15 years old, male patient was diagnosed BPDCN in January 2019 with generalized lymphadenopathy and maculopapular skin rash on trunk. AML BFM 2013 was given but one month after last cycle, he relapsed with macular rash. BPDCN was confirmed, Patient was treated with two cycles of FLAG-IDA and a cycle of Hyper-CYVAD regimens, no remission obtained. Tagraxofusp-erzs was planned but patient had COVID-19 pneumonia with no worsening of his ECOG score. Venetoclax initiated 100 mg/day and increased to 400 mg/day in a week, no toxicity and no tumor lysis syndrome was occurred. In combination first cycle of tagraxofusp- erz was given at a dose of 12 mcg/kg/day/5 days. A reversible moderate hepatotoxicity and myelosuppression occurred. There was no sign of capillary leak syndrome which is the most serious and not so rare side effect. On the second day of second cycle, capillary leak syndrome was diagnosed with sudden weight gain, fluid retention, hypoalbuminemia and slight hypotension. Therapy was ceased. Patient recovered with the supportive treatment including inotropic agent within 5 days. Responses after chemotherapy and side effects are outlined at Table -1 and supplemental Table S1.

The patient admitted to stem cell transplantation unit on July 2020 with remission. Myeloablative regimen was busulfan, melphalan and cyclophosphamide. Neutrophile engraftment occured on 10 th day of transplant. He followed for six months in remission. At 6 th month of transplant BPDCN blasts were positive for both bone marrow and central nervous system. Patient was considered ineligible for intensive chemotherapy and repeated tagraxofusp-erz due to lack of experience in relapsed cases and previous capillary leak syndrome side effect. Azacitidine plus venetoclax combination which was shown to be effective in AML for elderly (3) was considered safe rather than standard chemotherapy. Azacitidine was given 100 mg/m2/day for five days sc and oral venetoclax 400 mg/day. He had no response and died.