Aporphinoid antagonists of 5-HT2A receptors: further evaluation of ring A substituents and the size of ring C

Semisynthetic Transformations on (+)-Boldine Reveal a 5-HT2A/2CR Antagonist

Aporphine alkaloids have shown affinity for serotonin receptors (5-HTRs), and there has been a recent upsurge of interest in aporphines as 5-HT_2CR ligands. 1,2,9,10-Tetraoxygenated aporphine alkaloids in particular have demonstrated good affinity for 5-HTRs. In continued efforts to understand the impacts of structural modification of the 1,2,9,10-tetraoxygenated aporphine template on affinity, selectivity, and activity at 5-HT₂R subtypes, we used (+)-boldine (8) as a semisynthetic feedstock in the preparation of C-2-alkoxylated (+)-predicentrine analogues. Compound 10n, which contains a benzyloxy group at C-2, has been identified as a novel 5-HT_2CR ligand with strong affinity (4 nM) and moderate selectivity versus 5-HT_2BR and 5-HT_2AR (12-fold and 6-fold, respectively). Compound 10n functions as an antagonist at 5-HT_2A and 5-HT_2C receptors. Computational experiments indicate that several hydrophobic interactions as well as strong H-bond and salt bridge interactions between the protonated amine moiety in 10n and Asp134 are responsible for the potent 5-HT_2CR affinity of this compound. Furthermore, compound 10n displays favorable predicted drug-like characteristics, which is encouraging toward future optimization.

Mechanism Exploration of Arylpiperazine Derivatives Targeting the 5-HT2A Receptor by In Silico Methods

As a G-protein coupled receptor, the 5-hydroxytryptamine 2A (5-HT_2A) receptor is known for its critical role in the cognitive, behavioural and physiological functions, and thus is a primary molecular target to treat psychiatric diseases, including especially depression. With purpose to explore the structural traits affecting the inhibitory activity, currently a dataset of 109 arylpiperazine derivatives as promising 5-HT_2A antagonists was built, based on which the ligand-based three-dimensional quantitative structure-activity relationship (3D-QSAR) study by using both comparative molecular field analysis (CoMFA) and comparative molecular similarity indices analysis (CoMSIA) approaches was carried out. The resultant optimal CoMSIA model displays proper validity and predictability with cross-validated correlation coefficient Q² = 0.587, non-cross-validated correlation coefficient R²_ncv = 0.900 and predicted correlation coefficient for the test set of compounds R²_pre = 0.897, respectively. Besides, molecular docking was also conducted to investigate the binding mode between these ligands and the active site of the 5-HT_2A receptor. Meanwhile, as a docking supplementary tool to study the antagonists' conformation in the binding cavity, molecular dynamics (MD) simulation was also performed, providing further elucidation about the changes in the ligand-receptor complex. Lastly, some new molecules were also newly-designed based on the above results that are potential arylpiperazine antagonists of 5-HT_2A receptor. We hope that the present models and derived information may be of help for facilitating the optimization and design of novel potent antagonists as antidepressant drugs as well as exploring the interaction mechanism of 5-HT_2A antagonists.

Synthesis and evaluation of aporphine analogs containing C1 allyl isosteres at the h5-HT(2A) receptor

A series of C1 aporphine analogs related to compound 5 and that contain substituted allylic, alkynyl, nitrile, ester and benzyl groups was synthesized and evaluated for affinity at h5HT2A and α1A receptors in functional activity assays that measure calcium release. The presence of branched allylic substituent groups diminished affinity for the h5HT2A receptor. Likewise, the alkynyl, nitrile and ester derivatives evaluated displayed lower 5-HT2A receptor affinity as compared to 5. Hydrophobic, steric and electronic effects impact the affinity of p-substituted benzyl derivatives 8i-8k but in different ways. High hydrophobicity and size favor 5-HT2A affinity whereas, high electronegativity disfavors 5-HT2A affinity. p-Bromobenzyl analog 8k was identified as a 5-HT2A receptor selective ligand, with the highest 5-HT2A receptor affinity of any aporphine known to date. Most of the other analogs were selective for the 5-HT2A versus the α1A receptor. ChemScore binding energies from docking studies correlated qualitatively with the observed trends in affinity for 8i-8k, although the binding energies were not well differentiated quantitatively.

Total Synthesis of 4,5-Didehydroguadiscine: A Potent Melanogenesis Inhibitor from the Brazilian Medicinal Herb, Hornschuchia obliqua

The first total synthesis of the 7,7-dimethylaporphinoid, 4,5-didehydroguadiscine (6), originally isolated from the stems and roots of Hornschuchia oblique (Annonaceae), was achieved by the condensation of homopiperonylamine (7) with an α,α-dimethylphenylacetic acid derivative (8) and subsequent Pschorr reaction of the resulting benzylisoquinoline intermediate (22). The reported (13)C NMR data were partially revised on the basis of the analysis of HMBC spectra measured under different conditions. The melanogenesis inhibitory activity (IC50 = 4.7 μM) of 6 was 40 times stronger than that of arbutin (174 μM), which was used as reference standard. Furthermore, 6 was the most potent natural melanogenesis inhibitor within this class of compounds.

C4 phenyl aporphines with selective h5-HT(2B) receptor affinity

A group of aporphine alkaloids related to (±)-nantenine (1) and bearing a C4 phenyl and various C1 or N-substituents, was synthesized and evaluated for affinity to h5-HT receptors. In general, unlike nantenine, the analogs lack affinity for the h5-HT(2A) receptor and other 5-HT receptors but bind selectively to the h5-HT(2B) receptor. With regards to 5-HT(2B) affinity, there appears to be a low tolerance for bulky C1 or N-substituents when the C4 phenyl moiety is present. Compound 5a had the highest 5-HT(2B) affinity of the compounds tested, was found to be an antagonist and is selective vs other CNS receptors.

Semisynthetic Studies on and Biological Evaluation of N-Methyllaurotetanine Analogues as Ligands for 5-HT Receptors

N-Methyllaurotetanine (1) has been reported to display good affinity for the 5-HT1A receptor, but no structure-affinity studies have been performed to date. The commercially available alkaloid boldine (2) was used as the starting material for synthesis of various C-9 alkoxy analogues of N-methyllaurotetanine in order to gauge the effect of C-9 alkylation on affinity and selectivity at 5-HT1A, 5-HT2A, and 5-HT7 receptors. Mitsunobu reactions were implemented in the alkylation steps leading to the analogues. Modest improvement in 5-HT1A affinity was observed upon alkylation for most analogues. Thus, the C-9 hydroxy group of 1 is not critical for affinity to the 5-HT1A receptor. Some analogues displayed high affinity for the 5-HT7 receptor, comparable to N-methyllaurotetanine, with moderate selectivity vs 5-HT1A and 5-HT2A receptors.