Mdm2 inhibitors synergize with topoisomerase II inhibitors to induce p53-independent pancreatic cancer cell death.

Pancreatic ductal adenocarcinoma (PDAC) represents the fourth leading cause of cancer death in the western world, with a 5-year survival rate below 5%. Murine double minute 2 (Mdm2) is an important negative regulator of the tumor suppressor p53. Reactivation of wild-type p53 is a promising treatment strategy, and inhibitors of Mdm2 have already entered clinical trials. To investigate the effects of Mdm2 inhibitors in PDAC, we used a murine cell line platform with a genetically defined status of p53. Here, we describe that Mdm2 inhibitors can act on a subset of murine PDAC cell lines p53 independently. Furthermore, we observed that Mdm2 inhibitors increase the sensitivity of murine PDAC cell lines toward topoisomerase II inhibitors by inducing effector caspase-independent cell death. The combination of Mdm2 inhibitors with topoisomerase II inhibitors acts independent of the survival factor NFκB/RelA. Mechanistically, Mdm2 inhibitors increase topoisomerase II inhibitor-induced DNA double-strand breaks. We show that Mdm2 binds to Nbs1 of the Mre11-Rad50-Nijmegen breakage syndrome (Nbs) 1 DNA repair complex. In addition, we provide evidence that Mdm2 inhibitors delay DNA repair. These findings may help to design novel therapeutic strategies to overcome therapeutic resistance of PDAC.