Peer, T.

Abstract RATIONALE: Hypereosinophilic syndrome (HES) is a group of rare diseases characterized by persistent hypereosinophilia causing organ damage/dysfunction. Idiopathic and lymphocytic HES (I-HES/L-HES) subgroups exclude clonal and reactive HES. We evaluated real-world clinical outcomes of patients with I-HES/L-HES. METHODS: This retrospective observational cohort study of patients with I-HES/L-HES collected data from the TriNetX Linked Network of de-identified Electronic Health Records plus Claims in the US. Patients had a HES diagnosis (ICD-10-CM: D72.11) between 10/01/2020–09/12/2023, with ≥12 months continuous enrollment before their first HES diagnosis record (index date [ID]). Patients with myeloproliferative/reactive HES, or a diagnosis of eosinophilic granulomatosis with polyangiitis after HES diagnosis, were excluded. Baseline was defined as 12 months prior to ID. Year 1 was defined as 12 months after ID. Patients were followed up until either 09/12/2023, disenrollment from insurance, death, or 24 months after ID, whichever came first. Clinical response was defined as ≥3 months without HES symptoms and a blood eosinophil (bEOS) count ≤750 cells/µL. Complete hematological remission was defined as a bEOS count ≤500 cells/µL. Partial remission was defined as a bEOS count <50% of baseline value, but >500 cells/µL. Kaplan-Meier curves estimated time to first HES flare and hematological remission. Analyses were descriptive. RESULTS: At ID, patients with I-HES/L-HES (N=110) had a mean (standard deviation [SD]) age of 43.8 (21.3) years; 66 (60.0%) were male, 58 (52.7%) were White and 30 (27.3%) were Black or African American. Mean (SD) follow-up was 15.1 (7.7) months. Median (95% confidence interval [CI]) time from ID to first flare (n=47) was 18.0 (11.0, not estimated) months (Table 1). Median (95% CI) time to first complete or partial hematological remission (n=52) was 15.3 (10.8, 21.6) months. The proportion of patients with a definite or probable HES flare was lower at Year 1 (10/69 [14.5%]) versus baseline (40/110 [36.4%]). Few patients achieved clinical response at baseline (10/110 [9.1%]) and Year 1 (5/69 [7.2%]). Among patients with bEOS data, mean (SD) bEOS count was 2235.3 (4351.7) cells/µL at baseline (n=51) and 833.6 (1396.0) cells/µL at Year 1 (n=11). The proportion of patients with a bEOS count <500 cells/µL was 20/51 (39.2%) at baseline and 7/11 (63.6%) at Year 1. CONCLUSION: Despite a reduced proportion of patients experiencing flares within a year after diagnosis, many did not experience hematological remission or clinical response by Year 1; suggesting high disease burden with existing treatment and the need for novel therapies to better manage I-HES/L-HES.