Overcoming resistance to anti-PD-1 with tumor agnostic NBTXR3: From bench to bed side.
Authors: Seiwert Tanguy Y. , Shen Colette , Frakes Jessica M. , Niu Jiaxin , Weiss Jared , Caudell Jimmy J. , Hu Yun , Barsoumian Hampartsoum B. , Thariat Juliette , Bonvalot Sylvie , Papai Zsuzsanna , Cortez Maria Angelica , Zhang Ping , Jameson Katherine LaRoque , Said Patricia , Paris Sébastien , Welsh James William 
Published: June 2021

2591 Background: Despite recent advances, resistance to immune checkpoint inhibitors (ICI), observed in over 80% of treated patients, is currently the main challenge in immuno-oncology. Intense efforts are being made to identify combination therapies that could improve ICI response rates. NBTXR3, a novel radioenhancer administered by direct intratumoral injection (ITI), is designed at the nanoscale to increase radiation therapy (XRT) dose deposit within tumor cells and subsequent tumor cell killing, without increasing toxicity to surrounding healthy tissue. Here we present evidence that NBTXR3 activated by XRT primes the immune system, producing an anti-tumor response, including activation of the cGAS-STING pathway, that overcomes anti-PD-1 resistance both in mice models and patients. Methods: Abscopal assays were conducted in immunocompetent mice. Anti-PD-1 sensitive or resistant tumor cell lines, were injected in both flanks of mice. Intratumoral injection of NBTXR3 (or vehicle) followed by XRT was performed in right flank (primary) tumors only. Some mice also received anti-PD-1 injections. Tumor growth was monitored, and tumor immune cell infiltrates analyzed by immunohistochemistry (IHC). Separately, in the phase II/III randomized Act.in.Sarc [NCT02379845] trial patients with locally advanced soft tissue sarcoma (STS) received either NBTXR3+XRT or XRT alone followed by tumor resection. Pre- and post-treatment tumor samples from patients in both groups were analyzed by IHC and Digital Pathology for immune biomarkers. The safety and efficacy (RECIST 1.1/iRECIST) of NBTXR3 plus stereotactic body radiotherapy (SBRT) in combination with anti-PD-1 is being evaluated in three cohorts of patients with advanced cancers [NCT03589339]. Results: Pre-clinical studies demonstrated that NBTXR3+XRT induces an immune response not observed with XRT alone and enhances systemic control. IHC showed significant increase of CD8+ T-cell infiltrates in both NBTXR3+XRT treated and untreated tumors compared to XRT alone. Similarly, increased CD8+ T-cell and decreased FOXP3+ Treg density (pre- vs post-treatment) was observed in tumor tissues from STS patients treated with NBTXR3+XRT. Furthermore, NBTXR3+XRT in combination with anti-PD-1 improved local and systemic control in mice bearing anti-PD-1 resistant lung tumors, as well as reduced the number of spontaneous lung metastases. Preliminary efficacy data from the first in human trial of NBTXR3+XRT in combination with anti-PD-1 showed tumor regression in the majority of patients (8/9). Of note, tumor regression was observed in 6/7 pts who had progressed on prior anti-PD-1. Conclusions: The clinical efficacy of NBTXR3+XRT has been demonstrated as a single agent. We now demonstrate that it potentiates anti-PD-1 treatment to overcome resistance mechanisms. These results highlight the potential of NBTXR3+XRT to positively impact the immuno-oncology field. Clinical trial information: NCT03589339.