The Effects of Tasimelteon in Participants With REM Behavior Disorder (RBD)

REM behavior disorder (RBD) is characterized by abnormal behaviors that emerge from REM sleep and can lead to injury and disturbed sleep. Most patients have frequent events - typically more than once per week. Abnormalities can be seen almost nightly and consist of intermittent loss of the normal atonia of REM sleep. This phenomenon is used as diagnostic criterion even in the absence of an overt clinical event during the night.

RBD has serious consequences for the health of the patient. Besides risk of sometimes severe injury, a direct consequence of a violent nocturnal movement, it often leads to sleep disruption. Furthermore, it is commonly seen in association with Parkinson's disease and many experts in the field consider it a prodrome of neurodegenerative conditions. Other comorbidities may include a higher risk of cerebral hemorrhage as well as stroke. Multiple factors may contribute to the risk of RBD. Aside from neurodegenerative conditions, RBD is seen in association with disorders of REM sleep regulations: narcolepsy, post-traumatic stress disorder, or with use of selective serotonin reuptake inhibitors (SSRIs).

In healthy individuals, REM sleep is closely linked to circadian phase, with a peak a little after the nadir of the core body temperature, and thus also around the time when melatonin secretion is maximal. Studies using a forced desynchrony protocol suggest that the circadian system has a primary effect of REM sleep regulation with a modifying effect from the homeostatic factors. Various other factors affect REM sleep, including complex interactions with the serotonergic system, primarily from the raphe nuclei in the medulla, which inhibit the REM generating pontine tegmentum nuclei. Clinically, patients treated with antidepressants, particularly with serotononergic properties (particularly SSRIs), tend to suppress REM sleep and may also lead to REM without atonia and/or trigger RBD events.

The melatonin MT1 and MT2 receptors likely both affect the NREM/REM ratio with activation of the MT2 leading to earlier and more abundant NREM sleep, while MT1 receptors favoring REM sleep. Furthermore, RBD is common in patients with Parkinson's disease, and a reduced number of melatonin receptors have been found in the areas involved in the neurodegeneration: a recent study found a reduced number of MT1 receptor expression in the striatum and amygdala, and a reduced MT2 receptor expression in the substantia nigra and amygdala. In addition to circadian phase shift, activation of melatonin MT1 and MT2 receptors has been implicated as a potential protective mechanism against multiple other progressive neurodegenerative disorders, while MT2 receptors have been implicated in neurogenesis. Thus, REM suppression and/or disruption, as a result of the neurodegenerative process, that also involves impaired MT1 and MT2 receptor function may be a key mechanism for RBD pathophysiology and potential therapeutic target.