GDNF drug trial — your questions answered

The GDNF trial results offer hope that it may be possible to restore the cells damaged in Parkinson’s but they also raise a number of questions.

Dr Beckie Port
Mar 7, 2019 · 7 min read

We’re proud of what we’ve achieved so far. Our ultimate ambition is to find a cure and improve life for everyone affected by Parkinson’s, and we strongly believe GDNF has exciting potential that still needs exploring.

The trial has advanced our understanding of the potential effects of GDNF upon damaged brain cells, proved that delivering an experimental therapy in this way is feasible and acceptable to patients, and shown that it is possible to deliver drugs with precision to the brain.

These are all important steps forward that will advance and accelerate the development of new treatments for Parkinson’s. But the results from the trial are not clearcut, so here we answer your questions.

Q. When will GDNF be available as a treatment for Parkinson’s?

GDNF is still an experimental drug and is not available for people with Parkinson’s. Before any new medicine can be approved by regulatory authorities and made available to patients it must be proven to safe and effective in large-scale clinical trials.

For GDNF, this is likely to mean a further phase 2 trial, and then if that is successful, large-scale phase 3 trials — which together are likely to take several years at a minimum.

The results show promise that GDNF may have protective or even restorative effects but they are far from conclusive. Further, carefully planned studies are needed to explore the effects of GDNF in more detail. We’re fully committed to exploring the potential GDNF could hold. That’s why we’re working with the partners involved — the scientific team, companies and other charities — to explore routes to further studies.

We’re also committed to exploring other potential routes to new treatments and a cure. Our Virtual Biotech is investing in the best ideas and rapidly turning them into treatments that can be tested and progressed.

Q. Is it ethical to ask people to take part in a clinical trial that involves brain surgery if they will only receive a placebo treatment?

Any research study that involves people must be rigorously reviewed by an independent ethics committee. The job of the ethics committee is to protect the dignity, rights, safety and well being of people taking part in research.

Importantly, around a third of the people who sit on ethics committees are patients or members of the public, and they are there to think about the proposed research from the participants’ point of view.

Access to GDNF was a serious consideration in this trial and the research team decided that all participants in the study should have the chance to receive the active treatment. To allow this, the study was split into three distinct stages.

The initial safety and feasibility study involved 6 participants to ensure that the surgery, device and drug were safe and acceptable, and that the drug delivery system was working as intended.

Next, forming the main part of the study, was a 9-month double-blind placebo-controlled trial. 35 participants were randomly assigned by computer to receive monthly infusions of either GDNF or a placebo (an artificial form of the cerebrospinal fluid that surrounds the brain).

Once participants had completed the 9-month double-blind study, they were offered the opportunity to continue into an open-label extension study in which all participants received GDNF for a further 9 months. During this part of the trials, both the participants themselves and the researchers knew that everyone would be receiving GDNF. At the end of the trial, the participants also had the opportunity to find out if they had originally been in the GDNF or placebo group.

Q. Participants seemed to experience significant benefits in the documentary, why don’t the scientific results reflect this?

The scientific results do show an improvement in Parkinson’s assessments during both the initial 9-month double-blind phase and the extension period when all participants received GDNF.

The problem is that at the critical 9-month point when the double-blind trial finished, there was not a clear enough difference between the GDNF and placebo groups to prove that treatment with GDNF is better than placebo.

During the 9 month extension period, assessments showed a continued improvement in Parkinson’s symptoms in both groups. However, because everyone knew that they were receiving GDNF during this period and there was no comparison group, the improvement seen — while encouraging — is not scientifically conclusive.

While the results are not clear-cut we believe they hold real promise for people with Parkinson’s that deserves further exploration. For GDNF to move forward as quickly as possible we need to make sure that future studies address the questions raised by these results and deliver definitive results.

Q. Why has it taken so long to publish these results?

Since the clinical trials of GDNF came to an end in February 2017 the research team have worked tirelessly to analyse and understand the results so that they can be shared with the wider Parkinson’s community.

This has taken longer than expected partly because of the sheer volume and complexity of the data which took time to understand, but also because it is more challenging to publish ‘negative’ findings so this has added to the delays.

We’re working to speed up the publication of research results and accelerate progress towards new and better treatments by supporting initiatives like the AMRC Open Research.

Q. What has happened to the participants since the trials ended?

41 people with Parkinson’s underwent robot-assisted surgery to have four tubes carefully placed into their brains, which allowed GDNF to be infused directly to the affected brain areas via a port in the side of their head with pinpoint accuracy.

Many participants have continued to do extremely well and report that they are still feeling the benefits of participating in the trials.

They have all had the opportunity to discuss their personal results and future treatment options in one-to-one sessions with the trial team in Bristol. Depending on their personal preferences, some participants have chosen to have their port and peripheral tubing removed, others have elected to keep theirs.

You can read more of their stories at

Q. Why can’t the participants who benefited from GDNF continue to receive the treatment?

GDNF is still an experimental and unproven therapy. Its safety in the long-term is unknown so it can only be provided through a rigorously conducted clinical trial.

Before starting any clinical trial, individuals are given comprehensive and clear information to help them make an informed choice about whether to take part. This includes information about what is likely to happen at the end of the trial and whether they will be able to continue to receive the experimental treatment.

In this case, it was made clear that it may be possible for participants to continue to receive GDNF as part of ongoing clinical trials but this was not guaranteed and would be dependent on the results.

Because the results were inconclusive, the path to further trials is much more challenging. Unfortunately, it has not been possible to initiate further studies that would have provided the participants with an opportunity to continue receiving GDNF.

Q. Why didn’t Pfizer fund further trials?

In January 2018, after careful and lengthy consideration Pfizer confirmed that they would pull out of neurological research across a range of conditions, and therefore not be pursuing the development of GDNF for Parkinson’s.

This was very disappointing news for everyone involved in the current GDNF trials programme and especially for the participants.

Pfizer is only one of dozens of potential pharma partners for further clinical trials, and MedGenesis have continued to seek investment to take GDNF forwards into further studies and we are supporting MedGenesis’ efforts to take GDNF into further clinical trials.

We know, from our work with pharmaceutical companies across the globe, that there are many others still ready to devote tremendous time, effort and funding towards the development of better treatments for Parkinson’s.

For instance, our tireless work with the nine pharmaceutical companies who are members of our Critical Path for Parkinson’s consortium is helping smooth the path for all future drugs by making clinical trials smarter and faster.

Q. When would any future GDNF trial actually happen?

We want GDNF to move forward as quickly as possible so we need to make sure that the next clinical trial addresses the questions raised by these findings and delivers definitive results.

This means working with the international research community and with people affected by Parkinson’s to reflect and learn from these results and to get the plan for further studies absolutely right.

There are a huge number of factors to consider — from what dose to use and how best to measure the effects of GDNF, to assembling the right team to deliver it successfully. You can read more about these factors in our previous blog “GDNF — the results explained”.

While there are not yet any further trials of GDNF underway, there are many other studies in the UK looking for participants right now. These range widely, from simple questionnaires you can complete at home to trials testing experimental treatments that have the potential to improve symptoms or even slow, stop or reverse the condition. And it’s not just people with the condition that can take part, people without Parkinson’s are needed too.

It’s easy to browse these opportunities on our website using our Take Part Hub

Support our vital research

We’re proud pioneers behind the GDNF trial and we’ll leave no stone unturned in our search for a cure. But we can’t do it without you.

Donate now and make life-changing breakthroughs happen.

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Parkinson’s UK

Get the latest research news, discover more about…

Dr Beckie Port

Written by

Research Communications Manager at @ParkinsonsUK. Ex-researcher in oncology and virology.

Parkinson’s UK

Get the latest research news, discover more about Parkinson’s and read about how others are getting involved. For information and support, visit

Dr Beckie Port

Written by

Research Communications Manager at @ParkinsonsUK. Ex-researcher in oncology and virology.

Parkinson’s UK

Get the latest research news, discover more about Parkinson’s and read about how others are getting involved. For information and support, visit

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