The idea was so tantalizing. Drugs in the GLP-1 class, which includes Wegovy and Ozempic, have proved miraculous in treating weight loss and other diseases. And some researchers hoped that the drugs could also help with some of the most difficult diseases to treat — those of the brain, like Parkinson’s.

But now, at least for Parkinson’s, that hope seems dimmed. A rigorous study that randomly assigned Parkinson’s patients to take exenatide, a relative of Ozempic, showed absolutely no benefit or slowing of the course of the degenerative disease after 96 weeks.

And there were no effect on patient symptoms, no effect on brain scans, no subgroup that showed any benefit. No matter how the researchers sliced the data the results were the same.

The study, published Tuesday in The Lancet, is bad news for the half million Americans who have been diagnosed with Parkinson’s disease. Symptoms include tremors, stiffness and difficulty with balance. Patients also may develop dementia. Treatments, including medications and deep brain stimulation, can help with symptoms. But no treatment has been shown to slow the disease’s progress.

“It’s hugely disappointing,” said Dr. Thomas Foltynie of University College London, who led the trial. “We were expecting we would come through and we would get a positive result.”

Parkinson’s experts shared his sentiment.

“This is a sobering moment,” said Dr. Michael S. Okun, a Parkinson’s disease expert at the University of Florida and the national medical adviser for the Parkinson’s Foundation. “This is a really well done study and it came up empty-handed.”