Arm 1 (Initial Phase): 

We will open enrollment to subjects aged 2 months to 17 years with STXBP1-E and seizures. For SLC6A1, seizures occur later in the course (typically middle of 1st decade) and so seizures will not be an entry criteria. Subjects will be considered eligible to participate in this study if each one of the following inclusion criteria is satisfied at screening (and prior to dosing). 

1. Diagnosed with STXBP1-E or SLC6A1-NDD; confirmed by laboratory report (i.e., a genetic test with a pathogenic or likely pathogenic mutation of STXBP1-E orSLC6A1-NDDand a clinical picture consistent with the disorder, as determined by the Investigator). Children with the appropriate clinical picture, a de novo variant of uncertain significance in STXBP1-E or SLC6A1-NDD will also be eligible for enrollment, at the discretion of the Investigator. If the mutant is classified as definitively non-pathogenic, we would not enroll the child. “Appropriate clinical picture” is at the discretion of the Investigator. 
2. Is between 2 months and 17 years of age, inclusive.
3. For children with STXBP1-E, the child must have had at least one seizure in the past 30 days prior to enrollment. (If there is high demand for the study and we have several subjects to choose, we will prefer to enroll children with a high number of seizures in the past month.)
4. Is in general good health, aside from neurological consequences of STXBP1-E or SLC6A1-NDD, as determined by having no concurrent medical illness, in the opinion of the site investigator, that places the subject at increased risk of adverse drug reactions or that will interfere with study follow-up.
5. Has normal laboratory test results (≤ 1.5 × upper limit of normal [ULN]) for serum aminotransferase (aspartate aminotransferas [AST] and alanine aminotransferase [ALT]) concentrations and ammonia.
6. Has normal renal function, with estimated glomerular filtration rate > 90 mL/minute/1.73 m2 at Screening (using the Chronic Kidney Disease Epidemiology Collaboration equation).
7. Has a platelet count > 150 × 103/μL at Screening.
8. Has a QT interval corrected with Fridericia's formula (QTcF) < 450 msec on the Screening EKG.
9. Parent or guardian is able to comprehend and willing to sign an informed consent form (ICF). 

   
Arm 2 (Expansion Phase): 
   
We will open enrollment to subjects aged 0 months to 15 years with a monogenetic developmental and epileptic encephalopathy and seizures. 

1. Diagnosed with a monogenetic developmental and epileptic encephalopathy; confirmed by laboratory report (i.e., a genetic test with a pathogenic or likely pathogenic mutation of a monogenetic developmental and epileptic encephalopathy and a clinical picture consistent with the disorder, as determined by the Investigator). Children with the appropriate clinical picture, a de novo variant of uncertain significance in a monogenetic developmental and epileptic encephalopathy will also be eligible for enrollment, at the discretion of the Investigator. If the mutant is classified as definitively non-pathogenic, we would not enroll the child. “Appropriate clinical picture” is at the discretion of the Investigator. 
2. Is between 0 months and 15 years of age, inclusive.
3. The child must have had at least one seizure in the past 30 days prior to enrollment. (If there is high demand for the study and we have several subjects to choose, we will prefer to enroll children with a high number of seizures in the past month.)
4. Is in general good health, aside from neurological consequences of their monogenetic developmental and epileptic encephalopathy, as determined by having no concurrent medical illness, in the opinion of the site investigator, that places the subject at increased risk of adverse drug reactions or that will interfere with study follow-up.
5. Has normal laboratory test results (≤ 1.5 × upper limit of normal [ULN]) for serum aminotransferase (aspartate aminotransferas [AST] and alanine aminotransferase [ALT]) concentrations and ammonia.
6. Has normal renal function, with estimated glomerular filtration rate > 90 mL/minute/1.73 m2 at Screening (using the Chronic Kidney Disease Epidemiology Collaboration equation).
7. Has a platelet count > 150 × 103/μL at Screening.
8. Has a QT interval corrected with Fridericia's formula (QTcF) < 450 msec on the Screening EKG.
9. Parent or guardian is able to comprehend and willing to sign an informed consent form (ICF). 

Detailed eligibility will be discussed with the study team.