Background/Purpose: Macrophage activating syndrome (MAS) is a rare, potentially fatal complication of systemic juvenile idiopathic arthritis (sJIA) and adult-onset Still’s disease (AOSD). MAS (a form of secondary HLH) is associated with overproduction of proinflammatory cytokines, such as interferon gamma (IFNγ). The REAL-HLH study assessed the real-world utilization of emapalumab, an anti-IFNγ antibody, among patients in the US. Treatment patterns and outcomes in patients with MAS secondary to Still’s disease and treated with emapalumab are presented.

Methods: A retrospective medical chart review conducted across 33 US hospitals identified patients treated with≥1 dose of emapalumab between November 20, 2018 and October 31, 2021. Data extracted on the subset of patients with MAS secondary to Still’s disease from the time of emapalumab initiation to end of data availability, death, or study end (December 31, 2021) were analyzed.

Results: Of the 105 patients identified, 10 had Still’s disease (sJIA, n=9; AOSD, n=1). Most patients were female (8/10; 80%) and white (6/10; 60.0%). At diagnosis, mean (SD) age was 5.6 (6.4) years, and 70% (7/10) of patients met the 2016 ACR MAS diagnostic criteria. At diagnosis, the patient with AOSD was 22 years of age. At time of emapalumab initiation, all patients had received or were receiving other HLH-related therapies, including corticosteriods and anakinra; 60% of patients were in the intensive care unit. Emapalumab was mainly initiated to treat refractory (4/10; 40%), recurrent (3/10; 30%), or progressive (2/10; 20%) disease. Median (range) time from diagnosis to emapalumab initiation was 13.0 (1, 101) days, and median (range) treatment duration was 65.5 (25, 367) days. Emapalumab treatment doses are shown in the Table. Median (range) number of emapalumab doses was 15.5 (2, 35). The majority of patients achieved normal levels of ferritin (5/10; 50%), fibrinogen (6/10; 60%), platelets (8/10; 80%), alanine transaminase (8/10; 80%), absolute neutrophil count (9/10; 90%), and absolute lymphocyte count (9/10; 90%) during treatment. Median time to first normalization of these laboratory parameters ranged from 7 to 46 days. Overall survival and 12-month survival probability following emapalumab initiation was 90% (9/10) for patients with Still’s disease. One patient died due to uncontrolled viremia, which was deemed by the investigator to be unrelated to the clinical condition for which emapalumab was used.

Conclusion: This is the first study to report real-world treatment patterns and outcomes among patients with MAS secondary to Still’s disease treated with emapalumab. A phase 3 clinical trial of emapalumab in patients with sHLH/MAS and underlying rheumatologic disease is ongoing (NCT05001737).

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/treatment-patterns-and-outcomes-in-patients-with-macrophage-activation-syndrome-secondary-to-stills-disease-treated-with-emapalumab-the-real-hlh-study/