Human thymidine phosphorylase (dThdPase) is an angiogenic factor identical to platelet-derived endothelial cell growth factor (PD-ECGF). Thymidine phosphorylase is also a converting enzyme of the prodrug 5'-deoxy-5-fluorouridine (5'-DFUR) to 5-fluorouracil (5-FU) in tumors. To assess the role of dThdPase in targeting chemotherapy, we examined the relationship between the expression of dThdPase and the sensitivity of 5'-DFUR in cancer cell lines, and also examined whether transfection of dThdPase cDNA enhanced the drug-sensitivity to 5'-DFUR with or without angiogenesis in breast cancer cells. Thirteen human cancer cell lines consisting of 4 breast cancer, 6 gastric cancer, and 3 colon cancer cell lines were used. Expression of dThdPase was assessed by reverse transcriptase-polymerase chain reaction (RT-PCR) and enzyme-linked immunosorbent assay (ELISA). In vitro drug-sensitivity was assessed by MTT assay, and anti-tumor effect in vivo was assessed using nude mouse xenografts. Intratumoral microvessel density was evaluated by immunohistochemical staining to factor VIII related antigen. Transfection of dThdPase cDNA was performed using pcDNA3 expression vector encoding its cDNA by the lipofection method. An inverse relationship between the expression of dThdPase and the IC50 values of 5'-DFUR was observed (p=0.1278, ρ=−0.440) in the 13 cancer cell lines. Transfection of dThdPase cDNA into MCF-7 breast cancer cells resulted in an approximately 2.6- and 10-fold increase of the expression of dThdPase mRNA and its enzyme activity, respectively, compared to the control vector alone. The sensitivity to 5'-DFUR in the transfected cells was increased approximately 20-fold compared to the parent cells and control vector alone, and the sensitivity to 5-FU was also somewhat increased. In contrast, the sensitivity to ADM, CDDP, and VP-16 was not different between the transfected and control cells. In nude mice xenografts of the transfected cells, treatment with 5'-DFUR had a significant anti-tumor effect compared to those of the untreated transfected cells and control vector alone treated with 5'-DFUR (p<0.01). Intratumoral microvessel density in the transfected cells was not significantly increased with or without treatment with 5'-DFUR compared to control vector alone. The high expression of dThdPase was correlated with an increase in the sensitivity to 5'-DFUR in gastrointestinal and breast cancer cell lines. The introduction of dThdPase cDNA in breast cancer cells enhanced the sensitivity to 5'-DFUR without an increase of tumor angiogenesis, and targeting chemotherapy of dThdPase may be a good tumor-specific and personalized therapy for improving the poor prognosis of cancer patients who show high expressions of dThdPase.

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