PARP Inhibitors Sensitize to Floxuridine Figure 5. Small molecule PARP inhibitors sensitize HCT-8 and HT29 cells to FdUrd but not 5-FU. HCT-8 or HT29 cells were plated as single cells, exposed to the indicated concentrations of 5-FU or FdUrd along with 3 mM ABT-888, 300 nM AZD2281, or vehicle for 24 h. Following washing, 3 mM ABT-888 or 300 nM AZD2281 were re-added, and the cells were cultured for 10 d and stained with Coomassie Blue. doi:10.1371/journal.pone.0028862.g005 HT29 cells depleted of MLH1 or MSH2 were severely sensitized to FdUrd by AZD2281, and were modestly resistant to FdUrd alone. For the second model system, we employed the paired colon cells lines, HCT-116.ch2 and HCT-116.ch3 [51]. These cell lines were derived from parental HCT-116 cells, which have biallelic inactivating MLH1 mutations that render them MMR-deficient [74]. The HCT-116.ch3 cells contain an additional chromosome 3, which encodes a functional MLH1 that restores MMR. The HCT-116.ch2 cells, which are used as a control, contain an additional chromosome 2 and like the parental cells are MMR- deficient. Consistent with previously published results, the MMR deficient HCT-116.ch2 cells were modestly more resistant to FdUrd than were the HCT-116.ch3 cells (Fig. 6B), which are MMR proficient [75]. Notably, however, AZD2281 robustly sensitized both cell lines to FdUrd. Taken together, these results demonstrate that colon cancer cells with defects in the MMR pathway can also be sensitized to FdUrd by a small molecule PARP inhibitor. Discussion 5-FU is among the most widely used anticancer chemotherapy agents, and it (or the 5-FU prodrug capecitabine) is the backbone PLoS ONE | www.plosone.org 6 December 2011 | Volume 6 | Issue 12 | e28862