Abstract 6278: Glutaminase inhibition in combination with ionizing radiation improves treatment response in head and neck squamous cell carcinoma models
Authors: Wicker Christina A. , Hunt Brian G. , Palackdharry Sarah , Elaban William R. , Mills Gordon B. , Wise-Draper Trisha , Waltz Susan , Takiar Vinita 
Published: September 2020

Abstract
Head and neck cancer is the 6th most common cancer worldwide. Ionizing radiation (IR) is an important aspect of cancer treatment. However, its efficacy is limited by radioresistance, resulting in locoregional failure rates as high as 50%. The pathways used by cells to evade the cellular damage caused by IR are poorly understood. Initial Reverse Phase Protein Array analyses in our laboratory demonstrated that IR induced metabolic pathways, including those involving the Krebs cycle. Glutamate is a key substrate of the Krebs cycle, and essential for the production of biomolecules necessary for tumor proliferation. Glutaminase is the enzyme that converts glutamine to glutamate. Analysis of The Cancer Genome Atlas's transcriptome database revealed that glutaminase overexpression (upper 25%) in patients with HNSCC was associated with significantly reduced patient survival (p&lt;0.03).
In this study, we aimed to identify whether glutaminase inhibition with CB-839 could enhance cellular response to IR. To do this, we used three representative human-derived HNSCC cell lines: CAL-27, FaDu, and HN5. We first established that proliferation of all 3 HNSCC cell lines was glutamine dependent (20-50% increase, n=3 per line, p&lt;0.01). Moreover, a synergistic decrease in cell survival was observed in clonogenic assays with HNSCC cell lines treated with IR and CB-839 relative to either treatment alone (n=3 per line, p&lt;0.05). In 3D cell culture, which better represents the host microenvironment, combinatorial treatment significantly reduced HNSCC spheroid size (11-26% decrease, n=3 per line, p&lt;0.0001). To understand the mechanism behind this reduced cell growth and survival, we examined CB-839's effect on cellular metabolism using Seahorse MitoStress test assays. CB-839 significantly reduced spare respiratory capacity and OCR/ECAR ratio in HNSCC cell lines examined (p&lt;0.001, p&lt;2.0 × 10−6, n&gt;3). Combination of CB-839 with IR significantly inhibited tumor growth in Cal-27 xenograft mice relative to vehicle (p&lt;0.01), CB-839 (p&lt;0.05) or IR (p&lt;0.01) (p values representative of day 20).
In summary, adaptive resistance to IR and locoregional failure is a major hurdle in the successful treatment of HNSCC. Overall, these data suggest that combinatorial treatment with IR and the glutaminase inhibitor, CB-839, lead to an enhanced response over either treatment alone and provide pre-clinical data to justify further investigation for clinical use in patients with HNSCC.
Citation Format: Christina A. Wicker, Brian G. Hunt, Sarah Palackdharry, William R. Elaban, Gordon B. Mills, Trisha Wise-Draper, Susan Waltz, Vinita Takiar. Glutaminase inhibition in combination with ionizing radiation improves treatment response in head and neck squamous cell carcinoma models [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 6278.