First in Human Safety Study of FX-345 in Adults With Sensorineural Hearing Loss

Inclusion Criteria: * Adult aged 18-67 years (inclusive) * Documented medical history consistent with acquired, adult onset, sensorineural hearing loss * At the Screening, Lead-in (Visit 2) and Treatment (Day 1) Visits, a pure tone average of 40- 80 dBHL at 500Hz, 1000Hz, 2000Hz, and 4000Hz in the potential study ear to be injected * Female participants must not be pregnant, breastfeeding, or lactating. Women of child-bearing potential must agree to use a highly effective contraceptive method and must have a negative urine pregnancy test. * Male participants must refrain from donating sperm and agree to be either abstinent or use a barrier method of contraception Exclusion Criteria: * Randomization in a FX-322 (laduviglusib and sodium valproate) clinical trial * Perforation of tympanic membrane or other tympanic membrane disorders that would interfere with the delivery and safety assessment of an intratympanic medication or reasonably be suspected to affect tympanic membrane healing after injection in study ear. This includes a current tympanostomy tube. * Any conductive hearing loss of greater than 15 dB at a single frequency or greater than 10 dB at two or more contiguous octave frequencies in the study ear at the Screening, Lead-in (Visit 2) or Treatment (Day 1) Visits, based on the investigator's judgment. * Active chronic middle ear disease or a history of major middle ear surgery, as an adult, in the ear to be injected. * Within 3 months of screening visit any of the following: 1) an intratympanic injection in either ear 2) treatment with steroids 3) onset of sudden sensorineural hearing loss * Evidence of or previous diagnosis of traumatic brain injury, Meniere's disease, or genetic hearing loss * History of head or neck radiation, significant systemic autoimmune disease, and/or chronic, recurrent clinically significant vestibular symptoms * Exposure to another investigational drug within 28 days prior to screening visit