Key Inclusion Criteria:

* Subject must have been diagnosed SPS per the following criteria:

* Rigidity of limb and axial (trunk) muscles prominent in the abdominal and thoracolumbar paraspinal areas and making bending difficult
* Clinical or electrophysiological evidence of continuous contraction of agonist and antagonist muscles
* Episodic spasms precipitated by unexpected noises, tactile stimuli, or emotional upset * Absence of any other neurologic disease that could explain the stiffness and rigidity
* High titer serum anti-GAD65 antibodies shown at screening -OR- seropositive for anti-glycine antibodies. If anti-GAD65 antibodies are lower than the high titer threshold peripherally but positive in the cerebrospinal fluid (CSF), the subject can be included. A prior documented high titer anti-GAD65 antibody level may be acceptable subject to sponsor review.

* Active symptoms with inadequate response to at least one immunomodulatory therapy.
* Stiffness index ≥2.
* At least 20 of the 25 enrolled subjects should be ambulatory.

Key Exclusion Criteria:

* Bedridden subjects for more than 3 months.
* History of CNS or spinal cord tumor, metabolic or infectious cause of myelopathy, genetically inherited progressive CNS disorder, sarcoidosis, non-SPS progressive neurologic condition or progressive multifocal leukoencephalopathy (PML).
* History of stroke, seizure, dementia, Parkinson's disease, cerebellar diseases, psychosis, aphasia, and any other neurologic disorder that is of a nature and severity that the investigator considers would increase the risk for the subject.
* Cardiac ejection fraction ≤ 40%.

Note: Other protocol defined Inclusion/Exclusion Criteria may apply.

Eligibility last updated 11/14/2024. Questions regarding updates should be directed to the study team contact.