Background: Immune checkpoint inhibitors (ICIs) targeting PD-1 are an effective treatment for a variety of cancers. However, the majority of patients (pts) exhibit resistance to ICIs. Overcoming this resistance represents a major challenge in immuno-oncology. Emerging evidence suggests radiation therapy (RT) produces an immunomodulatory effect that may act synergistically with ICIs. However, RT dose and ultimate efficacy are limited by toxicity to surrounding healthy tissues. NBTXR3, a novel radioenhancer administered by direct intratumoral injection (ITI), is designed at the nanoscale to increase RT dose deposit within tumor cells and subsequent tumor cell killing, without increasing toxicity to surrounding healthy tissue. Preclinical data suggest NBTXR3/RT can trigger a local and systemic anti-tumor immune response and overcome anti-PD-1 resistance. NBTXR3/RT combined with anti-PD-1 may prime the immune system to increase the proportion of ICI responders, or convert ICI non-responders to responders. Methods: This is a multicenter, open-label, phase I trial [NCT03589339] to evaluate NBTXR3/RT/anti-PD-1 in 3 cohorts: (1) Locoregional recurrent or recurrent and metastatic head and neck squamous cell carcinoma (HNSCC) amenable to HN re-irradiation, and metastases from any primary cancer eligible for anti-PD-1 (nivolumab or pembrolizumab) treatment specifically localized in the lung (2) or liver (3), respectively. Stereotactic body RT (SBRT) is delivered at tumor-site selective doses per standard practice. The primary objective is NBTXR3/RT/anti-PD-1 recommended phase 2 dose in each cohort. Secondary objectives are anti-tumor response (objective response rate), safety and feasibility of NBTXR3 injection. Results: Nine pts have been treated: 3 HNSCC, 4 lung, 2 liver. 7/9 pts were anti-PD-1 non-responders. Overall tumor regression was observed in 8/9 pts. NBTXR3/RT/anti-PD-1 resulted in tumor regression in 6/7 pts who had progressed on prior anti-PD-1. A complete response in the injected lymph node lasting over 1 year was observed in 1 anti-PD-1 naïve pt. 2 SAEs related to anti-PD-1 and possibly related to NBTXR3 (G5 pneumonitis, G4 hyperglycemia) were observed in 1 anti-PD-1 naïve HNSCC pt and considered DLTs. This pt also experienced 2 other SAEs related to anti-PD-1 (G4 diabetic ketoacidosis, G4 acute kidney injury). SBRT-related safety profile was as expected. Updated results will be presented. Conclusions: Data from this first-in-human phase I trial evaluating NBTXR3/RT/anti-PD-1 in pts with advanced cancers, show NBTXR3 ITI is feasible and well-tolerated. NBTXR3/RT/anti-PD-1 demonstrated promising signs of efficacy. Of particular interest, NBTXR3/RT can overcome ICI resistance in pts having progressed on prior anti-PD-1, supporting further development of NBTXR3 in combination with anti-PD-1 as well as other ICIs. Clinical trial information: NCT03589339