Buspirone analogues as ligands of the 5-HT1A receptor. 1. The molecular structure of buspirone and its two analogues

Positron Emission Tomography (PET) Imaging Tracers for Serotonin Receptors

Serotonin (5-hydroxytryptamine, 5-HT) and 5-HT receptors (5-HTRs) have crucial roles in various neuropsychiatric disorders and neurodegenerative diseases, making them attractive diagnostic and therapeutic targets. Positron emission tomography (PET) is a noninvasive nuclear molecular imaging technique and is an essential tool in clinical diagnosis and drug discovery. In this context, numerous PET ligands have been developed for "visualizing" 5-HTRs in the brain and translated into human use to study disease mechanisms and/or support drug development. Herein, we present a comprehensive repertoire of 5-HTR PET ligands by focusing on their chemotypes and performance in PET imaging studies. Furthermore, this Perspective summarizes recent 5-HTR-focused drug discovery, including biased agonists and allosteric modulators, which would stimulate the development of more potent and subtype-selective 5-HTR PET ligands and thus further our understanding of 5-HTR biology.

Nitroimidazoles Part 9. Synthesis, molecular docking, and anticancer evaluations of piperazine-tagged imidazole derivatives

New piperazine-tagged imidazole derivatives were synthesized via reaction of 1-alkyl/aryl-5-bromo-2-alkyl-4-nitro-1H-imidazoles 1–3 with piperazine nucleophiles. Nine selected compounds were assessed for their antiproliferative inhibition potency against five human cancer cell lines (MCF-7, PC3, Du145, HepG2 and Dermal/Fibroblast). Compounds 7 and 10 are the most potent anticancer agents on HepG2 cell line with IC50 values of (5.6 ± 0.5 µm) and (29.6 ± 7.6 µm) respectively, and on MCF-7 with IC50 values of (32.1 ± 5.6 µm) and (46.2 ± 8.2 µm) respectively. The molecular docking of compounds 7 and 10 has been studied, and the results reveal that the newly designed piperazine-tagged imidazole derivatives bind to the hydrophobic pocket and polar contact with high affinity.

Design, Synthesis, and Biological Evaluation of a Series of 5- and 7-Hydroxycoumarin Derivatives as 5-HT1A Serotonin Receptor Antagonists

We have designed and synthesized a series of 60 new 5- and 7-hydroxycoumarin derivatives bearing the piperazine moiety with the expected binding to 5-HT_1A and 5-HT_2A receptors. Molecular docking of all investigated compounds revealed subnanomolar estimates of 5-HT_1AR K_i for three ligands and 5-HT_2AR Ki for one ligand as well as numerous low nanomolar estimates of K_i for both receptors. Intrigued by these results we synthesized all 60 new derivatives using microwave-assisted protocols. We show that three new compounds show a relatively high antagonistic activity against the 5HT_1A receptor, although lower than the reference compound WAY-100635. These compounds also showed relatively low binding affinities to the 5-HT_2A receptor. We also provide a detailed structure-activity analysis of this series of compounds and compare it with previously obtained results for an exhaustive series of coumarin derivatives.

Synthesis, in vitro binding studies and docking of long-chain arylpiperazine nitroquipazine analogues, as potential serotonin transporter inhibitors

It is well known that 6-nitroquipazine exhibits about 150-fold higher affinity for the serotonin transporter (SERT) than quipazine and recently we showed quipazine buspirone analogues with high to moderate SERT affinity. Now we have designed and synthesized several 6-nitroquipazine buspirone derivatives. Unexpectedly, their SERT binding affinities were moderate, and much lower than that of the previously studied quipazine buspirone analogues. To explain these findings, docking studies of both groups of compounds into two different homology models of human SERT was performed using a flexible target-ligand docking approach (4D docking). The crystal structures of leucine transporter from Aquifex aeolicus in complex with leucine and with tryptophan were used as templates for the SERT models in closed and outward-facing conformations, respectively. We found that the latter conformation represents the most reliable model for binding of buspirone analogues. Docking into that model showed that the nitrated compounds acquire a rod like shape in the binding pocket with polar groups (nitro- and imido-) at the ends of the rod. 6-Nitro substituents gave steric clashes with amino acids located at the extracellular loop 4, which may explain their lower affinity than corresponding quipazine buspirone analogues. The results from the present study may suggest chemical design strategies to improve the SERT modulators.

Classification of 5-HT(1A) receptor ligands on the basis of their binding affinities by using PSO-Adaboost-SVM

In the present work, the support vector machine (SVM) and Adaboost-SVM have been used to develop a classification model as a potential screening mechanism for a novel series of 5-HT_1A selective ligands. Each compound is represented by calculated structural descriptors that encode topological features. The particle swarm optimization (PSO) and the stepwise multiple linear regression (Stepwise-MLR) methods have been used to search descriptor space and select the descriptors which are responsible for the inhibitory activity of these compounds. The model containing seven descriptors found by Adaboost-SVM, has showed better predictive capability than the other models. The total accuracy in prediction for the training and test set is 100.0% and 95.0% for PSO-Adaboost-SVM, 99.1% and 92.5% for PSO-SVM, 99.1% and 82.5% for Stepwise-MLR-Adaboost-SVM, 99.1% and 77.5% for Stepwise-MLR-SVM, respectively. The results indicate that Adaboost-SVM can be used as a useful modeling tool for QSAR studies.

Quantitative and Molecular Analysis of Buspirone Hydrochloride Polymorphs

Quantitative and molecular analysis have been performed on two main polymorphs of buspirone hydrochloride (BUS-HCl). Quantitative analysis of solid-state composition of pharmaceutical powders is necessary to ensure safety and efficacy of drug substance and to validate the production processes. In this study, X-ray powder diffraction and differential scanning calorimetry have been used for the quantitative analysis of two polymorphic forms of BUS-HCl. In addition, single crystal X-ray study of Form 1 revealed its crystal structure, however, a similar study on Form 2 was not possible due to the difficulties encountered in producing it. Molecular analysis including partial charge analysis has been performed by using Gaussian simulation package to find the electro-negativity pattern in molecule. 1H and 13C solid-state NMR spectra of polymorphs were recorded and also regression equations and QM theory were developed to predict the 1H and 13C NMR spectra through the use of atomic environmental descriptors. The NMR differences between the two polymorphs were discussed by using prediction and experimental results and description of each NMR shift for carbon and hydrogen atoms.

Intrinsic activity and comparative molecular dynamics of buspirone analogues at the 5-HT1A receptors

In CNS, the 5-hydroxytryptamine(1A) (5-HT(1A)) receptors exist in two different populations with different behavioural and physiological effects: (1) somatodendritic autoreceptors located pre-synaptically of 5-HT containing neurons and (2) receptors located post-synaptic to 5-HT containing neurons. Clinical studies have shown that 5-HT(1A) partial agonists have anxiolytic properties, while antagonists of pre-synaptical autoreceptors shorten the onset time of selective serotonin reuptake inhibitors (SSRIs). In the present study, the pre- and post-synaptic activity of structural analogues of buspirone was evaluated in animal models. A three dimensional model of the 5-HT(1A) receptor was used to study their interaction modes and helical displacements upon receptor binding. The predicted receptor-ligand interactions indicated similarities in the receptor binding modes for all buspirone analogues, and no clear relationship between receptor contact residues and activity at pre- and post-synaptic receptors. Comparative molecular dynamics (MD) simulations for 650ps indicated that pre-synaptic antagonistic behaviour is connected to large displacements of transmembrane helix (TMH) 7 upon binding, while pre-synaptic agonistic behaviour is connected to large displacements of TMH2 and small displacements of TMH7. Post-synaptic partial agonist behaviour is connected to large displacements of TMH4 and TMH5 upon binding, while post-synaptic antagonists only slightly displace these helices.

Rigid analogues of buspirone and gepirone, 5-HT1A receptors partial agonists

Rigid analogues of buspirone and gepirone, 5-HT1A receptors partial agonists, were obtained. The compounds exhibited very low affinity to the receptors. Their structural features resembled to a large extent the arrangement of the respective structural elements found in the solid state of buspirone and in the theoretical structure of NAN-190 (5-HT1A postsynaptic antagonist) rigid analogue exhibiting high affinity to the receptor. The obtained results would thus suggest that the bioactive conformation of buspirone might not be the extended one. That would additionally suggest that either both groups of compounds could occupy different areas at the receptor binding sites (or bind to different receptor states) or the constrained structure of 2 does not represent well 5-HT1A receptor binding site requirements.

Interaction of 1,2,4-substituted piperazines, new serotonin receptor ligands, with 5-HT1A and 5-HT2A receptors

In the present paper, we describe affinities to 5-HT1A and 5-HT2A receptors of several new 1,2,4-trisubstituted piperazine derivatives. The affinities were compared with those described earlier for 1,4-disubstituted piperazines and the influence of the third (methyl) substituent on the affinity to both receptors is discussed. The difference between two- and three-substituted derivatives was rationalised in terms of molecular modelling of the respective ligand-receptor complexes. Additionally, the functional activity of some 1,2,4-trisubstituted piperazines for 5-HT1A receptor was examined in behavioural and biochemical models. The obtained results have shown that some trisubstituted compounds exhibited a higher affinity to 5-HT2A receptors than their respective disubstituted analogues (with the affinity to 5-HT1A receptors remaining the same or somewhat improving). The molecular dynamics simulations suggested that the presence of the third substituent in the piperazine ring of those compounds may induce stabilising effect on the ligand-receptor complexes. The results of the in vivo studies have shown that some of the examined trisubstituted piperazines (10-13, 16, 17) exhibited properties of postsynaptic 5-HT1A partial agonists. Moreover, compounds 13 and 16 exhibited features of 5-HT1A presynaptic agonists in in vitro test, and compound 16 also in in vivo tests.

Molecular dynamics of 5-HT1A and 5-HT2A serotonin receptors with methylated buspirone analogues

In the present study experimentally determined ligand selectivity of three methylated buspirone analogues (denoted as MM2, MM5 and P55) towards 5-HT1A and 5-HT2A serotonin receptors was theoretically investigated on a molecular level. The relationships between the ligand structure and 5-HT1A and 5-HT2A receptor affinities were studied and the results were found to be in agreement with the available site-directed mutagenesis and binding affinity data. Molecular dynamics (MD) simulations of ligand-receptor complexes were performed for each investigated analogue, docked twice into the central cavity of 5-HT1A/5-HT2A, each time in a different orientation. Present results were compared with our previous theoretical results, obtained for buspirone and its non-methylated analogues. It was found that due to the presence of the methyl group in the piperazine ring the ligand position alters and the structure of the ligand-receptor complex is modified. Further, the positions of derivatives with pyrimidinyl aromatic moiety and quinolinyl moiety are significantly different at the 5-HT2A receptor. Thus, methylation of such derivatives alters the 3D structures of ligand-receptor complexes in different ways. The ligand-induced changes of the receptor structures were also analysed. The obtained results suggest, that helical domains of both receptors have different dynamical behaviour. Moreover, both location and topography of putative binding sites for buspirone analogues are different at 5-HT1A and 5-HT2A receptors.

Molecular dynamics of buspirone analogues interacting with the 5-HT1A and 5-HT2A serotonin receptors

Three-dimensional (3-D) models of the human serotonin 5-HT1A and 5-HT2A receptors were constructed, energy refined, and used to study the interactions with a series of buspirone analogues. For both receptors, the calculations showed that the main interactions of the ligand imide moieties were with amino acids in transmembrane helix (TMH) 2 and 7, while the main interactions of the ligand aromatic moieties were with amino acids in TMH5, 6 and 7. Differences in binding site architecture in the region of highly conserved serine and tyrosine residues in TMH7 gave slightly different binding modes of the buspirone analogues at the 5-HT1A and 5-HT2A receptors. Molecular dynamics simulations of receptor-ligand interactions indicated that the buspirone analogues did not alter the interhelical hydrogen bonding patterns upon binding to the 5-HT2A receptor, while interhelical hydrogen bonds were broken and others were formed upon ligand binding to the 5-HT1A receptor. The ligand-induced changes in interhelical hydrogen bonding patterns of the 5-HT1A receptor were followed by rigid body movements of TMH2, 4 and 6 relative to each other and to the other TMHs, which may reflect the structural conversion into an active receptor structure.

Ligand induced conformational states of the 5-HT(1A) receptor

It has been shown that G-protein coupled receptors have seven transmembrane alpha-helices, but the structural changes occurring in a G-protein coupled receptor as a response on agonist stimulus and the molecular events leading to blockade of the signal transduction by antagonists are not well understood. In the present study, the AMBER 5.0 force field was used for comparative molecular dynamics simulations of a 5-HT(1A) receptor model in the absence of ligand, in complex with a 5-HT(1A) receptor agonist (R)-8-hydroxy-2-(di-n-propylamino)tetralin [(R)-8-OH-DPAT], in complex with a selective 5-HT(1A) receptor antagonist (S)-N-tert-butyl-3-[4-(2-methoxyphenyl)piperazin-1-yl ]-2-phenylpropanamide [(S)-WAY100135], and in complex with the partial agonist, buspirone. In the simulations, the agonist induced larger conformational changes into transmembrane helix 3 and 6 than into the other helices, while the main conformational differences between the agonist bound receptor and the antagonist bound receptor were in transmembrane helix 5 and 6. During the simulations, all the three ligands constrained the helical movements compared to those observed in the receptor without any ligand.

Ligand-5-HT1A receptor interaction

In the present paper the general structure and pharmacophore of some 5-HT1A receptor ligands are described. For several compounds (approximately 15) the intrinsic activity in lower lip retraction (postsynaptic, rat) and hypothermia (presynaptic, mice) tests was determined. For the identified functional presynaptic agonists and antagonists the influence on the brain serotonin level was examined. No direct correlation between the functional intrinsic activity and the influence on the level was observed. Molecular modelling revealed the possibility of the existence of different binding sites for different examined compounds.

13C cross-polarization magic angle spinning NMR and gauge-independent atomic orbital, coupled Hartree-Fock calculations of buspirone analogues. Part 2. Hydrochlorides and perchlorates of 1-arylpiperazine-4-alkylimides

13C cross-polarization (CP) magic angle spinning (MAS) solid state NMR spectra of hydrochlorides and perchlorates of buspirone analogues (2-5) were recorded. In the spectra for each compound, one set of signals appeared, in agreement with single crystal X-ray diffraction data indicating the presence of one molecule per crystal unit. The resonances of 2-5 hydrochlorides were assigned by comparison with the solution chemical shifts. For perchlorate 2b and diperchlorate 2c, the reasonable assignment of signals was made with the aid of the theoretical studies. Ab initio calculations of the carbon shieldings were performed by means of the GIAO-CHF method for two model systems: perchlorate and diperchlorate of quinoline-(N-methyl)piperazine. As no remarkable differences between carbon chemical shifts of hydrochlorides 3-5 in solid state and in solution were observed, it was concluded that in solution these compounds adopted the same conformation as in the solid state.

The novel buspirone analogue, 8-[4-[2-(1,2,3,4-tetrahydroisoquinolinyl)[butyl]-8-azaspiro [4.5 ]decane-7,9-dione, with anxiolytic-like and antidepressant-like effects in rats

In the conflict drinking test, used as a model to examine anxiolytic-like activity, the novel buspirone analogue 8-[4-[2-(1,2,3,4-tetrahydroisoquinolinyl)]butyl)-8-azaspiro[ 4.5]decane-7,9-dione (MM199) (0.62-2.5 mg/kg) and buspirone (0.62-5 mg/kg), significantly increased the punished drinking in water-deprived rats, without affecting water consumption or perception of the stimulus. The anticonflict activity of MM199 (1.25 mg/kg) was blocked by (S)-WAY 100135 (20 mg/kg), a 5-hydroxytrypatmine1A (5-HT1A) receptor antagonist. In the forced swimming test, used as a model to examine the antidepressant-like activity, MM199 (5-20 mg/kg) reduced the immobility time, while buspirone (5-20 mg/kg) had no such effect. The reduced immobility induced by MM199 (20 mg/kg) was antagonized by (S)-WAY100135 (10 mg/kg). The above findings suggest that MM199 possesses potent anxiolytic- and antidepressant-like properties which are mediated by activation of 5-HT1A receptors.

The ligand-binding site of buspirone analogues at the 5-HT1A receptor

A three-dimensional model of the 5-HT1A receptor in man was constructed by molecular-modelling techniques and used to study the molecular interactions of a series of buspirone analogues with the 5-HT1A receptor by molecular-mechanical-energy minimization and molecular-dynamics simulations. The receptor has seven trans-membrane alpha helices (TMHs) organized according to the electron-density-projection map of visual rhodopsin, and includes all loops between TMHs and the N- and C-terminal parts. The best fit between the buspirone analogues and the receptor model was obtained with the quinolinyl part of the ligand molecules interacting with amino acids in TMH6, the imide group interacting with amino acids in TMH2, TMH3 and TMH7, and the carbonyl groups hydrogen-bonded with Ser86 and Ser393. The ligand-binding rank order deduced from the experimentally determined inhibition constant was reproduced by calculation of receptor-binding energies of the buspirone analogues. The models suggest that steric hindrance and repulsive forces between the receptor and the imide group of the buspirone analogues are the most important determinants of ligand-binding affinity for discriminating between these ligands.

Conformational flexibility of serotonin1A receptor ligands from crystallographic data. Updated model of the receptor pharmacophore

Preparation and affinity to 5-HT1A and 5-HT2A receptors of new buspirone analogues 7-17 are reported. The compounds possess high to low affinity to 5-HT1A and moderate to low to 5-HT2A receptors. The crystal structures have been determined for compounds 11, 12, 13, and 14. For low affinity ligand (15) of 5-HT1A receptor conformational analysis was performed and compared with similar analyses performed for know high (buspirone 1) and very high (WY-48,723 2) affinity ligands of the receptor. Structure-activity relationship is discussed for the affinity to 5-HT1A receptor. A three-point pharmacophore explaining interactions of buspirone-like molecules with the receptor binding site is proposed.

8-[4-[2-(1,2,3,4-Tetrahydroisoquinolinyl]butyl-8-azaspiro[4.5]decane-7,9-dione: a new 5-HT1A receptor ligand with the same activity profile as buspirone

A new analog of buspirone (1), i.e., 8-[4-[2-(1,2,3,4-tetrahydroisoquinolinyl)]butyl]-8-azaspiro- [4.5]decane-7,9-dione (6a), was synthesized. In was demonstrated that buspirone and its analog 6a were equipotent 5-HT(1A) ligands. Several behavioral models showed that 6a had essentially the same functional profile at 5-HT(1A) receptors as buspirone. The obtained results permit a conclusion that the basic nitrogen atom and terminal, bulky cycloimide moiety, but not the 2-pyrimidinyl group, of buspirone are directly involved in the formation of the bioactive complex with 5-Ht1A receptors.