[Adult T-cell leukemia/lymphoma treated with mogamulizumab followed by allogeneic stem cell transplantation after bridging therapy with valemetostat].

A 64-year-old man with acute adult T-cell leukemia lymphoma (ATLL) was treated with biweekly CHOP with poor response. Mogamulizumab was administered as salvage therapy followed by valemetostat bridging therapy until allogeneic transplantation. Due to concerns about potential development of severe GVHD, mogamulizumab was withdrawn 70 days before transplantation and valemetostat was withdrawn 15 days before transplantation. Nevertheless, the patient developed acute GVHD (grade III (skin stage 3 gastrointestinal stage 2) 49 days after transplantation. The aGVHD was refractory to primary and secondary therapy, but symptoms improved with human mesenchymal stem cell treatment. Although valemetostat bridging therapy controlled ATLL and allowed for a treatment-free interval after mogamulizumab therapy, it delayed regulatory T cell (Treg) recovery and caused severe aGVHD. Further improvements are needed in the management of severe GVHD after mogamulizumab administration, such as monitoring of Tregs and residual mogamulizumab concentrations.