6069 Background: New therapeutic approaches are needed for elderly or frail head and neck squamous cell carcinoma (HNSCC) patients (pts) ineligible for standard of care treatment. NBTXR3, a crystalline solution of hafnium oxide nanoparticles may represent such an option. Injected intratumorally, NBTXR3 enters tumor cells and yields an increased cell-localized energy deposit upon exposure to radiotherapy (RT), leading to increased tumor cell death compared to the same dose of RT alone. Methods: Phase I study of NBTXR3 activated by RT in pts ≥70 years old or ≥65 years old and unable to receive cisplatin, eligible for exclusive RT with stage III or IV HNSCC of the oral cavity or oropharynx [NCT01946867]. A 3+3 dose escalation design was implemented with dose levels corresponding to 5%, 10%, 15% and 22% of baseline tumor volume, followed by an expansion phase. Pts received an intratumoral (IT) injection of NBTXR3 and intensity modulated RT (IMRT; 70 Gy/35 fractions/7 weeks). Determination of Recommended Phase 2 Dose (RP2D) and Dose Limiting Toxicities (DLT) were primary endpoints of phase I. Absence of NBTXR3 leakage and preliminary efficacy using RECIST 1.1 principles were also evaluated. Results: The dose-escalation is complete. Nineteen pts were enrolled: 3 at 5%, 3 at 10%; 5 at 15% and 8 at 22% with no observed DLT or SAE related to NBTXR3 or IT injection. One grade 1 NBTXR3-related AE (asthenia at 22%) and four IT injection-related AE (grade 2 oral pain; grade 1 tumor hemorrhage; grade 1 asthenia, and grade 1 injection site hemorrhage) were reported. RT-related toxicity was as expected with IMRT. RP2D has been determined to be 22%. CT-scan assessment between 24h and 7 weeks post-IT injection demonstrated absence of NBTXR3 leakage in the surrounding tissues. Among 13 evaluable pts treated at doses ≥10%, 9 achieved a complete response of the injected lesion. Conclusions: These results show that NBTXR3 activated by RT is safe and well tolerated at all doses with preliminary encouraging efficacy results. It thus represents a promising future treatment for frail and elderly pts with locally advanced HNSCC with limited therapeutic options. Expansion phase has started at the RP2D. Clinical trial information: NCT01946867.

6051 Background: The non-surgical standard of care (SOC) for the treatment of locally advanced head and neck squamous cell carcinoma (LA HNSCC) patients is concurrent chemoradiation with high dose cisplatin or cetuximab in case of contra-indication to cisplatin. However elderly patients, and those with poor performance status, comorbidities, and/or intolerance, may not benefit from these SOC treatments and represent a high unmet need. New approaches are thus needed to improve clinical outcomes without adding toxicity. NBTXR3, a novel radioenhancer, composed of functionalized hafnium oxide nanoparticles, is injected once intratumorally and activated by radiotherapy (RT).NBTXR3 increases the RT energy deposit inside tumor cells and subsequently increases tumor cell death compared to RT alone, while sparing healthy tissues. We present here the results of the dose expansion part of the phase I study evaluating NBTXR3 plus intensity modulated radiation therapy (IMRT) in this population. Methods: Patients with stage III-IVA or T3/T4 (AJCC/UICC TNM staging system 8th ed.) HNSCC of the oropharynx or oral cavity, ineligible to cisplatin or cetuximab and amenable for RT, received a single intratumoral injection of NBTXR3 and IMRT (70 Gy in 35 fractions /7 weeks). A classical 3 + 3 dose escalation design has tested four doses of NBTXR3, equivalent to 5, 10, 15, and 22% of baseline theoretical tumor volume. The RP2D established as 22% of baseline tumor volume is further tested in the dose expansion part. The primary endpoints of the dose expansion part are objective response rate (ORR) and complete response rate (CRR) of the primary tumor, by imaging according to RECIST 1.1. Safety is also evaluated. Results: As of August 13, 2020, 43 patients have been treated in the phase I dose expansion part. The median age was 70.7 years old (range: 50.7- 89.9), 70% of patients had cardiac disorder risk, 44% had gastrointestinal disorder risk and 44% metabolic and nutrition disorder risk. The median tumor volume was 42.8 mL (range: 1.3 - 222.3). At a median time of 7.8 months after NBTXR3 injection, the ORR of the primary lesion was 83.9% and the CRR 67.7% in the evaluable population for efficacy (N = 31). Three patients (7%) experienced at least one serious adverse event (AE) related to the injection procedure and/or NBTXR3 which represented less than 1% of all reported AEs. RT-related toxicity was as expected with IMRT. Three deaths due to AEs related to RT and other causes were reported. The recruitment is ongoing and updated efficacy and safety results will be presented. Conclusions: NBTXR3 intratumoral administration followed by IMRT may represent an option in elderly patients or patients with multiple comorbidities with LA-HNSCC who have limited therapeutic options. NBTXR3 activated by RT showed promising anti-tumor efficacy, supporting further evaluation in a phase III randomized trial. Clinical trial information: NCT01946867.

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6573 Background: The standard of care non-surgical approach for locally advanced head and neck squamous cell carcinoma (LA HNSCC) patients (pts) is concurrent chemoradiation with high dose cisplatin or cetuximab in case of contra-indication. Older age is a contra-indication to cisplatin, and cetuximab might not improve survival in older pts. It is therefore urgently needed to develop new treatment options for elderly pts with LA HNSCC. NBTXR3 are hafnium oxide nanoparticles that can enhance the efficacy of radiotherapy (RT) by increasing locally the deposited dose. In this phase I clinical trial we aimed to evaluate the feasibility and safety of NBTXR3 administered as intratumoral (IT) injection prior to RT in LA HNSCC elderly pts. Methods: Pts with stage III-IV LA HNSCC of the oropharynx or oral cavity ineligible for platinum-based chemoradiation received a single IT injection of NBTXR3 into a selected primary tumor and intensity modulated RT (IMRT; 70 Gy/35 fractions/7 weeks) [NCT01946867]. A 3+3 dose escalation design, tested NBTXR3 dose levels equivalent to 5, 10, 15, and 22% of baseline tumor volume, followed by a dose expansion at the Recommended Phase II Dose (RP2D). Primary endpoints included RP2D determination, and early dose limiting toxicities (DLT). NBTXR3 intratumoral bioavailability and anti-tumor activity (RECIST 1.1) were also evaluated. Results: Enrollment was completed at all dose escalation levels: 5% (3 pts), 10% (3 pts), 15% (5 pts), and 22% (8 pts). No early DLT or SAE related to NBTXR3 or injection were observed. The median follow-up from NBTXR3 administration is 7.6 months. One AE (Grade 1) related to NBTXR3 and four AEs (Grade 1-2) related to the injection were observed. RT-related toxicity was as expected with IMRT. CT-scan assessment showed a good dispersion of NBTXR3 throughout the injected tumor and not in surrounding healthy tissues. The RP2D was determined to be 22%. Preliminary efficacy was evaluated in pts who received the intended dose of NBTXR3 and RT. A complete response of the injected lesion was observed in 9/13 (69%) evaluable pts at doses ≥10% (2 unconfirmed) and an overall complete response in 5/13 (38%) evaluable pts at doses ≥10%. Preliminary safety and efficacy data of the dose expansion cohort at the RP2D will also be presented. Conclusions: NBTXR3 activated by RT was well tolerated at all tested doses and demonstrated promising preliminary anti-tumor activity. Recruitment is ongoing in the dose expansion cohort. These results demonstrate that further testing of NBTXR3 in this population is warranted. Clinical trial information: NCT01946867 .

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