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A diabetes drug can slow the progress of Parkinson’s disease, and seems to target the underlying cause of the condition, not just its symptoms. The finding adds weight to the theory that the two conditions work in a similar way.

Parkinson’s disease leads to the loss of brain cells that make dopamine, a chemical that helps control body movements. Standard treatments for the condition try to replace the missing dopamine. “That goes a good way to improving symptom control, but it does nothing for the underlying disease pathology,” says of the National Hospital for Neurology and Neurosurgery in London.

There is still no cure for Parkinson’s. While replacing dopamine can improve the tremors and stiffness, it doesn’t stop the brain from continuing to deteriorate. In an attempt to slow this, Foltynie and his colleagues have turned to a drug typically used to treat type 2 diabetes, called exenatide.

This drug comes from a class of compounds originally isolated from the venom of a lizard called the gila monster. Not only can these help control blood sugar levels – which is useful for people with diabetes – this kind of drug also seems to protect neurons from toxins.

Improved symptoms

Foltynie’s colleagues gave exenatide to 31 people with moderate Parkinson’s disease over 48 weeks. The participants injected themselves with the drug every day – except the day before assessments – while 29 people with a similar level of disease did the same with a placebo.

Eight weeks after the end of the trial, Foltynie’s team assessed all the volunteers’ symptoms. Those who had received the placebo had deteriorated over the course of the year – by an average of three points on a 200-point scale, a typical rate of decline for people with Parkinson’s.

But those who had taken exenatide showed an average improvement of one point, putting them, on average, four points ahead of the placebo group. Brain scans also showed that those taking the drug showed less degeneration. However, these effects were subtle – they didn’t perform better on cognitive tests, and there wasn’t a visible improvement in their day-to-day symptoms.

Shared roots

“You can say with a reasonable level of certainty that the drug is slowing the disease down, if only by a small amount,” says , a neuropharmacologist at the charity Parkinson’s UK.

The fact that a diabetes drug seems to help Parkinson’s adds to a growing body of research suggesting that neurodegenerative diseases like Alzheimer’s and Parkinson’s may work in a similar way to diabetes, and that neurons can become unresponsive to insulin in the same way that cells in the pancreas do in type 2 diabetes.

This can affect how cells produce energy, causing them to starve and leading to inflammation. Exenatide and drugs like it seem to protect cells from this damage, says Foltynie.

Exenatide is thought to be generally safe, although its side effects can include an upset stomach and weight loss. The drug isn’t yet recommended for Parkinson’s, says Foltynie. “We need to remain scientific and have proof that the benefits outweigh the risks.”

Read more: Are Alzheimer’s and diabetes the same disease?