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Tagraxofusp (TAG), a first-in-class CD123-targeted therapy, is a recombinant fusion protein of human interleukin-3 conjugated to a truncated diphtheria toxin payload. Blastic plasmacytoid dendritic cell neoplasm (BPDCN) is an aggressive orphan hematologic cancer with a poor prognosis, and is derived from plasmacytoid dendritic cells that overexpress interleukin-3 receptor subunit alpha (IL3RA or CD123). This open-label phase I/II study evaluated the efficacy and safety of TAG in 11 Japanese BPDCN patients, of whom seven were treatment-naïve (TN) and four had relapsed/refractory (R/R) disease. In the phase I portion, seven patients (five TN, two R/R) were treated, and no dose-limiting toxicity was observed, with 12 μg/kg/day tolerated. In the phase II portion, four patients (two TN, two R/R) were treated. Among the seven TN BPDCN patients, the rate of complete response (CR) + clinical CR (CRc: CR with minimal residual skin abnormality) was 57.1% (90% confidence interval [CI], 22.5-87.1), and the lower limit of the 90% CI (22.5%) exceeded the pre-specified threshold of 10%. Common adverse events included increased alanine aminotransferase (81.8%) and aspartate aminotransferase (72.7%), hypoalbuminemia, hypokalemia, and capillary leak syndrome (54.5%). The results indicate that TAG was effective and had a manageable safety profile in Japanese BPDCN patients.