Study of AD128 Versus Placebo in Obstructive Sleep Apnea

Inclusion Criteria: * Patients who are able to understand the nature of the study and to give free informed consent * AHI ≥ 15 on screening/baseline PSG * Any of the following conditions should be met: 1. Documented prior PSG within 1 year demonstrating AHI of 15 or higher 2. Documented Continuous Positive Airway Pressure (CPAP) intolerance or poor compliance (compliance is defined as self-reported use of CPAP 4 hours per night for 70% of nights) or CPAP-naïve. 3. Patients who have been using CPAP at least 4 hours nightly for at least 70% of the nights are eligible only if CPAP is not used for 2 weeks prior to the screening/study baseline PSG. * Epworth Sleepiness Scale (ESS) score ≥ 4 for patients not using CPAP * Previous surgical treatment for OSA is allowed if ≥ 1 year prior to enrollment. * BMI between 18.5 and 40.0 kg/m2, inclusive Exclusion Criteria: * History of narcolepsy. * Clinically significant craniofacial malformation. * Clinically significant cardiac disease or hypertension requiring more than 3 medications for control. * Clinically significant neurological disorder, including epilepsy/convulsions * History of schizophrenia, schizoaffective disorder or bipolar disorder according to - Diagnostic and Statistical Manual of Mental Disorders-V (DSM V) or International Classification of Disease X edition criteria. * History of attempted suicide or suicidal ideation within 1 year prior to screening, or current suicidal ideation. * Positive history for abuse of drugs or substance use disorder as defined in DSM-V within 12 months prior to Screening Visit. * A significant illness or infection requiring medical treatment in the past 30 days. * Clinically significant cognitive dysfunction. * Untreated narrow angle glaucoma. * Women who are pregnant or nursing. * History of using oral or nasal devices for the treatment of OSA may enroll as long as the devices are not used during participation in the study. * History of using devices to affect participant sleeping position for the treatment of OSA, e.g. to discourage supine sleeping position, may enroll as long as the devices are not used during participation in the study. * History of oxygen therapy. * Use of medications from the list of disallowed concomitant medications. * Treatment with strong cytochrome P450 3A4 (CYP3A4) inhibitors, or monoamine oxidase inhibitors (MAOI) or linezolid within 14 days of the start of treatment, or concomitant with treatment. * Use of another investigational agent within 30 days or 5 half-lives, whichever is longer, prior to dosing. * Central apnea index \> 5/hour on baseline PSG * Any condition that in the investigator's opinion would present an unreasonable risk to the participant, or which would interfere with their participation in the study or confound study interpretation. * Patients considered by the investigator, for any reason, unsuitable candidates to receive AD128 treatment or unable or unlikely to understand or comply with the dosing schedule or study evaluations.