Regenxbio reveals initial data on Duchenne gene therapy's muscle function improvement

Regenxbio recently announced promising results from its gene therapy study targeting Duchenne muscular dystrophy (DMD). The biotech firm revealed that its treatment, RGX-202, demonstrated improved muscle function in several young male patients across two different doses. The company initiated the pivotal phase of its clinical trial by dosing the first patient, with plans to file for accelerated approval with the FDA by 2026. If RGX-202 is approved, it would join Sarepta's Elevidys as the second gene therapy available for DMD and could potentially cater to a younger demographic.

On Monday, Regenxbio's stock saw an 8% increase following the news. In the trial, two patients who received the full dose of RGX-202 exhibited notable improvements: an eight-year-old patient gained a 1-point increase in the North Star Ambulatory Assessment (NSAA) score after nine months, while a 12-year-old showed a significant 10-point improvement over the same period. Additionally, three patients who received a lower dose of the therapy experienced a mean increase of 2.33 points in their NSAA scores after one year. Across all five patients, enhancements in muscle function were also noted in activities such as standing up, climbing stairs, and walking or running, compared to both baseline scores and natural history controls. Despite the encouraging preliminary data, Regenxbio will need to confirm these results in a larger study.

CEO Curran Simpson expressed optimism about the therapy's potential impact, highlighting that the patient community is eagerly awaiting new advancements. Simpson believes RGX-202 represents a significant step forward in second-generation gene therapy for DMD.

Duchenne muscular dystrophy is a genetic disorder characterized by the absence or near absence of dystrophin, a crucial muscle protein. Traditional gene therapy faces challenges since the dystrophin gene is too large to be delivered using adeno-associated virus (AAV) vectors. To overcome this, developers focus on delivering microdystrophin genes, which are truncated versions of dystrophin. Regenxbio's approach includes an additional part of the dystrophin gene known as the C-terminal domain.

The company's trial has expanded into an open-label Phase 1/2/3 study, aiming to enroll approximately 30 ambulatory patients aged one year and above. The primary endpoint of the pivotal phase will assess microdystrophin expression at week 12, diverging from the NSAA-focused primary endpoints used in Sarepta and Pfizer's studies. Notably, Regenxbio's trial does not include a placebo group, as an approved gene therapy is already on the market. The FDA has reportedly supported the use of natural history comparisons for the trial.

Recently, Pfizer reported that despite achieving microdystrophin expression, its gene therapy did not yield functional improvements and subsequently ceased its DMD program. In contrast, Sarepta's Elevidys received FDA approval based on microdystrophin expression as well as secondary and exploratory endpoints, although the approval process was contentious.

Simpson noted that when Regenxbio seeks accelerated approval, functional data will be reviewed, mirroring the process seen with Elevidys. Importantly, Regenxbio's study has reported no serious adverse events, adding to the positive outlook for RGX-202's potential in treating Duchenne muscular dystrophy.