Plonmarlimab, a novel anti‐GM‐CSF blocking antibody, ameliorates disease progression in the pre‐clinical model of macrophage activation syndrome

Abstract Objectives We aimed to characterize and investigate the safety and efficacy of Plonmarlimab, a novel anti‐granulocyte‐macrophage colony‐stimulating factor (anti‐GM‐CSF) neutralizing antibody, on the treatment of macrophage activation syndrome (MAS), a life‐threatening systemic inflammatory disease, in pre‐clinical models. Methods The binding affinity was evaluated using Biacore. The neutralizing activity was measured through the blockade of ligand–receptor interaction, inhibition of STAT5 phosphorylation and suppression of TF‐1 cell proliferation. The efficacy of Plonmarlimab was evaluated in a humanized MAS model, which was established by engrafting human umbilical cord blood (UCB) cells into NOG‐EXL mice. Additionally, the safety profile of Plonmarlimab was investigated in cynomolgus monkeys. Results At the molecular level, Plonmarlimab showed sub‐nanomolar binding affinity with human GM‐CSF and effectively blocked the binding of GM‐CSF to its receptor. At the cellular level, Plonmarlimab dose‐dependently inhibited intracellular STAT5 phosphorylation and suppressed GM‐CSF‐induced TF‐1 proliferation. In the UCB‐engrafted NOG‐EXL MAS mouse model, Plonmarlimab treatment significantly ameliorated disease progression, demonstrated by the improvements in body weight loss, anaemia and some histopathological features. Furthermore, Plonmarlimab was well tolerated up to 150 mg/kg weekly in monkeys with no reported adverse effects. Conclusions Plonmarlimab is a highly potent GM‐CSF blocking antibody and has demonstrated promising efficacy in a pre‐clinical MAS model with a favourable safety profile, supporting its clinical development.