Long-term evaluation of the novel radioenhancer NBTXR3 plus radiotherapy in patients with locally advanced soft tissue sarcoma treated in the phase II/III Act.In.Sarc trial.

11544 Background: NBTXR3, a novel radioenhancer activated by radiotherapy (RT) demonstrated superior efficacy, as preoperative treatment, in patients with locally advanced soft tissue sarcoma (LA STS), compared to RT alone. Primary endpoint of pCR rate was 16% vs 8% (p=0.044) and R0 margin rate was 77% vs 64% (p=0.042) (Bonvalot et al. Lancet Oncol. 2019). No modification of the early safety profile of RT was observed, leading to market authorization. Here we report on the long-term safety, limb function and quality of life. Methods: This phase II/III randomized (1:1), international trial included adult patients with LA STS of the extremity or trunk wall, requiring preoperative RT (NCT02379845). Patients were treated with either a single intratumoral injection of NBTXR3 (volume equivalent to 10% of tumor volume, at 53.3g/L) plus EBRT (arm A), or EBRT alone (arm B) (50 Gy in 25 fractions), followed by surgery. The primary and main secondary efficacy endpoints were previously reported. Safety of NBTXR3+RT, as preoperative treatment, was evaluated as secondary endpoint. We present the safety analyses done in the “all treated population”, with data recorded during at least a two-year follow-up. Important parameters related to HR-QoL, including functional outcome were studied using the EQ-5D, RNLI, TESS and MSTS questionnaires. Results: Patients had at least two-year follow-up and the lost to follow-up rate was very low (1.9%). RT-related SAEs were observed in 11.2% (10/89) vs 13.3% (12/90) in A vs B. Post-treatment AEs, any grade, were observed in 51.7% (46/89) vs 57.8% (52/90) and serious post-treatment AEs in 13.5% (12/89) vs 24.4% (22/90) of patients in A vs B. Second primary cancer was observed in 1 patient in arm A and 6 patients in arm B. Long-term safety continues to demonstrate that NBTXR3 plus RT has no impact on post-surgical wound complications (24.7% vs 36.7%, A vs B). Furthermore, the evaluation of radiation late toxicities in limbs such as fibrosis (4.5% vs 7.7%), arthrosis (2.2% vs 0.0%) and edema (6.7% vs 2.2%) that may alter limb function showed no difference between arms. Accordingly, HR-QoL evaluation yielded no difference in functional outcome. In addition, the intratumoral injection of NBTXR3 did not induce cancer cell seeding at the former tumor site. Finally, sequelae or chronic tissue disturbances at the former tumor localization were similar in both treatment arms, confirming that the increase of energy dose deposit and the physical presence of NBTXR3 did not impact post-treatment limb functions. Conclusions: The long-term safety results demonstrate that the addition of NBTXR3 to EBRT neither added toxicity nor modified the bystander effect of RT. The results presented here associated with the efficacy data reported previously reinforce the favorable benefit-risk ratio of the use of NBTXR3 in patients with LA STS. Clinical trial information: NCT02379845.