Benzamides derived from 1,2-diaminocyclopropane as novel ligands for human D2 and D3 dopamine receptors

In vitro characterization of [125I]HY-3-24, a selective ligand for the dopamine D3 receptor

Introduction

Dopamine D3 receptor (D3R) ligands have been studied for the possible treatment of neurological and neuropsychiatric disorders. However, selective D3R radioligands for in vitro binding studies have been challenging to identify due to the high structural similarity between the D2R and D3R. In a prior study, we reported a new conformationally-flexible benzamide scaffold having a high affinity for D3R and excellent selectivity vs. D2R. In the current study, we characterized the in vitro binding properties of a new radioiodinated ligand, [125I]HY-3-24.

Methods

In vitro binding studies were conducted in cell lines expressing D3 receptors, rat striatal homogenates, and rat and non-human primate (NHP) brain tissues to measure regional brain distribution of this radioligand.

Results

HY-3-24 showed high potency at D3R (Ki = 0.67 ± 0.11 nM, IC50 = 1.5 ± 0.58 nM) compared to other D2-like dopamine receptor subtypes (D2R Ki = 86.7 ± 11.9 nM and D4R Ki > 1,000). The Kd (0.34 ± 0.22 nM) and Bmax (38.91 ± 2.39 fmol/mg) values of [125I]HY-3-24 were determined. In vitro binding studies in rat striatal homogenates using selective D2R and D3R antagonists confirmed the D3R selectivity of [125I]HY-3-24. Autoradiography results demonstrated that [125I]HY-3-24 specifically binds to D3Rs in the nucleus accumbens, islands of Calleja, and caudate putamen in rat and NHP brain sections.

Conclusion

These results suggest that [125I]HY-3-24 appears to be a novel radioligand that exhibits high affinity binding at D3R, with low binding to other D2-like dopamine receptors. It is anticipated that [125I]HY-3-24 can be used as the specific D3R radioligand.

Design and Synthesis of Conformationally Flexible Scaffold as Bitopic Ligands for Potent D3-Selective Antagonists

Previous studies have confirmed that the binding of D₃ receptor antagonists is competitively inhibited by endogenous dopamine despite excellent binding affinity for D₃ receptors. This result urges the development of an alternative scaffold that is capable of competing with dopamine for binding to the D₃ receptor. Herein, an SAR study was conducted on metoclopramide that incorporated a flexible scaffold for interaction with the secondary binding site of the D₃ receptor. The alteration of benzamide substituents and secondary binding fragments with aryl carboxamides resulted in excellent D₃ receptor affinities (Ki = 0.8-13.2 nM) with subtype selectivity to the D₂ receptor ranging from 22- to 180-fold. The β-arrestin recruitment assay revealed that 21c with 4-(pyridine-4-yl)benzamide can compete well against dopamine with the highest potency (IC₅₀ = 1.3 nM). Computational studies demonstrated that the high potency of 21c and its analogs was the result of interactions with the secondary binding site of the D₃ receptor. These compounds also displayed minimal effects for other GPCRs except moderate affinity for 5-HT₃ receptors and TSPO. The results of this study revealed that a new class of selective D₃ receptor antagonists should be useful in behavioral pharmacology studies and as lead compounds for PET radiotracer development.

Synthesis of 1-isopropyl-3-acyl-5-methyl-benzimidazolone derivatives and their antimicrobial activity

A series of N-acylated analogues of 1-isopropyl-3-acyl-5-methyl-benzimidazolone were synthesized. Bioassay results indicated that analogues 5-07 and 5-19 exhibited the most potency against Bacillus cereus, Bacillus subtilis, Staphylococcus aureus, Escherichia coli and Pseudomonas aeruginosa. Analogues 5-02, 5-07, 5-12, 5-15, 5-19, 5-20 and 5-25 could effectively inhibit the spore germination of Botrytis cinerea. The relationship between structure and their antimicrobial activity (SAR) has also been discussed according to aliphatic acids and aromatic acids derivatives, respectively. This implied that the N-acylated derivatives of 5-methyl-benzimidazolone might be potential antimicrobial agents.

Molecular Modeling of the Three-Dimensional Structure of Dopamine 3 (D3) Subtype Receptor: Discovery of Novel and Potent D3 Ligands through a Hybrid Pharmacophore- and Structure-Based Database Searching Approach

The dopamine 3 (D3) subtype receptor has been implicated in several neurological conditions, and potent and selective D3 ligands may have therapeutic potential for the treatment of drug addiction, Parkinson's disease, and schizophrenia. In this paper, we report computational homology modeling of the D3 receptor based upon the high-resolution X-ray structure of rhodopsin, extensive structural refinement in the presence of explicit lipid bilayer and water environment, and validation of the refined D3 structural models using experimental data. We further describe the development, validation, and application of a hybrid computational screening approach for the discovery of several classes of novel and potent D3 ligands. This computational approach employs stepwise pharmacophore and structure-based searching of a large three-dimensional chemical database for the identification of potential D3 ligands. The obtained hits are then subjected to structural novelty screening, and the most promising compounds are tested in a D3 binding assay. Using this approach we identified four compounds with K(i) values better than 100 nM and eight compounds with K(i) values better than 1 microM out of 20 compounds selected for testing in the D3 receptor binding assay. Our results suggest that the D3 structural models obtained from this study may be useful for the discovery and design of novel and potent D3 ligands. Furthermore, the employed hybrid approach may be more effective for lead discovery from a large chemical database than either pharmacophore-based or structure-based database screening alone.

Synthesis and structure-affinity relationships of novel N-(1-ethyl-4-methylhexahydro-1,4-diazepin-6-yl)pyridine-3-carboxamides with potent serotonin 5-HT3 and dopamine D2 receptor antagonistic activity

A structurally original series of N-(1-ethyl-4-methylhexahydro-1,4-diazepin-6-yl)pyridine-3-carboxamides derived from the corresponding benzamide 5 were prepared and evaluated for their binding affinity for the dopamine D(2) and serotonin 5-HT(3) receptors using rat striatum and rat cortical membrane, respectively. Many of the synthesized pyridine-3-carboxamides exhibited nanomolar binding affinity for the serotonin 5-HT(3) receptor along with moderate to high binding affinity for the dopamine D(2) receptor. Introduction of the more lipophilic bromine atom and methylamino group at the 5- and 6-positions of the pyridine ring, respectively, enhanced the affinity for the dopamine D(2) receptor while keeping a potent serotonin 5-HT(3) receptor binding affinity. As a result of structure-affinity relationships, the 5-bromo-2-methoxy-6-methylaminopyridine-3-carboxamide 53 was selected as the most promising product showing a high binding affinity for both receptors. Compound 53 affinity for the dopamine D(2) and serotonin 5-HT(3) receptors was much more potent than that of metoclopramide (dopamine D(2) receptor; 23.3 nM vs 444 nM, serotonin 5-HT(3) receptor; 0.97 nM vs 228 nM). Optical resolution of the racemate 53 brought about a dramatic change in the pharmacological profile with (R)-53 exhibiting a strong affinity for both the dopamine D(2) and serotonin 5-HT(3) receptors, while the corresponding (S)-53 had a potent serotonin 5-HT(3) receptor binding affinity and a moderate dopamine D(2) receptor binding affinity. X-ray crystallographic study of (R)-53 revealed the existence of two energically stable conformers just like two mirror images. This may account for (R)-53 high affinity for both the dopamine D(2) and serotonin 5-HT(3) receptors. Pharmacologically, (R)-53 [AS-8112] showed a potent antagonistic activity for both the dopamine D(2) and serotonin 5-HT(3) receptors in vivo tests and dose-dependently inhibited both the incidence and frequency of emetic episodes induced by cisplatin (ferrets) and morphine (dogs) with ID(50) values of 27.1 microg/kg, po and 136 microg/kg, po, respectively. On the basis of this pharmacological profile, (R)-53 is now under further investigation as a potential broad antiemetic agent.

Synthesis and structure-activity relationships of 4-amino-5-chloro-N-(1,4-dialkylhexahydro-1,4-diazepin-6-yl)-2-methoxybenzamide derivatives, novel and potent serotonin 5-HT3 and dopamine D2 receptors dual antagonist

In search of a dopamine D2 and serotonin 5-HT3 receptors dual antagonist as a potential broad antiemetic agent, a number of benzamides were prepared from 4-amino-5-chloro-2-methoxybenzoic acid derivatives and 6-amino-1,4-dialkylhexahydro-1,4-diazepines and evaluated for their binding affinity for the dopamine D2 and the serotonin 5-HT3 receptors using rat brain synaptic and rat cortical membranes, respectively. From the results of both in vitro receptor binding and in vivo biological assays for the dopamine D2 receptor, 1-ethyl-4-methylhexahydro-1,4-diazepine ring was selected as an optimum amine moiety. Introduction of one methyl group on the nitrogen atom at the 4-position and/or modification of the substituent at the 5-position of the 4-amino-5-chloro-2-methoxybenzoyl moiety caused a marked increase in the dopamine D2 receptor binding affinity along with a potent 5-HT3 receptor binding affinity. Among the compounds, 5-chloro-N-(1-ethyl-4-methylhexahydro-1,4-diazepin-6-yl)-2-methoxy-4-methylaminobenzamide (82), 5-bromo (110), and 5-iodo (112) analogues exhibited a much higher affinity for the dopamine D2 receptor than that of metoclopramide (IC50=17.5-61.0 nM vs. 483 nM). In particular, 82 showed a potent antagonistic activity for both receptors in vivo tests. Optical resolution of the racemate 82 brought about a dramatic change in the pharmacological profile with the (R)-enantiomer exhibiting a strong affinity for both the dopamine D2 and the 5-HT3 receptors, while the corresponding (S)-enantiomer had a potent and selective serotonin 5-HT3 receptor binding affinity.