Synthesis and binding affinity of new 1,4-disubstituted triazoles as potential dopamine D3 receptor ligands

Dopamine Receptor Ligand Selectivity-An In Silico/In Vitro Insight

Different dopamine receptor (DR) subtypes are involved in pathophysiological conditions such as Parkinson's Disease (PD), schizophrenia and depression. While many DR-targeting drugs have been approved by the U.S. Food and Drug Administration (FDA), only a very small number are truly selective for one of the DR subtypes. Additionally, most of them show promiscuous activity at related G-protein coupled receptors, thus suffering from diverse side-effect profiles. Multiple studies have shown that combined in silico/in vitro approaches are a valuable contribution to drug discovery processes. They can also be applied to divulge the mechanisms behind ligand selectivity. In this study, novel DR ligands were investigated in vitro to assess binding affinities at different DR subtypes. Thus, nine D2R/D3R-selective ligands (micro- to nanomolar binding affinities, D3R-selective profile) were successfully identified. The most promising ligand exerted nanomolar D3R activity (Ki = 2.3 nM) with 263.7-fold D2R/D3R selectivity. Subsequently, ligand selectivity was rationalized in silico based on ligand interaction with a secondary binding pocket, supporting the selectivity data determined in vitro. The developed workflow and identified ligands could aid in the further understanding of the structural motifs responsible for DR subtype selectivity, thus benefitting drug development in D2R/D3R-associated pathologies such as PD.

Recent advances in dopamine D2 receptor ligands in the treatment of neuropsychiatric disorders

Dopamine is a biologically active amine synthesized in the central and peripheral nervous system. This biogenic monoamine acts by activating five types of dopamine receptors (D_1-5 Rs), which belong to the G protein-coupled receptor family. Antagonists and partial agonists of D₂ Rs are used to treat schizophrenia, Parkinson's disease, depression, and anxiety. The typical pharmacophore with high D₂ R affinity comprises four main areas, namely aromatic moiety, cyclic amine, central linker and aromatic/heteroaromatic lipophilic fragment. From the literature reviewed herein, we can conclude that 4-(2,3-dichlorophenyl), 4-(2-methoxyphenyl)-, 4-(benzo[b]thiophen-4-yl)-1-substituted piperazine, and 4-(6-fluorobenzo[d]isoxazol-3-yl)piperidine moieties are critical for high D₂ R affinity. Four to six atoms chains are optimal for D₂ R affinity with 4-butoxyl as the most pronounced one. The bicyclic aromatic/heteroaromatic systems are most frequently occurring as lipophilic appendages to retain high D₂ R affinity. In this review, we provide a thorough overview of the therapeutic potential of D₂ R modulators in the treatment of the aforementioned disorders. In addition, this review summarizes current knowledge about these diseases, with a focus on the dopaminergic pathway underlying these pathologies. Major attention is paid to the structure, function, and pharmacology of novel D₂ R ligands, which have been developed in the last decade (2010-2021), and belong to the 1,4-disubstituted aromatic cyclic amine group. Due to the abundance of data, allosteric D₂ R ligands and D₂ R modulators from patents are not discussed in this review.

The 1,2,3-triazole ring as a bioisostere in medicinal chemistry

1,2,3-Triazole is a well-known scaffold that has a widespread occurrence in different compounds characterized by several bioactivities, such as antimicrobial, antiviral, and antitumor effects. Moreover, the structural features of 1,2,3-triazole enable it to mimic different functional groups, justifying its wide use as a bioisostere for the synthesis of new active molecules. Here, we provide an overview of the 1,2,3-triazole ring as a bioisostere for the design of drug analogs, highlighting relevant recent examples.

1,4-Disubstituted aromatic piperazines with high 5-HT2A/D2 selectivity: Quantitative structure-selectivity investigations, docking, synthesis and biological evaluation

Simultaneous targeting of dopamine D2 and 5-HT2A receptors for the treatment of schizophrenia is one key feature of typical and atypical antipsychotics. In most of the top-selling antipsychotic drugs like aripiprazole and risperidone, high affinity to both receptors can be attributed to the presence of 1,4-disubstituted aromatic piperazines or piperidines as primary receptor recognition elements. Taking advantage of our in-house library of phenylpiperazine-derived dopamine receptor ligands and experimental data, we established highly significant CoMFA and CoMSIA models for the prediction of 5-HT2A over D2 selectivity. Subsequently, the models were applied to identify the selective candidates 55-57 from our newly synthesized library of GPCR ligands comprising a pyrazolo[1,5-a]pyridine head group and a 1,2,3-triazole based linker unit. The test compound 57 showed subnanomolar a Ki value (0.64 nM) for 5-HT2A and more than 10- and 30-fold selectivity over the dopamine receptor isoforms D2S and D2L, respectively.

Using click chemistry toward novel 1,2,3-triazole-linked dopamine D3 receptor ligands

The dopamine D3 receptor (D3R) is a target of pharmacotherapeutic interest in a variety of neurological disorders including schizophrenia, Parkinson's disease, restless leg syndrome, and drug addiction. A common molecular template used in the development of D3R-selective antagonists and partial agonists incorporates a butylamide linker between two pharmacophores, a phenylpiperazine moiety and an extended aryl ring system. The series of compounds described herein incorporates a change to that chemical template, replacing the amide functional group in the linker chain with a 1,2,3-triazole group. Although the amide linker in the 4-phenylpiperazine class of D3R ligands has been previously deemed critical for high D3R affinity and selectivity, the 1,2,3-triazole moiety serves as a suitable bioisosteric replacement and maintains desired D3R-binding functionality of the compounds. Additionally, using mouse liver microsomes to evaluate CYP450-mediated phase I metabolism, we determined that novel 1,2,3-triazole-containing compounds modestly improves metabolic stability compared to amide-containing analogues. The 1,2,3-triazole moiety allows for the modular attachment of chemical subunit libraries using copper-catalyzed azide-alkyne cycloaddition click chemistry, increasing the range of chemical entities that can be designed, synthesized, and developed toward D3R-selective therapeutic agents.

Synthesis, pharmacological evaluation and molecular modeling studies of triazole containing dopamine D3 receptor ligands

A series of 2-methoxyphenyl piperazine analogues containing a triazole ring were synthesized and their in vitro binding affinities at human dopamine D2 and D3 receptors were evaluated. Compounds 5b, 5c, 5d, and 4g, demonstrate high affinity for dopamine D3 receptors and moderate selectivity for the dopamine D3 versus D2 receptor subtypes. To further examine their potential as therapeutic agents, their intrinsic efficacy at both D2 and D3 receptors was determined using a forskolin-dependent adenylyl cyclase inhibition assay. Affinity at dopamine D4 and serotonin 5-HT1A receptors was also determined. In addition, information from previous molecular modeling studies of the binding of a panel of 163 structurally-related benzamide analogues at dopamine D2 and D3 receptors was applied to this series of compounds. The results of the modeling studies were consistent with our previous experimental data. More importantly, the modeling study results explained why the replacement of the amide linkage with the hetero-aromatic ring leads to a reduction in the affinity of these compounds at D3 receptors.