RP-A501 (AAV9.LAMP2B)

Danon disease is a rare X-linked disorder caused by loss-of-function variants in the LAMP2 gene (Xq24), which encodes lysosome-associated membrane protein 2. It is characterized by a triad of severe cardiomyopathy, skeletal myopathy, and cognitive impairment, and all known pathogenic variants affect at least the LAMP-2B isoform. A mother with a pathogenic variant has a 50% chance of transmitting it in each pregnancy, and an affected father transmits it to all of his daughters and none of his sons. De novo variants account for more than one third of cases in the 2023 international consensus statement, 40% in one study cited by GeneReviews, and half of the probands in a Japanese survey of 20 families. The consensus statement describes penetrance as likely 100% in both sexes. No estimate of incidence or birth prevalence and no newborn or population screening series was found, and GeneReviews states that the general population prevalence is not known. The available prevalence estimates span about two orders of magnitude. Orphanet lists a prevalence below 1 per 1,000,000 and more than 100 families described in the literature. A 2026 congress abstract that applied three methods reported a range from 240 (United States patients with at least 1 instance of ICD-10 code E74.05 in the TriNetX electronic health record database) to 11,807 (loss-of-function LAMP2 variants in population genomic databases). The 2022 proposal for the ICD-10-CM code extrapolated from a hypertrophic cardiomyopathy prevalence of approximately 1 in 500 and a LAMP2 variant frequency of approximately 1% to 4% of those cases to 10,000 to 25,000 cases in the United States (1 in 12,000 to 1 in 50,000 individuals). Rocket Pharmaceuticals stated a prevalence of 15,000 to 30,000 in the United States and Europe in its Form 10-K filed in February 2026, and in October 2026 presented a modeled United States population of 10,000 to 11,000 (about 4,000 male and 6,000 to 7,000 female), derived from a genotype-based analysis of more than 450 patient records and a phenotype-based analysis of claims and health record databases covering more than 310 million lives. The company reported more than 900 identified United States patients as of July 2026 (45% male and 55% female), which it describes as a diagnosis rate under 10%. The frequency of LAMP2 variants in hypertrophic cardiomyopathy, on which several of these estimates rest, also differs by source: 1% to 4% in the consensus statement, below 1% in a 2022 expert consensus table, 2 of 197 index cases (1%) in a French cohort, 2 of 50 probands (4%) in a United States pediatric cohort, 4 of 64 probands (6%) in a Chinese pediatric cohort, and 4 of 2068 participants with unexplained increased left ventricular wall thickness enrolled across 22 countries or regions.

In the consensus statement, the median age at onset is approximately 14 years in males and ranges between 17 and 35 years in females; males typically progress to end-stage hypertrophic cardiomyopathy in the second or third decade of life, and females reach the end stage a decade or more later. In the International Danon Disease Registry (116 participants, 52 male and 64 female; median follow-up 5.82 years), the median age at diagnosis was 12.4 years in males and 21.9 years in females. A heart failure outcome (heart transplantation, left ventricular assist device, or death) occurred in 21 males (40.4%) and 14 females (21.9%), and heart transplantation in 17 males (32.7%) and 13 females (20.3%). Among those who reached the outcome, the median age was 19 years in males and 26 years in females, and the actuarial median time from birth to the outcome was 25 years in males and 63 years in females. The registry table prints deaths in 7 males (13.5%) and 2 females (3.1%) and a total of 6 (5.2%). In a 2011 registry of 82 participants from 36 families, the text gives mean ages at transplantation and at death of 18.1 and 19.8 years in men and 34.5 and 36.0 years in women, and the population table of the same article prints 20.8 and 20.1 years in men and 32.3 and 40.2 years in women; with 63 published cases added, 59.0% of men and 51.6% of women reached transplantation or death. A systematic review of 146 published cases (90 male and 56 female) found the composite of death, heart transplantation, or ventricular assist device in 37% of males at a median age of 21 years and in 32% of females at a median age of 38 years. In a European registry of 57 participants (30 male and 27 female), 17 (30%) died, at a mean age of 23 years in males and 45 years in females; the mean age at first heart failure hospitalization was 18 years in men, and for women it is given as 28 years in the published abstract and 34 years in the body of the accepted manuscript. In a Japanese survey of 39 participants, 20 of the 21 who had died (95%) died of cardiac failure or sudden cardiac arrest, at a mean age of 19 years in males and 37 years in females. Rates of change in untreated males, the reference for the single-arm trials of RP-A501, differ by source. In mixed-effects models of the International Danon Disease Registry, left ventricular mass in males increased by 28.424 g per year of age in 20 participants without a heart failure outcome and by 34.183 g per year in 13 participants with one, left ventricular ejection fraction decreased by 0.888 and 2.461 percentage points per year, and left ventricular mass in females did not change significantly (P=0.5037 and P=0.482). The company's October 2026 presentation cites unpublished data from the same registry: left ventricular mass increasing by 39.86 ± 4.01 g per year and BNP by 42.17 ± 18.46 pg/mL per year in untreated males. An earlier abstract from the registry (112 participants) reported that BNP did not change over time. In a prospective study of 9 males (mean age 11.6 years at enrollment), mean NT-proBNP rose from 932.2 to 1903.6 pg/mL over a mean follow-up of 18.6 months, a statistically significant increase, and mean high-sensitivity troponin T rose from 41.1 to 78.4 ng/L, which was not significant; among the pediatric participants, the average paired differences in left ventricular ejection fraction (-5.9) and left ventricular mass (+64 g) were not statistically significant.

Extracardiac disease is more frequent in males. In the systematic review of 146 cases, males presented with the triad of cognitive impairment, skeletal myopathy, and hypertrophic cardiomyopathy in 42% of cases and females with isolated cardiac disease in 73%; cardiomyopathy was hypertrophic in 96.2% of males, and hypertrophic in 70.3% and dilated in 29.3% of females. In the European registry, all males and 10 of 27 females had at least 1 extracardiac manifestation, clinically significant skeletal myopathy was present in 60% of males and 3% of females, and liver transaminases were elevated in 83% of males and 6% of females. The consensus statement reports mild cognitive impairment and learning difficulties in at least 80% of males and between 6% and 47% of females, supraventricular and ventricular arrhythmias in approximately 60% of individuals, and heart block or sinus node dysfunction requiring a pacemaker in 35% to 50%. A Wolff-Parkinson-White pattern was reported in 68% of men and 27% of women in the 2011 registry and in 19.8% of the 116 participants of the International Danon Disease Registry. A systematic review of ophthalmic findings in 70 patients found retinal abnormalities in 81% and macular involvement in 62%, and in a psychiatric evaluation of 13 participants, 9 (69.2%) met criteria for at least one psychiatric disorder. Quality of life evidence consists of one prospective study of 9 males and one abstract of 10 participants. In the prospective study, average total scores on the Pediatric Cardiac Quality of Life Inventory and on a 23-question pediatric quality of life questionnaire were below 60 of 100 at baseline and at follow-up for both children and parents (on the second instrument, a baseline total of 53.4 ± 17.6 reported by children and 44.9 ± 8.6 reported by parents); the authors cite mean totals between 60 and 80 for children with congenital heart disease not requiring catheter or surgical correction and 68.9 ± 16 for children with cancer on treatment. In the abstract, Kansas City Cardiomyopathy Questionnaire summary scores were lower in participants with an implanted defibrillator than in those without. The authors of both reports state that these instruments are not validated in Danon disease, and no health state utility value, Voice of the Patient report, or published study of caregiver time, work loss, or out-of-pocket cost was found. Caregiver evidence is limited to statements by a patient organization and by families quoted in a company presentation; in the Japanese survey, 10 of 20 mothers (50%) were themselves symptomatic.

No approved therapy treats the cause of the disease. The consensus statement states that there are “no available therapies to specifically treat Danon disease” and that management is largely based on expert opinion and extrapolation from hypertrophic cardiomyopathy guidelines, and no controlled trial of a heart failure drug in Danon disease was found. In the International Danon Disease Registry, 45 of 116 participants (38.8%) had a cardiovascular implantable electronic device, which was a defibrillator in 43 (37.1%), and in the European registry 38 of 57 participants (67%) received an implantable device. In a series of 7 participants, 1 died at age 12 from ventricular fibrillation refractory to defibrillator therapy. Heart transplantation was performed in 30 of 116 registry participants (25.9%), and a 2024 review cites 17.6%. Among 38 recipients from 8 centers, the median age at transplantation was 20.2 years, 2 died, 2 underwent retransplantation, and estimated 5-year graft survival was 87.1% (95% CI 63.6% to 95.9%). In the European registry, 5 of 8 male recipients of a transplant or assist device (62%) died within one year of the procedure, and no death was reported among females receiving a transplant during a mean follow-up of 10.6 years. No cost of illness or healthcare resource utilization study specific to Danon disease was found, and the available cost evidence consists of benchmarks from other populations. For heart transplantation in the United States for all diagnoses, Milliman estimated 2025 billed charges of $1,918,700 per transplant, covering 30 days before transplantation to 180 days after discharge and including $1,220,400 for the hospital transplant admission. Among 2896 pediatric heart transplant recipients (2002 to 2016), total hospitalization costs in 2016 dollars were $906,454 in the second era studied and $767,221 in the third; among 2873 pediatric recipients, total hospitalization costs were $755,345 with a ventricular assist device and $457,086 without mechanical circulatory support. The hospitalization cost of left ventricular assist device implantation in adults was $234,808 in 2011. Median charges per pediatric heart failure-related hospitalization were $72,087 in 2009, the mean cost of an adult hospitalization with primary heart failure was $11,552 in 2014, and adjusted total hospital charges for a defibrillator placement admission were $132,455 in adults without congenital heart disease and $147,002 in adults with it. Adults with obstructive hypertrophic cardiomyopathy in commercial and Medicare supplemental claims had incremental costs of approximately $20,000 per year compared with matched controls, and approximately $35,000 per year when symptomatic. For diagnosis, the 2026 Florida Medicaid independent laboratory fee schedule lists $392.06 for procedure code 81439, a hereditary cardiomyopathy sequencing panel, and a sponsored program offers genetic testing at no charge to patients in the United States aged 40 years or younger with a high suspicion of an inherited cardiomyopathy.

As of October 2026, clinical evidence for RP-A501 comes from two uncontrolled trials in male participants with Danon disease. RP-A501-0219 was a non-randomized, open-label phase 1 trial of a single intravenous infusion, with sequential cohorts by age and dose. It treated 7 participants between June 2019 and March 2022: 5 aged 15 years or older and 2 aged 11 to 14 years. Five participants (3 adults or older adolescents and 2 children) received 6.7 × 10¹³ GC/kg, and 2 adults or older adolescents received 1.1 × 10¹⁴ GC/kg. All received transient immunomodulation, which differed by cohort: a steroid alone in 1 participant, a steroid with tacrolimus in 2, rituximab with a steroid and tacrolimus in 2, and rituximab with a steroid and sirolimus in 2. The trial is published, with data over 24 to 54 months. The phase 1 results cited in this report come from the published abstract and from company presentations, announcements, and securities filings. The company has continued to report long-term follow-up of the same participants, most recently from a data extraction of July 31, 2026, at a median follow-up of 5.7 years (range 4.3 to 7.1 years). RP-A501-0123 is a single-arm, open-label phase 2 trial that the company describes as pivotal. It had dosed 9 participants as of October 2026. Its results are preliminary and are reported by the company alone, from a data extraction of August 7, 2026, with cohort 4 safety data as of September 4, 2026. No peer-reviewed report of phase 2 results was found.

Phase 1 efficacy results cover 6 of the 7 participants. The seventh, who received the higher dose and had a left ventricular ejection fraction below 40% at enrollment (reported as 35% in documents of 2021 and 2023 and as 32% in the 2026 Form 10-K), had progressive heart failure and underwent heart transplantation 5 months after infusion. The company attributes the transplantation to progression of Danon disease and describes the participant as “not evaluable for efficacy”. In the 6 participants with normal ejection fraction at baseline, the published abstract reports cardiac LAMP2 protein expression, a reduction from baseline in or stabilization of left ventricular mass index, preservation of ejection fraction, and a reduction in or stabilization of cardiac troponin I and NT-proBNP. In the July 31, 2026 data extraction, the company reports that at about 12 months (month 18 in 2 participants) all 6 had LAMP2 protein expression of Grade 1 or higher and a decrease in left ventricular mass index of 10% or more, with individual decreases of 11% to 32%. At the most recent visit (month 36 to month 84), the participant-level table gives changes in left ventricular mass index of -32%, -31%, -12%, -7%, -47%, and +3%: a decrease of 10% or more in 4 of 6 participants and a value below baseline in 5 of 6. The participant with the 3% increase at month 36 had a 15% decrease at month 12, and left ventricular posterior wall thickness in that participant had increased by 66% at month 36. The same presentation states that LAMP2 protein expression was durable in all participants and records Grade 0 staining in 1 participant at the 30-month and 36-month assessments, with vector RNA and DNA persisting through 60 months. From baseline to the most recent visit, cardiac troponin I decreased by 31% to 99% and NT-proBNP by 31% to 90% in all 6 participants, NYHA class changed from II to I in 5 participants and remained II in 1, and the KCCQ-12 score increased by 7 to 56 points. At the earlier data cut of April 19, 2024, the median reductions were 24% for left ventricular mass index, 84% for cardiac troponin I, and 57% for natriuretic peptide (attributed to NT-proBNP in the 2024 Form 8-K and to BNP in the 2026 Form 10-K), and the median KCCQ improvement was 27 points. Overall survival was 100% at the July 31, 2026 extraction. One further participant underwent heart transplantation 4.8 years after infusion, after that extraction.

RP-A501-0123 enrolls males aged 8 years or older with a pathogenic or likely pathogenic LAMP2 variant, left ventricular hypertrophy, a left ventricular ejection fraction of 50% or more, NYHA class II to III symptoms, and elevated high-sensitivity troponin I. The design aligned with the FDA in September 2023 was 12 participants at 6.7 × 10¹³ GC/kg. Six participants were dosed at that level before the trial was paused in May 2025: 1 aged 15 years or older (cohort 1) and 3 aged 8 to 14 years (cohort 2), who received two doses of rituximab with sirolimus and corticosteroids, and 2 (cohort 3) whose regimen also contained the C3 inhibitor pegcetacoplan. The pivotal population is now the 12 participants of cohort 4, who receive commercial-grade product at 3.8 × 10¹³ GC/kg with three doses of rituximab, sirolimus, and corticosteroids, without pegcetacoplan. The company states that the dose was recalibrated to account for the higher full-to-empty capsid ratio of the current product. Three of the 12 had been dosed as of October 2026, and 9 remained to be enrolled and treated. The registry lists one primary outcome at 12 months that requires both myocardial LAMP2 protein expression of Grade 1 or higher by immunohistochemistry and a decrease of 10% or more in left ventricular mass index. The company describes these as co-primary endpoints intended to support accelerated approval and defines success as at least 7 of 12 participants meeting both criteria. The company expects to complete dosing by mid-2027, to report topline results in mid-2028, and to file a biologics license application in late 2028; the registry gives an estimated primary completion of June 2028. Preliminary efficacy data are limited to the 4 participants of cohorts 1 and 2, at most recent visits of 18 to 36 months. Left ventricular mass index changed by -36%, -35%, +9%, and -30% (a decrease of 10% or more in 3 of 4 participants), NT-proBNP by -19%, -7%, -19%, and -30%, and cardiac troponin I by -49%, -14%, +122%, and -14%. NYHA class improved to class I in 3 participants and remained class II in 1, and the KCCQ-12 overall score changed by 0, +2, +16, and +35 points. Data from the 2 participants of cohort 3 were excluded. No myocardial LAMP2 protein expression result, 12-month responder status, event-free survival result, or cohort 4 efficacy result was found for the phase 2 trial, and the company states that “results from earlier cohorts may not predict outcomes at the recalibrated dose under the modified protocol”.

In phase 1, 1 of the 2 participants given 1.1 × 10¹⁴ GC/kg had grade 4 complement-mediated thrombotic microangiopathy with thrombocytopenia and acute kidney injury. The event was treated with eculizumab and transient hemodialysis, and kidney function returned to baseline within three weeks. This was the participant who underwent transplantation at 5 months, which the company states was not associated with the event. Three participants (2 at the lower dose and 1 at the higher dose) had a grade 3 glucocorticoid-related exacerbation of skeletal myopathy that resolved with steroid taper, and a 2022 presentation also lists grade 3 transaminase increase in 3 participants and grade 3 Salmonella sepsis in 1. The FDA placed the program on clinical hold in May 2021 and lifted the hold on August 16, 2021, after the protocol was modified. The higher dose was discontinued and an ejection fraction below 40% became an exclusion criterion. The 2 pediatric participants, treated afterward with rituximab, sirolimus, and a lower corticosteroid dose, had no serious adverse event related to RP-A501. The published abstract reports complete resolution of side effects at 24 to 54 months. In phase 2, the company's table of Grade 3 or higher serious adverse events related to RP-A501 (data extraction of August 7, 2026) lists at least 1 such event in 3 of 4 participants (75.0%) in cohorts 1 and 2, in 2 of 2 (100%) in cohort 3, and in 1 of 3 (33.3%) in cohort 4. Thrombotic microangiopathy is listed in 3 participants dosed at 6.7 × 10¹³ GC/kg: 2 of 4 (50.0%) in cohorts 1 and 2 and 1 of 2 (50.0%) in cohort 3. A 2025 news report of the company's conference call states that the C3 inhibitor was added after at least two cases of thrombotic microangiopathy and that one case was linked to a genetic mutation that confers complement sensitivity. In cohorts 1 and 2, acute kidney injury, renal failure, hepatic enzyme increase, cardiac arrest, posterior reversible encephalopathy syndrome, and rhabdomyolysis are each listed in 1 participant. In May 2025, the 2 participants of cohort 3 had serious adverse events involving capillary leak syndrome with multi-organ damage, and 1 died on May 25, 2025, after an acute systemic infection, which the European Union trial record describes as bacteremia and sepsis. The company's announcement of May 27, 2025 described one affected participant, and its 2026 filings describe two. Dosing was halted on May 7, 2025, and the FDA placed the trial on clinical hold on May 23, 2025, and lifted the hold in August 2025. The 2026 Form 10-Q states that an investigation concluded the events were likely the result of the combination of the C3 inhibitor and RP-A501; the 2026 Form 10-K also describes the investigation as tracing the events to the protocol amendment that added the C3 inhibitor. The modified protocol lowered the dose to 3.8 × 10¹³ GC/kg, discontinued prophylactic use of the C3 inhibitor, set a lower threshold for giving the C5 inhibitor eculizumab, and required the first 3 participants to be treated sequentially at least four weeks apart. The company describes the modified protocol as including enhanced eligibility criteria, and a 2026 commentary states that the eligibility criteria remained unchanged. In the first 3 participants dosed under the modified protocol, the company reports no thrombotic microangiopathy, capillary leak, or significant complement-mediated adverse event through 3 months, 1 Grade 3 increase in gamma-glutamyltransferase related to RP-A501 that resolved, and a decrease in platelet count at week 8 to 9 that was not deemed clinically significant and resolved after medication changes. In an announcement of September 15, 2026, the company reported that the FDA had confirmed continued enrollment and dosing under the modified protocol. The incidence of all adverse events, including non-serious events and events below Grade 3, was not found for the phase 2 trial.

The phase 2 trial has no concurrent control arm. The company states that a global natural history study, running concurrently, will serve as the external comparator. RP-NI-A501-0223 is a company-sponsored observational study with prospective and retrospective data collection in males aged 8 years or older and females aged 8 to 50 years, at 15 locations in 6 countries. Its registry record estimates enrollment of 60 participants and a primary completion date of December 2029 and states that a subset of participants will be used as an external control arm; the company reported more than 50 participants enrolled in its second quarter 2026 results announcement. RP-NI-A501-0122 is a completed retrospective chart review of 59 participants (21 males and 38 females) at 15 locations in 5 European countries, which a September 2023 company presentation lists among the natural history data sets for the external comparator. No results were found for either study. The September 2023 presentation refers to “Appropriate matching to ensure robust comparisons”. No document found states the number of participants in the external control subset, the matching variables, the selection rules, the statistical hypothesis, the sample size justification, or the statistical method of comparison with the pivotal trial. The natural history comparison shown in the October 2026 company presentation uses unpublished data from the International Danon Disease Registry, from 22 untreated males for left ventricular mass and 20 for BNP.

The following evidence was not found as of October 2026. No randomized or concurrently controlled comparison has been reported, and no direct or indirect comparison of RP-A501 with heart transplantation was found. No clinical data were found for females or for children younger than 8 years. Experience with reduced ejection fraction is the 1 phase 1 participant who underwent transplantation at 5 months, and the phase 2 trial requires an ejection fraction of 50% or more. Experience at 3.8 × 10¹³ GC/kg is about 3 months of safety follow-up in 3 participants, and no phase 1 participant received that dose. The registered outcome measures of the phase 2 trial are myocardial LAMP2 protein expression, left ventricular mass index, cardiac biomarkers, KCCQ-12 score, NYHA class, event-free survival, and safety, and a 2023 journal article states that extracardiac symptoms, including skeletal myopathy, retinal impairment, and intellectual disability, will be largely unaffected by gene therapy. Follow-up extends to a maximum of 7.1 years in phase 1 and 36 months in phase 2, and a 2026 commentary states that longer-term follow-up is required to assess durability of gene expression, late toxicity, and clinical outcomes, including survival free from heart transplantation. Both trials give a single infusion, and the same commentary describes the inability to re-dose after neutralizing antibody formation as a limitation of current AAV platforms.

No price, economic model, cost-effectiveness analysis, budget impact analysis, or health technology assessment of RP-A501 has been published, and no payer coverage policy or prior authorization criteria naming RP-A501 was found. RP-A501 is investigational. The company presentation of October 2026 schedules completion of dosing in the 12-participant pivotal cohort for mid-2027, topline results for mid-2028, and a biologics license application for late 2028, and it marks launch in mid-2029. The Form 10-K filed in February 2026 states that the company has not commenced efforts to have its product candidates reimbursed. The company's population funnel starts from a modeled 10,000 to 11,000 people with Danon disease in the United States, about 4,000 of them male, and more than 20,000 across the United States and five European countries; the Form 10-K states a prevalence of 15,000 to 30,000 in the United States and Europe, and the page of the sponsored genetic testing program states approximately 15,000 in the United States, citing company data on file. The presentation reports more than 900 identified United States patients as of July 2026, a count that includes people who have received a heart transplant or have died, and a diagnosis rate under 10%. The company states that its initial focus is males with symptomatic hypertrophic cardiomyopathy and that estimated prevalence and identified patient records “do not establish the number of patients eligible for treatment”. It gives an illustrative, non-risk-adjusted global peak annual opportunity of more than $1 billion with the initial indication and more than $2 billion with potential lifecycle expansions, based on internal assumptions that include pricing, access, and uptake. One third-party forecast was identified: GlobalData analysts, cited in August 2025 by a publication GlobalData owns, estimated $364 million in sales by 2031 if the product is approved. The pivotal trial enrolls males aged 8 years and older with a pathogenic or likely pathogenic LAMP2 variant, left ventricular hypertrophy with a left ventricular ejection fraction of at least 50%, New York Heart Association class II to III, and elevated high-sensitivity troponin I, and it excludes an anti-AAV9 neutralizing antibody titer above 1:40 and prior organ transplantation. No source reports how many of the modeled or identified males meet these criteria. Confirming eligibility requires genetic testing and anti-AAV9 antibody testing: the 2026 Florida Medicaid independent laboratory fee schedule lists $392.06 for procedure code 81439, a hereditary cardiomyopathy sequencing panel, no published cost of anti-AAV9 antibody testing was found, and the prescribing information for Fayuvi, an approved AAV9 gene therapy, states that an FDA-authorized test for anti-AAV9 total binding antibodies is not currently available.

List prices of approved one-time gene therapies are the available price benchmarks. Among systemic adeno-associated virus products, the reported prices are $2.125 million for Zolgensma (wholesale acquisition cost, Novartis, 2019), $3.5 million for Hemgenix (2022 news report), $3.2 million for Elevidys (Sarepta Therapeutics, 2023), $2.7 million for Genglycos (wholesale acquisition cost reported by Managed Healthcare Executive, 2026), and $3.95 million for Fayuvi (list price reported by EMJ, 2026). Prices of other one-time gene therapies are $425,000 per eye for Luxturna (Spark Therapeutics, 2018), $2.59 million for Itvisma (reported by Managed Healthcare Executive, 2025), $3.0 million for Skysona (bluebird bio, 2022), $2.2 million for Casgevy (Vertex Pharmaceuticals, 2023), $3.1 million for Lyfgenia (bluebird bio, 2023), and $4.25 million for Lenmeldy (Orchard Therapeutics, 2024). A 2026 conference analysis of gene therapies authorized by the FDA as of January 1, 2026 found a wholesale acquisition cost at market entry for 26 therapies and a median inflation-adjusted cost of $2,316,863 for single-use therapies. The Institute for Clinical and Economic Review (ICER) stated a value-based price benchmark of $1.1 million to $1.9 million for Zolgensma and a health benefit price benchmark of approximately $3,000,000 for Hemgenix after capping cost offsets at $150,000 per year, and its methods for single and short-term therapies add a 50/50 shared savings analysis. The cost offsets a model would consider are documented in other populations. For heart transplantation in the United States for all diagnoses, Milliman estimated 2025 billed charges of $1,918,700 per transplant and notes that billed charges are unlikely to equal the amount paid. Among pediatric heart transplant recipients for all diagnoses, total hospitalization costs in 2016 dollars were $767,221 in the most recent era studied, and $755,345 with a ventricular assist device compared with $457,086 without mechanical circulatory support. Adjusted total hospital charges for a defibrillator placement admission were $132,455 in adults without congenital heart disease, median charges per pediatric heart failure-related hospitalization were $72,087 in 2009, the mean cost of an adult hospitalization with primary heart failure was $11,552 in 2014, and annual median total medical costs for heart failure care were estimated at $24,383 per patient. In the International Danon Disease Registry, 17 of 52 males (32.7%) received a heart transplant and 17 (32.7%) a defibrillator over a median follow-up of 5.86 years. The 2023 international consensus statement expects extracardiac symptoms to be largely unaffected by gene therapy and lists the inability to re-treat among its challenges.

Payer management precedents quoted for other one-time gene therapies are pay-over-time and outcomes-based agreements of up to 5 years for Zolgensma, reimbursement of up to 80% of the cost of Zynteglo if transfusion independence is not achieved and maintained up to two years after infusion, outcomes-based contracts for Lyfgenia tied to hospitalizations with three years of follow-up, and delivery through networks of Qualified Treatment Centers for Zynteglo and Lenmeldy; Rocket Pharmaceuticals describes a limited network of Qualified Treatment Centers for its own approved gene therapy, Kresladi. The Cell and Gene Therapy Access Model of the Centers for Medicare & Medicaid Services is a voluntary model in which the agency negotiates outcomes-based agreements with manufacturers on behalf of state Medicaid agencies; its initial focus is gene therapy for sickle cell disease, and states began participation between January 2025 and January 2026. In August 2021, 18 large commercial health plans and state Medicaid programs imposed restrictions in 53.5% and 68.3%, respectively, of their coverage policies for 11 cell and gene therapies. Between 2017 and 2022, yearly Medicaid spending on gene and RNA therapies increased from $148.3 million to $879.7 million and yearly treatments from 327 to 1638, and starting in 2021, 11 states carved spinal muscular atrophy drugs out of managed Medicaid. Budget impact for RP-A501 therefore cannot be estimated from public data as of October 2026.

RP-A501 is an investigational gene therapy developed by Rocket Pharmaceuticals. It consists of a recombinant adeno-associated virus serotype 9 (AAV9) capsid containing a full-length human LAMP2B transgene and is given as a single intravenous infusion. It is not approved in the United States or authorized in the European Union, and no regulatory submission was found. In the United States it holds orphan drug designation (February 1, 2019), Fast Track designation (February 2019), regenerative medicine advanced therapy designation (announced February 7, 2023), and rare pediatric disease designation; in the European Union it holds PRIME designation (announced May 31, 2023) and orphan designation (August 16, 2023). The FDA placed the phase 1 trial on clinical hold in May 2021, with the lifting of that hold announced on August 16, 2021, and placed the phase 2 trial on clinical hold on May 23, 2025, lifting it in August 2025. As of October 2026, the company expects to complete dosing in the pivotal cohort by mid-2027, to report topline results in mid-2028, and to file a biologics license application in late 2028, and its presentation marks launch in mid-2029. Nine of the 12 participants of the pivotal cohort remained to be enrolled and treated, and the company describes the mid-2028 target as provisional. The application is intended to support accelerated approval, with success defined as at least 7 of 12 participants meeting both co-primary criteria at 12 months. The company states that meeting this criterion “would not, by itself, establish that RP-A501 will receive regulatory approval”, and its Form 10-K states that the FDA may require confirmatory clinical trials as a condition of accelerated approval.

The evidence comes from two open-label trials in males that have no control arm. In phase 1, 5 participants received 6.7 × 10¹³ GC/kg and 2 received 1.1 × 10¹⁴ GC/kg, efficacy is reported for 6 of the 7, and median follow-up is 5.7 years (range 4.3 to 7.1 years). At about 12 months, all 6 had myocardial LAMP2 protein expression of Grade 1 or higher and a decrease in left ventricular mass index of 10% or more (11% to 32%). At the most recent visit, the decrease was 10% or more in 4 of 6, cardiac troponin I had decreased by 31% to 99% and NT-proBNP by 31% to 90%, and 1 participant had Grade 0 LAMP2 staining at 30 and 36 months. In phase 2, 9 participants have been dosed and efficacy data cover 4, at 18 to 36 months: left ventricular mass index decreased by 10% or more in 3 and increased by 9% in 1, and no LAMP2 protein expression result has been reported. These are biopsy, imaging, and laboratory measures, and registry authors report that left ventricular hypertrophy in males “stabilizes before HF outcome”. The phase 1 publication covers 24 to 54 months; later phase 1 data and every phase 2 result are reported by the company alone. All efficacy data come from participants dosed at 6.7 × 10¹³ GC/kg or higher under immunomodulation regimens that differ from that of the pivotal cohort, which receives 3.8 × 10¹³ GC/kg. Three participants have received that dose, with about 3 months of safety follow-up and no efficacy result. The planned external comparator is a company-sponsored natural history study without reported results, and no document found states the method of comparison.

No randomized or concurrently controlled comparison of RP-A501 was found. Mixed-effects models of the International Danon Disease Registry give an increase in left ventricular mass in untreated males of 28.424 g per year of age in 20 participants without a heart failure outcome and 34.183 g per year in 13 with one. The company's October 2026 comparison cites unpublished data from the same registry: 39.86 ± 4.01 g per year in 22 untreated males, and an increase in BNP of 42.17 ± 18.46 pg/mL per year in 20, whereas an earlier registry abstract of 112 participants reported that BNP did not change over time. In the registry, a heart failure outcome occurred in 21 of 52 males (40.4%); in phase 1, 2 of 7 participants underwent heart transplantation, 5 months and 4.8 years after infusion. After heart transplantation, estimated 5-year graft survival was 87.1% (95% CI 63.6% to 95.9%) among 38 recipients, and in a European registry 5 of 8 male recipients of a transplant or assist device (62%) died within one year of the procedure. No direct or indirect comparison of RP-A501 with heart transplantation or with another gene therapy was found. No registered interventional trial of another product for Danon disease was found; an AAV2i8 vector carrying the LAMP2B transgene holds European orphan designation (May 22, 2025), and PPL-002, a gene-modified hematopoietic stem and progenitor cell therapy with preclinical data, holds FDA orphan drug designation. Approved single-dose intravenous AAV gene therapies for other diseases have labeled doses of 1.1 × 10¹⁴ vg/kg for Zolgensma and 3.0 × 10¹³ vg/kg for Fayuvi (both AAV9), 1.33 × 10¹⁴ vg/kg for Elevidys at 10 to 70 kg, and 2 × 10¹³ gc/kg for Hemgenix. The Zolgensma and Elevidys labels carry boxed warnings for serious liver injury and acute liver failure.

In phase 1, thrombotic microangiopathy occurred in 1 of 2 participants given 1.1 × 10¹⁴ GC/kg, as a grade 4 event managed with eculizumab and hemodialysis, and that dose was discontinued. In phase 2 at 6.7 × 10¹³ GC/kg it is listed in 2 of 4 participants (50.0%) in cohorts 1 and 2 and in 1 of 2 (50.0%) in cohort 3. Both participants of cohort 3, whose regimen contained the C3 inhibitor pegcetacoplan, had capillary leak syndrome with multi-organ damage, and 1 died on May 25, 2025, after bacteremia and sepsis; the investigation concluded that the events were likely the result of the combination of the C3 inhibitor and RP-A501. In phase 1, 3 of 7 participants had a grade 3 glucocorticoid-related exacerbation of skeletal myopathy. The modified protocol uses 3.8 × 10¹³ GC/kg with three doses of rituximab, sirolimus, and corticosteroids, without a prophylactic C3 inhibitor; in its first 3 participants, the company reports no thrombotic microangiopathy or capillary leak through 3 months and 1 Grade 3 increase in gamma-glutamyltransferase related to RP-A501 that resolved. Participants are hospitalized for infusion and monitoring, these 3 were discharged 1 to 2 weeks after infusion, immunosuppressive therapy is discontinued 2 to 3 months after infusion, and trial follow-up lasts 5 years. For class context, a random-effects meta-analysis of AAV gene therapy reported thrombotic microangiopathy in 4.7% and treatment-related death in 4.7%. The Zolgensma label states that thrombotic microangiopathy generally occurred within the first two weeks after infusion and calls for platelet counts at least weekly for the first month; the Elevidys label calls for weekly liver function monitoring for the first 3 months and proximity to a healthcare facility for at least 2 months. The rituximab, sirolimus, and eculizumab labels warn of fatal infections.

No approved therapy treats the cause of Danon disease; the 2023 international consensus statement states that there are “no available therapies to specifically treat Danon disease”. Heart transplantation was performed in 30 of 116 registry participants (25.9%), including 17 of 52 males (32.7%). Among the 38 recipients cited above, the median age at transplantation was 20.2 years, 2 died, and 2 underwent retransplantation. For a formulary decision, the following remain unknown: the price, for which no announcement, economic model, or health technology assessment was found (prices reported for two approved systemic AAV9 products, Zolgensma and Fayuvi, are $2.125 million and $3.95 million); the labeled population, given trial eligibility limited to males aged 8 years or older with an ejection fraction of 50% or more and an anti-AAV9 neutralizing antibody titer no higher than 1:40; the pivotal result, with 3 of 12 participants dosed; durability beyond a maximum follow-up of 7.1 years, given that a 2026 commentary describes the inability to re-dose after neutralizing antibody formation; the effect on extracardiac disease, which the consensus statement expects to be largely unaffected; the effect on survival or transplant-free survival, with event-free survival a secondary outcome followed to 60 months; outcomes in females and in children younger than 8 years, for whom no clinical data were found; and the number of eligible people, given prevalence estimates from 240 to 11,807 in one 2026 analysis, a company model of 10,000 to 11,000 people in the United States (about 4,000 male), more than 900 identified United States patients, and no source reporting how many meet the trial criteria.