ATSN-201 advances to registrational trial for X-linked retinoschisis

Atsena Therapeutics has initiated enrollment in the Phase 3 cohort of its Phase 1/2/3 LIGHTHOUSE Trial evaluating ATSN-201, an investigational adeno-associated virus (AAV)-based gene therapy for X-linked retinoschisis (XLRS). This next step follows interim 6-month data from Part B of the study that demonstrated structural and functional improvements broadly consistent with Part A findings. Results were presented May 1, 2026, at the Foundation Fighting Blindness Retinal Therapeutics Innovation Summit in Denver, Colorado.

XLRS remains without any approved treatment. The disease — caused by loss-of-function mutations in the RS1 gene encoding retinoschisin — affects an estimated 30,000 males in the United States and European Union combined, typically manifesting in the first decade of life as bilateral macular schisis that causes uncorrectable visual acuity loss and, in many cases, progressive vision decline into adulthood.¹,² The initiation of Part C (Phase 3) of the LIGHTHOUSE Trial marks the first time an XLRS gene therapy has advanced to a registrational study.

"Enrollment in the pivotal Part C cohort is now underway, and this marks a significant milestone for patients with XLRS and for Atsena. ATSN-201 is the first gene therapy to advance to a registrational trial for XLRS, a disease with no approved treatments. We remain on track for a BLA filing in 2028," Kenji Fujita, MD, Chief Medical Officer of Atsena, said in a press release.

Trial Design and Current Status

The LIGHTHOUSE Trial (NCT05878860) is structured as a continuous Phase 1/2/3 study following alignment with the US Food and Drug Administration (FDA) through a Regenerative Medicine Advanced Therapy (RMAT) meeting, after which the agency agreed to the proposed study design, endpoints, and patient population as a potential basis for a Biologics License Application (BLA).³ The trial encompasses 6 cohorts across 3 sequential parts.

Part A (cohorts 1–3) enrolled 9 adult male patients across 3 dose levels — low, high, and medium — and has been fully reported with up to 12 months of follow-up for the earliest cohort. Part B (cohorts 4–5) enrolled 9 adults across 2 injection-volume arms and an untreated control arm, plus 3 pediatric patients ages 8 to 12 years. Part C (cohort 6) is the Phase 3 pivotal portion, targeting enrollment of 76 patients with XLRS across North American and European clinical sites. Enrollment is expected to complete by the end of the first quarter of 2027.

In Part C, patients are randomized to either immediate treatment with ATSN-201 or a 12-month observation period followed by the option to receive treatment. The primary endpoint is microperimetry at 52 weeks, as agreed with the FDA and European Medicines Agency (EMA), with best-corrected visual acuity and optical coherence tomography (OCT) as key secondary endpoints. Supportive data from Part C are intended to underpin a BLA filing anticipated in 2028.

Key Findings from Part B Interim Analysis

At the 6-month interim analysis of Part B, the structural and functional response kinetics in cohort 4 (the 2 adult treated arms) were reported to be consistent with those observed in Part A cohorts 1 through 3. Foveal schisis closure at Month 6 was confirmed by OCT in 4 of 6 treated adult patients (67%). Notably, none of the 3 untreated control subjects and none of the 6 untreated contralateral eyes demonstrated foveal schisis closure, providing an internal comparator that reinforces the interpretation of the treatment effect as biologically plausible, though the small sample size limits definitive attribution.

Microperimetry response in the Part B adult cohort at Month 6 was reported to closely mirror that observed at the same time point in Part A, suggesting consistency in functional outcomes across sequentially enrolled cohorts. However, the company has not yet disclosed the magnitude of the microperimetry change or p-values for Part B, and these data should be interpreted within the context of an open-label, non-randomized Phase 1/2 design.

The pediatric cohort (cohort 5, ages 8–12) exhibited a clean safety profile with no serious adverse events (SAEs), no subretinal deposits, no macular hole formation, no retinal detachment, and no study discontinuations.

Subretinal deposits were observed in a subset of adult cohort 4 subjects but resolved with transient steroid treatment, consistent with the known immunogenicity profile of AAV-based gene therapies delivered to the subretinal space.⁴

Clinical Context: Disease Burden and Unmet Need

XLRS is an X-linked recessive condition caused exclusively by mutations in RS1 (Xp22.2-p22.1), which codes for retinoschisin.¹,⁵ More than 290 mutations of RS1 variants that have been identified, the majority being missense mutations.⁶ The condition is characterized by bilateral, symmetric foveal splitting (schisis cavities) visible on OCT and a characteristic "spoke-wheel" pattern on fundus examination, along with a selective reduction in the b-wave amplitude on electroretinography.⁶

Affected males typically present in early childhood with visual acuity ranging from approximately 20/60 to 20/120.² Visual function often deteriorates during the first 2 decades, may stabilize in mid-adulthood, and then frequently declines again in the fifth and sixth decades, sometimes with atrophic progression.⁷ Phenotypic variability is substantial even among patients carrying the same mutation.⁸ Current management is limited to refraction correction and monitoring; off-label use of topical carbonic anhydrase inhibitors (e.g., dorzolamide) has been reported to reduce schisis cavity depth in some patients, but evidence supporting sustained visual benefit is limited.⁸,⁹

No pharmacologic therapy has received regulatory approval for XLRS in any jurisdiction, and ATSN-201 would represent the first if approved.

Drug and Drug-Class Background

ATSN-201 is a recombinant AAV-based gene replacement therapy designed to deliver a codon-optimized human RS1 transgene (hRS1syn) under the control of a photoreceptor-specific GRK1 promoter. Its distinguishing characteristic is the delivery capsid: AAV.SPR (laterally spreading), a novel capsid engineered to spread beyond the subretinal injection bleb margin and transduce foveal cones without requiring direct foveal injection or surgical detachment of the fovea. This design addresses a technical limitation of conventional benchmark AAV capsids, which remain confined to the original bleb, necessitating either larger injection volumes or direct subfoveal delivery with its attendant procedural risks.¹⁰

In non-human primate preclinical studies, AAV.SPR demonstrated transgene expression substantially beyond injection bleb margins at clinically relevant doses, with a favorable safety profile relative to benchmark capsids.¹⁰

ATSN-201 is administered via a one-time unilateral subretinal injection. The treated eye receives the investigational therapy; the contralateral eye serves as an internal control — an important structural feature of the trial design that partially offsets the absence of blinding in this open-label study.

The broader context for ATSN-201 is the precedent established by voretigene neparvovec-rzyl (Luxturna; Spark Therapeutics), which in December 2017 became the first FDA-approved ocular gene therapy and the first U.S.-approved gene therapy for a genetic disease, indicated for biallelic RPE65 mutation-associated retinal dystrophy.¹¹ Luxturna used an AAV2 subretinal delivery approach and achieved regulatory approval based on a pivotal Phase 3 trial employing functional vision endpoints.¹¹,¹² Since then, regulators and investigators have progressively refined acceptable endpoints for inherited retinal disease (IRD) gene therapy trials. The FDA has now accepted microperimetry as a primary trial endpoint for IRD programs in cases where foveal function is preserved — an approach directly applicable to XLRS, where central retinal sensitivity is the key functional domain at risk.¹²

ATSN-201 has received the following FDA designations: RMAT, Fast Track, Rare Pediatric Disease, and Orphan Drug. It has also received Orphan Designation from the EMA.

Interpretive Perspective

The 6-month Part B data are encouraging in terms of consistency with Part A findings, but several considerations merit attention before drawing strong conclusions. The Part B adult treated cohort (n = 6) is small, and the primary findings — schisis closure in 4 of 6 treated eyes versus 0 of 3 controls and 0 of 6 untreated contralateral eyes — are supportive but come from an open-label study without masked outcome assessment. The magnitude and statistical characterization of microperimetry outcomes have not been publicly detailed in the interim analysis, and the extent to which the observed improvements translate to patient-meaningful functional gains in daily visual tasks has yet to be rigorously characterized.

The internal comparator design (untreated contralateral eyes and control arm subjects) strengthens causal inference compared with historical controls alone, but the study was not powered for formal hypothesis testing in Parts A and B. The pivotal Phase 3 cohort (n = 76) will be the definitive assessment.

Pediatric safety data at 6 months are early but appear reassuring. XLRS disproportionately impacts children — it is among the most common causes of juvenile macular degeneration in males — and any effective therapy would ideally be available to this population.¹ Longer follow-up in the pediatric cohort and assessment of efficacy endpoints in younger patients, in whom the retina may be more responsive to therapy but also more surgically vulnerable, will be important.

The resolution of subretinal deposits with corticosteroids in adult Cohort 4 patients warrants monitoring. Subretinal inflammation and vector-related immune responses remain recognized risks in AAV-based subretinal gene therapy, and the incidence, severity, and durability of such responses across larger populations will require characterization in Part C.⁴

Enrollment in Part C is expected to complete by end of Q1 2027, and the Phase 3 data supporting the BLA are targeted for a 2028 filing.

References

1. GeneReviews: Leroy BP, Boon CJF, Bhattacharya SS. X-Linked Congenital Retinoschisis. In: Adam MP, Feldman J, Mirzaa GM, et al., eds. GeneReviews®. Seattle (WA): University of Washington, Seattle; 1993–2024. Updated November 2020. https://www.ncbi.nlm.nih.gov/books/NBK1222/
2. Hahn LC, van Schooneveld MJ, Wesseling NL, et al. X-Linked Retinoschisis: Novel Clinical Observations and Genetic Spectrum in 340 Patients. Ophthalmology. 2022;129(1):66–78. https://www.sciencedirect.com/science/article/pii/S0161642021007405
3. ClinicalTrials.gov. ATSN-201 Gene Therapy in RS1-Associated X-linked Retinoschisis (LIGHTHOUSE). NCT05878860. https://clinicaltrials.gov/study/NCT05878860
4. Igoe JM, Lam BL, Gregori NZ. Update on Clinical Trial Endpoints in Gene Therapy Trials for Inherited Retinal Diseases. J Clin Med. 2024;13(18):5512. doi:10.3390/jcm13185512. https://pmc.ncbi.nlm.nih.gov/articles/PMC11431936/
5. MedlinePlus Genetics. X-linked juvenile retinoschisis. National Library of Medicine. https://medlineplus.gov/genetics/condition/x-linked-juvenile-retinoschisis/
6. Orphanet. X-linked retinoschisis. https://www.orpha.net/en/disease/detail/792
7. Zhour A, Stutzle S, Kühlewein L, et al. Long-term functional and structural outcomes in X-linked retinoschisis: implications for clinical trials. Front Ophthalmol. 2023. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10310546/
8. Intrafamilial Phenotype Variability in Two Male Siblings, With X-linked Juvenile Retinoschisis and Dorzolamide Treatment Effect in the Natural History of the Disease. Front Med (Lausanne). 2019. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6433199/
9. EyeWiki. X-linked Retinoschisis. American Academy of Ophthalmology. https://eyewiki.org/X-linked_Retinoschisis
10. Atsena Therapeutics. About AAV.SPR. https://atsenatx.com/our-approach/laterally-spreading-aav/
11. Filkins, K. Ophthalmology Times. LIGHTHOUSE study of ATSN-201 expanded to continuous phase 1/2/3 trial following FDA feedback. Published November 2025. https://www.ophthalmologytimes.com/view/lighthouse-study-of-atsn-201-expanded-to-continuous-phase-1-2-3-trial-following-fda-feedback
12. Chung DC, Bertelsen M, Lorenz B, et al. The Natural History of Inherited Retinal Dystrophy Due to Biallelic Mutations in the RPE65 Gene. Am J Ophthalmol. 2019;199:58–70.

Note: This article was generated with AI using company-issued press release materials and available conference presentations.