Utilization of an Active Site Mutant Receptor for the Identification of Potent and Selective Atypical 5-HT2C Receptor Agonists.

Agonism of the 5-HT2C receptor represents one of the most well-studied and clinically proven mechanisms for pharmacological weight reduction. Selectivity over the closely related 5-HT2A and 5-HT2B receptors is critical as their activation has been shown to lead to undesirable side effects and major safety concerns. In this communication, we report the development of a new screening paradigm that utilizes an active site mutant D134A (D3.32) 5-HT2C receptor to identify atypical agonist structures. We additionally report the discovery and optimization of a novel class of nonbasic heterocyclic amide agonists of 5-HT2C. SAR investigations around the screening hits provided a diverse set of potent agonists at 5-HT2C with high selectivity over the related 5-HT2A and 5-HT2B receptor subtypes. Further optimization through replacement of the amide with a variety of five- and six-membered heterocycles led to the identification of 6-(1-ethyl-3-(quinolin-8-yl)-1H-pyrazol-5-yl)pyridazin-3-amine (69). Oral administration of 69 to rats reduced food intake in an ad libitum feeding model, which could be completely reversed by a selective 5-HT2C antagonist.

Development of 1H-Pyrazolo[3,4-b]pyridines as Metabotropic Glutamate Receptor 5 Positive Allosteric Modulators.

The metabotropic glutamate receptor 5 (mGluR5) is an attractive target for the treatment of schizophrenia due to its role in regulating glutamatergic signaling in association with the N-methyl-d-aspartate receptor (NMDAR). We describe the synthesis of 1H-pyrazolo[3,4-b]pyridines and their utility as mGluR5 positive allosteric modulators (PAMs) without inherent agonist activity. A facile and convergent synthetic route provided access to a structurally diverse set of analogues that contain neither the aryl-acetylene-aryl nor aryl-methyleneoxy-aryl elements, the predominant structural motifs described in the literature. Binding studies suggest that members of our new chemotype do not engage the receptor at the MPEP and CPPHA mGluR5 allosteric sites. SAR studies culminated in the first non-MPEP site PAM, 1H-pyrazolo[3,4-b]pyridine 31 (BMT-145027), to improve cognition in a preclinical rodent model of learning and memory.

Evidence for Classical Cholinergic Toxicity Associated with Selective Activation of M1 Muscarinic Receptors.

The muscarinic acetylcholine receptor subtype 1 (M1) receptors play an important role in cognition and memory, and are considered to be attractive targets for the development of novel medications to treat cognitive impairments seen in schizophrenia and Alzheimer's disease. Indeed, the M1 agonist xanomeline has been shown to produce beneficial cognitive effects in both Alzheimer's disease and schizophrenia patients. Unfortunately, the therapeutic utility of xanomeline was limited by cholinergic side effects (sweating, salivation, gastrointestinal distress), which are believed to result from nonselective activation of other muscarinic receptor subtypes such as M2 and M3. Therefore, drug discovery efforts targeting the M1 receptor have focused on the discovery of compounds with improved selectivity profiles. Recently, allosteric M1 receptor ligands have been described, which exhibit excellent selectivity for M1 over other muscarinic receptor subtypes. In the current study, the following three compounds with mixed agonist/positive allosteric modulator activities that are highly functionally selective for the M1 receptor were tested in rats, dogs, and cynomologous monkeys: (3-((1S,2S)-2-hydrocyclohexyl)-6-((6-(1-methyl-1H-pyrazol-4-yl)pyridin-3-yl)methyl)benzo[h]quinazolin-4(3H)-one; 1-((4-cyano-4-(pyridin-2-yl)piperidin-1-yl)methyl)-4-oxo-4H-quinolizine-3-carboxylic acid; and (R)-ethyl 3-(2-methylbenzamido)-[1,4'-bipiperidine]-1'-carboxylate). Despite their selectivity for the M1 receptor, all three compounds elicited cholinergic side effects such as salivation, diarrhea, and emesis. These effects could not be explained by activity at other muscarinic receptor subtypes, or by activity at other receptors tested. Together, these results suggest that activation of M1 receptors alone is sufficient to produce unwanted cholinergic side effects such as those seen with xanomeline. This has important implications for the development of M1 receptor-targeted therapeutics since it suggests that dose-limiting cholinergic side effects still reside in M1 receptor selective activators.

High-Throughput Screening for Allosteric Modulators of GPCRs.

The continued evolution of our understanding of G protein-coupled receptor (GPCR) signaling has revealed new opportunities for drug discovery. Specifically, biased agonism at GPCRs and allosteric modulation of GPCRs both represent emerging areas of GPCR biology that hold promise for the development of novel GPCR-targeted therapeutics that may provide greater therapeutic efficacy and/or improved side-effect profiles. To obtain initial chemical leads, high-throughput screening (HTS) of a large compound library for the desired activity is often deployed during the early stages of a discovery program. The identification of allosteric modulators, in particular, poses significant challenges for HTS. We describe several HTS protocols designed for the identification of GPCR ligands, with a particular focus on the identification of allosteric modulators.

Structure activity relationship studies of 3-arylsulfonyl-pyrido[1,2-a]pyrimidin-4-imines as potent 5-HT6 antagonists.

Comprehensive structure activity relationship (SAR) studies were conducted on a focused screening hit, 2-(methylthio)-3-(phenylsulfonyl)-4H-pyrido[1,2-a]pyrimidin-4-imine (1, IC50: 4.0 nM), as 5-HT6 selective antagonists. Activity was improved some 2-4 fold when small, electron-donating groups were added to the central core as observed in 19, 20 and 26. Molecular docking of key compounds in a homology model of the human 5-HT6 receptor was used to rationalize our structure-activity relationship (SAR) findings. In pharmacokinetic experiments, compound 1 displayed good brain uptake in rats following intra-peritoneal administration, but limited oral bioavailability.

Discovery of a novel series of quinolone α7 nicotinic acetylcholine receptor agonists.

High throughput screening led to the identification of a novel series of quinolone α7 nicotinic acetylcholine receptor (nAChR) agonists. Optimization of an HTS hit (1) led to 4-phenyl-1-(quinuclidin-3-ylmethyl)quinolin-2(1H)-one, which was found to be potent and selective. Poor brain penetrance in this series was attributed to transporter-mediated efflux, which was in turn due to high pKa. A novel 4-fluoroquinuclidine significantly lowered the pKa of the quinuclidine moiety, reducing efflux as measured by a Caco-2 assay.

Design, optimization, and in vivo evaluation of a series of pyridine derivatives with dual NK1 antagonism and SERT inhibition for the treatment of depression.

A series of substituted pyridines, ether linked to a phenylpiperidine core were optimized for dual NK(1)/SERT affinity. Optimization based on NK(1)/SERT binding affinities, and minimization of off-target ion channel activity lead to the discovery of compound 44. In vivo evaluation of 44 in the gerbil forced swim test (a depression model), and ex-vivo NK(1)/SERT receptor occupancy data support the potential of a dual acting compound for the treatment of depression.

Development of a high-throughput calcium flux assay for identification of all ligand types including positive, negative, and silent allosteric modulators for G protein-coupled receptors.

In recent years, the increased use of cell-based functional assays for G protein-coupled receptors in high-throughput screening has enabled the design of robust assays to identify allosteric modulators (AMs) in addition to the more traditional orthosteric agonists and antagonists. In this article, the authors describe a screening format able to identify all ligand types using a triple-add assay that measures changes in cytosolic calcium concentration with three separate additions and reads in the same assay plate. This triple-add assay captures more small molecule ligand types than previously described assay formats without a significant increase in screening cost. Finally, the customizability of the triple-add assay to suit the needs of various AM screening programs is demonstrated.

Conformationally restricted homotryptamines. Part 4: Heterocyclic and naphthyl analogs of a potent selective serotonin reuptake inhibitor.

A series of hybrid molecules containing the cyclopropylmethylamino side chain found in homotryptamine (1S,2S)-2c and an isosteric heteroaryl or naphthyl core were prepared and their binding affinities for the human serotonin transporter determined. The most potent isosteres were CN-substituted naphthalenes. These results demonstrate that isosteric aromatic cores which lack an H-bond donor site may be substituted for the indole nucleus without substantial loss in hSERT binding.

Interaction of the novel antipsychotic aripiprazole with 5-HT1A and 5-HT 2A receptors: functional receptor-binding and in vivo electrophysiological studies.

BACKGROUND: Aripiprazole (7-{4-[4-(2,3-dichlorophenyl)-1-piperazinyl]butoxy}-3,4-dihydro-2(1H)-quinolinone) is a novel antipsychotic with a mechanism of action that differs from current typical and atypical antipsychotics. Aripiprazole interacts with a range of receptors, including serotonin [5-hydroxytryptamine (5-HT)] and dopamine receptors. MATERIALS AND METHODS: This study examined aripiprazole's interactions with 5-HT systems in vitro and in vivo to further clarify its pharmacologic properties. RESULTS: Aripiprazole produced increases in [(35)S]GTPgammaS binding to rat hippocampal membranes. Its potency (pEC(50) = 7.2) was similar to that of ziprasidone (7.1) and greater than that of 5-HT (6.7) and buspirone (6.4), a 5-HT(1A)-receptor partial agonist, whereas its intrinsic activity was similar to that of ziprasidone and buspirone. The stimulatory effect of aripiprazole was blocked by WAY-100635, a 5-HT(1A)-receptor antagonist. In in vivo electrophysiology studies, aripiprazole produced a dose-related reduction in the firing rate of 5-HT-containing dorsal raphe neurons in rats, which was both prevented and reversed by WAY-100635 administration. Aripiprazole showed a high affinity for human 5-HT(1A) receptors (K (i) = 4.2 nM) using parietal cortex membrane preparations. In membranes from cells expressing human recombinant receptors, aripiprazole bound with high affinity to 5-HT(2A) receptors (K (i) = 3.4 nM), moderate affinity to 5-HT(2C) (K (i) = 15 nM) and 5-HT(7) (K (i) = 39 nM) receptors, and low affinity to 5-HT(6) receptors (K (i) = 214 nM) and 5-HT transporter (K (i) = 98 nM). In addition, aripiprazole potently blocked 5-HT(2A)-receptor-mediated increases in intracellular Ca(2+) levels in a rat pituitary cell line (IC(50) = 11 nM). DISCUSSION: These results support a partial agonist activity for aripiprazole at 5-HT(1A) receptors in vitro and in vivo, and suggest important interactions with other 5-HT-receptor subtypes. This receptor activity profile may contribute to the antipsychotic activity of aripiprazole in humans.

Identification of a potent and selective 5-HT(6) antagonist: one-step synthesis of (E)-3-(benzenesulfonyl)-2- (methylsulfanyl)pyrido[1,2-a]pyrimidin-4-ylidenamine from 2-(benzenesulfonyl)-3,3-bis(methylsulfanyl)acrylonitrile.

(E)-3-(Benzenesulfonyl)-2-(methylsulfanyl)pyrido[1,2-a]pyrimidin-4-ylidenamine (7) was found to be a potent and selective 5-HT(6) antagonist. A one-step synthesis of this compound is described.