We have recently identified 1,8-naphthyridin-2(1H)-one-3-carboxamide as a new scaffold very suitable for the development of new CB2 receptor potent and selective ligands. In this paper we describe a number of additional derivatives in which the same central scaffold has been variously functionalized in position 1 or 6. All new compounds showed high selectivity and affinity in the nanomolar range for the CB2 receptor. Furthermore, we found that their functional activity is controlled by the presence of the substituents at position C-6 of the naphthyridine scaffold. In fact, the introduction of substituents in this position determined a functionality switch from agonist to antagonists/inverse agonists. Finally, docking studies showed that the difference between the pharmacology of these ligands may be in the ability/inability to block the Toggle Switch W6.48(258) (χ1 g+ → trans) transition.

The CB(2) receptor activation can be exploited for the treatment of diseases such as chronic pain and tumors of immune origin, devoid of psychotropic activity. On the basis of our already reported 1,8-naphthyridin-4(1H)-on-3-carboxamide derivatives, new 1,8-naphthyridin-2(1H)-on-3-carboxamide derivatives were designed, synthesized, and tested for their affinities toward the human CB(1) and CB(2) cannabinoid receptors. Some of the reported compounds showed a subnanomolar CB(2) affinity with a CB(1)/CB(2) selectivity ratio greater than 200 (compounds 6, 12, cis-12, 13, and cis-13). Further studies revealed that compound 12, which presented benzyl and carboxy-4-methylcyclohexylamide substituents bound in the 1 and 3 positions, exerted a CB(2)-mediated inhibitory action on immunological human basophil activation. On the human T cell leukemia line Jurkat the same derivative induced a concentration-dependent decrease of cell viability. The obtained results suggest that 1,8-naphthyridin-2(1H)-on-3-carboxamides represent a new scaffold very suitable for the development of new promising CB(2) agonists.