Structure–Activity Relationship Study of Indole-2-carboxamides Identifies a Potent Allosteric Modulator for the Cannabinoid Receptor 1 (CB1)

Abstract

The cannabinoid CB1 receptor is involved in complex physiological functions. The discovery of CB1 allosteric modulators generates new opportunities for drug discovery targeting the pharmacologically important CB1 receptor. 5-chloro-3-ethyl-N-(4-(piperidin-1-yl)phenethyl)-1H-indole-2-carboxamide (ORG27569; 1) represents a new class of indole-2-carboxamides that exhibit allostery of CB1. To better understand the SAR, a group of indole-2-carboxamide analogs were synthesized and assessed for allostery of the CB1 receptor. We found that within the structure of indole-2-carboxamides, the presence of the indole ring is preferred for maintaining the modulator's high binding affinity for the allosteric site, but not for generating allostery on the orthosteric site. However, the C3 substituents of the indole-2-carboxamides significantly impact the allostery of the ligand. A robust CB1 allosteric modulator 5-chloro-N-(4-(dimethylamino)phenethyl)-3-pentyl-1H-indole-2-carboxamide (11j) was identified. It showed an equilibrium dissociation constant (KB) of 167.3 nM with a markedly high binding cooperativity factor (α=16.55) and potent antagonism of agonist-induced GTPγS binding.

Structure–activity relationships of substituted 1H-indole-2-carboxamides as CB1 receptor allosteric modulators

“…29 Reduction of 9 by triethylsilane in trifluoroacetic acid at room temperature gave 10 which was hydrolyzed in aqueous sodium hydroxide and ethanol to furnish the intermediate 11 . 26, 27 Amide coupling between 11 and 4- or 3-nitrophenylethylamine in the presence of BOP provided 12 and 13 which were reduced by Raney nickel-catalyzed transfer hydrogenation using hydrazine in ethanol at 50 °C to give 14 and 15 . Reductive amination of amines 14 and 15 with the corresponding aldehydes provided the target compounds 16 - 20 .…”

“…25 Subsequent work found that longer alkyl side chains with up to 9 carbon units at the C3 position of indole ring A could retain activity, whereas linkers other than an ethylene between the amide bond and the phenyl ring B resulted in total loss of activity. 26-28 In an attempt to expand our understanding of the structure-activity relationship on this scaffold, we have designed additional analogs by (i) exploring different substituents at the 4-, 3-, 2-positions of the phenyl ring B, (ii) examining several rigid cyclic ring linkers between the phenyl and the indole rings, and (iii) investigating the effects of shorter alkyl side chain at the C3 position and halogenations at the C5 position of the indole ring A. Here, we report the synthesis of these 1H-indole-2-carboxamide analogs and the evaluation of CB1 and CB2 activities in fluorometric imaging plate reader (FLIPR) based calcium mobilization assays.…”

Synthesis and biological evaluation of indole-2-carboxamides bearing photoactivatable functionalities as novel allosteric modulators for the cannabinoid CB1 receptor

“…The obtained photophore-bearing ethanamines 12, 16 and 22 were then employed in the synthesis of corresponding indole-2-carboxamides 26–29 through the methods (Scheme 2) as we described earlier[32]. The required indole-2-carboxylic acids 23–25 were prepared according to our previous report [33].…”

“…7–9 , Figure 2) reported previously [31–33]. Compounds 7 [31], 8 [33] and 9 [32, 35] have been demonstrated to have either comparable or improved allosteric effects to 1 [31–33]. Therefore, they were selected along with 1 as the parent compounds to introduce photoactivatable functionalities.…”

“…Earlier studies of the structure-activity relationship (SAR) of indole-2-carboxamides revealed that the substitution on the phenyl group of 1 can be structurally varied without significant reduction in allosteric modulatory activity [32–35]. Therefore, benzoyl, azido, and trifluoromethyldiazirinyl (TFMD) groups were respectively employed to replace the C4-substituents on the phenyl ring of compounds 1 7, 8 and 9 to introduce photoactivatable functionalities (i.e., benzophenone, phenyl azide, and phenyltrifluoromethyl diazirine).…”