A novel approach to thymidylate synthase as a target for cancer chemotherapy

Tumor cell resistance to fluoropyrimidines and other inhibitors of thymidylate synthase (TS) is a serious problem often associated with increased intracellular TS. Clinically, another problem that arises from the use of TS inhibitors is toxicity, which develops, in part, because normal cells may be adversely affected by doses of inhibitor that do not impact tumor cells. To circumvent this problem, we have devised a new strategy called enzyme-catalyzed therapeutic activation (ECTA), which takes advantage of overexpressed TS to enzymatically generate cytotoxic moieties preferentially in tumor cells. We show herein that tumor cells expressing elevated levels of TS are preferentially sensitive to NB1011, a phosphoramidate derivative of (E)-5-(2-bromovinyl)-2'-deoxyuridine. We find support for the proposed mechanism of NB1011 in the following results: 1) positive relationship between TS protein level and sensitivity to NB1011 in engineered HT1080 tumor cells, designed to express defined levels of TS protein; 2) NB1011 activity is enhanced on tumor cells which express endogenous elevated TS; 3) cytotoxicity of NB1011 is blocked by raltitrexed (Tomudex); 4) NB1011 selection of TS-overexpressing MCF7TDX tumor cells results in recovery of cell populations and clones with diminished TS levels (and restored sensitivity to raltitrexed). A preliminary comparison of TS mRNA levels in multiple normal tissues versus colon tumor samples suggests that selective tumor cytotoxicity of NB1011 may be possible in the clinical setting. Because NB1011 cytotoxicity is dependent upon activation by TS, its proposed mechanism of action is distinct from current TS-targeted drugs, which require inhibition of TS to be effective.