Abstract 4137: Inhibition of protein synthesis with polyamine depletion in high-risk neuroblastoma

The MYC family of oncogenes coordinately deregulate cellular programs that link cell cycle progression with the requisite biomass and energy creation. Neuroblastoma is a MYC-driven, commonly lethal pediatric tumor, with a high proportion of tumors containing MYCN amplification. Polyamine synthesis is a pathway downstream of MYC, with polyamines functioning, in part, to support protein synthesis. ODC1 encodes ornithine decarboxylase, the rate-limiting enzyme in polyamine synthesis, and we found that ODC1 is co-amplified with MYCN in a subset of high-risk neuroblastomas (~8%). Difluoromethylornithine (DFMO) is an irreversible inhibitor or ornithine decarboxylase and is an approved agent for the treatment of Trypanosomiasis. We have previously shown that DFMO inhibits tumor progression and synergizes with chemotherapy in murine mouse models of neuroblastoma, and we postulate that inhibition of protein synthesis is a predominant mechanism of anti-tumor activity. The effects of polyamine depletion on specific components of protein translational machinery have yet to be defined. Here, we show inhibition of global protein synthesis of up to 90% after treatment with various concentrations of DFMO in vitro, with a larger decrease in protein synthesis generally correlating with the presence of MYCN amplification. We found that colony formation is inhibited at DFMO exposures as low as 150 to 500 μM DFMO, which are achievable concentrations in adults based on previous clinical trials. We also show via isoelectric focusing that treatment of neuroblastoma cells in vitro with DFMO leads to a marked decrease in the proportion of hypusinated, or active, eIF5A (an elongation factor important in translation of polyproline stretches and possibly broader influence on translation initiation). To interrogate influences of polyamine depletion on the cap-dependent translation eIF4F complex, we evaluated for changes in the phosphorylation of 4E-BP1 in DFMO treatment conditions, though no changes in phosphorylation were seen compared to controls. Studies to further elucidate the protein synthetic components affected by polyamine depletion are ongoing, as are studies to assess for selective effects of DFMO on polyproline-containing proteins via eIF5A. Further elucidation of the mechanisms of DFMO activity will identify opportunities for drug synergy and provide a responder hypothesis to test in upcoming pivotal Phase 2 and 3 clinical trial of DFMO in the treatment of neuroblastoma. Citation Format: Andrea T. Flynn, Kangning Liu, Annette Vu, Michael D. Hogarty. Inhibition of protein synthesis with polyamine depletion in high-risk neuroblastoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 4137.