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 mechanism in PDA cells has generated encouraging pre-clinical results. Ubiquitin-specific peptidase 11 (USP11), an enzyme that interacts with BRCA2, was recently discovered to play a key role in DNA double-strand break repair and may be a novel therapeutic target. Using a systematic high-throughput approach, we biochemically screened 2000 FDA approved and pharmacologically active compounds for inhibition of USP11 enzymatic activity. We identified six active small molecules that inhibit USP11 enzymatic activity. An in vitro drug sensitivity assay demonstrated that one of these USP11 inhibitors, mitoxantrone, affected PDA cell survival with an IC50 of less than 10 nM. Across six different PDA cell lines, two with defects in the Fanconi Anemia/BRCA2 pathway (Hs766T and Capan-1), mitoxantrone is 40 to 20,000-fold more potent than GEM, with increased endogenous USP11 mRNA levels associated with increased sensitivity to mitoxantrone. USP11 silencing in PDA cells also enhanced sensitivity to GEM. These findings establish a model for rapid discovery of FDA approved compounds by complementing in vitro biochemical experiments with cell culture studies. Further, they provide a strong rationale to study mitoxantrone in pre-clinical and early-phase clinical settings for the treatment of PDA.