Copyrine alkaloids: synthesis, spectroscopic characterization, and antimycotic/antimycobacterial activity of A- and B-ring-functionalized sampangines

Biological Activity of Naturally Derived Naphthyridines

Marine and terrestrial environments are rich sources of various bioactive substances, which have been used by humans since prehistoric times. Nowadays, due to advances in chemical sciences, new substances are still discovered, and their chemical structures and biological properties are constantly explored. Drugs obtained from natural sources are used commonly in medicine, particularly in cancer and infectious diseases treatment. Naphthyridines, isolated mainly from marine organisms and terrestrial plants, represent prominent examples of naturally derived agents. They are a class of heterocyclic compounds containing a fused system of two pyridine rings, possessing six isomers depending on the nitrogen atom's location. In this review, biological activity of naphthyridines obtained from various natural sources was summarized. According to previous studies, the naphthyridine alkaloids displayed multiple activities, i.a., antiinfectious, anticancer, neurological, psychotropic, affecting cardiovascular system, and immune response. Their wide range of activity makes them a fascinating object of research with prospects for use in therapeutic purposes.

Synthesis and biological evaluation of novel 8- substituted sampangine derivatives as potent inhibitor of Zn2+-Aβ complex mediated toxicity, oxidative stress and inflammation

A series of 8-substituted sampangine derivatives have been designed, synthesized and tested for their ability to inhibit cholinesterase and penetrate the blood-brain barrier. Their chelating ability toward Zn²⁺ and other biologically relevant metal ions was also demonstrated by isothermal titration calorimetry. The new derivatives exhibited high acetylcholinesterase inhibitory activity, high blood-brain barrier penetration ability and high chelating selectivity for Zn²⁺. Moreover, compound 10 with the strongest binding affinity to Zn²⁺ was selected for further research. Western blotting analysis, transmission electron microscopy, DCFH-DA assay and paralysis experiment indicated that compound 10 suppressed the formation of Zn²⁺-Aβ complexes, alleviated the Zn²⁺ induced neurotoxicity and inhibited the production of ROS catalyzed by Zn²⁺ in Aβ42 transgenic C. elegans. Furthermore, compound 10 also inhibited the expressions of pro-inflammatory cytokines, such as NO, TNF-α, IL-6 and IL-1β, induced by Zn²⁺ + Aβ_1-42 in BV2 microglial cells. In general, this work provided new insights into the design and development of potent metal-chelating agents for Alzheimer's disease treatment.

Novel sampangine derivatives as potent inhibitors of Cu2+-mediated amyloid-β protein aggregation, oxidative stress and inflammation

A series of 11-substituted sampangine derivatives have been designed, synthesized, and tested for their ability to inhibit cholinesterase. Their chelating ability and selectivity for Cu²⁺ over other biologically relevant metal ions were demonstrated by isothermal titration calorimetry. Their blood-brain barrier permeability was also tested by parallel artificial membrane permeation assay. Among the synthesized derivatives, compound 11 with the strong anti-acetylcholinesterase activity, high blood-brain barrier penetration ability and high binding affinity to Cu²⁺ was selected for further research. Western blotting analysis, transmission electron microscopy, DCFH-DA assay and paralysis experiment indicated that compound 11 suppressed the formation of Cu²⁺-Aβ complexes, alleviated the Cu²⁺ induced neurotoxicity and inhibited the production of ROS catalyzed by Cu²⁺ in Aβ42 transgenic C. elegans. Moreover, compound 11 also inhibited the expressions of proinflammatory cytokines, such as NO, TNF-α, IL-6 and IL-1β, induced by Cu²⁺ + Aβ_1-42 in BV2 microglial cells. In general, this work provided new insights into the design and development of potent metal-chelating agents for AD treatment.

Acetylcholinesterase inhibitors for the treatment of Alzheimer's disease - a patent review (2016-present)

Introduction - AD, the most common form of dementia, has a multifactorial etiology, and the current therapy (AChEIs and memantine) is unable to interrupt its progress and fatal outcome. This is reflected in the research programs that are oriented toward the development of new therapeutics able to operate on multiple targets involved in the disease progression.Areas covered - The patents from 2016 to present regarding the use of AChEIs in AD, concerns the development of new AChEIs, multitarget or multifunctional ligands, or the associations of currently used AChEIs with other compounds acting on different targets involved in the AD.Expert opinion - The development of new multitarget AChEIs promises to identify compounds with great therapeutic potential but requires more time and effort in order to obtain drugs with the optimal pharmacodynamic profile. Otherwise, the research on new combinations of existing drugs, with known pharmacodynamic and ADME profile, could shorten the time and reduce the costs to develop a new therapeutic treatment for AD. From the analyzed data, it seems more likely that a response to the urgent need to develop effective treatments for AD therapy could come more quickly from studies on drug combinations than from the development of new AChEIs.

Oxoisoaporphines and Aporphines: Versatile Molecules with Anticancer Effects

Cancer is a disease that involves impaired genome stability with a high mortality index globally. Since its discovery, many have searched for effective treatment, assessing different molecules for their anticancer activity. One of the most studied sources for anticancer therapy is natural compounds and their derivates, like alkaloids, which are organic molecules containing nitrogen atoms in their structure. Among them, oxoisoaporphine and sampangine compounds are receiving increased attention due to their potential anticancer effects. Boldine has also been tested as an anticancer molecule. Boldine is the primary alkaloid extract from boldo, an endemic tree in Chile. These compounds and their derivatives have unique structural properties that potentially have an anticancer mechanism. Different studies showed that this molecule can target cancer cells through several mechanisms, including reactive oxygen species generation, DNA binding, and telomerase enzyme inhibition. In this review, we summarize the state-of-art research related to oxoisoaporphine, sampangine, and boldine, with emphasis on their structural characteristics and the relationship between structure, activity, methods of extraction or synthesis, and anticancer mechanism. With an effective cancer therapy still lacking, these three compounds are good candidates for new anticancer research.

Structural Simplification of Natural Products

Natural products (NPs) are important sources of clinical drugs due to their structural diversity and biological prevalidation. However, the structural complexity of NPs leads to synthetic difficulties, unfavorable pharmacokinetic profiles, and poor drug-likeness. Structural simplification by truncating unnecessary substructures is a powerful strategy for overcoming these limitations and improving the efficiency and success rate of NP-based drug development. Herein, we will provide a comprehensive review of the structural simplification of NPs with a focus on design strategies, case studies, and new technologies. In particular, a number of successful examples leading to marketed drugs or drug candidates will be discussed in detail to illustrate how structural simplification is applied in lead optimization of NPs.

α-Pyrones, secondary metabolites from fungus Cephalotrichum microsporum and their bioactivities

Cephalotrichum microsporum (SYP-F 7763) was a fungus isolated from the rhizosphere soil of traditional Chinese medicine Panax notoginseng. The EtOAc extract of Cephalotrichum microsporum cultivated on sterilized moistened-rice medium was separated by various chromatographic techniques, which yielded 11 metabolites (1-11) of this fungus. On the basis of the widely spectroscopic data, the chemical structures of isolated metabolites were determined, most of which were α-pyrones, including 5 compounds (4-7, and 10) unreported. In the anti-bacterial bioassay, compound 1 displayed significant inhibitory effects on three pathogenic bacteria, MR S. aureus, S. aureus, and B. cereus. α-Pyrones 2, 3, and 5-7 also displayed moderate inhibitory effects on MR S. aureus, S. aureus, and B. subtilis, which could be the major anti-bacterial constituents of Cephalotrichum microsporum. Additionally, compounds 1, 4, and 5 displayed significant cytotoxicity on five human cancer cell lines, with the IC₅₀ values < 20 μM, which are more effective than positive control 5-fluorouracil. Therefore, α-pyrones were important secondary metabolites of Cephalotrichum microsporum, which displayed anti-bacterial and anti-tumor activities.

Electrophilic carbocyclization reactions of 2-(2-alkynylphenyl)amino-1,4-naphthoquinones

Electrophilic carbocylization reactions for the synthesis of 5H-benzo[b]naphtho [2,3-f]azepine-6,11-diones are described.

Strategies in the discovery of novel antifungal scaffolds

The development of next-generation antifungal agents with novel chemical scaffolds and new mechanisms of action is vital due to increased incidence and mortality of invasive fungal infections and severe drug resistance. This review will summarize current strategies to discover novel antifungal scaffolds. In particular, high-throughput screening, drug repurposing, antifungal natural products and new antifungal targets are focused on. New scaffolds with validated antifungal activity, their discovery and optimization process as well as structure–activity relationships are discussed in detail. Perspectives that could inspire future antifungal drug discovery are provided.

New Perspectives in the Chemistry of Marine Pyridoacridine Alkaloids

Secondary metabolites from marine organisms are a rich source of novel leads for drug development. Among these natural products, polycyclic aromatic alkaloids of the pyridoacridine type have attracted the highest attention as lead compounds for the development of novel anti-cancer and anti-infective drugs. Numerous sophisticated total syntheses of pyridoacridine alkaloids have been worked out, and many of them have also been extended to the synthesis of libraries of analogues of the alkaloids. This review summarizes the progress in the chemistry of pyridoacridine alkaloids that was made in the last one-and-a-half decades.

Sampangine (a Copyrine Alkaloid) Exerts Biological Activities through Cellular Redox Cycling of Its Quinone and Semiquinone Intermediates

The cananga tree alkaloid sampangine (1) has been extensively investigated for its antimicrobial and antitumor potential. Mechanistic studies have linked its biological activities to the reduction of cellular oxygen, the induction of reactive oxygen species (ROS), and alterations in heme biosynthesis. Based on the yeast gene deletion library screening results that indicated mitochondrial gene deletions enhanced the sensitivity to 1, the effects of 1 on cellular respiration were examined. Sampangine increased oxygen consumption rates in both yeast and human tumor cells. Mechanistic investigation indicated that 1 may have a modest uncoupling effect, but predominately acts by increasing oxygen consumption independent of mitochondrial complex IV. Sampangine thus appears to undergo redox cycling that may involve respiratory chain-dependent reduction to a semi-iminoquinone followed by oxidation and consequent superoxide production. Relatively high concentrations of 1 showed significant neurotoxicity in studies conducted with rat cerebellar granule neurons, indicating that sampangine use may be associated with potential neurotoxicity.

The discovery of novel antifungal scaffolds by structural simplification of the natural product sampangine

Structural simplification of the natural product sampangine led to the discovery of two novel antifungal compounds with excellent activity and low toxicity.

Palladium(ii)-catalysed regioselective synthesis of 3,4-disubstituted quinolines and 2,3,5-trisubstituted pyrroles from alkenes via anti-Markovnikov selectivity

A novel strategy has been identified for the regioselective synthesis of 3,4-disubstituted quinolines and 2,3,5-trisubstituted pyrroles through anti-Markovnikov selectivity.

A novel approach to the pyridoacridine ring system: synthesis of the topoisomerase inhibitor 13-deazaascididemin

A novel approach to the pyridoacridine ring system of the ascididemin-type marine alkaloids is presented. This approach allows for the introduction of the ring A of the alkaloids by using a simple aromatic aldehyde building block. The viability of this approach is demonstrated with the synthesis of AK37, a bioactive deaza analogue of the alkaloid ascididemin. Starting from 3-cyano-4-methylquinoline, a sequence of regioselective homolytic benzoylation, annulation of a bromopyridine ring, and radical cyclization leads to the pentacyclic ring system.

Sampangine inhibits heme biosynthesis in both yeast and human

The azaoxoaporphine alkaloid sampangine exhibits strong antiproliferation activity in various organisms. Previous studies suggested that it somehow affects heme metabolism and stimulates production of reactive oxygen species (ROS). In this study, we show that inhibition of heme biosynthesis is the primary mechanism of action by sampangine and that increases in the levels of reactive oxygen species are secondary to heme deficiency. We directly demonstrate that sampangine inhibits heme synthesis in the yeast Saccharomyces cerevisiae. It also causes accumulation of uroporphyrinogen and its decarboxylated derivatives, intermediate products of the heme biosynthesis pathway. Our results also suggest that sampangine likely works through an unusual mechanism-by hyperactivating uroporhyrinogen III synthase-to inhibit heme biosynthesis. We also show that the inhibitory effect of sampangine on heme synthesis is conserved in human cells. This study also reveals a surprising essential role for the interaction between the mitochondrial ATP synthase and the electron transport chain.

Azaphenanthrene alkaloids with antitumoral activity from Anaxagorea dolichocarpa Sprague & Sandwith (Annonaceae)

Phytochemical investigation of Anaxagorea dolichocarpa Sprague & Sandwith led to isolation of three azaphenanthrene alkaloids: eupolauramine, sampangine and imbiline 1. Their chemical structures were established on the basis of spectroscopic data from IR, HR-ESI-MS, NMR (including 2D experiments) and comparison with the literature. Sampangine and imbiline 1 are being described in the Anaxagorea genus for the first time. Eupolauramine and sampangine show concentration-dependent antitumoral activity in leukemic cells K562 with IC₅₀ of 18.97 and 10.95 µg/mL, respectively.

Isolation and synthesis of antiproliferative eupolauridine alkaloids of Ambavia gerrardii from the Madagascar Dry Forest

Investigation of the Madagascan endemic plant Ambavia gerrardii for antiproliferative activity against the A2780 ovarian cancer cell line led to the isolation of the three new alkaloids 8-hydroxyeupolauridine (1), 9-methoxyeupolauridine 1-oxide (2), and 11-methoxysampangine (3) and the three known alkaloids 4-6. The structures of 1 and 2 were confirmed by synthesis. Compounds 3, 4, and 6 showed moderate to good antiproliferative activities, with IC50 values of 10.3, 3.5, and 0.60 μM, respectively, against the A2780 human ovarian cancer cell line and with IC50 values of 0.57, 1.77, and 0.58 μM, respectively, against the H460 human lung cancer cell line.

Alkaloids as potential anti-tubercular agents

An increasing incidence of deaths due to tuberculosis and the known drawbacks of the current existing drugs including the emergence of multi drug-resistant strains have led to a renewed interest in the discovery of new anti-tubercular agents with novel modes of actions. The recent researches focused on natural products have shown a useful way to obtain a potentially rich source of drug candidates, where alkaloids have been found more effective. The present review focuses on current epidemiology of tuberculosis, synergy of the disease with HIV, current therapy, available molecular targets and, highlights why natural products especially alkaloids are so important. The review summarizes alkaloids found active against mycobacteria from the mid-1980s to late 2008 with special attention on the study of structure-activity relationship (SAR).

Role of heme in the antifungal activity of the azaoxoaporphine alkaloid sampangine

Sampangine, a plant-derived alkaloid found in the Annonaceae family, exhibits strong inhibitory activity against the opportunistic fungal pathogens Candida albicans, Cryptococcus neoformans, and Aspergillus fumigatus. In the present study, transcriptional profiling experiments coupled with analyses of mutants were performed in an effort to elucidate its mechanism of action. Using Saccharomyces cerevisiae as a model organism, we show that sampangine produces a transcriptional response indicative of hypoxia, altering the expression of genes known to respond to low-oxygen conditions. Several additional lines of evidence obtained suggest that these responses could involve effects on heme. First, the hem1Delta mutant lacking the first enzyme in the heme biosynthetic pathway showed increased sensitivity to sampangine, and exogenously supplied hemin partially rescued the inhibitory activity of sampangine in wild-type cells. In addition, heterozygous mutants with deletions in genes involved in five out of eight steps in the heme biosynthetic pathway showed increased susceptibility to sampangine. Furthermore, spectral analyses of pyridine extracts indicated significant accumulation of free porphyrins in sampangine-treated cells. Transcriptional profiling experiments were also performed with C. albicans to investigate the response of a pathogenic fungal species to sampangine. Taking into account the known differences in the physiological responses of C. albicans and S. cerevisiae to low oxygen, significant correlations were observed between the two transcription profiles, suggestive of heme-related defects. Our results indicate that the antifungal activity of the plant alkaloid sampangine is due, at least in part, to perturbations in the biosynthesis or metabolism of heme.

Hetero Analogues of the Antimicrobial Alkaloids Cleistopholine and Sampangine

Hetero analogues of the alkaloids cleistopholine and sampangine were prepared in order to investigate the significance of the (aza)quinoid partial structures for antimicrobial activity. Several analogues containing amino or sulfone groups showed high antimicrobial activities, indicating that the (aza)quinoid partial structures of the alkaloids are not an indispensable requisite for antimicrobial activity.

Induction of apoptosis by the plant alkaloid sampangine in human HL-60 leukemia cells is mediated by reactive oxygen species

Sampangine is a plant-derived copyrine alkaloid extracted from the stem bark of Cananga odorata. This azaoxoaporphine alkaloid primarily exhibits antifungal and antimycobacterial activities but also displays in vitro antimalarial activity against Plasmodium falciparum and it is cytotoxic to human malignant melanoma cells. Recently, sampangine was described as a pro-apoptotic agent, but the biochemical pathway leading to cell death remained unclear. Considering that sampangine possesses an iminoquinone moiety, potentially functioning as an oxidizing agent, we have investigated the implication of an oxidant stress on sampangine-induced cytotoxicity. We show that the treatment of human HL-60 leukemia cells for 48 h with sampangine induced an important oxidative burst. Real time flow cytometry measurements indicated that the production of oxidative species is very rapid, within minutes following the drug addition. Quenching of reactive oxygen species by the antioxidants N-acetyl cystein, vitamin C and vitamin E abolishes the pro-apoptotic activity of sampangine. The drug-induced production of reactive oxygen species is associated with cell cycle perturbations and mitochondrial alterations. This study shed light on the mechanism of action of sampangine and provides novel opportunities to use azaoxoaporphine alkaloids as lead compounds for the design of pro-apoptotic anticancer agents.

Identification of heteroarylenamines as a new class of antituberculosis lead molecules

Enamine-containing analogues of heteroarylquinones were prepared based on initial screening data observed against Mycobacterium tuberculosis H37Rv (Mtb). Several analogues showed moderate to good inhibitory activity, with one analogue (7) also demonstrating acceptable toxic selectivity (MIC 0.39 microg/mL, SI 15). Activity towards a range of single-drug-resistant strains of Mtb was suggestive of a novel mechanism of action for 7.

Bioactive Marine Alkaloids

The chapter deals with bioactive marine alkaloids. The chemistry and biological activities of pyridoacridines, pyrroloacridines, indoles, β-carbolines, pyrroles, isoquinolines, and tyrosine derived alkaloids have been discussed and reviewed.