RAMELTEON

12.1 Mechanism of Action Ramelteon is a melatonin receptor agonist with both high affinity for melatonin MT 1 and MT 2 receptors and relative selectivity over the MT 3 receptor. The activity of ramelteon at the MT 1 and MT 2 receptors is believed to contribute to its sleep- promoting properties, as these receptors, acted upon by endogenous melatonin, are thought to be involved in the maintenance of the circadian rhythm underlying the normal sleep-wake cycle. Ramelteon has no appreciable affinity for the GABA receptor complex or for receptors that bind neuropeptides, cytokines, serotonin, dopamine, noradrenaline, acetylcholine, and opiates. Ramelteon also does not interfere with the activity of a number of selected enzymes in a standard panel. The major metabolite of ramelteon, M-II, is pharmacologically active and has approximately one tenth and one fifth the binding affinity of the parent molecule for the human MT 1 and MT 2 receptors, respectively. However, M-II circulates at higher