Study of SDX in Patients With Idiopathic Hypersomnia (IH)

Inclusion Criteria: 1. At least 18 years of age at the time of consent 2. Body Mass Index (BMI) ≤35 kg/m2 3. Documented primary diagnosis of IH according to the International Classification of Sleep Disorders (ICSD-3) criteria 4. At the Screening Visit and Baseline Visit (start of OLTP), Epworth Sleepiness Scale (ESS) scores ≥11 5. Average nightly Total Sleep Time (TST) of ≥7 hours, per subject history and confirmed during screening. 6. Subject must be in general good health defined as the absence of any clinically relevant abnormalities as determined by the Investigator based on physical and neurological examinations, vital signs, ECGs, medical history, and clinical laboratory values (hematology, chemistry, and urinalysis) at Screening. 7. If currently treated with nicotine replacement therapy, must have been taking the same regimen and dose for at least 2 months prior to screening and must agree to take the same dose during the study. 8. Have used a medically acceptable method of contraception for at least 2 months prior to the first dose of study drug and consent to use a medically acceptable method of contraception from the first dose of study drug, throughout the entire study period, and for 90 days after the last dose of study drug. Exclusion Criteria 1. Hypersomnia due to another medical, behavioral, or psychiatric disorder condition (eg, narcolepsy, depression disorders, multiple sclerosis, Parkinson's disease, stroke). 2. Clinically significant sleep-related breathing disorders, including sleep apnea, treatment with Continuous Positive Airway Pressure (CPAP) therapy, Obstructive Apnea Hypopnea Index (AHI) \>15 episodes per hour, or hypoventilation. 3. Clinically significant parasomnias (eg, sleep walking, rapid eye movement \[REM\] sleep behavior disorder, etc). 4. Periodic Limb Movement Disorder (PLMD) Arousal Index (PLMA-I) \>15 during Screening PSG, a historical diagnosis of PLMD (last 10 years), or a PLMD diagnosis older than 10 years with current (last 60 days) treatment or symptoms of rhythmic movements involving one or both legs during sleep. 5. Occupation requiring nighttime shift work or variable shift work with early work start times (before 6 AM), if this occurs more than once per week. 6. Planned travel during the study that includes more than 3 time zones, or planned travel that includes 3 time zones on more than 2 occasions during the study. 7. Going to sleep for the night later than 1 AM at a frequency of more than once per week. 8. Current or past (within 1 year) major depressive episode according to DSM-5 criteria. 9. Any history of attempted suicide (lifetime) or clinically significant suicidal ideation, in the opinion of the Investigator, based on the C-SSRS assessment at Screening. 10. Any clinically significant unstable medical abnormality, chronic disease (eg, asthma or diabetes), or a history of a clinically significant abnormality of the cardiovascular, central nervous system, 11. Any of the following out-of-range vital signs at Screening: systolic blood pressure outside 90-145 mmHg; diastolic blood pressure outside 50-90 mmHg; resting heart rate outside 40-100 beats per minute. 12. History or presence of abnormal ECGs, which in the Investigator's opinion is clinically significant, including the following: 1. ECG findings of ischemia or infarct 2. Complete bundle branch blocks 3. Symptomatic arrhythmias as ventricular arrhythmias (non- sustained ventricular tachycardia (VT), multifocal or frequent premature ventricular contractions), bundle branch block, axis deviation, or abnormal or any predominantly non-sinus- conducted rhythm. 4. QTcF \>450 msec for males or \>470 msec for females, on Screening ECG. 5. PR interval outside the range of 120 to 220 msec on Screening ECG 13. Estimated glomerular filtration rate (GFR) at Screening \<60 mL/min/1.73 m2. 14. Malignant neoplastic disease requiring therapy within 2 years prior to Screening or during the study, or clinically relevant as judged by the Investigator. 15. Uncontrolled thyroid disorder as evidenced by thyroid stimulating hormone (TSH) ≤0.8 x the lower limit of normal (LLN) or ≥1.25 x the upper limit of normal (ULN) for the reference laboratory at Screening. 16. Laboratory value for aspartate aminotransferase (AST) or alanine aminotransferase (ALT) \>3 x upper limits of normal (ULN). 17. Excessive caffeine use during the 10 days prior to first dose of study drug or anticipated excessive use defined as \>600 mg/day of caffeine during the treatment periods of the study. 18. Treatment or planned treatment with prohibited medications (including medications that may affect daytime sleepiness and nighttime sleep) or unwilling to refrain from any prohibited medications. Treatment must have been discontinued 14 days or 5 half-lives, whichever is longer, prior to the first dose of study medication (and at least 30 days for sedating antidepressants; at least 14 days for CNS stimulants). 19. Current or past (within 12 months prior to Screening) substance use disorder (including alcohol and psychoactive cannabinoids) according to DSM-5 criteria; current or past history of substance abuse treatment (including alcohol), or unwilling to refrain from substance use (including alcohol) during the study. 20. Nicotine dependence that has an effect on sleep (eg, a subject who routinely awakens at night to smoke). 21. Evidence of substance or alcohol use or has a positive urine or breath alcohol or positive urine drug screen at Screening.