5-Fluorouracil (FU) is a halogenated nucleobase analog that is widely used in
chemotherapy. Here we show that
5-hydroxymethyl-2'-deoxyuridine (hmUdR) synergistically enhances the activity of FU in cell lines derived from solid
tumors but not normal tissues. While the cytotoxicity of FU and hmUdR was not directly related to the amount of the modified bases incorporated into cellular
DNA, incubation with this combination resulted in dramatic increase in the number of single strand breaks in replicating
cancer cells, leading to
NAD-depletion as consequence of
poly(ADP-ribose) synthesis and S phase arrest. Cell death resulting from the base/
nucleoside combination did not occur by apoptosis, autophagy or necroptosis. Instead, the cells die via
necrosis as a result of
NAD depletion. The FU-related
nucleoside analog,
5-fluoro-2'-deoxyuridine, also displayed synergy with hmUdR, whereas hmUdR could not be replaced by
5-hydroxymethyluracil. Among other 5-modified
deoxyuridine analogs tested,
5-formyl-2'-deoxyuridine and, to a lesser extent,
5-hydroxy-2'-deoxyuridine, also acted synergistically with FU, whereas 5-hydroxyethyl-2'-deoxyuridine did not. Together, our results have revealed an unexpected synergistic interaction between
deoxyuridine analogs and FU in a
cancer cell-specific manner, and suggest that these novel base/
nucleoside combinations could be developed into improved FU-based
chemotherapies.