Abstract 1630: Modulation of the neuroblastoma microenvironment via polyamine blockade

Abstract Despite improvements in therapy over the last few decades, neuroblastoma (NB) still accounts for a considerable portion of childhood cancer-related mortalities and 5-year survival rates in patients with high-risk disease remains poor (40-50%). Amplification of MYCN and dysregulation in downstream Myc-related pathways result in NBs with high-risk features. Notably, MYCN-amplified NBs have elevated polyamine (PA) levels, and the gate-keeper enzyme in the PA synthesis pathway, ornithine decarboxylase (ODC1), is a direct target of Myc. Yet, ODC1 activity can be irreversibly inhibited by the FDA-approved drug difluoromethylornithine (DFMO), and exposure of NB cell lines in vitro to DFMO reduces cellular growth. In a mouse model of MYCN-driven NB (TH-MYCN+/+ transgenic mice), inhibition of PA synthesis with DFMO led to reductions in NB burden and extension of survival that appeared more profound than that predicted by the in vitro activity of the drug, suggesting that in addition to tumor-intrinsic effects, DFMO may have significant effects on the NB tumor microenvironment (TME). We therefore sought to characterize the tumor microenvironment of NB in TH-MYCN+/+ mice in the presence or absence of DFMO-mediated PA blockade. Tumors from DFMO treated and untreated mice were dissected, mechanically and enzymatically dissociated, and the number and frequencies of various TIL subsets were assessed using an optimized flow cytometry-based protocol. Our results indicate that DFMO reduces tumor growth and results in distinct and reproducible alterations in the cellular composition of the NB TME, the most profound of which was a significant increase in the frequency of NK cells. Moreover, we found an increase in the percentage of tumor cells expressing NK cell ligands. Concordantly, we also witnessed a shift in the NK cell expression of activating and inhibitory receptors. These findings are consistent with the hypothesis that PA blockade induces distinct TME changes that predispose to more efficient immune control of NB growth. To probe the contribution of NK cells to this process, we are now testing whether antibody-mediated depletion of NK cells in DFMO-treated TH-MYCN+/+ mice results in loss of the extension in survival afforded by PA blockade. In parallel, we are also performing transcriptomic analysis on tumor and NK cells sorted from DFMO-treated and untreated tumors, to ascertain whether specific signaling pathways may be being altered by exposure to DFMO. It is our hope that these studies will complement the data being accrued from phase I/II clinical studies using DFMO in various therapeutic strategies for NB, and will allow for an increased understanding of how to more effectively combine PA blockade with other immunotherapies for this disease. Citation Format: Adriana D. Benavides, Annette Vu, Gabrielle M. Ferry, Michael D. Hogarty, Hamid Bassiri. Modulation of the neuroblastoma microenvironment via polyamine blockade [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1630. doi:10.1158/1538-7445.AM2017-1630