Abstract 572: Inhibition of IDO1 with epacadostat enhances anti-tumor efficacy of PD-1 blockade in a syngeneic glioblastoma (GBM) model
Authors: Reardon David A. , Gokhale Prafulla C. , Klein Sarah R. , Jones Kristen L. , Kirschmeier Paul T. , Speranza Maria , Koblish Holly , Scherle Peggy , Leopold Lance , Newton Robert , Freeman Gordon J. 
Published: July 2017

Purpose: To determine if epacadostat, an oral indoleamine 2,3-dioxygenase (IDO1) inhibitor has therapeutic benefit against GBM when administered as single agent and with PD-1 blocking antibody.

Methods: An initial survival experiment was performed to assess efficacy and was followed by an identical repeat experiment for validation. 1X105 luciferized GL261 cells, a murine GBM tumor line derived from intracerebral methylcholanthrene implantation, were stereotactically implanted intracranially in albino syngeneic C57BL/6 mice. Mice with increasing bioluminescence on days 3 and 6 were randomized (n=8/group) to receive treatment beginning on day 6: anti-PD-1 (332.8H3, mouse IgG1; 500 μg intraperitoneal (IP) on day 6, 250 μg q 3 days X 7); epacadostat (Incyte Corporation, orally dosed at 300 mg/kg/day for 5 days on/2days off for 3 weeks); anti-PD-1 + epacadostat; and control therapy (isotype IgG antibody IP and 0.5% methocel in water). Tumor response assessments were performed by quantifying bioluminescence and survival. A re-challenge experiment was performed in long-term survivors to assess for tumor immune responses capable of preventing relapse. All long-term surviving mice (defined as ≥ 100 days) from the efficacy experiment were injected with 1X105 GL261 cells in the contralateral hemisphere and followed for survival.

Results: In both preclinical efficacy experiments, median survival in the epacadostat monotherapy group did not differ from controls (approximately 30 days). Four of 8 mice (50%) treated with anti-PD-1 were long-term survivors in both efficacy experiments. In the epacadostat plus anti-PD-1 combination group, 81% of the mice were long-term survivors (7 of 8 in experiment 1 and 6 of 8 in experiment 2). Of note, none of the long-term surviving mice developed evidence of tumor; thus the median survival among the anti-PD-1 and epacadostat plus anti-PD-1 combination groups were both &gt; 100 days. In the re-challenge study, all of the mice who underwent GL261 re-inoculation survived &gt; 100 more days with no evidence of tumor recurrence.

Conclusions: IDO1 inhibition with epacadostat increased the eradication rate of anti-PD-1 therapy in an orthotopic syngeneic GBM model and long term survivors rejected tumor following orthotopic re-challenge. Further combinatorial studies incorporating IDO inhibitor therapy for GBM, including mechanistic studies, are warranted.

Citation Format: David A. Reardon, Prafulla C. Gokhale, Sarah R. Klein, Kristen L. Jones, Paul T. Kirschmeier, Maria Speranza, Holly Koblish, Peggy Scherle, Lance Leopold, Robert Newton, Gordon J. Freeman. Inhibition of IDO1 with epacadostat enhances anti-tumor efficacy of PD-1 blockade in a syngeneic glioblastoma (GBM) model [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 572. doi:10.1158/1538-7445.AM2017-572