Our trial of ondansetron as a treatment for hallucinations in Parkinson's is restarting

What is the TOP HAT trial?

This major trial funded through Parkinson's UK's Virtual Biotech programme is investigating a drug called ondansetron in people with Parkinson's or dementia with Lewy bodies who experience visual hallucinations.

Ondansetron is a drug that is already approved and in use in the UK, mainly to help reduce nausea in people undergoing cancer treatment. The drug first showed potential for treating visual hallucinations in Parkinson's in the early 1990s.

The TOP HAT trial is testing whether ondansetron is better than a placebo at reducing the frequency and severity of visual hallucinations in people with Parkinson’s and people with dementia with Lewy bodies.

Why was the trial paused and why is it restarting?

After the first 100 participants had taken part in the study a preliminary analysis was carried out to look at how well the study drug was tolerated and address any safety concerns as promptly as possible.

After looking at this data, the independent Data Monitoring and Ethics Committee for the trial recommended that recruitment should stop as they thought there were more safety issues in those who received ondansetron than the placebo. The safety of participants is always the top priority in clinical trials, and the trial team moved quickly to stop the study in early 2024 on this advice.

Since the trial was halted the team have worked tirelessly to gather the outstanding data and investigate the safety issues that were identified. 

Thanks to this painstaking work, they were able to identify that the safety concerns were actually due to data recording errors.

Now that these question marks have been resolved, the team has regulatory approval to restart the trial.

Recruitment is back underway now

Chief Investigator, Professor Suzanne Reeves from University College London, explains the aims of the TOP HAT trial: "Up to 75% of people with Parkinson's may experience hallucinations at some point but current treatment options can cause side effects and increase mortality (risk of death).

"We desperately need better treatments to help people who experience hallucinations as part of their Parkinson's and small studies have suggested that ondansetron could have potential.

"That's why we are so dedicated to completing this important study. We were making good progress before the trial was paused and we’re delighted to be able to resume.

"We are now keen to hear from people with either Parkinson’s or dementia with Lewy bodies who experience hallucinations and would like to take part."

You can find further information about the TOP HAT study on our Take Part Hub. You can take part at research sites across England, Scotland, Northern Ireland and Wales. Visit the Take Part Hub.