Regenxbio reveals first functional data for DMD gene therapy, promising FDA path

Regenxbio's stock experienced a 5% surge on Monday following the release of promising early-stage data on its Duchenne muscular dystrophy (DMD) gene therapy, RGX-202. The company also received positive feedback from the FDA, potentially accelerating regulatory approval in the future.

In Regenxbio's Phase I/II AFFINITY DUCHENNE trial, all five participants showed signs of positively influencing the disease's progression. Key indicators of this progress included stable or improved performance on the North Star Ambulatory Assessment (NSAA) and timed function tests, which measure the time taken to stand or walk/run 10 meters. Based on these results, the trial has been expanded into an open-label pivotal Phase I/II/III study, with the goal of supporting an FDA filing in 2026 through the accelerated approval pathway.

RGX-202 is a microdystrophin gene therapy that incorporates the functional elements of the C-terminal domain found in natural dystrophin. According to Regenxbio, the data demonstrate functional improvements exceeding those observed in natural history benchmarks. The recent data release included 12-month results from three patients aged 4 to 10 at dose level 1 (1x10¹⁴ GC/kg) and nine-month results from two patients aged 8 and 12 at dose level 2 (2x10¹⁴ GC/kg). These results were compared against external natural history controls matched for age and baseline function.

Regenxbio's CEO, Curran Simpson, highlighted the clear drug effect at dose level 1 and an even more pronounced effect at dose level 2 during an investor call. One standout outcome was the performance of a 12-year-old patient who improved his NSAA score by 10 points. The company noted that there was no matched natural history control data for this patient on this specific measure. Clinical development lead, Jahannaz Dastgir, remarked that this patient's performance was exceptional, exceeding expectations.

Dastgir added that older children entering the study would likely show better function for Duchenne patients, emphasizing that the 12-year-old's improvement in NSAA was significant. She indicated that all children in the study will be closely monitored to identify any unique factors contributing to their positive responses to the treatment.

Conversely, an eight-year-old patient on the second dose level showed only a 1-point improvement on the NSAA compared to baseline, and a 2.8-point improvement against external natural history controls, where the condition typically worsened over the same period.

The biomarker data indicated that RGX-202 microdystrophin expression levels were among the highest reported in approved or investigational gene therapies for ambulatory patients aged 8 and older. At 12 weeks, patients showed mean microdystrophin expression levels ranging from 10.4% to 39.7% of normal control levels.

The safety profile of RGX-202 appeared promising, with no serious adverse events reported among the 11 treated patients. The most common side effects included nausea, vomiting, and fatigue.

If successful, RGX-202 could become the second DMD gene therapy approved in the US, following Sarepta Therapeutics' Elevidys (delandistrogene moxeparvovec-rokl). Elevidys initially received accelerated approval last year with a restricted label, which was later expanded by the FDA in June to include both non-ambulatory boys and those aged four years and older. Elevidys generated $191 million in US sales in the third quarter, with Sarepta projecting $2 billion in sales for the next year.

On the other hand, Pfizer's gene therapy program was discontinued earlier this year following a failed pivotal trial. Fordadistrogene movaparvovec did not meet the primary endpoint of the Phase III CIFFREO study, nor did it achieve several key secondary endpoints.