Among the 153 stiff person syndrome patients in Kyverna’s retrospective natural history study, the majority showed less than 20% improvement in Timed 25-Foot Walk speed on existing immunotherapies — a benchmark that makes the 100% immunotherapy-free rate at Week 16 in KYSA-8 less a promising signal than a categorical discontinuity. That contrast is the regulatory argument Kyverna just handed the FDA, and the agency accepted it: a single-arm trial, no randomized comparator, is now formally sufficient to support a BLA.
The pre-BLA alignment matters because single-arm approvals in rare neurological disease live or die on the credibility of the historical comparator. A 153-patient multicenter natural history study anchored to the same endpoint — T25FW — is not a casual data pull. It was designed to do exactly this: establish that T25FW tracks disability over time in SPS and that existing therapies leave it essentially unmoved. The FDA’s acceptance of that package as BLA-adequate suggests the agency is treating the natural history cohort as the de facto control arm. That’s a meaningful regulatory design precedent for autoimmune CAR-T broadly.
The durability data from Germany reinforce what a single-arm approval would need to hold up post-market: two patients treated through the individual hospital pathway have now maintained efficacy at 15 and 26 months, respectively, without chronic immunotherapy. One year of follow-up from KYSA-8, expected in the second half of 2026, will need to show the cohort tracks toward that durability range. If the T25FW gains regress at month nine or ten, the entire single-arm case softens — not just for SPS but for the generalized myasthenia gravis Phase 3 already enrolling across 15 sites. The gMG Phase 2 data showing 100% of patients hitting clinically meaningful MG-ADL and QMG reductions at one year is encouraging, but Phase 3 co-primary endpoints demand replication at scale, and enrollment pace at 15 sites will determine whether a gMG BLA follows SPS inside this decade.
The one number to watch through the rest of 2026 is the one-year T25FW responder rate from KYSA-8. It is the structural load-bearing element of this BLA, and if it holds at the same magnitude as Week 16, Kyverna’s 2027 launch timeline becomes a serious operational question rather than an aspirational one.
Jon Napitupulu is Director of Media Relations at The Clinical Trial Vanguard. Jon, a computer data scientist, focuses on the latest clinical trial industry news and trends.
