An experimental new drug for stiff person syndrome restores mobility

A living drug could dial back the debilitating symptoms of stiff person syndrome.

After just a single infusion, patients with the autoimmune disorder saw meaningful improvements in mobility, stiffness and disability, neurologist Amanda Piquet reported April 21 at the annual American Academy of Neurology meeting in Chicago.

The drug, an experimental cell therapy called miv-cel, stands out from current treatment options for several reasons — including how well it appears to work. “The results of this trial are truly remarkable,” Piquet, of the University of Colorado Anschutz in Aurora, said at an April 13 news conference. “The magnitude and consistency of functional improvement observed is unprecedented.”

Stiff person syndrome, or SPS, which may affect more than 5,000 people in the United States, is a mysterious disorder perhaps best known for afflicting singer Celine Dion. In people with SPS, their own antibodies can go rogue, attacking the brain and spinal cord. Symptoms include intense muscle contractions, chronic pain and spasms so strong they break bones. There are no cures for the disorder and no treatments approved by the U.S. Food and Drug Administration.

Current therapies include drugs to treat symptoms and immune therapies approved for diseases other than SPS. But these have limited benefits, Piquet said, and over time, patients often lose mobility, requiring walking aids or wheelchairs.

With the new therapy, developed by Kyverna Therapeutics, a biopharmaceutical company based in Emeryville, Calif., doctors looked beyond symptom relief and zeroed in on the root problem: rogue antibodies. Scientists borrowed a strategy from oncology called CAR T cell therapy, where patients’ immune cells are genetically modified to attack a specific target. Miv-cel eliminates B cells, the body’s antibody factories. Wiping out these cells gives the immune system a factory reset, Piquet said, ultimately clearing out harmful antibodies.

In a Phase II clinical trial with 26 patients to determine effectiveness, the strategy seemed successful. About four months after treatment, patients’ walking speed improved, and eight of 12 people who had relied on a walking aid no longer needed one, Piquet reported. In a video presented during her talk on April 21, Piquet highlighted one patient’s improvements. Before treatment, he was walking stiffly and slowly with a walker. After treatment, he was able to quickly stroll. “And at his last follow-up,” she said, “he is running.”

Stanford University neurologist Paul George called the results “very exciting.” The work represents an advance in a disorder where no good treatment options exist, he said at the news conference.

The most common serious side effect Piquet’s team reported was a low white blood cell count. And scientists don’t yet know whether patients will need multiple infusions of the drug. “Obviously the hope and dream is once and done, right?” Piquet said. “We would love that.”

Kyverna plans to request FDA approval for miv-cel in the first half of 2026. If approved, the therapy would be the first CAR T cell therapy for autoimmune disease.