Clinical research in MPS continues to move at pace internationally, with a number of important regulatory developments over the past few months. While progress can feel slow at times, we are now seeing an increasing number of therapies reaching late-stage review, particularly in MPS II Hunter and MPS III Sanfilippo.
These developments reflect both the growing scientific understanding of these conditions and the challenges involved in bringing advanced therapies through regulatory approval. We want to share an update on some of the most recent key announcements and what they mean for our community.
MPS II Hunter
There have been several important updates in MPS II over the past few months, reflecting the ongoing progress and complexity of developing new treatments for this condition.
Most recently, Denali Therapeutics’ AVLAYAH™ (tividenofusp alfa) received accelerated approval from the US Food and Drug Administration (FDA) for the treatment of neurological manifestations of MPS II, with specific age-related eligibility criteria.
This marks a significant milestone, as it is the first therapy designed to deliver treatment across the blood–brain barrier, with the aim of addressing neurological aspects of the condition in addition to systemic symptoms.
What does accelerated approval in the US mean and why is this different in the UK?
Accelerated approval allows a treatment to be approved earlier for serious conditions with urgent unmet needs, based on biomarker data that suggests benefit, even while longer‑term clinical studies are still ongoing. Further trial data is then required to confirm that the treatment leads to meaningful clinical outcomes.
However, the UK and Europe have different regulatory requirements and do not currently accept early biomarker data alone.
Instead, they require additional clinical evidence from ongoing trials, such as the COMPASS study, before making approval and access decisions.
The MPS Society are actively engaging with key stakeholders and will keep you updated with key developments.
MPS III Sanfilippo
In MPS III, research continues globally, with several gene therapy and disease-modifying programmes in early and mid-stage clinical development. At present, there are no approved disease-modifying treatments for MPS III, but research activity remains active and continues to focus on improving delivery to the brain and targeting the underlying disease mechanisms.
UX111 (MPS IIIA)
There has recently been an important update relating to UX111, a gene therapy for MPS IIIA developed by Ultragenyx.
The FDA has recently accepted a resubmitted Biologics License Application (BLA) and assigned a review timeline with a decision expected in September 2026. This follows an earlier review process where additional information was requested, and the company has since provided updated data and manufacturing details as part of the resubmission.
The application is now back under formal FDA review, representing an important step forward in the regulatory pathway, although approval has not yet been granted.
UX111 is beginning early discussions with NICE (National Institute for Health and Care Excellence), which reviews new treatments to decide whether they should be available on the NHS. A scoping meeting planned for 24 April 2026 will help NICE agree what evidence and questions to focus on in any future review, and we will keep the community updated as we hear more.
Tralesinidase Alfa (MPS IIIB)
You will likely have heard about Leni, who along with her family have been doing an amazing job at raising funds and awareness.
Following a meeting with the FDA in February 2026, Spruce Biosciences is preparing to submit a Biologics License Application (BLA) for Tralesinidase Alfa (TA‑ERT) later in 2026. This application is a formal request asking regulators to review the evidence from clinical studies to decide whether the treatment can be approved for use.
What does this mean for the UK?
It is important to remember that FDA decisions apply only to the United States.
Even where a treatment is approved, it does not automatically become available in the UK.
Any potential access in the UK would require:
Review of safety and clinical effectiveness
Separate assessment by UK regulatory authorities
In the UK, two organisations are involved in decisions about new treatments:
| MHRA | NICE |
|---|---|
| The Medicines and Healthcare products Regulatory Agency checks whether a medicine is safe, works as expected, and meets quality standards before it can be licensed for use. | The National Institute for Health and Care Excellence then looks at whether the treatment should be available on the NHS, considering how well it works, who may benefit, and whether it represents good value for the health system. |
At this stage, there are no immediate changes to treatment availability in the UK arising from these FDA updates.
However, these developments are important in shaping the wider global research landscape and may influence future regulatory discussions.
Potential gene therapy for MPS IIIB
Researchers at the University of Edinburgh, led by Professor Brian Bigger, are exploring a potential gene therapy for children with MPSIIIB. The approach aims to deliver a missing gene into patients’ blood stem cells, but progressing to clinical trials will depend on securing significant funding.
At this point, there is no confirmed information about the logistics of any future trial, including whether it would involve UK sites or what eligibility criteria might apply. These details will only become available as the programme advances. We will keep you updated as we hear more.
We will keep you updated
We will continue to monitor clinical trial progress and regulatory decisions across all MPS conditions and share updates where they are relevant to our community.
If you have any questions about specific trials or emerging therapies, please contact our Support Team, who can help guide you to further information and support.
