Orexin (hypocretin) receptor agonists and antagonists for treatment of sleep disorders. Rationale for development and current status.

Orexin A and orexin B are hypothalamic neuropeptides initially identified as endogenous ligands for two orphan G-protein coupled receptors (GPCRs). They play critical roles in the maintenance of wakefulness by regulating function of monoaminergic and cholinergic neurons that are implicated in the regulation of wakefulness. Loss of orexin neurons in humans is associated with narcolepsy, a sleep disorder characterized by excessive daytime sleepiness and cataplexy, further suggesting the particular importance of orexin in the maintenance of the wakefulness state. These findings have encouraged pharmaceutical companies to develop drugs targeting orexin receptors as novel medications of sleep disorders, such as narcolepsy and insomnia. Indeed, phase III clinical trials were completed last year of suvorexant, a non-selective (dual) antagonist for orexin receptors, for the treatment of primary insomnia, and demonstrate promising results. The New Drug Application (NDA) for suvorexant has been submitted to the US FDA. Thus, the discovery of a critical role played by the orexin system in the regulation of sleep/wakefulness has opened the door of a new era for sleep medicine.