Tricyclic Pyrazoles. Part 8. Synthesis, Biological Evaluation and Modelling of Tricyclic Pyrazole Carboxamides as Potential CB2 Receptor Ligands with Antagonist/Inverse Agonist Properties.

Previous studies have investigated the relevance and structure-activity relationships (SARs) of pyrazole derivatives in relation with cannabinoid receptors, and the series of tricyclic 1,4-dihydroindeno[1,2-c]pyrazoles emerged as potent CB2receptor ligands. In the present study, novel 1,4-dihydroindeno[1,2-c]pyrazole and 1H-benzo[g]indazole carboxamides containing a cyclopropyl or a cyclohexyl substituent were designed and synthesized to evaluate the influence of these structural modifications towards CB1and CB2receptor affinities. Among these derivatives, compound 15 (6-cyclopropyl-1-(2,4-dichlorophenyl)-N-(adamantan-1-yl)-1,4-dihydroindeno[1,2-c]pyrazole-3-carboxamide) showed the highest CB2receptor affinity (Ki= 4 nM) and remarkable selectivity (KiCB1/KiCB2= 2232), whereas a similar affinity, within the nM range, was seen for the fenchyl derivative (compound 10: Ki= 6 nM), for the bornyl analogue (compound 14: Ki= 38 nM) and, to a lesser extent, for the aminopiperidine derivative (compound 6: Ki= 69 nM). Compounds 10 and 14 were also highly selective for the CB2receptor (KiCB1/KiCB2> 1000), whereas compound 6 was relatively selective (KiCB1/KiCB2= 27). The four compounds were also subjected to GTPγS binding analysis showing antagonist/inverse agonist properties (IC50for compound 14 = 27 nM, for 15 = 51 nM, for 10 = 80 nM and for 6 = 294 nM), and this activity was confirmed for the three more active compounds in a CB2receptor-specific in vitro bioassay consisting in the quantification of prostaglandin E2 release by LPS-stimulated BV2 cells, in the presence and absence of WIN55,212-2 and/or the investigated compounds. Modelling studies were also conducted with the four compounds, which conformed with the structural requirements stated for the binding of antagonist compounds to the human CB2receptor.