AbstractThis study aimed to identify a recommended phase II dose and evaluate the safety, tolerability, pharmacokinetics/pharmacodynamics, and preliminary clinical activity of JNJ‐63709178, a CD123/CD3 dual‐targeting antibody, in patients with relapsed or refractory acute myeloid leukemia. Intravenous (i.v.) and subcutaneous (s.c.) administration of JNJ‐63709178 were evaluated. The i.v. infusions were administered once every 2 weeks (cohorts 1–5 [n = 17]) or twice weekly (cohorts 6–11 [n = 36]). A twice‐weekly s.c. dosing regimen with step‐up dosing was also studied (s.c. cohorts 1–2 [n = 9]). Treatment‐emergent adverse events (TEAEs) greater than or equal to grade 3 were observed in 11 (65%) patients in cohorts 1–5 and 33 (92%) patients in cohorts 6–11. At the highest i.v. dose (4.8 μg/kg), 5 (71%) patients discontinued treatment due to TEAEs. For s.c. administration (n = 9), eight (89%) patients experienced TEAEs greater than or equal to grade 3 and injection site reactions (≤ grade 3) emerged in all patients. At 4.8 μg/kg (i.v. and s.c.), the mean maximum serum concentrations were 30.3 and 3.59 ng/ml, respectively. Increases in multiple cytokines were observed following i.v. and s.c. administrations, and step‐up dosing strategies did not mitigate cytokine production or improve the safety profile and led to limited duration of treatment. Minimal clinical activity was observed across all cohorts. The i.v. and s.c. dosing of JNJ‐63709178 was associated with suboptimal drug exposure, unfavorable safety profiles, limited clinical activity, and inability to identify a recommended phase II dose.