Abstract LB-386: Novel multimodal precision immunotherapy by co-targeting the HER3 oncogenic signaling circuitry and PD-1 for head and neck squamous cell carcinoma
Authors: Wang Zhiyong , Goto Yusuke , Gilardi Mara , Allevato Michael , Wu Victoria , Saddawi-konefka Robert , Alvarado Diego , Molinolo Alfredo , Gutkind J. Silvio 
Published: September 2020

Abstract
In prior studies, we have shown that persistent activation of the PI3K/mTOR signaling circuitry is the most frequently dysregulated signaling pathway in HNSCC (&gt;80% of all HPV- and HPV+ cases), and that mTOR inhibitors (mTORi) exert potent antitumor activity in multiple experimental HNSCC model systems and in a recent Phase 2 clinical trial. In this regard, the overreliance on PI3K/mTOR signaling for tumor growth may in turn expose a cancer vulnerability that can be exploited therapeutically. However, the immunosuppressive effects of mTORi limit the potential benefits of their combination with new immune oncology (IO) agents. These include immune check point blockade with pembrolizumab and nivolumab (anti-PD-1), which have recently demonstrated anti-tumor activity in HNSCC, albeit only &lt;20% of the patients show durable responses. There is urgent need to search for alternative approaches for HNSCC treatment. We conducted a kinome-wide siRNA screen to identify new therapeutic targets for HNSCC. This revealed that the ERBB3 gene, encoding HER3, is among the top 20 kinases whose knockdown (KD) decreases HNSCC cell proliferation. A counter screen analysis revealed that ERBB3 was the gene whose KD results in the highest reduction of mTOR pathway activation (indicated by pS6). Remarkably, HER3 KD specifically reduced activation of mTOR but not ERK in cells that do not harbor PIK3CA mutations, and immunoprecipitation of HER3 showed direct binding between HER3 and PI3K. Furthermore, a blocking antibody targeting HER3 (CDX-3379) potently inhibited the tumor growth of PIK3CA wild type but not PIK3CA mutant HNSCC cells. CDX-3379 also showed anti-tumor activity in 4MOSC1, our recently reported tobacco-associated syngeneic HNSCC model. Of importance, we found that HER3 is not expressed in immune cells, but CDX-3379 treatment led to a rapid increase in the accumulation of pro-immunogenic IL-2 and IL-7, concomitant with a reduction in multiple pro-tumorigenic and immune suppressive cytokines (e.g., IL-10, VEGF, G-CSF, GM-CSF, IL-6). This provided a rationale to combine HER3i with IO agents. Indeed, the combination of HER3 and PD-1 blockade elicited a remarkable beneficial effect, with an increase of tumor-infiltrating CD8+ T cells and ~70% of the mice exhibiting complete and durable (&gt;6 months) responses and consequently a significant increase in overall survival. Overall, our findings suggest that persistent tyrosine phosphorylation of HER3 underlies aberrant PI3K/mTOR signaling in HNSCC harboring wild type PIK3CA, and that targeting HER3 may exert its anti-tumor effect by both reducing cancer-driving mTOR activity and reversing an immune evasive tumor microenvironment. These findings also support that co-targeting the HER3 signaling circuitry combined with PD-1 blockade may represent a novel multimodal precision therapeutic approach for HNSCC aimed at achieving durable responses and cancer remission.
Citation Format: Zhiyong Wang, Yusuke Goto, Mara Gilardi, Michael Allevato, Victoria Wu, Robert Saddawi-konefka, Diego Alvarado, Alfredo Molinolo, J. Silvio Gutkind. Novel multimodal precision immunotherapy by co-targeting the HER3 oncogenic signaling circuitry and PD-1 for head and neck squamous cell carcinoma [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 LB-386.