Impact of Emapalumab on Patients with Hemophagocytic Lymphohistiocytosis

Rationale In 2018, emapalumab was introduced for treatment of hemophagocytic lymphohistiocytosis (HLH). Emapalumab targets cytosolic and receptor-bound interferon-gamma to attenuate hyperinflammation. Methods A large limited de-identified dataset from Epic Cosmos was used to examine treatment rates with emapalumab. Demographics, length of stay, and overall death rates were analyzed. Results Since emapalumab was introduced, 41,750 patients have been diagnosed with primary or secondary HLH. Treatment with emapalumab steadily increased from 0.73% in 2019 to 1.9% in 2024. Death rate of HLH in 2018 prior to introduction of emapalumab was 4%. The death rate in 2024 was 1.1%. Prevalence of HLH was not affected by race or age. Overall prevalence across all ethnicities was 0.01%. Prevalence of HLH in patients under 10 years and between 10 to 19 years of age was 0.01%, but after age 19 years the prevalence decreased to between 0.002 and 0.006%. 39% of patients diagnosed with HLH not treated with emapalumab had a length of stay greater than 14 days compared with 13% of treated patients. Conversely, 69% of treated patients had a length of stay between 4 to 6 days, compared with 18% of untreated patients. Discussion Treatment rates of emapalumab have slowly increased since introduction but remain low despite benefits of treatment. Prevalence of HLH remains consistent among races but is lower in patients aged >20 years. Length of hospital stay was significantly reduced in patients treated with emapalumab. Death rates from HLH decreased from 4% to 1.1% in the time period since emapalumab became available.