This study is researching 2 experimental drugs, REGN1908 and REGN1909, which are called REGN1908-1909 when mixed together (called "study drug") to reduce eye allergy signs and symptoms from cat allergy.

The aim of the study is to see how safe and effective the study drug is at lowering allergic eye signs and symptoms compared with placebo. A placebo looks like a treatment but does not contain any real medicine.

The study is 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 body makes antibodies against the study drug (which could make the drug less effective or could lead to side effects)

Key Inclusion Criteria:

1. Documented or participant-reported history of moderate to severe cat allergy for at least 2 years with bothersome ocular symptoms being near cat(s)
2. Positive SPT with cat allergen extract (mean wheal diameter at least 5 mm greater than the negative control) at screening visit 1
3. Positive Allergen-specific Immunoglobulin E (sIgE) tests for cat allergen and Fel d 1 (both ≥0.7 kUa/L) at screening visit 1
4. Must be able to complete the screening CACs to confirm moderate to severe cat allergen-induced allergic conjunctivitis as described in the protocol
5. Must not have lived with a cat(s) in the home for the past 3 years and must be willing to have no exposure to cat(s) at home throughout the study duration as described in the protocol

Key Exclusion Criteria:

1. Participation in a prior clinical study and received either REGN1908-1909, REGN1908, or REGN1909 antibodies as described in the protocol
2. Inability to complete or termination of the screening or confirmatory CAC (where applicable) due to a safety concern (eg, anaphylaxis), per PI judgement
3. Significant and/or severe allergies, ocular, nasal, or systemic disease causing symptoms (eg, ocular itching, ocular redness, etc) that are expected to coincide or potentially interfere with the study CAC assessments, as described in the protocol
4. Persistent chronic or recurring acute infection requiring treatment with systemic antibiotics, antivirals, or antifungals, or any untreated respiratory infections within 4 weeks prior to screening visit 1 as described in the protocol
5. Presence of ocular conditions associated with acute or chronic vision loss, or any other ophthalmic disease or abnormality that may affect the study outcomes or participant safety, per Principal Investigator (PI) judgement as described in the protocol
6. Uncontrolled asthma or on Global Initiative for Asthma (GINA) steps 4 to 5 as described in the protocol
7. Abnormal lung function as judged by the investigator with Forced expiratory volume in 1 second (FEV1) <70% of predicted at screening
8. History of cat immunotherapy (eg, subcutaneous immunotherapy or any other route) in the 3 years prior to screening visit 1
9. Ongoing Allergen immunotherapy (AIT) with any allergen other than cat at screening visit 1 as described 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