Regeneron is conducting a study of an investigational new drug called DB-OTO. DB-OTO is a gene therapy that is being developed to treat pediatric and adult participants who have severe-to profound and profound hearing loss due to changes in the otoferlin gene.

The purpose of this study is to:

• Learn about the safety of DB-OTO
• Determine how well DB-OTO is tolerated (does not cause ongoing discomfort)
• Evaluate the efficacy of DB-OTO (how well DB-OTO works)

Former Sponsor Decibel Therapeutics

Key Inclusion Criteria:

1. Willingness to provide written informed consent (by at least one parent/legal guardian for pediatric participants, and with participant to provide assent, when applicable, or by the adult participant) and willingness to comply with trial protocol

2. Willingness to consent to genetic testing for the participant (by at least one parent/legal guardian for pediatric participants, and with participant to provide assent, when applicable, or by the adult participant) in order to evaluate a panel of hearing loss-related genes

3. Willingness to consent to vaccinations for the participant (by at least one parent/legal guardian for pediatric participants, and with participant to provide assent, when applicable, or by the adult participant) in accordance with the country-specific, age-appropriate immunization schedule, as described in the protocol

4. Participant able to perform all necessary assessments to qualify for enrollment and dosing in the corresponding cohort at the time the participant or parent/legal guardian signing the informed consent form (and participant providing assent, when applicable)

5. Presence of biallelic, likely pathogenic or pathogenic OTOF variants

6. No clinically significant laboratory findings on clinical laboratory tests at time of Screening as described in the protocol

7. Audiological Criteria:

   1. Investigator diagnoses the participant with profound sensorineural hearing loss (SNHL; >90 dB HL) based on behavioral and physiologic measurements (ABR) of inner ear function
   2. Outer hair cell presence is confirmed via presence of otoacoustic emissions (≥6 dBSNR) at ≥3 frequencies from 1 to 8 kHz in the ear(s) to be infused with DB-OTO. Alternatively, for participants >24 months of age, outer hair cell presence can be confirmed via presence of the cochlear microphonic in the ear(s) to be infused with DB-OTO.

8. No evidence from measures of hearing loss that show a dependence on body temperature

9. From study start and for the duration of the short-term follow-up period (48 weeks): Female participants of childbearing potential and fertile males, must agree to use highly effective contraception. Female participants must agree not to become pregnant. Fertile male participants must agree not to father a child or donate sperm, for 48 weeks and in cases of early withdrawal, for at least 12 months after DB-OTO administration.

Key Exclusion Criteria:

1. History of prior treatment with gene therapy
2. Surgical anatomy that would preclude or meaningfully impact the planned surgical approach as indicated by medical imaging (eg, Computed Tomography [CT] or Magnetic Resonance Imaging [MRI]) in the ear(s) to be infused with DB-OTO
3. History or presence of other permanent or untreatable hearing loss conditions
4. Prior or current history of malignancies
5. Prior or current history of meningitis
6. History or presence of cochlear implants in the ear(s) to be infused with DB-OTO
7. History of risk factor(s) for auditory neuropathy not caused by OTOF pathogenic variants including but not limited to: prematurity, low birth weight, hyperbilirubinemia, Neonatal Intensive Care Unit (NICU) admission, and/or low Apgar scores 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