Inclusion Criteria:

• Patient and/or authorized representative must be willing and able to give informed consent/assent and any authorizations required by local law for participation in the study.
• Patient and their caregiver must be willing and able (in the Investigator’s opinion) to comply with all protocol requirements.
• Patient must be between 2 and 18 years (inclusive) of age at Screening.
• Patient must have DS as defined by:
  • Onset prior to 12 months of age with recurrent focal motor or hemiconvulsive or generalized tonic-clonic seizures, which are often prolonged and triggered by hyperthermia;
  • No past history of causal magnetic resonance imaging (MRI) lesion (MRI not required to confirm absence of lesion);
  • No other known etiology;
  • Normal development at seizure onset.
• Patient must have a documented pathogenic, likely pathogenic variant, or variant of uncertain significance in the SCN1A gene. Patients who have SCN1A testing results of Negative (no variants of clinical significance identified) cannot be enrolled.
• Patient has had at least 2 prior treatments for epilepsy that either had lack of adequate seizure control (requiring an additional antiepileptic drug [AED]) or had to be discontinued due to an AE(s).
• Patient must be experiencing 4 or more convulsive seizures (Hemiclonic, Focal With Motor Signs, Focal To Bilateral Tonic Clonic Convulsion, Generalized Tonic Clonic Convulsion, Tonic, Tonic/Atonic [Drop Attacks], and Clonic) during the initial 28 days of the Observation Period.
• Patient must currently be taking at least one AED at a dose which has been stable for at least 4 weeks prior to Screening.
• All epilepsy medications or interventions for epilepsy (including ketogenic diet or vagal nerve stimulator) must have been stable (including total daily dose and setting) for at least 4 weeks prior to Screening. Vagal nerve stimulator implantation must have occurred at least 6 months prior to Screening.
• Any marijuana- or cannabinoid-based product or medication is allowed but treatment must have been stable for at least 4 weeks prior to Screening, including supplier, ratio, and dose.
• Patient must meet age-appropriate institutional guidelines for intrathecal (IT) drug administration procedures.
• Patient and/or family (or caretaker) must be sufficiently fluent in English or local language to be able to complete questionnaires relevant to this study.
• Patient must have negative pregnancy test at Screening and at Baseline (Day -1) if female of childbearing potential.

Exclusion Criteria:

• Patient has one of the following mutations in the SCN1A gene:
  • Thr226Met, Leu263Val, Val422Leu, Thr1174Ser, Trp1204Arg, Pro1345Ser, Gln1489Lys, Phe1499Leu, Arg1575Cys, Val1611Phe, Leu1624Pro, Arg1648Cys, Leu1649Gln, Leu1670Trp, Gly1674Arg, or Asp1866Tyr.
• Patient has a known pathogenic mutation in another gene that causes epilepsy. (The pathogenic mutation must be homozygous in cases of known recessive disease).
• Patient is currently being treated with an AED acting primarily as a sodium channel blocker, as maintenance treatment including:
  • phenytoin, carbamazepine, oxcarbazepine, lamotrigine, lacosamide, or rufinamide.
• Patient has clinically significant unstable medical conditions other than epilepsy.
• Patient has had clinically relevant symptoms or a clinically significant illness in the 4 weeks prior to Screening or prior to dosing on Day 1, other than epilepsy.
• Patient has had clinically relevant symptoms or a clinically significant illness (in the Judgement of the Investigator) in the 4 weeks prior to Screening through prior to dosing on Day 1, other than epilepsy. This includes the presence of an active infection requiring systemic antiviral or antimicrobial therapy.
• Patient has a history of brain or spinal cord disease (other than epilepsy or DS) or a history of bacterial meningitis or brain malformation.
• Patient has a spinal deformity or other condition that may alter the free flow of CSF or has an implanted CSF drainage shunt.
• Patient has clinically significant (in the judgment of the Investigator) abnormal laboratory values at Screening or prior to dosing on Day 1.
• Patient has aspartate aminotransferase or alanine aminotransferase > 2.5-fold upper limit of normal (ULN), serum creatinine >ULN or platelet count < 100 platelets per µL at Screening and upon repeat testing.
• Patient has clinically relevant abnormalities in the 12-lead ECG measured at Screening or prior to dosing on Day -1.
• Patient has a psychiatric or behavioral disorder which, in the opinion of the Investigator, may interfere with the patient’s participation in the study.
• Patient is currently taking or within 4 weeks prior to Day 1, Visit 1 has taken any anticoagulant (including but not limited to heparins, warfarin and other vitamin K antagonists, dabigatran, rivaroxaban and apixaban), or within 7 days prior to Day 1, Visit 1 has taken any antiplatelet (including but not limited to aspirin, non-steroidal anti-inflammatory drugs, clopidogrel, ticlopidine. and dipyridamole).
• Patient is a female of childbearing potential, or patient is a fertile male with female partner(s) of childbearing potential, unless willing to ensure that they or their partners use highly effective contraception throughout the duration of the study and for at least 6 months after their last dose of STK-001.
• Female patient who is pregnant, lactating, or planning pregnancy during the course of the study and for 3 months thereafter.
• Patient who is currently enrolled in or has been part of a blinded clinical study involving an investigational medicinal product within  2 months (or 5 half-lives, whichever is longer) prior to Screening.
• Patient has been part of an interventional clinical study in which an investigational gene therapy or in which STK-001 was administered.
• Patient has any other significant disease or disorder which, in the opinion of the Investigator, may either put the patient at risk because of participation in the study, may influence the results of the study, or may affect the patient’s ability to participate in the study.

Eligibility last updated 1/20/22. Questions regarding updates should be directed to the study team contact.