Antitumoral and antileishmanial dioncoquinones and ancistroquinones from cell cultures of Triphyophyllum peltatum (Dioncophyllaceae) and Ancistrocladus abbreviatus (Ancistrocladaceae)

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.

Naphthylisoquinoline alkaloids: novel agents against the causative pathogens of eumycetoma and actinomycetoma-en route to broad-spectrum antimycetomal drugs

Mycetoma is a devastating neglected tropical infection of the subcutaneous tissues. It is caused by fungal and bacterial pathogens recognized as eumycetoma and actinomycetoma, respectively. Mycetoma treatment involves diagnosing the causative microorganism as a prerequisite to prescribing a proper medication. Current therapy of fungal eumycetoma causative agents, such as Madurella mycetomatis, consists of long-term antifungal medication with itraconazole followed by surgery, yet with usually unsatisfactory clinical outcomes. Actinomycetoma, on the contrary, usually responds to treatment with co-trimoxazole and amikacin. Therefore, there is a pressing need to discover novel broad-spectrum antimicrobial agents to circumvent the time-consuming and costly diagnosis. Using the resazurin assay, a series of 23 naphthylisoquinoline (NIQ) alkaloids and related naphthoquinones were subjected to in vitro screening against two fungal strains of M. mycetomatis and three bacterial strains of Actinomadura madurae and A. syzygii. Seven NIQs, mostly dimers, showed promising in vitro activities against at least one strain of the mycetoma-causative pathogens, while the naphthoquinones did not show any activity. A synthetic NIQ dimer, 8,8'''-O,O-dimethylmichellamine A (18), inhibited all tested fungal and bacterial strains (IC50 = 2.81-12.07 µg/mL). One of the dimeric NIQs, michellamine B (14), inhibited a strain of M. mycetomatis and significantly enhanced the survival rate of Galleria mellonella larvae infected with M. mycetomatis at concentrations of 1 and 4 µg/mL, without being toxic to the uninfected larvae. As a result, broad-spectrum dimeric NIQs like 14 and 18 with antimicrobial activity are considered hit compounds that could be worth further optimization to develop novel lead antimycetomal agents.

Dimeric (Poly)Hydroxynaphthazarins, Metabolites of Echinoderms and Lichens: The History of the Synthesis and Structure Elucidation

This review provides information on the synthesis and revision of the structures of natural dimeric (poly)hydroxynaphthazarins, metabolites of echinoderms and lichens, and on the refinement of the direction and mechanism of reactions in the synthesis of some of these compounds.

Carnivory on demand: phosphorus deficiency induces glandular leaves in the African liana Triphyophyllum peltatum

Triphyophyllum peltatum, a rare tropical African liana, is unique in its facultative carnivory. The trigger for carnivory is yet unknown, mainly because the plant is difficult to propagate and cultivate. This study aimed at identifying the conditions that result in the formation of carnivorous leaves. In vitro shoots were subjected to abiotic stressors in general and deficiencies of the major nutrients nitrogen, potassium and phosphorus in particular, to trigger carnivorous leaves' development. Adventitious root formation was improved to allow verification of the trigger in glasshouse-grown plants. Among all the stressors tested, only under phosphorus deficiency, the formation of carnivorous leaves was observed. These glandular leaves fully resembled those found under natural growing conditions including the secretion of sticky liquid by mature capture organs. To generate plants for glasshouse experiments, a pulse of 55.4 μM α-naphthaleneacetic acid was essential to achieve 90% in vitro rooting. This plant material facilitated the confirmation of phosphorus starvation to be essential and sufficient for carnivory induction, also under ex vitro conditions. Having established the cultivation of T. peltatum and the induction of carnivory, future gene expression profiles from phosphorus starvation-induced leaves will provide important insight to the molecular mechanism of carnivory on demand.

Biological Potential of Carnivorous Plants from Nepenthales

Since Charles Darwin and his book carnivorous plants have aroused interest and heated debate. In addition, there is growing interest in this group of plants as a source of secondary metabolites and in the application of their biological activity. The aim of this study was to trace the recent literature in search of the application of extracts obtained from families Droseraceae, Nepenthaceae, and Drosophyllaceae to show their biological potential. The data collected in the review clearly indicate that the studied Nepenthales species have great biological potential in terms of antibacterial, antifungal, antioxidant, anti-inflammatory, and anticancer use. We proposed that further investigations should include: (i) bioactivity-guided investigations of crude plant extract to connect a particular type of action with a specific compound or a group of metabolites; (ii) a search for new bioactive properties of carnivorous plants; (iii) establishment of molecular mechanisms associated with specific activity. Furthermore, further research should be extended to include less explored species, i.e., Drosophyllum lusitanicum and especially Aldrovanda vesiculosa.

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.

Activity of New Synthetic (2-Chloroethylthio)-1,4-naphthoquinones in Prostate Cancer Cells

Development of resistance to currently available standard therapies in advanced prostate cancer (PCa) emphasizes the need for novel therapeutic options. Here, we report the synthesis of new hybrid molecules consisting of 2-chloroethylthio and 1,4-naphthoquinone pharmacophores and describe their activity in PCa. In screening analyses, the introduction of one 2-chloroethylthio group improved the anticancer properties of 1,4-naphthoquinones, whereas the introduction of a second 2-chloroethylthio moiety rather decreased activity. Two most promising of the synthesized compounds, 30 and 32, were highly active in different human PCa cell lines harboring varying resistance profiles at nanomolar concentrations. The generated data suggest that the compounds are capable of mitochondria targeting, cytotoxic ROS induction, and DNA damage, which resulted in apoptosis presumably executed in a caspase-dependent manner. The substances synergized with the clinically approved PARP inhibitor olaparib and resensitized AR-V7-expressing PCa cells to antiandrogen enzalutamide, as well as to a combination of enzalutamide and an AKT inhibitor. This was at least in part exerted via down-regulation of AR-V7 expression and inhibition of AR signaling. Mild antagonism was observed in combination with platinum- or taxane-based chemotherapy, which was putatively related to treatment-induced activation of p38, JNK1/2, ERK1/2, MEK1/2, and AKT, functioning as potential pro-survival factors. Thus, the synthesized (2-chloroethylthio)-1,4-naphthoquinone derivatives exhibit promising anticancer properties in vitro, suggesting their further development as potential therapeutics for the treatment of castration-resistant PCa.

Multiple Myeloma Inhibitory Activity of Plant Natural Products

A literature search on plant natural products with antimyeloma activity until the end of 2020 resulted in 92 compounds with effects on at least one human myeloma cell line. Compounds were divided in different compound classes and both their structure-activity-relationships as well as eventual correlations with the pathways described for Multiple Myeloma were discussed. Each of the major compound classes in this review (alkaloids, phenolics, terpenes) revealed interesting candidates, such as dioncophyllines, a group of naphtylisoquinoline alkaloids, which showed pronounced and selective induction of apoptosis when substituted in position 7 of the isoquinoline moiety. Interestingly, out of the phenolic compound class, two of the most noteworthy constituents belong to the relatively small subclass of xanthones, rendering this group a good starting point for possible further drug development. The class of terpenoids also provides noteworthy constituents, such as the highly oxygenated diterpenoid oridonin, which exhibited antiproliferative effects equal to those of bortezomib on RPMI8226 cells. Moreover, triterpenoids containing a lactone ring and/or quinone-like substructures, e.g., bruceantin, whitaferin A, withanolide F, celastrol, and pristimerin, displayed remarkable activity, with the latter two compounds acting as inhibitors of both NF-κB and proteasome chymotrypsin-like activity.

Synthesis and Evaluation of Antimicrobial and Cytotoxic Activity of Oxathiine-Fused Quinone-Thioglucoside Conjugates of Substituted 1,4-Naphthoquinones

A series of new tetracyclic oxathiine-fused quinone-thioglycoside conjugates based on biologically active 1,4-naphthoquinones and 1-mercapto derivatives of per-O-acetyl d-glucose, d-galactose, d-xylose, and l-arabinose have been synthesized, characterized, and evaluated for their cytotoxic and antimicrobial activities. Six tetracyclic conjugates bearing a hydroxyl group in naphthoquinone core showed high cytotoxic activity with EC50 values in the range of 0.3 to 0.9 μM for various types of cancer and normal cells and no hemolytic activity up to 25 μM. The antimicrobial activity of conjugates was screened against Gram-positive bacteria (Staphylococcus aureus, Bacillus cereus), Gram-negative bacteria (Pseudomonas aeruginosa and Escherichia coli), and fungus Candida albicans by the agar diffusion method. The most effective juglone conjugates with d-xylose or l-arabinose moiety and hydroxyl group at C-7 position of naphthoquinone core at concentration 10 µg/well showed antimicrobial activity comparable with antibiotics vancomicin and gentamicin against Gram-positive bacteria strains. In liquid media, juglone-arabinosidic tetracycles showed highest activity with MIC 6.25 µM. Thus, a positive effect of heterocyclization with mercaptosugars on cytotoxic and antimicrobial activity for group of 1,4-naphthoquinones was shown.

Epoxide containing molecules: A good or a bad drug design approach

Functional group modification is one of the main strategies used in drug discovery and development. Despite the controversy of being identified for many years as a biologically hazardous functional group, the introduction of an epoxide function in a structural backbone is still one of the possible modifications being implemented in drug design. In this manner, it is our intention to prove with this work that epoxides can have significant interest in medicinal chemistry, not only as anticancer agents, but also as important drugs for other pathologies. Thus, this revision paper aims to highlight the biological activity and the proposed mechanisms of action of several epoxide-containing molecules either in preclinical studies or in clinical development or even in clinical use. An overview of the chemistry of epoxides is also reported. Some of the conclusions are that effectively most of the epoxide-containing molecules referred in this work were being studied or are in the market as anticancer drugs. However, some of them in preclinical studies, were also associated with other different activities such as anti-malarial, anti-arthritic, insecticidal, antithrombotic, and selective inhibitory activity of FXIII-A (a transglutaminase). As for the epoxide-containing molecules in clinical trials, some of them are being tested for obesity and schizophrenia. Finally, drugs containing epoxide groups already in the market are mostly used for the treatment of different types of cancer, such as breast cancer and multiple myeloma. Other diseases for which the referred drugs are being used include heart failure, infections and gastrointestinal disturbs. In summary, epoxides can be a suitable option in drug design, particularly in the design of anticancer agents, and deserve to be better explored. However, and despite the promising results, it is imperative to explore the mechanisms of action of these compounds in order to have a better picture of their efficiency and safety.

Novel Catechol Derivatives of Arylimidamides as Antileishmanial Agents

Two novel bis-arylimidamide derivatives with terminal catechol moieties (9a and 10a) and two parent compounds with terminal phenyl groups (DB613 and DB884) were synthesized as dihydrobromide salts (9b and 10b). The designed compounds were hybrid molecules consisting of a catechol functionality embedded in an arylimidamide moiety. All compounds were examined for in vitro antiparasitic activity upon promastigotes of Leishmania major and L. infantum as well as axenic amastigotes of L. major. It was shown that conversion of terminal phenyl groups into catechol moieties resulted in more than 10-fold improvement in potency, coupled with lower cytotoxicity against fibroblast cells, compared to the corresponding parent compounds. The furan-containing analog 9a exhibited the highest activity with submicromolar IC₅₀ values, ranging from 0.29 to 0.36 μm, which is comparable in efficacy to the reference drug amphotericin B (IC₅₀ 0.28 - 0.33 μm). The results justify further study of this class of compounds. It seems that the combination of catechol chelating groups with potent antiparasitic agents could improve the efficacy by presenting novel hybrid compounds.

Compounds from African Medicinal Plants with Activities Against Selected Parasitic Diseases: Schistosomiasis, Trypanosomiasis and Leishmaniasis

Parasitic diseases continue to represent a threat on a global scale, particularly among the poorest countries in the world. This is particularly because of the absence of vaccines, and in some cases, resistance against available drugs, currently being used for their treatment. In this review emphasis is laid on natural products and scaffolds from African medicinal plants (AMPs) for lead drug discovery and possible further development of drugs for the treatment of parasitic diseases. In the discussion, emphasis has been laid on alkaloids, terpenoids, quinones, flavonoids and narrower compound classes of compounds with micromolar range activities against Schistosoma, Trypanosoma and Leishmania species. In each subparagraph, emphasis is laid on the compound subclasses with most promising in vitro and/or in vivo activities of plant extracts and isolated compounds. Suggestions for future drug development from African medicinal plants have also been provided. This review covering 167 references, including 82 compounds, provides information published within two decades (1997-2017).

Axially Chiral Dimeric Naphthalene and Naphthoquinone Metabolites, from Root Cultures of the West African Liana Triphyophyllum peltatum

Root cultures of the West African liana Triphyophyllum peltatum were initiated from stem explants of in vitro cultivated shoots. From these organ cultures, three new binaphthalenes, one binaphthoquinone, and two (bi)naphthalene glucosides were isolated, with substitution patterns related to those of the naphthylisoquinoline alkaloids, which are the "normal" main metabolites of T. peltatum. The structures of the diglucoside dioncoquinoside A (1) and of the axially chiral biaryls triphyoquinols A1 (3), A2 (4), and B (5), triphyoquinoside A (6), and triphyoquinone A (7) were elucidated by spectroscopic analysis (HRESIMS, 1D and 2D NMR) and by application of electronic circular dichroism (ECD) spectroscopy in combination with the exciton chirality method and quantum-chemical ECD calculations. The root cultures likewise produced the known alkaloids dioncophylline A (8), 5'-O-demethyldioncophylline A (9), dioncopeltine A (10), habropetaline A (11), and 5'-O-methyldioncophylline D (12a/b), the naphthalene glucoside plumbaside A (2), and the naphthoquinones plumbagin (13), droserone (14), and 8-hydroxydroserone (15).

Cytotoxic quinones from the roots of Aloe dawei

Seven naphthoquinones and nine anthraquinones were isolated from the roots of Aloe dawei by chromatographic separation. The purified metabolites were identified by NMR and MS analyses. Out of the sixteen quinones, 6-hydroxy-3,5-dimethoxy-2-methyl-1,4-naphthoquinone is a new compound. Two of the isolates, 5,8-dihydroxy-3-methoxy-2-methylnaphthalene-1,4-dione and 1-hydroxy-8-methoxy-3-methylanthraquinone showed high cytotoxic activity (IC₅₀ 1.15 and 4.85 µM) on MCF-7 breast cancer cells, whereas the others showed moderate to low cytotoxic activity against MDA-MB-231 (ER Negative) and MCF-7 (ER Positive) cancer cells.

Natural product based leads to fight against leishmaniasis

The growing incidence of parasitic resistance against generic pentavalent antimonials, specifically for visceral disease in Indian subcontinent, is a serious issue in Leishmania control. Notwithstanding the two treatment alternatives, that is amphotericin B and miltefosine are being effectively used but their high cost and therapeutic complications limit their use in endemic areas. In the absence of a vaccine candidate, identification, and characterization of novel drugs and targets is a major requirement of leishmanial research. This review describes current drug regimens, putative drug targets, numerous natural products that have shown promising antileishmanial activity alongwith some key issues and strategies for future research to control leishmaniasis worldwide.

Three new polyketides from marine-derived fungus Aspergillus glaucus HB1-19

Two new benzyl derivatives, aspergentisyl A (1) and aspergentisyl B (2), as well as one new naphthoquinone derivative, aspergiodiquinone (3), together with seven known prenylated benzaldehyde derivatives (4-10) were isolated from the marine-derived fungus Aspergillus glaucus HB1-19. The structures of these compounds were characterized based on 1D and 2D NMR spectra analyses and comparison with those reported in the literature. In addition, each isolate was tested for its 1,1-diphenyl-2-picrylhydrazyl radical-scavenging property and all these compounds except compound 3 exhibited strong radical-scavenging activity.

Bioactive natural products derived from the Central African flora against neglected tropical diseases and HIV

This review discusses the medicinal potential of bioactive metabolites isolated from medicinal plants in Central Africa for the treatment of neglected tropical diseases and HIV. A correlation is established between the biological activities of the isolated compounds and the uses of the plants in traditional medicine. Insight is provided on how secondary metabolites from medicinal plants in Central Africa could be exploited for drug discovery.

Highly selective antiplasmodial naphthylisoquinoline alkaloids from Ancistrocladus tectorius

Naphthylisoquinoline alkaloids, named ancistectorine A1, N-methylancistectorine A1, ancistectorine A2, 5-epi-ancistectorine A2, ancistectorine A3, ancistectorine B1, and ancistectorine C1, have been isolated from twigs of the Chinese plant Ancistrocladus tectorius. The structural elucidation succeeded by chemical, spectroscopic, and chiroptical methods. Three of these compounds exhibited excellent, and specific, antiplasmodial activities, comparable with that of the as yet most active representative, dioncophylline C. Moreover, the antitumoral activities of two of the main alkaloids in this species was tested.

Perspectives on medicinal properties of plumbagin and its analogs

Plumbagin is one of the simplest plant secondary metabolite of three major phylogenic families viz. Plumbaginaceae, Droseraceae, and Ebenceae, and exhibits highly potent biological activities, including antioxidant, antiinflammatory, anticancer, antibacterial, and antifungal activities. Recent investigations indicate that these activities arise mainly out of its ability to undergo redox cycling, generating reactive oxygen species and chelating trace metals in biological system. The compound is endowed with a property to inhibit the drug efflux mechanism in drug-resistant bacteria, thereby allowing intracellular accumulation of the potent drug molecules. An interesting bioactivity exhibited by this compound is the elimination of stringent, conjugative, multidrug-resistant plasmids from several bacterial strains including opportunistic bacteria, such as Acinetobacter baumannii. Moreover, plumbagin effectively induces apoptosis and causes cell cycle arrest, which is, in part, due to the inactivation of NF-κB in cancer cells. Therefore, it has been suggested that designing "hybrid drug molecules" of plumbagin by combining it with other appropriate anticancer agents may lead to the generation of novel and potent anticancer drugs with pleiotropic action against human cancers. This comprehensive review is an attempt to understand the chemistry of plumbagin and catalog its biological activities reported to date.