入鹿山 容子

Structurally diverse small compounds are utilized to obtain hit compounds that have suitable pharmacophores in appropriate three-dimensional conformations for the target drug receptors. We have focused on the 1,3,5-trioxazatriquinane skeleton, which has a rigid bowl-like structure enabling the diverse orientation of side chain units, leading to a novel small-scale focused library based on the skeleton. In the library screening for the orexin receptor, some of the compounds showed orexin receptor antagonistic activity with a high hit rate of 7%. By optimizing the hit compounds, we discovered a potent dual orexin receptor antagonist, 38b, and a selective orexin 1 receptor antagonist, 41b carrying the same plane structure. Both compounds showed reasonable brain permeability and beneficial effects when administered intraperitoneally to wild-type mice. Docking simulations of their eutomers, (-)-38b and (+)-41b, with orexin receptors suggested that the interaction between the 1,3,5-trioxazatriquinane core structure and the hydrophobic subpocket in orexin receptors enables a U-shape structure, which causes tight van der Waals interactions with the receptors similar to SB-334867, a selective orexin 1 receptor antagonist. These results indicate that the library approach utilizing the 1,3,5-trioxazatriquinanes bearing multiple effective residues (TriMERs) might be useful for the hit discovery process targeting not only opioid and orexin receptors but other G-protein coupled receptors.

A novel series of 1-amino-tetralin derivatives were designed and synthesized based on the putative binding mode of the naphthalene-type orexin receptor agonist 5 and their agonist activities against orexin receptors were evaluated. The introduction of N-methyl-(3-methoxyphenyl)acetamide unit onto the 1-amino-tetralin skeleton remarkably enhanced the potency of the agonist. The asymmetric synthesis of 6 revealed that (-)-6 having a (S)-1-amino-tetralin skeleton showed a OX2R selective agonist activity (EC50 = 2.69 nM for OX2R, OX1R/OX2R = 461) yet its enantiomer (R)-(+)-6 showed a potent OX1/2R dual agonist activity (EC50 = 13.5 nM for OX1R, 0.579 nM for OX2R, OX1R/OX2R = 23.3). These results suggested that upward orientation of the amide side chain against the tetralin scaffold (S-configuration) would be selective for OX2R activation, and the downward orientation (R-configuration) would be significant for dual agonist activity. To our best knowledge, there have been no reports thus far that the stereochemistry of one carbon center on the agonist structure regulates the orexin receptor selectivity. Our results would provide important information for the development of OX1R selective agonists.

To investigate the contribution of hydrogen bonding between the 14-hydroxy group and the 6-amide chain on the binding affinity of nalfurafine toward KOR and OX1R, we prepared the 14-H and 14-dehydrated nalfurafine and their five-membered D-ring nalfurafine (D-nor-nalfurafine) derivatives. The 14-H and 14-dehydrated nalfurafine derivatives showed almost the same affinity for KOR as nalfurafine and more potent affinity for OX1R. On the other hand, 14-H and 14-dehydrated D-nor-nalfurafine derivatives showed weak affinity for KOR and almost no affinity for OX1R. The conformational analyses suggested that the 6-amide chains of the nalfurafine derivatives are mainly oriented just at or downward from the C-ring, while those of the D-nor-nalfurafine derivatives were mainly oriented toward the upper side of the C-ring even in the absence of the 14-hydroxy group. We postulated that the ion-dipole interaction between the 6-amide and the 16-nitrogen might stabilize the upwardly oriented 6-amide group. These results suggested that the 14-hydroxy group and the ion-dipole interaction would play important roles in the orientation of the 6-amide group, which might control the affinity between KOR and OX1R.

A novel series of naphthalene derivatives were designed and synthesized based on the strategy focusing on the restriction of the flexible bond rotation of OX2R selective agonist YNT-185 (1) and their agonist activities against orexin receptors were evaluated. The 1,7-naphthalene derivatives showed superior agonist activity than 2,7-naphthalene derivatives, suggesting that the bent form of 1 would be favorable for the agonist activity. The conformational analysis of 1,7-naphthalene derivatives indicated that the twisting of the amide unit out from the naphthalene plane is important for the enhancement of activity. The introduction of a methyl group on the 2-position of 1,7-naphthalene ring effectively increased the activity, which led to the discovery of the potent OX2R agonist 28c (EC50 = 9.21 nM for OX2R, 148 nM for OX1R). The structure-activity relationship results were well supported by a comparison of the docking simulation results of the most potent derivative 28c with an active state of agonist-bound OX2R cryo-EM SPA structure. These results suggested important information for understanding the active conformation and orientation of pharmacophores in the orexin receptor agonists, which is expected as a chemotherapeutic agent for the treatment of narcolepsy.

The five-membered D-ring nalfurafine (d-nor-nalfurafine) derivatives with a 16-sulfonamide group were synthesized. Conversion of the 16-cyclopropylmethyl group to the 16-benzenesulfonamide group in the d-nor-nalfurafine derivatives drastically improved the orexin 1 receptor (OX1R) antagonist activities. The intramolecular hydrogen bond between the 14-hydroxy and the 16-sulfonamide groups may play an important role in increasing the probability that the 6-amide group would be located at the lower side of the C-ring, leading to an active conformation for OX1R. The assay results and the conformational analyses of the 14-OH, 14-H, and 14-dehydrated d-nor-nalfurafine derivatives suggested that the 14- and 16-substituents of the d-nor-nalfurafine derivatives had a greater effect on the affinities for the OX1R than did the 14- and 17-substituents of nalfurafine derivatives.