Data from Thymidine Kinase 1 Loss Confers Trifluridine Resistance without Affecting 5-Fluorouracil Metabolism and Cytotoxicity

Abstract

<div>Abstract<p>Acquired resistance to therapeutic drugs is a serious problem for patients with cancer receiving systemic treatment. Experimentally, drug resistance is established in cell lines <i>in vitro</i> by repeated, continuous exposure to escalating concentrations of the drug; however, the precise mechanism underlying the acquired resistance is not always known. Here, it is demonstrated that the human colorectal cancer cell line DLD1 with acquired resistance to trifluridine (FTD), a key component of the novel, orally administered nucleoside analogue-type chemotherapeutic drug trifluridine/tipiracil, lacks functional thymidine kinase 1 (TK1) expression because of one nonsense mutation in the coding exon. Targeted disruption of the <i>TK1</i> gene also conferred severe FTD resistance, indicating that the loss of TK1 protein expression is the primary cause of FTD resistance. Both FTD-resistant DLD1 cells and DLD1-<i>TK1</i><sup>−/−</sup> cells exhibited similar 5-fluorouracil (5-FU) sensitivity to that of the parental DLD1 line. The quantity of cellular pyrimidine nucleotides in these cells and the kinetics of thymidylate synthase ternary complex formation in 5-FU–treated cells is similar to DLD1 cells, indicating that 5-FU metabolism and cytotoxicity were unaffected. The current data provide molecular-based evidence that acquired resistance to FTD does not confer 5-FU resistance, implying that 5-FU–based chemotherapy would be effective even in tumors that become refractory to FTD during trifluridine/tipiracil treatment. <i>Mol Cancer Res; 16(10); 1483–90. ©2018 AACR</i>.</p></div>