Depemokimab in Participants With Hypereosinophilic Syndrome, Efficacy, and Safety Trial

This is a 52-week, randomized, placebo-controlled, double-blind, parallel group, multicenter study of depemokimab in adults with uncontrolled HES receiving standard of care (SoC) therapy. The study will recruit patients with a confirmed diagnosis of HES and who are on stable HES therapy for at least 4 weeks prior to randomization (Visit 2). Eligible participants must have uncontrolled HES with a history of repeated flare (≥2 flares in the previous 12 months) and blood eosinophil count of ≥1,000 cells/ microliter (μL) during Screening. Historical HES flares are defined as documented HES-related worsening of clinical symptoms or blood eosinophil counts requiring an escalation in therapy. Participants who meet the inclusion and exclusion criteria will be randomized in a 2:1 ratio to receive either depemokimab or placebo while continuing their SoC HES therapy.

Inclusion Criteria: * Participants who are greater than or equal (\>=) 40 kilogram (kg) at Screening Visit 1. * Participants who have a documented diagnosis of HES prior to Visit 2. * A history of 2 or more HES flares within the past 12 months prior to Visit 1. * A female participant is eligible to participate if she is not pregnant or breastfeeding, and one of the following conditions applies: a) woman of non-childbearing potential (WONCBP) Or b) woman of childbearing potential (WOCBP) and using a contraceptive method that is highly effective, with a failure rate of less than (\<) 1 percentage (%). * Capable of giving signed informed consent. Exclusion Criteria: * Participants with HES disease manifestations which in the opinion of the investigator may put the participant at unacceptable risk from study participation or confound interpretation of efficacy or safety data. * Participants with chronic or ongoing active infections requiring systemic treatment or a pre-existing parasitic infestation within 6 months prior to Visit 1. * Participants with a known immunodeficiency (e.g., Human Immunodeficiency Virus \[HIV\]), other than that explained by the use of OCS or other therapy taken for HES. * Participants with a history of or current lymphoma. * Participants with current malignancy or previous history of cancer in remission for less than 5 years prior to Visit 1. Participants that had localized carcinoma (i.e., basal or squamous cell) of the skin which was resected for cure will not be excluded. * Participants with a haematologic malignancy with hypereosinophilia in which HES is not the primary diagnosis, e.g., chronic myeloid leukaemia, myelodysplastic syndrome, chronic eosinophilic leukaemia-not otherwise specified. * Cirrhosis or current unstable liver or biliary disease per investigator assessment. * Participants who have severe or clinically significant cardiovascular disease uncontrolled with standard treatment. * Participants with current diagnosis of vasculitis. * Hypereosinophila with no clinical symptoms and/or proof of organ dysfunction. * Clinical diagnosis of Eosinophilic granulomatosis with polyangiitis (EGPA). * Participants with an allergy/ intolerance to a monoclonal antibody or biologic, or any of the excipients of the investigational product. * Participants who have a previous documented failure with anti-interleukin (IL)-5/5R therapy. * Participants who have received monoclonal antibodies (mAb) within 30 days or 5 half-lives, whichever is longer, prior to Visit 1. * Participants who test positive for the FIP1L1-PDGFRα fusion gene. * QT interval corrected for heart rate according to Fridericia's formula (QTcF) ≥450 milliseconds (msec) or QTcF ≥480 msec for participants with Bundle Branch Block at Screening Visit 1. * Participants who are not responsive to OCS based on clinical response or blood eosinophil counts in the opinion of the Investigator. * Participants who are pregnant or breastfeeding.