Abstract 400: MEK inhibition enhances immune checkpoint blockade treatment of murine models of neuroblastoma

Background: The tumor microenvironment (TME) plays an important role in neuroblastoma (NBL) biology. Infiltrations of tumor-associated macrophages with alterations in patterns of pro-inflammatory genes are associated with poor prognosis in NBL. However, the role of regulatory T-cells in NBL remains unknown. As several genes in MAPK pathway are among recurrent mutations in neuroblastomas, and the production of pro-inflammatory mediators in immune cells is MAPK-dependent, we hypothesized targeted kinase inhibitor in combination with check-point blockade could exert synergistic effects on the TME9s response to tumor cells. Therefore, we assessed the efficacy of combined therapy using a transgenic MYCN non-amplified neuroblastoma murine model driven by SV40 large T antigen (NB-Tag). Methods: Three human neuroblastoma cell lines and two NB-Tag derived mouse cell-lines were used for in vitro cell proliferation assay. For in vivo tumor growth models, combinations of cyclophosphamide, topotecan, trametinib, anti-CTLA4, and anti-PD1 therapies were studied in NB-Tag transgenic and transplantable subcutaneous (NB-SQ) murine models. Results: In vitro studies demonstrated trametinib had the highest anti-proliferative activity compared to other kinase inhibitors, and it effectively blocked cell cycle arrest in G1 phase. These anti-proliferative effects could not be rescued by co-culturing tumor cells with murine or human macrophages. In NB-Tag mice, which develop neuroblastoma spontaneously at 12 weeks, daily oral administration of trametinib (0.6mg/Kg) at pre-tumoral age (10 wk) significantly impaired tumor growth by 17 weeks (1424 mm3 in controls vs. 43 mm3 in treated mice). Treatment of 15 week-old NB-Tag mice (visible tumor by MRI) with trametinib after chemotherapy administration (5-days of Cyclophosphamide + Topotecan) also significantly impaired tumor regrowth (volume four weeks post-chemo, 491 vs. 42 mm3, p = 0.037), and more importantly, treatment increased the survival of NB-Tag mice compared to control (median survival: control = 24.5 wk, treated = 35 wk, p Conclusions: Our results provide strong evidence that MEK inhibition combined with checkpoint blockade significantly inhibited tumor formation in a syngeneic subcutaneous model. These findings indicate opportunities to enhance antitumor immunity with the potential to produce durable clinical responses in children with neuroblastomas. Citation Format: Sakunthala Muthugounder, Long Hung, Randall Chan, Jin Kim, Soheila Shirinbak, Hiroyuki Shimada, Shahab Asgharzadeh. MEK inhibition enhances immune checkpoint blockade treatment of murine models of neuroblastoma. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 400. doi:10.1158/1538-7445.AM2015-400