Background: Hearing loss with cisplatin (CP) chemotherapy is common. DB-020 is a formulation of thiosulfate for intratympanic (IT) injection being developed to reduce CP ototoxicity. The primary objective of this Phase 1 study was to evaluate the safety and tolerability of repeated IT injections of DB-020. A secondary objective was to compare hearing changes with DB-020 vs. placebo. Methods: Subjects scheduled for at least 3 cycles of CP and cumulative exposure of ≥280 mg/m² were randomized to blinded, bilateral, IT injection with DB-020 (12% or 25%) in one ear and placebo in the other, once every 3 or 4 weeks, up to 3 hours before receiving CP. Subjects with moderate or severe hearing loss at baseline were excluded. Ototoxicity was defined by American Speech-Language-Hearing criteria: ≥20 dB threshold increase at any one test frequency, or ≥10 dB threshold increase in any two adjacent frequencies, or loss of response at three consecutive frequencies where responses were previously obtained. Severe ototoxicity was ≥20 dB threshold increase in any two adjacent frequencies. A pre-specified interim analysis evaluated safety, ototoxicity (with Mcnemar’s test using the last observation carried forward), and average threshold shifts (from air conduction audiometry, analyzed with a mixed model with repeated measures). Results: Nineteen subjects were randomized, with mean age of 57 years (84% male and 100% white) and mean cumulative CP dose 248 mg/m². Most (95%) had head and neck cancers (5% had lung cancer). Seventeen subjects had both baseline and follow-up audiometry, and 16/17 had baseline audiograms within 5 dB of the median threshold for age-matched controls. Free CP systemic levels were similar to reference values. Ear pain (15/19 subjects, 78.9%) and tinnitus (8/19 subjects, 42.1%) were common. Ear pain was more common in DB-020 treated ears, and tinnitus was more common in placebo treated ears. There were no tympanic perforations, no serious adverse events in the category of ear and labyrinth disorders, and no deaths. Ototoxicity was significantly more common and more severe in placebo treated ears (Table). As the overall study objectives were met at this interim analysis, no further enrollment was pursued. Conclusions: In this initial clinical trial experience there was no significant safety signal, and DB-020 IT injections showed a meaningful reduction in cisplatin ototoxicity. These results warrant further clinical development. Clinical trial information: NCT04262336.