Influence of chain length and N-alkylation on the selective serotonin receptor ligand 9-(aminomethyl)-9,10-dihydroanthracene

Multitargeting nature of muscarinic orthosteric agonists and antagonists

Muscarinic receptors (mAChRs) are typical members of the G protein-coupled receptor (GPCR) family and exist in five subtypes from M₁ to M₅. Muscarinic receptor subtypes do not sufficiently differ in affinity to orthosteric antagonists or agonists; therefore, the analysis of receptor subtypes is complicated, and misinterpretations can occur. Usually, when researchers mainly specialized in CNS and peripheral functions aim to study mAChR involvement in behavior, learning, spinal locomotor networks, biological rhythms, cardiovascular physiology, bronchoconstriction, gastrointestinal tract functions, schizophrenia, and Parkinson’s disease, they use orthosteric ligands and they do not use allosteric ligands. Moreover, they usually rely on manufacturers’ claims that could be misleading. This review aimed to call the attention of researchers not deeply focused on mAChR pharmacology to this fact. Importantly, limited selective binding is not only a property of mAChRs but is a general attribute of most neurotransmitter receptors. In this review, we want to give an overview of the most common off-targets for established mAChR ligands. In this context, an important point is a mention the tremendous knowledge gap on off-targets for novel compounds compared to very well-established ligands. Therefore, we will summarize reported affinities and give an outline of strategies to investigate the subtype’s function, thereby avoiding ambiguous results. Despite that, the multitargeting nature of drugs acting also on mAChR could be an advantage when treating such diseases as schizophrenia. Antipsychotics are a perfect example of a multitargeting advantage in treatment. A promising strategy is the use of allosteric ligands, although some of these ligands have also been shown to exhibit limited selectivity. Another new direction in the development of muscarinic selective ligands is functionally selective and biased agonists. The possible selective ligands, usually allosteric, will also be listed. To overcome the limited selectivity of orthosteric ligands, the recommended process is to carefully examine the presence of respective subtypes in specific tissues via knockout studies, carefully apply “specific” agonists/antagonists at appropriate concentrations and then calculate the probability of a specific subtype involvement in specific functions. This could help interested researchers aiming to study the central nervous system functions mediated by the muscarinic receptor.

Cyclisation reactions of N-cinnamoyl-9-aminoanthracenes

N-Cinnamoyl-9-aminoanthracenes cyclise with PPA or triflic acid to form novel 2-azahexacyclo[10.6.6.0(1,5).0(6,11).0(13,18).0(19,24)]tetracosa-6(11),7,9,13,15,17,19(24),20,22-nonaen-3-ones. In contrast, both N-cinnamoyl-N-methyl-9-(2-aminomethyl)anthracene and N-cinnamoyl-9-(2-aminoethyl)anthracene undergo an intramolecular Diels-Alder cycloaddition.

Involvement of sigma-1 receptors in the antidepressant-like effects of dextromethorphan

Dextromethorphan is an antitussive with a high margin of safety that has been hypothesized to display rapid-acting antidepressant activity based on pharmacodynamic similarities to the N-methyl-D-aspartate (NMDA) receptor antagonist ketamine. In addition to binding to NMDA receptors, dextromethorphan binds to sigma-1 (σ1) receptors, which are believed to be protein targets for a potential new class of antidepressant medications. The purpose of this study was to determine whether dextromethorphan elicits antidepressant-like effects and the involvement of σ1 receptors in mediating its antidepressant-like actions. The antidepressant-like effects of dextromethorphan were assessed in male, Swiss Webster mice using the forced swim test. Next, σ1 receptor antagonists (BD1063 and BD1047) were evaluated in conjunction with dextromethorphan to determine the involvement of σ receptors in its antidepressant-like effects. Quinidine, a cytochrome P450 (CYP) 2D6 inhibitor, was also evaluated in conjunction with dextromethorphan to increase the bioavailability of dextromethorphan and reduce exposure to additional metabolites. Finally, saturation binding assays were performed to assess the manner in which dextromethorphan interacts at the σ1 receptor. Our results revealed dextromethorphan displays antidepressant-like effects in the forced swim test that can be attenuated by pretreatment with σ1 receptor antagonists, with BD1063 causing a shift to the right in the dextromethorphan dose response curve. Concomitant administration of quinidine potentiated the antidepressant-like effects of dextromethorphan. Saturation binding assays revealed that a Ki concentration of dextromethorphan reduces both the Kd and the Bmax of [(3)H](+)-pentazocine binding to σ1 receptors. Taken together, these data suggest that dextromethorphan exerts some of its antidepressant actions through σ1 receptors.

9-Aminomethyl-9,10-dihydroanthracene (AMDA) analogs as structural probes for steric tolerance in 5-HT2A and H1 receptor binding sites

Synthesis, radioligand binding and molecular modeling studies of several 9-aminomethyl-9,10-dihydroanthracene (AMDA) analogs were carried out to determine the extent of the steric tolerance associated with expansion of the tricyclic ring system and amine substitution at 5-HT(2A) and H(1) receptors. A mixture of (7,12-dihydrotetraphene-12-yl)methanamine and (6,11-dihydrotetracene-11-yl)methanamine in a 75-25% ratio was found to have an apparent K(i) of 10nM at the 5-HT(2A) receptor. A substantial binding affinity for (7,12-dihydrotetraphene-3-methoxy-12-yl)methanamine at the 5-HT(2A) receptor (K(i)=21 nM) was also observed. Interestingly, this compound was found to have 100-fold selectivity for 5-HT(2A) over the H(1) receptor (K(i)=2500 nM). N-Phenylalkyl-AMDA derivatives, in which the length of the alkyl chain varied from methylene to n-butylene, were found to have only weak affinity for both 5-HT(2A) and H(1) receptors (K(i)=223 to 964 nM). Our results show that large rigid annulated AMDA analogs can be sterically accommodated within the proposed 5-HT(2A) binding site.

Synthesis, structure-affinity relationships, and modeling of AMDA analogs at 5-HT2A and H1 receptors: structural factors contributing to selectivity

Histamine H(1) and serotonin 5-HT(2A) receptors present in the CNS have been implicated in various neuropsychiatric disorders. 9-Aminomethyl-9,10-dihydroanthracene (AMDA), a conformationally constrained diarylalkyl amine derivative, has affinity for both of these receptors. A structure-affinity relationship (SAFIR) study was carried out studying the effects of N-methylation, varying the linker chain length and constraint of the aromatic rings on the binding affinities of the compounds with the 5-HT(2A) and H(1) receptors. Homology modeling of the 5-HT(2A) and H(1) receptors suggests that AMDA and its analogs, the parent of which is a 5-HT(2A) antagonist, can bind in a fashion analogous to that of classical H(1) antagonists whose ring systems are oriented toward the fifth and sixth transmembrane helices. The modeled orientation of the ligands are consistent with the reported site-directed mutagenesis data for 5-HT(2A) and H(1) receptors and provide a potential explanation for the selectivity of ligands acting at both receptors.

Potential modes of interaction of 9-aminomethyl-9,10-dihydroanthracene (AMDA) derivatives with the 5-HT2A receptor: a ligand structure-affinity relationship, receptor mutagenesis and receptor modeling investigation

The effects of 3-position substitution of 9-aminomethyl-9,10-dihydroanthracene (AMDA) on 5-HT 2A receptor affinity were determined and compared to a parallel series of DOB-like 1-(2,5-dimethoxyphenyl)-2-aminopropanes substituted at the 4-position. The results were interpreted within the context of 5-HT 2A receptor models that suggest that members of the DOB-like series can bind to the receptor in two distinct modes that correlate with the compounds' functional activity. Automated ligand docking and molecular dynamics suggest that all of the AMDA derivatives, the parent of which is a 5-HT 2A antagonist, bind in a fashion analogous to that for the sterically demanding antagonist DOB-like compounds. The failure of the F340 (6.52)L mutation to adversely affect the affinity of AMDA and the 3-bromo derivative is consistent with the proposed modes of orientation. Evaluation of ligand-receptor complex models suggest that a valine/threonine exchange between the 5-HT 2A and D 2 receptors may be the origin of selectivity for AMDA and two substituted derivatives.

Methoxy-substituted 9-aminomethyl-9,10-dihydroanthracene (AMDA) derivatives exhibit differential binding affinities at the 5-HT(2A) receptor

The effects of methoxy-substitution at the 1-, 2-, 3-, and 4-positions of 9-aminomethyl-9,10-dihydroanthracene (AMDA) on h5-HT(2A) receptor affinity were determined. Racemic mixtures of these compounds were found to show the following affinity trend: 3-MeO > 4-MeO > 1-MeO approximately 2-MeO. Comparison of the effects of these substitutions, with the aid of computational molecular modeling techniques, suggest that the various positional and stereochemical isomers of the methoxy-substituted AMDA compounds interact differently with the h5-HT(2A) receptor. It is predicted that for the compounds with higher affinities, the methoxy oxygen atom is able to interact with hydrogen bond-donating sidechains within alternative h5-HT(2A) receptor binding sites, whereas the lower-affinity isomers lack this ability.

GPCR antitarget modeling: pharmacophore models for biogenic amine binding GPCRs to avoid GPCR-mediated side effects

G protein-coupled receptors (GPCRs) form a large protein family that plays an important role in many physiological and pathophysiological processes. However, the central role that the biogenic amine binding GPCRs and their ligands play in cell signaling poses a risk in new drug candidates that reveal side affinities towards these receptor sites. These candidates have the potential to interfere with the physiological signaling processes and to cause undesired effects in preclinical or clinical studies. Here, we present 3D cross-chemotype pharmacophore models for three biogenic amine antitargets: the alpha(1A) adrenergic, the 5-HT(2A) serotonin, and the D2 dopamine receptors. These pharmacophores describe the key chemical features present within these biogenic amine antagonists and rationalize the biogenic amine side affinities found for numerous new drug candidates. First applications of the alpha(1A) adrenergic receptor model reveal that these in silico tools can be used to guide the chemical optimization towards development candidates with fewer alpha(1A)-mediated side effects (for example, orthostatic hypotension) and, thus, with an improved clinical safety profile.

Structural determinants for high 5-HT 2A receptor affinity of spiro[9,10-dihydroanthracene]-9,3 ′ -pyrrolidine (SpAMDA)

The synthesis and 5-HT(2A) receptor affinities of ring altered derivatives of spiro[9,10-dihydroanthracene]-9,3(')-pyrrolidine (4), a structurally unique tetracyclic 5-HT(2A) receptor antagonist, are described. The characteristics of the parent compound prove to be necessary for optimal 5-HT(2A) receptor affinity. However, expansion of the size of the pyrrolidine and central rings produce compounds with reasonably high 5-HT(2A) receptor affinities. In addition, the parent compound is shown to have high 5-HT(2) receptor selectivity.

Ring substituted analogues of 5-aminomethyl-10,11-dihydro-dibenzo[a,d]cycloheptene (AMDH): potential modes of binding to the 5-HT(2A) receptor

The synthesis and 5-HT(2A) receptor affinities of 2-substituted-5-aminomethyl-10,11-dihydrodibenzo[a,d]cycloheptene (AMDH) derivatives are described. Comparison of the effects of substitution on affinities allowed assignment of potential binding modes in comparison with DOB-like agonists/antagonists and 3-substituted 1-(aminomethyl)-9,10-dihydroanthracene structures.

Geometry-affinity relationships of the selective serotonin receptor ligand 9-(aminomethyl)-9,10-dihydroanthracene

With the exception of its two aromatic rings and basic nitrogen atom, 9-(aminomethyl)-9,10-dihydroanthracene (AMDA; 1) is remarkably devoid of the pharmacophore features usually associated with high-affinity receptor ligands such as the heteroatom hydrogen bonding features of the endogenous ligand serotonin. AMDA does contain a phenylethylamine skeleton within a tricyclic ring system, and the presence of the second aromatic group is necessary for optimal receptor affinity. The structural requirements for the binding of AMDA at 5-HT(2A) receptors were investigated with respect to the geometric relationship between the two aromatic rings. It appears that the geometry of the AMDA parent is in the optimal range for fold angle between aromatic moieties. Evaluation of conformationally constrained derivatives of AMDA suggests that a chain extended trans, gauche form is most likely responsible for high affinity.

Insights into the structure and function of 5-HT(2) family serotonin receptors reveal novel strategies for therapeutic target development

5-HT(2) family serotonin receptors, principal sites of action of serotonin in the brain, represent major molecular targets for drugs used in treating a variety of diseases including schizophrenia, depression, anxiety, eating disorders, obsessive-compulsive disorder, chronic pain conditions and obesity. The 5-HT(2) family of receptors has three members: 5-HT(2A), 5-HT(2B) and 5-HT(2C). Therefore, it is likely that subtype-selective compounds will be needed to avoid serious side effects and to enhance therapeutic indices. Unfortunately, recent insights into the structure and function of 5-HT(2A) receptors have revealed that structurally-diverse agonists and antagonists have distinct modes of interacting with 5-HT(2A) receptors, complicating efforts at structure-based drug-design. These distinct binding modes would not have been predicted based on conventional structure-activity relationships or static docking models. Fortunately, these complicated binding modes can be predicted and simulated using molecular dynamics, allowing for the possibility of structure-based drug design. Thus, provided appropriately sophisticated drug design strategies are employed, it is likely that uniquely valuable medications will result which could have great potential for treating a variety of mental and physical illnesses.