Phase 1 Study of AAV9.LAMP2B Gene Therapy in Danon Disease

Authors, Journal, Affiliations, Type, DOI

• Authors: Barry Greenberg, Matthew Taylor, Eric Adler, Steven Colan, David Ricks, Paul Yarabe, et al.
• Journal: New England Journal of Medicine, 2025;392(10):972–983
• Affiliations: University of California San Diego; University of Colorado Anschutz; Boston Children's Hospital; Rocket Pharmaceuticals (sponsor); Children's Hospital of Philadelphia
• Type: Phase 1, open-label, nonrandomized dose-escalation study with long-term follow-up
• DOI: https://doi.org/10.1056/NEJMoa2412392
• Funding: Rocket Pharmaceuticals
• Trial Registry: NCT03882437 (parent study); NCT06092034 (ongoing Phase 2)

Overview

RP-A501 (recombinant AAV9 carrying the full-length LAMP2B transgene) was administered as a single IV infusion to 7 male patients with Danon disease across three dose cohorts; a comprehensive immunomodulatory regimen (prednisone + tacrolimus/sirolimus + rituximab) was co-administered. At 24–54 months of follow-up, all 7 patients were alive; in the 6 with preserved baseline LVEF (≥40%), LVM index fell a median 23%, troponin I fell 84%, and natriuretic peptides fell 57%, with confirmed LAMP2 protein expression in endomyocardial biopsies. The adult patients now exceed the typical Danon disease transplant/death age of 19–21 years — providing the longest published follow-up for AAV cardiac gene therapy. Key safety signals included grade 4 complement-mediated thrombotic microangiopathy (1 patient, high-dose/low-LVEF), grade 3 steroid-induced myopathy (3 patients), and salmonella sepsis from immunosuppression.

Keywords

Danon disease, LAMP2B, LAMP2, AAV9, gene therapy, RP-A501, hypertrophic cardiomyopathy, lysosomal autophagy, Wolff-Parkinson-White syndrome, thrombotic microangiopathy, immunomodulatory regimen, cardiac troponin I, LVM index

Key Takeaways

Background and Disease Context

• Danon disease: X-linked dominant disorder caused by LOF variants in LAMP2 (lysosomal-associated membrane protein 2), a mediator of autophagy
• LAMP2 deficiency → accumulation of autophagic vacuoles, myofibrillar disarray, fibrosis → severe HCM ± Wolff-Parkinson-White/conduction disease
• Males: median age at cardiac hypertrophy diagnosis ~13 years; typical age at transplantation or death 19–21 years; skeletal myopathy and neurocognitive deficits also present
• Females: later/variable presentation; heart failure and death from adolescence to ~50 years
• LAMP2B isoform (one of 3 splice isoforms) contributes most substantially to cardiomyocyte autophagy
• No disease-directed therapy exists prior to this study; ICD may fail due to extreme hypertrophy; transplantation not curative

Study Design

• Phase 1, open-label, nonrandomized; three treatment cohorts:
  • Cohort 1: Adults/adolescents ≥15 years (n=3); low-dose 6.7×10¹³ gc/kg
  • Cohort 2: Adults/adolescents ≥15 years (n=2); high-dose 1.1×10¹⁴ gc/kg
  • Cohort 3: Pediatric 8–14 years (n=2); low-dose 6.7×10¹³ gc/kg (added after initial safety evaluation of cohorts 1–2 showed LVEF <40% exclusion needed)
• Immunomodulatory regimen for all: prednisone + tacrolimus or sirolimus + rituximab (adapted per cohort)
• Key exclusion criteria: anti-AAV9 neutralising antibody titers >1:40; cardiopulmonary instability; prior organ transplantation; LVEF <40% (added after cohorts 1–2)
• All patients had NYHA class II at baseline; all had marked LVH (median LVM 438 g; median LVM index 98 g/m²·⁷); troponin I elevated in all (median 0.86 ng/mL; ULN 0.04); no myocardial LAMP2 staining on baseline biopsy
• Outcomes assessed: safety; endomyocardial biopsy (LAMP2 protein expression, vector copy number, RNA); echocardiography/MRI; troponin I; BNP/NT-proBNP; KCCQ-12; 6MWT

Safety

Adverse Events Overview

• All 7 patients alive at data cutoff (up to 4.5 years); all serious AEs resolved without sequelae
• Most AEs mild/moderate; occurred predominantly within first 5 months; only 3 nonserious AEs reported beyond 12 months
• Most common AEs: vomiting (19 events), headache (18 events), myopathy (13 events), elevated ALT (12 events)
• All arrhythmic AEs assessed as nonserious, mild/moderate, related to underlying Danon disease; no meaningful changes in PR interval, QRS, or preexcitation morphology over follow-up

Serious Adverse Events

• Grade 4 thrombotic microangiopathy (TMA) — Patient 5 (high-dose, LVEF 32%):
  • Complement-mediated TMA with grade 4 thrombocytopenia and acute kidney injury requiring renal replacement therapy
  • Full recovery of platelet count and renal function within 4 weeks of RRT initiation
  • Also had grade 4 VT and grade 4 HF (attributed to Danon disease progression)
  • Heart transplantation at 5 months post-infusion; explanted heart showed severe hypertrophy and fibrosis without micro-thrombi or lymphocytic infiltration
  • Contribution of TMA to cardiac deterioration cannot be excluded
  • Mechanism: Complement activation (soluble C5b-9 complex) was more robust in adult/adolescent cohorts; anti-capsid antibodies mediating TMA (per separate published cohort analysis ref 33)
  • Clinical implication: LVEF <40% is now an exclusion criterion for enrollment
• Grade 3 skeletal myopathy (3 adult patients): Glucocorticoid-induced exacerbation of underlying Danon disease myopathy; resolved with glucocorticoid discontinuation
• Grade 3 salmonella sepsis (1 patient): Related to immunomodulatory regimen
• Grade 2 DVT (1 patient): Related to immunomodulatory regimen
• No treatment-related serious AEs in the 2 pediatric patients — pediatric patients had lower complement activation (peak sC5b-9 345–422 ng/mL vs higher in adults; C3/C4 normal)

Efficacy (6 evaluable patients, Patient 5 excluded)

Myocardial Transduction and LAMP2 Expression

• LAMP2 protein expression confirmed by immunohistochemistry in 6/6 evaluable patients at 12 months and 5/6 at 24–36 months
• Median vector copy number: 0.251/diploid nucleus (range 0.042–1.36)
• Median LAMP2B transcript: 3.72×10⁵ copies/μg RNA
• RNA and vector copy numbers largely sustained up to 4.5 years — consistent with non-dividing cardiomyocytes preventing episomal dilution
• Patient 1 (less intensive immunosuppression): lost LAMP2 protein on IHC at 24 and 36 months but retained vector DNA, RNA, and reduced autophagic vacuoles → possible immune-mediated protein suppression or assay limitation; clinical benefit persisted through 54 months

Cardiac Structure (LVM Index)

• Median LVM index reduction: 23% (range 7–48%) over 24–54 months
• 5/6 patients had ≥10% reduction at 12 months
• Pediatric cohort: ≥10% reduction observed as early as 6 months, sustained at 24 months
• Median change in septal wall thickness: −8% (range −52 to +12%)
• Median change in posterior wall thickness: −24% (range −49 to +1%)
• Natural history context: Danon disease untreated → ~8% annualized LVM increase

Cardiac Biomarkers

• Troponin I: median −84% (range −33 to −99%) over 24–54 months; ≥80% reduction in 4/6 patients
• Natriuretic peptides: median −57% (range −93 to +16%) over 24–54 months
• Both were markedly elevated at baseline in all patients

Functional and Symptomatic Outcomes

• NYHA class improved in all 6 evaluable patients
• KCCQ-12 ≥5-point improvement (clinically meaningful threshold in adults) in all 6 evaluable patients
• Most patients reported full participation in school, work, and leisure activities post-treatment
• Adult patients are now 21–24 years old — beyond the typical Danon disease transplant/death age of 19–21 years

Immunological Response

• Anti-LAMP2B binding antibodies detected in several patients — without clinical immunological sequelae (in contrast to Pompe GC301 where no anti-GAA antibodies were detected at all)
• No LAMP2B-specific T-cell response (IFN-γ ELISpot negative in all patients)
• Anti-AAV9 neutralising antibodies and total antibodies developed post-infusion (expected)

Phase 2 Study

• Global Phase 2 registrational study (NCT06092034) ongoing
• Dose: 6.7×10¹³ gc/kg in males ≥8 years with Danon disease

Limitations of the Document

• Small sample (n=7, n=6 evaluable for efficacy): No control arm; open-label; patient/investigator-reported outcomes not blinded
• No CMR for all patients: Echocardiographic LVM assessment more susceptible to variability; fibrosis quantification not available for all
• KCCQ-12 not validated in paediatric populations: Only 2 pediatric patients anyway
• Skeletal myopathy confounds 6MWT: Doses used unlikely to address skeletal myopathy; 6MWT not considered reliable for cardiac improvement measurement
• Sponsor (Rocket Pharmaceuticals) involvement: Confidential disclosure agreements with authors; data-management and statistical analysis involvement
• Female patients excluded: Considerable phenotypic heterogeneity in females; study limited to males
• Complement activation mechanism not fully understood: TMA mediating anti-capsid antibodies confirmed separately; management of complement in future patients requires characterisation
• Patient 1 LAMP2 protein loss: Whether immune-mediated or assay limitation unclear; long-term durability requires further follow-up

Key Concepts Mentioned

• concepts/AAV-Gene-Delivery — AAV9 systemic delivery; immunomodulatory regimen; complement-mediated TMA; adult cardiomyocyte non-dividing = durable episomal expression
• concepts/Danon-Disease — LAMP2 LOF, hypertrophic cardiomyopathy, natural history, WPW, gene therapy

Key Entities Mentioned

• entities/Rocket-Pharmaceuticals — sponsor of RP-A501 development and trial

Wiki Pages Updated

• wiki/sources/aav9-danon-nejm-2025.md — created (this file)
• wiki/concepts/AAV-Gene-Delivery.md — updated with Danon disease clinical data
• wiki/concepts/Danon-Disease.md — created (new concept page)
• wiki/sourceindex.md — new entry added
• wiki/wikiindex.md — Danon-Disease concept entry added; AAV-Gene-Delivery description updated