The study will evaluate the safety, tolerability, and potential efficacy of CGF166 and the associated delivery procedures in patients with severe-to-profound bilateral hearing loss. Eligible patients are required to have documented, non-fluctuating hearing loss. Part A will include a safety and tolerability cohort (N=3). Patient dosing will be staggered; dosing the next patient in a cohort will be based on a safety review of all available data through 4 weeks post-dose of the previously dosed patient(s). Part B includes a volumetric escalation design to evaluate infusion volumes of the same CGF166 concentration (5.0 x 10E11vp/mL) in 4 cohorts of patients (n=3/cohort; total of 12 patients). Part C is an expansion cohort of the highest safe and tolerable dose identified in Part B, for further assessment of efficacy.