Two diagnoses of exclusion
Narcolepsy type 2 is excessive daytime sleepiness with the sleep-study findings of narcolepsy but without cataplexy, and with orexin levels that are typically normal. Idiopathic hypersomnia is overwhelming sleepiness with long, unrefreshing sleep and severe difficulty waking — and the name says the rest, because idiopathic means the cause is unknown.
Both are defined substantially by what they are not. That makes them harder to study, harder to diagnose consistently, and historically less attractive to drug developers than a condition with a measurable deficiency. Patients with these diagnoses have generally been offered the same wake-promoting drugs as everyone else, with the same partial results.
Why an orexin agonist might still work
The reasoning does not depend on these patients being secretly orexin-deficient. Orexin signalling promotes wakefulness in anyone; it is a general mechanism, not a narcolepsy-specific repair. Raising orexin tone above baseline can therefore push the wake system harder in a brain whose orexin system is functioning normally.
Put simply, in narcolepsy type 1 the drug is replacing something missing. In narcolepsy type 2 and idiopathic hypersomnia it is turning up something that is already there. Those are different pharmacological propositions, and there was no guarantee the second one would produce a clinically meaningful effect.
What the data has shown so far
Two results have moved this question. Alkermes' alixorexton was the first oral OX2R agonist to show efficacy in a large Phase 2 study in narcolepsy type 2, and it is now testing that finding in a registrational Phase 3 study. Centessa's cleminorexton produced Phase 2a data across 55 participants that was the first showing of an oral OX2R agonist working across narcolepsy type 1, type 2 and idiopathic hypersomnia together.
Takeda's TAK-360 is running three parallel Phase 2 studies covering all three conditions, and Alkermes has a separate Phase 2 study, Vibrance-3, in idiopathic hypersomnia.
What is riding on it
Narcolepsy type 1 is a rare disease. Narcolepsy type 2 and idiopathic hypersomnia together represent a substantially larger population, and the wider hypersomnia and fatigue space beyond them is larger still.
If orexin agonists work in people with an intact orexin system, the class is not a narcolepsy treatment that happens to have adjacent uses. It is a wakefulness drug class, and the narcolepsy indications are its first and cleanest proof. That is the reading behind the programmes now pointed at fatigue in multiple sclerosis and Parkinson's disease, at ADHD, and at neurodegenerative and neuropsychiatric disease.