FDA Reverses Rejection of Regenxbio’s Navsunli Gene Therapy for Hunter Syndrome

The U.S. Food and Drug Administration has withdrawn its February 2026 complete-response letter rejecting Regenxbio’s Navsunli (RGX-121) gene therapy for mucopolysaccharidosis II (Hunter syndrome). The agency now considers the existing Phase 1/2 data sufficient for accelerated approval, and Regenxbio will file a new biologics license application in the third quarter of 2026 after a Type A meeting scheduled for July.

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Hunter syndrome affects fewer than ten U.S. patients annually, prompting developers to rely on external controls and surrogate biomarkers. Under former commissioner Marty Makary and CBER director Vinay Prasad, the FDA required a sham-controlled Phase 3 study, citing concerns about eligibility criteria, comparability of external controls, and the validity of surrogate endpoints.

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• Curran Simpson, CEO, Regenxbio

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• Kyle Diamantas, Acting FDA Commissioner

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• Marty Makary, former FDA Commissioner (relevant to the original rejection)

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• Vinay Prasad, former CBER director

FDA Executes Shock Reversal on Regenxbio’s Navsunli Childhood Gene Therapy

• Sean McCutcheon, analyst, Raymond James

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• FDA Office of Cellular and Gene Therapies

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• February 2026 – FDA issues complete-response letter rejecting Navsunli.

FDA Executes Shock Reversal on Regenxbio’s Navsunli Childhood Gene Therapy

• April 2026 – Regenxbio meets with then-Commissioner Makary; appeal encouraged.

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• May 2026 – FDA staff reverses stance during collaborative discussion.

FDA Executes Shock Reversal on Regenxbio’s Navsunli Childhood Gene Therapy

• July 2026 – Planned Type A meeting to review longer-term biomarker data.

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• Q3 2026 – Regenxbio to resubmit accelerated-approval BLA.

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• Phase 1/2 trial enrolled 13 patients; three-year follow-up data deemed sufficient.

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• Over 30 patients have received Navsunli across studies, with no confirmed safety signal.

FDA Executes Shock Reversal on Regenxbio’s Navsunli Childhood Gene Therapy

• Shares rose 12-17 % after the reversal; one report noted a 37 % pre-market surge.

FDA Executes Shock Reversal on Regenxbio’s Navsunli Childhood Gene Therapy

• UniQure’s Huntington-disease gene therapy received a similar FDA pivot in the same period.

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The reversal signals a broader FDA shift toward flexible trial designs for ultra-rare pediatric diseases, potentially accelerating access to life-saving therapies. It also offers a template for global regulators, reducing development timelines from years to months for conditions with tiny patient populations.

FDA Executes Shock Reversal on Regenxbio’s Navsunli Childhood Gene Therapy

The FDA stated that Navsunli’s existing clinical data meet criteria for accelerated approval and that no additional untreated control arm is required. Regenxbio emphasized continued collaboration with the agency and a focus on delivering the therapy quickly. Analyst Sean McCutcheon interpreted the move as evidence of a more flexible regulatory approach.

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Some experts argue that the absence of a placebo-controlled arm and reliance on surrogate biomarkers may limit confidence in efficacy and safety, especially after a safety scare involving a related Regenxbio therapy (RGX-111) that prompted a temporary clinical hold.

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Sources differ on the magnitude of the stock reaction (12-17 % vs. 37 %). The FDA’s public statement does not address the tumor incident in the RGX-111 program, leaving uncertainty about how safety concerns may affect Navsunli’s review.

FDA Executes Shock Reversal on Regenxbio’s Navsunli Childhood Gene Therapy

• “We are encouraged by recent signals from the new FDA leadership reinforcing a commitment to address the unique nature of rare diseases and use the accelerated approval pathway to bring transformative therapies to patients with serious, unmet medical needs,” — Curran Simpson, CEO, Regenxbio

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• “While there are still some wrinkles to smooth out with FDA based on the justifications provided in the Navsunli [rejection], we think this is a clear indication of a new approach which is significantly more flexible and portends a favorable outcome for Regenxbio and patients with Hunter Syndrome,” — Sean McCutcheon, analyst, Raymond James

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• “on an expedited basis, with labeling discussions to begin shortly following the resubmission.” — Curran Simpson, CEO, Regenxbio

FDA Executes Shock Reversal on Regenxbio’s Navsunli Childhood Gene Therapy

• “During those discussions, Makary encouraged the company to formally appeal the decision, assuring executives that the application would receive a review conducted with "fresh eyes," Simpson said.” — Curran Simpson, CEO, Regenxbio

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• “acknowledged the existing NAVSUNLI [RGX-121] clinical data is sufficient to be considered for the accelerated approval pathway and that the Company does not need to enroll additional patients or conduct additional studies, including the FDA’s previously recommended incorporation of an untreated control arm,” — U.S. Food and Drug Administration

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Regenxbio will present longer-term biomarker and clinical data at the July Type A meeting. After FDA feedback, the company plans a third-quarter 2026 resubmission, followed by labeling discussions. If the accelerated review proceeds without further objections, Navsunli could reach the market by late 2026, offering the first disease-modifying option for Hunter syndrome.

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