This study will assess the safety and efficacy of JZP258 (XYWAV) on sleepiness, polysomnography, and functional outcomes in patients with idiopathic hypersomnia (IH) or narcolepsy.

This prospective, multicenter, single-arm, open-label interventional study will assess the safety and efficacy of JZP258 on sleepiness, polysomnography measurements (eg, sleep stage transitions, sleep patterns, and sleep quality), daytime and nighttime symptoms, pharmacokinetics (in narcolepsy), and patient-reported outcomes that include subjective sleep quality and quality of life in patients with IH or narcolepsy.

Key Inclusion Criteria:

• Is 18 to 75 years of age (inclusive) at the time of signing the informed consent form (ICF)
• Has a primary diagnosis of IH meeting International Classification of Sleep Disorders, Third Edition (ICSD-3) criteria or narcolepsy (Type 1 or Type 2) meeting ICSD-3 or Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-V) criteria.
• If not currently taking oxybate medication, has clinically significant symptoms of Excessive daytime sleepiness (EDS) with an ESS score > 10 at Screening Visit 1. If currently taking oxybate medication, has an ESS score > 10 at the Baseline Visit 2 polysomnography (after washout period). Note: Not applicable for participants entering the Narcolepsy >9-gram cohort (see additional criteria below for the Narcolepsy >9-gram cohort).
• If currently treated with anticataplectics and/or alerting agents, has been taking the same dosage for at least 1 month prior to Screening Visit 1 and has no current plans to adjust the dosage during the study period. Note: Alerting agents refer to either traditional stimulants (eg, amphetamines, methylphenidates) or wake-promoting agents (eg, modafinil, pitolisant, solriamfetol).

Additional Inclusion Criteria for Participants in the Narcolepsy >9-Gram Cohort

• Is on a current treatment regimen including XYWAV at a dosage of 9 grams

Key Exclusion Criteria:

• Shows evidence of a previous untreated or inadequately treated sleep disorder considered by the investigator to negatively impact the conduct of the study, including sleep-disordered breathing, parasomnias, circadian rhythm sleep disorders, or restless legs syndrome determined by a previous sleep-laboratory diagnosis or interview utilizing modules of the Diagnostic Interview for Sleep Patterns and Disorders.

• Shows evidence of untreated or inadequately treated sleep-disordered breathing during Baseline Visit 2 polysomnography defined as an Apnea/Hypopnea Index (AHI) > 10, using the US Centers for Medicare and Medicaid Services rules (CMS.gov).

• Has a history or presence of any unstable or clinically significant medical condition (chronic pain condition that may impact sleep), behavioral or psychiatric disorder (including active suicidal ideation or current or past [within 1 year] major depressive episode), or history or presence of another neurologic disorder or surgical history that might affect the participant's safety and/or interfere with the conduct of the study, in the opinion of the investigator.

• Has recently taken (ie, within 1 month prior to screening), is taking, or plans to take any of the following:

  • A substance or medication contraindicated with XYWAV use, ie. specifically alcohol and sedative hypnotics
  • A medication with a known drug-drug interaction with XYWAV
  • A medication that may have similar EEG effects to XYWAV
  • Medications known to have clinically significant CNS sedative effects
  • Other medications, natural health products, or substances from which the participant experiences clinically significant sedation

Additional Exclusion Criterion for Participants in the Narcolepsy >9-Gram Cohort

• Any past or current medical conditions or experience that, in the investigator's clinical judgment, would preclude further increases in the current oxybate medication dosage beyond 9 grams