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

Nanotherapeutics for Nose-to-Brain Drug Delivery: An Approach to Bypass the Blood Brain Barrier

Treatment of neurodegenerative diseases or other central nervous system (CNS) disorders has always been a significant challenge. The nature of the blood-brain barrier (BBB) limits the penetration of therapeutic molecules to the brain after oral or parenteral administration, which, in combination with hepatic metabolism and drug elimination and inactivation during its journey in the systemic circulation, decreases the efficacy of the treatment, requires high drug doses and often induces adverse side effects. Nose-to-brain drug delivery allows the direct transport of therapeutic molecules by bypassing the BBB and increases drug concentration in the brain. The present review describes mechanisms of nose-to-brain drug delivery and discusses recent advances in this area with especial emphasis on nanotechnology-based approaches.

Design, synthesis and biological evaluation of novel serotonin and dopamine receptor ligands being 6-bromohexyl saccharine derivatives

Due to numerous side effects of current antidepressants, the search for new, safer bioactive compounds is still a valid research topic in medical chemistry. In our research we decided to synthesize and determine SAR for new hexyl arylpiperazines (LACPs) derivated with saccharin moiety. High biological activity has been explained using molecular modelling methods. The compounds obtained show high affinity for the 5-HT_1A (compound 18, K_i = 4 nM - antagonist mode) and D₂ (compound 15, K_i = 7 nM - antagonist mode) receptor, and in some cases also 5-HT₇ receptor (compound 17, K_i = 20 nM). A preliminary ADME analysis showed that the compounds exhibit CNS drugability properties. We have proved that carbon-chain lengthening may have a beneficial effect on increasing the activity towards serotonin and dopamine receptors.

Novel 4-aryl-pyrido[1,2-c]pyrimidines with dual SSRI and 5-HT(1A) activity. Part 5

A series of novel 4-aryl-pyrido[1,2-c]pyrimidine derivatives containing a 1-(2-quinoline)piperazine moiety was synthesized. The chemical structure of new compounds was confirmed by FT-IR, (1)H NMR, (13)C NMR and HRMS spectra as well as elemental analysis. Affinity of the novel pyrido[1,2-c]pyrimidine derivatives for 5-HT1A, 5-HT2A receptors and serotonin transporter (SERT) was evaluated in an in vitro radioligand binding assay. Tested compounds showed moderate to high affinity for 5-HT1AR and SERT and low affinity for 5-HT2AR. Selected ligands were subjected to in vivo tests, such as induced hypothermia and the forced swimming test in mice, which determined presynaptic agonistic activity of the ligands 8d, 8e, 9d and 9e and presynaptic antagonistic activity of the ligands 8a, 8b, 9a, 9b. Additionally, metabolic stability evaluation was performed for selected ligands, proving that a para-substitution in the 4-aryl-pyrido[1,2-c]pyrimidine moiety leads to an increase in stability, whereas a substitution in the ortho-position lowers the stability.

The molecular interactions of buspirone analogues with the serotonin transporter

A major problem with the selective serotonin reuptake inhibitors (SSRIs) is the delayed onset of action. A reason for that may be that the initial SSRI-induced increase in serotonin levels activates somatodendritic 5-HT(1A) autoreceptors, causing a decrease in serotonin release in major forebrain areas. It has been suggested that compounds combining inhibition of the serotonin transport protein with antagonistic effects on the 5-HT(1A) receptor will shorten the onset time. The anxiolytic drug buspirone is known as 5-HT(1A) partial agonist. In the present work, we are studying the inhibition of the serotonin transporter protein by a series of buspirone analogues by molecular modelling and by experimental affinity measurements. Models of the transporter protein were constructed using the crystal structure of the Escherichia coli major facilitator family transporter-LacY and the X-ray structure of the neurotransmitter symporter family (NSS) transporter-LeuT(Aa) as templates. The buspirone analogues were docked into both SERT models and the interactions with amino acids within the protein were analyzed. Two putative binding sites were identified on the LeuT(Aa) based model, one suggested to be a high-affinity site, and the other suggested to be a low-affinity binding site. Molecular dynamic simulations of the LacY based model in complex with ligands did not induce a helical architecture of the LacY based model into an arrangement more similar to that of the LeuT(Aa) based model.

High-throughput modeling of human G-protein coupled receptors: amino acid sequence alignment, three-dimensional model building, and receptor library screening

The current study describes the development of a computer package (GPCRmod) aimed at the high-throughput modeling of the therapeutically important family of human G-protein coupled receptors (GPCRs). GPCRmod first proposes a reliable alignment of the seven transmembrane domains (7 TMs) of most druggable human GPCRs based on pattern/motif recognition for each of the 7 TMs that are considered independently. It then converts the alignment into knowledge-based three-dimensional (3-D) models starting from a set of 3-D backbone templates and two separate rotamer libraries for side chain positioning. The 7 TMs of 277 human GPCRs have been accurately aligned, unambiguously clustered in three different classes (rhodopsin-like, secretin-like, metabotropic glutamate-like), and converted into high-quality 3-D models at a remarkable throughput (ca. 3s/model). A 3-D GPCR target library of 277 receptors has consequently been setup. Its utility for "in silico" inverse screening purpose has been demonstrated by recovering among top scorers the receptor of a selective GPCR antagonist as well as the receptors of a promiscuous antagonist. The current GPCR target library thus constitutes a 3-D database of choice to address as soon as possible the "virtual selectivity" profile of any GPCR antagonist or inverse agonist in an early hit optimization process.

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.

Key issues in the computational simulation of GPCR function: representation of loop domains

Some key concerns raised by molecular modeling and computational simulation of functional mechanisms for membrane proteins are discussed and illustrated for members of the family of G protein coupled receptors (GPCRs). Of particular importance are issues related to the modeling and computational treatment of loop regions. These are demonstrated here with results from different levels of computational simulations applied to the structures of rhodopsin and a model of the 5-HT2A serotonin receptor, 5-HT2AR. First, comparative Molecular Dynamics (MD) simulations are reported for rhodopsin in vacuum and embedded in an explicit representation of the membrane and water environment. It is shown that in spite of a partial accounting of solvent screening effects by neutralization of charged side chains, vacuum MD simulations can lead to severe distortions of the loop structures. The primary source of the distortion appears to be formation of artifactual H-bonds, as has been repeatedly observed in vacuum simulations. To address such shortcomings, a recently proposed approach that has been developed for calculating the structure of segments that connect elements of secondary structure with known coordinates, is applied to 5-HT2AR to obtain an initial representation of the loops connecting the transmembrane (TM) helices. The approach consists of a simulated annealing combined with biased scaled collective variables Monte Carlo technique, and is applied to loops connecting the TM segments on both the extra-cellular and the cytoplasmic sides of the receptor. Although this initial calculation treats the loops as independent structural entities, the final structure exhibits a number of interloop interactions that may have functional significance. Finally, it is shown here that in the case where a given loop from two different GPCRs (here rhodopsin and 5-HT2AR) has approximately the same length and some degree of sequence identity, the fold adopted by the loops can be similar. Thus, in such special cases homology modeling might be used to obtain initial structures of these loops. Notably, however, all other loops in these two receptors appear to be very different in sequence and structure, so that their conformations can be found reliably only by ab initio, energy based methods and not by homology modeling.

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.