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KMRC011 is a novel Toll‐like receptor 5 agonist under development as a treatment for acute radiation syndrome (ARS). The aim of this first‐in‐human study was to investigate the tolerability, pharmacokinetics, and pharmacodynamics of a single intramuscular dose of KMRC011 in healthy subjects. A randomized, single‐blind, placebo‐controlled, single dose‐escalation study was conducted with the starting dose of 5 μg. Eight (4 only for 5 μg cohort) subjects per cohort were randomly assigned to KMRC011 or placebo in a 3:1 ratio. Dose‐limiting toxicity (DLT) was assessed throughout the study. Serum concentrations of KMRC011, granulocyte colony‐stimulating factor (G‐CSF), and interleukin‐6 (IL‐6) were measured up to 48 h postdose. Based on safety review, the dose of KMRC011 escalated up to 20 μg, and consequently, a total of 4 dose levels (5, 10, 15, and 20 μg) were explored. The most common adverse event was injection site reaction, showing no dose‐related trend. Three DLTs (2 cases of hepatic enzyme increased and 1 of pyrexia) were observed; 1 in the 15 μg cohort and 2 in the 20 μg cohort. A developed method could not detect any KMRC011 in serum. KMRC011 15 μg and 20 μg showed significant increases of G‐CSF, IL‐6, and absolute neutrophil counts, compared with the placebo. A single intramuscular administration of KMRC011 ranging from 5 to 15 μg was tolerated in healthy subjects. Doses of KMRC011 equal to or greater than 15 μg exerted TLR5 agonist‐like activities by increasing serum G‐CSF and IL‐6. It suggests that KMRC011 has the potential for a treatment for ARS. [ABSTRACT FROM AUTHOR]