Sarepta Therapeutics presented interim findings from its Phase II clinical trial (Study 102) in Duchenne muscular dystrophy (DMD) for its novel gene transfer therapy, SRP-9001 (delandistrogene moxeparvovec), at the virtual 2021 American Academy of Neurology (AAN) conference held on 17-22 April.
The randomised, double-blind, placebo-controlled, Phase II study assessed the safety, efficacy and tolerability of a single dose of intravenous infusion of SRP-9001 in 41 patients with DMD aged 4-7 years. The patients were stratified by age into two groups, 4-5 years and 6-7 years. The gene therapy achieved its primary biological endpoint of change in micro-dystrophin protein expression at 12 weeks post-treatment. The primary functional endpoint of change in the North Star Ambulatory Assessment (NSAA) total score, however, was not met.
Despite this, the group of patients aged 4-5 years on SRP-9001 showed a statistically significant difference in the NSAA score at week 48 compared to the placebo group, at +4.3 in SRP-9001 versus +1.9 in placebo (P=0.0172). Four patients developed treatment-related serious adverse events, but no events led to trial discontinuation or death, which is not uncommon in gene therapy trials. Sarepta believes this positive finding and the manageable tolerability data support the further development of SRP-9001 into a Phase III trial.
SRP-9001 is an adeno-associated virus (AAV) mediated gene therapy that delivers a micro-dystrophin-encoding gene to the muscles and is intended as a curative treatment. The specific vector deployed in the gene transfer, AAVrh74, has been shown to achieve the efficient delivery of micro-dystrophin to skeletal muscle with tolerable immunogenicity in early studies.
Sarepta is currently leading the DMD market with a strong, extensive pipeline and three approved antisense oligonucleotide products, namely Exondys 51 (eteplirsen), Vyondys 53 (golodirsen) and Amondys 45 (casimersen). These products target patients with a confirmed mutation to exon 51, exon 53 and exon 45 respectively. Patients with mutations account for 28% of the overall DMD population.
The DMD community highly anticipates Sarepta’s pipeline gene therapy. This therapy seems to have a better prospect, as it targets most DMD patients and its trial design involved a placebo control group, unlike the previous single-arm trials that led to US Food and Drug Administration (FDA) approvals for Exondys in 2016 and Vyondys in 2019.
When Sarepta announced the topline results of SRP-9001’s Phase II study in January, it was first met with disappointment from the DMD community. This in turn significantly affected its shares in the stock market and caused Sarepta to lose half of its shares’ value. The company, however, attributed the failure to meet one of the primary endpoints to an imbalance within the participants in the placebo group. The company’s presentation at the AAN conference revealed that it is planning to initiate a Phase III trial after discussing the findings from the Phase II trial with the FDA and regulatory bodies.
Sarepta is looking to strengthen its leading position in this market with SRP-9001, especially after its main anticipated competitor, Pfizer’s PF-06939926, started enrolling patients for its Phase III in January despite prior safety concerns. PF-066939926 uses AAV9 to transfer a truncated human dystrophin gene that encodes dystrophin to the muscle tissues. In its previous clinical trial, however, serious adverse events such as dehydration, acute kidney injury and low platelet count led to three participants being hospitalised before the issues were resolved.
With Fast Track and Orphan Drug designations already in place, GlobalData expects SRP-9001 to gain approval in the US by 2024. Obtaining FDA approval is unlikely to be easy, however, given its inferior performance in Phase II trials. Sarepta will have to prove SRP-9001’s efficacy with more robust data and a definitive clinical trial design in Phase III.
If approved, GlobalData forecasts that SRP-9001’s global sales will reach $2.4bn by 2026. The only late-stage competitor to watch is Pfizer’s PF-06939926, which might precede it to the market given the unexpected setback faced by Sarepta and the initiation of Pfizer’s Phase III trial, which has a primary completion date of next September.