 with score 4+), suggesting that at least 9 of the
tumors were hENT1 deficient. We conclude that because hENT1 deficiency has previously been associated with
nucleoside drug resistance, immunohistochemical staining of hENT1 warrants further study as a predictive tool for guiding the appropriate use of
gemcitabine and
capecitabine in the treatment of
breast cancer.