Development

Regenxbio's hits Phase III endpoints with gene therapy for Duchenne muscular dystrophy

Edited by Robert McTiernan··5 min read

Regenxbio (Nasdaq: RGNX), the Rockville, Maryland-based gene therapy company, reported positive topline data from the pivotal Phase III portion of its AFFINITY DUCHENNE trial of RGX-202, a microdystrophin gene therapy for Duchenne muscular dystrophy, with 93% of participants achieving the primary biomarker endpoint. The RGX-202 Phase III results position the company to pursue accelerated approval from the US FDA, with a potential commercial launch targeted for 2027.

The study specifics

The AFFINITY DUCHENNE trial is a Phase III study evaluating a single intravenous dose of RGX-202 at 2×10¹⁴ genome copies per kilogram in ambulatory boys aged one year and older with Duchenne muscular dystrophy. The pivotal cohort enrolled 31 participants, with topline data cut as of April 16, 2026.

The trial met its primary endpoint with high statistical significance. A total 93% of participants achieved greater than 10% RGX-202 microdystrophin expression at Week 12 (p-value not disclosed in full, reported as highly significant). Mean microdystrophin expression across all participants reached 71.1%, with 80% of participants exceeding the 40% threshold. In older boys aged over eight years, mean expression averaged 41.6%, a subgroup of particular clinical relevance given the progressive nature of the disease in that age range.

Interim functional data from nine participants aged approximately five to twelve years at dosing, assessed at one year post-treatment, showed improvement across the North Star Ambulatory Assessment (NSAA) and all timed function tests — Time to Stand, 10 Meter Walk-run, and Time to Climb — when compared against external controls using propensity score weighting, the primary analysis method specified in the statistical analysis plan. Critically, microdystrophin expression at Week 12 correlated with functional improvement at one year as measured by NSAA change from baseline (correlation = 0.94, p = 0.0002) and NSAA change from baseline versus the cTAP predictive model (correlation = 0.92, p = 0.0005).

The safety data, drawn from all 31 participants, showed a manageable profile. Two serious adverse events were reported: one case of subacute myocarditis in an eight-year-old participant, whose most recent cardiac MRI confirmed no myocardial fibrosis and no change in ejection fraction; and one case of asymptomatic liver injury in a ten-year-old participant, with a gamma-glutamyl transferase peak of 123 U/L and normal bilirubin and ultrasound findings. Both resolved without sequelae. Mean GGT and total bilirubin did not exceed the upper limit of normal at up to one year post-treatment in the nine participants with that follow-up duration. Common drug-related adverse events — vomiting, fatigue, and nausea — were mild or moderate and resolved without sequelae. Regenxbio attributes the safety profile in part to a proactive short-course immune suppression regimen administered alongside dosing.

RGX-202 in the DMD gene therapy landscape

RGX-202 is an adeno-associated virus-based gene therapy designed to deliver a novel microdystrophin construct that includes the C-Terminal (CT) domain, a structural element absent from competing microdystrophin constructs. The company contends that inclusion of the CT domain more closely replicates naturally occurring dystrophin and may contribute to improved muscle protection and functional durability. RGX-202 is manufactured using a suspension-based process that the company reports delivers full-capsid purity levels above 80%, which it argues contributes to the favorable safety profile observed to date.

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The Duchenne gene therapy space has faced increased regulatory scrutiny following mixed efficacy results from Sarepta Therapeutics’ Elevidys confirmatory EMBARK study and subsequent safety concerns that led to temporary shipment pauses and label restrictions in 2025. In that context, the statistically significant correlation between microdystrophin expression and functional outcomes reported in the AFFINITY DUCHENNE study is a material distinction: the US FDA had previously indicated that surrogate endpoint acceptability for accelerated approval would depend on demonstrated correlation with clinical outcomes, a bar that prior programs struggled to clear convincingly. Cross-trial comparisons are limited by differences in patient age, enrollment criteria, construct design, and follow-up duration, but the directional functional data from RGX-202 at one year — particularly the performance relative to external controls across multiple timed tests — provides a more coherent mechanistic narrative than some earlier DMD gene therapy datasets.

Regulatory pathway and next steps

In recent discussions with the US FDA, the agency indicated that microdystrophin expression as a surrogate endpoint would be evaluated on the basis of its correlation with clinical outcomes — a standard the interim data appear to satisfy. The FDA noted that while a randomized controlled trial is preferred, externally controlled trials may be sufficient when the treatment effect is large enough to overcome the inherent limitations of that design. Regenxbio said it plans to discuss the full dataset with the FDA at a future meeting and is preparing for a potential accelerated approval submission targeting a commercial launch in 2027.

More than 20 additional participants have been enrolled in a separate confirmatory trial of RGX-202, with the company expecting to complete dosing across all 60 patients in both the pivotal and confirmatory trials by mid-2026. Regenxbio is also finalizing the design of an ex-US study to support global regulatory submissions.

The functional dataset remains limited — nine participants with one-year follow-up — and the absence of a concurrent randomized control group means the external comparator analysis carries inherent uncertainty. Whether the US FDA will accept an accelerated approval pathway based on the current evidence package, and on what timeline, will determine the near-term trajectory of the RGX-202 DMD treatment program.

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