A Phase II Study of Basmisanil in Children and Adolescents With Dup15q Syndrome

The primary goal of this Phase 2 clinical trial is to determine the effects of a selective negative allosteric modulator (NAM) of GABAA α5 receptor, Basmisanil (NTX-1511), on brain wave (EEG) characteristics in children and adolescents with Dup15q syndrome. The main questions this trial aims to answer are: * How does NTX-1511 affect brain waves (as measured by EEG) in subjects with Dup15q syndrome? * Does NTX-1511 impact other developmental features of subjects with Dup15q syndrome? Researchers will compare the effects of NTX-1511 with matching placebo (look-alike granules that contain no drug). Participants will: * Take NTX-1511 or matching placebo 3 times every day for 12 weeks * Visit the clinic 7 times over the course of 23 weeks

Key Inclusion Criteria: * Parent, caregiver, or legally authorized representatives has provided written informed consent for the study and is willing to comply with all requirements of the protocol * Male or female aged 2 to 14 years inclusive at time of informed consent signature * Documented duplication (3 copies) or triplication (4 copies) of the chromosome 15q11.2.-q13.1 (BP2-BP3) segment. * Dup15q syndrome Clinician Global Impression of Severity Scale (Dup15q CGI-S) overall severity score ≥ 3 (at least mildly impaired) Key Exclusion Criteria: * Pathogenic or likely pathogenic genomic variant outside of the BP2-BP3 segment. Individuals with an isodicentric 15q chromosome and a history of epilepsy * Uncontrolled epilepsy (in individuals with interstitial BP2-BP3 duplication or triplication) indicated by: - Use of rescue medication(s) to treat more than one seizure episode or seizure cluster per month, on average in the past 6 months, or -Concomitant chronic use of more than four anti-epileptic medications, or -Status epilepticus within the past 6 months requiring hospitalization for treatment of the status epilepticus, or -Any implanted devices to treat drug-resistant epilepsy * Changes to medications within 6-8 weeks prior to screening * Changes to non-pharmacological/behavioral therapies within 6 weeks prior to screening