Novel Therapy Shows Promise in Improving Mobility for Stiff Person Syndrome Patients, May Become First Approved Treatment

A novel CAR T-cell therapy has shown significant improvements in mobility for patients with stiff person syndrome (SPS), a rare and debilitating autoimmune disorder, according to results from a registrational trial announced by Kyverna Therapeutics on April 21, 2026.

The therapy, mivocabtagene autoleucel (miv-cel), demonstrated statistically significant and clinically meaningful improvements in mobility and disability scores in the KYSA-8 trial, which included 26 patients with SPS. After a single dose, patients achieved substantial improvements in various mobility and disability measures within 16 weeks, with nearly all patients achieving functional independence and remaining off chronic treatments.

On the timed 25-foot walk (T25FW), a key measure of mobility, patients treated with miv-cel experienced a median improvement of 46% at 16 weeks. 81% of patients achieved at least a 20% improvement on this metric. The therapy also met all secondary endpoints, including improvements in ambulation, sensitivity, disability, and distribution of stiffness across the body.

At the time of data readout, all patients had discontinued immunotherapies and none required rescue therapy. Kyverna Therapeutics stated this outcome highlights “miv-cel’s potential to provide unprecedented clinical benefit while significantly reducing or eliminating chronic treatment burden.”

Regarding safety, no high-grade cases of cytokine release syndrome or immune effector cell-associated neurotoxicity syndrome were observed. Cases of grade 3/4 neutropenia were reported but considered manageable.

The company plans to submit a biologics license application (BLA) for miv-cel to the U.S. Food and Drug Administration in the first half of 2026. If approved, it would be the first CAR T-cell therapy authorized for an autoimmune disease.

Analysts from William Blair described the results as “impressive” and stated in a note to investors that the data are “extremely positive and supportive of the upcoming BLA submission for miv-cel in SPS, setting it up to potentially be the first FDA-approved CAR-T for the treatment of an autoimmune disease.” Following the announcement, Kyverna’s stock rose by 23%, closing at $10.82 per share.

Stiff person syndrome is a rare neurological disorder characterized by progressive muscle stiffness and spasms, often leading to disability. You’ll see currently no approved treatments for the condition, and management typically involves immunosuppressive therapies and symptomatic treatments that do not alter disease progression.

The positive trial results suggest miv-cel could transform the treatment paradigm for SPS by offering a potential disease-modifying approach that reduces the need for ongoing immunotherapy. Kyverna Therapeutics will host a conference call to discuss these results and additional data.