Introduction and Mechanistic Framework
Adeno-associated virus (AAV) vectors have emerged as the leading platform for in vivo gene transfer, with seven AAV-based therapeutics approved globally as of 2024 and dozens in late-stage development 1921. To understand where AAV offers the clearest clinical advantage, it is essential to frame its mechanistic position relative to competing modalities. AAV delivers episomal gene addition, providing durable transgene expression in non-dividing cells without integration-associated genotoxicity 11. This contrasts fundamentally with antisense oligonucleotides (ASO) and small interfering RNA (siRNA), which mediate reversible gene knockdown requiring ongoing administration, and with genome-editing platforms (CRISPR/Cas, base editors, prime editors) that permanently alter endogenous loci but face delivery and off-target challenges 311. Enzyme replacement therapy (ERT) provides exogenous protein supplementation but demands lifelong infusions and often encounters tissue-penetration barriers, particularly at the blood-brain barrier 17.
The clinical advantage of AAV emerges most clearly in indications requiring durable gene addition in accessible, non-dividing tissues where alternative modalities face fundamental biological constraints. Conversely, AAV's inability to redose due to neutralizing antibody development, substantial dose requirements creating manufacturing challenges, and safety signals including hepatotoxicity and thrombocytopenia constrain its competitive position in other therapeutic areas 1131725.
Hemophilia: Paradigmatic Advantage in Liver-Secreted Coagulation Factors
Hemophilia B represents the therapeutic area with the strongest evidence for AAV superiority over all comparator modalities. Etranacogene dezaparvovec (Hemgenix), an AAV5 vector delivering the hyperactive factor IX (FIX) Padua variant under liver-specific promoter control, achieved FDA approval in November 2022 based on phase IIb/III data demonstrating sustained mean FIX activity of 47% at 26 weeks (range 33.2–57.0%), with complete cessation of bleeding events and elimination of prophylactic FIX replacement therapy through 26 weeks of follow-up 4. Critically, this efficacy was achieved despite the presence of preexisting anti-AAV5 neutralizing antibodies detected by sensitive luciferase assays, and the safety profile was exceptional, with no clinically significant liver enzyme elevations, no inflammatory markers requiring intervention, and no corticosteroid use—a unique profile among AAV programs 417.
The comparator landscape for hemophilia B underscores AAV's advantage. Extended half-life ERT products such as nonacog beta pegol (Rebinyn) require ongoing prophylaxis despite achieving median annualized bleeding rates of 0.66 over five years, with 72.9-hour half-life still necessitating regular infusions 17. No ASO or siRNA programs targeting hemophilia B have advanced beyond preclinical stages in the available dataset 18, and gene-editing approaches remain in early development 18. The binary choice between one-time AAV infusion achieving near-normal FIX levels versus lifelong ERT prophylaxis establishes a clear clinical advantage for AAV in patients without contraindications 17.
Hemophilia A presents a more nuanced competitive landscape. Valoctocogene roxaparvovec (Roctavian), an AAV5-factor VIII (FVIII) vector approved by FDA in June 2023 and EMA in August 2022, demonstrated sustained FVIII activity in the mild-to-normal range through nearly four years of follow-up in the high-dose cohort, with mean annualized bleeding rate of zero in year one and 1.2 over total observation 1719. The AFFINE study in 75 adults showed total annualized bleeding rate reduction from 4.73 to 1.24 (p=0.004), with 84% achieving greater than 5% FVIII activity at month 15 and 88% experiencing no treated bleeds 17. However, critical limitations emerged. The prophylactic corticosteroid study revealed that FVIII activity was approximately one-third of pivotal study levels, and the BENEGENE-2 lead-in study documented 58.1% screening failure rates, with 82.1% of failures attributable to AAV6 neutralizing antibody positivity 17. This contrasts with extended half-life FVIII products such as efanesoctocog alfa (Altuviiio), which achieved 82% zero joint bleeds with once-weekly dosing, zero percent inhibitor development, and no antibody-based exclusions 17. The competitive positioning thus depends on patient eligibility, with AAV offering clear advantage in antibody-negative individuals accepting one-time high-risk intervention over weekly prophylaxis, while ERT remains superior for antibody-positive populations or those preferring reversible therapy 17.
Spinal Muscular Atrophy: Age-Dependent Advantage in Presymptomatic Populations
Onasemnogene abeparvovec (Zolgensma), an AAV9 vector delivering the survival motor neuron (SMN1) gene, received FDA approval in May 2019 for pediatric patients younger than two years with bi-allelic SMN1 mutations 2. The NURTURE study established AAV's strongest competitive position in presymptomatic SMA, demonstrating 100% survival without permanent ventilation at median 4.9 years follow-up, with 92% achieving independent walking and 88% reaching maximum Children's Hospital of Philadelphia Infant Test of Neuromuscular Disorders (CHOP-INTEND) scores when treated at six weeks of age or younger 17. Notably, 84% achieved sitting without support within World Health Organization age-appropriate windows 17. These outcomes substantially exceed those achieved with delayed nusinersen (Spinraza) treatment, where the SHINE extension study documented only 5% walking with assistance in the delayed treatment cohort versus 92% independent walking with presymptomatic AAV administration 17.
However, AAV's advantage narrows substantially in symptomatic or older SMA populations. The SMART study enrolled 24 patients aged 18 months to 9 years (mean 4.69 years) weighing 8.5 to 21 kg, with 21 previously treated with nusinersen or risdiplam. At week 52, mean Hammersmith Functional Motor Scale Expanded (HFMSE) increased only 3.7 points and Revised Upper Limb Module (RULM) increased 2.0 points, while adverse event burden was substantial: 100% experienced any adverse event, 62.5% serious adverse events, and 87.5% developed transaminase elevations requiring median 175 days of corticosteroid therapy 17. The STRENGTH study in older patients (mean age 7.4 years) who had discontinued nusinersen or risdiplam showed HFMSE increase of only 1.05 points at week 52 with stabilization rather than improvement in other motor scores, and all patients experienced at least one adverse event 17.
The competitive landscape includes nusinersen, which requires intrathecal administration every four months indefinitely but demonstrated mean HFMSE of 26.0 for early treatment versus 21.2 for delayed treatment at four years in CHERISH-SHINE, and risdiplam, an oral SMN2 splicing modifier providing an alternative for patients ineligible for AAV or preferring reversible therapy 17. The absence of gene-editing programs in clinical development for SMA is notable 18, suggesting AAV and ASO/small-molecule approaches dominate without genome-editing competition. AAV's clearest advantage thus resides in the narrow therapeutic window of presymptomatic infants younger than six weeks, where one-time dosing achieves unprecedented motor outcomes, while symptomatic or older patients may derive comparable benefit from ongoing ASO or small-molecule therapy with lower acute risk 17.
Inherited Retinal Dystrophies: Uncontested Dominance in Immune-Privileged Tissue
Inherited retinal diseases represent the therapeutic area with AAV's most uncontested advantage. Voretigene neparvovec (Luxturna), an AAV2 vector delivering RPE65 via subretinal injection, received FDA approval in December 2017 for RPE65-mediated Leber congenital amaurosis and retinitis pigmentosa, establishing proof-of-concept for durable photoreceptor transduction 518. The immune-privileged nature of the subretinal space, the requirement for durable gene addition in non-dividing photoreceptors, and the absence of blood-retinal barrier for local delivery create ideal conditions for AAV 1718.
Recent clinical data further validate AAV's position. The OCU400 phase I/II study in 18 retinitis pigmentosa and 4 Leber congenital amaurosis patients (ages 18–77 years) demonstrated 100% (9/9) improvement or preservation in visual function versus untreated eyes (p=0.01) at two years, with 83% showing best-corrected visual acuity (BCVA) stabilization or improvement and 75% showing multi-luminance mobility testing (MLMT) stabilization or improvement 17. The OPGx-001 study treating three adults (ages 19, 26, 34) with severe baseline LCA5-associated disease showed sustained visual improvements at 12 months, including average 0.35 logMAR (3.5-line) visual acuity improvement, full-field sensitivity testing improvement of 0.86 log units in treated eyes versus 0.16 in control eyes (0.7 log units better interocular difference), and increased pupillary light response, with no serious adverse events or dose-limiting toxicities 17.
The competitive landscape confirms AAV's monopoly: the drug database contains 20 AAV programs across multiple genetic targets (RPE65, LCA5, NR2E3, GUCY2D, CEP290, RDH12, NMNAT1, AIPL1, RPGRIP1, KCNJ13) versus zero ASO programs, zero siRNA programs, and one gene-editing program (EDIT-101, which uses AAV as the delivery vehicle for CRISPR/Cas9) 18. The absence of ERT alternatives reflects fundamental biological constraints: photoreceptors require cell-autonomous gene expression that cannot be achieved through systemic protein replacement 1718. Geographic distribution of retinal gene therapy development is global, with strong representation across the United States, Europe, and emerging China programs 18, and no alternative modality has demonstrated comparable visual function preservation or improvement 17.
Duchenne Muscular Dystrophy: Mixed Evidence and Competitive Landscape
Duchenne muscular dystrophy illustrates the complexity of AAV's competitive positioning when clinical translation is inconsistent despite compelling molecular evidence. Delandistrogene moxeparvovec (Elevidys), an AAVrh74 vector delivering micro-dystrophin, received FDA accelerated approval in June 2023 for ambulatory patients aged 4 years and older based on the EMBARK study 24. The primary endpoint (North Star Ambulatory Assessment change at 52 weeks) was not met, with mean difference of 0.65 points (p=0.2441) 24. However, key secondary endpoints showed statistically significant improvements: time to rise from floor improved by 0.64 seconds (p=0.0025), 10-meter walk/run improved by 0.42 seconds (p=0.0048), and four-stair climb improved by 0.36 seconds (p=0.0412) 24. Two-year data demonstrated divergence, with North Star Ambulatory Assessment change of +2.63 versus −0.25 points in placebo (p<0.01), and sustained micro-dystrophin expression from 34.29% at week 12 to 45.68% at week 64 17.
Early-stage programs suggest potential for superior efficacy with optimized constructs. The SGT-003 (Solid Biosciences) AAV9-micro-dystrophin program achieved 110% of normal micro-dystrophin expression by Western blot at 90 days in three patients, with substantial biomarker reductions (creatine kinase −57%, lactate dehydrogenase −60%, serum titin −42%) and cardiac function improvement, with excellent safety profile showing only mild-to-moderate adverse events 17. RGX-202 (Regenxbio/Ultragenyx) AAV8-micro-dystrophin demonstrated dose-dependent efficacy, with the pivotal dose (2×10¹⁴ genome copies per kilogram) achieving North Star Ambulatory Assessment improvement of +4.5 points at 12 months, 6.8 points better than natural history, with 20.8% to 77.2% micro-dystrophin expression and no serious adverse events 17.
However, serious safety signals have emerged. In 2025, two Duchenne-treated individuals died from acute liver failure following Elevidys administration, prompting Sarepta to suspend shipments for non-ambulatory patients and pause the ENVISION trial pending evaluation of immunosuppressive regimens 25. Pfizer discontinued development of PF-06939926 following safety concerns including kidney injury, thrombocytopenia, and two treatment-related deaths 25. These events underscore the narrow therapeutic index of high-dose AAV in muscle disorders 25.
The competitive landscape includes 23 antisense oligonucleotide exon-skipping programs targeting exons 8, 23, 35, 43, 44, 45, 50, 51, 52, 53, and 55, with four approved products (eteplirsen, golodirsen, casimersen, viltolarsen) 18. The ESSENCE trial of casimersen showed dystrophin increase from 0.93% to 1.74% (p<0.001), substantially lower than AAV micro-dystrophin levels but with established safety profiles and mutation-specific targeting applicable to 5% to 15% of Duchenne patients per ASO 1718. Gene-editing approaches have failed clinically, with fordadistrogene movaparvovec missing its primary endpoint and currently on dosing pause due to a fatal serious adverse event in a separate pediatric trial 17. AAV thus offers mutation-agnostic high-level micro-dystrophin expression potentially applicable to more than 90% of Duchenne patients, but inconsistent clinical translation, variable safety, and high manufacturing demands (2×10¹⁴ genome copies per kilogram doses) create uncertainty about whether AAV will displace or complement exon-skipping ASOs 1718.
Central Nervous System Disorders: Niche Applications with Limited Alternatives
Eladocagene exuparvovec (Upstaza), a modified AAV2 vector delivering the DDC gene encoding aromatic L-amino acid decarboxylase, received FDA approval in November 2024 for AADC deficiency, representing the first FDA-approved gene therapy administered directly to the brain via bilateral intraputaminal injection 19. The approval establishes proof-of-concept for AAV's capability to achieve durable enzyme expression in CNS parenchyma where intrathecal ASO delivery and systemic ERT face blood-brain barrier limitations 19. The drug database confirms AAV's monopoly in AADC deficiency, with two AAV programs (one approved, one phase II) and zero ASO, siRNA, or gene-editing competitors 18.
However, AAV has not achieved comparable success in all CNS indications. Amyotrophic lateral sclerosis illustrates the limitations: no AAV programs are in clinical development within the available dataset 1718. The ASO tofersen (QALSODY) for SOD1-ALS achieved approval but demonstrated modest benefits only with early treatment initiation. The VALOR study with open-label extension showed the primary endpoint (28-week ALSFRS-R change) was not met (tofersen −6.98 versus placebo −8.14, difference 1.2 points, p=0.97), though 12-month combined analysis showed significant benefits with early versus delayed start (ALSFRS-R difference 3.5 points, respiratory function 9.2% difference) 17. Critically, safety was concerning, with 36.5% serious adverse events, 17.3% discontinuation due to adverse events, and 6.7% serious neurologic events including myelitis, radiculitis, and aseptic meningitis 17. The BIIB105 ASO for ATXN2-ALS failed to demonstrate efficacy despite achieving target knockdown, with no reduction in plasma neurofilament or improvement in clinical outcomes over 40-plus weeks, and development was terminated 17. These failures suggest neither ASO nor AAV approaches have identified optimal therapeutic targets in ALS, with tissue penetration alone insufficient without appropriate target selection 17.
Metachromatic leukodystrophy further illustrates CNS indication complexity: atidarsagene autotemcel (Libmeldy/Lenmeldy), an ex vivo lentiviral hematopoietic stem cell gene therapy, received approval across the United States and Europe, while ERT (TAK-611) failed to meet primary and secondary endpoints due to blood-brain barrier limitations 1718. No in vivo AAV programs for metachromatic leukodystrophy are represented in the dataset 18, suggesting that systemic enzyme delivery crossing the blood-brain barrier and long-term expression in multiple tissues favors HSC-based approaches over in vivo AAV delivery 17. AAV's advantage in CNS disorders thus resides in focal parenchymal delivery for enzyme deficiencies (AADC) or specific neurodegenerative targets accessible via direct injection, rather than systemic CNS diseases requiring broad tissue distribution 1718.
Constraints, Future Directions, and Conclusions
Pre-existing neutralizing antibodies represent AAV's most significant constraint, with 82.1% of screening failures in the BENEGENE-2 hemophilia A lead-in study attributable to AAV6 antibody positivity 17. Immunosuppression strategies including corticosteroids, rituximab, sirolimus, plasmapheresis, and anti-thymocyte globulin have been deployed 25, but the inability to redose limits AAV to one-time intervention 14. At the ASGCT 2025 Annual Meeting, Regeneron presented novel B-cell immunomodulation strategies for successful AAV vector re-administration in non-human primates, aiming to temporarily block factors leading to neutralizing antibody development 22, suggesting future solutions may enable repeat dosing.
Next-generation capsid engineering efforts leveraging directed evolution, rational design, and machine learning aim to generate customized AAV capsids evading pre-existing antibodies and achieving tissue-specific targeting 19. Self-complementary AAV constructs, insulator elements preventing unintended host-gene activation, and removal of bacterial backbone sequences improve transgene expression kinetics and biosafety 19. Integration of gene-editing systems with AAV delivery platforms (exemplified by EDIT-101 for CEP290-mediated LCA10) positions AAV as the delivery vehicle for CRISPR/Cas9, base editors, and prime editors 18.
Manufacturing scalability and cost remain critical challenges. Hemophilia programs use 3 to 6×10¹³ vector genomes per kilogram, SMA uses 1.1×10¹⁴ vector genomes per kilogram, and Duchenne programs range from 1.33×10¹⁴ to 2×10¹⁴ vector genomes per kilogram 17. These high doses create manufacturing bottlenecks and contribute to prohibitive pricing, with ICER identifying $1 million to $3 million one-time costs as a core challenge for health system budget impact 26. A 2025 microcosting study quantified first-year delivery costs for hemophilia gene therapy at €28,696 for hemophilia A and €20,511 for hemophilia B, encompassing personnel, hospital visits, laboratory tests, medications, and consumables 27, emphasizing that infrastructure requirements extend beyond vector manufacturing.
In conclusion, AAV gene transfer demonstrates clearest clinical advantage in hemophilia B (near-normal factor expression with complete bleed cessation and excellent safety), presymptomatic SMA (unprecedented motor outcomes in the narrow therapeutic window of infants younger than six weeks), and inherited retinal dystrophies (durable photoreceptor transduction in immune-privileged tissue). AAV competes effectively but faces viable alternatives in hemophilia A (variable efficacy and high screening failures versus extended half-life ERT), Duchenne muscular dystrophy (high dystrophin expression but inconsistent clinical translation versus mutation-specific ASOs), and symptomatic SMA (stabilization in older patients with high adverse event burden versus established ASO/small-molecule therapies). AAV shows minimal presence or disadvantage in amyotrophic lateral sclerosis, metachromatic leukodystrophy (ex vivo lentiviral HSC gene therapy approved; no in vivo AAV programs), and systemic CNS diseases requiring broad tissue distribution. The competitive advantage is context-dependent, strongest in indications requiring durable gene addition in non-dividing cells accessible via direct delivery and lacking viable ASO/siRNA/ERT alternatives, while narrowing where alternative modalities offer comparable efficacy with lower manufacturing burden, reversible mechanisms, or superior safety profiles 1718192124252627.