Use of plasmapheresis to lower anti-AAV antibodies in nonhuman primates with pre-existing immunity to AAVrh74

Patients with pre-existing immunity to adeno-associated virus (AAV) are currently unable to receive systemic gene transfer therapies. In this nonhuman primate study, we investigated the impact of immunosuppression strategies on gene transfer therapy safety and efficacy and analyzed plasmapheresis as a potential pretreatment for circumvention of pre-existing immunity or redosing. In part 1, animals received delandistrogene moxeparvovec (SRP-9001), an AAVrh74-based gene transfer therapy for Duchenne muscular dystrophy. Cohort 1 (control, n = 2) received no immunosuppression; cohorts 2-4 (n = 3 per cohort) received prednisone at different time points; and cohort 5 (n = 3) received rituximab, sirolimus, and prednisone before and after dosing. In part 2, cohorts 2-4 underwent plasmapheresis before redosing; cohort 5 was redosed without plasmapheresis. We analyzed safety, immune response (humoral and cell-mediated responses and complement activation), and vector genome distribution. After 2 or 3 plasmapheresis exchanges, circulating anti-AAVrh74 antibodies were reduced, and animals were redosed. Plasmapheresis was well tolerated, with no abnormal clinical or immunological observations. Cohort 5 (redosed with high anti-AAVrh74 antibody titers) had hypersensitivity reactions, which were controlled with treatment. These findings suggest that plasmapheresis is a safe and effective method to reduce anti-AAV antibody levels in nonhuman primates prior to gene transfer therapy. The results may inform human studies involving redosing or circumvention of pre-existing immunity.

Long-term safety and functional outcomes of delandistrogene moxeparvovec gene therapy in patients with Duchenne muscular dystrophy: A phase 1/2a nonrandomized trial

INTRODUCTION/AIMS

Delandistrogene moxeparvovec is indicated in the United States for the treatment of ambulatory pediatric patients aged 4 through 5 years with Duchenne muscular dystrophy (DMD) with a confirmed mutation in the DMD gene. Long-term delandistrogene moxeparvovec microdystrophin protein (a shortened dystrophin that retains key functional domains of the wild-type protein) expression may positively alter disease progression in patients with DMD. We evaluated long-term safety and functional outcomes of delandistrogene moxeparvovec in patients with DMD.

METHODS

An open-label, phase 1/2a, nonrandomized controlled trial (Study 101; NCT03375164) enrolled ambulatory males, ≥4 to <8 years old, with DMD. Patients received a single intravenous infusion (2.0 × 1014 vg/kg by supercoiled quantitative polymerase chain reaction) of delandistrogene moxeparvovec and prednisone (1 mg/kg/day) 1 day before to 30 days after treatment. The primary endpoint was safety. Functional outcomes were change from baseline in North Star Ambulatory Assessment (NSAA) and timed function tests.

RESULTS

Four patients (mean age, 5.1 years) were enrolled. There were 18 treatment-related adverse events; all occurred within 70 days posttreatment and resolved. Mean NSAA total score increased from 20.5 to 27.5, baseline to year 4, with a mean (standard deviation) change of +7.0 (2.9). Post hoc analysis demonstrated a statistically significant and clinically meaningful 9-point difference in NSAA score, relative to a propensity-score-weighted external control cohort (least-squares mean [standard error] = 9.4 [3.4]; P = .0125).

DISCUSSION

Gene transfer therapy with delandistrogene moxeparvovec treatment is well tolerated, with a favorable safety profile. Functional improvements are sustained through 4 years, suggesting delandistrogene moxeparvovec may positively alter disease progression.

Gene therapy with bidridistrogene xeboparvovec for limb-girdle muscular dystrophy type 2E/R4: phase 1/2 trial results

Limb-girdle muscular dystrophy 2E/R4 is caused by mutations in the β-sarcoglycan (SGCB) gene, leading to SGCB deficiency and consequent muscle loss. We developed a gene therapy approach based on functional replacement of the deficient SCB protein. Here we report interim results from a first-in-human, open-label, nonrandomized, phase 1/2 trial evaluating the safety and efficacy of bidridistrogene xeboparvovec, an adeno-associated virus-based gene therapy containing a codon-optimized, full-length human SGCB transgene. Patients aged 4-15 years with confirmed SGCB mutations at both alleles received one intravenous infusion of either 1.85 × 1013 vector genome copies kg-1 (Cohort 1, n = 3) or 7.41 × 1013 vector gene copies kg-1 (Cohort 2, n = 3). Primary endpoint was safety, and secondary endpoint was change in SGCB expression in skeletal muscle from baseline to Day 60. We report interim Year 2 results (trial ongoing). The most frequent treatment-related adverse events were vomiting (four of six patients) and gamma-glutamyl transferase increase (three of six patients). Serious adverse events resolved with standard therapies. Robust SGCB expression was observed: Day 60 mean (s.d.) percentage of normal expression 36.2% (2.7%) in Cohort 1 and 62.1% (8.7%) in Cohort 2. Post hoc exploratory analysis showed preliminary motor improvements using the North Star Assessment for Limb-girdle Type Muscular Dystrophies maintained through Year 2. The 2-year safety and efficacy of bidridistrogene xeboparvovec support clinical development advancement. Further studies are necessary to confirm the long-term safety and efficacy of this gene therapy. ClinicalTrials.gov registration: NCT03652259 .

Delandistrogene Moxeparvovec Gene Therapy in Ambulatory Patients (Aged ≥4 to <8 Years) with Duchenne Muscular Dystrophy: 1-Year Interim Results from Study SRP-9001-103 (ENDEAVOR)

OBJECTIVE

Delandistrogene moxeparvovec is approved in the USA for the treatment of ambulatory patients (4-5 years) with Duchenne muscular dystrophy. ENDEAVOR (SRP-9001-103; NCT04626674) is a single-arm, open-label study to evaluate delandistrogene moxeparvovec micro-dystrophin expression, safety, and functional outcomes following administration of commercial process delandistrogene moxeparvovec.

METHODS

In cohort 1 of ENDEAVOR (N = 20), eligible ambulatory males, aged ≥4 to <8 years, received a single intravenous infusion of delandistrogene moxeparvovec (1.33 × 1014  vg/kg). The primary endpoint was change from baseline (CFBL) to week 12 in delandistrogene moxeparvovec micro-dystrophin by western blot. Additional endpoints evaluated included: safety; vector genome copies; CFBL to week 12 in muscle fiber-localized micro-dystrophin by immunofluorescence; and functional assessments, including North Star Ambulatory Assessment, with comparison with a propensity score-weighted external natural history control.

RESULTS

The 1-year safety profile of commercial process delandistrogene moxeparvovec in ENDEAVOR was consistent with safety data reported in other delandistrogene moxeparvovec trials (NCT03375164 and NCT03769116). Delandistrogene moxeparvovec micro-dystrophin expression was robust, with sarcolemmal localization at week 12; mean (SD) CFBL in western blot, 54.2% (42.6); p < 0.0001. At 1 year, patients demonstrated stabilized or improved North Star Ambulatory Assessment total scores; mean (SD) CFBL, +4.0 (3.5). Treatment versus a propensity score-weighted external natural history control demonstrated a statistically significant difference in least squares mean (standard error) CFBL in North Star Ambulatory Assessment, +3.2 (0.6) points; p < 0.0001.

INTERPRETATION

Results confirm efficient transduction of muscle by delandistrogene moxeparvovec. One-year post-treatment, delandistrogene moxeparvovec was well tolerated, and demonstrated stabilized or improved motor function, suggesting a clinical benefit for patients with Duchenne muscular dystrophy. ANN NEUROL 2023;94:955-968.

Expression and function of four AAV-based constructs for dystrophin restoration in the mdx mouse model of Duchenne muscular dystrophy

Robust expression of shortened, functional dystrophin provided impetus to develop adeno-associated virus (AAV)-based constructs for clinical application. Because several cassettes are being tested in clinical trials, this study compared the efficacies of four shortened dystrophin-promoter combinations with implications for outcomes in clinical trials: MHCK7 or MCK promoter with a shortened dystrophin transgene containing the N-terminus and spectrin repeats R1, R2, R3 and R24 (rAAVrh74.MHCK7.micro-dystrophin and rAAVrh74.MCK.micro-dystrophin, respectively); shortened dystrophin construct containing the neuronal nitric oxide (nNOS) binding site (rAAVrh74.MHCK7.DV.mini-dystrophin); and shortened dystrophin containing the C-terminus (rAAVrh74.MHCK7.micro-dystrophin.Cterm). Functional and histological benefit were examined at 4 weeks following intramuscular delivery in mdx mice. rAAVrh74.MHCK7.micro-dystrophin provided the most robust transgene expression and significantly increased specific force output in the tibialis anterior muscle. Muscle environment was normalized (i.e. reductions in central nucleation), indicating functional and histological advantages of rAAVrh74.MHCK7.micro-dystrophin. Thus, promoter choice and transgene design are critical for optimal dystrophin expression/distribution for maximal functional improvement.

Expression of SRP-9001 dystrophin and stabilization of motor function up to 2 years post-treatment with delandistrogene moxeparvovec gene therapy in individuals with Duchenne muscular dystrophy

Introduction: Delandistrogene moxeparvovec (SRP-9001) is an investigational gene transfer therapy designed for targeted expression of SRP-9001 dystrophin protein, a shortened dystrophin retaining key functional domains of the wild-type protein. Methods: This Phase 2, double-blind, two-part (48 weeks per part) crossover study (SRP-9001-102 [Study 102]; NCT03769116) evaluated delandistrogene moxeparvovec in patients, aged ≥4 to <8 years with Duchenne muscular dystrophy. Primary endpoints (Part 1) were change from baseline (CFBL) in SRP-9001 dystrophin expression (Week 12), by Western blot, and in North Star Ambulatory Assessment (NSAA) score (Week 48). Safety assessments included treatment-related adverse events (TRAEs). Patients were randomized and stratified by age to placebo (n = 21) or delandistrogene moxeparvovec (n = 20) and crossed over for Part 2. Results: SRP-9001 dystrophin expression was achieved in all patients: mean CFBL to Week 12 was 23.82% and 39.64% normal in Parts 1 and 2, respectively. In Part 1, CFBL to Week 48 in NSAA score (least-squares mean, LSM [standard error]) was +1.7 (0.6) with treatment versus +0.9 (0.6) for placebo; p = 0.37. Disparity in baseline motor function between groups likely confounded these results. In 4- to 5-year-olds with matched baseline motor function, CFBL to Week 48 in NSAA scores was significantly different (+2.5 points; p = 0.0172), but not significantly different in 6-to-7-year-olds with imbalanced baseline motor function (-0.7 points; p = 0.5384). For patients treated with delandistrogene moxeparvovec in Part 2, CFBL to Week 48 in NSAA score was +1.3 (2.7), whereas for those treated in Part 1, NSAA scores were maintained. As all patients in Part 2 were exposed to treatment, results were compared with a propensity-score-weighted external control (EC) cohort. The LSM difference in NSAA score between the Part 2 treated group and EC cohort was statistically significant (+2.0 points; p = 0.0009). The most common TRAEs were vomiting, decreased appetite, and nausea. Most occurred within the first 90 days and all resolved. Discussion: Results indicate robust expression of SRP-9001 dystrophin and overall stabilization in NSAA up to 2 years post-treatment. Differences in NSAA between groups in Part 1 were not significant for the overall population, likely because cohorts were stratified only by age, and other critical prognostic factors were not well matched at baseline.

Assessing the value of delandistrogene moxeparvovec (SRP-9001) gene therapy in patients with Duchenne muscular dystrophy in the United States

Background: Delandistrogene moxeparvovec (SRP-9001) is an investigational gene therapy that may delay progression of Duchenne muscular dystrophy (DMD), a severe, rare neuromuscular disease caused by DMD gene mutations. Early cost-effectiveness analyses are important to help contextualize the value of gene therapies for reimbursement decision making. Objective: To determine the potential value of delandistrogene moxeparvovec using a cost-effectiveness analysis. Study design: A simulation calculated lifetime costs and equal value of life years gained (evLYG). Inputs included extrapolated clinical trial results and published utilities/costs. As a market price for delandistrogene moxeparvovec has not been established, threshold analyses established maximum treatment costs as they align with value, including varying willingness-to-pay up to $500,000, accounting for severity/rarity. Setting: USA, healthcare system perspective Patients: Boys with DMD Intervention: Delandistrogene moxeparvovec plus standard of care (SoC; corticosteroids) versus SoC alone Main outcome measure: Maximum treatment costs at a given willingness-to-pay threshold Results: Delandistrogene moxeparvovec added 10.30 discounted (26.40 undiscounted) evLYs. The maximum treatment cost was approximately $5 M, assuming $500,000/evLYG. Varying the benefit discount rate to account for the single administration increased the estimated value to #$5M, assuming $500,000/evLYG. Conclusion: In this early economic model, delandistrogene moxeparvovec increases evLYs versus SoC and begins to inform its potential value from a healthcare perspective.

Testing preexisting antibodies prior to AAV gene transfer therapy: rationale, lessons and future considerations

Given the increasing number of gene transfer therapy studies either completed or underway, there is growing attention to the importance of preexisting adaptive immunity to the viral vectors used. The recombinant viral vectors developed for gene transfer therapy share structural features with naturally occurring wild-type virus. Antibodies generated against viral vectors obtained through a previous exposure to wild-type virus can potentially compromise transgene expression by blocking transduction, thereby limiting the therapeutic efficacy of the gene transfer therapy; they may also pose potential safety concerns. Therefore, systemic gene transfer delivery requires testing patients for preexisting antibodies. Two different assays have been used: (1) binding assays that focus on total antibodies (both neutralizing and non-neutralizing) and (2) neutralizing assays that detect neutralizing antibodies. In this review we focus on adeno-associated virus-based gene therapies, describing the immune response that occurs to naturally occurring adeno-associated viruses, the implications for patients with this exposure, the assays used to detect preexisting immune responses, and strategies to circumvent preexisting adaptive immunity to expand the patient base that could benefit from such therapies.

Unmet needs and evolving treatment for limb girdle muscular dystrophies

Limb-girdle muscular dystrophies (LGMDs) represent a major group of muscle disorders. Treatment is sorely needed and currently expanding based on safety and efficacy adopting principles of single-dosing gene therapy for monogenic autosomal recessive disorders. Gene therapy has made in-roads for LGMD and this review describes progress that has been achieved for these conditions. This review first provides a background on the definition and classification of LGMDs. The major effort focuses on progress in LGMD gene therapy, from experimental studies to clinical trials. The disorders discussed include the LGMDs where the most work has been done including calpainopathies (LGMD2A/R1), dysferlinopathies (LGMD2B/R2) and sarcoglycanopathies (LGMD2C/R5, LGMD2D/R3, LGMD2E/R4). Early success in clinical trials provides a template to move the field forward and potentially apply emerging technology like CRISPR/Cas9 that may enhance the scope and efficacy of gene therapy applied to patient care.

Assessment of Systemic Delivery of rAAVrh74.MHCK7.micro-dystrophin in Children With Duchenne Muscular Dystrophy: A Nonrandomized Controlled Trial

Question

Is rAAVrh74.MHCK7.micro-dystrophin gene transfer safe and well tolerated in patients with Duchenne muscular dystrophy?

Findings

In this nonrandomized controlled trial of 4 young patients with Duchenne muscular dystrophy, rAAVrh74.MHCK7.micro-dystrophin gene transfer was well tolerated, with minimal adverse events, and was associated with robust micro-dystrophin expression, reduced serum creatine kinase levels, and functional improvement as measured by the North Star Ambulatory Assessment.

Meaning

These results indicated the safe systemic delivery of micro-dystrophin transgene and targeted expression of functional micro-dystrophin protein product, suggesting the potential for rAAVrh74.MHCK7.micro-dystrophin to provide clinically meaningful functional improvement that is greater than the standard of care.

Importance

Micro-dystrophin gene transfer shows promise for treating patients with Duchenne muscular dystrophy (DMD) using recombinant adeno-associated virus serotype rh74 (rAAVrh74) and codon-optimized human micro-dystrophin driven by a skeletal and cardiac muscle-specific promoter with enhanced cardiac expression (MHCK7).

Objective

To identify the 1-year safety and tolerability of intravenous rAAVrh74.MHCK7.micro-dystrophin in patients with DMD.

Design, Setting, and Participants

This open-label, phase 1/2a nonrandomized controlled trial was conducted at the Nationwide Children’s Hospital in Columbus, Ohio. It began on November 2, 2017, with a planned duration of follow-up of 3 years, ending in March 2021. The first 4 patients who met eligibility criteria were enrolled, consisting of ambulatory male children with DMD without preexisting AAVrh74 antibodies and a stable corticosteroid dose (≥12 weeks).

Interventions

A single dose of 2.0 × 10¹⁴ vg/kg rAAVrh74.MHCK7.micro-dystrophin was infused through a peripheral limb vein. Daily prednisolone, 1 mg/kg, started 1 day before gene delivery (30-day taper after infusion).

Main Outcomes and Measures

Safety was the primary outcome. Secondary outcomes included micro-dystrophin expression by Western blot and immunohistochemistry. Functional outcomes measured by North Star Ambulatory Assessment (NSAA) and serum creatine kinase were exploratory outcomes.

Results

Four patients were included (mean [SD] age at enrollment, 4.8 [1.0] years). All adverse events (n = 53) were considered mild (33 [62%]) or moderate (20 [38%]), and no serious adverse events occurred. Eighteen adverse events were considered treatment related, the most common of which was vomiting (9 of 18 events [50%]). Three patients had transiently elevated γ-glutamyltransferase, which resolved with corticosteroids. At 12 weeks, immunohistochemistry of gastrocnemius muscle biopsy specimens revealed robust transgene expression in all patients, with a mean of 81.2% of muscle fibers expressing micro-dystrophin with a mean intensity of 96% at the sarcolemma. Western blot showed a mean expression of 74.3% without fat or fibrosis adjustment and 95.8% with adjustment. All patients had confirmed vector transduction and showed functional improvement of NSAA scores and reduced creatine kinase levels (posttreatment vs baseline) that were maintained for 1 year.

Conclusions and Relevance

This trial showed rAAVrh74.MHCK7.micro-dystrophin to be well tolerated and have minimal adverse events; the safe delivery of micro-dystrophin transgene; the robust expression and correct localization of micro-dystrophin protein; and improvements in creatine kinase levels and NSAA scores. These findings suggest that rAAVrh74.MHCK7.micro-dystrophin can provide functional improvement that is greater than that observed under standard of care.

Trial Registration

ClinicalTrials.gov Identifier: NCT03375164

Evaluation of the Lipid-binding Properties of Recombinant Dystrophin Spectrin-like Repeat Domains R1-3

Recombinant micro-dystrophin genes are designed to treat Duchenne muscular dystrophy (DMD) by retaining dystrophin domains believed to play key functional roles while fitting the packaging capacity of adeno-associated virus vectors. Domains R1-3 are important for muscle force generation and for association with the sarcolemma, but the nature of this interaction is not fully understood. We measured lipid-binding affinity of 3 peptides containing different spectrin-like repeat modules (R1-3; R1-2; and R1, 2, 22). Lipid-binding affinity was highest with R1-3, suggesting that the complete R1-R3 region could be beneficial and should be considered for inclusion in micro-dystrophin constructs.

Preclinical Systemic Delivery of Adeno-Associated α-Sarcoglycan Gene Transfer for Limb-Girdle Muscular Dystrophy

Limb-girdle muscular dystrophy type 2D/R3 (LGMD2D/R3) is a progressive muscular dystrophy that manifests with muscle weakness, respiratory abnormalities, and in rare cases cardiomyopathy. LGMD2D/R3 is caused by mutations in the SGCA gene resulting in loss of protein and concomitant loss of some or all components of the dystrophin-associated glycoprotein complex. The sgca-null (sgca^−/−) mouse recapitulates the clinical phenotype of patients with LGMD2D/R3, including dystrophic features such as muscle necrosis and fibrosis, elevated serum creatine kinase (CK), and reduction in the generation of absolute muscle force and locomotor activity. Thus, sgca^−/− mice provide a relevant model to test the safety and efficacy of gene transfer. We designed a self-complementary AAVrh74 vector containing a codon-optimized full-length human SGCA (hSGCA) transgene driven by a muscle-specific promoter, shortened muscle creatine kinase (tMCK). In this report, we test the efficacy and safety of scAAVrh74.tMCK.hSGCA in sgca^−/− mice using a dose-escalation design to evaluate a single systemic injection of 1.0 × 10¹², 3.0 × 10¹², and 6.0 × 10¹² vg total dose compared with vehicle-treatment and wild-type mice. In sgca^−/− mice, treatment with scAAVrh74.tMCK.hSGCA resulted in robust expression of α-sarcoglycan protein at the sarcolemma membrane in skeletal muscle at all doses tested. In addition, scAAVrh74.tMCK.hSGCA was effective in improving the histopathology of limb and diaphragm muscle of sgca^−/− mice, as indicated by reductions in fibrosis, central nucleation, and normalization of myofiber size. These molecular changes were concomitant with significant increases in specific force generation in the diaphragm and tibialis anterior muscle, protection against eccentric force loss, and reduction in serum CK. Locomotor activity was improved at all doses of vector-treated compared with vehicle-treated sgca^−/− mice. Lastly, vector toxicity was not detected in a serum chemistry panel and by gross necropsy. Collectively, these findings provide support for a systemic delivery of scAAVrh74.tMCK.hSGCA in a clinical setting for the treatment of LGMD2D/R3.

Dose-Escalation Study of Systemically Delivered rAAVrh74.MHCK7.micro-dystrophin in the mdx Mouse Model of Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is a rare, X-linked, fatal, degenerative neuromuscular disease caused by mutations in the DMD gene. More than 2,000 mutations of the DMD gene are responsible for progressive loss of muscle strength, loss of ambulation, and generally respiratory and cardiac failure by age 30. Recently, gene transfer therapy has received widespread interest as a disease-modifying treatment for all patients with DMD. We designed an adeno-associated virus vector (rAAVrh74) containing a codon-optimized human micro-dystrophin transgene driven by a skeletal and cardiac muscle-specific promoter, MHCK7. To test the efficacy of rAAVrh74.MHCK7.micro-dystrophin, we evaluated systemic injections in mdx (dystrophin-null) mice at low (2 × 10¹² vector genome [vg] total dose, 8 × 10¹³ vg/kg), intermediate (6 × 10¹² vg total dose, 2 × 10¹⁴ vg/kg), and high doses (1.2 × 10¹³ vg total dose, 6 × 10¹⁴ vg/kg). Three months posttreatment, specific force increased in the diaphragm (DIA) and tibialis anterior muscle, with intermediate and high doses eliciting force outputs at wild-type (WT) levels. Histological improvement included reductions in fibrosis and normalization of myofiber size, specifically in the DIA, where results for low and intermediate doses were not significantly different from the WT. Significant reduction in central nucleation was also observed, although complete normalization to WT was not seen. No vector-associated toxicity was reported either by clinical or organ-specific laboratory assessments or following formal histopathology. The findings in this preclinical study provided proof of principle for safety and efficacy of systemic delivery of rAAVrh74.MHCK7.micro-dystrophin at high vector titers, supporting initiation of a Phase I/II safety study in boys with DMD.

Assessment of rAAVrh.74.MHCK7.micro-dystrophin Gene Therapy Using Magnetic Resonance Imaging in Children With Duchenne Muscular Dystrophy

This case-control study uses magnetic resonance imaging and spectroscopy to evaluate the association between treatment with recombinant adeno-associated virus serotype rh74 (rAAVrh74) and muscle quality in children with Duchenne muscular dystrophy.

Current Clinical Applications of In Vivo Gene Therapy with AAVs

Hereditary diseases are caused by mutations in genes, and more than 7,000 rare diseases affect over 30 million Americans. For more than 30 years, hundreds of researchers have maintained that genetic modifications would provide effective treatments for many inherited human diseases, offering durable and possibly curative clinical benefit with a single treatment. This review is limited to gene therapy using adeno-associated virus (AAV) because the gene delivered by this vector does not integrate into the patient genome and has a low immunogenicity. There are now five treatments approved for commercialization and currently available, i.e., Luxturna, Zolgensma, the two chimeric antigen receptor T cell (CAR-T) therapies (Yescarta and Kymriah), and Strimvelis (the gammaretrovirus approved for adenosine deaminase-severe combined immunodeficiency [ADA-SCID] in Europe). Dozens of other treatments are under clinical trials. The review article presents a broad overview of the field of therapy by in vivo gene transfer. We review gene therapy for neuromuscular disorders (spinal muscular atrophy [SMA]; Duchenne muscular dystrophy [DMD]; X-linked myotubular myopathy [XLMTM]; and diseases of the central nervous system, including Alzheimer’s disease, Parkinson’s disease, Canavan disease, aromatic l-amino acid decarboxylase [AADC] deficiency, and giant axonal neuropathy), ocular disorders (Leber congenital amaurosis, age-related macular degeneration [AMD], choroideremia, achromatopsia, retinitis pigmentosa, and X-linked retinoschisis), the bleeding disorder hemophilia, and lysosomal storage disorders.

Clinical development on the frontier: gene therapy for duchenne muscular dystrophy

Introduction: The development of adeno-associated virus (AAV) vectors as safe vehicles for in vivo delivery of therapeutic genes has been a major milestone in the advancement of gene therapy, enabling a promising strategy for ameliorating a wide range of diseases, including Duchenne muscular dystrophy (DMD).Areas covered: Based on experience with the development of a gene transfer therapy agent for DMD, we discuss ways in which gene therapy for rare disease challenges traditional clinical development paradigms, and recommend a step-wise approach for design and evaluation to support broader applicability of gene therapy.Expert opinion: The gene therapy development approach should intentionally design the therapeutic construct and the clinical study to systematically evaluate agent delivery, safety, and efficacy. Rigorous preclinical work is essential for establishing an effective gene delivery platform and determining the efficacious dose. Clinical studies should thoroughly evaluate transduction, on-target transgene expression at the tissue and cellular level, and functional efficacy.

Gene Delivery for Limb-Girdle Muscular Dystrophy Type 2D by Isolated Limb Infusion

In a previous limb-girdle muscular dystrophy type 2D (LGMD2D) clinical trial, robust alpha-sarcoglycan gene expression was confirmed following intramuscular gene (SGCA) transfer. This paved the way for first-in-human isolated limb infusion (ILI) gene transfer trial to the lower limbs. Delivery of scAAVrh74.tMCK.hSGCA via an intravascular route through the femoral artery predicted improved ambulation. This method was initially chosen to avoid safety concerns required for large systemic vascular delivery viral loads. ILI methods were adopted from the extensive chemotherapy experience for treatment of malignancies confined to the extremities. Six LGMD2D subjects were enrolled in a dose-ascending open-label clinical trial. Safety of the procedure was initially assessed in the single limb of a non-ambulant affected adult at a dose of 1 × 1012 vg/kg. Subsequently, ambulatory children (aged 8-13 years) were enrolled and dosed bilaterally with either 1 × 1012 vg/kg/limb or 3 × 1012 vg/kg/limb. The six-minute walk test (6MWT) served as the primary clinical outcome; secondary outcomes included muscle strength (maximum voluntary isometric force testing) and SGCA expression at 6 months. All ambulatory participants except one had pre- and post-treatment muscle biopsies. All four subjects biopsied had confirmed SGCA gene delivery by immunofluorescence, Western blot analysis (14-25% of normal), and vector genome copies (5.4 × 103-7.7 × 104 vg/μg). Muscle strength in the knee extensors (assessed by force generation in kilograms) showed improvement in two subjects that correlated with an increase in fiber diameter post gene delivery. Six-minute walk times decreased or remained the same. Vascular delivery of AAVrh74.tMCK.hSGCA was effective at producing SGCA protein at low doses that correlated with vector copies and local functional improvement restricted to targeted muscles. Future trials will focus on systemic administration to enable targeting of proximal muscles to maximize clinical benefit.

Long-term treatment with eteplirsen in nonambulatory patients with Duchenne muscular dystrophy

This analysis aims to describe the outcomes of two nonambulatory patients with Duchenne muscular dystrophy (DMD) who participated in two clinical studies. The two consecutive trials of eteplirsen (studies 201 and 202) were conducted in patients with DMD (N = 12) and confirmed genetic mutations amenable to exon 51 skipping.In study 201, 12 patients were randomized to receive once-weekly, double-blind intravenous infusions of eteplirsen 30 or 50 mg/kg or placebo for 24 weeks; patients then received open-label eteplirsen during weeks 25 through 28. All 12 patients continued onto open-label extension study 202 and received long-term treatment with eteplirsen. We compared cardiac, pulmonary, and upper limb function and dystrophin production in the nonambulatory twin patients versus the 10 ambulatory patients through 240 combined treatment weeks.Ten study patients remained ambulatory through both studies, while the identical twin patients both experienced early, rapid loss of ambulation. The twin patients had greater disease severity at baseline (6-minute walk test [6MWT], 330 and 256 m) versus the other patients (n = 10; 6MWT range, 341-418 m). They maintained cardiac and upper limb function through combined week 240, with outcomes similar to those of the patients who remained ambulatory. Dystrophin production was confirmed following eteplirsen treatment.Despite the loss of ambulation, other markers of disease progression remained relatively stable in the eteplirsen-treated twin patients and were similar to those of the ambulatory patients.

AAV-mediated follistatin gene therapy improves functional outcomes in the TIC-DUX4 mouse model of FSHD

Facioscapulohumeral muscular dystrophy (FSHD) is an autosomal dominant or digenic disorder linked to derepression of the toxic DUX4 gene in muscle. There is currently no pharmacological treatment. The emergence of DUX4 enabled development of cell and animal models that could be used for basic and translational research. Since DUX4 is toxic, animal model development has been challenging, but progress has been made, revealing that tight regulation of DUX4 expression is critical for creating viable animals that develop myopathy. Here, we report such a model - the tamoxifen-inducible FSHD mouse model called TIC-DUX4. Uninduced animals are viable, born in Mendelian ratios, and overtly indistinguishable from WT animals. Induced animals display significant DUX4-dependent myopathic phenotypes at the molecular, histological, and functional levels. To demonstrate the utility of TIC-DUX4 mice for therapeutic development, we tested a gene therapy approach aimed at improving muscle strength in DUX4-expressing muscles using adeno-associated virus serotype 1.Follistatin (AAV1.Follistatin), a natural myostatin antagonist. This strategy was not designed to modulate DUX4 but could offer a mechanism to improve muscle weakness caused by DUX4-induced damage. AAV1.Follistatin significantly increased TIC-DUX4 muscle mass and strength even in the presence of DUX4 expression, suggesting that myostatin inhibition may be a promising approach to treat FSHD-associated weakness. We conclude that TIC-DUX4 mice are a relevant model to study DUX4 toxicity and, importantly, are useful in therapeutic development studies for FSHD.

An Isolated Limb Infusion Method Allows for Broad Distribution of rAAVrh74.MCK.GALGT2 to Leg Skeletal Muscles in the Rhesus Macaque

Recombinant adeno-associated virus (rAAV)rh74.MCK.GALGT2 is a muscle-specific gene therapy that is being developed to treat forms of muscular dystrophy. Here we report on an isolated limb infusion technique in a non-human primate model, where hindlimb blood flow is transiently isolated using balloon catheters to concentrate vector in targeted leg muscles. A bilateral dose of 2.5 × 1013 vector genomes (vg)/kg/limb was sufficient to induce GALGT2-induced glycosylation in 10%-60% of skeletal myofibers in all leg muscles examined. There was a 19-fold ± 6-fold average limb-wide increase in vector genomes per microgram genomic DNA at a bilateral dose of 2.5 × 1013 vg/kg/limb compared with a bilateral dose of 6 × 1012 vg/kg/limb. A unilateral dose of 6 × 1013 vg/kg/limb showed a 12- ± 3-fold increase in treated limb muscles compared to contralateral untreated limb muscles, which received vector only after release into the systemic circulation from the treated limb. Variability in AAV biodistribution between different segments of the same muscle was 125% ± 18% for any given dose, while variability between the same muscle for any given treatment dose was 45% ± 7%. These experiments demonstrate that treatment of muscles throughout the leg with rAAVrh74.MCK.GALGT2 can be accomplished safely using an isolated limb infusion technique, where balloon catheters transiently isolate the limb vasculature, but that intra- and inter-muscle transduction variability is a significant issue.

Systemic Delivery of Dysferlin Overlap Vectors Provides Long-Term Gene Expression and Functional Improvement for Dysferlinopathy

Dysferlinopathies comprise a family of disorders caused by mutations in the dysferlin (DYSF) gene, leading to a progressive dystrophy characterized by chronic muscle fiber loss, fat replacement, and fibrosis. To correct the underlying histopathology and function, expression of full-length DYSF is required. Dual adeno-associated virus vectors have been developed, defined by a region of homology, to serve as a substrate for reconstitution of the full 6.5 kb dysferlin cDNA. Previous work studied the efficacy of this treatment through intramuscular and regional delivery routes. To maximize clinical efficacy, dysferlin-deficient mice were treated systemically to target all muscles through the vasculature for efficacy and safety studies. Mice were evaluated at multiple time points between 4 and 13 months post treatment for dysferlin expression and functional improvement using magnetic resonance imaging and magnetic resonance spectroscopy and membrane repair. A systemic dose of 6 × 1012 vector genomes resulted in widespread gene expression in the muscles. Treated muscles showed a significant decrease in central nucleation, collagen deposition, and improvement of membrane repair to wild-type levels. Treated gluteus muscles were significantly improved compared to placebo-treated muscles and were equivalent to wild type in volume, intra- and extramyocellular lipid accumulation, and fat percentage using magnetic resonance imaging and magnetic resonance spectroscopy. Dual-vector treatment allows for production of full-length functional dysferlin with no toxicity. This confirms previous safety data and validates translation of systemic gene delivery for dysferlinopathy patients.

Eteplirsen treatment for Duchenne muscular dystrophy: Exon skipping and dystrophin production

OBJECTIVE

To describe the quantification of novel dystrophin production in patients with Duchenne muscular dystrophy (DMD) after long-term treatment with eteplirsen.

METHODS

Clinical study 202 was an observational, open-label extension of the randomized, controlled study 201 assessing the safety and efficacy of eteplirsen in patients with DMD with a confirmed mutation in the DMD gene amenable to correction by skipping of exon 51. Patients received once-weekly IV doses of eteplirsen 30 or 50 mg/kg. Upper extremity muscle biopsy samples were collected at combined study week 180, blinded, and assessed for dystrophin-related content by Western blot, Bioquant software measurement of dystrophin-associated immunofluorescence intensity, and percent dystrophin-positive fibers (PDPF). Results were contrasted with matched untreated biopsies from patients with DMD. Reverse transcription PCR followed by Sanger sequencing of newly formed slice junctions was used to confirm the mechanism of action of eteplirsen.

RESULTS

Reverse transcription PCR analysis and sequencing of the newly formed splice junction confirmed that 100% of treated patients displayed the expected skipped exon 51 sequence. In treated patients vs untreated controls, Western blot analysis of dystrophin content demonstrated an 11.6-fold increase (p = 0.007), and PDPF analysis demonstrated a 7.4-fold increase (p < 0.001). The PDPF findings were confirmed in a re-examination of the sample (15.5-fold increase, p < 0.001). Dystrophin immunofluorescence intensity was 2.4-fold greater in treated patients than in untreated controls (p < 0.001).

CONCLUSION

Taken together, the 4 assays, each based on unique evaluation mechanisms, provided evidence of eteplirsen muscle cell penetration, exon skipping, and induction of novel dystrophin expression.

CLASSIFICATION OF EVIDENCE

This study provides Class II evidence of the muscle cell penetration, exon skipping, and induction of novel dystrophin expression by eteplirsen, as confirmed by 4 assays.

Single-Dose Gene-Replacement Therapy for Spinal Muscular Atrophy

BACKGROUND

Spinal muscular atrophy type 1 (SMA1) is a progressive, monogenic motor neuron disease with an onset during infancy that results in failure to achieve motor milestones and in death or the need for mechanical ventilation by 2 years of age. We studied functional replacement of the mutated gene encoding survival motor neuron 1 (SMN1) in this disease.

METHODS

Fifteen patients with SMA1 received a single dose of intravenous adeno-associated virus serotype 9 carrying SMN complementary DNA encoding the missing SMN protein. Three of the patients received a low dose (6.7×10¹³ vg per kilogram of body weight), and 12 received a high dose (2.0×10¹⁴ vg per kilogram). The primary outcome was safety. The secondary outcome was the time until death or the need for permanent ventilatory assistance. In exploratory analyses, we compared scores on the CHOP INTEND (Children's Hospital of Philadelphia Infant Test of Neuromuscular Disorders) scale of motor function (ranging from 0 to 64, with higher scores indicating better function) in the two cohorts and motor milestones in the high-dose cohort with scores in studies of the natural history of the disease (historical cohorts).

RESULTS

As of the data cutoff on August 7, 2017, all 15 patients were alive and event-free at 20 months of age, as compared with a rate of survival of 8% in a historical cohort. In the high-dose cohort, a rapid increase from baseline in the score on the CHOP INTEND scale followed gene delivery, with an increase of 9.8 points at 1 month and 15.4 points at 3 months, as compared with a decline in this score in a historical cohort. Of the 12 patients who had received the high dose, 11 sat unassisted, 9 rolled over, 11 fed orally and could speak, and 2 walked independently. Elevated serum aminotransferase levels occurred in 4 patients and were attenuated by prednisolone.

CONCLUSIONS

In patients with SMA1, a single intravenous infusion of adeno-associated viral vector containing DNA coding for SMN resulted in longer survival, superior achievement of motor milestones, and better motor function than in historical cohorts. Further studies are necessary to confirm the safety and efficacy of this gene therapy. (Funded by AveXis and others; ClinicalTrials.gov number, NCT02122952 .).

MicroRNA-29 overexpression by adeno-associated virus suppresses fibrosis and restores muscle function in combination with micro-dystrophin

Duchenne muscular dystrophy (DMD) is caused by dystrophin deficiency resulting in progressive muscle weakness and fibrotic scarring. Muscle fibrosis impairs blood flow, hampering muscle repair and regeneration. Irrespective of the success of gene restoration, functional improvement is limited without reducing fibrosis. The levels of miR-29c, a known regulator of collagen, are reduced in DMD. Our goal is to develop translational, antifibrotic therapy by overexpressing miR-29c. We injected the gastrocnemius muscle with either self-complementary AAV.CMV.miR-29c or single-stranded AAV.MCK.micro-dystrophin alone or in combination in the mdx/utrn+/- mouse, a DMD mouse model. Treatment of 3-month-old mdx/utrn+/- mice with AAV.miR-29c showed a reduction in collagen and increased absolute and specific force compared with untreated animals, but neither parameter reached WT levels. Combinatorial gene delivery in 3-month-old mdx/utrn+/- mice further decreased fibrosis, and showed a reduction of transcript levels for Col1A, Col3A, fibronectin, and Tgfb1. In addition, absolute and specific force was normalized and equivalent to WT. However, protection against eccentric contraction fell short of WT levels at this time point. When this same mouse model was treated with miR-29c/micro-dystrophin combinatorial therapy at 1 month of age, there was complete normalization of specific and absolute force and protection against eccentric contraction-induced injury was comparable to WT. These findings highlight the potential for miR-29c as an important addition to the armamentarium for translational gene therapy, especially when used in combination with micro-dystrophin in DMD.

Induction of T-Cell Infiltration and Programmed Death Ligand 2 Expression by Adeno-Associated Virus in Rhesus Macaque Skeletal Muscle and Modulation by Prednisone

Use of adeno-associated virus (AAV) to transduce genes into skeletal muscles can be associated with T-cell responses to viral capsid and/or to transgenic protein. Intramuscular mononuclear cell infiltrates primarily consisting of CD8+ T cells and also containing FOXP3+ regulatory T cells were present in rhesus macaque skeletal muscle treated with rAAVrh74.MCK.GALGT2 by vascular delivery. Administration of oral prednisone prior to AAV gene delivery and throughout the study reduced such infiltrates by 60% at 24 weeks post AAV delivery compared with AAV-treated animals not receiving prednisone, regardless of the presence of pre-existing AAV serum antibodies at the time of treatment. The majority of CD8+ T cells in AAV-treated muscles expressed activated caspase 3 and programmed cell death protein 1 (PD1), suggesting ongoing programmed cell death. AAV-transduced skeletal muscles also had elevated expression of programmed death ligand 2 (PDL2) on skeletal myofibers, and this increase in expression extended to muscles where transgene was not overexpressed. These data demonstrate that prednisone can reduce the extent of intramuscular T-cell infiltrates in AAV-treated muscles, which may aid in achieving long-term transgene expression, as may the induction of PDL2 expression on skeletal myofibers to promote PD1-mediated programmed T-cell death.

Systemic AAV-Mediated β-Sarcoglycan Delivery Targeting Cardiac and Skeletal Muscle Ameliorates Histological and Functional Deficits in LGMD2E Mice

Limb-girdle muscular dystrophy type 2E (LGMD2E), resulting from mutations in β-sarcoglycan (SGCB), is a progressive dystrophy with deteriorating muscle function, respiratory failure, and cardiomyopathy in 50% or more of LGMD2E patients. SGCB knockout mice share many of the phenotypic deficiencies of LGMD2E patients. To investigate systemic SGCB gene transfer to treat skeletal and cardiac muscle deficits, we designed a self-complementary AAVrh74 vector containing a codon-optimized human SGCB transgene driven by a muscle-specific promoter. We delivered scAAV.MHCK7.hSGCB through the tail vein of SGCB^-/- mice to provide a rationale for a clinical trial that would lead to clinically meaningful results. This led to 98.1% transgene expression across all muscles that was accompanied by improvements in histopathology. Serum creatine kinase (CK) levels were reduced following treatment by 85.5%. Diaphragm force production increased by 94.4%, kyphoscoliosis of the spine was significantly reduced by 48.1%, overall ambulation increased by 57%, and vertical rearing increased dramatically by 132% following treatment. Importantly, no adverse effects were seen in muscle of wild-type mice injected systemically with scAAV.hSGCB. In this well-defined model of LGMD2E, we have demonstrated the efficacy and safety of systemic scAAV.hSGCB delivery, and these findings have established a path for clinically beneficial AAV-mediated gene therapy for LGMD2E.

Follistatin Gene Therapy for Sporadic Inclusion Body Myositis Improves Functional Outcomes

Sporadic inclusion body myositis, a variant of inflammatory myopathy, has features distinct from polymyositis/dermatomyositis. The disease affects men more than women, most commonly after age 50. Clinical features include weakness of the quadriceps, finger flexors, ankle dorsiflexors, and dysphagia. The distribution of weakness is similar to Becker muscular dystrophy, where we previously reported improvement following intramuscular injection of an isoform of follistatin (FS344) by AAV1. For this clinical trial, rAAV1.CMV.huFS344, 6 × 10¹¹ vg/kg, was delivered to the quadriceps muscles of both legs of six sporadic inclusion body myositis subjects. The primary outcome for this trial was distance traveled for the 6-min walk test. The protocol included an exercise regimen for each participant. Performance, annualized to a median 1-year change, improved +56.0 m/year for treated subjects compared to a decline of -25.8 m/year (p = 0.01) in untreated subjects (n = 8), matched for age, gender, and baseline measures. Four of the six treated subjects showed increases ranging from 58-153 m, whereas two were minimally improved (5-23 m). Treatment effects included decreased fibrosis and improved regeneration. These findings show promise for follistatin gene therapy for mild to moderately affected, ambulatory sporadic inclusion body myositis patients. More advanced disease with discernible muscle loss poses challenges.

Duchenne muscular dystrophy: CRISPR/Cas9 treatment

A novel approach to gene correction by genome editing shows great promise as a treatment for Duchenne muscular dystrophy (DMD). CRISPR/Cas9 delivered by adeno-associated virus to a mouse model for DMD demonstrated improvement in function and histology.

Defective membrane fusion and repair in Anoctamin5-deficient muscular dystrophy

Limb-girdle muscular dystrophies are a genetically diverse group of diseases characterized by chronic muscle wasting and weakness. Recessive mutations in ANO5 (TMEM16E) have been directly linked to several clinical phenotypes including limb-girdle muscular dystrophy type 2L and Miyoshi myopathy type 3, although the pathogenic mechanism has remained elusive. ANO5 is a member of the Anoctamin/TMEM16 superfamily that encodes both ion channels and regulators of membrane phospholipid scrambling. The phenotypic overlap of ANO5 myopathies with dysferlin-associated muscular dystrophies has inspired the hypothesis that ANO5, like dysferlin, may be involved in the repair of muscle membranes following injury. Here we show that Ano5-deficient mice have reduced capacity to repair the sarcolemma following laser-induced damage, exhibit delayed regeneration after cardiotoxin injury and suffer from defective myoblast fusion necessary for the proper repair and regeneration of multinucleated myotubes. Together, these data suggest that ANO5 plays an important role in sarcolemmal membrane dynamics. Genbank Mouse Genome Informatics accession no. 3576659.

Follistatin Gene Therapy Improves Ambulation in Becker Muscular Dystrophy

Follistatin is a ubiquitous secretory propeptide that functions as a potent inhibitor of the myostatin pathway, resulting in an increase in skeletal muscle mass. Its ability to interact with the pituitary activin-inhibin axis and suppress the secretion of follicle-stimulating hormone (FSH) called for caution in its clinical applicability. This limitation was circumvented by the use of one of the alternatively spliced follistatin variants, FS344, undergoing post-translational modification to FS315. This follistatin isoform is serum-based, and has a 10-fold lower affinity to activin compared to FS288. Preclinical studies of intramuscular delivery of the follistatin gene demonstrated safety and efficacy in enhancing muscle mass. We herein review the evidence supporting the utility of follistatin as a genetic enhancer to improve cellular performance. In addition, we shed light on the results of the first clinical gene transfer trial using the FS344 isoform of follistatin in subjects with Becker muscular dystrophy as well as the future directions for clinical gene therapy trials using follistatin.

β-Sarcoglycan gene transfer decreases fibrosis and restores force in LGMD2E mice

Limb-girdle muscular dystrophy type 2E (LGMD2E) results from mutations in the β-sarcoglycan (SGCB) gene causing loss of functional protein and concomitant loss of dystrophin-associated proteins. The disease phenotype is characterized by muscle weakness and wasting, and dystrophic features including muscle fiber necrosis, inflammation and fibrosis. The Sgcb-null mouse recapitulates the clinical phenotype with significant endomysial fibrosis providing a relevant model to test whether gene replacement will be efficacious. We directly addressed this question using a codon optimized human β-sarcoglycan gene (hSGCB) driven by a muscle-specific tMCK promoter (scAAVrh74.tMCK.hSGCB). Following isolated limb delivery (5 × 10(11) vector genome (vg)), 91.2% of muscle fibers in the lower limb expressed β-sarcoglycan, restoring assembly of the sarcoglycan complex and protecting the membrane from Evans blue dye leakage. Histological outcomes were significantly improved including decreased central nucleation, normalization of muscle fiber size, decreased macrophages and inflammatory mononuclear cells, and an average of a 43% reduction in collagen deposition in treated muscle compared with untreated muscle at end point. These measures correlated with improvement of tetanic force and resistance to eccentric contraction. In 6-month-old mice, as indicated by collagen staining, scAAVrh74.tMCK.hSGCB treatment reduced fibrosis by 42%. This study demonstrates the potential for gene replacement to reverse debilitating fibrosis, typical of muscular dystrophy, thereby providing compelling evidence for movement to clinical gene replacement for LGMD2E.

Human α7 Integrin Gene (ITGA7) Delivered by Adeno-Associated Virus Extends Survival of Severely Affected Dystrophin/Utrophin-Deficient Mice

Duchenne muscular dystrophy (DMD) is caused by mutations in the DMD gene. It is the most common, severe childhood form of muscular dystrophy. We investigated an alternative to dystrophin replacement by overexpressing ITGA7 using adeno-associated virus (AAV) delivery. ITGA7 is a laminin receptor in skeletal muscle that, like the dystrophin-glycoprotein complex, links the extracellular matrix to the internal actin cytoskeleton. ITGA7 is expressed in DMD patients and overexpression does not elicit an immune response to the transgene. We delivered rAAVrh.74.MCK.ITGA7 systemically at 5-7 days of age to the mdx/utrn(-/-) mouse deficient for dystrophin and utrophin, a severe mouse model of DMD. At 8 weeks postinjection, widespread expression of ITGA7 was observed at the sarcolemma of multiple muscle groups following gene transfer. The increased expression of ITGA7 significantly extended longevity and reduced common features of the mdx/utrn(-/-) mouse, including kyphosis. Overexpression of α7 expression protected against loss of force following contraction-induced damage and increased specific force in the diaphragm and EDL muscles 8 weeks after gene transfer. Taken together, these results further support the use of α7 integrin as a potential therapy for DMD.

AAV.Dysferlin Overlap Vectors Restore Function in Dysferlinopathy Animal Models

OBJECTIVE

Dysferlinopathies are a family of untreatable muscle disorders caused by mutations in the dysferlin gene. Lack of dysferlin protein results in progressive dystrophy with chronic muscle fiber loss, inflammation, fat replacement, and fibrosis; leading to deteriorating muscle weakness. The objective of this work is to demonstrate efficient and safe restoration of dysferlin expression following gene therapy treatment.

METHODS

Traditional gene therapy is restricted by the packaging capacity limit of adeno-associated virus (AAV), however, use of a dual vector strategy allows for delivery of over-sized genes, including dysferlin. The two vector system (AAV.DYSF.DV) packages the dysferlin cDNA utilizing AAV serotype rh.74 through the use of two discrete vectors defined by a 1 kb region of homology. Delivery of AAV.DYSF.DV via intramuscular and vascular delivery routes in dysferlin deficient mice and nonhuman primates was compared for efficiency and safety.

RESULTS

Treated muscles were tested for dysferlin expression, overall muscle histology, and ability to repair following injury. High levels of dysferlin overexpression was shown for all muscle groups treated as well as restoration of functional outcome measures (membrane repair ability and diaphragm specific force) to wild-type levels. In primates, strong dysferlin expression was demonstrated with no safety concerns.

INTERPRETATION

Treated muscles showed high levels of dysferlin expression with functional restoration with no evidence of toxicity or immune response providing proof of principle for translation to dysferlinopathy patients.

Translation from a DMD exon 5 IRES results in a functional dystrophin isoform that attenuates dystrophinopathy in humans and mice

Most mutations that truncate the reading frame of the DMD gene cause loss of dystrophin expression and lead to Duchenne muscular dystrophy. However, amelioration of disease severity can result from alternate translation initiation beginning in DMD exon 6 that leads to expression of a highly functional N-truncated dystrophin. This novel isoform results from usage of an internal ribosome entry site (IRES) within exon 5 that is glucocorticoid-inducible. IRES activity is confirmed in patient muscle by both peptide sequencing and ribosome profiling. Generation of a truncated reading frame upstream of the IRES by exon skipping leads to synthesis of a functional N-truncated isoform in both patient-derived cell lines and in a new DMD mouse model, where expression protects muscle from contraction-induced injury and corrects muscle force to the same level as control mice. These results support a novel therapeutic approach for patients with mutations within the 5’ exons of DMD.

Plasmapheresis eliminates the negative impact of AAV antibodies on microdystrophin gene expression following vascular delivery

Duchenne muscular dystrophy is a monogenic disease potentially treatable by gene replacement. Use of recombinant adeno-associated virus (AAV) will ultimately require a vascular approach to broadly transduce muscle cells. We tested the impact of preexisting AAV antibodies on microdystrophin expression following vascular delivery to nonhuman primates. Rhesus macaques were treated by isolated limb perfusion using a fluoroscopically guided catheter. In addition to serostatus stratification, the animals were placed into one of the three immune suppression groups: no immune suppression, prednisone, and triple immune suppression (prednisone, tacrolimus, and mycophenolate mofetil). The animals were analyzed for transgene expression at 3 or 6 months. Microdystrophin expression was visualized in AAV, rhesus serotype 74 sero-negative animals (mean: 48.0 ± 20.8%) that was attenuated in sero-positive animals (19.6 ± 18.7%). Immunosuppression did not affect transgene expression. Importantly, removal of AAV binding antibodies by plasmapheresis in AAV sero-positive animals resulted in high-level transduction (60.8 ± 18.0%), which is comparable with that of AAV sero-negative animals (53.7 ± 7.6%), whereas non-pheresed sero-positive animals demonstrated significantly lower transduction levels (10.1 ± 6.0%). These data support the hypothesis that removal of AAV binding antibodies by plasmapheresis permits successful and sustained gene transfer in the presence of preexisting immunity (natural infection) to AAV.

Eteplirsen for the treatment of Duchenne muscular dystrophy

OBJECTIVE

In prior open-label studies, eteplirsen, a phosphorodiamidate morpholino oligomer, enabled dystrophin production in Duchenne muscular dystrophy (DMD) with genetic mutations amenable to skipping exon 51. The present study used a double-blind placebo-controlled protocol to test eteplirsen's ability to induce dystrophin production and improve distance walked on the 6-minute walk test (6MWT).

METHODS

DMD boys aged 7 to 13 years, with confirmed deletions correctable by skipping exon 51 and ability to walk 200 to 400 m on 6 MWT, were randomized to weekly intravenous infusions of 30 or 50 mg/kg/wk eteplirsen or placebo for 24 weeks (n = 4/group). Placebo patients switched to 30 or 50 mg/kg eteplirsen (n=2/group) at week 25; treatment was open label thereafter. All patients had muscle biopsies at baseline and week 48. Efficacy included dystrophin-positive fibers and distance walked on the 6MWT.

RESULTS

At week 24, the 30 mg/kg eteplirsen patients were biopsied, and percentage of dystrophin-positive fibers was increased to 23% of normal; no increases were detected in placebo-treated patients (p≤0.002). Even greater increases occurred at week 48 (52% and 43% in the 30 and 50 mg/kg cohorts, respectively), suggesting that dystrophin increases with longer treatment. Restoration of functional dystrophin was confirmed by detection of sarcoglycans and neuronal nitric oxide synthase at the sarcolemma. Ambulation-evaluable eteplirsen-treated patients experienced a 67.3 m benefit compared to placebo/delayed patients (p≤0.001).

INTERPRETATION

Eteplirsen restored dystrophin in the 30 and 50 mg/kg/wk cohorts, and in subsequently treated, placebo-controlled subjects. Duration, more than dose, accounted for dystrophin production, also resulting in ambulation stability. No severe adverse events were encountered.

Micro-dystrophin and follistatin co-delivery restores muscle function in aged DMD model

Pharmacologic strategies have provided modest improvement in the devastating muscle-wasting disease, Duchenne muscular dystrophy (DMD). Pre-clinical gene therapy studies have shown promise in the mdx mouse model; however, studies conducted after disease onset fall short of fully correcting muscle strength or protecting against contraction-induced injury. Here we examine the treatment effect on muscle physiology in aged dystrophic mice with significant disease pathology by combining two promising therapies: micro-dystrophin gene replacement and muscle enhancement with follistatin, a potent myostatin inhibitor. Individual treatments with micro-dystrophin and follistatin demonstrated marked improvement in mdx mice but were insufficient to fully restore muscle strength and response to injury to wild-type levels. Strikingly, when combined, micro-dystrophin/follistatin treatment restored force generation and conferred resistance to contraction-induced injury in aged mdx mice. Pre-clinical studies with miniature dystrophins have failed to demonstrate full correction of the physiological defects seen in mdx mice. Importantly, the addition of a muscle enhancement strategy with delivery of follistatin in combination with micro-dystrophin gene therapy completely restored resistance to eccentric contraction-induced injury and improved force. Eccentric contraction-induced injury is a pre-clinical parameter relevant to the exercise induced injury that occurs in DMD patients, and herein, we demonstrate compelling evidence for the therapeutic potential of micro-dystrophin/follistatin combinatorial therapy.

Update on the treatment of Duchenne muscular dystrophy

Duchenne muscular dystrophy is the most severe childhood form of muscular dystrophy caused by mutations in the gene responsible for dystrophin production. There is no cure, and treatment is limited to glucocorticoids that prolong ambulation and drugs to treat the cardiomyopathy. Multiple treatment strategies are under investigation and have shown promise for Duchenne muscular dystrophy. Use of molecular-based therapies that replace or correct the missing or nonfunctional dystrophin protein has gained momentum. These strategies include gene replacement with adeno-associated virus, exon skipping with antisense oligonucleotides, and mutation suppression with compounds that "read through" stop codon mutations. Other strategies include cell therapy and surrogate gene products to compensate for the loss of dystrophin. All of these approaches are discussed in this review, with particular emphasis on the most recent advances made in each therapeutic discipline. The advantages of each approach and challenges in translation are outlined in detail. Individually or in combination, all of these therapeutic strategies hold great promise for treatment of this devastating childhood disease.

Impaired regeneration in LGMD2A supported by increased PAX7-positive satellite cell content and muscle-specific microrna dysregulation

INTRODUCTION

Recent in vitro studies suggest that CAPN3 deficiency leads initially to accelerated myofiber formation followed by depletion of satellite cells (SC). In normal muscle, up-regulation of miR-1 and miR-206 facilitates transition from proliferating SCs to differentiating myogenic progenitors.

METHODS

We examined the histopathological stages, Pax7 SC content, and muscle-specific microRNA expression in biopsy specimens from well-characterized LGMD 2A patients to gain insight into disease pathogenesis.

RESULTS

Three distinct stages of pathological changes were identified that represented the continuum of the dystrophic process from prominent inflammation with necrosis and regeneration to prominent fibrosis, which correlated with age and disease duration. Pax7-positive SCs were highest in the fibrotic group and correlated with down-regulation of miR-1, miR-133a, and miR-206.

CONCLUSIONS

These observations, and other published reports, are consistent with microRNA dysregulation leading to inability of Pax7-positive SCs to transit from proliferation to differentiation. This results in impaired regeneration and fibrosis.

Emerging drugs for Duchenne muscular dystrophy

INTRODUCTION

Duchenne muscular dystrophy (DMD) is the most common, severe childhood form of muscular dystrophy. Treatment is limited to glucocorticoids that have the benefit of prolonging ambulation by approximately 2 years and preventing scoliosis. Finding a more satisfactory treatment should focus on maintaining long-term efficacy with a minimal side effect profile.

AREAS COVERED

Authors discuss different therapeutic strategies that have been used in pre-clinical and clinical settings.

EXPERT OPINION

Multiple treatment approaches have emerged. Most attractive are molecular-based therapies that can express the missing dystrophin protein (exon skipping or mutation suppression) or a surrogate gene product (utrophin). Other approaches include increasing the strength of muscles (myostatin inhibitors), reducing muscle fibrosis and decreasing oxidative stress. Additional targets include inhibiting NF-κB to reduce inflammation or promoting skeletal muscle blood flow and muscle contractility using phosphodiesterase inhibitors or nitric oxide (NO) donors. The potential for each of these treatment strategies to enter clinical trials is a central theme of discussion. The review emphasizes that the goal of treatment should be to find a product at least as good as glucocorticoids with a lower side effect profile or with a significant glucocorticoid sparing effect.

mdx(⁵cv) mice manifest more severe muscle dysfunction and diaphragm force deficits than do mdx Mice

Duchenne muscular dystrophy (DMD) is characterized by progressive skeletal muscle dysfunction leading to premature death by the third decade of life. The mdx mouse, the most widely used animal model of DMD, has been extremely useful to study disease mechanisms and to screen new therapeutics. However, unlike patients with DMD, mdx mice have a very mild motor function deficit, posing significant limitations for its use as a platform to assess the impact of treatments on motor function. It has been suggested that an mdx variant, the mdx(5cv) mouse, might be more severely affected. Here, we compared the motor activity, histopathology, and individual muscle force measurements of mdx and mdx(⁵cv) mice. Our study revealed that mdx(⁵cv) mice showed more severe exercise-induced fatigue, Rotarod performance deficits, and gait anomalies than mdx mice and that these deficits began at a younger age. Muscle force studies showed more severe strength deficits in the diaphragm of mdx(⁵cv) mice compared to mdx mice, but similar force generation in the extensor digitorum longus. Muscle histology was similar between the two strains. Differences in genetic background (genetic modifiers) probably account for these functional differences between mdx strains. Overall, our findings indicate that the mdx and mdx(⁵cv) mouse models of DMD are not interchangeable and identify the mdx(⁵cv) mouse as a valuable platform for preclinical studies that require assessment of muscle function in live animals.

Aminoglycoside-induced mutation suppression (stop codon readthrough) as a therapeutic strategy for Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is the most common, lethal, X-linked genetic disease, affecting 1 in 3500 newborn males. It is caused by mutations in the DMD gene. Owing to the large size of the gene, the mutation rate in both germline and somatic cells is very high. Nearly 13-15% of DMD cases are caused by nonsense mutations leading to premature termination codons in the reading frame that results in truncated dystrophin protein. Currently there is no cure for DMD. The only available treatment is the use of glucocorticoids that have modest beneficial effects accompanied by significant side effects. Different therapeutic strategies have been developed ranging from gene therapy to exon skipping and nonsense mutation suppression to produce the full-length protein. These strategies have shown promise in the mdx mouse model of muscular dystrophy where they have been reported to ameliorate the dystrophic phenotype and correct the physiological defects in the membrane. Each of these molecular approaches are being investigated in clinical trials. Here we review nonsense mutation suppression by aminoglycosides as a therapeutic strategy to treat DMD with special emphasis on gentamicin-induced readthrough of disease-causing premature termination codons.

AAV-mediated gene therapy to the isolated limb in rhesus macaques

The development of a nonhuman primate (NHP) model for vascular delivery of therapeutic transgenes with adeno-associated viral (AAV) vectors is crucial for successfully treating muscular dystrophies. Current animal models for Duchenne muscular dystrophy (DMD) gene therapy have species limitations related to assessing function, immune response, and distribution of the micro- and minidystrophin transgenes in a clinically relevant manner. In addition, there are many forms of muscular dystrophy for which there are no available disease models. NHPs provide the ideal model to optimize vector delivery across a vascular barrier and provide accurate dose estimates for local or broadly targeted gene therapy studies. The vascular anatomy NHPs more clearly parallels humans providing an appropriate substrate for translational experiments. Here we outline the development of a rhesus macaque isolated focal limb perfusion (IFLP) protocol targeting the vascular bed of the gastrocnemius. This protocol serves as a model with broad implications for other muscle diseases along with the capability of targeting multiple muscle groups. To overcome the partial homogeneity between portions of the human microdystrophin transgene and those of the NHP dystrophin gene, we utilized a FLAG tag for tracking distribution of microdystrophin. We also provide methods for assessing transduction efficiency of microdystrophin.FLAG following the IFLP vascular delivery protocol.

Sustained alpha-sarcoglycan gene expression after gene transfer in limb-girdle muscular dystrophy, type 2D

OBJECTIVE

The aim of this study was to attain long-lasting alpha-sarcoglycan gene expression in limb-girdle muscular dystrophy, type 2D (LGMD2D) subjects mediated by adeno-associated virus (AAV) gene transfer under control of a muscle specific promoter (tMCK).

METHODS

rAAV1.tMCK.hSGCA (3.25 × 10¹¹ vector genomes) was delivered to the extensor digitorum brevis muscle of 3 subjects with documented SGCA mutations via a double-blind, randomized, placebo controlled trial. Control sides received saline. The blind was not broken until the study was completed at 6 months and all results were reported to the oversight committee.

RESULTS

Persistent alpha-sarcoglycan gene expression was achieved for 6 months in 2 of 3 LGMD2D subjects. Markers for muscle fiber transduction other than alpha-sarcoglycan included expression of major histocompatibility complex I, increase in muscle fiber size, and restoration of the full sarcoglycan complex. Mononuclear inflammatory cells recruited to the site of gene transfer appeared to undergo programmed cell death, demonstrated by terminal deoxynucleotide transferase-mediated deoxyuridine triphosphate nick-end labeling and caspase-3 staining. A patient failing gene transfer demonstrated an early rise in neutralizing antibody titers and T-cell immunity to AAV, validated by enzyme-linked immunospot on the second day after gene injection. This was in clear distinction to other participants with satisfactory gene expression.

INTERPRETATION

The findings of this gene replacement study in LGMD2D subjects have important implications not previously demonstrated in muscular dystrophy. Long-term, sustainable gene expression of alpha-sarcoglycan was observed following gene transfer mediated by AAV. The merit of a muscle-specific tMCK promoter, not previously used in a clinical trial, was evident, and the potential for reversal of disease was displayed.

Molecular therapeutic strategies targeting Duchenne muscular dystrophy

Since the discovery of the gene for Duchenne muscular dystrophy more than 20 years ago, scientists have worked to apply molecular principles for restoration of the dystrophin protein and correction of the underlying physiologic defect that predisposes muscle fibers to injury. Recent studies provide realistic hope that molecular therapies may help patients who have this disorder. At present, only corticosteroids can improve walking ability and increase quality of life for boys with this disease. The results are modest and encumbered by side effects. The authors review 3 molecular therapeutic approaches that have been introduced into the clinic: (1) gene replacement therapy, (2) mutation suppression, and (3) exon skipping.