7058 Background: Patients with myelofibrosis (MF) who fail or are intolerant to JAK inhibitors (JAKi) have no standard treatment options. CD123 is expressed on a variety of malignancies, including MF. CD123+ plasmacytoid dendritic cells (pDCs), in the MPN microenvironment, including chronic myelomonocytic leukemia and MF, may be tumor-promoting. Monocytosis in MF associated with rapid disease progression and short survival, suggesting an accelerated disease phase. Notably, monocytes share a common precursor with CD123+ pDCs. Tagraxofusp, a novel CD123 targeted therapy, demonstrated high activity in patients with BPDCN, an aggressive hematologic malignancy derived from CD123+ pDCs, and is FDA approved in BPDCN. As such, tagraxofusp may offer a novel therapeutic approach in MF. Methods: Multicenter, 2-stage Ph 1/2 trial enrolling patients (pts) with MF relapsed, refractory, or intolerant to JAKi. Objectives: determine optimal dose, evaluate safety and efficacy. Stage 1 dose escalation: IV tagraxofusp (7, 9, and 12 mcg/kg/day) dosed daily days 1-3 every 21 days (C1-4), 28 days (C5-7), and 42 days (C8+). Stage 2 (ongoing): pts receive optimal S1 dose (12 mcg/kg/day; no MTD). Results: 23 r/r pts treated. Median age 69 (55-81); 57% female. DIPSS Plus: 4% INT-1, 55% INT-2, 41% high. Baseline platelets: median 59 K/uL (15-579); 70% (16/23) <100 K/uL, 8 pts <50 K/uL. 87% (20/23) baseline splenomegaly (palpable ≥5 cm below left costal margin by physical exam). Most common TRAEs: headache (22%), hypoalbuminaemia (22%), ALT incr. (17%) and thrombocytopenia (17%). Most common ≥Gr3 TRAE thrombocytopenia (2%). Capillary leak syndrome in 1 pt (4%; Gr3). 57% (8/14) of pts with baseline spleen ≥5cm BCM spleen responses: 43% (6/14) had ≥29% and 21% (3/14) had ≥45% reduction. 100% of pts with baseline spleen ≥5cm and monocytosis splenomegaly reductions: 80% (4/5) had ≥29% and 40% (2/5) had ≥45%. 6 pts (3 monocytosis pts and 5 pts platelets <100 K/uL) had 6 mos+ duration, 9 pts ongoing. Conclusions: Tagraxofusp demonstrated single agent activity (reduction in splenomegaly) and manageable safety in R/R MF, including pts with monocytosis, an unmet medical need. Given the presence of CD123+ pDCs, tagraxofusp may offer a novel targeted approach in MF. Updated data to be presented. Registrational designs are being evaluated. Clinical trial information: NCT02268253.