A diabetes drug that seemed to slow Parkinson's down

Every medicine currently prescribed for Parkinson's treats the symptoms. Levodopa restores movement, sometimes dramatically, but the nerve cells keep dying underneath. Nothing on the market slows that down. Finding a treatment that does — a disease-modifying treatment — is the central unsolved problem of the field.

This trial tested an unlikely candidate: a drug developed for type 2 diabetes.

Why a diabetes drug?

Lixisenatide belongs to a family called GLP-1 drugs — the same class as semaglutide, now familiar from weight-loss coverage. They were designed to act on the pancreas, but their receptors also appear on nerve cells in the brain.

Over the past fifteen years, laboratory work has suggested these drugs might do something useful there: reduce inflammation, improve how cells manage energy, and help nerve cells survive stress. In animal models of Parkinson's, they protected dopamine-producing cells. An earlier small trial of a related drug, exenatide, produced encouraging results that the field had been arguing about ever since.

The logic of repurposing is straightforward: these drugs are already approved, their safety profile is known from millions of patients, and they are relatively inexpensive. If one of them slowed Parkinson's, it could reach people quickly.

What they did

The trial enrolled 156 people with Parkinson's diagnosed within the previous three years, across a network of hospitals in France, with 78 assigned to each group. Participants were randomly assigned to a daily injection of lixisenatide or a matching placebo injection, and neither they nor the assessing doctors knew which was which — a double-blind design.

Everyone stayed on their normal Parkinson's medication throughout. The trial ran for 12 months, followed by a two-month washout period in which everyone stopped the trial drug.

The primary endpoint was the change in the motor examination portion of the MDS-UPDRS at 12 months, scored while participants were on their usual medication. Higher scores mean worse symptoms.

What they found

The headline is the first row. The placebo group followed the expected trajectory for early Parkinson's: gradual worsening over the year. The lixisenatide group did not measurably worsen at all.

The difference of 3.08 points was statistically significant (95% confidence interval 0.86 to 5.30, p=0.007).

After the two-month washout, when nobody had taken the study drug for eight weeks, the gap was still visible: motor scores off medication were 17.7 in the lixisenatide group against 20.6 on placebo. That detail matters, because it hints the drug had done something lasting rather than simply masking symptoms while it was in the bloodstream. But the confidence interval on that washout difference runs from 0.1 to 5.8 — it only just excludes zero, and a range that wide is consistent with an effect that is real and large, or real and negligible.

The side effects were not incidental. Nearly half the treatment group experienced nausea against 12% on placebo, and 13% vomited against 3%. Some participants needed their dose reduced.

Why it matters

A 3-point difference on a scale that runs past 100 is not something a person would necessarily feel after one year. Its importance is as a proof of principle.

If the effect is real and it continues to accumulate, three points a year compounds. Over five years, that is the difference between independence and dependence for some people. The reason to take this seriously is not the size of the gap at 12 months but the possibility that the underlying process was slowed.

There is also a strategic argument. GLP-1 drugs are manufactured at enormous scale and are already in the world's pharmacies. A repurposed treatment does not need the decade a novel compound requires.

What this doesn't tell us

Whether the drug slowed the disease or improved symptoms. This is the crux. A drug that makes symptoms better looks identical, on a rating scale, to a drug that slows the damage — for a while. The washout result leans toward genuine modification, but two months is a short washout for a disease that moves over decades.

Whether it works in people further along. Everyone here was within three years of diagnosis. Results in early disease often do not transfer to people who have had Parkinson's for fifteen years.

Whether the tolerability is workable. A trial can support participants through nausea with close monitoring. Sustaining a daily injection with that side-effect profile for years, in ordinary life, is a different proposition.

Whether it holds up at scale. 156 participants is a reasonable phase 2 trial. It is not a phase 3 trial. The history of Parkinson's research is full of promising mid-size trials that did not survive a larger one — as next week's issue shows.

What to watch next

The obvious next step is a larger, longer trial — more participants, more sites, several years rather than one.

The wider question is whether this is a class effect. Lixisenatide is one of several GLP-1 drugs. Exenatide has been tested in Parkinson's for years, and results from a larger trial of it are due. If the whole class helps, the case becomes much stronger. If lixisenatide works and exenatide does not, the field has a puzzle.