Abstract 2922: The radioprotector GC4419 enhances the response of squamous cell carcinoma of the head and neck tumors to ionizing radiation and enhances radioimmune therapy

Locoregional recurrence (LRR) is the major cause of morbidity and mortality in patients with squamous cell carcinoma of the head and neck (HNSCC). For those patients that experience surgically unresectable LRR, median overall survival (OS) is less than one year. Approaches utilizing stereotactic ablative radiotherapy (SAbR) and immune oncology agents such as Nivolumab (α-PD-1 agonist) in the salvage setting have become alternatives to conventional chemoradiation therapy with one randomized, Phase II clinical trial combining these two modalities currently recruiting (clinicaltrials.gov identifier: NCT02684253). Despite improvements in OS, rates of acute oral mucositis (OM) and late toxicities (skin fibrosis) remain a concern with SAbR, particularly in the context of re-irradiation. Here we present pre-clinical evidence that GC4419 (Galera Therapeutics), a highly selective superoxide dismutase mimetic that recently received FDA Breakthrough therapy status following a Phase II, randomized, placebo controlled trial for reducing the duration and incidence of severe OM in patients undergoing chemoradiation therapy for locally advanced HNSCC (NCT02508389) also enhances the anti-tumor radiation response of HNSCC tumors to irradiation. GC4419 slows the growth of a panel of HNSCC cell lines in vitro, and utilizing clonogenic survival assays, demonstrates single agent anti-cancer activity at physiologically achievable concentrations, while also enhancing the response of HNSCC lines to irradiation. In tumor growth delay (TGD) experiments utilizing the syngeneic HNSCC tumor model, AT-84, when GC4419 is delivered starting 30-60 minutes prior to irradiation with biologically equivalent fractionation schedules of 17 Gy x 1 fxn, 10.24 Gy x 2 fxn, or 5 Gy x 5 fxn, a significant enhancement of the radiation response is achieved. Furthermore, when GC4419 is combined with radiation and α-PD-1 inhibitor, a further enhancement of the radiation response is observed, indicating that GC4419 compliments radiotherapy combined with immune oncology therapies. This potentiation of the anti-tumor response with GC4419 is more pronounced at doses that exceed the threshold to be considered for intensity modulated radiation therapy (IMRT). Results from clinical trials and the accompanying pre-clinical data strongly suggest that GC4419 should be combined with radio-immune therapy to not only enhance local tumor control, but that the potential for normal tissue protection with SAbR also creates the opportunity for dose escalation and may further improvement in treatment outcome. Citation Format: Brock J. Sishc, Elizabeth M. Polsdofer, Yuanyuan Zhang, Debabrata Saha, Michael D. Story. The radioprotector GC4419 enhances the response of squamous cell carcinoma of the head and neck tumors to ionizing radiation and enhances radioimmune therapy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 2922.