Efficacy Of Add-On Treosulfan And Melphalan High-Dose Therapy In Patients With High-Risk Metastatic Ewing Sarcoma: Report From The International Ewing 2008r3 Trial.

11501 Background: Ewing 2008R3 (EudraCT2008-003658-13) was conducted in 12 countries. It evaluated the effect of treosulfan and melphalan high dose chemotherapy followed by re-infusion of autologous hematopoietic stem cells (HDTreoMel) on event-free (EFS, primary endpoint) and overall survival (OS) in high-risk Ewing Sarcoma (EwS). Methods: Phase III, open label, prospective, multi-center, randomized controlled clinical trial. Eligible patients (pts) had disseminated EwS with metastases to bone and/or other sites, excluding pts with only pleuropulmonary metastases. Pts received 6 cycles of VIDE induction and 8 cycles of VAC consolidation therapy. Patients were randomized to receive additional HDTreoMel chemotherapy or no further treatment (control), They were further stratified by number of bone metastases (1, 2-5, > 5). One-sided adaptive inverse-normal 4-stage design, changed after the 1st interim analysis via Müller-Schäfer method. Initial sample size 185 pts, type I error rate 2.5%, power 80%. Results: 109 pts were randomized between 2009 and 2018: 55 were randomized to HDTreoMel. With a median follow-up of 3.3 years, the primary endpoint EFS was not significantly different between HDTreoMel and control in the adaptive design (HR 0.85, 95% CI 0.55-1.32, intention-to-treat). 3-year (3y) EFS was 20.9 % (95% CI 11.5-37.9%) in HDTreoMel and 19.2 % (95% CI 10.8-34.4%) in control pts. Results were similar in the per protocol collective. Subgroup analyses showed that independent of treatment, male patients had a worse outcome than female patients: 3y EFS 13.3% (95% CI 5.7-31.1%) vs 25.2% (95% CI 15.5-40.8%); p = 0.07. Patients aged < 14 had a better outcome when treated in the HDTreoMel group: 3y EFS 39.3% (95% CI 20.4-75.8%) vs 9% (95% CI 2.4-34%); p = 0.016; HR 0.40 (0.19-0.87). These effects were similar in the per protocol collective. Severe toxicities of hematology, gut, general condition and infection were more pronounced in the HDTreoMel group (p < 0.05). Conclusions: In patients with very high risk EwS, additional HDTreoMel was of no benefit for the entire cohort of patients. HDTreoMel may be of benefit for children age < 14. This observation is supported by comparable results from a non-randomized trial EE99 R3 (Ladenstein et al. JCO, 2010). Clinical trial information: NCT00987636 .