Final safety and efficacy results from a phase 1/2 study of tagraxofusp, a CD123-targeted therapy, for myelofibrosis.

Patients with myelofibrosis (MF) who are resistant to or relapse after Janus kinase inhibitor (JAKi) therapy have limited treatment options and typically poor prognoses. CD123 is overexpressed in various myeloid malignancies, including MF. Tagraxofusp is a first-in-class CD123-targeted therapy, and the only drug approved globally for the rare myeloid malignancy blastic plasmacytoid dendritic cell neoplasm. We conducted a phase 1/2 trial to determine optimal dosing, and evaluate safety and efficacy of tagraxofusp monotherapy in treatment-naïve (n = 5) MF and patients with MF resistant/refractory to JAKi (n = 25) and not eligible for stem cell transplant. There were no dose-limiting toxicities. The recommended phase 2 dose of tagraxofusp was 12 μg/kg per day for 3 consecutive days per cycle. In the safety population (n = 36) treated at 12 μg/kg per day, the most frequent grade ≥3 treatment-emergent adverse events were thrombocytopenia (19%), anemia (22%), and dyspnea (11%). Capillary leak syndrome occurred in 11% of patients, all during cycle 1 with resolution in all patients. Thirty patients treated at 12 μg/kg per day were efficacy evaluable. Of 18 (n = 2 treatment-naïve, n = 16 relapsed/refractory) patients with baseline splenomegaly, 2 relapsed/refractory patients had spleen volume reduction ≥35%. In relapsed/refractory patients, 40% had total symptom score (TSS) ≥50%, and median overall survival (OS) was 19.3 months. In patients who were treatment naïve, 40% had TSS ≥50%, and median OS was 26.6 months. In this trial, tagraxofusp monotherapy in MF was well tolerated, without cumulative myelotoxicity, and with symptom score improvements, warranting further investigation in combination therapy. This trial was registered at www.clincaltrials.gov as #NCT02268253.