Assess the efficacy and safety of a 10% intravenous immunoglobulin (IGIV) in patients with stiff person syndrome (SPS).

SPS is an ultra-rare, progressive autoimmune neurological disorder defined by fluctuating muscle rigidity and painful spasms in the trunk and limbs. Currently, treatment focuses on symptomatic relief through GABA-enhancing drugs, and immunotherapy which includes off-label use of IVIG or immunosuppressive agents. QIVIGY is a 10% IGIV licensed in the US for treatment of adults with primary humoral immunodeficiency.

NCT07552987 is a phase III, double-blind, randomized, placebo-controlled, parallel group multi-center study assessing the efficacy and safety of QIVIGY administered every 4 weeks to adults with SPS, for 24 weeks, followed by an optional open label extension (OLE) for an additional 24 weeks. Approximately 38 subjects will be enrolled. Subjects will be 18-70 years old, with a documented SPS diagnosis, GAD65 or glycine-receptor antibody positive, a distribution of stiffness score of ≥2, paravertebral stiffness and torso/lower extremity predominance, and not bed-bound or wheelchair-dependent. Subjects will be randomized at the Baseline Visit in a 1:1 ratio to receive either 2 g/kg or an equivalent volume of placebo (each infusion over 2-5 days).

From weeks 24 to 48, subjects will receive QIVIGY 2 g/kg every 4 weeks in the OLE.

Subjects will continue to receive stable symptomatic therapy throughout the trial.

The primary efficacy endpoint is the proportion of participants showing an improvement on the Timed 25-Foot Walk from baseline to week 24. Secondary efficacy endpoints include changes in the distribution of stiffness index, heightened sensitivity scale, modified Rankin scale, Neuro-QOL, and Pain Interference score. Safety endpoints include adverse events.

This randomized, first multi-center, double-blind, placebo-controlled trial is designed to evaluate the safety and efficacy of high-dose QIVIGY in patients with classic SPS, addressing an important unmet need in this ultra-rare disorder.