Pemigatinib therapy in myeloid/lymphoid neoplasm with FGFR1 rearrangement manifesting as aggressive B-cell lymphoma.

Myeloid/lymphoid neoplasm with FGFR1 rearrangement (MLNF) is one of the rare hematologic malignancies with variable clinical presentations, including a chronic phase resembling myeloproliferative neoplasms and an acute phase presenting as myeloid/lymphoid leukemia or lymphoma, often associated with eosinophilia. The prognosis of MLNF has been poor, and allogeneic hematopoietic stem cell transplantation (HSCT) remains the only curative approach, although selective FGFR1 inhibitors, such as pemigatinib, have recently emerged as therapeutic options. Nevertheless, the efficacy of pemigatinib in aggressive or blast-phase MLNF remains unclear. Herein, we report a case of a 67-year-old woman initially diagnosed with Richter's syndrome. The patient achieved a complete response with six cycles of rituximab-based chemoimmunotherapy followed by acalabrutinib maintenance. Two years later, the patient developed leukocytosis, eosinophilia, and recurrent lymphadenopathy. Bone marrow examination revealed disease recurrence with marked eosinophilia, and FGFR1 rearrangement was confirmed by fluorescence in situ hybridization. The rearrangement was also confirmed from the lymph node specimen of the initial diagnosis; thus, we revised the diagnosis to relapsed MLNF. The patient received pemigatinib, but rapid disease progression was observed. The patient was ineligible for HSCT and salvage chemotherapies were unsuccessful, resulting in death four months later. The present case report highlights a rare lymphoma-like clinical presentation of MLNF, and we discuss the therapeutic options, including pemigatinib.