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Objective: Extramedullary relapse (EMR) is rare in acute promyelocytic leukemia (APL) and, there is a lack of information on its management. Current practices for EMR in APL are always to adopt strategies from other subtypes of Acute lymphoblastic leukemia (ALL) and Acute myeloid leukemia (AML). Gilteritinib, a highly selective FLT3 inhibitor, has demonstrated a remarkable effect on EMR in FLT3-mutant AML. Therefore, it is worthwhile exploring if FLT3 mutation can be a therapeutic target and assessing the efficacy of Gilteritinib on FLT3-mutant EMR in APL.Methods: We described three cases of FLT3-mutant EMR in APL, comprising two isolated EMR cases and one systemic relapse. The patients underwent treatment with Gilteritinib-based regimens based on FLT3 mutation.Results: All three patients achieved complete regression of EMR, and no signs of tumor lysis syndrome during Gilteritinib-based therapy, only patient 1 showed mild granulocytopenia. They all maintained molecular complete remission (mCR) during the follow-up period.Conclusions: The Gilteritinib-based regimen shows a high and sustained therapeutic effect with minimal adverse effects, and provides a valuable experience for further evaluation in EMR APL patients.

ABSTRACTBackground: The V617F mutation of Janus-associated kinase 2 (JAK2) is common in myeloproliferative neoplasms (MPN). JAK2 V617F mutation can be detected in patients with de novo acute myeloid leukemia (AML), but de novo acute promyelocytic leukemia (APL) with JAK2 V617F mutation is rare.Case presentation: We report a case of APL with both the t(15;17) translocation as well as the JAK2 V617F mutation that transformed into MPN (PV/ET).Conclusions: A de novo APL patient presented initially with JAK2 V617F. After ATRA and ATO dual induction and chemotherapy consolidation, the patient achieved complete remission (CR) with undetectable PML/RARα. However, the JAK2 V617F remained positive, and the patient developed MPN (PV/ET) 22 months later, which responded well to interferon therapy.AML, acute myeloid leukemia; APL, acute promyelocytic leukemia; ATRA, all-trans retinoic acid; ATO, arsenic trioxide; BM, bone marrow; CR, complete remission; ET, essential thrombocythemia; Hb, hemoglobin; JAK2, Janus-associated kinase 2; MPN, myeloproliferative neoplasms; PLT, platelets; PMF, primary myelofibrosis; PML/RARα; PV, polycythemia vera; WBC, white blood cells.