This study is researching two experimental medicines, freneslerbart and mevonlerbart. Freneslerbart and mevonlerbart can be given alone or together. When given together, the medicine is called "REGN-2Cat". These medicines are also known as the "study drugs".

The aim of the study is to see what side effects freneslerbart, mevonlerbart, and REGN-2Cat may have and if they help reduce eye allergy symptoms caused by cat hair/dander compared with a placebo.

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

• How much study drug is in the blood at different times
• Whether the body makes antibodies against the study drug (which could make the drug less effective or could lead to side effects)
• How long the study drug effect lasts

Key Inclusion Criteria:

1. Part A: adult participants aged 18 years and older. For part B: pediatric participants ≥8 years and <18 years, and adult participants aged 18 years and older.
2. Positive SPT to cat allergen extract
3. Positive specific Immunoglobulin E (sIgE) tests for cat and Fel d 1
4. Positive CAC criteria

Key Exclusion Criteria:

1. Significant and/or severe environmental allergies causing symptoms (eg, ocular itching, ocular redness, etc) outside of the challenge setting that are expected to coincide or may interfere with study assessments
2. Presence of any ophthalmic disease/ abnormality/ condition that may interfere with study assessments, affect the study outcomes or participant safety
3. A clinical history of asthma with treatment of asthma requiring systemic (oral or parenteral) corticosteroid treatment as defined in the protocol

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