Abstract
A 79-year-old man presented with respiratory distress associated with a mediastinal mass and pleural effusion, and was diagnosed as having adult T-cell leukemia/lymphoma. The patient was highly refractory to anticancer drugs and radiotherapy from the time of onset and had progressive respiratory deterioration. However, his condition became stable with the administration of valemetostat for 11 days, and subsequent low-dose-anticancer agents led to a rapid improvement accompanied by high fever and a surge in C-reactive protein. In this case, the in vivo priming effect of valemetostat on tumor cells may have increased the sensitivity of these cells to conventional anti-cancer drugs.