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Page last updated: 17th March 2023

ZOLGENSMA™ is the trademark name for Onasemnogene abeparvovec, which is manufactured by the pharmaceutical company Novartis Gene Therapies.

Zolgensma is a gene therapy that delivers a healthy copy of the SMN1 gene to motor neurons. It is made from parts of a virus called AAV9 (adeno-associated virus 9) that transports SMN1 around the body to many different cells, helping to restore some of the SMN protein that is missing in SMA.

AAV9 cannot produce copies of itself and is therefore unable to be passed from person to person. It’s only used to treat 5q variant spinal muscular atrophy (5q SMA).

Zolgensma is injected into the bloodstream (intravenously). A small flexible tube (cannula) is placed into a vein in the arm or leg using a needle. The needle is then removed leaving the cannula in place for the infusion.

Since 2014, there are published data from clinical trials in humans, using this method of delivery, with children treated up to 2 years of age. Several trials of Zolgensma in a wider range of children who have SMA are currently ongoing, including those who are older and heavier than in the initial trials.

Novartis Gene Therapies was also conducting clinical trials to see whether intrathecal administration of Zolgensma (i.e., directly into the fluid bathing the spinal cord and brain) would address the needs of those who are older than 2 years. These were put on hold, but were started again in August 2021. This is the STEER trial.

To create a gene therapy, components of a virus are used. This is because viruses have a natural ability to travel around the body once they gain access. However, most viruses also have the capacity to replicate themselves to produce many copies. That is why gene therapies only include the parts of a virus that allow it to travel around the body, not those parts that allow it to replicate.

Zolgensma is made from harmless parts of the AAV9 virus, which has been ‘gutted’ creating something like an empty envelope that can have something inserted. As the purpose of Zolgensma is to increase SMN protein, the SMN1 gene is put into the empty AAV9 envelope. When introduced into the blood, Zolgensma is able to travel through the bloodstream and reach the brain and motor neurons, delivering SMN protein to where it is needed.

Viruses are usually combatted by our immune systems. For example, we may initially suffer after contracting a flu virus, but when our immune system starts to work, we fight the virus and get better. After exposure to a particular virus, our bodies produce proteins called antibodies that will very specifically recognise and neutralise that same virus in the future. When this happens, we are said to have developed an immunity to that particular virus, which will help to prevent us from becoming ill again.

Our immune system responds to parts of AAV9 virus used in Zolgensma in the same way as to a flu virus. As soon as Zolgensma is introduced into a child, their body starts to produce antibodies against it. Despite this, Zolgensma can still produce the required SMN protein. If a child were to have a second injection of Zolgensma, it would not work, as they will have developed an immunity to it. This means that only a single injection of the treatment can be effectively used.

The AAV9 virus that is used to create Zolgensma is naturally found in the environment; so some children, including children with SMA, will have developed an immunity to AAV9 and will therefore not benefit from Zolgensma.

Children being considered for treatment with Zolgensma will first have their blood tested to see if they have this natural immunity. This is the AAV9 antibody test. If they show a response, they are unlikely to benefit from Zolgensma and are therefore ineligible for treatment.

In June 2021, Novartis Gene Therapies told us:

"A small number of children may have developed AAV9 antibodies before they are tested. Depending on what is called the ‘antibody titre level’, the presence of these antibodies could mean that it is not possible to administer zolgensma. Recent evidence suggests that only a small number of children may become ineligible for this reason (around 5 in 100 children tested). If AAV9 antibody titres are reported to be above 1:50, your child may be re-tested. AAV9 antibody results are valid for 30 days."

Any drug, including ones that are commonly prescribed or sold over the counter, can cause side effects. Side effects of medications are reported and recorded during their clinical trials. They are also picked up through the ongoing systems that are in place to monitor medications and their use in the ‘real world’. All possible side effects are listed in the Patient Information Leaflet that is given with any medication. They are grouped as:

  • very common
  • common
  • uncommon
  • rare.

For Zolgensma, possible side effects listed are:

Very common: may affect more than 1 in 10 people

  • increases in liver enzymes seen in blood tests.

Common: may affect up to 1 in 10 people

  • vomiting
  • fever.

An infection (e.g., cold, flu or bronchiolitis) before or after Zolgensma treatment may lead to more serious complications.

These and any rare or very rare side effects are listed in Novartis Gene Therapies’ Patient Information Leaflet which covers other questions as well (see Q 10 below).

Professor Muntoni has told us:

"No, they only have a similar name because one is an adeno viral vector, the other is an adeno-associated viral vector; although the names look similar, they’re not even like cats and dogs, they’re even more different ones. So, no there is no problem."

Zolgensma is a gene therapy that has been engineered to contain the SMN1 gene. Once Zolgensma has been injected, it travels around the body, reaching the body’s cells and increasing the amount of SMN protein that they can produce. Increasing SMN in this way, can counteract some of the negative effects caused by the low amount of this important protein. However, zolgensma is not a cure:

Timing is critical. Although the treatments can rapidly increase SMN protein production after administration, some irreversible damage may have already occurred in the nervous system. This may happen even before a baby is born.

It is not known whether any of the treatments will have continuing effects as they are all so relatively new (see clinical trials information).

No, they will not pass on the new healthy SMN1 gene. Zolgensma gene therapy does not integrate into the DNA of human cells. This is what makes Zolgensma safe to use – if it were to insert itself into DNA, it could disrupt the function of healthy genes and cause them to malfunction. Importantly, Zolgensma also does not integrate into the DNA of reproductive cells known as the “germline” (e.g., sperm/egg cells); therefore, the healthy SMN1 gene that has been engineered into Zolgensma cannot be passed on to the next generation.

In May 2020, the European Commission approved dosing guidance for Zolgensma to cover babies and young children who have SMA up to 21 kg.

In June 2020, a group of leading European clinicians published a consensus statement that stated,

"Since the risk of gene therapy increases with the dose administered and since the dose is directly proportional with the weight, patients above 13.5 kg should only be treated in specific circumstances…."

For all treatments, possible benefits have to be considered along with possible risks. One of the challenges for Zolgensma is the increased potential risk of children who are heavier needing an increasingly large viral load to deliver an effective treatment. Clinicians and researchers know from experience of other clinical trials that this increased viral load could in itself create some severe side effects and outcomes.

On 9th January 2023, NHS England announced a pause in the Zolgensma programme which takes effect across the UK while the Medicines and Healthcare products Regulatory Agency (MHRA) investigated safety concerns. The programme was reinstated in March 2023. You can read more about this on these pages:

UK Investigation into Zolgensma safety concerns 9th January – 28th March 2023

Novartis has started a global clinical trial of Zolgensma called SMART with 24 patients weighing between 8 and 21 kgs.

Yes, results from an observational study of the efficacy and safety of Zolgensma, which included children older than 24 months at treatment and who had previously received nusinersen, were published in the Lancet medical journal in October 2021. You can read a report of this study here.

For more information about any of the above topics, we suggest you talk to your clinician. You can also read Novartis Gene Therapies’ Patient Information Leaflet. This covers:

1. What Zolgensma is and what it is used for

2. What you need to know before your child is given Zolgensma

3. How Zolgensma is given

4. Possible side effects

5. How to store Zolgensma

6. Contents of the pack and other information

There are now three licenced drugs funded by the NHS for people who have SMA; until 2018, there were none. Different eligibility criteria apply to each one. There have been no studies to date that directly compare the treatments. You should discuss treatment options with your child’s clinical team in your specialist neuromuscular centre. Together, you will be able to discuss risks and benefits of any of the treatments that are potentially possible and make an informed decision as to which one is best for your child.