A Study to Test the Safety of Pozelimab in Pediatric Participants 1 to 5 Years of Age With a Rare Disease Called CHAPLE (Complement Hyperactivation, Angiopathic Thrombosis, Protein-losing Enteropathy) Disease

An Open-Label, Single-Arm Study Evaluating the Safety, Pharmacokinetics, Pharmacodynamics, and Immunogenicity of Pozelimab in Pediatric Patients 1 to 5 Years of Age With CD55-Deficient Protein-Losing Enteropathy (CHAPLE Disease)

This study is researching a drug called pozelimab (called "study drug"). The main aim of this study is to monitor the safety and tolerability of the study drug.

The study is focused on young children 1 to 5 years of age, who have CHAPLE disease. CHAPLE is a very rare hereditary disease that can cause potentially life-threatening symptoms related to the stomach and intestines (gastrointestinal symptoms), and symptoms related to the heart and blood vessels (cardiovascular symptoms).

The study is also looking at several other research questions, including:

• What side effects may happen from taking the study drug
• How much study drug is in the blood at different times
• Whether the study drug blocks Complement 5 (C5) in the body
• Whether the study drug changes the level of a substance called CH50 measured in the blood
• Whether the study drug changes the levels of albumin and other proteins
• Whether the body makes antibodies against study drug, which could make the study drug less effective or could lead to side effects

Key Inclusion Criteria:

1. Clinical diagnosis of CD55-deficient CHAPLE disease as described in the protocol
2. Parent(s)/legal guardian(s) are willing and able to comply with participant's clinic visits and study-related procedures, including participant's completion of the full series of meningococcal vaccinations required per protocol.
3. Parent(s)/legal guardian(s) are willing to provide written informed consent
4. Participant has active CHAPLE disease as described in the protocol

Key Exclusion Criteria:

1. History of meningococcal infection
2. No documented meningococcal quadrivalent (serotype ACWY) vaccination prior to screening and participant's parent(s)/legal guardian(s) are unwilling for participant to undergo vaccination during the study as described in the protocol
3. No documented vaccination for Haemophilus influenzae and Streptococcus pneumoniae if applicable based on local practice or guidelines prior to screening as described in the protocol
4. Prior treatment with a complement inhibitor as described in the protocol
5. Presence of a concomitant disease that leads to hypoproteinemia or secondary intestinal lymphangiectasia as described in the protocol

Note: Other Protocol Defined Inclusion/ Exclusion Criteria Apply