5-Fluorouracil（5-FU）是一種應用廣泛的抗腫瘤藥物，近年來隨著抗藥性的產生，限制了其在臨床上的用途及發展。因此，本研究主要探討乙型反式胸腺核苷合成  （rTSβ）在乳癌的表現與5-FU抗藥性間的關聯性。我們利用五株不同的乳癌細胞株，分別為 MCF-1、MCF-7、BT-20、60055及T47D進行研究。首先，利用西方墨點法及免疫細胞染色法來測定rTSβ的表現量，再用細胞毒性分析法測定5-FU對五株乳癌細胞株的毒殺作用，以不同的濃度：1.6μM至50 mM，分別與五株細胞株培養72小時後，製作劑量-反應曲線圖計算5-FU對五株細胞株的IC50。Leucovorin為亞葉酸，在臨床使用上可與5-FU合併以增強5-FU的細胞毒殺作用。因此，另一組加入5-FU同時又加入leucovorin（LV）1.25 mM，進行合併用藥評估。由實驗結果得之，五株乳癌細胞株除了T47D外，其餘的細胞株都有rTSβ的表現，且有rTSβ表現的細胞株其5-FU之IC50的濃度比T47D明顯高了2至78倍。換句話說，有rTSβ表現量的細胞株，其會明顯降低5-FU的細胞毒殺作用，產生5-FU的抗藥性，然而其機轉尚未清楚確立。
回溯有關5-FU抗藥性的研究，大多數是以TS與大腸癌細胞株為主，本研究希望藉由乳癌細胞株來建立rTSβ與5-FU抗藥性的關聯性，以改善5-FU在乳癌之輔助性療法的效果，並藉由rTSβ的表現量，可做為5-FU在臨床上的用藥治療指標。另外結果顯示，合併使用Leucovorin可增強5-FU的療效。
 

5-Fluorouracil (5-FU) is a commonly used therapeutic drug in breast cancer. In this study, we investigated a novel correlation between expression of antisense thymidylate synthase (rTSβ) in breast cancer and resistance to 5-FU. We used five breast cancer cell lines (MCF-1, MCF-7, BT-20, 60055, T47D) in this study. First, we measured rTS expression levels by western blotting and immunocytochemistry. Next, we investigated sensitivity of the cells to 5-FU by cytotoxic assay. The cells were exposed to 5- FU at concentrations from 1.6M to 50 mM for 72 hr. Dose-response curves from these five breast cancer cell lines were plotted and IC50s were measured. Our results showed that four breast cell lines except T47D overexpressed rTSβ. The IC50 of 5-FU for T47D was 0.013M, which was 2 to 78 times lower than that of other four cell lines, showing significantly more sensitive to 5-FU. Therefore, our data demonstrated that a relationship between expression of rTSβ could result in reduced 5-FU cytotoxic effect and increase resistance to 5-FU. Although the mechanism of resistance is not yet clear, our results provide a focus for future studies to elucidate the mechanism in which drug resistance in breast cancer could be mediated by rTSβ expression and to improve the efficacy of 5-FU used in the clinical adjuvant chemotherapy of breast cancer.