HANGZHOU – November 04, 2026 -- TJ Biopharma's GM-CSF antibody plonmarlimab achieved an 83.3% overall response rate at Week 8 in the Phase 2 PRIME-1 study of macrophage activation syndrome (MAS), according to results now published in the Journal of Hematology & Oncology.
PRIME-1 enrolled 18 patients who had failed high-dose glucocorticoids
The open-label, single-arm, multi-center study treated patients who were refractory to, or had relapsed after, high-dose glucocorticoid therapy. Fifteen of 18 patients achieved a complete or partial response. Median time to response was 2.1 weeks, with the earliest response recorded on Day 5.
Glucocorticoid doses fell 79.3% from baseline
The least-squares mean reduction in glucocorticoid dose reached 79.3% at Week 8. A total of 83.3% of patients cut their dose by at least 50%. Glucocorticoid sparing is a central commercial and clinical point in MAS, where treatment-related adverse effects are a documented concern.
No Grade 3 or higher infections were observed. No adverse events led to permanent treatment discontinuation or death during the study.
Unmet need in China remains high, with no MAS-specific therapy approved
Published reports from China indicate only about 40% of MAS patients respond to first-line glucocorticoids. Mortality in relapsed or refractory cases can exceed 60%. No therapy is currently approved specifically for MAS in China.
MAS is a life-threatening hyperinflammatory complication of autoimmune rheumatic diseases. Professor Qiong Fu of Renji Hospital, Shanghai Jiao Tong University School of Medicine, a co-corresponding author, said the findings "warrant further investigation" and she expects validation in later studies.
Ferritin and IL-18 fell during treatment, pointing to a possible biomarker
Ferritin and interleukin-18 (IL-18) levels declined markedly on treatment, and IL-18 changes were significantly correlated with ferritin changes. The authors indicate IL-18 may serve as a pharmacodynamic biomarker for response to GM-CSF therapy.
Phase 3 is under way as the pipeline widens to gouty arthritis and sepsis
Plonmarlimab holds Breakthrough Therapy Designation from the Center for Drug Evaluation of China's National Medical Products Administration for MAS. TJ Biopharma has initiated a Phase 3 study in relapsed or refractory MAS associated with rheumatic diseases. Chief Medical Officer Dr. Isaac Meng said larger studies aim to further establish efficacy and safety.
The company is also running PRIME-2, a Phase 2 study in gouty arthritis. It has received investigational new drug clearance for PRIME-3, a Phase 2 study in sepsis.
Chief Scientific Officer Dr. Xi Chen said earlier GM-CSF development largely targeted T cell-driven diseases. He said the PRIME-1 data are consistent with the company's preclinical observations on macrophage-driven inflammation and support development across indications.