Abstract B203: Polyamine blockade modulates the neuroblastoma microenvironment

Abstract

Neuroblastoma (NB) accounts for a considerable portion of childhood cancer-related mortalities. Despite improvements in therapy over the last few decades, 5-year survival rates in patients with high-risk disease remain poor (40-50%). High-risk disease results from amplification of MYCN and dysregulation in downstream Myc-related pathways. Polyamine (PA) synthesis is one such pathway, as MYCN-amplified high-risk NBs have elevated polyamine levels, and the gate-keeper enzyme in this pathway, ornithine decarboxylase (ODC1), is a direct target of Myc. In a mouse model of MYCN-driven NB (TH-MYCN+/+ transgenic mice), inhibition of PA synthesis with an irreversible inhibitor of ODC1, difluoromethylornithine (DFMO), led to reductions in NB burden that seemed out of proportion to DFMO's ability to limit tumor growth in vitro, suggesting that PA blockade may result in additional tumor cell-extrinsic effects. Consistent with this possibility, PAs have been shown to favor the differentiation and function of suppressive, tumor-supportive cell types, while PA depletion results in reversal of immune suppression via increased number and function of tumor-infiltrating leukocytes (TILs) and concomitant inhibition of infiltration and differentiation of suppressive cell types. However, these previous studies focused on investigating the effects of PA depletion in vitro or in vivo in tumor-induction models or immune-deficient mice. 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 at terminal disease were dissected, mechanically and enzymatically dissociated, and the number and frequencies of various TIL subsets were assessed using an optimized flow cytometry-based protocol. Results indicate that DFMO therapy reduces tumor growth and results in distinct and reproducible alterations in the cellular composition of the NB tumor microenvironment at terminal disease. Specifically, we found an increased frequency of dendritic cells, natural killer cells, and a late accumulation of CD4-negative invariant natural killer T cells and granulocytic tumor-associated myeloid cells. Increases in these specific TIL subsets are consistent with the hypothesis that polyamine blockade induces distinct tumor cell-extrinsic changes in the microenvironment that allow for more efficient immune control of NB. We 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 employ PA blockade in the immunotherapy of this disease. Citation Format: Adriana D. Benavides, Annette T. Vu, Jennie B. Altman, Gabrielle M. Ferry, Michael D. Hogarty, Hamid Bassiri. Polyamine blockade modulates the neuroblastoma microenvironment. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2015 Nov 5-9; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2015;14(12 Suppl 2):Abstract nr B203.