Jason Gotlib

Disease Overview The eosinophilias encompass a broad range of non‐hematologic (secondary or reactive) and hematologic (primary or clonal) disorders with the potential for end‐organ damage. Diagnosis Hypereosinophilia (HE) has generally been defined as a peripheral blood eosinophil count greater than 1.5 × 10 9 /L, and may be associated with tissue damage. After the exclusion of secondary causes of eosinophilia, diagnostic evaluation of primary eosinophilias relies on a combination of various tests. They include morphologic review of the blood and marrow, standard cytogenetics, fluorescence in situ hybridization, molecular testing and flow immunophenotyping to detect histopathologic or clonal evidence for an acute or chronic hematolymphoid neoplasm. Risk Stratification Disease prognosis relies on identifying the subtype of eosinophilia. After evaluation of secondary causes of eosinophilia, the 2022 World Health Organization and International Consensus Classification endorse a semi‐molecular classification scheme of disease subtypes. This includes the major category “myeloid/lymphoid neoplasms with eosinophilia and tyrosine kinase gene fusions” (MLN‐eo‐TK), and the MPN subtype, “chronic eosinophilic leukemia” (CEL). Lymphocyte‐variant HE is an aberrant T‐cell clone‐driven reactive eosinophila, and idiopathic hypereosinophilic syndrome (HES) is a diagnosis of exclusion. Risk‐Adapted Therapy The goal of therapy is to mitigate eosinophil‐mediated organ damage. For patients with milder forms of eosinophilia (e.g., <1.5 × 10 9 /L) without symptoms or signs of organ involvement, a watch and wait approach with close follow‐up may be undertaken. Identification of rearranged PDGFRA or PDGFRB is critical because of the exquisite responsiveness of these diseases to imatinib. Pemigatinib was recently approved for patients with relapsed or refractory FGFR1 ‐rearranged neoplasms. Corticosteroids are first‐line therapy for patients with lymphocyte‐variant HE and HES. Hydroxyurea and interferon‐α have demonstrated efficacy as initial treatment and in steroid‐refractory cases of HES. Mepolizumab, an interleukin‐5 (IL‐5) antagonist monoclonal antibody, is approved by the U.S Food and Drug Administration for patients with idiopathic HES. Cytotoxic chemotherapy agents, and hematopoietic stem cell transplantation have been used for aggressive forms of HES and CEL, with outcomes reported for limited numbers of patients. Targeted therapies such as the IL‐5 receptor antibody benralizumab, IL‐5 monoclonal antibody depemokimab, and various tyrosine kinase inhibitors for MLN‐eo‐TK, are under active investigation.