This study is researching an experimental drug called cemiplimab (called "study drug") and the combination of experimental drugs of fianlimab and cemiplimab (called "study drugs"). The study is focused on children and young adults with recurrent or progressive High-Grade Glioma (HGG) or ependymoma. "Recurrent" means that the cancer came back after treatment. "Progressive" means that the tumor has grown or spread.

The aim of the study is to see how safe, tolerable, and effective cemiplimab and the combination of fianlimab and cemiplimab are.

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

• What side effects may happen from receiving the study drug(s)
• Do the study drug(s) help study participants live longer without their tumors growing or spreading
• How much of the study drug(s) is in the blood at different times
• Whether the body makes antibodies against the study drug(s) (which could make the study drug[s] less effective or lead to side effects)

Key Inclusion Criteria:

1. Participant must be diagnosed with recurrent/progressive HGG or PF-A ependymoma with unequivocal progression on Magnetic Resonance Imaging (MRI) as described in the protocol
2. Participant must have histologically confirmed (at initial diagnosis or relapse) HGG or PF-A ependymoma
3. Participant must be an adequate medical candidate for surgical resection as described in the protocol
4. Karnofsky Performance Status (KPS) score ≥50 (in participants ≥16 years) or Lansky Performance Status (LPS) score ≥50 (in participants <16 years) as described in the protocol
5. Adequate organ function as described in the protocol

Key Exclusion Criteria:

1. Active autoimmune disease requiring systemic immunosuppressive therapy in the past 2 years
2. Active, serious medical illness, infection or other systemic illness which would limit participation in the trial
3. Has not yet recovered from any acute toxicities resulting from prior therapy
4. History of myocarditis
5. Prior treatment with antibodies to Programmed Cell Death Protein -1 (PD-1), Programmed Cell Death Protein Ligand -1 (PD-L1), Lymphocyte Activation Gene 3 (LAG3), or Cytotoxic T-Lymphocyte Associated protein 4 (CTLA-4)
6. Treatment with high dose systemic corticosteroids as described in the protocol
7. History of interstitial lung disease (eg, idiopathic pulmonary fibrosis, organizing pneumonia) or active, noninfectious pneumonitis that required immune-suppressive doses of glucocorticoids to assist with management

Note: Other protocol defined Inclusion/ Exclusion Criteria apply

All Individual Patient Data (IPD) that underlie publicly available results will be considered for sharing

When Regeneron has:

• received marketing authorization from major health authorities (e.g., FDA, European Medicines Agency (EMA), Pharmaceuticals and Medical Devices Agency (PMDA), etc.) for the product and indication or has globally discontinued development of the product for all indications on or after April 2020 and has no plans for future development
• made the study results publicly available (e.g., scientific publication, scientific conference, clinical trial registry)
• the legal authority to share the data, and
• ensured the ability to protect participant privacy

Qualified researchers can submit a proposal for access to individual patient or aggregate level data from a Regeneron-sponsored clinical trial through Vivli. Regeneron's Independent Research Request Evaluation Criteria can be found at: https://www.regeneron.com/sites/default/files/Regeneron-External-Data-Sharing-Policy-and-Independent-Research-Request-Evaluation-Criteria.pdf