Kyverna Therapeutics has reported one-year data showing that clinical improvements with its single-dose CD19 CAR T-cell therapy mivocabtagene autoleucel remained durable in patients with stiff person syndrome, strengthening the evidence being assembled for an ongoing United States regulatory submission. In the 26-patient KYSA-8 registrational study, median improvement in the Timed 25-Foot Walk reached 49% at 12 months, while 92% of patients remained free of chronic immunotherapies. Longer-term follow-up from a separate generalized myasthenia gravis program also showed sustained responses, extending the company’s argument that deep B-cell depletion could potentially provide prolonged disease control after a single treatment.
Kyverna Therapeutics plans to include the latest stiff person syndrome results in its rolling Biologics License Application, which remains scheduled for completion during the fourth quarter of 2026. The data are encouraging, particularly because there are currently no FDA-approved therapies specifically for stiff person syndrome, but the evidence comes from a small single-arm trial and requires careful interpretation until regulators assess the complete clinical, manufacturing and safety package.
One-year KYSA-8 results show mobility improvements persisting after a single miv-cel treatment
KYSA-8 enrolled 26 adults with stiff person syndrome who had experienced an inadequate response to at least one previous immunotherapy. Each participant received one infusion containing 100 million miv-cel CAR T cells, with the study evaluating mobility through the Timed 25-Foot Walk as one of its primary measures.
The original primary analysis showed a median 46% improvement from baseline in walking time at Week 16. At the 12-month assessment, the improvement remained at 49%, suggesting that the mobility benefit observed shortly after treatment had not meaningfully deteriorated during longer follow-up. Among patients who achieved a clinically meaningful improvement of more than 20% during the primary analysis, 95% maintained that response at one year.
Functional changes were also visible in patients with more substantial baseline disability. Twelve participants required a walking aid before receiving miv-cel, and 67% of those patients continued to walk without assistance at the one-year assessment. More than one-third of the overall study population completed the Timed 25-Foot Walk in less than five seconds, which Kyverna Therapeutics described as comparable to typical performance among healthy adults.
Secondary measures evaluating disability, stiffness, hypersensitivity and mobility also remained significantly improved at 12 months. These included the Modified Rankin Scale, Hauser Ambulation Index, Distribution-of-Stiffness Index and Heightened Sensitivity Scale, giving investigators several independent measures suggesting that the treatment effect extended beyond a single walking-speed endpoint.
The findings are clinically relevant because stiff person syndrome is a progressive autoimmune neurological disorder associated with severe muscle stiffness, painful spasms and declining mobility. Kyverna Therapeutics estimates that approximately 6,000 diagnosed patients live with the disease in the United States, while current management relies on symptomatic treatment and off-label immunotherapies such as intravenous immunoglobulin, rituximab and plasmapheresis.
Freedom from chronic immunotherapy adds another dimension to the SPS durability findings
One of the more notable outcomes was the continued reduction in dependence on chronic immunotherapy. At 12 months, 92% of patients remained free of ongoing immunotherapies for stiff person syndrome, compared with all 26 patients being treatment-free at the earlier Week 16 analysis.
That finding matters because miv-cel is being developed around the concept of resetting B-cell-driven autoimmunity rather than continually suppressing immune activity through repeated medication. The therapy is an autologous CD19-targeted CAR T product using a fully human CAR construct with CD28 co-stimulation, designed to produce deep depletion of B cells involved in generating disease-driving antibodies.
Kyverna Therapeutics has described this approach as potentially enabling prolonged drug-free remission, although the current trials do not yet establish that patients are cured or permanently free of disease. One year represents meaningful follow-up for an emerging autoimmune CAR T approach, but longer observation will be required to determine how often symptoms recur as B-cell populations recover.
Safety also remained relatively favorable through the one-year follow-up. Kyverna Therapeutics reported no high-grade cytokine release syndrome, no immune effector cell-associated neurotoxicity syndrome and no cases of immune effector cell-associated hemophagocytic lymphohistiocytosis-like syndrome. More than 100 patients have now received miv-cel across autoimmune-disease studies, according to the company, providing a broader but still developing safety database.
These findings are particularly relevant as safety has become an increasingly important question across autoimmune CAR T development. Cell therapies can produce powerful immune effects, and establishing that efficacy can be achieved without severe cytokine or neurological complications will be important if the modality is to expand beyond oncology into chronic autoimmune diseases.
Generalized myasthenia gravis results suggest the durability signal may extend beyond SPS
Kyverna Therapeutics simultaneously reported longer-term follow-up from the seven-patient Phase 2 portion of KYSA-6 in generalized myasthenia gravis. All seven patients achieved clinically meaningful improvements at 24 weeks in both the Myasthenia Gravis Activities of Daily Living scale and Quantitative Myasthenia Gravis score.
Five patients have now reached at least one year of follow-up, extending to approximately 18 months in some cases, and all five maintained clinically meaningful improvements across major disease measures. Minimal symptom expression, defined as an MG-ADL score of zero or one, was maintained in 57% of participants at their latest assessment.
Treatment independence again emerged as an important component of the dataset. All seven patients were free of immunotherapies at 24 weeks, including nonsteroidal immunosuppressants, high-dose steroids, FcRn inhibitors and complement inhibitors, while six of seven remained off immunosuppressive therapies at their latest follow-up.
The Phase 2 population is too small to establish definitive efficacy, but it provides supportive evidence for the ongoing randomized Phase 3 portion of KYSA-6. That study is expected to enroll approximately 60 patients and compare miv-cel with standard care using MG-ADL and QMG as co-primary endpoints, with enrollment expected to finish by mid-2027.
The two datasets together suggest that the biological effect of miv-cel may not be confined to one neurological autoimmune disorder. Kyverna Therapeutics is also developing the therapy in non-active secondary progressive multiple sclerosis, and miv-cel has received FDA Regenerative Medicine Advanced Therapy designation in stiff person syndrome, generalized myasthenia gravis and that progressive multiple sclerosis setting.
Rolling FDA submission puts the stiff person syndrome program close to its regulatory test
Kyverna Therapeutics initiated its rolling stiff person syndrome BLA earlier this year after reaching alignment with the FDA during pre-submission discussions. The Chemistry, Manufacturing and Controls module has already been submitted, and management continues to target completion of the remaining application during the fourth quarter.
The company plans to seek Priority Review under the program’s RMAT designation, potentially positioning miv-cel for a commercial launch in 2027 if the application is accepted and ultimately approved. Kyverna Therapeutics has already begun launch preparation, including commercial-site activation, supply planning, payer discussions, patient advocacy engagement and recruitment of commercial leadership.
The regulatory strategy nevertheless carries risk because KYSA-8 is a 26-patient, single-arm Phase 2 registrational trial rather than a conventional randomized pivotal study. The FDA will need to determine whether the magnitude, consistency and durability of improvement are sufficient in the context of the disease’s rarity, lack of approved therapies and natural history.
Full one-year KYSA-8 data are scheduled for presentation at the joint ACTRIMS-ECTRIMS meeting in Toronto in October. More detailed generalized myasthenia gravis findings are expected at the American Association of Neuromuscular & Electrodiagnostic Medicine meeting on September 29, providing additional opportunities to examine the topline results in greater depth.
Kyverna shares reverse early gains despite positive miv-cel durability results
Kyverna Therapeutics shares initially rose strongly after the data release, gaining nearly 10% during early trading after also advancing in premarket activity. That enthusiasm subsequently reversed, with the stock trading around $6.75 to $6.80 by late morning, roughly 4% below the previous close of $7.07, despite unusually heavy trading volume.
There was no obvious negative clinical finding in the topline announcement that directly explains the reversal, making attribution uncertain. The stock movement may instead reflect expectations already embedded ahead of the scheduled one-year update, questions investors want answered in the full datasets or broader trading dynamics around a small-cap biotechnology company.
The financial position gives Kyverna Therapeutics time to navigate the regulatory process. The company held approximately $199.4 million in cash, cash equivalents and marketable securities at the end of June and expects its existing resources, together with available financing capacity, to support operations into 2028. Second-quarter research and development spending totaled $24.8 million, while the company reported a net loss of $38.3 million.
Analyst sentiment remains broadly positive despite the share-price volatility. S&P Global data compiled by StockAnalysis show six analysts with a Strong Buy consensus and an average price target around $30.40, although those projections remain highly uncertain given the company’s dependence on regulatory decisions and relatively small clinical datasets.
Miv-cel has now demonstrated that its initial stiff person syndrome mobility improvements can persist through one year while most patients remain free from chronic immunotherapy. The next question is whether that durability, combined with the observed safety profile and the disease’s high unmet need, will be sufficient for the FDA to accept a single-arm CAR T dataset as the basis for the first approved treatment specifically for stiff person syndrome.
