Chronic graft-versus-host disease (cGvHD) occurs in 20-50% of recipients following allogeneic haematopoietic stem cell transplantation (allo-HSCT). Corticosteroids (CS) remain the first line treatment but have suboptimal efficacy and carry the risk of long-term side effects. New agents with a better safety profile and higher efficacy, ideally combining immunomodulatory and antifibrotic properties, are urgently needed. From 2016 to 2020, we conducted a phase 2 clinical trial with ATO in chronic GvHD (ClinicalTrials.gov ID NCT02966301 - GMED16-001). This study aimed to evaluate the efficacy and safety of a first-line combination of arsenic trioxide (ATO) and CS in adult patients with cGvHD who require systemic therapy after a first allo-HSCT for a haematological disease. In this prospective, national, multicenter, single-arm, open-label phase II study, conducted in 5 university hospital centers in France, ATO was initiated within 10 days of starting CS at 1 mg/kg/day. Patients received 11 infusions of ATO per 4-week cycle, at a dose of 0.15 mg/kg per infusion. Patients received 1 or 2 cycles of treatment depending on the clinical response and the assessment of the treating physician. The results were published in July 2022 in a paper entitled "High Response Rate and Corticosteroid Sparing with ATO-Based First-Line Therapy in Chronic Graft-versus-Host Disease after Allogeneic Hematopoietic Stem Cell Transplantation". The GMED16-001 study demonstrated convincing efficacy and safety characteristics of the intravenous drug (Trisenox®/Arscimed®): the first-line use of ATO in addition to corticosteroids was associated with a high clinical response rate and correlated with rapid corticosteroid (CS) sparing in moderate to severe cGvHD after allo-HSCT. We performed a post-hoc analysis of the data to determine the best schedule for treating cGvHD patients with ATO. Our post-hoc analysis leads us to the conclusion that two cycles of ATO, at 0.15 mg/kg/day, 5 days per week for 4 weeks each, separated by a 4-week therapy-free interval, appears to be the most promising treatment schedule.