Substituted pentacyclic carbazolones as novel muscarinic allosteric agents: Synthesis and structure-affinity and cooperativity relationships

Two series of pentacyclic carbazolones, 22 and 23, have been synthesized utilizing a facile intramolecular Diels-Alder reaction and are allosteric modulators at muscarinic acetylcholine receptors. Their affinities and cooperativities with acetylcholine and the antagonist N-methylscopolamine (NMS) at M-1-M-4 receptors have been analyzed and compared. All of the synthesized compounds are negatively cooperative with acetylcholine. In contrast, the majority of the compounds exhibit positive cooperativity with NMS, particularly at M-2 and M-4 receptors. The subtype selectivity, in terms of affinity, was in general M-2 > M-1 > M-4 > M-3. The largest increases in affinity produced by a single substitution of the core structure were given by the 1-OMe (22b) and 1-Cl (22d) derivatives. The position of the N in the ring did not appear to be important for binding affinity or cooperativity. Two compounds 22y and 23i, both trisubstituted analogues, were the most potent compounds synthesized, with dissociation constants of 30-100 nM for the M-2 NMS-liganded and unliganded receptor, respectively. The results indicate that the allosteric site, like the primary binding site, is capable of high-affinity interactions with molecules of relatively low molecular weight.