3,4-Dihydroxy- and 3,4-methylenedioxy- phenanthrene-type alkaloids with high selectivity for D2 dopamine receptor

Aporphines: A privileged scaffold in CNS drug discovery

Aporphine alkaloids embedded in 4H-dibenzo[de,g]quinoline four-ring structures belong to one of the largest subclasses of isoquinoline alkaloids. Aporphine is a privileged scaffold in the field of organic synthesis and medicinal chemistry for the discovery of new therapeutic agents for central nervous system (CNS) diseases, cancer, metabolic syndrome, and other diseases. In the past few decades, aporphine has attracted continuing interest to be widely used to develop selective or multitarget directed ligands (MTDLs) targeting the CNS (e.g., dopamine D_1/2/5, serotonin 5-HT_1A/2A/2C and 5-HT₇, adrenergic α/β receptors, and cholinesterase enzymes), thereby serving as valuable pharmacological probes for mechanism studies or as potential leads for CNS drug discovery. The aims of the present review are to highlight the diverse CNS activities of aporphines, discuss their SAR, and briefly summarize general synthetic routes, which will pave the way for the design and development of new aporphine derivatives as promising CNS active drugs in the future.

First Total Syntheses of Natural Phenanthrene Alkaloids, Uvariopsamine, Noruvariopsamine, 8-Hydroxystephenanthrine, 8-Methoxyuvariopsine, Thalihazine, and Secophoebine, and Their Potential as Antimalarial Agents

The first total syntheses of natural phenanthrene alkaloids, namely, uvariopsamine (1), noruvariopsamine (2), 8-hydroxystephenanthrine (3), 8-methoxyuvariopsine (4), thalihazine (5), and secophoebine (6), have been realized. In addition, their in vitro antimalarial activity against the multidrug-resistant K1 strain of Plasmodium falciparum and in vitro cytotoxic activity against the human nasopharynx carcinoma (KB), small cell lung cancer (NCI-H187), and breast cancer (MCF7) human cancer cell lines were investigated. All the phenanthrene alkaloids showed significant antiplasmodial activity (IC₅₀ 1.07-7.41 µM), and most compounds displayed low to no toxicity against the three cancer cell lines tested. Particularly, 3 exhibited the best antimalarial activity with an IC₅₀ value of 1.07 µM, no toxicity to NCI-H187 (IC₅₀ > 50 µM), and low toxicity against KB (IC₅₀ 24.53 µM) and MCF7 (IC₅₀ 42.67 µM) cell lines.

A unified total synthesis of benzo[d][1,3]dioxole-type benzylisoquinoline alkaloids of aporphines, coptisines, and dibenzopyrrocolines

The first total synthesis of (S)-(+)-ovigerine, (S)-(+)-N-formylovigerine, and (6aS,6a'S)-(+)-ovigeridimerine of aporphine alkaloids with a benzo[d][1,3]dioxole structure feature was established. The strategy was based upon the well-known Pd-catalyzed arylation to set the aporphine framework, and Noyori asymmetric hydrogenation followed by diastereoselective resolution to achieve excellent enantioselectivity. By slightly modifying the total synthetic route and strategically combining it with a aza-Michael addition, Bischler-Napieralski reaction and N-arylation, this methodology was also applied to the total syntheses of benzo[d][1,3]dioxole-type benzylisoquinoline alkaloids of coptisines and dibenzopyrrocolines, including two impatiens, tetrahydrocoptisine, and quaternary coptisine bromide of coptisines and two dibenzopyrrocoline analogues, with the syntheses of all of these target compounds being efficient. Among the nine synthesized compounds, the total syntheses of the three aporphines and the two impatiens, all with ee values of greater than 99%, were reported for the first time. This work also represents the first unification of synthetic routes for the total synthesis of benzo[d][1,3]dioxole-type aporphines, coptisines, and dibenzopyrrocolines.

Synthesis of Aporphine Analogues via Palladium-Catalyzed Intramolecular Aryl-Aryl Dehydrogenative Coupling

Reported herein is an intramolecular dehydrogenative coupling of two inert aryl C-H bonds for the synthesis of aporphine analogues. The process represents a novel tool for the preparation of aporphines via palladiun-catalyzed C-H bond activation. The present reaction is compatible with various functional groups, and the coupling products have been further applied for the synthesis of natural products aporphine and zenkerine.

Metabolic Profiling of Nuciferine In Vivo and In Vitro

Nuciferine (NF) is one of the main constituents of Lotus (Nelumbo nucifera) leaves which have been widely used in both food and drug formulations in China. Although possessing a broad spectrum of bioactivities, the metabolic characteristics of NF are inadequately unknown after oral gavage with this NF. The present study was performed to characterize its metabolism in vivo and in vitro. After NF oral gavage with mice, a total of 55 metabolites, containing 14 novel phase I metabolites and 18 novel phase II metabolites, were identified with high-resolution mass spectrometry-based metabolomics. Recombinant enzyme screenings showed that multiple cytochrome P450s, two UDP-glucuronosyltransferases (UGT1A4, UGT1A9), and several sulfotransferases (SULTs) participated in the metabolism of NF. In silico prediction and molecular docking of NF to the polymorphic enzymes (CYPs) provided additional support for the above experiments. This research details metabolic characteristics and provides an important reference basis for further application of NF.

Identification of C10 nitrogen-containing aporphines with dopamine D1 versus D5 receptor selectivity

New aporphines containing C10 nitrogen substituents (viz. nitro, aniline or amide moieties), were synthesized and evaluated for affinity at human serotonin 5-HT_1A and 5-HT_2A receptors and at human dopamine D₁, D₂ and D₅ receptors. Two series of analogs were investigated: series A which contain a sole C10 nitrogen substituent on the tetracyclic aporphine core and series B which are 1,2,10-trisubstituted aporphines. Remarkably, compounds from both series lacked affinity for the D₅ receptor, thus attaining D₁ versus D₅ selectivity. Compound 20c was the most potent D₁ ligand identified. Docking studies at D₁ and D₅ receptors indicate that the binding mode of 20c at the D₁ receptor allows for stronger hydrophobic contacts, (primarily with Phe residues) as compared to the D₅ receptor, accounting for its D₁ versus D₅ selectivity. Considering the lack of affinity for the D₅ receptor (and low affinity at other receptors tested), compound 20c represents an interesting starting point for further structural diversification of aporphines as sub-type selective D₁ receptor tools.

1-(2'-Bromobenzyl)-6,7-dihydroxy-N-methyl-tetrahydroisoquinoline and 1,2-Demethyl-nuciferine as Agonists in Human D2 Dopamine Receptors

Certain D₂-like dopamine receptor (DR) agonists are useful therapeutically as antiparkinsonian drugs, whereas D₂-like DR antagonists or partial agonists are proven effective as antipsychotics. Two isoquinoline derivatives, 1-(2'-bromobenzyl)-6,7-dihydroxy-N-methyl-tetrahydroisoquinoline (Br-BTHIQ, 1) and 1,2-demethyl-nuciferine (aporphine, 2), were herein synthesized, and their dopaminergic affinity in cloned human D₂R, D₃R, and D₄R subtypes and their behavior as agonists/antagonists were evaluated. They showed affinity values (K_i) for hD₂, hD₃, and hD₄ DR within the nanomolar range. The trends in affinity were hD₄R ≫ hD₃R > hD₂R for Br-BTHIQ (1) and hD₂R > hD₄R > hD₃R for 1,2-demethyl-nuciferine (2). The functional assays of cyclic adenosine monophosphate signaling at human D₂R showed a partial agonist effect for Br-BTHIQ (1) and full agonist behavior for aporphine (2), with half maximal effective concentration values of 2.95 and 10.2 μM, respectively. Therefore, both isoquinolines 1 and 2 have emerged as lead molecules for the synthesis of new therapeutic drugs that ultimately may be useful to prevent schizophrenia and Parkinson's disease, respectively.

Synthesis and Structure-Activity Relationships of a Series of Aporphine Derivatives with Antiarrhythmic Activities and Acute Toxicity

Some aporphine alkaloids, such as crebanine, were found to present arrhythmic activity and also higher toxicity. A series of derivatives were synthesized by using three kinds of aporphine alkaloids (crebanine, isocorydine, and stephanine) as lead compounds. Chemical methods, including ring-opening reaction, bromination, methylation, acetylation, quaternization, and dehydrogenation, were adopted. Nineteen target derivatives were evaluated for their antiarrhythmic potential in the mouse model of ventricular fibrillation (VF), induced by CHCl₃, and five of the derivatives were investigated further in the rat model of arrhythmia, induced by BaCl₂. Meanwhile, preliminary structure-activity/toxicity relationship analyses were carried out. Significantly, N-acetamidesecocrebanine (1d), three bromo-substituted products of crebanine (2a, 2b, 2c), N-methylcrebanine (2d), and dehydrostephanine (4a) displayed antiarrhythmic effects in the CHCl₃-induced model. Among them, 7.5 mg/kg of 2b was able to significantly reduce the incidence of VF induced by CHCl₃ (p < 0.05), increase the number of rats that resumed sinus rhythm from arrhythmia, induced by BaCl₂ (p < 0.01), and the number of rats that maintained sinus rhythm for more than 20 min (p < 0.01). Therefore, 2b showed remarkably higher antiarrhythmic activity and a lower toxicity (LD50 = 59.62 mg/kg, mice), simultaneously, indicating that 2b could be considered as a promising candidate in the treatment of arrhythmia. Structural-activity analysis suggested that variationsin antiarrhythmic efficacy and toxicity of aporphines were related to the C-1,C-2-methylenedioxy group on ring A, restricted ring B structural conformation, N-quaternization of ring B, levoduction of 6a in ring C, and the 8-, 9-, 10-methoxy groups on ring D on the skeleton.

The first total syntheses of (±)-norphoebine, dehydrophoebine, oxophoebine, dehydrocrebanine, oxocrebanine and uthongine and their cytotoxicity against three human cancer cell lines

The first total syntheses of (±)-norphoebine, dehydrophoebine, oxophoebine, dehydrocrebanine, oxocrebanine and uthongine have been achieved. The crucial step involved the formation of ring C by a microwave-assisted direct biaryl coupling to produce the aporphine skeleton in high yields. The synthetic alkaloids were evaluated for their cytotoxicity against three human cancer cell lines MCF7, KB and NCI-H187. The results showed that uthongine was the best candidate of the series and it exhibited cytotoxicity against a human breast cancer MCF7 line with an IC50 = 3.05 μM.

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.