Synthesis of a novel series of tricyclic dihydrofuran derivatives: discovery of 8,9-dihydrofuro[3,2-c]pyrazolo[1,5-a]pyridines as melatonin receptor (MT1/MT2) ligands.

Novel tricyclic dihydrofuran derivatives were designed, synthesized, and evaluated as melatonin receptor (MT(1)/MT(2)) ligands based on the previously reported 1,6-dihydro-2H-indeno[5,4-b]furan 1a. By screening the central tricyclic cores, we identified 8,9-dihydrofuro[3,2-c]pyrazolo[1,5-a]pyridine as a potent scaffold with a high ligand-lipophilicity efficiency (LLE) value. Subsequent optimization of the side chains led to identification of the potent MT(1)/MT(2) agonist 4d (MT(1), K(i) = 0.062 nM; MT(2), K(i) = 0.420 nM) with good oral absorption and blood-brain barrier (BBB) penetration in rats. The oral administration of compound 4d exhibited a sleep-promoting action in freely moving cats at 0.1 mg/kg.

Molecular cloning and pharmacological characterization of monkey MT1 and MT2 melatonin receptors showing high affinity for the agonist ramelteon.

Melatonin receptor agonists such as melatonin and ramelteon [(S)-N-[2-(1,6,7,8-tetrahydro-2H-indeno-[5,4-b]furan-8-yl)ethyl]-propionamide; TAK-375] have sleep-promoting effects in humans. In preclinical models, these effects are more similar to those observed in monkeys than in other species. However, in contrast to the human melatonin receptors, the pharmacological characteristics of the monkey melatonin receptors have yet to be elucidated. In this study, we cloned the cynomolgus monkey MT(1) and MT(2) melatonin receptors based on rhesus monkey genome sequences and then characterized the monkey melatonin receptors and compared their pharmacological properties with those of the human homologs. The overall amino acid sequences of the monkey MT(1) and MT(2) melatonin receptors showed high homology to the human MT(1) (95%) and MT(2) (96%) receptors, respectively. Saturation binding experiments with 2-[(125)I]iodomelatonin revealed that the dissociation constants (K(d)) for the monkey MT(1) and MT(2) melatonin receptors were 19.9 and 70.4 pM, respectively. In ligand competition assays using 2-[(125)I]iodomelatonin, ramelteon displayed approximately 3- to 7-fold higher affinities than melatonin for the recombinant monkey MT(1) and MT(2) melatonin receptors and monkey suprachiasmatic nucleus membranes. This higher affinity of ramelteon compared with melatonin has also been observed in human melatonin receptors. Furthermore, ramelteon inhibited pituitary adenylate cyclase-activating polypeptide-27-stimulated cAMP production with higher potency than melatonin. In conclusion, this information will help us to understand the pharmacological effects of melatonin receptor agonists in monkeys.