Combinatorial Approaches to Enhance DNA Damage Following Enzyme-mediated Depletion of L-Cyst(e)ine for Treatment of Pancreatic Cancer.

Abstract Pancreatic ductal adenocarcinoma (PDAC) represents one of the deadliest forms of cancer with very few available therapeutic options. We previously reported that an engineered human enzyme, Cyst(e)inase, that degrades L-cysteine and cystine, inhibits growth of multiple cancer cells including PDAC both in vitro and in vivo. Here, we show that Cyst(e)inase treatment leads to increased clustered oxidative DNA damage, DNA single strand breaks, apurinic/apyrimidinic sites and DNA double strand breaks (DSBs) in PDAC cells sensitive to intracellular depletion of L-Cys that is associated with reduced survival. BRCA2 deficient PDAC cells exhibited increased DSBs and enhanced sensitivity to Cyst(e)inase. Blocking a second antioxidant pathway (thioredoxin/thioredoxin reductase) using Auranofin or inhibiting DNA repair using the PARP inhibitor, Olaparib, led to significant increases in DSBs following Cyst(e)inase treatment in all PDAC cells examined. Cyst(e)inase plus Olaparib also synergistically inhibited growth of sensitive and resistant PDAC cells in both xenograft and allograft tumor models. Collectively, these results demonstrate an important role for oxidative DNA damage and ultimately DNA DSBs in the anticancer action of Cyst(e)inase. The data further show the potential for combining agents that target alternate antioxidant pathways or by targeting DNA repair pathways or genetic liabilities in DNA repair pathways to enhance the therapeutic action of Cyst(e)inase for PDAC. Cyst(e)inase is an engineered human enzyme shown to inhibit growth of multiple cancer cells. In this study, Saha et al. found that combining agents that target alternate antioxidant pathways or by targeting DNA repair pathways or genetic liabilities in DNA repair pathways improves therapeutic action of Cyst(e)inase in pancreatic cancer.