Structure−Activity Relationship and Studies on the Molecular Mechanism of Leishmanicidal N,C-Coupled Arylisoquinolinium Salts

Structural variety and pharmacological potential of naphthylisoquinoline alkaloids

Naphthylisoquinoline alkaloids are a fascinating class of natural biaryl compounds. They show characteristic mono- and dimeric scaffolds, with chiral axes and stereogenic centers. Since the appearance of the last comprehensive overview on these secondary plant metabolites in this series in 1995, the number of discovered representatives has tremendously increased to more than 280 examples known today. Many novel-type compounds have meanwhile been discovered, among them naphthylisoquinoline-related follow-up products like e.g., the first seco-type (i.e., ring-opened) and ring-contracted analogues. As highlighted in this review, the knowledge on the broad structural chemodiversity of naphthylisoquinoline alkaloids has been decisively driven forward by extensive phytochemical studies on the metabolite pattern of Ancistrocladus abbreviatus from Coastal West Africa, which is a particularly "creative" plant. These investigations furnished a considerable number of more than 80-mostly new-natural products from this single species, with promising antiplasmodial activities and with pronounced cytotoxic effects against human leukemia, pancreatic, cervical, and breast cancer cells. Another unique feature of naphthylisoquinoline alkaloids is their unprecedented biosynthetic origin from polyketidic precursors and not, as usual for isoquinoline alkaloids, from aromatic amino acids-a striking example of biosynthetic convergence in nature. Furthermore, remarkable botanical results are presented on the natural producers of naphthylisoquinoline alkaloids, the paleotropical Dioncophyllaceae and Ancistrocladaceae lianas, including first investigations on the chemoecological role of these plant metabolites and their storage and accumulation in particular plant organs.

Synthesis and pharmacodynamic evaluation of naphthalene derivatives against influenza A virus in vitro and in vivo

Influenza A virus is a highly mutable pathogenic pathogen that could cause a global pandemic. It is necessary to find new anti-influenza drugs to resist influenza epidemics due to the seasonal popularity of a certain area every year. Naphthalene derivatives had potential antiviral activity. A series of naphthalene derivatives were synthesized via the metal-free intramolecular hydroarylation reactions of alkynes. Evaluation of their biological efficacy showed that compound 2-aminonaphthalene 4d had better antiviral activity in vitro than ribavirin. By studying the mechanism of action of 2-aminonaphthalene 4din vivo and in vitro, we found that 4d had antiviral activity to three different subtype influenza viruses of A/Weiss/43 (H1N1), A/Virginia/ATCC2/2009 (H1N1) and A/California/2/2014 (H3N2). Compound 4d had the best effect after viral adsorption, and mainly played in the early stage of virus replication. 2-Aminonaphthalene 4d could reduce the replication of virus by inhibiting the NP and M proteins of virus. Compound 4d cut down ROS accumulation, autophagy and apoptosis induced by influenza virus. Inflammatory response mediated by RIG-1 pathway were suppressed in the cell and mice. In addition, the pathological changes of lung tissue and virus titer in mice were reduced by the administration of 4d. Therefore, naphthalene derivative 4d is a potential drug for the treatment of influenza A virus infection.

Asymmetric Synthesis of Axially Chiral Molecules via Organocatalytic Cycloaddition and Cyclization Reactions

Atropisomeric molecules are present in many natural products, biologically active compounds, chiral ligands and catalysts. Many elegant methodologies have been developed to access axially chiral molecules. Among them, organocatalytic cycloaddition and cyclization have attracted much attention because they have been widely used in the asymmetric synthesis of biaryl/heterobiaryls atropisomers via construction of carbo- and hetero-cycles. This strategy has undoubtedly become and will continue to be a hot topic in the field of asymmetric synthesis and catalysis. This review aims to highlight the recent advancements in this field of atropisomer synthesis by using different organocatalysts in cycloaddition and cyclization strategies. The construction of each atropisomer, its possible mechanism, the role of catalysts, and its potential applications are illustrated.

Korupensamine A, but not its atropisomer, korupensamine B, inhibits SARS-CoV-2 in vitro by targeting its main protease (Mpro)

By combining docking and molecular dynamics simulations, we explored a library of 65 mostly axially chiral naphthylisoquinoline alkaloids and their analogues, with most different molecular architectures and structural analogues, for their activity against SARS-CoV-2. Although natural biaryls are often regarded without consideration of their axial chirality, they can bind to protein targets in an atroposelective manner. By combining docking results with steered molecular dynamics simulations, we identified one alkaloid, korupensamine A, that atropisomer-specifically inhibited the main protease (M^pro) activity of SARS-CoV-2 significantly in comparison to the reference covalent inhibitor GC376 (IC₅₀ = 2.52 ± 0.14 and 0.88 ± 0.15 μM, respectively) and reduced viral growth by five orders of magnitude in vitro (EC₅₀ = 4.23 ± 1.31 μM). To investigate the binding pathway and mode of interaction of korupensamine A within the active site of the protease, we utilized Gaussian accelerated molecular dynamics simulations, which reproduced the docking pose of korupensamine A inside the active site of the enzyme. The study presents naphthylisoquinoline alkaloids as a new class of potential anti-COVID-19 agents.

Asian Ancistrocladus Lianas as Creative Producers of Naphthylisoquinoline Alkaloids

This book describes a unique class of secondary metabolites, the mono- and dimeric naphthylisoquinoline alkaloids. They occur in lianas of the paleotropical Ancistrocladaceae and Dioncophyllaceae families, exclusively. Their unprecedented structures include stereogenic centers and rotationally hindered, and thus likewise stereogenic, axes. Extended recent investigations on six Ancistrocladus species from Asia, as reported in this review, shed light on their fascinating phytochemical productivity, with over 100 such intriguing natural products. This high chemodiversity arises from a likewise unique biosynthesis from acetate-malonate units, following a novel polyketidic pathway to plant-derived isoquinoline alkaloids. Some of the compounds show most promising antiparasitic activities. Likewise presented are strategies for the regio- and stereoselective total synthesis of the alkaloids, including the directed construction of the chiral axis.

N,C-Coupled naphthylisoquinoline alkaloids: a versatile new class of axially chiral natural products

Covering: up to April 2021During the past decades, a plethora of natural products with restricted rotation about a biaryl axis have been discovered, among them the naphthylisoquinoline (NIQ) alkaloids, mostly C,C-coupled and having remarkable bioactivities. Within this fascinating class of naturally occurring biaryl compounds, NIQ alkaloids bearing an N,C-heterobiaryl axis have attracted particular attention. They are structurally and biosynthetically unprecedented, with interesting stereochemical implications and biological activities. In contrast to existing articles and reviews about axially chiral - yet C,C-coupled - natural products, this is the first, comprehensive review on the new subclass of N,C-coupled NIQs, their isolation and structural elucidation, their N,C-axial chirality, their biosynthetic origin, their promising antiparasitic and antileukemic activities, and their total synthesis.

Palladium-mediated synthesis and biological evaluation of C-10b substituted Dihydropyrrolo[1,2-b]isoquinolines as antileishmanial agents

The development of new molecules for the treatment of leishmaniasis is, a neglected parasitic disease, is urgent as current anti-leishmanial therapeutics are hampered by drug toxicity and resistance. The pyrrolo[1,2-b]isoquinoline core was selected as starting point, and palladium-catalyzed Heck-initiated cascade reactions were developed for the synthesis of a series of C-10 substituted derivatives. Their in vitro leishmanicidal activity against visceral (L. donovani) and cutaneous (L. amazonensis) leishmaniasis was evaluated. The best activity was found, in general, for the 10-arylmethyl substituted pyrroloisoquinolines. In particular, 2ad (IC₅₀ = 3.30 μM, SI > 77.01) and 2bb (IC₅₀ = 3.93 μM, SI > 58.77) were approximately 10-fold more potent and selective than the drug of reference (miltefosine), against L. amazonensis on in vitro promastigote assays, while 2ae was the more active compound in the in vitro amastigote assays (IC₅₀ = 33.59 μM, SI > 8.93). Notably, almost all compounds showed low cytotoxicity, CC₅₀ > 100 μg/mL in J774 cells, highest tested dose. In addition, we have developed the first Perturbation Theory Machine Learning (PTML) algorithm able to predict simultaneously multiple biological activity parameters (IC₅₀, K_i, etc.) vs. any Leishmania species and target protein, with high values of specificity (>98%) and sensitivity (>90%) in both training and validation series. Therefore, this model may be useful to reduce time and assay costs (material and human resources) in the drug discovery process.

Dimeric naphthylisoquinoline alkaloids: polyketide-derived axially chiral bioactive quateraryls

This is the first review on dimeric naphthylisoquinolines, a group of structurally intriguing, biosynthetically unique, and pharmacologically promising alkaloids.

Vibrational spectroscopic characterization of arylisoquinolines by means of Raman spectroscopy and density functional theory calculations

Seven new AIQ antimalarial agents were investigated using FT-NIR and deep-UV resonance Raman spectroscopy.

Quinolone Amides as Antitrypanosomal Lead Compounds with In Vivo Activity

Human African trypanosomiasis (HAT) is a major tropical disease for which few drugs for treatment are available, driving the need for novel active compounds. Recently, morpholino-substituted benzyl amides of the fluoroquinolone-type antibiotics were identified to be compounds highly active against Trypanosoma brucei brucei Since the lead compound GHQ168 was challenged by poor water solubility in previous trials, the aim of this study was to introduce structural variations to GHQ168 as well as to formulate GHQ168 with the ultimate goal to increase its aqueous solubility while maintaining its in vitro antitrypanosomal activity. The pharmacokinetic parameters of spray-dried GHQ168 and the newly synthesized compounds GHQ242 and GHQ243 in mice were characterized by elimination half-lives ranging from 1.5 to 3.5 h after intraperitoneal administration (4 mice/compound), moderate to strong human serum albumin binding for GHQ168 (80%) and GHQ243 (45%), and very high human serum albumin binding (>99%) for GHQ242. For the lead compound, GHQ168, the apparent clearance was 112 ml/h and the apparent volume of distribution was 14 liters/kg of body weight (BW). Mice infected with T. b. rhodesiense (STIB900) were treated in a stringent study scheme (2 daily applications between days 3 and 6 postinfection). Exposure to spray-dried GHQ168 in contrast to the control treatment resulted in mean survival durations of 17 versus 9 days, respectively, a difference that was statistically significant. Results that were statistically insignificantly different were obtained between the control and the GHQ242 and GHQ243 treatments. Therefore, GHQ168 was further profiled in an early-treatment scheme (2 daily applications at days 1 to 4 postinfection), and the results were compared with those obtained with a control treatment. The result was statistically significant mean survival times exceeding 32 days (end of the observation period) versus 7 days for the GHQ168 and control treatments, respectively. Spray-dried GHQ168 demonstrated exciting antitrypanosomal efficacy.

Pathogen- and Host-Directed Antileishmanial Effects Mediated by Polyhexanide (PHMB)

BACKGROUND

Cutaneous leishmaniasis (CL) is a neglected tropical disease caused by protozoan parasites of the genus Leishmania. CL causes enormous suffering in many countries worldwide. There is no licensed vaccine against CL, and the chemotherapy options show limited efficacy and high toxicity. Localization of the parasites inside host cells is a barrier to most standard chemo- and immune-based interventions. Hence, novel drugs, which are safe, effective and readily accessible to third-world countries and/or drug delivery technologies for effective CL treatments are desperately needed.

METHODOLOGY/PRINCIPAL FINDINGS

Here we evaluated the antileishmanial properties and delivery potential of polyhexamethylene biguanide (PHMB; polyhexanide), a widely used antimicrobial and wound antiseptic, in the Leishmania model. PHMB showed an inherent antileishmanial activity at submicromolar concentrations. Our data revealed that PHMB kills Leishmania major (L. major) via a dual mechanism involving disruption of membrane integrity and selective chromosome condensation and damage. PHMB's DNA binding and host cell entry properties were further exploited to improve the delivery and immunomodulatory activities of unmethylated cytosine-phosphate-guanine oligodeoxynucleotides (CpG ODN). PHMB spontaneously bound CpG ODN, forming stable nanopolyplexes that enhanced uptake of CpG ODN, potentiated antimicrobial killing and reduced host cell toxicity of PHMB.

CONCLUSIONS

Given its low cost and long history of safe topical use, PHMB holds promise as a drug for CL therapy and delivery vehicle for nucleic acid immunomodulators.

Modelling antibiotic and cytotoxic isoquinoline effects in Staphylococcus aureus, Staphylococcus epidermidis and mammalian cells

Isoquinolines (IQs) are natural substances with an antibiotic potential we aim to optimize. Specifically, IQ-238 is a synthetic analog of the novel-type N,C-coupled naphthylisoquinoline (NIQ) alkaloid ancisheynine. Recently, we developed and tested other IQs such as IQ-143. By utilizing genome-wide gene expression data, metabolic network modelling and Voronoi tessalation based data analysis - as well as cytotoxicity measurements, chemical properties calculations and principal component analysis of the NIQs - we show that IQ-238 has strong antibiotic potential for staphylococci and low cytotoxicity against murine or human cells. Compared to IQ-143, systemic effects are less pronounced. Most enzyme activity changes due to IQ-238 are located in the carbohydrate metabolism. Validation includes metabolite measurements on biological replicates. IQ-238 delineates key properties and a chemical space for a good therapeutic window. The combination of analysis methods allows suggestions for further lead development and yields an in-depth look at staphylococcal adaptation and network changes after antibiosis. Results are compared to eukaryotic host cells.

Anti-trypanosomal activities and structural chemical properties of selected compound classes

Potent compounds do not necessarily make the best drugs in the market. Consequently, with the aim to describe tools that may be fundamental for refining the screening of candidates for animal and preclinical studies and further development, molecules of different structural classes synthesized within the frame of a broad screening platform were evaluated for their trypanocidal activities, cytotoxicities against murine macrophages J774.1 and selectivity indices, as well as for their ligand efficiencies and structural chemical properties. To advance into their modes of action, we also describe the morphological and ultrastructural changes exerted by selected members of each compound class on the parasite Trypanosoma brucei. Our data suggest that the potential organelles targeted are either the flagellar pocket (compound 77, N-Arylpyridinium salt; 15, amino acid derivative with piperazine moieties), the endoplasmic reticulum membrane systems (37, bisquaternary bisnaphthalimide; 77, N-Arylpyridinium salt; 68, piperidine derivative), or mitochondria and kinetoplasts (88, N-Arylpyridinium salt; 68, piperidine derivative). Amino acid derivatives with fumaric acid and piperazine moieties (4, 15) weakly inhibiting cysteine proteases seem to preferentially target acidic compartments. Our results suggest that ligand efficiency indices may be helpful to learn about the relationship between potency and chemical characteristics of the compounds. Interestingly, the correlations found between the physico-chemical parameters of the selected compounds and those of commercial molecules that target specific organelles indicate that our rationale might be helpful to drive compound design toward high activities and acceptable pharmacokinetic properties for all compound families.

Identification of multiple cellular uptake pathways of polystyrene nanoparticles and factors affecting the uptake: relevance for drug delivery systems

Nanoparticles may address challenges by human diseases through improving diagnosis, vaccination and treatment. The uptake mechanism regulates the type of threat a particle poses on the host cells and how a cell responds to it. Hence, understanding the uptake mechanisms and cellular interactions of nanoparticles at the cellular and subcellular level is a prerequisite for their effective biomedical applications. The present study shows the uptake mechanisms of polystyrene nanoparticles and factors affecting their uptake in bone marrow-derived macrophages, 293T kidney epithelial cells and L929 fibroblasts. Labeling with the endocytic marker FM4-64 and transmission electron microscopy studies show that the nanoparticles were internalized rapidly via endocytosis and accumulated in intracellular vesicles. Soon after their internalizations, nanoparticles trafficked to organelles with acidic pH. Analysis of the ultrastructural morphology of the plasma membrane invaginations or extravasations provides clear evidence for the involvement of several uptake routes in parallel to internalize a given type of nanoparticles by mammalian cells, highlighting the complexity of the nanoparticle-cell interactions. Blocking the specific endocytic pathways by different pharmacological inhibitors shows similar outcomes. The potential to take up nanoparticles varies highly among different cell types in a particle sizes-, time- and energy-dependent manner. Furthermore, infection and the activation status of bone marrow-derived macrophages significantly affect the uptake potential of the cells, indicating the need to understand the diseases' pathogenesis to establish effective and rational drug-delivery systems. This study enhances our understanding of the application of nanotechnology in biomedical sciences.

Rh(iii)-catalyzed synthesis of 1-substituted isoquinolinium salts via a C–H bond activation reaction of ketimines with alkynes

An efficient synthesis of highly substituted isoquinolinium salts from ketimines and alkynes via a Rh(iii)-catalyzed C–H bond activation and annulation reaction is described.

A novel Leishmania major amastigote assay in 96-well format for rapid drug screening and its use for discovery and evaluation of a new class of leishmanicidal quinolinium salts

In most laboratories, the screening for leishmanicidal compounds is carried out with Leishmania promastigotes or axenic amastigotes. However, the best approach to identify leishmanicidal compounds is the use of amastigotes residing in macrophages. Reporter gene-based assays are relatively new tools in the search for drugs against eucaryotic protozoa, permitting the development of faster, more automated assays. In this paper, we report on the establishment of a rapid screening assay in a 96-well format. A luciferase-transgenic (Luc-tg) Leishmania major strain was generated and used to infect bone marrow-derived macrophages (BMDM). Amastigote-infected BMDM were treated with different compound concentrations. Cells were lysed with a luciferin-containing buffer, and the resulting luminescence was measured to determine the half-maximal inhibitory concentration (IC50). To validate this new amastigote screening assay, a library of a new class of quinolinium salts was synthesized and tested for leishmanicidal activity. Some of the quinolinium salts showed very promising activities, with IC50s against intracellular amastigotes (IC50 < 1 μg/ml) and selectivity indices (SI > 20) that match the criteria of World Health Organization (WHO) for hits. Compound 21c (IC50 = 0.03 μg/ml; SI = 358) could become a new lead structure for the development of improved chemotherapeutic drugs against L. major. In summary, we describe the establishment of a new 96-well format assay with Luc-transgenic L. major for the rapid screening of compounds for leishmanicidal activity against intracellular amastigotes and its application to the identification of a new class of quinolinium salts with most promising leishmanicidal activity.

Antileishmanial bis-arylimidamides: DB766 analogs modified in the linker region and bis-arylimidamide structure-activity relationships

Analogs of the lead antileishmanial bis-arylimidamide DB766 were prepared that possess unsymmetrical substitutions on the diphenylfuran linker, and an additional compound was synthesized that contains isopropoxy groups meta to the central furan. These agents all displayed nanomolar in vitro potency against intracellular Leishmania with selectivity indexes >100 compared to J774 macrophages. While the unsymmetrical analogs were toxic to mice when given ip at 30 mg/kg/day, the compound bearing the meta isopropoxy groups was well tolerated by mice and showed activity in a murine model of visceral leishmaniasis when administered ip at 30 mg/kg/day for five days.

Modeling antibiotic and cytotoxic effects of the dimeric isoquinoline IQ-143 on metabolism and its regulation in Staphylococcus aureus, Staphylococcus epidermidis and human cells

BACKGROUND

Xenobiotics represent an environmental stress and as such are a source for antibiotics, including the isoquinoline (IQ) compound IQ-143. Here, we demonstrate the utility of complementary analysis of both host and pathogen datasets in assessing bacterial adaptation to IQ-143, a synthetic analog of the novel type N,C-coupled naphthyl-isoquinoline alkaloid ancisheynine.

RESULTS

Metabolite measurements, gene expression data and functional assays were combined with metabolic modeling to assess the effects of IQ-143 on Staphylococcus aureus, Staphylococcus epidermidis and human cell lines, as a potential paradigm for novel antibiotics. Genome annotation and PCR validation identified novel enzymes in the primary metabolism of staphylococci. Gene expression response analysis and metabolic modeling demonstrated the adaptation of enzymes to IQ-143, including those not affected by significant gene expression changes. At lower concentrations, IQ-143 was bacteriostatic, and at higher concentrations bactericidal, while the analysis suggested that the mode of action was a direct interference in nucleotide and energy metabolism. Experiments in human cell lines supported the conclusions from pathway modeling and found that IQ-143 had low cytotoxicity.

CONCLUSIONS

The data suggest that IQ-143 is a promising lead compound for antibiotic therapy against staphylococci. The combination of gene expression and metabolite analyses with in silico modeling of metabolite pathways allowed us to study metabolic adaptations in detail and can be used for the evaluation of metabolic effects of other xenobiotics.

Phenolic analogs of the N,C-coupled naphthylisoquinoline alkaloid ancistrocladinium A, from Ancistrocladus cochinchinensis (Ancistrocladaceae), with improved antiprotozoal activities

The first N,8'-coupled naphthylisoquinoline alkaloids with free phenolic OH groups, 4'-O-demethylancistrocladinium A and 6,4'-O-didemethylancistrocladinium A, have been isolated from the leaves and bark of the Vietnamese liana Ancistrocladus cochinchinensis, along with its known, non-phenolic parent compound, ancistrocladinium A, and four C,C-coupled representatives. The structure elucidation was achieved by chemical, spectroscopic, and chiroptical methods. The mono-phenolic alkaloid showed excellent activities in particular against the pathogen causing Chagas' disease, Trypanosoma cruzi.

Aspergiolides C and D: spirocyclic aromatic polyketides with potent protein kinase c-Met inhibitory effects

Variation of the cultivation conditions for Aspergillus glaucus led to the discovery of two novel spirocyclic aromatic polyketides, aspergiolides C (3) and D (4). Their constitutions were elucidated by a combination of spectroscopic methods and isotope-labeling experiments. Aspergiolides C (3) and D (4) occur as racemic mixtures, the resolution of which was succeeded by HPLC on a chiral phase. The absolute configurations of their enantiomers were assigned online, from the peaks in the chromatogram, by a combination of HPLC-CD and quantum chemical CD calculations. Both compounds were found to inhibit the kinase activities of the receptor tyrosine kinases (RTKs) c-Met, Ron, and c-Src with low-micromolar IC(50)s. The enantiomers of 3 were resolved by HPLC on a chiral phase. Both enantiomers showed a comparable inhibition of the HGF-induced autophosphorylation of c-Met and of subsequent cell migration.

Alterations to the structure of Leishmania major induced by N-arylisoquinolines correlate with compound accumulation and disposition

Naphthylisoquinoline alkaloids equipped with a N,C-hetero-‘biaryl’ axis, and, in particular, simplified synthetic analogues thereof, kill intracellular Leishmania major at concentrations in the low submicromolar range, while being significantly less toxic to their major host cell, the macrophage, at the same concentrations. To further investigate their mechanism of action we evaluated the morphological and ultrastructural changes induced by specific N-arylisoquinolines in L. major, and the correlation of these changes with compound accumulation and disposition by the parasite. After 24 h of treatment with the synthetic arylisoquinolinium salts 3 or 4, dramatic structural changes and cell death were observed. Furthermore, the auto-fluorescent derivative salt 3 accumulates continually in intracellular compartments. Our results thus suggest that the leishmanicidal effect of arylisoquinolinium salts may involve their ability to accumulate and precipitate in intracellular organelles, form a huge vacuole and eventually promote cell lysis.

Convergence in the biosynthesis of acetogenic natural products from plants, fungi, and bacteria

This review deals with polyketides to which nature has developed different biosynthetic pathways in the course of evolution. The anthraquinone chrysophanol is the first example of an acetogenic natural product that is, in an organism-specific manner, formed via more than one polyketide folding mode: In eukaryotes, like e.g., in fungi, in higher plants, and in insects, it is synthesized via folding mode F, while in prokaryotes it originates through mode S. It has, more recently, even been found to be synthesized by a third pathway, named mode S'. Thus, chrysophanol is the first polyketide synthase product that originates through a divergent-convergent biosynthesis (depending on the respective producing organisms). A second example of a striking biosynthetic convergence is the isoquinoline alkaloids. While all as yet investigated representatives of this large family of plant-derived metabolites (more than 2500 known representatives!) are formed from aromatic amino acids, the biosynthetic origin of naphthylisoquinoline alkaloids like dioncophylline A is unprecedented in following a route to isoquinolines in plants: we have shown that such naphthylisoquinolines represent the as yet only known polyketidic di- and tetrahydroisoquinolines, originating from acetate and malonate units, exclusively. Both molecular halves, the isoquinoline part and the naphthalene portion, are even synthesized from a joint polyketide precursor, the first proven case of the F-type folding mode in higher plants. The biosynthetic origins of the natural products presented in this paper were elucidated by feeding (13)C(2)-labeled acetate (or advanced precursors) to the respective producing organisms, with subsequent NMR analysis of their (13)C(2) incorporation patterns using the potent cryoprobe methodology, thus making the full polyketide folding pattern visible.