The US Food and Drug Administration (FDA) is allowing biotech company Regenxbio to reapply for licensing of a gene therapy for Hunter syndrome, in a reversal of its previous decision.
Hunter syndrome, or mucopolysaccharidosis type II (MPS II), is a rare X-linked disorder caused by mutations in the IDS gene that leads to problems in physical and mental development. Patients typically receive weekly enzyme replacement therapy to delay disease progression and address physical but not cognitive symptoms. Regenxbio, which is based in Rockville, Maryland, has developed Navsunli, a single-dose gene therapy that delivers a functional copy of IDS to the brain. The FDA rejected Regenxbio's application for accelerated approval of Navsunli in February 2026, but has now agreed that existing clinical data may support a resubmission.
'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,' said Curran Simpson, president and CEO of Regenxbio. 'We remain focused on bringing this important therapy to boys living with Hunter syndrome.'
Regenxbio first submitted a Biologics Licence Application for Navsunli in May 2025, a required step for the therapy to be manufactured and marketed in the USA. The FDA rejected the application, citing concerns about study design and whether changes in a biomarker could reliably predict clinical benefit. It instructed Regenxbio to conduct additional studies and to include a group of placebo-treated patients. The company noted that these requirements were challenging when working on rare diseases affecting small populations with limited treatment options.
Following an appeal by Regenxbio, the FDA acknowledged that data from its existing CAMPSIITE clinical trial was sufficient to be considered for the accelerated approval pathway. The agency asked Regenxbio to request a Type A meeting – an urgent discussion with regulators – to review existing clinical data before resubmitting the application, which would then be assessed on an expedited basis.
'The MPS community urgently needs new treatment options, and we appreciate the FDA's willingness to expedite the review of Navsunli,' commented Dr Scott Loiler, chief scientific officer of the National MPS Society. 'The Hunter syndrome community may soon have access to a potentially transformative one-time treatment.'
Regenxbio is among several companies affected by recent FDA decision reversals following the replacement of the FDA commissioner and the director of the Centre for Biologics Evaluation and Research. Two weeks ago, uniQure announced that the FDA agreed its existing Phase I/II data could support the resubmission of a licence application for its Huntington's disease gene therapy (see BioNews 1344). Replimune has also reported that its refiled application for an advanced melanoma treatment was accepted for review.
In February 2026, the FDA imposed a clinical hold on a Regenxbio gene therapy for Hurler syndrome, following detection of a brain tumour in a previously treated patient (see BioNews 1340). The hold was extended to Navsunli due to similarities in therapy design, but subsequently lifted.
Regenxbio plans to resubmit the revised Navsunli application during the third quarter of 2026.
Sources and References
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Regenxbio announces alignment with FDA on path forward for NAVSUNLI BLA resubmission for accelerated approval; first potential gene therapy for MPS II
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US FDA reverses course on Regenxbio's rare-disease gene therapy, backs accelerated approval bid
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Regenxbio to resubmit gene therapy as FDA backtracks on another drug rejection
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In another reversal for rare disease field, FDA to reconsider Regenxbio gene therapy
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In another FDA rethink, door opens for Regenxbio's gene therapy comeback
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