Acute myeloid leukemia developed through myeloproliferative features during immunosuppressive therapy for juvenile idiopathic arthritis.

A 17-year-old male with thrombocytosis and exacerbation of arthralgia during intensified immunosuppressive therapy with tocilizumab, prednisolone, and methotrexate for juvenile idiopathic arthritis (JIA) was referred to our department. Bone marrow examination revealed myelodysplastic syndrome/myeloproliferative neoplasm, unclassifiable (MDS/MPN-U). Peripheral myeloblasts disappeared temporarily after discontinuation of tocilizumab but progressed to acute myeloid leukemia six months after the development of MDS/MPN-U. The patient sustained complete remission after unrelated bone marrow stem cell transplantation, followed by chemotherapy. The arthralgia also improved after chemotherapy. The possibility of developing malignancies during immunosuppressive therapy in patients with JIA should be considered. J. Med. Invest. 71 : 335-339, August, 2024.