Ramelteon metabolite M-II is the primary metabolite of Ramelteon and exhibits IC50 values of 208 pM and 1470 pM for human melatonin receptors (MT1 and MT2), respectively. Ramelteon itself is a selective melatonin receptor agonist.

The affinity of Ramelteon metabolite M-II (M-II) for MT1 receptors is 10- and 2.5-fold lower than that of ramelteon and melatonin, respectively, and for MT2 receptors is approximately 5- and 1.5-fold lower. The selectivity of M-II for melatonin receptors over 215 targets, including other receptors, transporters, ion channels, and enzymes, was investigated. No significant affinities or activities were observed for these targets, except for the 5-HT2B receptor, with a Ki value of 1.75±0.23 μM. The potency of M-II for MT1 receptors is approximately 17- and 4.3-fold lower than that of ramelteon and melatonin, respectively, and for MT2 receptors is approximately 28- and 1.6-fold lower.

Ramelteon metabolite M-II 1 mg/kg significantly increases NREM sleep and significantly decreases wakefulness. Moreover, a lower dose of M-II (0.1 mg/kg) yield similar results (NREM, F1,7=121.9, p<0.01; wakefulness, F1,7=87.0, p<0.01), and decreased wakefulness is sustained for 6 h after the administration of either dose. After the administration of 0.01 mg/kg M-II, only NREM sleep is significantly increased . No significant differences in REM sleep are observed after the administration of M-II at any of the doses tested in this study.