The US Food and Drug Administration (FDA) will reconsider Regenxbio’s gene therapy for Hunter syndrome after determining the preexisting package adequate for a review, marking the latest easing in stringent regulatory proceedings at the agency.
Regenxbio said that it had completed a “collaborative discussion” with the FDA to discuss the next steps for the biotech’s Navsunli (clemidsogene lanparvovec-sngl). This open dialogue with the FDA was part of Regenxbio’s appeal to a complete response letter (CRL) that was issued in February 2026.
As per the letter’s publication in early March, the FDA had concerns about the study design and patient criteria. Now, just four months on from the original refusal, the agency has agreed that the existing clinical data is sufficient to be considered for the accelerated approval pathway, negating the need for enrolment of additional patients or initiation of additional studies.
With the path to accelerated approval now cleared, Regenxbio expects to meet with the FDA again in July, with a Biologics License Application (BLA) resubmission slated for Q3 2026.
In that meeting, the FDA will review existing longer-term biomarker and clinical data, a qualm also raised in the CRL from February. Since then, Denali Therapeutics’ enzyme replacement therapy Avlayh, which used biomarker-based data as clinical evidence, has been approved in the US for Hunter syndrome.
Hunter Syndrome, also known as mucopolysaccharidosis II (MPS II), is a rare, X-linked recessive disease caused by a deficiency in the lysosomal enzyme I2S. This leads to an accumulation of glycosaminoglycans (GAGs), including heparan sulfate (HS), in tissues that leads to dysfunction.
Approximately 2,000 patients worldwide are diagnosed with MPS II, with more than 500 babies born annually around the world with the disease.
Rare disease stance changes
Regenxbio’s positive announcement comes less than a week after the FDA also U-turned on uniQure’s Huntington’s disease gene therapy. The agency previously refused to accept uniQure’s filing for AMT-130 in November 2024, citing insufficient data from a Phase I/II trial. However, regulators at the agency last week decided the data package from the biotech is sufficient for a review.
Reacting to that decision, William Blair analysts highlighted that, taken together with other recent U-turns, the FDA is showing less rigidity with drug submissions in rare diseases. It is expected that Regenxbio’s Navsunli reversal will only add to that viewpoint.
Regulatory shifts have coincided with leadership changes at the FDA. Vinay Prasad left his role as head of the Center for Biologics Regulation and Evaluation (CBER) in March 2026, while Marty Makary officially resigned from his post as commissioner in May. Prasad, especially, incited controversy due to his approach to cell and gene therapy approvals. His strict stance on clinical trial endpoints led to significant tensions with drug developers and rare disease charities.
Acting Commissioner Kyle Diamantas, Acting CDER Director Mike Davis, and Acting CBER Director Karim Mikhail met with rare disease advocacy groups in an early June roundtable. The meeting intended to fix relationships and pledge an advancement for rare disease therapies.
