DNA Damage and Repair Mitoxantrone Targets Human Ubiquitin-Specific Peptidase 11 (USP11) and Is a Potent Inhibitor of Pancreatic Cancer Cell Survival

Pancreatic ductal adenocarcinoma (PDA) is the fourth leading cause of cancer-related death in the United States, with a 95% five-year mortality rate. For over a decade, gemcitabine (GEM) has been the established first-line treatment for this disease despite suboptimal response rates. The development of PARP inhibitors that target the DNA damage repair (DDR) system in PDA cells has generated encouraging results. Ubiquitinspecific peptidase 11 (USP11), an enzyme that interacts with the DDR protein BRCA2, was recently discovered to play a key role in DNA double-strand break repair and may be a novel therapeutic target. A systematic high-throughput approach was used to biochemically screen 2,000 U.S. Food and Drug Administration (FDA)-approved compounds for inhibition of USP11 enzymatic activity. Six pharmacologically active small molecules that inhibit USP11 enzymatic activity were identified. An in vitro drug sensitivity assay demonstrated that one of these USP11 inhibitors, mitoxantrone, impacted PDA cell survival with an IC50 of less than 10 nM. Importantly, across six different PDA cell lines, two with defects in the Fanconi anemia/ BRCA2 pathway (Hs766T and Capan-1), mitoxantrone is 40to 20,000-fold more potent than GEM, with increased endogenous USP11 mRNA levels associated with increased sensitivity to mitoxantrone. Interestingly, USP11 silencing in PDA cells also enhanced sensitivity to GEM. These findings establish a preclinical model for the rapid discovery of FDA-approved compounds and identify USP11 as a target of mitoxantrone in PDA. Implications: This high-throughput approach provides a strong rationale to study mitoxantrone in an early-phase clinical setting for the treatment of PDA. Mol Cancer Res; 11(8); 901–11. 2013 AACR.