On 17 September 2026, the U.S. Food and Drug Administration (FDA) approved FAYUVI (rebisufligene etisparvovec-hopf), previously known as UX111, for paediatric patients with MPS IIIA (Sanfilippo syndrome type A). This is the first approved therapy intended to address the underlying cause of MPS IIIA.
FAYUVI is a one-time intravenous gene therapy that uses an AAV9 viral vector to deliver a functioning copy of the SGSH gene, enabling production of the missing sulfamidase enzyme.
This approval represents a significant milestone for the global Sanfilippo community. We recognise the urgency within our MPS IIIA community to understand what this approval means for them and how they might explore accessing the therapy.
This guide is intended to help UK families understand the steps involved if they wish to explore accessing FAYUVI in the United States.
It is important to understand that exploring access to therapy does not guarantee that an individual will be eligible for, or able to receive, treatment. Decisions about suitability and eligibility will be made by the relevant clinical teams in your chosen treatment centre.
Important to remember
FAYUVI (rebisufligene etisparvovec, UX111) remains an investigational gene therapy in the UK and has not received MHRA marketing authorisation.
There is no formal NHS pathway to accessing this therapy – this document outlines the process that a family should undertake if they want to explore privately funded access to FAYUVI.
Starting this process does not guarantee access to FAYUVI. Eligibility will depend on a number of factors, including clinical assessment, AAV9 antibody status and the requirements of the individual US treatment centre.
Families considering this route should also ensure that they fully understand the total financial implications of receiving treatment in the United States, including costs beyond the price of FAYUVI itself.
At present there are no UK or EU sites approved for the delivery of FAYUVI.
Important: This information has been developed by the MPS Society to support UK families in understanding the potential route for accessing FAYUVI in the United States. It is not a direct communication from Ultragenyx or UK specialist clinical centres and should not be considered medical or clinical advice. Families should discuss their individual circumstances, potential eligibility and treatment options directly with their specialist clinical team. Ultragenyx has reviewed this information for factual accuracy only.
Key steps
1. Speak to your UK specialist centre
The first step is for families who are interested in exploring access to FAYUVI to contact their UK specialist centre and request a discussion with their clinical team.
Your UK clinician will talk to you about FAYUVI, including:
the potential benefits of treatment
the potential risks and disadvantages
whether exploring treatment in the United States may be appropriate for your child
the practical and clinical considerations associated with accessing treatment overseas.
This is an opportunity for families to discuss the treatment with a clinician who knows their child's medical history and to ask questions before deciding whether they wish to explore international access further.
2. Confirming how treatment will be funded
Families will need to confirm that they have funding available to meet the costs associated with treatment before referral to a US treatment centre
Families should be aware that the published US list price for FAYUVI relates to the drug itself. There may be significant additional costs associated with receiving treatment in the United States.
These could include costs associated with:
the treating hospital or clinical centre
clinical investigations and assessments
additional treatments required by the US centre
travel to and from the United States
accommodation and living expenses
the length of stay required in the United States
other medical and practical costs associated with treatment.
It is therefore essential that families understand the full potential cost of accessing treatment, rather than considering the drug price alone, and are able to confirm that these costs can be met.
We recommend that families contact US treatment sites to seek advice about the cost of accessing treatment in their facility.
You will not be accepted for treatment at a US centre unless you are able to confirm that funding is available.
3. Identifying a treatment centre in the United States
US treatment sites:
The family, working with their UK clinical team, will need to identify a US specialist centre willing to consider the referral.
It is important to be aware that not all US treatment centres will accept international patients.
Finding a centre willing to consider a referral does not, in itself, mean the child has been accepted for treatment.
4. AAV9 antibody testing
In parallel with identifying a potential US treatment centre, it may be possible to arrange AAV9 antibody screening. This can be coordinated by your UK specialist team and is part of the overall treatment pathway.
AAV9 antibody testing is available through a European site, with UK testing arrangements under development. Families should discuss testing options with their specialist centre.
Families should understand the significance and limitations of antibody testing:
If testing identifies AAV9 antibodies, this may mean that the individual is not eligible to receive FAYUVI.
If no AAV9 antibodies are detected, the result represents the individual's antibody status at that particular point in time.
A person may subsequently develop AAV9 antibodies. Therefore, an earlier negative result does not guarantee that antibody screening immediately before potential treatment will also be negative.
The treating US centre will ultimately determine what testing is required and how the results affect eligibility for treatment.
6. Referral and assessment by the US specialist centre
If a US specialist centre agrees to consider the referral, that centre will liaise with the child's UK specialist clinical team.
This is likely to involve sharing relevant clinical information, including:
the child's medical history
information about their disease progression
previous clinical assessments
relevant neurodevelopmental assessments
other investigations and clinical information requested by the US centre.
Decisions about whether an individual is eligible for treatment will therefore be made directly between the US treatment centre, the family and the referring clinical team as appropriate and will align with US prescribing information.
Ultragenyx does not determine referral, assessment or treatment decisions.
7. Further assessments may be required
Families should be prepared for the possibility that the US centre will request additional or repeat assessments before deciding whether a child is eligible for FAYUVI.
For example, further assessments may be required to evaluate the child's:
neurodevelopmental status
current clinical status
disease progression
suitability for treatment.
The exact assessments required will depend on the individual patient and the requirements of the US treatment centre.
FDA approval states that individuals under the age of 18 with preserved neurodevelopmental function may be eligible for treatment.
8. Immunosuppression and immunomodulation
Families have understandably asked about the medicines that may be required to manage the immune response associated with treatment.
The current FAYUVI approval information recommends an immunosuppression regimen involving corticosteroids.
However, an individual US specialist centre may recommend or require additional immunomodulatory treatment as part of its treatment protocol or based on an individual patient's circumstances.
Families should therefore discuss this directly with the prospective US treatment centre and understand that additional immunomodulatory treatment, monitoring or associated care could represent an additional cost.
