FdUMP[10] is a 10mer of FdUMP, the thymidylate synthase (TS) inhibitory metabolite of 5-fluorouracil (5-FU). We studied the mechanism by which FdUMP[10] inhibits cell growth and induces apoptosis. COMPARE analysis of the data from the NCI 60 cell line screen revealed unexpected mechanistic similarities for FdUMP[10] with camptothecins (CPTs), suggesting that FdUMP[10] possesses a mechanism of action possibly related to topoisomerase I (top1) inhibition. In this study, we found that FdUMP[10] is much more potent than 5-FU (160-fold and 600-fold, respectively) toward P388 (mouse leukemia) and HCT116 (human colon adenocarcinoma) cells. P388/CPT45 cells that are highly-resistant to CPT as a consequence of top1-deficiency are highly cross-resistant to FdUMP[10], but not to 5FU. Intense DNA fragmentation following exposure to FdUMP[10] was observed by alkaline elution in P388 and HCT116 cells. Interestingly, FdUMP[10] induced the trapping of top1-DNA complexes both in P388 and HCT116 cells when these complexes were detected with the Immuno Complex of Enzyme (ICE) bioassay. These top1-DNA complexes could only be partially reduced by co-treatment with the broad-spectrum caspase inhibitor Z-VAD.FMK in FdUMP[10] treated HCT116 cells, while Z-VAD.FMK almost had no effect on top1-DNA complexes formation in P388 cells treated with FdUMP[10]. In such case, we propose that the top1-DNA complexes result from trapping of top1 at misincorporation sites (uracil and fluorouracil incorporation) resulting from potent TS inhibition by FdUMP[10]. In conclusion, FdUMP[10] is a markedly more potent antitumor agent than 5-FU. Trapping of top1- DNA complexes in cells by FdUMP[10] could be one of the mechanisms of action. FdUMP[10] mainly targets the DNA pathway, and is mechanistically distinct from 5-FU which targets both RNA and DNA pathways. FdUMP[N] compounds may be more efficacious than 5-FU or alternative FPs in cancer chemotherapy.

[Proc Amer Assoc Cancer Res, Volume 45, 2004]