The evolution of dual-AAV strategies for congenital hearing loss: From concept to clinical potential
Industry Buzz
When it is proven to be safe, durable, deliverable in young children, and clearly improves long-term communication outcomes, not just hearing thresholds, then it becomes a routine referral.
—Julie Wei, MD, director of otolaryngology at Akron Children's Hospital
Adeno-associated virus (AAV) is used by gene therapy groups as a delivery vehicle. The wild-type virus carries a small DNA genome. Recombinant AAV swaps viral genes for a therapeutic gene cassette, then packages the cassette inside the viral capsid.
In treating some types of congenital hearing loss, the AAV is delivered into the inner ear, where cochlear cells take up the vector, and subsequently express the therapeutic gene from the cassette.[]
AAV size influences the suitability of the dual-vector approach due to its tight packaging limit. Multiple groups have measured an effective upper bound near 5.0 kb.
Oversized genomes package poorly or package as truncated fragments, which undermines functional expression. In addition, once regulatory elements enter the design, the coding sequence can exceed the AAV payload limit.[]
To circumvent these problems, a dual-vector approach has been developed. Dual AAV splits the payload across two vectors, then relies on reconstitution inside the target cell to generate a full-length transcript and protein. The dual-AAV technique has been used in OTOF-related congenital hearing loss.
Related: Targeting the cause, not the symptom: The new paradigm in pediatric hearing careHow dual AAV works in OTOF programs
The dual-AAV concept in the cochlea is built around one constraint: both vectors must enter the same inner hair cell. Program design centers on three features.[]
Vector choice and tropism: OTOF programs have used AAV serotypes with cochlear hair-cell transduction experience, including AAV1 in Regeneron’s DB-OTO, as described in product updates and publications.
Cell targeting: DB-OTO is described as hair-cell targeted, aligning expression with otoferlin biology at inner hair-cell synapses.
Local delivery: DB-OTO uses intracochlear administration. The procedure is framed as similar in approach to cochlear implant surgery in sponsor communications. Local delivery supports high cochlear exposure with limited systemic distribution.
Clinical proof point: DB-OTO and CHORD
By 2024, clinical studies were showing that functional OTOF delivery via AAV could partially restore auditory function.[] DB-OTO moved dual AAV from preclinical feasibility to measurable pediatric outcomes. Results of the CHORD trial on DB-OTO, reported in 2025 in The New England Journal of Medicine, demonstrated normalization of hearing sensitivity in 3 of 12 treated patients with OTOF-related deafness.[]
Preclinical studies provided the scientific basis for the approach, describing a hair-cell-specific AAV-based dual-vector therapy designed to restore hearing in OTOF-related loss. The program uses two AAV1 vectors to reconstitute functional OTOF expression.[]
Safety signals in early pediatric cohorts drive referral behavior. Regeneron’s February 24, 2025, update reported no adverse events or serious adverse events considered related to DB-OTO. Transient post-surgical vestibular adverse events, occurring in 5 of 12 participants, resolved within 6 days.[]
Why is dual AAV gaining attention right now?
Large-gene delivery in a closed organ now has human data. Dual AAV is not a niche workaround anymore. Dual AAV is a platform solution for large deafness genes, once human co-transduction looks feasible.
OTOF-related deafness is a synaptic transmission problem. Gene replacement targets the lesion. Audiology teams and otologists already understand the anatomy and the surgical access.
This strategy is drawing attention for several reasons clinicians care about:
Local delivery with limited systemic exposure
Single administration model
Clear mechanism for a monogenic synaptopathy
Objective endpoints (ABR, PTA) plus functional speech measures
Still a long way to go
Researchers commenting in The Lancet, while enumerating the promise of gene therapy for hearing disorders, recognize the technical and clinical challenges that remain.[] There is still a long way to go before AAV becomes more mainstream.
When asked what would make such biology-based therapies more routine in everyday practice, Julie Wei, MD, director of otolaryngology at Akron Children's Hospital, says, “When it is proven to be safe, durable, deliverable in young children, and clearly improves long-term communication outcomes, not just hearing thresholds, then it becomes a routine referral.”
Lauren Gross, AuD, audiologist at Akron Children's, further adds, “Genetic testing is recommended to parents of children that are diagnosed with hearing loss to try and determine an etiology of hearing loss. Not all parents proceed with genetic testing, potentially due to out-of-pocket costs, not interested in ruling out a genetic marker, emotional readiness, [or a] perceived limited benefit that the results will not change the treatment or management plan.”
However, she stresses, “Personally, genetic testing does not impact my daily clinical workflow unless a patient has been identified to have a specific genetic marker linked to progressive hearing loss. I currently monitor my patient’s hearing routinely but may move them to a more frequent monitoring schedule if a genetic marker linked to progressive hearing loss is identified. With emerging research on gene therapy, genetic testing may become more important and increasingly adopted, especially in the population of individuals with severe-to-profound hearing loss.”
The next act
So, what to watch for next in dual-AAV inner ear programs?
The CHORD study authors highlight the importance of demonstrating sustained benefit through follow-up, with ongoing assessments needed to prove multi-year stability. Furthermore, studies investigating bilateral dosing in a subset of patients will be valuable for assessing outcomes related to localization and classroom listening. Finally, transient vestibular effects tied to the procedure, not the vector, remain a key counseling point for pediatric teams.[]