BACKGROUND: Regorafenib has proven activity in patients with pretreated gastrointestinal stromal tumours and colorectal and hepatocellular carcinoma. We designed REGOBONE to assess the efficacy and safety of regorafenib for patients with progressive metastatic osteosarcoma and other bone sarcomas. This trial comprised four parallel independent cohorts: osteosarcoma, Ewing sarcoma, chondrosarcoma, and chordoma. In this Article, we report the results of the osteosarcoma cohort. METHODS: In this non-comparative, double-blind, placebo-controlled, phase 2 trial, patients aged 10 years or older with histologically confirmed osteosarcoma whose disease had progressed after treatment with one to two previous lines of chemotherapy for metastatic disease and an Eastern Cooperative Oncology Group performance status of 0 or 1 were enrolled. Patients were randomly assigned (2:1) to receive either oral regorafenib (160 mg/day, for 21 of 28 days) or matching placebo. Patients in both groups also received best supportive care. Randomisation was done using a web-based system and was stratified (permuted block) by age at inclusion (<18 vs ≥18 years old). Investigators and patients were masked to treatment allocation. Patients in the placebo group, after centrally confirmed progressive disease, could cross over to receive regorafenib. The primary endpoint was the proportion of patients without disease progression at 8 weeks. Analyses were done by modified intention to treat (ie, patients without any major entry criteria violation who initiated masked study drug treatment were included). All participants who received at least one dose of study drug were included in the safety analyses. This study is registered with ClinicalTrials.gov, number NCT02389244, and the results presented here are the final analysis of the osteosarcoma cohort (others cohorts are ongoing). RESULTS: Between Oct 10, 2014, and April 4, 2017, 43 adult patients were enrolled from 13 French comprehensive cancer centres. All patients received at least one dose of assigned treatment and were evaluable for safety; five patients were excluded for major protocol violations (two in the placebo group and three in the regorafenib group), leaving 38 patients who were evaluable for efficacy (12 in the placebo group and 26 in the regorafenib group). 17 of 26 patients (65%; one-sided 95% CI 47%) in the regorafenib group were non-progressive at 8 weeks compared with no patients in the placebo group. Ten patients in the placebo group crossed over to receive open-label regorafenib after centrally confirmed disease progression. 13 treatment-related serious adverse events occurred in seven (24%) of 29 patients in the regorafenib group versus none of 14 patients in the placebo group. The most common grade 3 or worse treatment-related adverse events during the double-blind period of treatment included hypertension (in seven [24%] of 29 patients in the regorafenib group vs none in the placebo group), hand-foot skin reaction (three [10%] vs none), fatigue (three [10%] vs one [3%]), hypophosphataemia (three [10%] vs none), and chest pain (three [10%] vs none). No treatment-related deaths occurred. CONCLUSIONS: Regorafenib demonstrated clinically meaningful antitumour activity in adult patients with recurrent, progressive, metastatic osteosarcoma after failure of conventional chemotherapy, with a positive effect on delaying disease progression. Regorafenib should be further evaluated in the setting of advanced disease as well as potentially earlier in the disease course for patients at high risk of relapse. Regorafenib might have an important therapeutic role as an agent complementary to standard cytotoxic chemotherapy in the therapeutic armamentarium against osteosarcoma. BACKGROUND: Bayer HealthCare.