Checkpoint kinase inhibitor AZD7762 strongly sensitises urothelial carcinoma cells to gemcitabine

Background: More effective chemotherapies are urgently needed for bladder cancer, a major cause of morbidity and mortality worldwide. We therefore explored the efficacy of the combination of gemcitabine and AZD7762, a checkpoint kinase 1/2 (CHK1/2) inhibitor, for bladder cancer. Methods: Viability, clonogenicity, cell cycle distribution and apoptosis were assessed in urothelial cancer cell lines and various non-malignant urothelial cells treated with gemcitabine and AZD7762. DNA damage was assessed by ?H2A.X and 53-BP1 staining and checkpoint activation was followed by Western blotting. Pharmacological inhibition of CHK1 and CHK2 was compared to downregulation of either CHK1 or CHK2 using siRNAs. Results: Combined use of gemcitabine and AZD7762 synergistically reduced urothelial carcinoma cell viability and colony formation relative to either single treatment. Non-malignant urothelial cells were substantially less sensitive to this drug combination. Gemcitabine plus AZD7762 inhibited cell cycle progression causing cell accumulation in S-phase. Moreover, the combination induced pronounced levels of apoptosis as indicated by an increase in the fraction of sub-G1 cells, in the levels of cleaved PARP, and in caspase 3/7 activity. Mechanistic investigations showed that AZD7762 treatment inhibited the repair of gemcitabine-induced double strand breaks by interference with CHK1, since siRNA-mediated depletion of CHK1 but not of CHK2 mimicked the effects of AZD7762. Conclusions: AZD7762 enhanced sensitivity of urothelial carcinoma cells to gemcitabine by inhibiting DNA repair and disturbing checkpoints. Combining gemcitabine with CHK1 inhibition holds promise for urothelial cancer therapy

DNA repair: the culprit for tumor-initiating cell survival?

The existence of “tumor-initiating cells” (TICs) has been a topic of heated debate for the last few years within the field of cancer biology. Their continuous characterization in a variety of solid tumors has led to an abundance of evidence supporting their existence. TICs are believed to be responsible for resistance against conventional treatment regimes of chemotherapy and radiation, ultimately leading to metastasis and patient demise. This review summarizes DNA repair mechanism(s) and their role in the maintenance and regulation of stem cells. There is evidence supporting the hypothesis that TICs, similar to embryonic stem (ES) cells and hematopoietic stem cells (HSCs), display an increase in their ability to survive genotoxic stress and injury. Mechanistically, the ability of ES cells, HSCs and TICs to survive under stressful conditions can be attributed to an increase in the efficiency at which these cells undergo DNA repair. Furthermore, the data presented in this review summarize the results found by our lab and others demonstrating that TICs have an increase in their genomic stability, which can allow for TIC survival under conditions such as anticancer treatments, while the bulk population of tumor cells dies. We believe that these data will greatly impact the development and design of future therapies being engineered to target and eradicate this highly aggressive cancer cell population

Checkpoint Signaling, Base Excision Repair, and PARP Promote Survival of Colon Cancer Cells Treated with 5-Fluorodeoxyuridine but Not 5-Fluorouracil

The fluoropyrimidines 5-fluorouracil (5-FU) and FdUrd (5-fluorodeoxyuridine; floxuridine) are the backbone of chemotherapy regimens for colon cancer and other tumors. Despite their widespread use, it remains unclear how these agents kill tumor cells. Here, we have analyzed the checkpoint and DNA repair pathways that affect colon tumor responses to 5-FU and FdUrd. These studies demonstrate that both FdUrd and 5-FU activate the ATR and ATM checkpoint signaling pathways, indicating that they cause genotoxic damage. Notably, however, depletion of ATM or ATR does not sensitize colon cancer cells to 5-FU, whereas these checkpoint pathways promote the survival of cells treated with FdUrd, suggesting that FdUrd exerts cytotoxicity by disrupting DNA replication and/or inducing DNA damage, whereas 5-FU does not. We also found that disabling the base excision (BER) repair pathway by depleting XRCC1 or APE1 sensitized colon cancer cells to FdUrd but not 5-FU. Consistent with a role for the BER pathway, we show that small molecule poly(ADP-ribose) polymerase 1/2 (PARP) inhibitors, AZD2281 and ABT-888, remarkably sensitized both mismatch repair (MMR)-proficient and -deficient colon cancer cell lines to FdUrd but not to 5-FU. Taken together, these studies demonstrate that the roles of genotoxin-induced checkpoint signaling and DNA repair differ significantly for these agents and also suggest a novel approach to colon cancer therapy in which FdUrd is combined with a small molecule PARP inhibitor