Novel N-(arylalkyl) indol-3-ylglyoxylylamides targeted as ligands of the benzodiazepine receptor: Synthesis, biological evaluation, and molecular modeling analysis of the structure-activity relationships

A series of N-(arylalkyl)indol-3-ylglyoxylylamides (4-8) was synthesized as ligands of the benzodiazepine receptor (BzR) and tested for their ability to displace [H-3]flumazenil from bovine brain membranes. The new compounds, bearing a branched (4) or a geometrically constrained benzyl/phenylethyl a mide side chain (5-8), represent the continuation of our research on N-benz ylindol-3-ylglyoxylylamides 1 (Da Settimo et al., 1996), N ' -phenylindol-3 -ylglyoxylohydrazides 2 (Da Settimo et al., 1998), and N-(indol-3-ylglyoxyl yl)alanine derivatives 3 (Primofiore et al., 1989). A few indoles belonging to the previously investigated benzylamides 1 and phenylhydrazides 2 were synthesized and tested to enrich the SARs in these two series. The affiniti es and the GABA ratios of selected compounds for clonal mammalian alpha (1) beta (2)gamma (2), alpha (3)beta (2)gamma (2), and alpha (5)beta (3 gamma2) BzR subtypes were also determined. It was hypothesized that the reduced fl exibility of indoles 4-8 would both facilitate the mapping of the BzR bindi ng cleft and increase the chances of conferring selectivity for the conside red receptor subtypes. In the series of indoles 4, the introduction of a me thyl group on the benzylic carbon with the R configuration improved affinit y of the 5-substituted (5-Cl and 5-NO2) derivatives, whereas it was detrime ntal for their 5-unsubtituted (5-H) counterparts. All S enantiomers were le ss potent than the R ones. Replacement of the methyl with hydrophilic subst ituents on the benzylic carbon lowered affinity. The isoindolinylamide side chain was tolerated if the 5-position was unsubstituted (K-i of 5a 123 nM) , otherwise affinity was abolished (5b, c). All the 2-indanylamides 6 and ( S)-1-indanylamides 8 were devoid of any appreciable affinity. The 5-Cl and 5-NO2 (R)-1-indanylamides 7b (K-i 80 nM) and 7c (K-i 28 nM) were the most p otent among the indoles 5-8 geometrically constrained about the side chain. The 5-H (R)-1-indanylamide 7a displayed a lower affinity (K-i 675 nM). The SARs developed from the new compounds, together with those collected from our previous studies, confirmed the hypothesis of different binding modes f or 5-substituted and 5-unsubstituted indoles, suggesting that the shape of the lipophilic pocket L-1 (notation in accordance with Cook's BzR topologic al model) is asymmetric and highlighted the stereoelectronic and conformati onal properties of the amide side chain required for high potency. Several of the new indoles showed selectivity for the alpha (1)beta (2)gamma (2) su btype compared with the alpha (3)beta (2)gamma (2) and alpha (5)beta (3)gam ma (2) subtypes (e.g.: 4t and 7c bind to these three BzR isoforms with Ki v alues of 14 nM, 283 nM, 239 nM, and 9 nM, 1960 nM, 95 nM, respectively). Th e GABA ratios close to unity exhibited by all the tested compounds on each BzR subtype were predictive of an efficacy profile typical of antagonists.