SKY-GJB2 First-in-Human Dosing: Pioneering Intracochlear Gene Therapy Amid Deep Evidence Void and Elevated Safety Bar

Skylark Bio Doses First Patient in SONIX Gene Therapy Trial for Hearing Loss

Skylark Bio has initiated its SONIX Phase I/II clinical trial, dosing the first patient with SKY-GJB2, a gene therapy aimed at treating hearing loss caused by GJB2 gene mutations in children. The trial is designed to evaluate the safety, tolerability, pharmacokinetics, and pharmacodynamics of a single unilateral intracochlear injection. It will also assess preliminary efficacy in pediatric patients aged between nine months and seven years. Initial study findings are anticipated by the end of 2026, with additional data expected throughout 2027. This marks a significant step towards providing new treatment options for genetic hearing loss.

• SKY-GJB2 is engineered to deliver a functional copy of the GJB2 gene directly to the supporting cells within the cochlea. This intervention aims to restore the natural expression of connexin 26, a protein essential for normal hearing function. Mutations in the GJB2 gene are known to disrupt this protein's activity and impair the crucial gap-junction network in the inner ear, leading to hearing loss.
• The SONIX Phase I/II trial is a multi-site study focused on pediatric patients ranging from nine months to seven years of age. It investigates the effects of a single unilateral intracochlear injection of SKY-GJB2. Beyond safety and tolerability, the trial will gather pharmacokinetic and pharmacodynamic data, alongside evaluating the preliminary efficacy of the gene therapy in this specific young patient cohort.
• Skylark Bio is committed to advancing genetic medicines for monogenic diseases, with SKY-GJB2 representing a key program. The company is also developing gene therapies for SLC26A4-related hearing loss and has investigational programs in the central nervous system. The recent regulatory approval of another AAV-delivered inner-ear therapy provides important external validation for Skylark Bio's innovative gene therapy approach in this specialized field.

The Burden of GJB2-Related Hearing Loss and Current Gaps

GJB2-related hearing loss remains one of the most prevalent forms of hereditary deafness, yet effective disease-modifying treatments are still elusive. Despite promising preclinical advances, a constellation of biological, technical, and translational barriers continues to limit the clinical applicability of gene-based and surgical interventions.

• AAV vector specificity and ototoxicity risk: Gene therapy approaches using adeno-associated viral (AAV) vectors have been constrained by limited cochlear cell specificity. Ectopic GJB2 expression in hair cells—driven by non-selective promoters—can induce ototoxicity, necessitating the development of highly targeted promoter constructs to confine transgene expression to the appropriate supporting cell populations.

• Transduction efficiency, immune responses, and durability: Cochlear delivery of AAV vectors has achieved only partial restoration of connexin 26 (Cx26) expression and auditory function in preclinical models. Persistent limitations include suboptimal transduction efficiency across the full cochlear epithelium, vector-triggered immune responses, and uncertainty around long-term transgene stability—all of which complicate translation to a viable therapeutic regimen.

• Inflammatory responses and hair cell damage from gene replacement: In wild-type murine models, Gjb2 gene replacement has been associated with sensory hair cell deficits and excessive inflammatory responses, resulting in iatrogenic hearing loss. These findings highlight a critical safety liability that must be resolved before gene replacement strategies can be responsibly advanced.

• Extent of cochlear damage limiting therapeutic efficacy: Cx26 deficiency drives broad sensory epithelial degeneration, and in cases where cochlear damage is already extensive at the time of intervention, gene therapy has demonstrated negligible functional benefit—underscoring the need for early detection and timely therapeutic windows.

• Timing dependency and the constraints of delayed intervention: Animal model data consistently demonstrate that early postnatal intervention yields significantly greater auditory recovery than delayed treatment. This timing sensitivity creates a narrow therapeutic window, posing logistical and diagnostic challenges for translating findings into neonatal clinical protocols.

• Anatomical, ethical, and regulatory barriers to human translation: Translating preclinical results to human application is further complicated by anatomical differences, immunological variability, and substantial ethical and regulatory constraints—particularly concerning embryonic or germline gene editing. International ethical frameworks diverge considerably, necessitating rigorous oversight before any such approaches enter clinical evaluation.

• Surgical complications in syndromic GJB2 presentations: In patients with KID syndrome (keratitis-ichthyosis-deafness syndrome), a GJB2 gain-of-function disorder, cochlear implantation carries elevated risk of skin flap necrosis and implant extrusion. These complications necessitate modified surgical techniques—including the use of ultra-thin receiver/stimulator devices—adding procedural complexity to an already challenging patient population.

Unpacking the SONIX Phase I/II Trial for SKY-GJB2

The evidence base for GJB2-related hearing loss draws from multiple study designs across diverse populations, collectively characterizing the genetic epidemiology, phenotypic variability, and diagnostic utility of GJB2 mutation screening. The following studies span prospective cohort designs, retrospective chart reviews, and tertiary-care outpatient screening programs, each contributing distinct methodological perspectives to the field.

Skylark's Gene Therapy: Charting a New Course for Pediatric Hearing Loss

The initiation of Skylark Bio's SONIX trial represents a pivotal moment in the quest to address genetic hearing loss, particularly for children affected by GJB2 gene mutations. These mutations are a leading cause of hereditary nonsyndromic deafness, often leading to significant cochlear developmental disorders if not addressed early. While cochlear implants offer a valuable intervention, especially for GJB2 patients who tend to have better outcomes compared to those with acquired deafness, gene therapy offers the tantalizing prospect of restoring natural hearing by correcting the underlying genetic defect.

This trial's focus on a single unilateral intracochlear injection in young children (9 months to 7 years) is strategically significant. Preclinical evidence suggests that early postnatal intervention is crucial for maximizing efficacy, as the window for preventing or reversing developmental disorders in the cochlea may be time-sensitive. However, this innovative delivery method is not without its challenges. Studies indicate that a substantial proportion of children, up to 50%, present with tissue plugs in the round window niche, which could potentially obstruct the precise delivery of the gene therapy to the inner ear and impact its overall effectiveness.

Furthermore, while GJB2-related hearing loss typically spares vestibular function, simplifying the therapeutic target, the long-term safety, immune response, and sustained expression of the delivered gene remain critical considerations for any gene therapy in a pediatric population. The anticipated data from this Phase I/II trial by late 2026 and throughout 2027 will be instrumental in understanding the balance between the profound potential for restorative hearing and the inherent complexities of inner ear gene delivery and long-term biological stability. This endeavor could redefine the treatment paradigm for a significant pediatric population, moving towards a future where genetic hearing loss is not just managed, but potentially cured.