What two FDA gene therapy reversals in one week means for rare disease sponsors

In the space of five days, the FDA reversed its position on two rare disease gene therapies it had previously rejected. On June 17, the agency agreed that UniQure's AMT-130 for Huntington's disease could support a BLA submission under accelerated approval based on three-year Phase 1/2 data. On June 22, it reopened the path for Regenxbio's navsunli, a gene therapy for the neurological form of Hunter syndrome (MPS II), just four months after rejecting it.

Taken together, these developments are part of a pattern that has been building since spring 2026 and has significant implications for every sponsor with a rare disease gene therapy program in development.

A pattern emerging

The reversals followed the departure of FDA leadership installed during the Trump administration, including Commissioner Marty Makary and CBER director Vinay Prasad. At least three gene therapy decisions have been reversed in recent months, including an earlier reversal on Ebvallo in May 2026.

What connects these cases beyond personnel changes is the agency's evolving posture on evidence requirements for gene therapies. On June 2, CBER issued draft guidance on "Leveraging Prior Knowledge in the Development of Human Gene Therapy Products", which supports the use of cross-referenced dossiers, shared toxicology data, and natural history studies to reduce duplicative evidence generation.

These signals point toward a regulatory environment that is becoming more willing to accept non-traditional evidence packages for gene therapies in rare diseases, particularly where the unmet need is severe and the disease biology is well characterized.

What the AMT-130 and navsunli cases reveal

Both therapies target genetically defined, progressive conditions with limited treatment options.

AMT-130 is an AAV-based gene therapy that reduces mutant huntingtin protein through a one-time neurosurgical administration into the striatum. Three-year results showed a 75% slowing of disease progression on the cUHDRS (p = 0.003) compared to propensity-matched external controls from the Enroll-HD registry. The FDA had previously concluded the data were insufficient. It has now granted RMAT, Breakthrough Therapy, and Fast Track designations, and UniQure plans to file its BLA in Q3 2026.

Navsunli targets MPS II, a condition where existing enzyme-replacement therapy cannot cross the blood-brain barrier. Children with the neurological form experience progressive cognitive decline with no approved treatment that addresses the central nervous system component.

These examples that the FDA is now accepting external control data and prior knowledge frameworks that it recently rejected. Additionally, the timelines are compressing. A BLA filing in Q3 2026 for a therapy that was in regulatory limbo just three months ago is not a standard development pace.

Why faster regulatory paths raise the stakes on recruitment

It is easy to read these reversals as good news for rare disease sponsors. In one sense, they are. The regulatory barrier is lower, and the evidence requirements may be more pragmatic. But this creates a downstream problem that many sponsors are not yet positioned to solve.

When regulatory timelines compress, the enrollment window compresses with them. A sponsor that expects an 18-month runway to identify, screen, and enroll patients in a confirmatory study may now have half that time. For genetically defined conditions like Huntington's disease (caused by a CAG repeat expansion in the huntingtin gene) or Hunter syndrome, the patient population is already small. Finding patients who meet eligibility criteria, confirming genotype, and completing enrollment within an accelerated window requires infrastructure that most programs do not have in place at the point of regulatory engagement.

This is where the real gap opens. The regulatory path is becoming more flexible, but the patient identification and recruitment infrastructure at most sponsors has not kept pace. Programs that rely on traditional site-based recruitment, manual chart reviews, or reactive outreach will struggle to match the pace that compressed timelines demand.

Genotype-first approaches become essential

For rare disease gene therapies, every eligible patient is defined by a genetic marker. The recruitment model should reflect that.

A genotype-first approach means identifying patients based on their genetic profile before clinical engagement, rather than screening broadly at sites and hoping to find eligible participants. This is an operational necessity when:

• The patient population is small and geographically dispersed
• Eligibility depends on specific genetic variants
• Regulatory timelines leave limited time for enrollment
• Confirmatory studies may require active comparators rather than placebo or sham controls, increasing the complexity of patient matching

The AMT-130 case illustrates this directly. The confirmatory study may use standard-of-care comparators instead of sham neurosurgical controls. This is a more practical design, but it still requires identifying and enrolling enough patients with confirmed CAG repeat expansions on a BLA-driven timeline.

What rare disease sponsors should watch

Several developments are worth tracking in the coming months:

• Further FDA reversals. The pattern is not likely to stop at three. Sponsors with programs that received CRLs or clinical holds in 2025 or early 2026 should re-engage with the agency to understand whether the evidentiary landscape has shifted for their indication.
• Draft guidance finalization. The CBER draft guidance on prior knowledge will shape how much sponsors can leverage cross-program data. Its final form will matter for platform gene therapy programs especially.
• International regulatory alignment. MHRA's flexible licensing framework and other adaptive approaches outside the US may create additional pathways, but also additional complexity in patient identification across geographies.
• Confirmatory study design expectations. The move away from sham controls toward standard-of-care comparators changes recruitment math. Sponsors should model enrollment feasibility early, not after the BLA is filed.

Conclusion

The FDA's pattern of reversals is a meaningful signal. The agency is recalibrating its approach to rare disease gene therapies, and the evidence requirements are becoming more pragmatic. For sponsors, this is a reason to accelerate, but not without preparation.

The programs that will move fastest are the ones that already have patient identification infrastructure in place: genotype-first recruitment, integrated genetic testing, and engagement models that can activate when the regulatory window opens. Waiting until after a BLA filing to solve the enrollment problem is no longer a viable strategy when timelines are measured in quarters, not years.

If your organization is developing a gene therapy for a genetically defined rare disease and needs to build that infrastructure, get in touch.